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