Substrate-Controlled Transformation of Azobenzenes to Indazoles and Indoles via Rh(III)-Catalysis

Abstract

Rh(III)-catalyzed substrate-controlled transformation of azobenzenes to indazoles and 2-acyl (NH) indoles is achieved via C-H functionalization. Generally, good functional groups tolerance, satisfying yields, and excellent regio-selectivity are achieved in this reaction. Mechanistically, the reaction with acrylates undergoes β-hydride elimination, while the reaction with vinyl ketones or acrylamides undergoes nucleophilic addition. Copper acetate was supposed to play different roles in the β-hydride elimination to furnish indazoles and nucleophilic addition of C-Rh bond to deliver 2-acyl (NH) indoles.

Recent Advances in Indazole-Containing Derivatives: Synthesis and Biological Perspectives

“…Xi et al [ 38 ] reported rhodium (III)-catalyzed C-H functionalization of azobenzenes 64 with acrylates 65 to furnish the corresponding 2 H -indazoles 66 in the presence of Cu(OAc) 2 with good functional group tolerance and satisfying yields ( Scheme 19 ). Notably, copper acetate played an important role for β-hydride elimination during the course of transformation.…”

One‐pot Synthesis of Oxindoles through C−H Alkylation and Intramolecular Cyclization of Azobenzenes with Internal Olefins

“…Interestingly, during further optimization of the coupling between azobenzenes and internal olefins, we found that applying of Cu(OAc) 2 additive instead of AcOH directly provided the unexpected 2,3‐disubstituted indole 7 a in 60% yield by the reaction of azobenzene 1 o and n ‐butyl maleate ( 5 b ) under cationic Rh(III) catalysis (Scheme ). This reaction might proceed that the C−Rh bond on intermediate presumably takes place nucleophilic addition toward N=N bond of azobenzene, which can undergo N−N bond cleavage and subsequent aromatization under oxidative conditions to give free‐(NH)‐indole product 7 a …”

Transition‐Metal‐Catalyzed Syntheses of Indazoles

“…Substrate controlled, an Rh(III)-catalyzed reaction of azobenzene in the presence of Cu(OAc) 2 to 2H-indazole via CÀ H bond activation method was developed by Xi and co-workers in 2017 (Scheme 39). [43] From the screening of a series of Rh, Ru and Pdbased catalysts, [Cp*RhCl 2 ] 2 (5 mol%) was identified as the optimum catalyst along with Cu(OAc) 2 (2 equiv.). Copper acetate was found to be more effective than the other copper and silver salts tried.…”