Spin-selective generation of triplet alkenylcarbenes: olefin cyclopropanation through visible light photosensitization of alkenyldiazo compounds

Abstract

Alkenyldiazo compounds are interesting precursors for generating alkenylcarbenoid intermediates, but their direct photochemical activation can result in competitive formation of 1H-pyrazoles via 1,5-electrocyclization and cyclopropenes via singlet carbene-mediated 2π-electrocyclization. Herein, we report the spin-selective generation of triplet free alkenylcarbenes from alkenyldiazo compounds through their energy transfer with visible light-activated 4DPAIPN. The spin state of free alkenylcarbenes can be controlled by introducing an electron-withdrawing group to the γ-position of alkenyldiazo reagents. This strategy is applied to the cyclopropanation with exogenous alkenes, providing a photochemical metal-free approach to synthesize alkenylcyclopropanes. The reaction tolerates many sensitive functional groups, such as free alcohol, secondary amine, carboxylic acid, and bioactive scaffolds.