High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating O and Br Radicals over Iron-Modified TiO Photocatalysts.

Catalytic ammoxidation of alcohols into nitriles is an essential reaction in organic synthesis. While highly desirable, conducting the synthesis at room temperature is challenging, using NH as the nitrogen source, O as the oxidant, and a catalyst without noble metals. Herein, we report robust photocatalysts consisting of Fe(III)-modified titanium dioxide (Fe/TiO) for ammoxidation reactions at room temperature utilizing oxygen at atmospheric pressure, NH as the nitrogen source, and NHBr as an additive. To the best of our knowledge, this is the first example of catalytic ammoxidation of alcohols over a photocatalyst using such cheap and benign materials. Various (hetero) aromatic nitriles were synthesized at high yields, and aliphatic alcohols could also be transformed into corresponding nitriles at considerable yields. The modification of TiO with Fe(III) facilitates the formation of active O radicals and increases the adsorption of NH and amino intermediates on the catalyst, accelerating the ammoxidation to yield nitriles. The additive NHBr impressively improves the catalytic efficiency the formation of bromine radicals (Br) from Br, which works synergistically with O to capture H from C-H, which is present in benzyl alcohol and the intermediate aldimine (RCH═NH), to generate the active carbon-centered radicals. Further, the generation of Br from the Br additive consumes the photogenerated holes and OH radicals to prevent over-oxidation, significantly improving the selectivity toward nitriles. This amalgamation of function and synergy of the Fe(III)-doped TiO and NHBr reveals new opportunities for developing semiconductor-based photocatalytic systems for fine chemical synthesis.