Carbon with silacyclopropanes

Dipolar reagents diazomethane and methylenetrimethylphosphorane act as carbon nucleophiles in their reactions with silacyclopropanes. Reaction of diazomethane with hexamethylsilacyclopropane gave silacyclobutane (40) in 70% yield (Scheme 5), whereas reaction with methylenetrimethylphosphorane afforded phosphorus ylide (41) in 78% yield (Scheme 6). These... 

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

Photochemically generated trimethylsilylphenylsilylene also adds to the carbon-carbon double bonds of many types of olefins (54). Thus, the photolysis of a hexane solution of tris(trimethylsilyl)phenylsilane (20) in the presence of isobutene by irradiation with a low-pressure mercury lamp produces, after subsequent treatment of the photolysis mixture with methanol, fert-butylphenyI(trimethylsilyl)methoxysilane in 52% yield, as the sole insertion product. Direct evidence for the formation of 1-trimeth-ylsilyl-l-phenyl-2,2-dimethyl-l-silacyclopropane in this photolysis can be obtained by NMR spectroscopic analysis of the reaction mixture. 

In this case, no product arising from the reaction of the silicon-carbon double-bonded intermediate with methanol can be observed at all. However, on prolonged irradiation of the solution two products, 1,1-dimethyl-2,3-benzo-5-trimethylsilyl-l-silacycIopentene (48) and 1-methoxy-dimethylsilyl-l-trimethylsilyl-2-phenylethane are obtained in 17 and 7% yield, in addition to the (Z)- and (E)-isomers (15 and 12% yield). The formation of the latter compound can best be understood by the transient formation of a silacyclopropane followed by reaction with methanol (98). The mechanism for the production of 48 in the prolonged irradiation of PhCH=CHSiMe2SiMe3 is not fully understood but is tentatively given in Scheme 16. 

Although x-ray diffraction remains the most reliable method for ascertaining the structures of three-membered rings with one silicon or tin, l3C NMR and 29Si NMR can be quite useful (Table 2). In silacyclopropanes, the chemical shifts for the ring carbons range from 10-23 ppm for silacyclopropenes, the range is 147-195 ppm. 

A silacyclopropane (96) with no substituents on its ring carbons was prepared in 85% yield by thermolysis of 7,7-di-r-butyl-7-silabicyclo[4.1.0]heptane in the presence of ethylene (Equation (41)), <91OM2095>. 

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