Trichlorosilane as the Reducing Reagent

Trichlorosilane as the Reducing Reagent 957 Table 32.1 Reduction of ketimines by trichlorosilane. 

In general, if a phosphine is to be protected as a phosphine oxide, the corresponding phosphorus oxide starting materials are employed. However, it is possible to oxidize a phosphine to a phosphine oxide with terf-butylhydroperoxide.26 Several reagents are suitable for reducing phosphine oxides back to phosphines. A mixture of trichlorosilane and an amine is most often used to this end, as the conditions are quite mild and only a small degree of racemization occurs if chiral phosphine oxides are used (Scheme 10).27... 

Silanes are widely recognized as efficient reagents for reduction of carbonyl and heterocarbonyl functionality. In the case of alkyl and arylsilanes, the reaction requires catalysis by Lewis acids or transition metal complexes 1, 3] however, with more Lewis acidic trichloro or trialkoxysilanes, an altemative metal free activation can be accomplished. Thus, it has been demonstrated that extracoordinate silicon hydrides, formed by the coordination of silanes to Lewis bases, such as tertiary amines 7a], DMF [7b] or MeCN, and so on [7], can serve as mild reagents for the reduction of imines to amines [8]. In the case of trichlorosilane, an inexpensive and relatively easy to handle reducing reagent, and DMF as a Lewis basic promoter, the intermediacy of hexacoordinate species has been confirmed by Si NMR spectro scopy [7b]. 

As pointed out above, acidic media cleave the triazenes to yield the dia-zonium salts. The diazonium salts can be further functionahzed as demonstrated in the case of the reduction to the hydrocarbon 327 in THF with the aid of ultrasound via a radical pathway [227]. A recently found reagent for this reduction was trichlorosilane [232] which does not only serve as a source of trace-quantity hydrochloric acid to cleave the triazene moiety, but also as a hydride donor cleanly reducing the diazonium ions (Scheme 46). 

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