(521cq) Visible Light Mediated Selective Oxidation of Alcohols Catalyzed By Organic Small Molecules in Batch and Flow

The selective oxidation of benzylic sp³ C-H bonds is an important class of functional group transformation of organic synthesis, and its aldehydes and ketones products are widely used in natural products, agricultural chemistry, fine chemicals and other fields. In contrast to traditional thermal reactions, photocatalytic strategy has great benefits for the environment through the utilization of clean energy. Inspired by the potential of photocatalysis, several precious metal complexes and elaborate organic dyes have been reported for the selective oxidation of alcohols. Nevertheless, the availability and limited reactivity of these photocatalysts hinder their application. Therefore, it is important and of great research significance to explore an environmentally friendly, mild and efficient benzylic sp³ C-H bonds selective oxidation.

Based on the previous work, a photocatalytic strategy via disulfide-mediated selective oxidation of alcohols was developed. Green and efficient oxidation can be realized under light irradiation, utilizing phenyl disulfides as the photocatalyst, 1-phenethyl ethanol as the template substrate, molecular oxygen as the oxidant. A variety of primary and secondary alcohols were converted into the corresponding aldehydes or ketones. The functional groups were well tolerated, with a maximum yield of 91% at room temperature with 3 hours of light irradiation. The reaction efficiency could be increased even further by combining a continuous-flow strategy. 1-Phenylethanol can be oxidized more efficiently and 98% yields can be obtained at the residence time of 4.3 min, so the efficiency of the reaction was improved by a factor of nearly forty. Relevant mechanistic studies have also been achieved to determine the role of oxygen and disulfides for this oxidation.