Recently, our group reported an innovative photocatalytic strategy for the generation of reactive and controllable β-oxypyridinium radical through mild visible-light photoredox catalyzed single-electron oxidation of an alkyne/pyridine N-oxide system. By applying such strategy, we have developed an ortho-alkylation reaction of pyridine N-oxides and a photoinduced oxidative cyclopropanation of ene-ynamides. As part of our continuing efforts in development of alkyne transformations by harnessing visible light energy, we reported here a photoredox catalyzed oxidative radical cyclization of general 1,n-enynes. Based on our previous research and mechanism studies, it is proposed that photoredox catalyzed single-electron oxidation of the enyne/pyridine N-oxide system leads to the generation of β-oxypyridinium vinyl radical, which furnished the radical cyclopropanation product (2 and 4). The acridinium photocatalyst has been identified as the privileged catalyst for the reaction. O-, N-, C-Linked enynes, and alkyl or aryl substituted alkynes are all competent substrates providing broad reaction scope and a branched functionality in the [n.1.0] bicyclic products (over 20 examples and up to 85% yield). The bicyclic species with a fused three-member ring are commonly found in natural products, bioactive compounds, and important synthetic intermediates. Their synthesis is typically approached using precious metal catalysts. This research provides an alternative metal-free visible-light induced approach for the synthesis of such valuable compounds, which is enabled by the facile generation of β-oxypyridinium vinyl radical. Moreover, an unexpected tricyclic product 3 was obtained, when dimethyl substituted 1,6-enyne was subjected to the photocatalytic condition. The formation of product 3 is rationalized by β-oxypyridinium vinyl radical involved radical translocation homolytic aromatic substitution sequences.

Photoredox catalyzed divergent radical cyclization of eneyne via facile generation of β-oxypyridinium vinyl radical

ACS Spring 2022

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