Copper’s rapid ascent in visible-light photoredox catalysis

Abstract

Visible-light photoredox catalysis offers a distinct activation mode complementary to thermal transition metal catalyzed reactions. The vast majority of photoredox processes capitalizes on precious metal ruthenium(II) or iridium(III) complexes that serve as single-electron reductants or oxidants in their photoexcited states. As a low-cost alternative, organic dyes are also frequently used but in general suffer from lower photostability. Copper-based photocatalysts are rapidly emerging, offering not only economic and ecological advantages but also otherwise inaccessible inner-sphere mechanisms, which have been successfully applied to challenging transformations. Moreover, the combination of conventional photocatalysts with copper(I) or copper(II) salts has emerged as an efficient dual catalytic system for cross-coupling reactions.

Functionalization of remote C(sp3)-H bonds enabled by copper-catalyzed coupling of O-acyloximes with terminal alkynes

“…Although most of radical-Cu(II) rebound processes involved activated secondary or tertiary benzylic carbons [42][43][44] , we have very recently shown that it is also possible to functionalize the primary radical in the presence of copper under photocatalysis conditions 45,46 . While dual photocatalyst/Cu catalytic system has emerged as a powerful tool for cross-coupling reactions 47 , the catalytic cycle depicted in Fig. 1e using copper as the only catalyst remained uncommon [48][49][50][51] .…”

Dual Benzophenone/Copper‐Photocatalyzed Giese‐Type Alkylation of C(sp3)−H Bonds

“…Seminal works by Kochi and co‐workers have shown that alkyl radicals rapidly react with Cu(OAc) 2 to generate putative metastable alkylcopper(III) species . More recently, such alkylcopper(III) species have been proposed as key intermediates in reactions exploiting dual copper/photoredox catalysis processes . Thus, in the early stage of the reaction, that is, when the Cu II concentration is still high, radical intermediate C might react with Cu(OAc) 2 to generate an alkylcopper species D , which may be viewed as a masked radical.…”

“…[13] More recently,s uch alkylcopper(III) species have been proposed as key intermediates in reactions exploiting dual copper/photoredox catalysis processes. [22] Thus, in the early stage of the reaction, that is, when the Cu II concentration is stillh igh, radical intermediate C might react with Cu(OAc) 2 to generate an alkylcopper species D,w hich may be vieweda samaskedr adical. This may be particularly important for limiting the polymerization in the early stage of the reaction, when the concentration of reducing Cu I/0 species is low.…”

Recent advances in photocatalyzed reactions using well-defined copper(I) complexes

“…Therefore, alternative solutions have to be developed. Among them, the use of first-row transition metals [8,9], particularly copper, is an interesting approach [10][11][12][13]. The high abundance, low price, low toxicity, and intrinsic properties (redox potential, four oxidation states, etc.)…”