Synthetic Methods Driven by the Photoactivity of Electron Donor-Acceptor Complexes

The association of an electron-rich substrate with an electron-accepting molecule can generate a new molecular aggregate in the ground state, called an electron donor-acceptor (EDA) complex. Even when the two precursors do not absorb visible light, the resulting EDA complex often does. In 1952, Mulliken proposed a quantum-mechanical theory to rationalize the formation of such colored EDA complexes. However, and besides a few pioneering studies in the 20th century, it is only in the past few years that the EDA complex photochemistry has been recognized as a powerful strategy for expanding the potential of visible-light-driven radical synthetic chemistry. Here, we explain why this photochemical synthetic approach was overlooked for so long. We critically discuss the historical context, scientific reasons, serendipitous observations, and landmark discoveries that were essential for progress in the field. We also outline future directions and identify the key advances that are needed to fully exploit the potential of the EDA complex photochemistry

Catalytic asymmetric C–C cross-couplings enabled by photoexcitation

Enantioselective catalytic processes are promoted by chiral catalysts that can execute a specific mode of catalytic reactivity, channeling the chemical reaction through a certain mechanistic pathway. Here, we show how by simply using visible light we can divert the established ionic reactivity of a chiral allyl–iridium(iii) complex to switch on completely new catalytic functions, enabling mechanistically unrelated radical-based enantioselective pathways. Photoexcitation provides the chiral organometallic intermediate with the ability to activate substrates via an electron-transfer manifold. This redox event unlocks an otherwise inaccessible cross-coupling mechanism, since the resulting iridium(ii) centre can intercept the generated radicals and undergo a reductive elimination to forge a stereogenic centre with high stereoselectivity. This photochemical strategy enables difficult-to-realize enantioselective alkyl–alkyl cross-coupling reactions between allylic alcohols and readily available radical precursors, which are not achievable under thermal activation. [Figure not available: see fulltext.

Recent advances in organic synthesis using light-mediated n-heterocyclic carbene catalysis

The combination of photocatalysis with other ground state catalytic systems have attracted much attention recently due to the enormous synthetic potential offered by a dual activation mode. The use of N-heterocyclic carbene (NHC) as organocatalysts emerged as an important synthetic tool. Its ability to harness umpolung reactivity by the formation of the Breslow intermediate has been employed in the synthesis of thousands of biologically important compounds. However, the available coupling partners are relatively restricted, and its combination with other catalytic systems might improve its synthetic versatility. Thus, merging photoredox and N-heterocyclic carbene (NHC) catalysis has emerged recently as a powerful strategy to develop new transformations and give access to a whole new branch of synthetic possibilities. This review compiles the NHC catalyzed methods mediated by light, either in the presence or absence of an external photocatalyst, that have been described so far, and aims to give an accurate overview of the potential of this activation modeL.M. acknowledges the Autonomous Community of Madrid (CAM) for the financial support (PEJD-2019-PRE/AMB-16640 and SI1/PJI/ 2019-00237) and for an “Atracción de Talento Investigador” contract (2017-T2/AMB-5037

Branch-Selective, Iridium-Catalyzed Hydroarylation of Monosubstituted Alkenes via a Cooperative Destabilization Strategy

Highly branch-selective, carbonyl-directed hydro­aryl­ations of mono­substituted alkenes are described. The chemistry relies upon a cationic Ir­(I) catalyst modified with an electron deficient, wide bite angle bis­phosphine ligand. This work provides a regio­isomeric alternative to the Murai hydro­aryl­ation protocol

Branch-Selective, Iridium-Catalyzed Hydroarylation of Monosubstituted Alkenes via a Cooperative Destabilization Strategy

Highly branch-selective, carbonyl-directed hydro­aryl­ations of mono­substituted alkenes are described. The chemistry relies upon a cationic Ir­(I) catalyst modified with an electron deficient, wide bite angle bis­phosphine ligand. This work provides a regio­isomeric alternative to the Murai hydro­aryl­ation protocol