Carbanions as Intermediates in the Formation of Grignard Reagents

Abstract

The reaction between certain aryl bromides RBr having substituents with group 14 elements and magnesium in THF to form the corresponding Grignard reagents RMgBr were found to be accompanied by unusual migrations of the group 14 functionality; it partially exchanges position with that of the original aryl bromide function. This migration is purely intramolecular for organosilicon derivatives but both intra- and intermolecular for the corresponding tin compounds. For example, 1-bromo-2-((trimethylsilyl)methyl)benzene (1b) gives rise to 1-(bromomagnesio)-2-((trimethylsilyl)methyl)benzene (2b) and to its intramolecular rearrangement product 1-((bromomagnesio)methyl)-2-(trimethylsilyl)benzene (3b), whereas the trimethylstannyl analogue 1a forms, in addition to the two corresponding Grignard reagents 2a and 3a formed by the intramolecular pathway, the intermolecular exchange products 1-(trimethylstannyl)-2-((trimethylstannyl)methyl)benzene (4a) and 1-(bromomagnesio)-2-((bromomagnesio)methyl)benzene (5‘). These ...