The cleavage of aryl-silicon bonds.

A summary of the methods of cleaving an aryl-silicon bond by nucleophilic and electrophilic reagents is given. Reports in which it is claimed that aluminium chloride cleaves an aryl-silicon bond are surveyed. It was found that, contrary to earlier reports, aryl-silicon bonds in arylsilicon chlorides are not cleaved by aluminium chloride, in anhydrous conditions, to give organoaluminium compounds. In the presence of water, acid cleavage occurs; for example, trichlorophenyl- silane gives benzene and silicon tetrachloride. In the presence of aluminium chloride, acetyl chloride converts trichlorophenyl- and tri- chloro-m-tolyl-silane into acetophenone and methyl-m-tolyl ketone respectively, and phosphorus trichloride converts trichlorophenyl- silane into dichlorophenylphosphine, but such reactions probably do not involve organoaluminium intermediates, as suggested by earlier workers. A survey is made of the literature concerning the resolution of asymmetric organosilicon compounds and the application of stereochemical studies to the elucidation of the mechanism of nucleophilic substitution at silicon. Optically-active p-methoxyphenylmethylphenylsilane was prepared by the reaction of p-methoxyphenyl-lithium and optically-active methyl-1-naphthylphenylsilyl-hydride and chloride. The coupling involves inversion of configuration when the chloride is used and retention when the hydride is used. The cleavage of the p-methoxyphenyl-silicon bond by bromine in benzene and carbon tetrachloride was studied and appears to proceed with inversion. This is taken to rule out a four-centre mechanism, and other possibilities are discussed. The reaction between a silicon hydride and chlorotriphenylmethane proceeds with retention of configuration and is consistent with the proposed four-centre mechanism. Several attempts were made to racemise optically-active organo-silicon compounds without replacing any of the substrates. All such attempts were unsuccessful. Two routes were devised to prepare p-(ethylmethylphenylsilyl)- benzoic acid, which has been independently resolved, from optically- active methyl-l-naphthylphenylsilane, but neither route was entirely stereospecific. An attempt is made to explain this lack of stereo-specificity.