Photoredox Reaction of 2-Mercaptothiazolinium Salts with Silyl Enol Ethers

Abstract

A method for the generation of free radicals from thiazolinium salts upon photocatalytic reduction is described. The thiazolinium salts are generated by treatment with methyl triflate of 2-mercaptothiazolines, which can be readily obtained from alkyl bromides and tosylates via a nucleophilic substitution reaction or by hydrothiolation of alkenes. Silyl enol ethers were used to trap the radicals, furnishing ketones after successive single-electron oxidation and elimination of the silyl cation.

Boron, silicon, nitrogen and sulfur-based contemporary precursors for the generation of alkyl radicals by single electron transfer and their synthetic utilization

“…In 2019, Dilman introduced the use of 2-mercaptothiazolinium salts as a new source of alkyl radicals. 150 In contrast to the previous examples of sulphur based radical precursors, these electron-deficient salts exhibit low reductive potential ( E ox 1/2 = −0.82 V vs. SCE for R = CH 2 SiMe 3 ) suitable for oxidative quenching. Methyl thiazolinium salts 232 could be prepared by direct methylation of the corresponding thiazolines with methyl triflate (Scheme 88, path a) or by reacting potassium 2-mercaptothiazoline with alkyl bromides followed by addition of methyl triflate (Scheme 88, path b).…”

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

“…183 Alkylated ketones 22−3a−d were likewise obtained by the Ir III -photocatalyzed reaction between a 2-mercaptothiazolinium salt (22−1, as alkyl radical precursor) and silyl enol ethers 22−2a−d (Scheme 22). 106 Lauryl peroxide (LPO, see Scheme 18) was adopted for the Ru-catalyzed three-component carbofluorination of styrenes as illustrated in Scheme 23a. The vinylic double bond of compound 23−1 derived from estrone was functionalized twice by using triethylamine trihydrofluoride Et 3 N•HF as the fluoride anion source to deliver the desired alkyl-fluorinated olefin 23−2 in 61% yield.…”

“…In this figure, the radical precursors were collected depending on the C­(sp 3 )-Y bond cleaved during the radical release. As apparent, the photochemically triggered cleavage of several C-heteroatom bonds like C–X, ,− C–O, − C–B, − C–S, − or C–N (Figure ) affords carbon-centered radicals. The alkyl radical generation is granted by the very versatile photochemical tool.…”

Photocatalytic C(sp3) radical generationviaC–H, C–C, and C–X bond cleavage

“…170 With the same Ir-photocatalyst for SET reduction, Dilman et al exploited the ketone synthesis using dihydrothiazolinium salts as Rc precursors, which could be simply prepared from alkyl bromides (Scheme 41B). 171 Upon being reduced by the photoexcited Ir(III), dihydrothiazoline radical fragmented into an Rc along with a thione byproduct. Silyl enol ether trapped the Rc, followed by oxidation and silyl group removal to give the ketone product.…”