Trichlorosilane (HSiCl3) - A cheap and convenient reducing agent

Abstract

(A) Trichlorosilane has often been used together with tertiary amines to reduce carbonyl groups of aromatic aldehydes, ketones, acids, amides, acid chlorides, and anhydrides to give the corresponding benzylic trichlorosilanes. This transformation was termed a 'reductive silylation' for the replacement of a carbonyl oxygen with H and SiCl3.4 This method provides a new way to form silicon-carbon bonds, and the benzylic trichlorosilane products can be further transformed to toluenes by base treatment. (Chemical Equation Presented) (B) Reduction of isocyanates with HSiCl3 gives the corresponding isocyanides in high yield under mild conditions.5 This provides a relatively simple method for the synthesis of vinyl isocyanides from alkenes.6 (Chemical Equation Presented) (C) Highly diastereoselective reduction of α-hydroxy ketones can be achieved using HSiCl3 as the reductant under neutral free-radical conditions.7 This reduction provides a diastereoselective, mild, one-electron alternative to the established two-electron methods that employ hydride reagents. (Chemical Equation Presented) (D) The catalytic enantioselective reduction of aryl ketones by HSiCl3 gives the corresponding alcohols with excellent enantioselectivity in the presence of catalytic amounts of N-formyl- α′-(2,4,6-tri-ethylphenyl)-L-proline as activator.8 (Chemical Equation Presented) (E) Asymmetric reduction of ketimines with trichlorosilane can be catalyzed by a new N-methyl-L-valine derived Lewis basic organo-catalyst, affording the respective secondary amines with high enantioselectivity.9 (Chemical Equation Presented) (F) The N-methylvaline derived Lewis basic formamide catalyzed reductive amination of α-chloroketones is a key step in the enantioselective synthesis of 1,2-diarylaziridines that had not been prepared previously as pure enantiomers.10 This provides an efficient and environmentally friendly methodology for the preparation of enantiopure aziridines. (Chemical Equation Presented) (G) The preparation of optically active alcohols from prochiral styrenes can be realized by the palladium-MOP complex catalyzed asymmetric hydrosilylation of styrenes with trichlorosilane.11 (Chemical Equation Presented) (H) Trichlorosilane is an often-used reducing agent for converting phosphine oxides to phosphines, which play an extremely important role as ligands in homogeneous catalysis.10 (Chemical Equation Presented). © Georg Thieme Verlag Stuttgart.