Metal-Free Trifluoromethylthiolation of Arylazo Sulfones

Abstract

A visible-light-driven protocol for the synthesis of aryl trifluoromethyl thioethers under photocatalyst- and metal-free conditions has been pursued. The procedure exploits the peculiar properties of arylazo sulfones (having electron-rich or electron-poor substituents on the (hetero)aromatic ring) as photochemical precursors of aryl radicals and S -trifluoromethyl arylsulfonothioates as easy-to-handle trifluoromethylthiolating agents.

Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

“…The peculiar arylazosulfones 137 - 1 which imparted both color and photoreactivity to molecules were found to be suitable substrates for arylation reactions under different photocatalytic conditions (Scheme A). Indeed, the CH 3 SO 2 N 2 moiety is dubbed as a dyed auxiliary group that allows the functionalized arylazosulfones to undergo, upon visible-light or UV-light irradiation, homolytic cleavage of the N–S bond and the loss of N 2 to deliver aryl radicals (Ar•) 137 - 4 or triplet aryl cations ( 3 Ar + ) 137 - 5 . − As such, photoactivated arylazosulfones 137 - 1 have been applied to a plethora of arylation protocols by Protti, Fagnoni, and others, including metal-free photoarylation of (hetero)­arenes (Scheme B, 1), radical arylation of isonitriles (Scheme B, 2), gold-promoted Suzuki synthesis of (hetero)­biaryls (Scheme B, 3), Mizoroki–Heck-type coupling with 1,1-diarylethylenes (Scheme B, 4), visible-light-driven synthesis of arylstannanes with hexamethylditin (Scheme B, 5), direct borylation with bis­(pinacolato)­diboron (B 2 pin 2 ) (Scheme B, 6), Arbuzov-like reaction with triorganophosphites (Scheme B, 7), trifluoromethylthiolation of S -trifluoromethyl arylsulfonothioates (Scheme B, 8), oxidative coupling with thiols under air (Scheme B, 9), thiolation with disulfides (Scheme B, 10), deutero deamination (Scheme B, 11), and electrochemical synthesis of unsymmetrical thioethers/selenides (Scheme B, 12) . Obviously, such bench-stable arylazo sulfones derived from anilines are great substitutes for aromatic diazonium salts.…”

“…Classic transformations of N -sulfonyl-1,2,3-triazole normally proceed via an active nitrogen anion and carbene intermediate under heating or with the aid of Lewis acid catalyst. Nearly no radical reaction from N -sulfonyl-1,2,3-triazole was reported because of the high energy barrier of the N–S bond homolytic cleavage (up to 34.6 kcal/mol) . Very recently, Liu, Xia, and co-workers found the first oxidative C­(sp 3 )–H amination of tetrahydrofuran/tetrahydrothiophene with N -sulfonyl-1,2,3-triazole through a radical desulfonylative pathway facilitated by air (Scheme A) .…”

Synthesis, Reactivity and Activation Modes of Fluoroalkyl Thiosulfonates and Selenosulfonates

“…In 2021, the group of Protti and Zhao employed arylazo sulfones as aryl radical sources under simple blue light irradiation to achieve the trifluoromethylthiolation or arenes (Scheme 9E). [32] One more step is mandatory to convert the aryldiazonium salts into arylazo sulfones but it however circumvents the use of a photocatalyst. Nonetheless, it should be pointed out that the yields are usually moderate or even low when the substrate is not a para-substituted benzene derivative.…”

Dyedauxiliary Group Strategy for the α-Functionalization of Ketones and Esters

“…Thus, different intermediates (aryl diazenyl, aryl radicals, sulfonyl radicals, and aryl cations) can be generated selectively, by tuning the reaction conditions (light sources, reaction media, coupling partner). Such behavior was exploited in the optimization of synthetic protocols for aryl-carbon − and aryl-heteroatom bond formation − as well as aryldiazenylation and sulfonylation of alkenes. , …”