Visible-Light-Promoted Metal-Free Ammoxidation of C(sp3)-H bonds
The metal-free activation of C(<i>sp</i><sup>3</sup>)-H bonds to value-added products is of paramount importance in organic synthesis. Herein, we report the use of the commercially available organic dye 2,4,6-triphenylpyrylium tetrafluoroborate (TPP) for the conversion of methylarenes to the corresponding aryl nitriles via a photo process. Applying this methodology, a variety of cyanobenzenes have been synthesized in good to excellent yield under metal- and cyanide-free conditions. We demonstrate the scope of the method with over 50 examples including late-stage functionalization of drug molecules (celecoxib) and complex structures such as L-menthol, amino acids and cholesterol derivatives. Further, the presented synthetic protocol is applicable for gram-scale reactions. In addition to methylarenes, selected examples for the cyanation of aldehydes, alcohols and oximes are demonstrated as well. Detailed mechanistic investigations have been carried out using time-resolved luminescence quenching studies, control experiments and NMR‑ spectroscopic as well as kinetic studies, all supporting the proposed catalytic cycle.
