Suppressing Reverse Radical Conversion via Diluting Local Hydrogen Ions for Efficient Photocatalyzed C–C Coupling between Benzyl Alcohol and Benzaldehyde

Abstract

We report the efficient C–C coupling between benzyl alcohol and benzaldehyde to hydrobenzoin using CdSe/CdS quantum dots as visible-light photocatalysts and sodium oxide as a cocatalyst. The optimized conditions resulted in a reaction with a selectivity of hydrobenzoin >95%, an apparent quantum yield of 23.6% under the excitation by 520 nm LED, and a turnover frequency of 56.8 mmol g^–1 h^–1. Reactant concentration-dependent experiments demonstrated that benzaldehyde can process fast proton-coupled electron transfer, improving the utilization of photogenerated electrons. Photoluminescence quenching experiments and reactions with deuterated benzaldehyde confirmed that due to the in situ produced hydrogen ions, the hydroxymethyl radicals would process efficient reverse conversion from hydroxymethyl radicals to benzyl alcohol, resulting in noneffective charge transfer. Formation of sodium benzyloxide can eliminate the in situ produced hydrogen ions around the hydroxymethyl radical, suppressing this reverse conversion and improving the utilization efficiency of transferred holes and electrons.