Reductions of Carboxylic Acids and Esters with NaBH4 in Diglyme at 162°C

Abstract Aromatic esters, including the extremely sterically hindered ester: t-amyl 2-chlorobenzoate, are readily reduced to the corresponding benzyl alcohols in high yield with NaBH4 in refluxing diglyme (162°C). In sharp contrast, aliphatic esters usually gave only low yields of alcohols. Instead, diglyme fragmentation products are formed which undergo transesterification reactions, producing complex product mixtures including products such as RCOOCH2CH2OCH3. The mechanism of this process involves sodium borohydride-induced SN2 cleavage of diglyme (hydride attack) at high temperatures. However, when the extremely electron rich, 3,4,5-trimethoxybenzoic acid is treated with NaBH4/diglyme at 162°C (with or without an equivalent of LiCl), no 3,4,5-trimethyoxybenzyl alcohol is formed. The electron rich and hindered ester, t-amyl-3,4,5-trimethoxybenzoate, also does not reduce under these conditions (with or without LiCl). However, both methyl and isopropyl 3,4,5-trimethoxybenzoate esters were converted into 3,4,5-trimethyoxybenzyl alcohol in good yields in NaBH4/diglyme/LiCl at 162°C. These reductions did not occur unless LiCl was present, illustrating the electron releasing effect of the three methoxy functions which reduce the carbonyl group's reactivity. Keywords: Sodium borohydrideReductionsDiglymeTransesterificationCarboxylic acidsEsters Acknowledgments The work was supported, in part, by the Morton Division of Rohm and Haas and the Grant Division of Ferro Corp.