Recent Applications of [Cu(dap)2]Cl in Visible Light-Mediated Photoredox Catalysis

Abstract

Since 2008, visible light-mediated photoredox catalysis has enjoyed a renaissance in organic synthesis. [1–5] The most commonly employed metal-based photoredox catalysts typically utilise second- and third-row transition metals featuring polypyridyl ligand systems such as [Ru(bpy) 3 ]Cl 2 (1). [1] In contrast, the synthetic applications of copper-based photoredox catalysts have received much less attention. [6] A class of photoactive homoleptic, phenanthroline-based copper(I) complexes, including [Cu (dap) 2 ]Cl (2), were first prepared some 35 years ago (Chart 1). [7,8] This class of complexes share several interesting properties that can be exploited to facilitate chemical transformations. The absorption of a photon in the visible range leads to a metal-to-ligand charge transfer (MLCT) excited state. Subsequent intersystem crossing (ISC) results in spin inversion and allows access to long-lived triplet photoexcited states. [9] In its photoexcited state, complex 2 is a strong reductant (-1.43 V versus saturated calomel electrode) capable of reducing organic acceptor molecules via single electron transfer (SET) processes. [8] The ensuing CuIIspecies can undergo a second SET process to oxidise an organic donor molecule, which reduces the catalyst back to its ground state and closes the catalytic cycle. [1] The coordinatively unsaturated nature of these copper(I) complexes typically provides lower coordinative stability relative to saturated octahedral complexes (e.g. 1). Also, in the ground state, these copper(I) complexes typically exhibit a distorted tetrahedral coordination geometry. Upon photoexcitation, oxidation of the metal centre occurs as a result of the population of the MLCT excited state. This causes a flattening distortion that results in a more square-planar coordination geometry. These two factors are primarily responsible for reducing excited state lifetimes. The present focus article aims to highlight recent applications of [Cu(dap) 2 ]Cl in organic synthesis.