Direct reduction of alcohols: Highly chemoselective reducing system for secondary or tertiary alcohols using chlorodiphenylsilane with a catalytic amount of indium trichloride

The direct reduction of alcohols using chlorodiphenylsilane as a hydride source in the presence of a catalytic amount of indium trichloride is described. Benzylic alcohols, secondary alcohols, and tertiary alcohols were effectively reduced to give the corresponding alkanes in high yields. A compound bearing both primary and secondary hydroxyl groups was reduced only at the secondary site to afford the primary alcohol after workup with Bu4NF. This system showed high chemoselectivity only for the hydroxyl group while not reducing other functional groups that are readily reduced by standard reducing systems. Thus alcohols bearing ester, chloro, bromo, or nitro groups, which are sensitive to LiAlH4 or Zn/H+, were selectively reduced only at the hydroxyl sites by the chlorodiphenylsilane/InCl3 system. NMR studies revealed the reaction course. The hydrodiphenylsilyl ether is initially formed and then, with InCl3 acting as a Lewis acid, forms an oxonium. complex, which accelerates the desiloxylation with donation of the hydrogen to the carbon.