Proton Transfer from Methanol to Carbanion Intermediates

Proton Transfer from Methanol to Carbanion Intermediates 

Eaborn and coworkers reported PKIE values for the protonation of various benzylic anions by methanol [23], Anions are generated in situ by the reaction of methanolic sodium methoxide with YC6H4CH2Si(CH3)3. 

Proton Transfer Associated with Methoxide Promoted Dehydrohalogenation Reactions 

The E2 mechanism for alkoxide-promoted dehydrohalogenations is generally thought to be concerted with the breaking of the C-H and C-X bonds and the formation of the O-H and w-bond occurring in the transition structure [26]  

Obtaining both an isotope effect, k /k, and an element effect, k fk, are the experimental evidence that the C-H and C-X bonds are breaking in the transition structure. Since measurement of heavy atom isotope effects requires special instrumentation, the element effect has taken the place of heavy atom isotope effects in most investigations. The element effect was first proposed by Bunnett in a 1957 paper dealing with the nucleophilic substitution reactions of activated aromatic compounds [27], and later applied to dehydrohalogenation mechanisms by Bartsch and Burmett [28]. The lack of any incorporation of deuterium prior to elimination has also been used as experimental evidence favoring the concerted mechanism [29]. The stereochemistry should be a trans-elimination. 

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