Tetraorganosilanes as Alkylating Agents

Studies on the applications of tetraorganosilanes as alkylating agents in organic synthesis have been carried out mostly during the last decade, and show that the applicable reactions proceed, as a rule, at temperatures below 0°C. Under these conditions, side reactions (polymerisation, isomerisation, etc.) usually do not occur, and thus the transfer of the organic moiety is effected with high selectivity. 

The most useful synthetic methods are those based on the transfer of unsaturated residues such as allyl, vinyl, propargyl, acetylene and allene groups. It has been found, however, that saturated alkyl groups are also capable of being transfered from a silicon atom to a carbon atom. It was found at first [572] that trimethylsilylacety-lenes reacted with acyl halides in the presence of aluminium chloride to give ketones of the acetylene series (Eq. 4.33)  

The transfer of the organic moiety from silicon apparently proceeds to the acyl cation formed by the action of aluminium chloride on the acyl halide. 

The reaction discovered by Birkofer gave impetus to the development of a great number of methods for the synthesis of ketones based on the transfer of various organic fragments from silicon to the acyl cation. Olah et al. [573] have developed a general method for the synthesis of ketones using tetraalkylsilanes. When boiled with carboxylic acid chlorides in the presence of aluminium chloride, they form ketones (Eq. 4.34)  

Tetramethylsilane is also used for ionic hydrogenation of olefins. To carry out this reaction, it is necessary to introduce equimolar amounts of olefins, tetramethylsilane, hydrogen chloride and aluminium chloride. The yield of saturated hydrocarbons is as high as 85-90%, e.g. Eq. 4.35 and 4.36 [574]  

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