Second-order nucleophilic substitution reactions of cyclobutane derivatives: A study of the stability of pentacoordinate carbon

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Content SECOND-ORDER NUCLEOPHILIC SUBSTITUTION l| REACTIONS OF CYCLOBUTANE DERIVATIVES, A STUDY OF THE STABILITY OF PENTACOORDINATE CARBON 'by T arek M ukhtar E lgom ati I ft A T h e s is P r e s e n te d t o th e FACULTY OF THE GRADUATE SCHOOL In P a r t i a l F u lf il lm e n t o f th e R equirem ents f o r th e Degree 'MASTER OF SCIENCE (C h em istry ) J a n u a ry 1973 UMI Number: EP41654 All rights reserved IN FO R M A TIO N T O ALL U SERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI EP41654 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 -1 3 4 6 - R u 5 UNIVERSITY O F S O U T H E R N CA LIFO RN IA TH E GRADUATE SCH O O L U N IV ER SITY PARK LO S A N G ELES. C A LIFO R N IA 9 0 0 0 7 c ' 7 3 This thesis, written by T A R E K M U K H .......................... . under the direction of hJ.$...Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of M a s te r o f S cie n ce Dean F e b ru a ry 1973 D ate.................................. THESIS COMMITTEE ' 1 To my p a r e n ts 11 ACKNOWLEDGMENTS ment o f s e a r c h , ! w ould l i k e to e x p re s s my deep a p p r e c ia ti o n to t h e G overn- L ib y a f o r t h e i r c o n tin u e d su p p o rt o f my e d u c a tio n and r e - and t o Dr. I v a r K. U g i, f o r h i s g u id a n c e and d i r e c t i o n . TABLE OF CONTENTS A cknow ledgm ents.................................................................................................................... L i s t o f T a b l e s ................................................................................................................. L i s t o f F i g u r e s ..................... ■ ...................................................... I . I n t r o d u c t i o n , D e f i n i t i o n o f th e P ro b lem , and D is c u s s io n o f th e P r e s e n t S ta tu s o f th e F i e l d . — The C l a s s i c a l L im itin g Mechanism o f N u c le o p h ilic S u b s t i t u t i o n . . . — The R e o r g a n iz a tio n o f th e C o n ceiv ab le T e tra h e d ro n b ip y ra m i- d a l (TBP) I n te r m e d ia te o f S - P r o c e s s e s . . .......................................... — R e g u la r and I r r e g u l a r P r o c e s s e s ................................................................... — F o u r- and Five-m em bered C y c lic S y ste m s................................................. — Six-m em bered C y c lic S y ste m s........................................■ ................................... —P o l a r i t y E f f e c t s and th e S t e r i c Course o f S^ R e a c tio n s .......... —The C o r r e la tio n o f th e S t e r i c Course o f S^ P ro c e s s e s and th e R e o rg a n iz a tio n Mechanism o f TBP I n t e r m e d i a t e s ............. —The Pathw ays o f F o rm atio n o f TBP P e n ta c o o r d in a te S p ecies from T e t r a c o o r d in a te S p e c i e s ........................ . —R e te n tiv e D isp lacem en t R e a c tio n s o f C y c lic Systems I I . The Model E x p e rim e n ts ............................................................................................. I I I . D is c u s s io n o f R e s u l t s ............................................................................................. IV . E x p e rim e n ta l S e c t i o n ................................................................................................ V.- R e f e r e n c e s .......................................... .............................................................................. LIST -OF TABLES 1. Summary o f t h e S t e r i c Course o f S u b s t i t u t i o n R eactions- o f T e tr a c o o r d in a te M o le c u la r Compounds v i a P e n ta c o o rd in a te I n t e r m e d i a t e s . . . . ........................ 2.- E q u ilib riu m Geometry B in d in g E n e rg ie s (BE) o f Walden S p ecies :and t h e i r P e rm u ta tio n I s o m e r s -................... v LIST OF FIGURES I . N u c le o p h ilic S u b s t i t u t i o n on Carbon v i a TBP (R e a c tio n l ) .................................................................................................................... 2a. B erry P s e u d o r o ta tio n (BBR) R e a c tio n M echanism .................................. 2b. Components o f M otion f o r th e BPR M echanism................................ 3. S in g le and M u ltip le T u r n s t i l e R o ta tio n (TR) P r o c e s s e s ................ U. Base C a ta ly z e d R e a c tio n (R e a c tio n 2 ) ................. , ....................................... 5. T herm al o r A cid C a ta ly z e d R e a c tio n (R e a c tio n 3 ) ............................... 6. N u c le o p h ilic S u b s t i t u t i o n o f Phosphonium Io n on C y clic Six-m em bered Ring (R e a c tio n . . . .............................................................. 7- N u c le o p h ilic S u b s t i t u t i o n w ith (TR) o f th e TBP I n t e r m e d ia te (R e a c tio n 5)............................................................................................... 8. N u c le o p h ilic S u b s t i t u t i o n o f TBP w hich Undergo R e o rg a n i z a t io n by S im ple and M u ltip le (TR) P ro c e s s e s (R e a c tio n 6 ) . . . 9- P o s s i b l e A pproaches t o a T e tra h e d ro n and th e R e s u ltin g I n te r m e d ia te s (R e a c tio n j ) . .................................................................................. 1 0 a. Model B a r r i e r S i t u a t i o n o f th e Edge A t t a c k .......................................... 10b. Model B a r r i e r S i t u a t i o n o f th e Face A t t a c k ...................... ................... I I . A d d itio n - e lim in a tio n M echanism....................................................................... 1 2a. T e t r a h e d r a l A tta c k I n t e r m e d i a t e ..................................................................... 12b. T e t r a h e d r a l A tta c k I n t e r m e d i a t e ..................................................................... 13. Two Competing Pathw ays f o r S u b s t i t u t i o n on a Model P hos p h a te (R e a c tio n 8 ) ........................................................... ......................................... 1^. Scheme 1 C o rresp o n d in g t o th e C l a s s i c a l SN2 M e c h a n is m ........ v i ■15. N u c le o p h ilic S u b s t i t u t i o n o f C y c lic F o u r- or F iv e - membered R ings In v o lv in g TBP I n te r m e d ia te s w hich can Undergo R e o r g a n iz a tio n by Sim ple and M u ltip le TR P ro c e s s e s (R e a c tio n 9 ) .......... •,.............................................................................. l 6 a , b , c . L igand Exchange P ro c e s s e s o f C y c lic Four-niembered R ings o f P hosphorus Compounds................................................................... l6 d . L igand Exchange P r o c e s s e s o f C y c lic Five-m em bered Rings o f Phosphorus Compounds . (R e a c tio n lO d ) ............................................ 17. R e d u c tio n o f C y c lic P h o sp h in e Oxide (R e a c tio n l l ) .................. 18. L igand Exchange P ro c e s s o f Four-m em bered R ings o f...P h o s p h o ru s Compounds (R e a c tio n 1 2 ) ............................................. 19.. Model C o n fig u ra tio n R e te n tio n 3^2 Mechanism f o r A lip h a tic C arb o n.......................................................................................................................... 20. P r e p a r a t i o n o f c is - 3 - e th o x y c y e lo b u ta n o l (R e a c tio n 1 3 ) . . . . 21. H o d el E x p erim en ts f o r B im o le c u la r S^. Mechanism o f C yclo- b u ta n e D e r i v a t i v e s ............................................ ......................................... 22. N u c le o p h ilic S u b s t i t u t i o n on A lc o h o l by Amine G roup............. 23. B im o le c u la r N u c le o p h ilic S u b s t i t u t i o n o f A lc o h o l.................... 2k. P o s s i b l e Pathw ays f o r th e N u c le o p h ilic S u b s t i t u t i o n o f Four-m em bered R ings v i a TBP, w hich Undergo R e o rg a n iz a tio n 2 by TR and (TR) P r o c e s s e s ......................................................................... .. vii INTRODUCTION, DEFINITION OF THE PROBLEM, AND DISCUSSION OF THE PRESENT STATUS' OF THE FIELD. 2 T he■C l a s s i c a l L im itin g M echanism -of N u c le o p h ilic S u b s t i t u t i o n . I . J? | The l i m i t i n g a l i p h a t i c n u c l e o p h i l i c s u b s t i t u t i o n r e a c t i o n mech-. lanism s t h a t .w ere in tr o d u c e d by In g o ld and c o - w o r k e r s h a v e b een a I I I p a r t o f th e b a s i c te x tb o o k know ledge ,of o rg a n ic chem ists, f o r . t h i r t y l y e a r s . ^ ^ C hem ists have u se d th e s e m odels f o r m e c h a n is tic th o u g h t 'a n d a r e .p r o v i d i n g c o n s i s t e n t i n t e r p r e t a t i o n and p r e d i c t i o n s f o r many 'r e a c t i o n s . i In th e case o f t h e •l i m i t i n g S ^ l •e l im i n a tio n a d d i t i o n mechanism jno doubt e x i s t s .c o n c e rn in g th e in te rm e d ia c y o f • carbonium .ion w ith * ( 2 ) jtrx c o o rd x n a te carb o n . Many■ q u e stx o n s have been r a i s e d wxth r e s p e c t s • ( 3 ) . . ito th e n a tu re o f p e n ta e o o r d in a te carb o n s p e c ie s in v o lv e d in th e i ' (3 -1 0 ) I l i m i t i n g S ^ 2 .an d S ^ i p r o c e s s e s and a ls o th e p e n ta e o o r d in a te i n - Ite rm e d ia te in n u c l e o p h ili c s u b s t i t u t i o n o f t e t r a c o o r d i n a t e p h osphorus jcompounds. I n t h e p r e s e n t d i s c u s s i o n , . th e a tte m p t w i l l be made t o u se ' th e e x i s t e n t in fo rm a tio n on p e n ta e o o rd in a te .p h o s p h o ru s compounds as a ! S ib a s is f o r u n d e rsta n d x n g th e c o n c e iv a b le p e n ta e o o r d in a te carb o n s p e c ie s . I The p r e s e n t l y a c c e p te d S^2 mechanism i s s u p p o rte d b y . e x p e rim e n ta l |e v id e n c e w hich i s p r i m a r i l y k i n e t i c and s te r e o c h e m ic a l in n a t u r e . ^1 ’ 16 ,1 1 ). gecon^. o rd e r k i n e t i c s have e s t a b l i s h e d t h a t b o th r e a c t i n g s p e - i j jc ie s ( l ) . a n d (2) p a r t i c i p a t e . i n fo rm a tio n o f - th e t r a n s i t i o n s t a t e o f j |t h e r a te - d e te r m in in g s te p and t h e w ell-know n s te r e o c h e m is tr y of.W alden, i | in v e r s i o n have c o n t r i b u te d to ; ..th e . s u p p o s itio n t h a t th e t r i g o n a l - 'b ip y ra m id a l carbon s p e c ie s (3 ) w ith a p i c a l e n t e r in g and le a v in g groups ;i s a t r a n s i t i o n s t a t e in th e r e a c t i o n 1 (F ig u re l ) . F ig u re 1. N u c le o p h ilic s u b s t i t u t i o n on carb o n v i a TBP. * R e a c tio n 1. The s te r e o c h e m ic a l p a t t e r n o f R e a c tio n ( l ) , F ig u re 1 , a p p l ie s gen e r a l l y to -S ^ 2 mechanisms where th e n u c le o p h ile Y a t t a c k s th e t e t r a h e dron a t t h e f a c e o p p o s ite th e le a v in g group X. I f t h i s i s th e c a s e , th e le a v in g group X w i l l ap p ea r i n an a p i c a l p o s i t i o n o f th e TBP s p e c ie s 3 :,. The R e o r g a n iz a tio n o f th e C o n ceiv ab le TBP I n te r m e d ia te o f P r o c e s s e s . A s t e r i c c o u rse d i f f e r e n t from t h a t o f R e a c tio n ( l ) w i l l be ob t a i n e d i f a c o n s t r a i n t p r e v e n ts t h e le a v in g group X from becom ing a p i c a l o r f o r c e s one o f th e lig a n d s L^, to become- a p i c a l w h ile th e TBP s p e c ie s i s b e in g form ed. The most im p o rta n t o f th e s e con- (12 13) s t r a i n t s * o r i g i n a t e s w ith th e p re s e n c e o f U-5 membered r i n g s . T h is f e a t u r e can be co n c e iv e d t o f o r c e one o f th e '.lig a n d s t h a t forms p a r t o f th e r i n g i n t o an a p i c a l p o s i t i o n i n th e TBP, w ith th e c o n se quence t h a t a r e o r g a n i z a t i o n TBP in t e r m e d i a te i s a p r e r e q u i s i t e f o r th e a p i c a l d e p a r tu r e o f X. I t i s n e c e s s a ry to in tr o d u c e t h e f u r t h e r c o n ce p t o f p e r m u ta tio n a l is o m e r iz a t io n o f th e TBP i n t e r m e d i a te . I R eg u lar .and I r r e g u l a r P ro c e s s e s i s | Here i t i s im p o rta n t t o d i s t i n g u i s h 'between, r e g u l a r and i r r e g u l a r I f/12 13) Ip e rm u ta tio n a l i s o m e r i z a t i o n . ! / ’ R eg u lar p r o c e s s e s ta k e p la c e j jw ith o u t bond making and b r e a k in g , w ith p r e s e r v a t i o n o f a l l t h e p r o x i - f Imi-ty. r e l a t i o n s h i p s d e f in e d by th e bond system - and. m a in te n a n c e o f co o r d i n a t i o n num ber. R eg u lar p r o c e s s e s a re p o s s i b l e o nly with- f l e x i b l e -m o lecu la r s k e l e t o n s . R e g u la r r e o r g a n i z a t i o n p r o c e s s e s o f-m o le c u le s w ith m onocentri.c s k e le to n s , -by .angle d e fo rm a tio n , such as th e p e n t a - ic o o rd in a te p h o s p h o ru s , c o rre sp o n d in p r i n c i p l e to th e c o n fo rm a tio n a l iin te r c o n v e r s i o n o f m o lecu le s w ith p o l y c e n t r i c s k e le to n s by r o t a t i o n | |ab o u t s k e l e t a l a b o n d s, such as t h e e th a n e d e r i v a t i v e s ; th e d if f .e r - ! (12 13) le re n c e s a r e due t o d i f f e r e n t ty p e s o f .s k e l e t a l f l e x i b i l i t y . ’ I r r e g u l a r r e o r g a n i z a t i o n p r o c e s s e s , . in c o n t r a s t t o t h e , r e g u l a r [ones, p ro c e e d w ith, opening and c l o s i n g o f c o v a le n t bonds v i a i n t e r - Im ediates which, d i f f e r from th e i n i t i a l and f i n a l compounds in t h e i r j c o o r d in a tio n numbers . ( l 3 a , l ^ .1 6 ) The p e r m u ta tio n a l is o m e r i z a t i o n o f s t a b l e phosphoranes, a n d -o f p o s t u l a t e d p h osphorane i n t e r - I m e d ia te s have b een th e s u b j e c t - o f r e c e n t i n t e n s i v e .s tu d ie s and d i s - j c u s s io n . Only two ( o r fo u r r e s p e c t i v e l y ) o f a l l I |th e c o n c e iv a b le m e c h a n is tic a l t e r n a t i v e s w hich a re g iv e n by th e s u b - I (]_2 13) [c la s s e s o f th e sym m etric group 5 - can be a c c e p te d as p ro v id in g 'r e a s o n a b le pathw ays f o r th e r e g u l a r r e o r g a n i z a t i o n p r o c e s s e s o f pen-^ ! (2 jt a c o o r d in a te m o le c u le s .with TBP s k e l e t o n , nam ely . B erry p s e u d o ro .ta tio n i ( 1 0 !(BPR, F ig u re 2 , and th e sim p le and m u l tip l e t u r n s t i l e . r o t a ti o n (TR) ’ ! 13) | (F ig u re 3 ) . In some c a s e s , th e TR m echanism p ro v id e s th e o n ly i n - F ig u r e 2 b . Components o f m o tio n f o r th e BPR m echanism . & (TB)‘ ✓ a ' !“ S , »» a K (TR)' a- a ' N . F ig u re 3 . S in g le and m u l t i p l e TR p r o c e s s e s . 7 (12 13) t e r p r e t a t i o n o f e x p e rim e n ta l d a ta . ’ F o u r- and Five-M em bered C y c lic Systems C y c lic p h o sp h o ran e d e r i v a t i v e s where t h e p h o sp h o ru s i s a p a r t o f 0 f o u r - o r five-m em bered r in g s undergo o n ly such TR p r o c e s s e s in w hich th e r i n g p a r t i c i p a t e s as th e p a i r . In th e p h o sp h o ran e d e r i v a t i v e s , t h e i r r e g u l a r re a rra n g e m e n t p r o c e s s e s ta k e p la c e th ro u g h t e t r a c o o r - d in a te or h e x a c o o rd in a te s p e c ie s . I t i s p o s s i b l e i n some c a s e s to d i f f e r e n t i a t e e x p e rim e n ta lly betw een r e g u l a r and i r r e g u l a r p ro c e s s e s by t h e s u b je c t io n o f re a rra n g e m e n t p r o c e s s e s o f p h o sp h o ran e d e r i v a t i v e s t o i n v e s t i g a t i o n by nmr. I f , f o r exam ple, t h e r e i s a s p i n - s p i n i n t e r a c t io n betw een a li g a n d and th e phosphorus c e n t r a l atom , th e n t h i s c o u p lin g i s n o t d e s tr o y e d by r e g u l a r p r o c e s s e s , b u t d is a p p e a r d u rin g i r r e g u l a r p r o c e s s e s , u n le s s thes'e a re in t r a m o l e c u la r and p ro c e e d e x tre m e ly r a p i d l y . In th e h y d r o ly s is o f p h o s p h a te e s t e r s and r e l a t e d r e a c t i o n s , t h e p o s s i b l e i r r e g u l a r p e rm u ta tio n is o m e r iz a tio n o f oxy- ph o sp h o ran e in t e r m e d i a te s sh o u ld n o t be o v e rlo o k e d . T here a re many e x p e rim e n ta l o b s e r v a tio n s t h a t can be i n t e r p r e t e d s o l e l y o n - th e b a s i s o f i r r e g u l a r is o m e r iz a tio n o f t h e i n t e r m e d i a te . Base c a t a l y s i s fa v o rs i r r e g u l a r p r o c e s s e s p ro c e e d in g v i a in t e r m e d i a te s w ith h ig h e r c o o rd in a t i o n number (s e e R e a c tio n 2 , F ig u re ^ ) . Therm al or a c id c a ta ly z e d r e a c t i o n s a re more l i k e l y t o in v o lv e i r r e g u l a r p r o c e s s e s v i a i n t e r m e d ia te s t h a t a re low er in c o o r d in a tio n number (s e e R e a c tio n 3 , F ig u re 5). 8 z z z I - - x L* I X PCT + Ye — ^ X— fp- Y — ^ J^P---- Y + Xs — / | — X ^ | X X 6 7 8 WV UV F ig u re U. Base c a t a ly z e d r e a c t i o n (R e a c tio n 2) :x- . ' x ' N , X , + Z (o r 2H) X P X -P^. (x 10 11 F ig u re 5* Therm al and a c id c a ta ly z e d r e a c t i o n (R e a c tio n 3 ) . 9 Six-M embered C y c lic System s T here is no c o n s t r a i n t in th e s.ix-membered and l a r g e r r i n g s as t h e r e i s in th e s m a ll r i n g s . G e n e r a lly , th e s.ix-membered r i n g s p r e f e r t h e d i e q u a t o r i a l p la c e m e n t, i f a p i c o p h i l i c i t y allo w s t h a t , b u t i t can a l s o adopt an a p i c o - e q u a t o r i a l a tta c h m e n t i f th e six-m em bered r i n g oc c u p ie s t h e p r e f e r r e d d i e q u a t o r i a l a rra n g e m e n t, th e n i t f u n c tio n s as p a r t o f th e t r i o in th e TR p ro c e s s (F ig u re 3 ) , and in such a c a s e , TR 2 and (TR) w i l l move i t i n t o an a p i c o - e q u a t o r i a l a tta c h m e n t w h ile 3 CTR) w i l l le a v e i t d i e q u a t o r i a l . I f one i n s i s t s on in v o k in g th e BPR (F ig u re 2) w ith a d i e q u a t o r i a l six-m em ber r i n g , i t s h o u ld be n o te d t h a t one must u se a r i n g member as a p iv o t w hich le a d s t o an a p ic o - e q u a t o r i a l p la c e m e n t; o th e r w is e , th e r i n g w ould have t o span th e two a p i c a l p o s i t i o n s o f th e new TBP s k e l e t a l arra n g e m e n t. I f th e six-m em bered r i n g o c c u p ie s th e a p i c o - e q u a t o r i a l p la c e m e n t, 2 3 and i t f u n c tio n s as th e p a i r o f TR p r o c e s s e s , th e n TR, (TR) , and (TR) w i l l n e c e s s a r i l y le a v e i t in th e a p i c o - e q u a t o r i a l p la c e m e n t, alth o u g h t h e r i n g te r m in i w i l l exchange t h e s k e l e t a l p o s i t i o n as a r e s u l t o f 3 2 TR and (.TR) , b u t n o t (TR) . The p a r t i c i p a t i o n o f th e r i n g in th e t r i o o f t h e TR o r (TR) can r e s u l t in a p i c o - e q u a t o r i a l p la cem e n t w ith exchange o f t e r m i n i , o r in d i e q u a t o r i a l p lacem en t o f th e r i n g , w h ile i n t h e (TR) t h e r i n g r e t a i n s i t s p la c e m e n t. In t h e 'six-m em bered c y c l i c s y s te m s , j u s t as i n th e c y c l i c c a s e , d is p la c e m e n ts o ccu r n o rm a lly w ith in v e r s i o n (R e a c tio n 1 , F ig u re l ) . An a p p a re n t e x c e p tio n i s i l l u s t r a t e d in R e a c tio n U, F ig u r e 6 , where t h e r e i s some r e t e n t i o n , r a lth o u g h in v e r s i o n i s p r e f e r r e d . I f one C 6H 5X / C 6H5“C H 2 > '\_C H _ ^ 3 ■ s : ------ G H , 2 O H 12 O H .C H _ 13a 13b. A (TR)^ p a i r CgH.-CH O H HO-P ■ C H 13c C 6H 5 \ / / p C^H CH, 6 K / \ CH, lU a lUb 'F ig u re 6. N u c le o p h ilic s u b s t i t u t i o n jof phosphonium io n on c y c l i c six-m em bered r in g ;,(R e a c tio n U) . • assumes h e re th e p r e f e r r e d d i e q u a t o r i a l p la cem e n t and keeps th e r i n g jin t h a t a t ta c h m e n t, th e n t h e r e s u l t can be i n t e r p r e t e d in te rm s o f nu i O j o l e o p h i l i c d is p la c e m e n t in v o lv in g a (TR) r e o r g a n i z a t i o n o f th e TBP j |i n t e r m e d i a t e . ( 1 3 ) , u s in g th e r in g as p a r t o f th e t r i o . Note t h a t in . i jR e a c tio n U, F ig u re 6 , t h e r e i s n o t much d i f f e r e n c e •in e l e c t r o n e g a t i - i * v ity o f . t h e le a v in g group X and o f th e li g a n d L , L , and L . i 1 2 3 i | H ere, t h e p r e f e r e n c e i s f o r a p i c a l p h e n y l, (L ) as in TBP 13b, | L i r a t h e r th a n a p i'c a l b e n z y l. Cx) as in TBP 13a. However, r e t e n t i o n r e - | 3 ‘q u ir e s a-(TR) , 13b, — > 13c — > li+b, w h ile i n v e r s i o n r e q u ir e s no '- p e rm u ta tio n a l. is o m e r i z a t i o n , 13a — > lU a. The a n a l y s i s o f th e ' ex p e r i m e n t a l d a t a on s t a b l e p e n ta e o o r d in a te m o le c u le s , in p a r t i c u l a r i j t h e i r nmr d a t a , i n d i c a t e s t h a t th e r e o r g a n i z a t i o n o f c y c l ic p e n t a - j f 2 . 2 ) j c o o r d in a te ta k e s p la c e , e i t h e r by (BPR) or- by sim p le o r m u ltip le j (TR) p r o c e s s e s a n d t h a t the- c y c l i c T B P ^ ^ m o le c u le s as a r u l e I (12) R e a r r a n g e ,by p r o c e s s e s o f TR ty p e . ;P o l a r i t y E f f e c t s and th e S t e r i c Course o f R e a c tio n s I ! j In p h o sp h o ru s c h e m is tr y , th e TBP 3 (F ig u re l ) w ith a p i c a l . X . i s ' (3 2 — 3U) ^ favored by th e " p o l a r i t y r u l e " ' when X is. s i g n i f i c a n t l y m o re ; ;e l e c t r o n e g a t i v e th a n th e li g a n d s L ^, and L^. In m ost norm al r e - j ja c tio n s o f th e ty p e -u n d e r c o n s i d e r a t i o n , X i s s u b s t a n t i a l l y more : (3 5 ) ;e l e c t r o n e g a t i v e ' th a n L^, an-d-the s te r e o c h e m ic a l p a t t e r n !of R e a c tio n 1 , F ig u re 1 i s in d e e d o b se rv e d . However, i f X, L , L , | 1 !and bo n o t d i f f e r s i g n i f i c a n t l y in e l e c t r o n e g a t i v i t y , a d i f f e r e n t ;s te r e o c h e m ic a l p a t t e r n may b e . o b se rv e d f o r t h e - r e a c t i o n . In g e n e r a l , 12 ! n u c le o p h ile s .a tta c k the. f a c e o f s k e l e t a l t e t r a h e d r a a n d -a p p e a r in th e (12 l 8 a p i c a l p o s i t i o n o f th e TBP p e n t a e o o r d i n a t e - i n t e r m e d i a t e ■ s p e c i e s . ’ . ’ 25,36 4o) t e t r a c o o r d i n a t e atom i s p h b sp h o ru s and th e p e n t a - c o o r d in a te T B P 'sp e c ie s i s an i n t e r m e d i a t e , th e n e x t s te p i s th e .d e c o m p o s i t i o n o f th e in t e r m e d i a te by d e p a r tu r e o f t h e le a v in g group X from jth e apex o f th e T B P .' A p ic a l d e p a r tu r e o f X 'fro m 3 (w ith o u t any p e r - i ^ m u ta tio n a l is o m e r iz a tio n o f 3 J g iv e s th e t e t r a c o o r d i n a t e - s p e c ie s 4. ( 2 2 ) |w hich i s -h e t e r o c h i r a l to 1 , t h e d is p la c e m e n t p ro c e e d s w ith i n - i . (l8 ,36c ,U2 ,^3) i -u • n - j . i o. -u • i - j . jv e rs io n . The concepts, o f h o m o c h i r a l i t y .and h e t e r o c h i r a l i t y • * (6 5 ) [in tro d u c e d by Lord K e lv in , a re most u s e f u l f o r d e f in in g r e t e n t i o n , j ! j lor .in v e r s io n , r e s p e c t i v e l y . j I 1 j A c e n t r a l o f c h i r a l i t y A i s h o m o c h ira l to a n o th e r o n e , B , i f A j I _ j jis more s i m i l a r t o B th a n t o th e a n tip o d e B;. o th e r w is e , A i s h e t e r o - j I j c h i r a l t o B . . C o n f ig u r a tio n a l s i m i l a r i t y is - b a s e d .u p o n a common m o le- | I | icular. s k e l e t o n , a n d . e s t a b l i s h i n g p a i r - w i s e e q u iv a le n c e s o f th e l i g - 1 jands o f A + B. | i 1 j E . g . , | H ^C— 1 i s h o m o c h ira l t o x-C— N CK3- y CK3- y C2H5 - C 2H5 3 I H ■ ; \ ! fbut . h e t e r o c h i r a l t o xC-^HNL .. ! I c aV/ 3 I ! C H 3 I i i i ■ - ■ j [A ■ d is p la c e m e n t o f X by Y in t e t r a h e d r o n .1 w i l l o c c u r w ith - r e te n tio n o f i \ « ; c o n f ig u r a tio n when th e p ro d u c t i s 1 +, s in c e k i s h o m o c h ira l (22) t o 1 , j .The d is p la c e m e n t w ith d e t e n t i o n , 1 — > 4 , v i a TBP (3 ) r e q u i r e s I r * * a# | o |t h e .p e r m u t a t i o n a l is o m e r iz a tio n o f th e l a t t e r by t r i p l e or' (TR) ,(1 2 ,1 3 ,U 6 ) i p ro c e s s.. or by seq u en ce o f t h r e e s i n g l e TR p r o c e s s e s o r ;t h r e e BPR. One TR o r BPR, o r an even number o f them , c a n n o t . r e s u l t i ’ . 3 : in d isp la c e m e n t w ith r e t e n t i o n . One o f th e (TR) . p r o c e s s e s w hich ^exchange Y and a lig a n d - L , w ith o u t a f f e c t i n g X and th e o th e r two 'l i g a n d s L, i s d e p ic te d in R e a c tio n 5 , F ig u re J . In t h i s i l l u s t r a t i o n , ( • ’Y and exchange p o s i t i o n t o g iv e TBP 5 , from w hich a p i c a l d e p a r tu r e ;o f X r e s u l t s in t e t r a h e d r o n F , h o m o c h ira l to 1 ( r e t e n t i o n ) . s ' 1 (TR)3 , j (Y l) 3 2 X ✓ Y X -X. 3 X 3 W 3a 5 j I F ig u re 7> R e a c tio n 5 . (R u c le o p h ilic s u b s t i t u t i o n w ith (TR.) i | o f th e TBP i n t e r m e d i a t e ) , i i j As a r u l e , th e n u c le o p h ile Y ,and th e le a v in g , g ro u p X i s more i ' - :e l e c t r o n e g a t i v e ( i . e . , m o re . a p i c o p h i l i c ) th a n , o r . Conse- |q u e n t l y , TBP 3 (w hich le a d s t o in v e r s i o n w ith o u t p e r m u ta tio n a l i s o - im e riz a tio n ) i s e n e r g e t i c a l l y f a v o re d o v e r TBP 3a (w hich le a d s t o r e t e n t i o n ) by th e p o l a r i t y r u l e . A ll th e s e e f f e c t s th e n fa v o r th e p a t - j t e r n o f R e a c tio n 1 , F ig u re 1 , i . e . , - d is p la c e m e n t w ith in v e r s i o n f o r |a c y c l i c system s w here X and Y a re more a p i c o p h i l i e th a n L^, , and j .■L^. I f X, t h e . le a v in g g ro u p and th e lig a n d s L^, , and do n o t ■ d iffe r s u b s t a n t i a l l y in a p i c o p h i l i c i t y , th e n in t h i s c a se t h e s i t u a - | j t i o n becomes more com plex, b u t n o t , th e y b e l i e v e , f o r r e a s o n s . g iv e n J | / - j n} i | in t h e p re v io u s • l i t e r a t u r e .. The p r e f e r e n c e : f o r th e - .a t ta c k by I i th e n u c le o p h ile Y a t th e t e t r a h e d r a l fa c e o p p o s ite X v a n is h e s ,... as j |Xj L^., L^, an d 'L ^ become com parable in a p i c o p h i l i c i t y .. A tta c k by Y j i. ! |on th e re m a in in g th r e e fa c e s o p p o s ite t o L ^, L^, and , r e s p e c t i v e l y , j can p l a c e . t h e s e lig a n d s in an a p i c a l p o s i t i o n o f a TBP, t o t h e e x t e n t t h a t t h i s i s a llo w e d by th e e l e c t r o n e g a t i v i t y .and th e s t e r i c r e q u i r e - ' m ents o f th e s e lig a n d s (th e a p i c a l p o s i t i o n s a r e .s u b je c t t o more s t e r i c ^h in d ran ce th a n th e e q u a t o r i a l p o s i t i o n s ) . Wow, t h e ,i n t e r m e d i a t e .TBP 1 lean decompose by th e r e v e r s e p ro c e s s , o r can .undergo c e r t a i n p e rm u ta - f ; j t i o n a l . i s o m e r i z a t i o n s t h a t w i l l b r in g X t o the. a p i c a l p o s i t i o n from j ) , jwhich. i t can d e p a r t ; .(se e R e a c tio n 6 , F ig u re 8 ). The mechanism s by ! w hich r e g u l a r p e r m u ta tio n a l is o m e r iz a t io n .of-TBP ta k e s p la c e h a v e ’ ! i b een d is c u s s e d b e f o r e . \ ! i . The C o r r e l a t i o n o f th e ' S t e r i c Course .o f P r o c e s s e s and the. R eo rg an - | ji z a t i o n Mechanism o f TBP I n te r m e d ia te s | ! H e re .one need o n ly t o summarize th e c o n d itio n s t h a t a re n e c e s s a r y j jto o b se rv e d is p la c e m e n ts w ith i n v e r s i o n o r .w i t h r e t e n t i o n o f t e t r a h e d r a l I c o n f ig u r a tio n by th e TR and BPR p r o c e s s e s , when th e n u c le o p h ile Y j 1 I Imust. e n t e r a t th e fa c e o p p o s ite th e le a v in g group X, -and when Y ean j * ' ’ ■ 1 e n t e r a t any o n e -o f th e f o u r . f a c e s o f th e ' t e t r a h e d r o n , . i . e . , o p p o s ite j X U.+ x L. X Hk k + X (TR)' (TR)' L X- S 1 + Y Y 3b , L2 * TR / J2 TR^ ' -'L' L„ L„ Y 3c I k + X L A 3 _4 s ’ L, ■ - K 3 TR. — r (TR)' X K L. 1 k + X -'L, F ig u re '8 . (R e a c tio n 6 ) . • N u c le o p h ilic s u b s t i t u t i o n of-TBP v h ie h undergo r e o r g a n i z a t i o n by sim p le and m u l tip l e (TR) p r o c e s s e s . 1 6 L^> Lg, L , o r X. I f Y must e n t e r a t th e fa c e o p p o s ite X, in v e r s i o n can r e s u l t w ith o u t any TR o r BPR, o r a f t e r (TR) , o r a f t e r a sequence o f an even n u m b er.o r s i n g l e TR o r BPR. R e te n tio n can o c c u r o n ly a f t e r (TR) o r a f t e r a sequence o f t h r e e s i n g l e TR o r BPR. R e te n tio n can n o t be th e r e s u l t o f one s i n g l e TR or one BPR, b e cause in th o s e c a s e s , th e le a v in g group X i s t r a n s f e r r e d t o an equa t o r i a l p o s i t i o n from w hich i t cannot d e p a r t t o form a t e t r a h e d r a l a r - 2 ran g em en t. I n v e r s io n can occu r only a f t e r (TR) i f Y e n t e r s th e fa c e o p p o s ite t o L^, and L^, o r a f t e r a sequence o f even numbers o f s in g le TR o r BPR. W hile r e t e n t i o n can o c c u r a f t e r one s i n g l e TR o r BPR (assum ing t h a t Y i s a b le t o move t o an e q u a t o r i a l p o s i t i o n as a r e s u l t o f t h a t is o m e r iz a tio n ) o r a (TR) , (s e e R e a c tio n 6 , F ig u re 9 ) . W ith th e s e c o n s id e r a tio n s i n m ind, i t sh o u ld be c l e a r from R e a c tio n 6 , F ig u re 9 , t h a t th e d is p la c e m e n t w i l l p ro c e e d w ith r e t e n t i o n when th e s i n g l e TR o r BPR can b r in g X to an a p i c a l p o s i t i o n , i . e . , when th e is o m e r iz a tio n r e a r r a n g e s 3b o r 3 c in such a way t h a t 3 i s p r o duced by th e a p i c a l d e p a r tu r e o f X. On t h e o th e r h a n d , d is p la c e m e n t w ith in v e r s i o n i s now p o s s i b l e o n ly a f t e r th o s e (TR) , or th e c o r r e s ponding seq u en ces o f two TR o r BPR, t h a t a re c a p a b le o f tr a n s f o r m in g TBP (3b) or (3c) i n t o TBP(.3) • The P e n ta e o o r d in a te S p e c ie s in v o lv e d in SN2 - The a ssu m p tio n t h a t TBP(3) must be a t r a n s i t i o n s t a t e and n o t an in t e r m e d i a te was due t o t h e n a t u r a l a v e r s io n t o presum e any s t a b i l i t y in a p e n ta e o o r d in a te carb o n c o m p o u n d .^5 However, some a u th o rs have p o in te d o u t t h a t in t e r m e d i a te s c o rre s p o n d in g to a w e ll i n th e p o t e n t i a l en erg y s u r f a c e m ight b e form ed i n th e c o u rse o f Sjj2 r e a c - 1 fjlTa 5 2 k 3 2 \ ------ k 3 2 OlTbQ s 5 1 7c\\ \ O n & r ) F ig u re 9. P o s s i b le ap p ro ac h es t o a te tr a h e d r o n and th e r e s u l t i n g in t e r m e d i a te (..Reaction 7 ) . • T able 1. Summary o f th e S t e r i c Course o f S u b s t i t u t i o n R e a c tio n s of i j T e t r a c o o r d in a te M o lecu lar Compounds v i a P e n ta c o o r d in a te j TBP I n te r m e d ia te s . ! I . j ■ P o s i t i o n o f E n try o f . Y... P e r m u ta tio n a l I s o m e r iz a tio n o f th e TBP I n te r m e d ia te s S te re o c h e m ic a l R e s u lts o f th e P -C e n te r 'o p p o site. X n o n e . in v e r s i o n o p p o s ite X. one. B PR 'or-one TR no r e a c t i o n o p p o s ite X two BPR o r two TR o r one. C t r )2 in v e r s io n o p p o s ite X t h r e e BPR o r t h r e e TR. o r . one (TR)^ in v e r s io n o p p o s ite Ll* L2 , or L3 none. no r e a c t i o n o p p o s ite L^, L2 , or L3 one BPR o r one TR r e t e n t i o n * o p p o s ite V L2 , d r •L3 two BPR o r two TR o r o n e ■ (TR)2 i n v e r s i o n o p p o s ite ■ Ll? l 2 , o r • L3 . t h r e e BPR o r th r e e TR o r c lll one (TR)^ \ . 'r e t e n t i o n ;*Assuming t h a t Y i s a b le to.m ove t o an e q u a t o r i a l p o s i t i o n . j ' ^ 19 T ab le 2. E q u ilib riu m Geometry B in d in g E n e rg ie s (BE) o f Walden S p e c ie s .and T h e ir P e rm u ta tio n Iso m e rs. L igands and bond - EQUATORIAL le n g th s c-L (L-L5 )(A °) APICAL BE ( k c a l/m o le ) a H ’ H ' H F F -969.1+ l . 160 1 , 1 6 0 1 . 1 6 0 1 . 1 +1+0 1 . 1 + 1+0 ■ b- H H H., F. F -9 6 7 .9 1 . 1 6 0 1 . i 6 o: 1 . 1 6 0 1.1+50 1.1+50 c H H H ; F F -9 6 8 .9 1 . 1 6 0 1 . 1 6 0 1 . 1 6 0 1,1+30 1.1+30 d H H H F 7 -9 6 9 .2 1 . 1 6 0 1 . 1 6 0 1 . 1 6 0 1.1+50 1 . 1+30 • e H H H F F -9 6 7 .5 1 . 1 6 0 l . l 60 1 . 1 6 0 1.1+70 1 . 1+20 f H H ( g ’j i - F - 9 9 3 .8 I . . I 50 1 . 1 5 0 1 . 3 8 0 1 . 1 7 0 1 . 1 lie g H F F H H -9 8 9 .0 1 . 1 5 0 1 . 3 8 0 1 . 3 8 0 1 . 2 0 0 1 .,200 h H' H H OH F -1 1 3 7 .9 1 . 6 0 1 . 6 0 1 . 6 0 1 . 1+60 1 . 1 +1+0 i H H O H H F - 1 1 6 7 .9 - 1 . 1 5 0 1.150- l.UOO 1 .1 7 0 1 . 1 +1+0 j • H H F H OH - 1 1 6 0 . 9 1 .U50 1 .150 1 . 1+10 1 . 1 7 0 1 . 1 + 1+0 k H .OH- F H H - 1161+.6 1 . 1 5 0 1.1+05 1 . 3 8 0 1 . 2 0 0 1 . 2 0 0 1 fj H H H (C H F -1 6 3 2 .9 1.170 1 . 1 7 0 1 . 1 7 0 1 . 5 0 0 1.1+50 20 ;T ab le 2 c o n tin u e d . -1 6 3 2 .9 1 .150 1 . 1 5 0 1.170 CN l.hBO l.i+oo 1 . 1 5 0 1 . 1 5 0 1 . 1 7 0 1 . 1 5 0 1 . 3 9 0 1.200 1 .200 ( 2T d ^ 4 6 ) t i o n s . ’ S e v e r a l exam ples o f r e l a t i v e l y s t a b l e p e n ta c o o r d in a te carb o n compounds, Cl,-6 ^ ' ^ and as w e ll as t h e o r e t i c a l work y i e l d i n g s t a b i l i t y e s tim a te s f o r model system s CH,_e ^ Q ( Q ) and CH^F^ have ap p e a re d i n th e l i t e r a t u r e . T able 1 c o n ta in s b in d in g en e rg y (BE) d a t a , o b ta in e d by th e u se o f th e C N D O /2 ^ ^ ap p ro x im a tio n , f o r p e n ta c o o r d in a te carb o n compounds ( 1 5 ) w ith a t r i g o n a l - b ip y ra m id a l s k e l e t a l s t r u c t u r e . The v a r io u s is o m e rs , n o t t r e a t e d in (15) (9) p r i o r c a l c u l a t i o n s a re in c lu d e d f o r each compound. The BE o f a as a f u n c ti o n o f C-F bond d is ta n c e shows t h a t any sm a ll d is p la c e m e n t o f th e F lig a n d s from t h e i r e q u i lib r i u m p o s i t i o n ( s i m i l a r l y f o r th e H lig a n d s ) le a d s t o an in c r e a s e in th e en erg y o f th e "Walden s p e c ie s " (s e e b - e , T ab le l ) , i . e . , i t can b e s t a t e d t h a t a c o rre sp o n d s t o a l o c a l minimum in th e p o t e n t i a l s u r f a c e a n d , by d e f i n i t i o n , i s an i n te r m e d i a t e , n o t a t r a n s i t i o n s t a t e . T his h o ld s f o r th e o th e r model s p e c ie s ( f - n ) o f T ab le 1 as w e l l . A pp ro x im atio n s c o n ta in e d i n th e CNDO/2 m ethod and th e n e g l e c t o f zero p o i n t e n e r g ie s a r e n o t c r i t i c a l f o r e s t a b l i s h i n g th e in te r m e d ia te n a tu re o f th e s p e c ie s and presu m ab ly f o r th e q u a l i t a t i v e com parison betw een th e BE f o r th e p e rm u ta tio n (9a) iso m e rs in T ab le 1. B e r t h i e r , D av id , and V e i l l a r d found t h a t .the p o t e n t i a l en erg y o f th e "Walden s p e c ie s " (a) i s low er th a n t h a t | o f CH I- + F , w hich may_ be d u e .to n e g le c te d d i f f e r e n c e s i n ze ro p o in t j e n e r g ie s o f (a ) and th e l a t t e r . These d i f f e r e n c e s become im por-, 11ant when e i t h e r - t h e number o r ty p e o f bonds o r lig a n d s change, i BE com parison betw een iso m e rs ( a ) - ( g ) , ( h ) - ( k ) , and ( l ) - ( o ) i' 'i n d i c a t e t h a t th e p o l a r i t y r u l e does n o t seem t o h o ld f o r p e n ta c o o r - ;d in a te carb o n compounds; in f a c t , th e o p p o s ite seems to be t r u e . ;T his phenomenon may. b e - d u e , in p a r t , t o th e f a c t th-at th e lig a n d s ;a r e •r e q u ir e d t o c a r r y more 1 e l e c t r o n d e n s it y th a n th e y w ould in some n e u t r a l s p e c ie s , combined w ith th e f a c t t h a t th e hydrogens c a r r y th e .m a jo r ity o f t h e .e x c e ss ch arg e s in c e f l u o r i n e a l r e a d y - e n t e r s w ith a ’n e g a tiv e c h a rg e . F u r t h e r , l a c k o f . d o r b i t a l s i n b a s i s , set; e lim in a te s ja pathw ay f o r e l e c t r o n d e n s it y to be p a s s e d b ack to th e c a rb o n atom j i n . t h e s e model c a l c u l a t i o n s . T h is r e s u l t s i n . t h e h ig h ly n e g a t iv e l y jc h arg ed atoms se e k in g a p i c a l s i t e s in an e f f o r t t o a v o id e a c h - o th e r . i [Such r e l a t i v e s t a b i l i t i e s o f th e p e rm u ta tio n iso m e rs o f th e t r i g o n a l - Ib ip y ra m id a l sy ste m , th o u g h , do n o t cause d i f f i c u l t i e s f o r u n d e r s ta n d i n g t h e s t e r i c c o u rs e , o f th e Sjj2 . and ' S ^ i r e a c t i o n s . T here a r e , two i ' ]t y p e s , o f l i m i t i n g b im o le c u la r m echanism s, % 2 and Sjji f o r .S p r o c e s s e s ) i • - ja t a s a t u r a t e d C-atom -w hich p r o c e e d - v i a ' a p e n t a c o o r d in a t e i n t e r m e d i - ja te (1 5 ) w ith a t r i g o n a l - b i p y r a m i d a l m o le c u la r s k e le t o n as a d d i t i o n - I ' ’ ' J e lim in a tio n - p r o c e s s e s accom panied-by a change in c o o r d in a tio n number iof c a rb o n : U 5 F o r b o th o f t h e s e . t y p e s o f b im o le c u la r S„ ■ > - W ip ro c e ss e s th e tr a n s f o r m a tio n o f th e t e t r a c o o r d i n a t e C-compound i n t o 'th e p e n ta c o o r d in a te s p e c ie s r e s u l t s from an a t t a c k on any fa c e ,;( s;ee. 3 ! [Figure 9 , R e a c tio n 7) o f th e t e t r a h e d r o n by th e n u c le o p h ile such t h a t 23 t h e e n t e r in g group assumes an a p i c a l p o s i t i o n in th e in t e r m e d i a te . The l a t t e r decom poses by a p i c a l d e p a r tu r e o f th e le a v in g g ro u p . Re c e n t l y , th e n e c e s s i t y f o r a p i c a l e n t r y and d e p a r tu r e h as b een con firm e d by Eschenm oser. e t . a l . ^ ^ T h e ir e x p e rim e n ts d e m o n stra te t h a t i f t h e s t e r i c re q u ire m e n ts f o r b o th a p i c a l e n t r y and d e p a r tu r e a re n o t f u l f i l l e d , no n u c l e o p h ili c s u b s t i t u t i o n v i a p e n ta c o o r d in a te s p e c ie s ta k e s p l a c e . O b se rv a tio n from th e s e ex p e rim e n ts a r e -well in (22 a c c o rd w ith th e c o n c e p ts d e v e lo p e d i n t h i s and p re c e d in g p a p e rs ’ 13b U9) 5 and in p a r t i c u l a r w ith th e p r o p e r t i e s o f c y c l i c p e n ta c o o r d in a te s p e c ie s w ith t h r e e - to five-m em bered r i n g s v e rs u s th o s e w ith six-m em bered r i n g s . S in c e a r e a s o n a b le TBP carb o n s p e c ie s can be p o s t u l a t e d , i t rem a in s to be d e te rm in e d how th e n u c le o p h ile .approaches t h e t e t r a c o o r d i n a t e carbon atom. The Pathw ays o f F o rm atio n o f TBP P e n ta c o o r d in a te Species- from T e t r a - c o o rd in a te S p e c i e s . Of th e two p o s s i b i l i t i e s , a p i c a l a p p ro a c h , i . e . , a t t a c k a t one o f t h e t e t r a h e d r a l f a c e s (s e e F ig u re 9 ) , i s th e o n ly one where th e incom ing group can a c h ie v e an a p i c a l p o s i t i o n w ith o u t re a rra n g e m e n t. E q u a t o r i a l a p p ro a c h , i . e . , a t t a c k a t one o f t h e t e t r a h e d r a l e d g e s , alw ays p la c e s th e incom ing m o iety i n . a n e q u a t o r i a l p o s i t i o n . De c o m p o sitio n o f a .p e n ta c o o r d in a t e m o le c u le by t h e d e p a r tu r e o f an a p i c a l li g a n d can le a d to two d i f f e r e n t s p e c ie s w ith a t e t r a h e d r a l s k e l e t o n , w hich are. n o t n e c e s s a r i l y is o m e r ic . In t h i s d e co m p o sitio n (37 t h e group w ith th e b e t t e r le a v in g p r o p e r t i e s d e p a r ts p r e f e r e n t i a l l y . 2k 3 9 ) A p ic a l e n tr y h a s b e e n fa v o re d f o r n u c l e o p h i l i c s u b s t i t u t i o n or s a t u r a t e d carbon as a p i c a l d e p a r tu r e was c o n s id e r e d r e a s o n a b le .^ 3 8 a ,b ) The m ic ro s c o p ic r e v e r s i b i l i t y p r i n c i p l e ^ t h e n demands t h a t th e n u c le o p h ile e n t e r a p i c a l l y . Only r e c e n t l y has th e s u b j e c t o f eq u a t o r i a l o r e d g e - a tta c k been r a i s e d a g a in . ^ ^ ) Model c a l c u l a t i o n s on © th e a t t a c k o f F upon CH^F i n d i c a t e t h a t t h e b a r r i e r f o r " e q u a t o r i a l a t t a c k " (edge attack ).- (F ig u re 10a) i s h ig h e r .by 20 k c a l/m o le th a n f o r ( I T ) " a p i c a l a t ta c k " ( f a c e a t t a c k ) (F ig u re 1 0 b ). S im ila r c o o r d in a tio n s a p p ly t o n u c l e o p h i l i c s u b s t i t u t i o n r e a c t i o n s o f p e n ta c o o r d in a te p h o s p h o ru s compounds. The e x p e rim e n ta l d a t a ^ ^ s u p p o rt th e assu m p tio n t h a t oxyphosphoranes a re i n t e r m e d i a te s in th e h y d r o ly s is o f c y c l i c p h o sp h ate e s t e r s and an alo g o u s system s when th e r i n g i s fiv e-m em b ered . McEwen (*H) e t . a l . o r i g i n a t e d th e co n cep t o f ph o sp h o ran e i n t e r m e d i a te s in th e d is p la c e m e n t r e a c t i o n s o f t e t r a c o o r d i n a t e p h o sp h o ru s compounds, (52a) w h ile W estheim er e t . a l . i n i t i a l l y c o n s id e re d th e co n cep t o f p e n ta c o o r d in a te t r a n s i t i o n s t a t e s as an a l t e r n a t i v e in th e s e r e a c t i o n s . S h o r tly t h e r e a f t e r , how ever, Ram irez e t . a l . ^ >5^,kka-d) e s t a b l i s h e d th e form er as th e m e c h a n is tic i n t e r p r e t a t i o n f o r th e s e r e a c t i o n s fo llo w e d by a s i m i l a r d e te r m in a tio n from W estheim er e t . (.1^4 5 2 c ~6 r ) a l . * The e v id e n c e f o r th e s e in t e r m e d i a te s i s n o t as com p e l l i n g i n th e ca se o f p h o sp h a te e s t e r s o f m o n o fu n c tio n a l a l i p h a t i c a l c o h o ls . However, th e sum o f a l l th e e v id e n c e s f a v o rs th e g e n e r a l f o r m u la tio n o f (R e a c tio n 8 , F ig u re l l ) . H H F ig u re 10 a. Model b a r r i e r s i t u a t i o n o f th e edge a t t a c k H e F. 1 F ig u r e 10b.. Model b a r r i e r s i t u a t i o n o f th e fa c e a t t a c k N + , T -----^ TBP -----^ T' + N' F ig u re 11. A d d it.io n - e lim in a tio n m echanism where T and T ' ~ f th e t e t r a c o o r d i n a t e p h o s p h a te d e r i v a t i v e s , TBP. = t h e t r i g o n a l b ip y ra m id a l oxyphosphorane ■ , a n d . N and-N ' = th e n u c l e o p h ile . i L i t t l e i s known ab o u t t h e dynam ics o f t h i s ty p e - o f p r o c e s s ; i j s u b s t a n t i a l d i f f e r e n c e s in . th e r e l a t i v e ; - r a t e s o f th e r e a c t i o n can be !e x p e c te d in d i f f e r e n t c a s e s . ; The s t a b i l i t y o f . an oxyphosphorane f 'e l a t i v e t o i t s - te t r a c o o r - ; d i n a t e - p r e c u r s o r o r d e c o m p o sitio n -product inelfeasp.s w ith th e number o f ;P -0 bonds and d e c re a s e s w ith th e a c c u m u la tio n o f s t e r i c b u lk in t h e iim m ediate v i c i n i t y o f th e p h osphorus atom . In f a c t , th e oxyphosphor- ianes t h a t have so f a r been i s o l a t e d .c o n ta in a t l e a s t two P -0 b o n d s , 'a n d .in such c a se s * t h e m o lecu le a l s o c o n ta in s o n e .o r more f o u r - o r S j 5five-m em bered r i n g s , w hich h e lp re d u c e th e l o c a l s t e r i c a l b u lk about I !th e p h o sp h o ru s. Among a c y c l i c oxyphosphoranes , four:-- o r fiv e-' I jP-0 bonds a r e •r e q u i r e d . t o pro d u ce i s o l a b l e b u t s t i l l s l i g h t l y u n s ta b l e 'o x y p h o sp h o ra n e s. j j A l l . ev id e n c e p o i n t s to w ard s a m echanism in w hich th e oxyphos- Ip h o r a n e .in te r m e d ia te i s form ed fro m ,th e t e t r a c o o r d i n a t e p r e c u r s o r by I ■ |a t t a c k o f th e n u c le o p h ile a t one o f th e f a c e s o f :th e t e t r a h e d r o n , Iw ith r e g a r d t o th e s k e le t o n o f th e i n t e r m e d i a t e c o r r e s p o n d s t o an ! . la p i c a l e n t r y ; th e d ec o m p o sitio n o f t h e , o x y p h o sp h o ra n e ■ i n t e r m e d i a t e - t o 27 ; form a t e t r a c o o r d i n a t e compound o c c u rs v i a th e d e p a rtu re o f an a p i c a l g ro u p , a c c o rd in g t o t h e m e c h a n is tic co n cep t o f In g o ld e t . a l . ^ ‘La,^)^ ( U lc -g ) t h a t w ere d e v e lo p e d f u r t h e r by McEwen e t . a l . The p o s t u l a t e d a p i c a l e n t r y and d e p a rtu re ' can be s u p p o rte d by argum ents s i m i l a r t o th o s e u se d f o r S^2 r e a c t i o n s o f t e t r a h e d r a l (1 2 ,1 8 ,1 9 ,3 3 ,3 6 ,3 7 - ^ 0 ,5 2 ) „ . .. . carb o n sy ste m s. 5 5 5 5 5 . M oreover, t h i s h y p o th e s is i s c o n s i s t e n t w ith th e o b s e r v a tio n t h a t a p i c a l bonds i n ph o sp h o ran e ( 2 2 ) d e r i v a t i v e s a r e w eaker and lo n g e r th a n com parable e q u a t o r i a l ^ (33,31+) . (2 3 b ,c ) bonds ’ h a v in g s m a lle r f o r c e c o n s t a n t s . ’ E q u a t o r i a l e n t r y and d e p a r tu r e has b e e n c o n s id e re d as an a l t e r - (19 52a) n a t i v e t o th e a p i c a l ca se by W estheim er e t . a l . ’ and was fa v o re d ( 2 5 ) by Cram e t . a l . f o r an alo g o u s p r o c e s s e s in v o lv in g s u l f u r compounds. When an e n t e r i n g group a p p ro ac h es a m o le c u le w ith a t e t r a h e d r a l s k e l e to n a t one o f th e t e t r a h e d r a l "ed g es" c o rre s p o n d in g t o e q u a t o r i a l e n t r y , th e n t h i s group and two o f th e lig a n d s o f th e t e t r a c o o r d i n a t e s p e c ie s become v e ry c lo s e p r i o r t o th e incom ing m o iety b e in g n e a r enough t o th e c e n t r a l atom t o undergo enough b en d in g o f th e c o r r e s - . po n d in g t e t r a h e d r a l bonds to w ard s t h e i r new a p i c a l p o s i t i o n s . T his problem i s fa c e d t o a f a r l e s s e r d e g re e w ith a p i c a l e n t r y and d e p a r t u r e . F u rth e rm o re , th e g r e a t e r le n g th o f a p i c a l bonds i n a t r i g o n a l b ip y r a m id a l i n d i c a t e s t h a t a group e n t e r i n g e q u a t o r i a l l y i n - o r d e r t o e f f e c t com parable b o n d in g i n t e r a c t i o n w ith t h e c e n t r a l atom. I t w ould b e d e s i r a b l e t o have d e t a i l e d q u a n t i t a t i v e com parisons betw een t h e a p i c a l e n t r y - d e p a r t u r e mechanism and i t s a l t e r n a t i v e s ; b a r r i e r s f o r t h e v a r io u s ty p e s o f n u c l e o p h i l i c a t t a c k o f an F an io n on ( l8 ) as a model sy ste m , w ere c a l c u l a t e d u s in g th e CNDO/2 ap p ro x im a- F ig u re 12a. T e tra h e d ra l a tta c k in te rm e d ia te s . I A = - l H BEST ro 00 F ig u re 12b. T e tra h e d ra l a tta c k in te rm e d ia te s . F- F N N F i ; j (12 3913) i 1 t i o n . • R e s u lts o f t h e s e c a l c u l a t i o n s i n d i c a t e t h a t th e b a r r i e r : ■ , i ]s i t u a t i o n s c a n , w ith some a p p ro x im a tio n , he r e p r e s e n t e d by fo rm u las j Il9 a -d shown, w ith t h e i r b in d in g en erg y in k c a l / m o l e . ‘ T hese m odel c a l c u l a t i o n s i n d i c a t e t h a t a p i c a l a t t a c k on th e FFF 1 i f a e e . v i a t h e b a r r i e r model s i t u a t i o n 19a i s t h e m ost f a v o r a b le mode I |o f n u c l e o p h i l i c a t t a c k o f Fe on POF . The l a t t e r , in y c o m p ariso n t o J i • ■ j ;a t t a c k on- t h e OFF f a c e ( v i a 19b) i s f a v o re d by 15 k c a l / m o l e , due i n j ith e main t o in c r e a s e d e l e c t r o s t a t i c r e p 'u ls io n p r i o r t o th e g a in i n | 1 I : | jenergy upon fo rm a tio n -of s u f f i c i e n t 0 bond c h a r a c t e r . E q u a t o r i a l a t - . j | \ I «tack on an FF edge (2c) I s d i s f a v o r e d by 30 k c a l/m o le r e l a t i v e t o j I * j i ■ j a p ic a l a t t a c k - v i a 2 a , w h ile F0 edge a t t a c k v i a 2 d -i s a p p ro x im a te ly j j69 k c a l/m o le h ig h e r in e n e rg y v e rs u s 1 9 a .. F o rm a tio n o f th e most s t a b l e j ! ; i i |TBP i s n o t alw ays fa v o re d k i n e t i c a l l y , as i n d i c a t e d by com parisons ! I ;of l ^ a and 19b in R e a c tio n 8 , F ig u re 13. j ' I | Quantum m e c h a n ic a l c o n s id e r a tio n s t h a t ta k e i n t o ac c o u n t d - > » i j ■ ;o rb ita l e f f e c t s ,c l a s s i f y th e lig a n d s o f a g iv e n s e t a t ta c h e d t o p e n t a - j : (12 ) I ic o o rd in a te p h o sp h o ru s i n t o a p i c o p h i l i c and a p ic o p h o b ic l i g a n d s , j (35) . . ' ' ■ c o rre s p o n d in g t o t h e m ost e l e c t r o n e g a t i v e . li g a n d s o f t h e - o r i g i n a l j [ / O O j p o l a r i t y r u l e . •’ However, b in d in g en e rg y d a t a (BE) o f .p e n ta c o - • io rd in a te p h o sp h o ru s compounds c a l c u l a t e d by th e u se o f th e CNDO/2 j ' ' i 'a p p ro x im a tio n s u g g e s t t h a t w ith .the a i d o f o n ly one l i g a n d p r o p e r t y , : * ' 5 \ ( !i . e . , e l e c t r o n e g a t i v i S y , one can g e t a t b e s t a ro u g h e s tim a te o f th e ’ ! i - (12 13) ■ , r e l a t i v e s t a b i l i t y o f s e v e r a l p e rm u ta tio n is o m e rs . ’ When more j {than two p e rm u ta tio n iso m ers a re c o n c e iv a b le , t h e sequence o f t h e i r I ? ! j r e l a t i v e s t a b i l i t i e s can n o t be p r e d i c t e d s a f e l y by t h e p o l a r i t y r u l e j i-in a l l c a se s . ( - 7 2 1 . 9 k c a l/m o le ) (-73U .6 k c a l/m o le ) F ig u re 13. Two com peting pathw ays f o r s u b s t i t u t i o n on a m odel p h o s p h a te (R e a c tio n 8 ). F ig u re ll+ , Scheme 1 . C o rresp o n d in g to th e c l a s s i c a l S^2 m echanism . • • i i I Scheme 1 i l l u s t r a t e s a p ro c e s s c o rre s p o n d in g t o th e - c l a s s i c a l ;S^2 m echanism , e x c e p t t h a t 2 3 a . i s a n . i n t e r m e d i a t e , n o t a t r a n s i t i o n j s t a t e . S in ce p e n t a c o o r d in a t e carb o n compounds w ith t h e m o re . e l e c t r o - i ;n e g a t i v e . l i g a n d s in th e e q u a t o r i a l p la n e a r e p r e f e r r e d , . t h e fo rm a tio n !o f a m ethane d e r i v a t i v e 26 n o t • o n ly • from 23a "but from th e iso m e rs U ( \ /■ ' A ; ! J23b-'lj-{}23d;) as w e ll i s t o be e x p e c te d . Y . 1 <L ’ x i x— cr; Y 1 ^ X---- C ' 1 N l3 Y x — cC 1 N l 3 L1 L2 023b() ft A l}23cij 923d|j i A lthough fo rm a tio n o f th e l a t t e r t h r e e in te rm e 'd ia te s may b e f a - I ■ A A iv o red o v er t h a t o f y23a[/, th e y do • n o t le a d t o a s u b s t i t u t i o n p ro c e s s jby a p i c a l . d e p a r tu r e i f none o f th e li g a n d s t o h a s . s u f f i c i e n t jle a v in g p r o p e r t i e s u n le s s r e o r g a n i z a t i o n by BPR o r TR .p ro c e s s e s forms i ii n te r m e d ia te s w ith a p i c a l le a v in g g ro u p . 33 R e te n tiv e D isp lacem en t R e a c tio n o f C y c lic System s i i When t h e p h osphorus atom o f p h o sp h in e o x id e s a n d -o f phosphonium j io n s i s i n c o r p o r a te d i n t o a s m a ll r i n g , i . e . * a .fo u r- o r fiv e-m em b ered I j r i n g , th e r e d u c t io n and.-d isp lacem en t r e a c t i o n s p ro c e e d , as a. r u l e , I I i |w ith r e t e n t i o n o f th e c o n f ig u r a tio n . a t . t h e r e a c t i v e c e n t e r >56-58) j I in c o n t r a s t t o th e an alo g o u s r e a c t i o n s - o f a c y c l i c .system s w h e re . i n - I ; , . ' . ■ ' I v e r s i o n i s , t h e norm al s te r e o c h e m ic a l p a t t e r n (R e a c tio n 1 , F ig u r e l ) . j i i ■The re a s o n s f o r t h i s d i f f e r e n c e betw een f o u r - and five-m em bered c y c l i c j i ' . i jsy sb em s, on t h e one h a n d , and a c y c l i c system s on th e o t h e r h a n d , , are' ; I (12) * j e a s i l y u n d e rs to o d in te rm s o f th e TR m echanism . Both TR and t h e j |BPR p r o c e s s e s a r e a c c e p ta b le m e c h a n is tic a l t e r n a t i v e s t o e x p l a i n - t h e ! ! i 1 ! -p .e rm u tatio n al is o m e r iz a t io n o f a c y c l i c TBP p h o s p h o ra n e s . I n f o u r - and j ! i Ifive-m em bered c y c l i c p h o s p h o ra n e s , th e r i n g must alw ays p la y t h e r o l e ! j ' ! |o f t h e " p a i r " in th e f b in g l e TR and m u l t i p l e •(TR)n p r o c e s s e s , and t h e j i ' ' . ■remaining t h r e e l i g a n d s th e r o l e o f th e " t r i o " . These p ro v id e th e j I (12 13) i b n l y ;g e n e r a l and c o n s i s t e n t o f th e e x p e rim e n ta l d a ta ’ and p r e - j i (12) ;su m ab ly , r e s u l t s from th e c irc u m s ta n c e t h a t th e lo w e s t b a r r i e r ifo r th e ' TR i s o b ta in e d when th e p a i r a n g le i s - s lig h t ly s m a lle r th a n . ! o j90 , w h ile th e t r i o a n g le s a re s l i g h t l y l a r g e r th a n 90 . The a n a l y s i s | jta k e s th e f a l l o w i n g . form in th e s i t u a t i o n u n d e r d i s c u s s i o n . . F o u r - , i jand as a .- r u le , five-m em bered r i n g s a r e u n a b l e , for- re a s o n s o f s t r a i n , 'i jto occupy t h e d i e q u a t o r i a l p o s i t i o n o f a TBP m o le c u la r s k e le t o n ;and-m ust occupy t h e a p i c a l - e q u a t o r i a l p o s i t i o n , even i f t h i s r e s u l t s :in v i o l a t i o n o f th e p o l a r i t y r u l e . C o n se q u e n tly , th e n u c le o p h ile ! i ;Y must a t t a c k th e s k e l e t a l t e t r a h e d r a l f a c e o p p o s ite one o f th e l i g - 3^ ands t h a t form p a r t o f th e s m a ll r i n g i n th e c y c l i c t e t r a c o o r d i n a t e p h o sp h o ru s compound (s e e 26 -* ■ 27 and 26 3 0 , in R e a c tio n 9 , F ig u re ^ w A < > ^ 1 5 ). S in c e th e n u c le o p h ile Y must e n t e r a p i c a l l y and one o f th e r i n g - lig a n d s must a l s o be a p i c a l , th e le a v in g group must alw ays b e eq u a t o r i a l , and t h e r e m ust b e p e r m u ta tio n a l is o m e r iz a t io n o f TBP 27 f o r d is p la c e m e n t o f X by Y, i f X has t o d e p a r t from th e a p i c a l p o s i t i o n . The c y c l i c TBP i n t e r m e d i a te 27 i s c a p a b le o f u n d e rg o in g a s i n g l e TR, a (TR)2 o r a (TR)^ p r o c e s s , ^"* '2 a n d , in g e n e r a l , (TR)2 and (TR)^ p r o c e s s e s .te n d t o have h ig h e r b a r r i e r s t h a n . t h e s i n g l e TR p r o c e s s . The TR w hich u s e s th e r i n g as t h e p a i r and w hich moves th e le a v in g group X t o an a p i c a l p o s i t i o n i s d e n o te d by t h e p e rm u ta tio n ( l 2)(X Y 3) and tr a n s f o r m s TBP(27) i n t o ( 2 8 ). A p ic a l d e p a r t u r e o f X from (28) /V /v — ' y i e l d s t h e t e t r a h e d r o n (29) w hich i s h o m o c h ira l t o ( 2 6 ) ( r e t e n t i o n ) . A/ 2 The (TR) w h ic h , a g a in , u s e s th e r i n g as t h e p a i r and moves X t o (1 2 1*3 PP) P an a p i c a l p o s i t i o n i s d e n o te d by p e rm u ta tio n * 5 (X Y 3) = (TR) and tr a n s f o r m s TBP(12) i n t o ( l6 ) ; a p i c a l d e p a r t u r e o f X from (31) y i e l d s 2 9 , th e a p i c a l a n tip o d e o f (29) and h e t e r o c h i r a l t o ( 2 6 ) ( i n - /V /V' 2 v e r s i o n ) . S in c e (TR) h a s , in g e n e r a l , a lo w er b a r r i e r th a n (TR) , r e t e n t i o n o f c o n f i g u r a t i o n , 21 2 7 ^" 28 29 i s th e e x p e c te d ste re o c h e m - O/ /V' i c a l p a t t e r n in c y c l i c t e t r a c o o r d i n a t e p h o sp h o ru s compounds w ith s m a ll r i n g s . N o tic e th e s te r e o c h e m ic a l consequence im posed by th e r i n g in t h i s c a s e . Due t o th e f a c t t h a t th e n u c le o p h ile m ust e n t e r o p p o s ite 2 on a p i c a l r i n g l i g a n d s and t h a t th e r i n g must b e th e " p a i r " , a (TR) can o n ly l e a d t o i n v e r s i o n w ith no s u b s t i t u t i o n , i n c o n t r a s t to th e (1 2 1 3 ) c ase f o r a c y c l i c p h o sp h o ra n e s. 5 ‘ I n te r c o n v e r s i o n o f d i a s t e r o - 26 L, X X 26 2T Y (TR)3 =/ ( 12 ) > L3 “ S X (TR)2 (XY3) s 'L, X 28 X 31 -X 2 30 F ig u re 15* N u c le o p h ilic s u b s t i t u t i o n o f c y c lic fo u r- o r five-m em bered rin g s in v o lv in g TBP in te rm e d ia te s which can .undergo re o r g a n iz a tio n by sim ple and m u ltip le TR p ro c e s s e s . R eaction 9. ( ce3 ) ------- (c h 3 )2 | > - ch2- c6h 5 > C £H . 32 6 5 OH0 ^ 3 * 2 1 (CH3>2 -P=0 \ 33 C6H5 F ig u re l 6 a . L igand exchange p r o c e s s e s o f c y c l i c .four-m em bered r i n g s o f p h o sp h o ru s compounds. R e a c tio n 10a. R' RJ I 2 ^ ( ch3 )2 3 *+ ‘C6H5 ( c h 3 ) 2 35 i ( c h 3 ) 2 ( C h3)2 36 OC2S5 T R R (ch3)2 ( ch3 ) 2 J — ] <CH3 S iC l -O SiC l. 31e 6H5 \ t K R ,(ch 3 ) 2 ( c h ) * ;— pr 3 2 \ 6 5 37a 0 S iC lo R (CH0 ) ( cr3 )2 3'S -P^ "S iC l, V 37d C6H5 R S iC l 37c OSiCl R1 r^ t (CH3 }2 (CH3 ) 2 ■P-"C6H5 37b SiC13 F ig u re l6 b . L igand exchange p r o c e s s e s o f c y c l i c four-m em bered r in g s o f p h o sp h o ru s compounds. R e a c tio n 1 0 b . F ig u re l 6 c . L igand exchange p r o c e s s e s o f c y c l i c four-m em bered r i n g s o f p h o sp h o ru s compounds. R e a c tio n 10 c. OH U o H. a CH a: L1 = CH3_ b: L1 = CH3 O H P==0 F ig u re l6 d . L igand exchange p r o c e s s e s o f c y c l i c five-m em bered r in g s o f p h o sp h o ru s compounds. R e a c tio n lO d. 38 iso m e rs by i n v e r s i o n a t p h o sp h o ru s in th e c y c l i c sy stem i s d u e , in I th i s i n t e r p r e t a t i o n , t o th e a p i c a l e n t r y and d e p a r tu r e o f th e n u c le o - | O iphil.e Y, t o th e o c c u rre n c e o f one (TR) and t o th e n eed t o in v o lv e j (12 13) ithe r i n g as t h e p a i r in th e TR p r o c e s s e s . * j. R e a c tio n s ( lO a ,b ,c and d , F ig u re 1 0 a, b , c , and d) a re 'r e p o r t e d exam ples in -which phosphonium compounds an d -p h o sp h id e o x id e s i jc o n ta in in g s m a ll r i n g s u n d erg o d is p la c e m e n ts a n d - r e d u c t i o n s , r e s p e c t - j jively-, w ith r e t e n t i o n o f th e p h o sp h o ru s c o n f i g u r a t i o n . | ( I n R e a c tio n 10 b , th e d is p la c e m e n t i s 36 3 5 , and th e r e d u c tio n !i s 35 - * ■ 3T -* • -3l+). i ~ t • R e a c tio n 11 d e s c r ib e s some o b s e r v a t i o n s 't h a t h a v e . d e f i e d i n t e r p r e t a t i o n in t h e p a s t . The r e d u c t io n o f th e c y c l i c p h o sp h in e ox id e ! jh2 p ro c e e d s t o g iv e p re d o m in a n tly 1+3 w ith . i n v e r s i o n , r a t h e r th a n n jw ith th e e x p e c te d r e t e n t i o n , \{k3d » H CH. ^ - c6H5 t ' 1 {!khiy >%C6H5 F ig u re 17. R e d u c tio n o f c y c l i c p h o sp h in e I o x id e . R e a c tio n 11. ! 39 T his r e s u l t can be e x p la in e d in one o f two w ays. E i t h e r p ro d u c ts U3 and kh a r e form ed a c c o rd in g t o t h e p a t t e r n o f R e a c tio n 9 , F ig u re 2 15, b u t in t h i s c a s e , th e b a r r i e r f o r (TR) m ust be lo w er th a n t h a t f o r t h e s i n g l e TR, hence 27 ^ 31 ^ 29 i s fa v o re d o v er 27 ^ 28 ^ 29, and i n v e r s i o n r e s u l t s ; a n o th e r i n t e r p r e t a t i o n i s t h a t th e h o m o c h ira l p ro d u c t i s form ed a c c o rd in g t o R e a c tio n 9» F ig u re 13: 26 ^ 27 ^ 28 ^ 2 9 , o r U2 -* ■ U3 , w h ile th e h e t e r o c h i r a l p r o d u c t ( in v e r s io n ) i s form ed by a p i c a l e n t r y and d e p a r t u r e w ith o u t r e o r g a n iz a tio n - o f th e TBP i n t e r m e d ia te . R e a c tio n 12, F ig u re 18, i s an i l l u s t r a t i o n o f c o n f i g u r a t i o n a t th e p h o sp h o ru s c e n t e r o f c h i r a l i t y w ith o u t any s u b s t i t u t i o n o f one group by a n o th e r . I n t h i s i n t e r p r e t a t i o n , t h e pathw ay i s 26 ■ + ■ 27 -* ■ 30 11, o f R e a c tio n 9, F ig u re 1 5 s w here t h e r e i s a (TR)^ p ro c e s s w ith th e s m a ll r i n g fo rm in g th e p a i r and th e o th e r li g a n d s th e t r i o . T h is i s fa v o re d b e c a u se i t b r in g s b ack th e m ost e l e c t r o n e g a t i v e l i g a n d OH (12) t o th e a p i c a l p o s i t i o n , U6a U6b. />✓ y-w I f a c y c lo b u ta n e d e r i v a t i v e k j u n d erg o es a n u c l e o p h i l i c s u b s t i t u t i o n u n d e r c o n d itio n s w here t h e b im o le c u la r S^2 p ro c e s s i s f a s t e r th a n th e S ^ l , i t w ould le a d t o a d e t e c t a b l e carbonium io n r e a r r a n g e m e n t . ^ " ^ I t w ould b e e x p e c te d t h a t p r o c e s s e s w ith c o n f i g u r a t i o n a l r e t e n t i o n , U7 50. S in c e d i e q u a t o r i a l t h r e e - t o five-m em bered r in g s a r e e n e r g e t i c a l l y d is f a v o r e d in tr i g o n a l - b i p y r a m i d a l m o le c u le s a p i c a l a t t a c k by Ye on h j can o n ly le a d to kQj a p i c a l d e p a r tu r e o f Xs i s o n ly p o s s i b l e a f t e r a re a rra n g e m e n t, ^9» w hich i s fa v o re d i f X p r e f e r s a p i c a l and Y p r e f e r s e q u a t o r i a l p la c e m e n t on th e t r i g o n a l b ip y ra m id a l s k e le t o n .__________________________ ___ _________________ _______ U o CH, CH, H --- ( 0H3>2 - p - - ch- c6h C6H5 O H -OH ( ch3 )2 U6a J - CH2 - C6H5 1^ OH (TR)' H CH, ( ch3 ) 2 .p^ - - ch2 - c6h 5 C 6h5 \i45b': A H CH, f (CH3 }2 (CH3 }2 ^ CH2 - C6 H5 PV OH 6 5 k6-b F i g u r e . 18. L ig an d exchange p ro c e s s o f four-m em bered r in g s o f p h o sp h o ru s compounds. R e a c tio n 12. I t w a s -th e o b je c t i v e o f . t h e p r e s e n t st.udy to, d e te rm in e w h e th e r I |th e second O rder n u c l e o p h i l i c s u b s t i t u t i o n o f c y c lo b u ta n e d e r i v a t i v e s jw ith a s te r e o c h e m ic a l l a b e l and a s u i t a b l e le a v in g group, p ro c e e d w ith c o n f i g u r a t i o n a l , r e t e n t i o n , o r i n v e r s i o n . A n u c l e o p h i l i c s u b s t i t u t i o n Jw ith p a r t i a l o r com plete r e t e n t i o n w ould b e e v id e n c e f o r a p ro c e s s I in v o lv in g an i n t e r m e d i a te w ith an a p i c o - e q u a t o r i a l r i n g in t e r m e d i a te ‘o f . s u f f i c i e n t l i f e t i m e t o undergo r e o r g a n i z a t i o n by BPR o r TR, i t jwould have s u p p o rt f o r t h e in te rm e d ia c y o f a p e n t a - c o o r d in a t e s p e c i e s . ^ In v e rsio n w o u ld .b e th e r e s u l t o f a p e n t a c o o r d in a t e t r a n s i t i o n s t a t e li *with a d i e q u a t o r i a l r i n g , o r a TBP in t e r m e d i a te w ith an a p i c o - e q u a to r - ! 2 ; i a l r i n g , u n d e rg o in g r e o r g a n i z a t i o n by (TR) . F ig u re 19- Model c o n f ig u r a tio n r e t e n t i o n S^2.mechanism f o r a l i p h a t i c carbon. THE MODEL EXPERIMENTS. The model reac tio n , t h a t w as.used f o r i n v e s t i g a t i o n o f th e s t e r i c < ■course o f second o rd e r n u c l e o p h ili c s u b s t i t u t i o n o f c y clo b u tan e d e r i v a t i v e s was b ased upon c i s - 3-ethoxy c y c lo b u ta n o l, 53. This i s p r e - | p a re d by th e a d d i tio n o f k e te n e t o e t h y l e t h e r and subsequent r e a c t i o n j w ith LiAlHi . The reduction,- 52 53 proceeds w ith e x c lu s iv e forma- 1 t i o n o f (gas c h ro m a to g ra p h ic a lly pure) c i s - isom er; none o f th e t r a n s - j isom er has been d e te c te d . Conversion o f 53 i n t o i t s P -n itro b e n z o a te /%/ ! •, • • i . • - ' 5 e s t e r , 5^, r e c r y s t a l l i z a t i o n o f th e l a t t e r to a c o n s ta n t m e ltin g p o i n t , ! fs* \ arid subsequent h ydrolysis,«/5^ 53, d id n o t change th e GLC o r NMR p ro - j ! ■ ’ ~ I p e r t i e s , o f -53. j ! N0 2 - V _ / - C00^ V > * OC2H5 S From 53 th e m ethyl s u lf o n a t e e s t e r , 55., was p re p a re d a c c o rd in g j t o th e procedure o f S e rv is and C ro sslan d . j i j ■ The m e sy la te , 5 5 was r e a c t e d w ith a v a r i e t y o f n u c le o p h ile s i I i under c o n d itio n s fa v o rin g S^ 2 s u b s t i t u t i o n , i . e . , w ith s tr o n g n u c le o - ; p h i l e s , i n s o lv e n ts t h a t do not promote an S ^l r e a c t i o n too s t r o n g l y . I ! In a l l c a s e s , p ro d u c ts r e s u l t e d whose fo rm atio n seems t o be due t o an j S^ 2 r e a c t i o n ; no p ro d u c ts were observed which would h a v e .in d i c a t e d , i . i • some p a r t i c i p a t i o n o f S ^l p r o c e s s e s . j ; The c i s - s te re o c h e m is try o f 55 p r o h i b i t s th e a c c e l e r a t i o n o f : ELT 1 by th e p a r t i c i p a t i o n o f th e ethoxy g r o u p . ' In th e p ro d u c ts 56 - [ i f f ~ j 5 9 , th e ethoxy group and th e s u b s t i t u e n t o r i g i n a t e s w ith th e n u c le o - s > - v » i + 0=C=CH2 ;5i:J > v 5 , 2 H5 LiAlHU HO <?521 o53:; F ig u re 20. P r e p a r a t io n o f ■ c i s - 3-ethoxycyclo'butanol. R eactio n 13- ! 'on h LiAlD i O H H H- OC_H, IO C JS, *COO- rOC„H Figure 21. Model experiments fo r him olecular S mechanism o f cyclobutane d e riv a tiv e s . Reaction lU. p h i l e are t r a n s , i n d i c a t i n g complete in v e rsio n , d u rin g th e p ro c e s s . F u rth e r j (353/) was r e a c t e d w ith d ie th y la z o d ic a r b o x y la te ,(160/), itr ip h e n o l phosphine,f] 6 lQ and p h th a lim id e , f]6 iQ in THF according to th e I fo llo w in g -scheme, in analogy to. a . r e a c t i o n r e p o r te d hy T. S o n o . ^ ^ ' g2h 5 - o2c - n = w- co 2 c 2h 5 + ( c6h ^ ) 3p 06o'i) > I61s) 2 5 53>~ C2H5°2_C-N- | ~ — .....-----------=* (]6 2 T ) co2c2h 5 c 2h 5o2 c-nh-?i ' *oc2h 5 > C ;i6 k? r/63 D cc) c : h,_ 2 2 5 '2 5 2 2 2 ; 6 5 3 H65/;! f 66j A + OC H / 0 067f) 2 -<>- »2% H NH„--< > -* 0 C 2H^. F ig u re 23. W ucleophilic s u b s t i t u t i o n on a lc o h o l by amino group. I.n t h i s r e a c t i o n , presum ably, f i r s t th e d i p o l a r adduct.^/6 2 :,] i s formed r e v e r s i b l y from 60 + 61. The ad d u ct, 62, combines w ith 53 /S/ /V -V to form th e p e n ta c o o rd in a te phosphorus compound, 6 3 , in which th e | ! re s id u e t h a t .is a tta c h e d to th e c i s - 3- e th o x y c y c lo b u ty l group can a c t as a le a v in g group, and i s r e p la c e a b l e by n u c le o p h ile s such as anion o f p h th a lim id e , Gb. Also h e r e , th e r e a c t i o n proceeds w ith in v e rs io n and g iv e s t r a n s - p r o d u c t. DISCUSSION OF RESULTS. i The c o n f i g u r a t i o n a l in v e rs io n t h a t i s observed in th e feimolecu- i i l a r n u c le o p h ilic s u b s t i t u t i o n r e a c t i o n s o f th e systems o f ty p e 69 i ' ~ I y i e l d i n g 7.0 can be e i t h e r i n t e r p r e t e d by assinning t h a t th e r e a c t i o n c 2h 5o » < ^ ^ > - * x > c 2h 5o 69 70a ! A/ /V F ig u re 2k. B im olecular n u c l e o p h ili c s u b s t i t u t i o n o f c i s - alcohol* "proceeds acc o rd in g t o th e c l a s s i c a l S^2 m e c h a n i s m ^ w i t h a p e n ta c o o r- | d in a te t r a n s i t i o n s t a t e , a d i e q u a t o r i a l h ig h l y s t r a i n e d U-membered i ■ r i n g , o r v i a a p e n ta c o o rd in a te in te r m e d ia te "with an a p i c o - e q u a t o r i a l j \ ' i I 2 ■ ’ ; r i n g , undergoing a (TR) r e o r g a n i z a ti o n which b rin g s th e le a v in g group ! ! ln to t h e a p l c a l p0si t i on8 , See B , , a8 t . t h e j d e p a rtu re o f th e le a v in g group. However, th e l a t t e r e x p la n a tio n i s un- > : l i k e l y , s in c e t h e r e i s no reaso n why an in t e r m e d i a te , 71> should n o t / ■V / 5 • jundergo a sim ple TR r e o r g a n i z a ti o n to y i e l d 70b v i a 72b. I n a given > ( 1 2 ) ~ ! system , a sim ple TR can be assumed t o fee more l i k e l y to. occur th a n f ' 2 ' a co rresp o n d in g (TR) p ro c e s s . i ! However, th e a n a l y t i c a l te c h n iq u e s t h a t were used in th e s e e x p e r i - > i 1 ments do n o t perm it th e e x c lu s io n o f th e f o r m ation of small amounts .of j 51 C2H5 \ H- *X W , (TR)' H fjT2al X C2H5°4 TR (o r BPR) V/ g2H5°<v , :?70TD:) F ig u re 25* P o s s ib le pathways f o r th e n u c l e o p h ili c s u b s t i t u t i o n / o f U-membered r i n g v i a TBP-which con undergo r e o r - g a n i z a ti o n by TR and (TR) p r o c e s s e s . i 52 th e r e t e n t i o n p r o d u c ts , 70b. F u r th e r i n v e s t i g a t i o n o f th e n u c le o - p h i l i c s u b s t i t u t i o n o f compound 6 9 w i l l fo llo w in th e n e a r f u t u r e . I t a ls o seems to be I n t e r e s t i n g t o devote a t t e n t i o n to th e s t e r e o chem ical outcome o f th e b im o le c u la r p ro c e s s e s o f a c y c lic system s. S t r e i t w e i s e r e t . a l . ^ ^ observed upon c a r e f u l exam ination o f th e S^2 r e a c t i o n o f o p t i c a l l y a c t i v e s e c - a l k y l h a l i d e s t h a t sm all amounts o f th e p ro d u ct had been formed w ith r e t e n t i o n . A lso, th e b im o le c u la r p ro c e s s e s o f th e compounds 7 3 and 7 * + should be s tu d ie d in o rd e r to a s s e s s th e p o s s i b i l i t y o f p a r t i c i p a t i o n o f a b im o le c u la r p ro c e ss 73 Ik v i a an in te r m e d ia te w ith 5 -c o o rd in a te carbon. In 73 and 7^»' th e AX yS-/ normal Sjj.2 p ro c e s s e s a re i n h i b i t e d by th e s te re o c h e m is try o f th e mole c u le s . Up t o now, on ly th e s o l v o l y s i s o f compounds o f ty p e s 73 and 7U have been s tu d ie d under c o n d itio n s t h a t fa v o r S I r e a c t i o n s . J -’ * EXPERIMENTAL SECTION. 5^ //T-t \ £ i s - 3 - e th o x y c y c lo b u ta n o l. To a s l u r r y o f 2 g (.0 6 mole) o f LiAlH^ in 200 ml e t h e r was added a t 0 ^1 0 ° over 1 h r , a s o l u t i o n of 2 0 g o f 3-ethoxycyclobutanone in 20 ml o f e t h e r . ' A fte r a d d i tio n was com plete, th e m ix tu re was s t i r r e d a t 25° f o r 1.5 h r and th e n cooled to 0°. In t u r n were added 2 ml of w a te r, 2 ml o f 15% sodium hydroxide s o l u t i o n and 6 ml o f w a te r. The m ix tu re warmed t o 2 5° over 1 h r and was f i l t e r e d , s o lv e n t was evapor a te d and th e p ro d u c t d i s t i l l e d , b .p . 9 5 ° / 2 0 mm. I r ^ 1 ^ , 2 . 9 5 j 3 - 3 5 5 3 4 0 , 3 4 7 , 7 .2 8 , 9 .0 y; nmr 6 5-0 (S, H, OH), 3 .3 - 4 .1 (m, 2H, meth- i n e s ) , 3.33 Cl, 2H, OCHg) 1 .5 -2 .9 (m, UH, r i n g CHg) , 1.12 ( t , 3H, CH ) g lc one isom er p u re (column DC 55° 4 m e te rs )) temp. 115°. C i.s -3 -e th o x y c y c lo b u ty l-p -n itro b e n z o a te . A s o l u t i o n o f 17*1 g o f 3 -eth o x y c y c lo b u ta n o l and 27-9 g P~ n itr o b e n z o y l c h lo rid e in 1 5 . 0 ml dry p y r id in e * a t 0 ° was s t i r r e d ov er n ig h t . 300 ml o f w a te r , 120 ml conc. H C l-ice and 300 ml o f benzene were added. The benzene la y e r was s e p a r a te d , washed s e v e r a l tim es w ith 1 0% sodium carb o n a te and d r ie d w ith sodium s u l f i t e and ev ap o rated in vacuo t o re c o v e r th e e s t e r which was r e c r y s t a l l i z e d from c y e lo - h e x a n e -e th y l a c e t a t e to g iv e l i g h t y ellow c r y s t a l s , m.p. 7 7° ( y ie l d 70/5), i r , 3 .3 h , 3 .5 , 5.85 y , nmr 6 8.3 (S, hH, benzene r i n g ) , 3 4 - 5 - 1 * p y rid in e was d r ie d by d i s t i l l i n g from a m ix tu re c o n ta in in g p -to lu e n e s u lf o n y l c h l o r i d e . 55 (m, 2H, m e th in e s ), 3-3 (q., 2H, OCH2 ) 1 . 6 - 3 . 1 (m, 2H, r i n g CH ) , 1 .2 ( t , 3H, CH ). H y d ro ly sis o f c i s - 3 - e th o x y c y c lo b u t y l- p - n itr o b e n z o a te . 5 . 0 g o f c i s - 3 - e th o x y c y c lo b u ty l-p -n itro b e n z o a te in 15 ml e th a n o l was r e f lu x e d fo r th r e e hours a f t e r a d d itio n o f 1.5 g KOH i n minimum p o s s i b le amount o f w a te r. 5 0 ml o f w ater was added and th e p ro d u ct was e x t r a c t e d w ith th r e e 50 ml p o r ti o n s o f e t h e r . The combined e x t r a c t was d r ie d over potassium carb o n a te and e v ap o ra ted in vacuo to re c o v e r .$ 9 gm [k0% y i e l d ) of c i s - 3 -e th o x y c y c lo b u ta n o l, b .p . 9 7 ° / 2 0 mm. I r , 2 .9 5 , 3 .3 5 , 3.U0, 3.U7, 7 .2 0 , 9 .0 y; nmr 6 5.0 (S, 1H, OH), 3 . 3 - ^ .1 (m, 2H, m e th in e s ) , 3.33 U , 2H, OCHg), 1 .5 -2 .9 (m, UH, r i n g CHg) , 1.12 ( t , 3H, CHg) , g lc p u re , column CD 55° m e te rs ). C is -3 -e th o x y c y c lo b u ty lm e s y la te . To 1.5 g o f c is -3 -e th o x y c y c lo b u ta n o l in 10 ml d i s t i l l e d d ic h lo r o - methane 1 . 9 6 g of trim ethylam ine*' and 1 . 6 5 g f r e s h ly d i s t i l l e d m esyl- c h lo rid e were added over about 10 m in u te s , keeping te m p e ra tu re s between 0 °-1 0 °. A f te r 20 m inutes o f s t i r r i n g th e r e a c t i o n m ixture was t r a n s f e r r e d to a s e p a ra to ry f u n n e l w ith th e h e lp o f more d ic h lo ro m e th a n e , and washed w ith c i s - w a te r , cold 10% HC1, s a t u r a t e d sodium b ic a r b o n a te s o l u t i o n , and f i n a l l y w ith s a t u r a t e d sodium c h lo rid e . Drying over * T r ie th y l amine was r e f lu x e d w ith p h t h a l i c anhydride fo llo w ed and d i s t i l l e d tw ice from KOH. I anhydrous sodium s u l f a t e , fo llo w ed by e v a p o r a t i o n .o f s o lv e n t i n vacuo, I j g a v e . 2 . 0 1 g o f crude m e s y la te , which was used w ith o u t ppi-lf i c a t i o n f o r 1 f u r t h e r experim ents. I r , 3 . 3 S 3 -k , 3 • 5 5 7•5 U; nmr 6 3 . 3 - ^ . 8 ( m , 2H, | m e th in e s ) , 3-3 ( a , 2H, OCH ) , e . g . ( 6 , 3H, CH ) 1 . 6 - 2 . 8 (m, >H, rin g iCH2 ) 1 .0 ( t , 3H, CH3 ) . I . f i i I | I T etram e th y l amxnQni'uin-p-nitro'benz.Qate . j i . : I : 8 . it g .p -n itro b e h z o ic acid.w as c a r e f u l l y n e u t r a l i z e d w ith sodium I i I [hydroxide a o lu tio n u sin g pH p a p e r. To.-the s o l u t i o n of sodium s a l t so j : ‘ i ; o b ta in e d was -added 1 0 . 5 ^- g o f s i l v e r n i t r a t e d is s o lv e d in 20 ml o f j |w a te r. The p p t o f s i l v e r p - n itr o b e n z o a te was f i l t e r e d under s u c tio n j ! . i ‘and was added to 5 - 5 g te tr a m e th y l ammonium c h lo rid e in 1 0 0 ml 1 : 1 j , i ; ethanol-:-water under s t i r r i n g ;a f t e r about 2 hours th e p r e c i p i t a t e d i | s i l v e r c h lo rid e was f i l t e r e d out and th e f i l t r a t e w a s .c o n c e n tra te d I : i / ,Q\ ' ■ ;under vacuo t o re c o v e r c r y s t a l s o f th e crude ammonium s a l t (m.p. 1 2?J) i J i ;which w a s - r e c r y s t a l l i z e d from e t h y l a c e t a t e - e t h a n o l ( 6 : 1 ) t o o b ta in i ! 1 [ c r y s t a l s , m..p. 1 2 6 °. 13.^ g was re c o v e re d (9Q% y i e l d ) , nmr 6 8 . 0 (q , j ' i ii+H, benzene r i n g ) , 3.0 (.8 , 12H , . (CH ') ^) . j 1 : i : I ; jN u c le o p h ilic d isp lacem en t on m e sy lates w ith q u a te rn a ry ammonium s a l t . ! I 2 . k g o f th e q u a te rn a ry s a l t and . 8 g - o f th e m e sy la te s were ta k en I i | 1 i ;in 1 0 ml chloroform-eth;amol (U :l) and l e f t t h r e e weeks under r e f l u x ! land s t i r r i n g (b a th temp. 50°)• A f te r u s u a l work-up th e p ro d u ct was > ! • ■ ' ' i !analyzed. Nmr 6 8.2 (S , l+H, benzene r i n g ) , 3 .3 (q , 2H, 0CH_) , 1 .5 - 2 .6 j ! ^ I 57 N u c le o p h ilic s u b s t i t u t i o n .on m e sy lates by a z i d e . I | 6.U g sodium a z id e was d is s o lv e d a t 50 in a m ix tu re c o n ta in in g j ' 27. g w ater and U3-7 g e th a n o l. 2.96 g o f m e sy la te s were added and ! o i th e r e a c t i o n m ixture l e f t o v e rn ig h t -at 50 under s t i r r i n g , a f t e r which i th e d isp lacem en t was- complete ( t i c ) . E x t r a c t i o n o f p ro d u c t w ith e t h e r | follow ed by d i s t i l l a t i o n gave th e a z id e , b . p . 6U°/20 mm, .which was . analyzed.. I r , 3 .0 , U.75, 6 .0 y. ; N u c le o p h ilic s u b s t i t u t i o n on a lc o h o l by amino g ro u p .^^ > 6 U ) In a 250 ml f l a s k , 2 g o f 3 -e th o x y c y c lo b u ta n o l was added to | 2.95 g o f d ie th y l a z o d i c a r b o x y l a t e , and 2 .k g o f p h th a lim id e in 25 ml i ! THF. Then U.U5 g o f t r i p h e n y l phosphine ;was d is s o lv e d in 5 ml THF- (6k) 1 f | and added, to. th e f l a s k , and l e f t to . s t i r f o r 2 hou rs. | The p ro d u c t m ixture was th e n t r a n s f e r r e d t o a 250 ml (fdhnji(j3otltom | f l a s k , d is s o lv e d in 25 ml e th a n o l, th e n 10 ml o f M a lc o h o lic h y d ra z in I I | h y d ra te and th e m ix tu re h e a te d under r e f l u x f o r 1 hour. The m ix tu re ; was e v a p o ra te d t o d r y n e s s , th e n th e r e s id u e warmed to 50° f o r 10 min- ! * i u te s w ith 25 ml o f ap p ro x im ate ly 2N h y d ro c h lo ric a cid .. Then i t was I ( 6 3 ) I cooled f o r 30 m in u tes. 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Creator Elgomati, Tarek Mukhtar (author)

Core Title Second-order nucleophilic substitution reactions of cyclobutane derivatives: A study of the stability of pentacoordinate carbon

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Degree Master of Science

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