Silenes with alkoxysilanes

Alkoxysilanes are frequently used as scavenger reagents for silenes. They add regiospecifically to the Si=C bond. In the case of silacyclobutadiene 296 the reaction was shown to be also stereospecific. Thus Z-404 is the sole product of the addition of trimethylsilyl methyl ether (TMSOMe) to 296 (equation 122)163. Characteristically Brook -type silenes do not react with alkoxysilanes like TMSOMe. 

Two reactions have come to be extensively used with silenes, arising from the need to trap the short-lived species cleanly and in high yield, as evidence either of their formation or of the extent of their formation. These are the addition of alcohols, usually methanol, across the double bond to yield an alkoxysilane, and the Diels-Alder reaction with a diene, often 2,3-dimethylbutadiene. Each is an example of the two different types of addition to the Si=C double bond. 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

Silenes also react efficiently with alcohols to give addition products. Indeed, addition is the most characteristic reaction of silenes and has been used for trapping silenes. Alcohols react regiospecifically to form alkoxysilanes. 

Leigh and his co-workers have studied the photochemical decomposition of the silacyclobutanes (224). In hydrocarbon solution with added methanol, (224) undergoes decomposition and the formation of the alkoxysilanes (225). These are formed via the intermediacy of the silenes (226) formed by photochemical elimination of ethene. 

With respect to nucleophilic attack, silaaromatics appear to behave in a manner analogous to silenes. The addition of methanol to 1-methylsilabenzene290,326 and to hexamethyl-l,4-disilabenzene314 yields the expected adducts [exclusively 1,4 in the latter case (equation 154)], as does the addition of methanol and tert-butyl alcohol to 1-methyl-2,3,4,5-tetraphenylsilabenzene128. It is possible that the addition occurs on the Dewar isomer of the silaaromatic. The lack of stereospecificity is presumably due to subsequent isomerization of the alkoxysilane in the presence of alcohol. 

---