Photocatalytic Oxidative Activation of Bicyclo[1.1.0]butanes for Formal [2σ+2π] Cycloadditions
Maxim Golfmann, Marius Reinhold, Jorn D. Steen, Malte S. Deike, Behrend Rodemann, Christopher Golz, Stefano Crespi, Johannes C. L. Walker
Abstract
High Resolution Image Download MS PowerPoint Slide 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.