4144
Vol. 8G
N . A . LEBEL, P. D. BEIRNE,AXI) P M, SCBRARIANIAN [ C O S T R I B U T I O S FROM T H E D E P A R T M E N T O F CHEMISTRY, W A Y S E STATE U S I V E R S I T Y ,
DETROIT2,
MICH.]
The Mechanism of Elimination Reactions. 11.' Kinetic Preference for exo-cis Bimolecular Eliminations with frans-2,3-Dihalonorbornanes B Y ?jORRlAN
A.
LEBEL,2a P A T R I C K
D
B E I R I V E , ~A ~N D
KECEIVED h f A Y
P 11
SUBRAMANIAN
11, 1964
By application of a combination of kinetic x r i d product analyses and specific deuterium labeling, f m m - 2 . 3 dihalonorbornanes have been shown t o undergo nearly exclusive eso-cis bimolecular dehydrohalogenations. This influence is sufficient t o cause the preferential eliiiiiiiation of hydrogen chloride rather than hydrogen bromide in the E2 reaction of endo-2-hronio-c..~~-~-cllloronorbor1iar1e. The r?sult is attributed t o a steric effect, and the mechanism is considered to involve a concerted pathway with considerable carbanion character.
The superiority in rate of bromide over chloride as the leaving group in concerted, bimolecular p-elimination reactions is generally accepted. For those studies in which product analyses have been made, dehydrohalogenations of vicinal bromochlorides usually give vinyl chlorides in predominant Systematic studies of E2 reactions5 have resulted in the recognition that orientation and the usual stereoelectronic preference for trans elimination may be altered by steric factors6 and geometrical considerations.1,i*8 Recently, an example of a favored cis elimination over trans elimination has been documented with cyclohexyl derivatives; the results are interpreted in terms of a carbanion process with steric hindrance to proton abstraction responsible for the relative r e a c t i ~ i t i e s . ~ Lye wish to report evidence for the favored eso-cis (relative to endo-cis) bimolecular eliminations of norbornyl derivatives, that this effect is probably steric in origin, and that the usual preferential E% elimination of hydrogen bromide over hydrogen chloride may even be altered. Pertinent to the arguments presented is the fact that such conclusions could have only been tenuous had they been obtained with 2-monosubstituted norbornanes because of complications arising from competing unimolecular processes with the e.yo \i?ith the 2,3-dihalonorbornanes El and SN side reactions are suppressed.
Results Diels-Alder addition of 1-bromo-2-chloroethylene and cyclopentadiene afforded a mixture of the four isomeric 3-bromo-6-chloro-2-norbornenes.Careful fractionation allowed isolation of the two cis isomers in pure form. Hydrogenation gave endo-cis- and eso( I ) Fspei- I J A m . C h e m Soc , 86, 3189 (1863). ( 2 ) ( a ) :.lived P. Sloan F o u n d a t i o n Fellow, 1961-196.5. (b) S a t i o n a l Science F o u n i a i i u n C o o p e r a t i v e G r a d u a t e Fellow, 1862-1964. (:i) J H i n e a n d P . B I.angfoi-d, J . A m Chrnz S o c , 1 8 , 3002 (19%). ( 4 ) H.1,. Coei-ing a n d H. H Espy. i b i d . . 78, 14.54 (1956) [.i I R e c e n t campi-ehensive reviews o f bimolecular &elimination r e a c t i o n s ar-egiven i n (a) J F B u n n e t t . Axg?iLm Chem I n l e ~ nE d . E n & , 1, 225 ( 1 9 6 2 ) : ( b ) J h i n e , "Physical Organic C h e m i s t r y , " AlcGran-Hill Book C o . , I n c , S e n Yul-k, S Y . , 1H62, p p 186-206. (c) I ) V B a n t h o r p e in " R e a c t i o n i n O)-yanic C h e m i s t r y , " Vol. 2 , " E l i m i n a t i o n R e a c t i o n s , " eviei- Publishing C o , I n c . , N e w I'ork, N Y . , 1R6:i. t X T h e effect of steric h i n d r a n c e u p o n o r i e n t a t i o n is s u m m a i i z e d in ref .ib a n d pel-tinent lite: a t u i e c i t a t i o n s a r e given .4nother viewpoint, which is n o t n e a i l y so d i v e r g e n t a s t h e p r o t a g o n i s t s seem t o s u g g e s t . :requires a m i n i m u m tht-eshold 01 s t r u c t u r a l c o m p l e x i t y befoi-e stei-ic effects c a n become d o m i n a n t in contl-ullinl: o ~ i e n t a t i o na, n d t h e case is best presented in i e f 5c ( 7 : S J Ciistiil a n d E F H o e g g e r , J A7n C h o n S a c . . 79, 334338 ( l l l . 5 7 ) . S J Cristol a n d S I. H a u s e , ibrd , 7 4 , 2193 ( 3 9 . 3 , a n d references c i t e d f 8 ! C H I I e P u y . R 1) T h u i - n , a n d G F. Rlorris, ibiif , 84, 1:314 (1962) , 9) F (> Boiniwell, R I , A r n o l d , a n d J B . B i r a n o n s k i . J Oiw C h m , 28, L ' l t i i i [ l W $ ) , ' 4 Koir., ~ hnur;u I P PKOOI:- A preferred era-cis bimolecular eliminatlon ' Takef r o m t'xt) 2 nr,rb
