Palladium or platinum complex catalysed reactions of carbonyl and imine compounds with disilanes

Abstract

The transition metal catalysed reactions of benzaldehydes and benzylideneamines with disilanes have been investigated. Palladium phosphine complexes catalyse the double silylation of the CO bond in benzaldehydes and the CN bond in benzylideneamines with 1,2-difluoro-1,1,2,2-tetramethyldisilane to yield α-(fluorodimethylsilyl)-α-(fluorodimethylsiloxy)toluene and N-methyl-N-(fluorodimethylsilyl)-α-(fluorodimethylsilyl)benzylamine respectively. When less activated disilanes such as 1,2-dichloro- and 1,2-dimethoxy-1,1,2,2-tetramethyldisilane were employed, the palladium phosphine complexes were less active and selective, resulting in extensive side reactions inclusive of 1,2-disiloxy-1,2-diphenylethane formation. The reaction of benzophenone with the difluorodisilane formed 2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane without affording the corresponding simple double silylation product. The formation of side products such as 1,2-disiloxy-1,2-diphenylethane in the reaction of benzaldehyde and 2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane in the reaction of benzophenone appears to suggest intermediacy of radical and silylene species. Tris(dibenzylideneacetone)diplatinum–etpo (etpo = 4-ethyl-1-phospha-2,6,7-trioxabicyclo[2.2.2]octane catalyst system was more active for unactivated disilanes, catalysing double silylation of benzaldehydes with hexamethyldisilane. The same catalyst system was found to catalyse the ortho silylation of benzylideneamines with disilanes via intramolecular C–H activation; both mono- and bis-silylated products were obtained. Reaction rates and product distributions are rationalised in terms of the steric and electronic properties of the disilanes, substrates and the catalyst used.