Date of Award
Doctor of Philosophy
Thomas J. Barton
In an attempt to generate allene via a thermally-induced [beta]-elimination of methoxytrimethylsilane, flash vacuum pyrolysis (FVP) of 1-methoxy-2-trimethylsilyl-2-propene was found to give isopropenyltrimethylsilane via a retro-ene elimination of formaldehyde. By rendering the retro-ene process unavailable through replacement of the methoxy group by a trimethylsiloxy group, [beta]-elimination of hexamethyldisiloxane was found to be a viable route to allenes;An attempt to generate 1,1-dimethyl-1-silaallene by pyrolytic elimination of methoxytrimethylsilane from (1-trimethylsilyl)vinyldimethylmethoxysilane led to the discovery that vinyldimethylalkoxysilanes extrude dimethylsilanone via an unprecedented homolytic 1,5-migration of hydrogen to form a 1,4-biradical. Other mechanistic possibilities including an initial 1,2-migration of silicon to generate a carbene, a homolytic loss of methyl group, and a retro-ene elimination of formaldehyde to form a silene, were ruled out on the basis of product analysis, isotope labeling, and kinetic investigations. Dimethylsilanone was also generated by a retro-ene decomposition of allyloxydimethylsilane. Substitution by two methyl groups at the allylic position of allyloxydimethylsilane was found to give rise to a competition between the retro-ene process and a homolytic cleavage of the C-O bond to form the highly stable 2-methyl-2-butenyl radical;FVP of (1-methoxyethenyl)pentamethyldisilane was found to generate vinylidene via an [alpha]-elimination of methoxypentamethyldisilane rather than generating 1,1-dimethyl-1-silaallene via a much more thermodynamically favorable [beta]-elimination of methoxytrimethylsilane. A detailed gas-phase kinetic investigation using simple model systems revealed that [alpha]-eliminations of alkoxysilanes to generate carbenes were favored over [beta]-eliminations of alkoxysilanes to generate silenes and olefins, due to entropically favored three-centered transition states for [alpha]-eliminations over four-centered transition states for [beta]-eliminations.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Bain, Sukhamaya, "Mechanistic and kinetic investigations of thermally-induced [alpha]- and [beta]-eliminations of alkoxysilanes " (1987). Retrospective Theses and Dissertations. 8614.