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).

Recent Progress in Metal-Catalyzed [2+2+2] Cycloaddition Reactions

“…In 2020, Jacobi von Wangelin, Chakraborty, and coworkers reported a photo-organo-iron cyclotrimerization of alkynes demonstrating the first dual catalytic protocol combining organic photoredox catalysis and iron catalysis for synthetic applications (Scheme 5). 65 A photostable organic dye 9,10-diphenylanthracene (DPA) allowed the photocatalytic reduction of FeCl 2 under visible light irradiation. The reaction proceeded under mild conditions (visible light, 22 °C, 1 h) in 48-99% yield with regiocontrol up to >99:1 to produce a broad array of trisubstituted arenes when using 1-5 mol% of the three inexpensive catalysts (dye, amine, and FeCl 2 ) for this efficient transformation.…”

H-BPin/KOtBu Promoted Activation of Cobalt Salt to a Heterotopic Catalyst for Highly Selective Cyclotrimerization of Alkynes

“…A dual organophotoredox/FeCl 2 catalytic system for cyclotrimerization of alkynes was further developed, wherein the active species was found to be Fe nanoparticles in nature and alkylacetylenes exhibited very good reactivities, albeit with low regiocontrol (Scheme 1b-iv). 13 In 2018, Miura, Shishido and co-workers reported a catalytic [2 + 2 + 2] cycloaddition of substituted alkynes using Pd−Au alloy (Scheme 1b-v). 14 However, this catalytic system gave poor regioselectivities in the intermolecular cyclotrimerization of terminal alkynes.…”

“…Thereafter, the same group reported a Mn-cluster catalyzed cyclotrimerization of phenylacetylene with 1:1 regioselectivity (Scheme b-iii). A dual organophotoredox/FeCl 2 catalytic system for cyclotrimerization of alkynes was further developed, wherein the active species was found to be Fe nanoparticles in nature and alkylacetylenes exhibited very good reactivities, albeit with low regiocontrol (Scheme b-iv) . In 2018, Miura, Shishido and co-workers reported a catalytic [2 + 2 + 2] cycloaddition of substituted alkynes using Pd–Au alloy (Scheme b-v) .…”

Organic-photoredox-catalyzed three-component sulfonylative pyridylation of styrenes

“…A selective radical–radical coupling of 45-II with pyridine could form intermediate 45-III, that was unstable in this photoinduced system and prone to undergo oxidative aromatization to furnish the final product 45 ( Scheme 3a ). Additionally, Stern–Volmer fluorescence quenching experiments showed that the excited state of DPA I ( E 1/2 = −1.77 V versus SCE in acetonitrile) 13 was only quenched by cyanopyridine 2 ( E 1/2 = −1.75 V versus SCE in acetonitrile) 14 , other than sodium sulfonate or alkene ( Scheme 3b ).…”