Polysulfide Anions as Visible Light Photoredox Catalysts for Aryl Cross-Couplings

Abstract

Herein, we disclose the use of polysulfide anions as visible light photoredox catalysts for aryl cross-coupling reactions. The reaction design enables single-electron-reduction of aryl halides upon photo-excitation of tetrasulfide dianions (S<sub>4</sub><sup>2–</sup>). The resulting aryl radicals are engaged in (hetero)biaryl cross-coupling, borylation, and hydrogenation in a redox catalytic regime involving S<sub>4</sub><sup>•–</sup>/S<sub>4</sub><sup>2–</sup> and S<sub>3</sub><sup>•–</sup>/S<sub>3</sub><sup>2–</sup> redox couples.

Visible Light-Induced Sulphide Anion-Catalysed Decarboxylative Alkylations in Batch and Continuous Flow

“…Satisfyingly, this catalytic system featured broad substrate scope and good functional group compatibility under the optimized reaction conditions (Table 1). Isocyanides substituted with electron-neutral (3, 4) and electron-donating (5) and electron-groups (6) all gave good to excellent yields. A variety of primary, secondary and even tertiary RAEs were all amenable substrates to deliver the corresponding decarboxylative alkylation products in moderate to excellent yields (51% -98%).…”

“…Almost every atom (except inert elements) can bond with sulphur atoms, and its valence state can vary from -2 to 6 [5] . The excellent electron-transfer ability and radical properties of sulphur-containing backbones make them ideal candidates for catalysis [6] , particularly in the photoredox pathway [7] . To date, numerous sulphur-containing photosensitizers have been synthesized and employed in visible-light-induced transformations, such as disodium benzophenondisulphonate (BPSS) [8] , thiazine dyes [9] , thiophene derivatives [10] and N-phenylphenothiazine (PTH) [11] .…”

Light-Induced Single-Electron Transfer Processes involving Sulfur Anions as Catalysts

“…49−54 Despite their wellexplored photophysical properties, 49,50,53 their use as photocatalysts for chemical reactions has been reported only recently by Chiba and co-workers. 55,56 They showed that polysulfide anions possess a high reduction power after visible-light excitation, capable of single-electron reduction of aryl halides to generate synthetically valuable aryl radicals. The resulting aryl radicals were exploited in aryl−aryl cross-coupling, borylation, and hydrogenation reactions.…”

Leveraging of Sulfur Anions in Photoinduced Molecular Transformations

“…Chiba recently disclosed that these polysulfide anions could serve as visiblelight photoredox catalysts to promote a series of aryl crosscoupling reactions. 44 For example, a heterobiaryl crosscoupling reaction between 4′-bromoacetophenone (10) and N-methylpyrrole (80) was promoted by a catalytic amount of K 2 S x in DMSO under irradiation with visible light (440 nm), affording biaryl 81 in good yield within 1.5 h (Scheme 11). The process is proposed to be initiated by SET reduction of aryl bromide 10 by photoexcited S 4 2− to form the radical anion of aryl bromide 82, which subsequently undergoes C−Br bond mesolysis to form aryl radical 83.…”