Unexpected formation of the first doubly benzylic 1,1-di-Grignard reagent

Reaction of 1-bromo-2-(1-chloro-1-trimethylsilylmethyl)benzene (3) with mag nesium in THF expectedly gave the corresponding 1,3-di-Grignard reagent 1b in 44 % yield, but in addition, the isomeric 1,1-di-Grignard reagent 4 was obtained in equal amounts. Compound 4 is remarkable for two reasons: it is one of the few examples of a 1,1-di-Grignard reagent, and its formation inv olves rearrangement by 1,3-migration of a trimethylsilyl group which provid es further evidence for the occurrence of carbanionic intermediates such as 11 during the formation reaction of Grignard reagents.