Cycloaddition reactions are a central focus of research in asymmetric synthesis. However, control of stereochemistry in photochemical cycloadditions remains a substantial challenge, with few general solutions. A small number of highly enantioselective organocatalytic and Lewis acid catalyzed photocycloadditions have been described in the past several years, but these methods have been applied exclusively to [2+2] photoreactions of enones. Our laboratory recently reported a dual catalyst system for highly enantioselective [2+2] photocycloaddition comprising a chiral Lewis acid and a transition metal photoredox catalyst. We report herein the efficient asymmetric formal [3+2] cycloaddition of aryl cyclopropyl ketones with olefins to generate highly substituted and stereodefined cyclopentane ring systems. This process is initiated via the one-electron reduction of the ketone to generate the corresponding radical anion. The overall redox neutral transformation is accomplished using a photocatalytic system comprising Ru(bpy)32+, Gd(OTf)3, i-Pr2NEt, and a PyBox ligand. A strongly coordinating ligand proved to be critical for both reactivity and selectivity.

Enantioselective photocatalytic [3+2] cycloadditions of aryl cyclopropyl ketones

251st National Meeting (2016)

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