Photocatalytic Oxidative Activation of Bicyclo[1.1.0]butanes for Formal [2σ+2π] Cycloadditions

We report the discovery of an example of oxidative bicyclo[1.1.0]butane activation. This reactivity stands in contrast to well-established strain-release nucleophile addition and reductive activation and opens up further possibilities for exploiting bicyclo[1.1.0]butanes in synthesis. Using a strongly oxidizing acridinium organophotocatalyst, the formal [2σ+2π] cycloaddition between bicyclo[1.1.0]butanes and alkenes or aldehydes leads to the corresponding bicyclo[2.1.1]hexanes or oxabicyclo[2.1.1]hexanes. The subtle interplay between bicyclo[1.1.0]butane and alkene identity dictates the mechanism of the reaction, with regiodivergent pathways providing complementary reaction products in up to >20:1 regioselectivity. A mechanistic investigation including electrochemical, photophysical, radical trapping, and computational studies support the oxidative mechanisms proposed.