Mechanistic aspects in the Grignard coupling reaction of bis(chloromethyl)dimethylsilane with trimethylchlorosilane

Abstract

The Grignard reactions of bis(chloromethyl)dimethylsilane (1) with trimethylchlorosilane (2) in THF give both the intermolecular C-Si coupling and intramolecular C-C coupling products. At beginning stage, 1 reacts with Mg to give the mono-Grignard reagent ClCH2Me2SiCH2MgCl (I) which undergoes the C-Si coupling reaction to give Me2Si(CH2SiMe3)(2) 3, or C-C coupling to a mixture of formula Me3SiCH2(SiMe2CH2CH2)(n)R-1 (n = 1, 2, 3, ..; 4a, R-1 = H: 4b, R-1 = SiMe3). in the reaction, two reaction pathways are involved: a) I reacts with 2 to give Me3SiCH2SiMe2CH2Cl 6 which further reacts with Mg to afford a Me2SiCH3Me2SiCH2MgCl (II) or b) I cyclizes intramolecularly to a silacyclopropane intermediate A, which undergoes a ring-opening polymerization by the nucleophilic attack of the intermediates I or II, followed by the termination reaction with H2O and 2, to give 4a and 4b, respectively. As the mole ratio of 2/1 increased from 2 to 16 folds, the formation of product 3 increased from 16% to 47% while the formation of polymeric products 4 was reduced from 60% to 40%. The intermolecular C-Si coupling reaction of the pathway a becomes more favorable than the intramolecular C-C coupling reaction of the pathways b at the higher mole ratio of 2/1.