Process for the methylation of silicon compounds

Silicon compounds which are synthesised from silicon, halogen and optionally carbon and hydrogen atoms as the only atoms and contain at least two silicon atoms per molecule, for example hexachlorodisilane or 1,1,2,2-tetrachloro-1,2-dimethyldisilane or mixtures of such silanes, are methylated by reacting such silicon compounds with tetramethylsilane in the presence of at least one organoaluminium compound of the general formula Ra-AlY3-a, in which R denotes identical or different alkyl radicals having 1 to 4 carbon atoms, Y denotes halogen, hydrogen or hydrocarbon radicals having 1 to 10 carbon atoms, which are bonded to the aluminium atom via oxygen and are free of aliphatic multiple bonds, and a is 1, 2 or 3, or of a reaction product formed in situ of such an aluminium compound with at least another constituent of the contents of the reaction vessel, at least one silane of the general formula Rb HcSiCl4-b-c, where R denotes identical or different hydrocarbon radicals having 1 to 10 carbon atoms, which are free of aliphatic multiple bonds, b is 0, 1, 2 or 3 and c is 1, 2 or 3, with the proviso that the sum of b + c is at most 4, and of hydrogen halide.