Although esters are normally stable to sodium borohydride, the reduction of ketones and esters by sodium borohydride is dramatically accelerated by U.V. irradiation. However, this does not generally apply to all ketones and esters, and also the photochemical reduction is dependent on solvent polarity. The quantum yield of photocatalyzed reduction of methyl benzoate with sodium borohydride are 0.5 in diglyme, 0.08 in ethanol, 0.01 in isopropyl alcohol, 0.03 in dimethyl formamide, and 0.04 in dimethyl sulfoxide solution. In diglyme solution, the photoreduction quantum yields are 0.5 for methyl benzoate, 0.03 for methyl 4-methoxy benzoate, 0.5 for methyl 2-chlorobenzoate, 0.05 for methyl 4-nitrobenzoate, 0.01 for methyl 2-Furancarboxylate, and 0.001 for 5-methyl-4-imidazole carboxylic acid ethyl ester. The corresponding benzoic acids and their sodium salts in diglyme solution and in aq. solution are inert to sodium borohydride upon U.V. irradiation.
The reduction of aliphatic carboxylic acid esters, particularly phenyl esters, with sodium borohydride is also accelerated by photoexcitation. The quantum yields of photocatalyzed reduction of cyclohexanecarboxylic acid esters with sodium borohydride are 1.0 for phenyl ester, 3 for 2-chlorophenyl ester, 4 for 4-chlorophenyl ester, 0.4 for 2-methylphenyl ester, 0.2 for 3-methylphenyl ester, 0.7 for 4-methylphenyl ester, and 0.05 for 4-methoxyphenyl ester.
The reduction of ketones by sodium borohydride is generally accelerated by U.V. irradiation.
When the (n,$π^*$) state is the lowest excited state in the molecule, the photocatalyzed reduction quantum yields of esters or ketones are enhanced, and when the (π, $π^*$) state is the lowest excited state or the charge transfer character is present in the molecule, the acceleration of reduction with sodium borohydride is negligible.