Visible Light-Induced Coupling Cyclization Reaction of α-Diazosulfonium Triflates with α-Oxocarboxylic Acids or Alkynes

Abstract

A photocatalyst-free radical cleavage of α-diazo sulfonium salts has been developed for the first time. The reaction provides an efficient method for the generation of diazomethyl radicals from α-diazosulfonium triflates under photochemical conditions. Utilizing the in situ generated diazomethyl radicals as key intermediate, the coupling cyclization reaction of α-diazosulfonium triflates with α-oxocarboxylic acids or alkynes has been achieved. The method affords a diverse set of important 2,5-disubstituted 1,3,4-oxadiazoles and 3,5-disubstituted-1H-pyrazoles with excellent regioselectivity in a single step. A reaction mechanism involving a radical pathway was further supported by control experiments and DFT calculations.

Visible light-mediated photolysis of organic molecules: the case study of diazo compounds

“…Finally, an intramolecular cyclization event affords 1,3,4-oxadiazoles 124 (Scheme 23b). 106 In the case of pyrazoles 126 , the diazo compound-derived radical 128 is proposed to react with the terminal alkyne 125 to afford intermediate 132 , followed by an intramolecular cyclization and a putative 1,2-spin-center-shift (SCS) 107 to produce radical intermediate 133 . Then, hydrogen abstraction from the solvent and tautomerization leads to pyrazoles 126 (Scheme 23b).…”

“…Then, hydrogen abstraction from the solvent and tautomerization leads to pyrazoles 126 (Scheme 23b). 106…”

“…105 Yuan, Zhao and co-workers reported the S-C homolytic bond cleavage of a-diazosulfonium triflates 122 in the presence of aoxocarboxylic acids 123 or alkynes 125 to afford 1,3,4-oxadiazoles 124 or pyrazoles 126, respectively (Scheme 23a). 106 The reaction mechanisms of these transformations are proposed to start from This journal is © The Royal Society of Chemistry 2023 the blue light-mediated homolytic cleavage of diazo compound 122 to afford radical cation 127 and the diazo compound-derived radical 128. In the case of 1,3,4-oxadiazoles 124, the radical cation 127 is believed to react with the sodium salt of the a-keto acid 123 via a single electron transfer (SET) leading to a decarboxylation event that affords radical intermediate 130.…”

“…Then, hydrogen abstraction from the solvent and tautomerization leads to pyrazoles 126 (Scheme 23b). 106 Finally, it is worth to mention that some additional synthetic strategies have described the blue light irradiation of diazo compounds in the presence of other reacting partners that apparently seem to react faster under blue light irradiation, thus generating radical intermediates that add onto the diazo compounds, rather than having these diazo compounds undergoing photolysis to generate the expected free carbenes. This is the case for the synthesis of 1,2,4-triazoles reported by Koenigs, Xuan and co-workers; 108 and the synthesis of functionalized allenes, reported by Zhou and co-worker 109 (not shown).…”

Electron Donor–Acceptor Complex Enabled Radical Cyclization of α-Diazodifluoroethyl Sulfonium Salt with Unactivated Alkynes

“…In particular, the groups of Alcarazo and Wang have disclosed the synthesis of α-diazo sulfonium salts and demonstrated their unique radical reactivity under photoredox catalysis conditions . Very recently, Zhao, Yuan and co-workers reported the radical [3 + 2] cycloaddition reaction of α-diazosulfonium triflates with alkynes for the synthesis of 3,5-disubstituted-1 H -pyrazoles under blue light conditions, but the dipolarophile scope is restricted to aromatic alkynes (Scheme b) . Despite these advances, the utilization of electron-deficient α-diazo sulfonium salts as potential partners in forming electron donor–acceptor (EDA) complexes has not been reported .…”