Combined Photoredox and Iron Catalysis for the Cyclotrimerization of Alkynes

Abstract

Successful combinations of visible-light photocatalysis with metal catalysis have recently enabled the development of hitherto unknown chemical reactions. Dual mechanisms from merging metal-free photocatalysts and earth-abundant metal catalysts are still in their infancy. We report a photoorgano-iron-catalyzed cyclotrimerization of alkynes by photoredox activation of a ligand-free Fe catalyst. The reaction operates under very mild conditions (visible light, 20 8C, 1 h) with 1-2 mol % loading of the three catalysts (dye, amine, FeCl 2). The merging of visible-light-driven photocatalysis with transition-metal catalysis constitutes a highly versatile approach to sophisticated organic transformations. Such approaches benefit from the bond-activation and bondformation events in the coordination sphere of the metal catalyst and the distinct reactivity patterns of photoactivated species and open-shell intermediates. [1, 2] Although several protocols involve the direct irradiation of metal complexes by UV/Vis light, the spatial separation of photocatalyst and metal-catalyst centers may enable more diverse mechanistic scenarios and facilitate reaction optimization and selectivity control. The general modes of photoactivation of metal complexes involve ligand dissociation, M À X homolysis (mostly by UV irradiation), [3-5] excitation of metal-to-ligand or ligand-to-metal charge-transfer bands (MLCT, LMCT), [5-9] and single-electron transfer (SET) reactions in the presence of suitable redox partners (Scheme 1, top). [1, 2] The successful merging of visible-light excitation and one-electron redox processes with conventional organometallic reaction mechanisms has very recently enabled the development of hitherto unknown chemical transformations. [1, 2] This rapidly emerging field of metalla-photoredox catalysis has so far mostly been realized with pyridine complexes of Ru and Ir as photoredox catalysts and late-transition-metal complexes as chemical cocatalysts (Co, Ni, Pd, Cu, Au with phosphine, bipyridine, Nheterocyclic carbene, or amine ligands, Scheme 1). [1, 10-13] The utilization of earth-abundant 3d metals as visible-light photocatalysts has gained great interest, but is generally hampered by exceptionally short lifetimes owing to low-lying metalcentered electronic states. [6, 7, 14] We wished to challenge this Scheme 1. General strategies for photo-and chemical activation of metal precatalysts (top), and dual photo-organo-iron catalysis described in this work (bottom).

Emerging concepts in photocatalytic organic synthesis

“…Key to the success is the modulation of the oxidation state of transition metal complexes by SET processes, radical additions, or photosensitization. This strategy is extensively studied using nickel complexes ( Milligan et al., 2019 ; Twilton et al., 2017 ) and was further expanded to a range of other transition metals ( Hopkinson et al., 2014 ; Skubi et al., 2016 ), including abundant first row metals such as cobalt ( Kojima and Matsunaga, 2020 ), copper ( Hossain et al., 2019 ), and iron ( Neumeier et al., 2020 ; Ouyang et al., 2019 ). Combining photo- and organocatalysis involves SET of an excited PC and, for example, enamine intermediates to enable the α-functionalization of aldehydes with high enantioselectivity ( Nicewicz and MacMillan, 2008 ).…”

Recent Advances in Asymmetric Iron Catalysis

“…In this scenario, many other challenges need to be addressed, particularly with the aim to design new chiral ligands to expand the portfolio of asymmetric iron-catalyzed C‒C bond forming reactions as in example (reductive) cross-coupling [ 91 ] or C‒H functionalization reactions. Furthermore, a deeper understanding of reaction mechanism and particularly of the role played by additives and ligands, will be crucial for the application of stereoselective iron-catalyzed transformations in continuous-flow processes [ 92 ] or the development of photoredox iron-catalyzed transformations [ 93 ].…”