Visible-light photoredox catalysis has been esteemed as one sustainable and attractive synthetic tool. In the past four years, a new yet challenging trend, visible-light-driven redox-neutral radical C-C cross-coupling involving putative radical intermediates, has been booming rapidly. Its advent brings a powerful platform to achieve non-classical C-C connections, and should lead to fundamental changes in retrosynthetic analysis. In this tutorial review, we highlight the recent achievements of visible-light-mediated redox-neutral radical C(sp)-C(sp), C(sp)-C(sp), and C(sp)-C(sp) bond formation, opening a new window for C-C cross-coupling through the photoredox electron shuttling cycle between two coupling partners. While radical-radical coupling steered by the persistent radical effect was proposed as a rational explanation for the redox-neutral photoredox events, alternative kinetically driven chain propagation and radical addition pathways cannot be ruled out. This tutorial review aims to highlight the recent achievements of photoredox-neutral radical C-C coupling in synthetic chemistry.

The gold-catalyzed C-H annulation of anthranil derivatives with alkynes offers a facile, flexible, and atom-economical one-step route to unprotected 7-acylindoles. An intermediate α-imino gold carbene, generated by an intermolecular reaction, promotes ortho-aryl C-H functionalization to afford the target products. The transformation proceeds with a broad range of substrates under mild conditions. Moreover, the obtained functionalized indole products represent a versatile platform for the construction of diverse indolyl frameworks.

Within the wide family of gold-catalyzed reactions, gold photocatalysis intrinsically features unique elementary steps. When gold catalysis meets photocatalysis, a valence change of the gold center can easily be achieved via electron transfer and radical addition, avoiding the use of stoichiometric sacrificial external oxidants. The excellent compatibility of radicals with gold catalysts opens the door to a series of important organic transformations, including redox-neutral C–C and C–X coupling, C–H activation, and formal radical–radical cross-coupling. The photocatalysis with gold complexes nicely complements the existing photoredox catalysis strategies and also opens a new avenue for gold chemistry. This review covers the achieved transformations for both mononuclear gold­(I) catalysts (with and without a photosensitizer) and dinuclear gold­(I) photocatalysts. Various fascinating methodologies, their value for organic chemists, and the current mechanistic understanding are discussed. The most recent examples also demonstrate the feasibility of both, mononuclear and dinuclear gold­(I) complexes to participate in excited state energy transfer (EnT), rather than electron transfer. The rare applications of gold­(III) photocatalysts, both homogeneous and heterogeneous, are also summarized.

An unprecedented and challenging radical-radical cross-coupling of α-aminoalkyl radicals with monofluoroalkenyl radicals derived from gem-difluoroalkenes was achieved. This first example of tandem C(sp(3) )-H and C(sp(2) )-F bond functionalization through visible-light photoredox catalysis offers a facile and flexible access to privileged tetrasubstituted monofluoroalkenes under very mild reaction conditions. The striking features of this redox-neutral method in terms of scope, functional-group tolerance, and regioselectivity are illustrated by the late-stage fluoroalkenylation of complex molecular architectures such as bioactive (+)-diltiazem, rosiglitazone, dihydroartemisinin, oleanic acid, and androsterone derivatives, which represent important new α-amino C-H monofluoroalkenylations.