Indium metal as a reducing agent in organic synthesis

The low first ionisation potential (5.8 eV) of indium coupled with its stab ility towards air and water, suggest that this metallic element should be a useful reducing agent for organic substrates. The use of indium metal for the reduction of C=N bonds in imines, the heterocyclic ring in benzo-fused nitrogen heterocycles, of oximes, nitro compounds and conjugated alkenes an d the removal of 4-nitrobenzyl protecting groups is described. Thus the het erocyclic ring in quinolines, isoquinolines and quinoxalines is selectively reduced using indium metal in aqueous ethanolic ammonium chloride. Treatme nt of a range of aromatic nitro compounds under similar conditions results in selective reduction of the nitro groups; ester, nitrile, amide and halid e substituents are unaffected. Likewise indium in aqueous ethanolic ammoniu m chloride is an effective method for the deprotection of 4-nitrobenzyl eth ers and esters. Indium is also an effective reducing agent under non-aqueou s conditions and alpha -oximino carbonyl compounds can be selectively reduc ed to the corresponding N-protected amine with indium powder, acetic acid i n THF in the presence of acetic anhydride or di-tert-butyl dicarbonate. Con jugated alkenes are also reduced by indium in THF-acetic acid.