Notizen: Photochemische Verdrängung eines an Eisen(0) koordinierten Aromaten durch ein Dien / Photochemical Replacement of an Aromatic Ligand by a Diene in an Iron(0) Complex

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).

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Combined Photoredox and Iron Catalysis for the Cyclotrimerization of Alkynes

Neumeier

¹

,

Chakraborty

²

,

Schaarschmidt

³

et al. 2020

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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).

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