Carbanions as Intermediates in the Synthesis of Grignard Reagents

Evidence for the occurrence of R⊖ in reaction (a) is provided by the isolation of compounds such as 2 in the reaction of 1 with Mg in THF after addition of Me3GeCl. On the basis of a series of comparative experiments it could be concluded that a carbanion intermediate is probably responsible for the stannyl migration.