Metal alkoxide-induced dehydrohalogenations

Kinetic studies indicate that carbanion mechanisms are more probable in elimination from alkenes than from saturated molecules. Hydrogen-deuterium 

The observation of primary kinetic hydrogen isotope effects for elimination from /3-bromostyrenes in alkoxide-alcohol media suggests these substrates follow an E2 rather than an E1 cB mechanism - . Certainly greater carbanion character is associated with the transition states than those of the corresponding saturated halides. The magnitude of the isotope effect (Table 27) decreases 

RELATIVE RATES OF anti- AND i rt-E L 1 MI N ATI O N FROM SOME HALOGENOALKENES 

Release of steric interaction between bulky substituents related cis across the carbon-carbon double bond or unfavourable electronic interactions between the basic reagent and the c -related negatively charged alpha substituent are suggestions which have been proposed to explain an increase or a reduction in the knintlksun ratio, respectively . Certainly these two effects exert a major influence on the rate ratios. 

Possibly the measurement of the chlorine isotope effect for a series of suitably substituted substrates may help to decide whether or not greater stereospecificity is associated with decreased carbanion character of the transition state for elimination from alkenes. Of course, such studies must be accompanied by labelling experiments to exclude the possibility that alkynes arise by an apparent alpha-elimination (Section 5.1) involving formation of a carbanion at the alpha carbon, which subsequently eliminates alpha halogen simultaneously with migration of a substituent from the beta carbon. Results with alcoholic media, however, tend to indicate this alternative mechanism is unimportant if a normal beta-elimination is possible. 

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The mechanisms of base-induced beta-eliminations leading to alkynes are highly dependent on the type of base employed, and for this reason the dehydrohalogenations induced by metal alkoxides are considered separately from those initiated by metal amides or metal alkyls. 

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