Silylenes as intermediates in the 'direct synthesis'
Butadiene trapping experiments demonstrated that the silylenoids, :SiMeCl and :SiC12, are intermediates in the Direct Synthesis. It is believed that two types of silylenoid are formed on the metal surface. One type reacts on the surface with methyl chloride to generate methylchlorosilanes. The second type is released from the surface where as the free silylene it may be trapped by butadiene; it is not important in methylchlorosilane production. The gas phase reaction between the silylenes, :SiMeX (X=Me,Cl) and :SiCl2, and halomethanes involves two parallel series of reactions: silylene insertion reactions and radical chain reactions. In the reaction between :SiMe2 ch1orofluorocarbons (CFCs) an efficient chain sequence dominated; the driving force being the formation of Si-Cl bonds. The radical chain initiation step is believed to involve the dissociation of a singlet silylene-CFC acceptor complex bound through chlorine to produce haloalkyl (major chain carrier) and ClMe2Si. radicals. Fluorine was never abstracted. The major cyclic adduct produced when :SiMeX (X=H,C1,Me) is trapped by butadiene, is the result of a 1,3-silyl shift in the initial vinylsilacyclopropane, not Si-C bond rupture. Silacyclopropanes containing a Si-H bond undergo a rapid reversible ring-opening 1,2-H shift generating an alkylsilylene. Thermolysis of 1,1-dimethylsilacyclopent-3-ene produced 1,1-dimethylsilacyclopent-2-ene, butadiene, and :SiMe2 via a non-concerted mechanism involving a vinylsilacyclopropane intermediate. Computer modelling demonstrated that no single reaction is rate-determining with all intermediates in a steady-state. Alkene elimination from 1-allyl-1-silacyclopent-3-enes involves a conformationally enhanced endocyclic retro-ene reaction generating a 2H-silole, followed by a fast 1,5-H shift giving a 1H-silole. When the allyl group is substituted, it isomerises via a 1,3-silyl shift. a-Aryl-chloromethylsilanes, ArMeXSiCH2Cl (X=H,Me), underwent isomerisation in the gas phase as a result of H?C1 exchange when X=H, and Ar?Cl exchange when X=Me. H?Cl exchange is believed to involve an 'inverse ylide', whereas Ar?Cl exchange is believed to be concerted, i.e. a dyotropic rearrangement.