Silacyclopentanes, polymerization

The chemistry of silacyclopentanes resembles closely that of acyclic alkylsilanes. Strong bases are needed to cleave the silicon-carbon bond, while electrophilic attack occurs readily with halogens and strong acids, e.g. sulfuric. In addition, aluminum chloride will induce polymerization of silacyclopentanes unless one of the groups on silicon is carbofunctional (Scheme 154) (67MI12000). If this is chloromethyl, then ring expansion occurs (Scheme 155). 

Polymerization of DMSB occurs with up to quantitative yield in the presence of catalytic platinum(O) complexes, whereas dimerization is the predominant process in the presence of phosphine-platinum(O) complexes (Scheme 15) <1995JA8873, 1965JOC2618, 1966JC1137, 1972JOM(44)291>. Dimerization and polymerization apparently have the same intermediate, 2,2-dimethyl-l-platina-2-silacyclopentane, which can be isolated in moderate yield (45%) <1995JA8873>. 

In 1995, Tanaka and co-workers isolated 2,2-diphenyl-l-platina-2-silacyclopentane (Scheme 45) and showed that such a complex is a viable intermediate for both dimerization and polymerization of SCBs <1995JA8873>. The reactivity of 1,1-disubstituted SCBs toward oxidative addition of transition metals increases in the series of methyl < phenyl < chloro. 

Gyclosilanes merit special comment because of their tendency to yield polymeric products. Three- and four-membered rings containing silicon polymerize readily under very mild conditions. Silacyclopentanes, on the other hand, give linear polymeric materials [MW (molecular weight) = 1000-2500] when heated with aluminum halides. 

The polymerization of 1,1-dimethyl-l-silacyclopentane (7) was carried out as described for I, but only the starting material was recovered. 

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