Cycloadditions Enabled by Chromium(III) Photoredox Catalysis

Photoredox catalysis has emerged as an integral fraction of modern organic methodology.[1] Since the early 2000’s there has been a renewed interest in photochemistry, with photoredox catalysis utilizing visible light becoming a rapidly expanding field. Various transition-metal complexes have been developed and proven efficient photocatalysts; however, the two most commonly employed metals in these systems are the incredibly expensive, rare-earth metals iridium and ruthenium. Previously, the Ferreira lab has indicated the promise of Cr(III)-polypyridyl complexes to perform as effective photooxidants in the presence of visible light.[2]

Our recent work describes a direct dearomative Cr(III) photocatalyzed (3+2) cycloaddition between indoles and vinyldiazo reagents.[3] The desired cycloaddition product is obtainable in high yields, with exclusive regioselectivity. This reaction is enabled by a newly developed Cr(III) photocatalyst, with the enhanced reactivity in the reaction directly attributed to a bathochromic shift in the catalyst, allowing for stronger absorption of the catalyst in the presence of visible light. The novel Cr(III) photocatalyst considerably out-performs our previously reported Cr(III) photocatalyst and has greater stability to reaction conditions than commonly used Ru(II) photocatalysts.