Visible Light Induced Reduction and Pinacol Coupling of Aldehydes and Ketones Catalyzed by Core/Shell Quantum Dots

We present an efficient and versatile
visible light-driven methodology
to transform aryl aldehydes and ketones chemoselectively either to
alcohols or to pinacol products with CdSe/CdS core/shell quantum dots
as photocatalysts. Thiophenols were used as proton and hydrogen atom
donors and as hole traps for the excited quantum dots (QDs) in these
reactions. The two products can be switched from one to the other
simply by changing the amount of thiophenol in the reaction system.
The core/shell QD catalysts are highly efficient with a turn over
number (TON) larger than 4 × 104 and 4 × 105 for the reduction to alcohol and pinacol formation, respectively,
and are very stable so that they can be recycled for at least 10 times
in the reactions without significant loss of catalytic activity. The
additional advantages of this method include good functional group
tolerance, mild reaction conditions, the allowance of selectively
reducing aldehydes in the presence of ketones, and easiness for large
scale reactions. Reaction mechanisms were studied by quenching experiments
and a radical capture experiment, and the reasons for the switchover
of the reaction pathways upon the change of reaction conditions are
provided.