The merger of transition metal and photocatalysis

| The merger of transition metal catalysis and photocatalysis, termed metallaphoto­ catalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non­traditional nucleophile partners. In addition, photocatalysis can aid fundamental organometallic steps through modulation of the oxidation state of transition metal complexes or through energy­transfer­ mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished. REVIEWS NATURE REVIEWS | CHEMISTRY VOLUME 1 | ARTICLE NUMBER 0052 | 1 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Change of oxidation state The importance of the metal oxidation state in transition metal catalysis is well established. Indeed, it has long been noted that the acceleration of several elementary steps in coupling protocols can be observed on modulation of the oxidation state of the metal catalyst. For example, in a stoichiometric setting, Hillhouse and others have demonstrated that oxidation of a metal centre can vastly increase the rate of reductive elimination, thereby allowing the rapid formation of C–N and C–O bonds6–9. More recently, these concepts have been exploited in catalytic variants: for example, the Chan–Evans–Lam reaction, in which oxidation of a copper intermediate enables a rapid reductive elimination step to forge aryl–heteroatom bonds10–12. Reductive elimination from Cu(ii) is itself sluggish, but reductive elimination from a Cu(iii) intermediate, which can be accessed using readily available and benign oxidants (such as atmospheric oxygen), is facile. Indeed, Sanford and co-workers have shown that this strategy of oxidation and subsequent reductive elimination from Cu(iii) can also be used to construct C(sp2)–F bonds using nucleophilic fluoride as the fluorine source13, thereby providing a solution to the long-standing challenge of C–F bond formation. As a second example, Yu and co-workers have demonstrated that oxidation of cyclometallated Pd(ii) intermediates to form Pd(iv) species can greatly accelerate C–C and C–X bond-forming reductive elimination steps14–16, which have often been found to be problematic at Pd(ii) centres. Overall, these results are mechanistically consistent, given that reductive elimination steps formally represent an oxidation of the pendant substituent by the metal centre. As such, a metal centre that is in a higher oxidation state would have a greater thermodynamic driving force to participate in reductive elimination. Not surprisingly, these findings have inspired a renaissance in high-valent metal catalysis, which in turn has led to the development of valuable new bond-forming reactions17,18. Excitation The photophysical and photochemical properties of metal complexes have been extensively studied for more than 50 years19. In fact, the properties of transition metal species in their excited states have been exploited in many important applications: energy storage20,21, organic light-emitting diodes22 and dye-sensitized solar cells23, to name only a few. Interestingly, with the exception of photoredox catalysis (see below)24, the invocation of the excited state of a transition metal in organic transformations remains limited25–28. As a recent exception, Fu and Peters have demonstrated that photoexcitation can be exploited to achieve room-temperature Ullmann coupling Nature Reviews | Chemistry M