Visible-Light-Promoted Hydrogenation of Azobenzenes to Hydrazobenzenes with Thioacetic Acid as the Reductant

Abstract

A catalyst- and metal-free hydrogenation of azobenzenes to hydrazobenzenes in the presence of thioacetic acid was achieved under visible light irradiation. The transformation was carried out under mild conditions in an air atmosphere at ambient temperature, generating a variety of hydrazobenzenes with yields up to 99%. The current process is compatible with a variety of substituents and is highly chemoselective for azo reduction when other unsaturated functionalities (carbonyl, alkenyl, alkynyl, etc.) are contained. Preliminary mechanistic study indicated that the transformation could be a radical process.

Recent Advances in Radical Reactions of Azo Compounds

“…A variety of functional groups could be tolerated under the mild conditions. Compared with their photocatalytic reaction, [23][24] this protocol provided a more straightforward and sustainable procedure for the reduction of azobenzenes, albeit in slightly lower yield.…”

“…Recently, the radical strategy based on photoredox and electrochemical catalysis has been successfully applied in the hydrogenation of azobenzenes. [16][17][18][19][20][21][22][23][24][25][26] The earliest example was disclosed by Shibata's group under the irradiation of a 300 W high-pressure Hg arc lamp in the presence of TiO 2 powder. [16] This pioneering work spurred numerous investigations into radical reduction of azobenzenes in the following years.…”

“…Recently, the radical strategy based on photoredox and electrochemical catalysis has been successfully applied in the hydrogenation of azobenzenes [16–26] . The earliest example was disclosed by Shibata's group under the irradiation of a 300 W high‐pressure Hg arc lamp in the presence of TiO 2 powder [16] .…”

“…Recently, Xia and co‐workers reported a similar reduction of azobenzenes 3 to hydrazobenzenes 4 using thioacetic acid as an effective reductant (Scheme 8). [24] The reaction proceeds in absence of transition metal catalyst. A variety of functional groups could be tolerated in the reaction, such as ester, trifluoromethyl, cyano, carbonyl, alkenyl and alkynyl.…”

Condition-Controlled O-Acylation and N–O Bond Reduction of Hydroximic Acids with Thioacetic Acid

“…In recent years, thioacetic acid has been used as a mild acylating reagent for peptides, amines, sulfoximines, and phenols . Furthermore, thioacetic acid could serve as a reducing reagent for the deoxygenation of sulfoxides and the reduction of azides and azobenzenes . On the basis of our previous research interests in the novel transformations of amide scaffolds, we postulated that the hydroxamic acid could undergo O -acylation with thioacetic acid to deliver the N -acyloxy amide, which has a lower N–O BDE.…”