Pyrazole-Mediated C–H Functionalization of Arene and Heteroarenes for Aryl–(Hetero)aryl Cross-Coupling Reactions

Abstract

Herein we introduce a transition-metal-free protocol that involves a commercially available, inexpensive pyrazole molecule to conduct C–C cross-coupling reactions at room temperature via a radical pathway. Using this method, an aryldiazonium salt has been coupled to a wide range of arenes and heteroarenes including benzene, mesitylene, thiophene, furan, benzoxazole to result the corresponding biaryl products. The full reaction mechanism is elucidated along with the crystallographic probation of an active initiator species. A potassium-stabilized deprotonated pyrazole steers single-electron transfer to the substrate and behaves as an initiator for the reaction.

Aromatization as the driving force for single electron transfer towards C–C cross-coupling reactions

“…26,27 Some previous reports including our recent work showcased the potential of aromatic heterocycles in their deprotonated form. 28 We reasoned that the deprotonated form of 9,10dihydrophenazine (L1, Scheme 1) may be suitable for promoting electron transfer to a substrate molecule. 28 The deprotonation of L1 is fully feasible under basic conditions as the measured pKa is 5.5 in DMSO medium.…”

“…28 We reasoned that the deprotonated form of 9,10dihydrophenazine (L1, Scheme 1) may be suitable for promoting electron transfer to a substrate molecule. 28 The deprotonation of L1 is fully feasible under basic conditions as the measured pKa is 5.5 in DMSO medium. We further anticipated that L1 can be converted to its fully aromatic form, phenazine, upon undergoing the 2e − /2H + process, so there could be a thermodynamic driving force behind such electron transfer processes.…”

Multielectron Redox Afforded by a Pincer Ligand Promoting Kumada Cross‐Coupling Reactions

“…The discussed SET from the redox‐active o ‐phenylenediamido backbone is an analogous event. Furthermore, we have also observed that electron rich anion of 3,5 dimethyl pyrazole can facilitate SET process to aryldiazonium chloride to break the bond homolytically resulting the generation of aryl radical [26] . However, the reduction potential of aryldiazonium salts is much positive to a moderately strong Ar−Br bond.…”

Selective Radical Transformations with Aryldiazonium Salts

“…In 2021, Adhikari's research group [58] reported the use of a new SED in synthesis of biaryl compounds from aryldiazonium salts 1 and (hetero)‐arene compounds 20 (Scheme 24b). The organic molecule used as additive was 3,5‐dimethylpyrazole 64 in conjunction with KO t Bu.…”