Gas phase thermal interconversions of silacyclobutanes, alkylsilylenes, and silacyclopropanes

A study of the mechanism of propene formation in the pyrolysis of 1-hydrido-1-silacyclobutanes was undertaken by pyrolyzing deuterium labelled compounds. Flash vacuum pyrolysis of 1,1-dideuterio-1-silacyclobutane and 1-deuterio-1-methyl-1-silacyclobutane afforded deuterated propene, indicating that the mechanism of propene formation proceeds via initial isomerization of the silacyclobutane to propylsilylene with a 1,2-deuterium (or 1,2-hydrogen) shift ((alpha)-elimination of a methyl group). The propylsilylene then undergoes decomposition to propene and silylene. Copyrolysis of 1,1-dideuterio-1-silacyclobutane with 2,3-dimethylbutadiene afforded a silylene adduct with deuterium incorporation supporting the proposed mechanism;Generation of several n-alkylsilylenes and 1-deuterio-n-alkylsilylenes from disilanes and silanes resulted in formation of alkenes; the deuterioalkylsilylenes formed alkenes with deuterium incorporation observed, and n-alkylsilylenes with C(,4) or longer carbon chains formed not only the expected n-alkenes, but also isomeric alkenes with formal double bond migration. Both of these phenomena are proposed to result from a silylene-silacyclopropane isomerization occurring via a reversible (beta)-C-H insertion by the alkylsilylene. Pyrolysis of alkylsilylene generators under various gas flow conditions, and copyrolysis with butadiene or 2,3-dimethylbutadiene as silylene trapping agents, failed to suppress the formation of isomeric n-alkene product mixtures;Decomposition of alkylsilylenes via (beta)-C-H insertion and silacyclopropane formation appears to occur to the exclusion of any other C-H insertion reactions. No products resulting from (gamma)-, (delta)-, or (epsilon)-C-H insertion could be found in any of the systems studied