Visible- and UV-Light-Induced Decarboxylative Radical Reactions of Benzoic Acids Using Organic Photoredox Catalysts

Abstract

Photoinduced decarboxylative radical reactions of benzoic acids with electron-deficient alkenes, diborane, and acetonitrile under organic photoredox catalysis conditions and mild heating afforded adducts, arylboronate esters, and the reduction product, respectively. The reaction is thought to involve single-electron transfer promoted the generation of aryl radicals via decarboxylation. A diverse range of benzoic acids were found to be suitable substrates for this photoreaction. Only our two-molecule organic photoredox system can work well for the direct photoinduced decarboxylation of benzoic acids.

Decarboxylation‐Initiated Intermolecular Carbon‐Heteroatom Bond Formation

“…Yoshimi and co‐workers, recently reported that a photoredox system based on biphenyl (BP)/1,4‐dicyanonaphthalene (DCN) promotes decarboxylative borylation of benzoic acids with bis(pinacolato)diboron (B 2 pin 2 ) under ultraviolet‐light irradiation (Scheme 96). [383] Electron‐rich, ‐neutral, as well as ‐poor phenylboronate esters were synthesized, although only in moderate yields. The authors proposed that under ultraviolet‐irradiation a SET process between excited‐state DCN* and BP initially occurs to deliver the anion radical [DCN] .− and the radical cation [BP] .+ .…”

Single Electron Activation of Aryl Carboxylic Acids

“…Inspired by visible-light-induced decarboxylation processes, the group of Yoshimi recently extended this concept to the intermolecular decarboxylative radical addition reaction between electron-poor alkenes and aryl carboxylic acids under mild conditions (30 C) (Kubosaki et al, 2020) (Scheme 29). As a result, a wide variety of electron-poor alkenes, such as acrylonitrile, acrylamide, acrylate, and phenyl vinyl sulfone, coupled smoothly with benzoic acids upon visible light or UV irradiation.…”

Decarboxylative Hydroxylation of Benzoic Acids

“…However, even with the low barrier for radical aromatic decarboxylation, other reactions such as hydrogen atom abstraction (HAT) and back electron transfer (BET) can be even faster [2] and result in undesired reactivity. Radical decarboxylation of benzoic acids has been successfully used to react with reactive radical acceptors such as (hetero)arenes, [21] acrylates, or diboron species, [22] but not to make C−O bonds, even with prior activation to activated esters [23]…”

“…Both electron‐poor and electron‐rich substrates, such as 4‐cyanobenzoate ( 3 ) and 3,5‐dimethylbenzoate ( 13 ) can outcompete TC to afford the corresponding phenols (Table 1). Electron neutral benzoates, which are often problematic for thermal decarboxylation due to the lack of electronic bias, [16] or electron deficient benzoates, which are often problematic for oxidative radical decarboxylation due to their high oxidation potential, [21b, 22] performed well. Heteroaryl carboxylates, such as isonicotinic carboxylates ( 6 , 14 ) and quinoxaline‐2‐carboxylate ( 15 ), are also compatible.…”