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submitted on 24.08.2020 and posted on 24.08.2020by Kathiravan Murugesan, Karsten Donabauer, Burkhard Koenig
The metal-free activation of C(sp3)-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.