Photolytic reactions of bis(trimethylsilyl)mercury and bis(trimethylgermyl)mercury with benzene, toluene, and anisole. Free-radical aromatic silylation and germylation

Abstract

Trimethylsilyl radicals produced by photolysis of bis(trimethylsilyl)mercury react with benzene or toluene to give the normal products of homolytic aromatic substitution. Thus the reaction with benzene gives, in addition to hexamethyldisilane, phenyltrimethylsilane and 2,5-cyclohexadienyltrimethylsilane, along with higher boiling products. With toluene the relative amounts of o-, m-, and p-tolyltrimethylsilanes are roughly in the satistically determined 2/2/1 ratio. The yield of hexamethyldisilane rises, and that of the aromatic substitution products falls when photolysis is carried out at higher temperatures.The reaction of trimethylsilyl radicals with benzene appears to be in competition with induced decomposition of the mercurial, from which some of the hexamethyldisilane is thought to arise. For small extents of reaction the ratio of the amount of phenyltrimethylsilane to that of hexamethyldisilane formed is linearly related to the initial concentration of the mercurial, while the absolute amount of phenyltrimethylsilane found in a given time remains constant. Possible reaction schemes accounting for the main results are considered.Analogous products are obtained from bis(trimethylgermyl)mercury, but the yields of aromatic substitution products are lower. Better yields of such products are obtained from naphthalene and phenanthrene.Anisole also gives the expected aromatic substitution products with both mercurials, but there appears to be an accompanying molecular reaction in this case to give other products, and it is suggested that absorption of light by the mercurial gives an excited [(Me3M)2Hg]* molecule, which can either break up into free Me3M radicals or react directly with a reactive substrate.