Synthetic Application of Bismuth LMCT Photocatalysis in Radical Coupling Reactions

Abstract

Ligand-to-metal charge transfer (LMCT) photocatalysis allows the activation and synthetic utilization of halides and other heteroatoms in metal complexes. Many metals are known to undergo LMCT but so far remain underutilized in the field of catalysis. A screening assay identifying LMCT activity helped us to expand this catalysis concept to the application of bismuth LMCT in organic radical coupling reactions. We demonstrate its application for the generation of two different radicals (chlorine and carboxyl) in net-oxidative as well as redox-neutral photochemical reactions. Detailed investigation of the model Giese-type coupling revealed BiCl4 – and BiCl5 2– as catalytically active bismuth species under 385 nm irradiation. Combined cyclic voltammetry and UV–vis studies gave insight into the reactivity of the highly reactive bismuth(II) catalyst fragment.

Chlorine-radical-mediated C–H oxygenation reaction under light irradiation

“…In recent years, several metal-catalysed reactions other than oxygenation that are initiated by HAT from C-H bonds to chlorine radicals have been reported. [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] 2.1.1 Iron(III) chloride. Imoto et al first reported the photocatalysis of FeCl 3 in the oxidation of ethylene glycol derivatives [44][45][46] and benzylic chlorination of toluene; the effect of water on this reaction was subsequently reported.…”

“…In recent years, several metal-catalysed reactions other than oxygenation that are initiated by HAT from C–H bonds to chlorine radicals have been reported. 29–43…”

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

“…Moreover, various electron-deficient alkenes and di- tert -butyl azodicarboxylate were demonstrated to be capable radical acceptors for this catalytic manifold. Analogous LMCT strategies employing other homoleptic metal–chloride complexes, including iron, 97–100 titanium, 101 and bismuth, 102 have also been recently reported (Scheme 3B).…”

Bismuth(III)-Catalyzed Oxidative Cross-Coupling of 3-Hydroxycarbazoles with Arenols under an Oxygen Atmosphere

“…20 The couplings of arylboronic acids were reported using organobismuth(III) species, and Bi(III)/ Bi(V) redox cycles were proposed for these reactions. 21−23 Recently, an aminooxygenation reaction 24 and a photoreaction 25 were developed using a BiCl 3 catalyst, which is an inorganic bismuth(III) compound. Nevertheless, further development of catalytic reactions using nontoxic Bi compounds is essential.…”

“…Triphenylbismuth, an organobismuth­(III) compound, behaves as a redox catalyst in the presence of a terminal oxidant in the oxidative cleavage of 1,2-diols and in the oxidation of hydrazones . The couplings of arylboronic acids were reported using organobismuth­(III) species, and Bi­(III)/Bi­(V) redox cycles were proposed for these reactions. − Recently, an aminooxygenation reaction and a photoreaction were developed using a BiCl 3 catalyst, which is an inorganic bismuth­(III) compound. Nevertheless, further development of catalytic reactions using nontoxic Bi compounds is essential.…”