Reduction of amides with NaBH4 in diglyme at 162 °C

Abstract

High temperature (162 °C) reductions of aromatic amides were studied to extend the useful range of functional group transformations by NaBH₄. Primary aromatic amides were reduced to the amines with NaBH₄–diglyme at 162 °C. Reduction proceeds via fast initial loss of hydrogen, followed by formation of the corresponding nitrile, which is then more slowly reduced to the amine. N-Methylbenzamide is not reduced under these conditions, but it is reduced to benzylmethylamine when LiCl is added to NaBH₄–diglyme at 162 °C. LiCl addition raised the rate of primary aromatic amide and aromatic nitrile conversions to both the nitrile, first, and the amine. An intermediate was isolated from the reaction of N-benzylformamide with NaBH₄–LiCl in diglyme at 162 °C. It was examined by ¹H NMR, atomic absorption, IR and thermal decomposition. Possible structures are proposed. A mechanism for the reduction of primary aromatic amides is proposed based on the initial evolution of one mole equivalent of hydrogen and formation of the nitrile prior to further reduction to amine.