W. Lu, et al.
Catalysis Communications 132 (2019) 105806
Fig. 3. Proposed mechanism for Cu O-NPs@rGO catalyzed oxidative coupling synthesis of α-ketoamides.
2
entries 5–7). With CH
2
Cl
2
, THF as the solvents, the reaction gave low
of the TEM images of the recycled and fresh catalyst, no obvious change
in the morphology and structure of the catalyst was observed. It was
indicated that Cu O-NPs@rGO possesses excellent recyclability in the
2
yields of 4%, 57% respectively (Table S2, entries 8,9). The above results
indicated that EtOH was the optimized solvent. For easy handing, 2 mL
of EtOH was then applied as a solvent in further research.
synthesis of α-ketoamides.
Moreover, the different usages of NIS and Cu
investigated in the oxidative coupling reaction from acetophenone
1 mmol) and morpholine (2 mmol). The optimal amounts were 1 mmol
NIS and 2 mg Cu O-NPs@rGO (Table S3). Therefore, the optimized
reaction conditions were as follows: acetophenone (1 mmol), morpho-
line (2 mmol), NIS (1 mmol) and Cu O-NPs@rGO (2 mg) in 2 mL EtOH
at room temperature under O atmosphere.
2
O-NPs@rGO were
Based on the previous research results [12,13,35] and our studies, a
possible mechanism for the formation of α-ketoamides was proposed in
Fig. 3. With great electrical conductivity and huge specific surface area
(
2
2
of graphene, the electron transfer between Cu O nanoparticles and
graphene was more easily proceeding, which facilitated the transfer of
electron in the catalytic process. Firstly, acetophenone and piperidine
generated enamine I. Intermediate II was then formed by the iodination
2
2
In order to investigate the generality of the catalytic system, a series
of secondary amines were examined in the synthesis of the α-ketoa-
mides. Several aliphatic secondary amines such as dimethylamine and
diethylamine were reacted with acetophenone to afford corresponding
α-ketoamides with good yields of 68% and 79% respectively (Table 2,
entries 1,2). Several cyclic secondary amines such as piperidine, pyr-
rolidine and morpholine that reacted with acetophenone gave excellent
yields of 96%, 89% and 95% (Table 2, entries 3–5). It was noteworthy
that cyclic secondary amines were obtained better yields than aliphatic
2
of NIS catalyzed by Cu O-NPs@rGO. Piperidine took placed nucleo-
philic substitution with intermediate II to generate the α-piperidinyl
substituted iminium III. During the process, the carbon-iodine bond
cleavage was promoted by Cu
2
O-NPs@rGO. Meanwhile, superoxide
promoted by the value state change
%−
radical O
from Cu
2
was formed from O
2
O to CuO in the Cu O-NPs@rGO, which accelerated the pro-
2
2
cess of the iminium III to radical intermediate IV. Then, intramolecular
cyclization took place to form pivotal aminodioxetane intermediate V
accomplished with the single electron transfer process of CuO to Cu
2
O
secondary amines. Some functional groups (Bre, NO
2
e, CH
3
e and
in the Cu O-NPs@rGO. Through OeO bond heterolysis, ring opening
2
HOe) on acetophenone that reacted with piperidine were also in-
vestigated in the oxidative coupling synthesis of α-ketoamides. All of
these acetophenone derivatives afforded the desire α-ketoamides in
good yields of 81–91% (Table 2, entries 6–9). And the corresponding
TONs/TOFs value of different substrates were shown in the Table 2. It
could be seen that the oxidative coupling of acetophenone and piper-
idine gave relatively higher TONs/TOFs of 84.6/2.6 (Table 2, entries 3).
reaction took place to generate intermediate VI. Finally, α-ketoamide
2
was obtained by CeN bond cleavage. Cu O-NPs@rGO continued to
attend for the next catalysis cycle.
4. Conclusions
The above results demonstrated that the Cu
2
O-NPs@rGO showed ex-
2
In conclusion, we developed a novel, efficient and recyclable Cu O-
cellent catalytic performance and great functional groups tolerance for
the direct oxidative coupling synthesis of various α-ketoamides.
One of the most important properties of the heterogeneous catalysts
NPs@rGO material to catalyze the direct oxidative coupling of aryl
methyl ketones and secondary amines to synthesize α-ketoamides with
2 2
O as clean oxidant. The Cu O-NPs@rGO was systematically char-
was their recyclability. Hence, the cycle experiments of the Cu
rGO were carried out by using acetophenone and piperidine as standard
substrates under optimum reaction condition. After the reaction fin-
2
O-NPs@
acterized by XRD, XPS, TEM, SEM, SEM-EDS, ICP-OES, which proved
that Cu O nanoparticles evenly and firmly supported on the rGO. But
more important, the heterogeneous synthetic methodology using fa-
cilely prepared Cu O-NPs@rGO, green oxidant and mild reaction con-
2
ished, Cu
2
O-NPs@rGO was recovered by centrifugation, washed with
2
ethanol and water, and finally dried under vacuum. The desired pro-
duct was obtained by column chromatography and calculated the exact
yields. And the concrete experimental results were shown in the Fig. S5.
It could be seen that no apparent decline in the yield after recycling
ditions, provides a simple and efficient approach to synthesize various
α-ketoamides in good to excellent yields. We believed that our study
could provide a reliable approach to develop copper-based carbon
catalysts and promote the application of heterogeneous catalyst into
organic reaction.
2
seven times (89%). Furthermore, the recycled Cu O-NPs@rGO was
characterized by TEM to observe its morphology and structure. The
corresponding image was displayed in Fig. S6. Through the comparison
5