Photocatalyzes reduction of carbonyl compounds

Abstract

The photocatalyzed reduction of carbonyl compound with various metal hydrides is investigated. The photoreduction quantum yields of methyl benzoate with sodium borohydride increase as the polarity of solvents decrease. The reduction of cyclohexanecarboxylic acid esters with sodium borohydride in diglyme, of which the lowest excited state is pure (n,$π^{\ast}$) excited state, is tremendously accelerated by photoexcitation. The photoreduction of tolyl cyclohexanecarboxylic acid esters with lithium borohydride showed the large quantum yields. But the photoreduction of cyclohexanecarboxylic acid esters with borane was not accelerated by irradiation with 254 nm light due to the decreased of electron density of carbonyl oxygen in the singlet (n,$π^{\ast}$) excited state. In the photoreduction of cyclohexanecarboxylic acid esters with lithium borohydride, being a basic borohydride, is more effective than acidic borane. The photoreduction of cyclohexanecarboxylic acid esters with β-naphthoxyborane was accelerated and showed the large quantum yields due to the easy coordination to the carbonyl oxygen of esters as electron rich site by the increase of acidity of-naphthol in reducing agents. On the other hand, the photoreduction of cyclohexanecarboxylic acid esters with lithium tri-β -naphthoxyborohydride was not accelerated due to the decrease of attacking ability of hydride to the carbonyl carbon of esters by the increase of acidity of -naphthol in reducing agents. The photoreduction of cyclohexanecarboxylic acid esters with catecholborane was accelerated by irradiation with 265 nm light showing small quantum yields. The photoreduction of ketones with sodium borohydride also accelerated in the (n,$π^{\ast}$) excited state. The photoreduction of cyclic ketones with lithium borohydride was dramatically accelerated, especially in the case of more hindered cyclic ketones, on irradiation of uv light.