The development of alkyne transformations extending from non-catalytic, transition-metal or acid catalyzed additions and cyclizations, to metathesis, metalations, and radical addition reactions, has been a long-standing and one of the essential research objectives in organic synthesis. Conceptually, a single-electron oxidation event of carbon-carbon triple bond could lead to the generation of alkyne radical cation, which presents a synthetically attractive intermediate. However, reported methods for alkyne radical cation generation are limited to -irradiation or the use of strongly oxidizing conditions; and its catalytical generation and applications in synthetically meaningful reactions still remain scarce.<br/>Recently, our group developed a novel photocatalytic strategy for the generation of reactive and controllable -oxypyridinium radical through mild visible-light photoredox catalyzed single-electron oxidation of an alkyne/pyridine <i>N</i>-oxide system. During these studies, it was found that ynamides, subgroups of alkynes, exhibited significantly lower oxidation protentional than aliphatic and aryl alkynes (Scheme a below). In this context, we reported here a acridinium photoredox catalyzed hydroamination reaction of ynamides with a variety of pyrazoles, triazoles, tetrazoles, and indazoles (24 examples, up to 85% yield). Our investigations, including electrochemical studies, radical-quenching experiments, and Stern–Volmer fluorescence quenching studies clearly indicate the intermediacy of a alkyne radical cation in these transformations.

Photoredox catalyzed hydroamination of ynamide via facile generation of alkyne radical cation

ACS Spring 2022

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