Toward an Understanding of Substituent Effects on the Efficacy and Decomposition of Iridium(III) Photoredox Catalysts

Abstract

In the last decade, visible-light photoredox catalysis has emerged as a powerful tool for the development of organic transformations that proceed under very mild conditions. This remarkable reactivity is enabled by the unique photophysical properties of the catalysts, which can undergo single-electron transfer (SET) processes upon excitation by light to generate organic radicals or activate other types of catalysts. However, the radical intermediates formed have been found to undergo Minisci-type addition to heteroarene groups on the ligands of the photocatalysts, and these modifications often ruin the desired electrochemical parameters. In keeping with the aim of designing a radical-proof photocatalyst, this work presents progress toward an understanding of the relationship between ligand substituents and efficacy of an Iridium(III) catalyst as it is used in a published nickel-photoredox-mediated decarboxylative arylation. Mass spectroscopy analysis of catalyst decomposition patterns revealed unexpected radical addition to the cyclometalated phenyl rings of phenyltetrazole ligands, as well as a surprising nickelmediated coupling of the N-boc radical to the photocatalyst itself. The catalyst scaffold was optimized to block these potential pathways, and the protected photocatalyst showed a four-fold increase in yield relative to the unprotected version in the N-boc proline reaction. Although photocatalyst decompositions in organic reactions are seldom discussed in the current literature, uncovering these potential side reactions and ways of eliminating them will continue to expand the applicability of photoredox catalysis in organic synthesis.