Scheme 2. Transition metal-catalyzed synthesis of symmetric diaryl ketones. a) Aerobic oxidation of terminal alkenes; b) decarbonylative homocoupling; c, d, e) self-carbonylation strategies; f) a denitrogenative self-carbonylation strategy (This work ).
The transition metal-catalyzed carbonylative self-coupling reactions with carbon monoxide (source of carbonyl) and aryl halides (arylating agents) are able to synthesize symmetrical biaryl ketones, as other demonstrative paths have also been reported in the literature.[13] More recently, Arndtsen developed an efficient strategy for palladium-catalyzed oxidative carbonylation of arenes using CO gas to synthesize biaryl ketones via C-H bond activation (Scheme 1d).[14] On the other hand, the carbonylative self-coupling of organometallic nucleophiles (Ar-M) into symmetrical biaryl ketones is accomplished through transition metal catalysts under gaseous CO with the moisture-sensitive ligands (Scheme 1e).[15]
Arylhydrazines have recently gained attention as prospective coupling partners because of their C-N bond stability, high dissociation energy, ease of availability, and high reactivity. In recent decades, arylhydrazines have developed as an efficient arylating partner in a variety of C-C and C-X bond formation reactions.[16] In 1857, Emil Fisher first synthesized phenyl hydrazine for the characterization of sugar molecules through a hydrazone synthesis. The various numerous catalytic methods are also accessible for the preparation of these compounds, including the synthesis of various arylhydrazine derivatives through the diazotization of anilines and they could also be synthesized from aryl halides and hydrazine using transition metal (Pd, Cu) catalysts.[17] Because of the environmental-friendly arylating source for the Pd-catalyzed coupling reactions with the releasing N2 and H2O as a by-product.
Our research group has been interested in emerging new carbonylation reactions via C-N bond activation for the last few years.[18] Recently, we reported Palladium-catalyzed oxidative alkoxycarbonylation and aminocarbonylation of aryl hydrazine, aryl halides, and aryl boronic acids with alcohols and tertiary amines to synthesize the esters and tertiary amide derivatives via sp2 and sp3 C-N bond activation.[19] On the basis of these works, we focused on emerging an applicable and cost-efficient method for Pd-catalyzed denitrogenative self-carbonylation of arylhydrazines to afford the symmetrical biaryl ketones through C-N bond cleavage. Herein, we demonstrate the first-time Palladium-catalyzed oxidative carbonylation of aryl hydrazine hydrochlorides to form the series of symmetric biaryl ketones in moderate to good yields via C(sp2)-N bond activation under CO/O2 with releasing N2and H2O as a by-product.
Results and Discussion
We begin our study for the carbonylative self-coupling using phenylhydrazine hydrochloride 1a as a model substrate to examine the effects of various optimized reaction conditions shown in Table 1.
Table 1. Optimization of Reaction conditions for carbonylative self-coupling of phenyl hydrazinea