The chemistry of organoborates. Part 5. Alkylation of alkynyltrialkylborate salts

Abstract

Alkynyltrialkylborates undergo β-alkylation with alkyl migration from boron to the α-carbon atom under mild conditions to give dialkylvinylboranes which were previously unobtainable. Oxidation of these intermediates provides a versatile route to regiospecifically α-substituted ketones. Hydrolysis yields alkenes in high yields as E–Z mixtures. The reaction is more stereoselective with borates derived from phenylethyne and with alkynyldialkyl(thexyl)borates. Formation of the major products appears to be kinetically controlled and, in all cases, the the major product has alkylating agent and the migrating group on the same side of the double bond. Whilst the rate of reaction is dependent on the alkylating agent, the rearrangement appears to have much less polar character than expected from reactions of carbocations, as the isomer ratio (E:Z) is essentially independent of the alkylating agent and solvent. MINDO/3 calculations are used to support a two-stage mechanism (Scheme 4) in which rate-determining electrophilic attack produces a bent vinyl cation-like intermediate; this then undergoes rapid, exothermic rearrangement with preferential retention of stereochemistry at the migration terminus.