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.