Photo-enhanced reduction of conjugated enones with $NaBH_4$

Abstract

The reduction of conjugated enones with sodium borohydride was carried out with and without irradiation. Reduction of some enones were accelerated on irradiation, but the others were not. Photoacceleration was observed in the cyclic cojugated enones with electron donating substituent on C-3. The photocatalyzed reduction seems to undergo through zwitter ionic species formed from the triplet state of an conjugated enone, followed by hydride attack to yield unsaturated or saturated alcohol. Generally, a conjugated enone has two excited triplet states, but only the (n, $\pi^*$) triplet state is effective. The highly viscous solvent was favored and aqueous solvent is not.