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

Photosensitized oxidative addition to gold(i) enables alkynylative cyclization of o-alkylnylphenols with iodoalkynes

International audienceThe well-established oxidative addition-reductive elimination pathway is the most followed one in transition metal catalyzed cross coupling reactions. While readily occurring with a series of transition metals, it does not take place with gold(I) complexes which have shown some reluctance to undergo oxidative addition unless special sets of ligands on gold(I), reagents or reaction conditions are used. A new possibility to overcome this hurdle has been devised. Upon visible light irradiation, an iridium photocatalyst triggers via triplet sensitization the oxidative addition of an al-kynyliodide onto a vinylgold(I) intermediate to deliver Csp 2-Csp coupling products after reductive elimination. Mechanistic and modeling studies support that an energy transfer takes place and not a redox pathway. This novel mode of activation in gold homogenous catalysis was applied in several dual catalytic processes. Alkynylbenzofuran derivatives were obtained from o-alkynylphenols and iodoalkynes in the presence of catalytic gold(I) and iridium(III) complexes under blue LED irradiation. Over the last two decades, homogeneous gold catalysis has been extensively used to efficiently and selectively promote a variety of cyclization processes. 1-3 The typical casting involves bifunctional substrates bearing an unsaturation prompt to electrophilic activation and a judiciously positioned internal nucleophile. A protodemetalation of the organogold intermediates to afford hydrofunctional-ized products generally terminates the catalytic cycles. 4 Pursuing the step economy principle and also aiming at higher level of molecular complexity, some in situ post-functionalization reactions of the organogold 5 intermediate have been devised such as electrophilic halogenation or cross-coupling reactions. Although palladium catalyzed cross coupling from an organogold(I) intermediate has bee