Dual-immobilized copper catalyst: Carbon nitride-supported copper nanoparticles catalyzed oxidation of propargylic alcohols

Article abstract of DOI:10.1016/j.tetlet.2015.01.177

Copper nanoparticles were supported and modified by carbon nitride, which was utilized as support and might work as a kind of immobilized N-donor ligand. The modified nanoparticle catalyst was evaluated with the oxidation of propargylic alcohols and showed highly catalytic efficiency as well as significant ligand or support effect in the reaction. The dual-immobilized nanoparticle catalyst could be recycled for 3 times at least without an obvious decrease in catalytic activities.

Full text of DOI:10.1016/j.tetlet.2015.01.177

Accepted Manuscript
Dual-immobilized copper catalyst: carbon nitride-supported copper nanoparti-
cles catalyzed oxidation of propargylic alcohols
Wei Lv, Jing Tian, Nan Deng, Yan Wang, Xiaoshu Zhu, Xiaoquan Yao
PII:
S0040-4039(15)00227-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.01.177
TETL 45847
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
26 October 2014
22 January 2015
27 January 2015
Please cite this article as: Lv, W., Tian, J., Deng, N., Wang, Y., Zhu, X., Yao, X., Dual-immobilized copper catalyst:
carbon nitride-supported copper nanoparticles catalyzed oxidation of propargylic alcohols, Tetrahedron Letters
(2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.01.177
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Dual-immobilized copper catalyst: carbon
nitride-supported copper nanoparticles
catalyzed oxidation of propargylic alcohols
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Wei Lv, Jing Tian, Yan Wang, Xiaoshu Zhu* and Xiaoquan Yao*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Dual-immobilized copper catalyst: carbon nitride-supported copper nanoparticles
catalyzed oxidation of propargylic alcohols
Wei Lva, Jing Tiana, Nan Denga, Yan Wanga, Xiaoshu Zhub,* and Xiaoquan Yaoa, ∗
a Department of Applied Chemistry, College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R.
China
b Analysisand Testing Center, Nanjing Normal University, Nanjing 210008, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
Copper nanoparticles were supported and modified by carbon nitride, which was utilized as
support and might work as a kind of immobilized N-donor ligand. The modified nanoparticle
catalyst was evaluated with the oxidation of propargylic alcohols and showed highly catalytic
efficiency as well as significant ligand or support effect in the reaction. The dual-immobilized
nanoparticle catalyst could be recycled for 3 times at least without obvious decrease in catalytic
activities.
Received in revised form
Accepted
Available online
Keywords:
Copper nanoparticles
Carbon nitride
2009 Elsevier Ltd. All rights reserved.
Immobilized ligand
Catalytic oxidation
Recently, much attention has been attracted to the use of
nanoparticles as catalysts in organic reactions.1 Because of their
easy preparation and relative stabilities in air, the nanoparticles of
coinage metals, such as copper, silver and gold, were widely
reported, and there are many excellent examples on their
applications as catalysts in organic reactions.2 In comparison with
most of examples using surfactant-free nanoparticles as catalysts,
there are few examples reported about the ligand effect on
coinage metal nanoparticle-catalyzed reactions. Recently, several
should be noted that the carbon nitride actually acted as a catalyst
support only in the reaction, and there was no ligand/support-
effect observed.7 Otherwise, Antonietti and his co-workers also
reported a Pd nanoparticles supported on a mesoporous graphitic
carbon nitride, Pd@mpg-C3N4, which was shown to be highly
active and promoted the selective formation of cyclohexanone
from the hydrogenation of phenol under atmospheric pressure of
hydrogen in aqueous media,6b but no ligand/support effect was
mentioned either.8 Considering the sp3- and sp2- hybridized
nitrogen-containing structure,4 we suspected that carbon nitride
might be used as a novel immobilized ligand or functionalized
support, to achieve a greener approach to the ligand-promoted,
metal nanoparticles-catalyzed reaction. As our first attempt,
carbon nitride-supported copper nanoparticle catalyst (CuNPs-
CN) was synthesized successfully following the reported method.
The average size of copper nanoparticles is ca.20-30 nm based on
the TEM image. 9, 10
ligand-promoted or ligand
&
support-promoted, coinage
nanoparticle-catalyzed reactions were reported by our groups and
the others,3 in which the utilization of ligands proved to be the
key role to the reaction. However, in these examples, the ligands
were introduced separately into the reaction mixture, which
resulted in many difficulties to recover and reuse the ligands with
the solid nanoparticle catalyst together. To resolve the problem,
using immobilized ligand or functionalized support instead might
be a possibility of choices.
Previously, a highly efficient, copper nanoparticles catalyzed
oxidation of propargylic alcohols to ynones was reported with
Mesoporous carbon nitride is a novel superior material and
has been wildly used as electrode materials because of its porous
structure and amine-containing molecules.4 On the other hand, its
use for organic catalysis reports has not been concentrated ever.5,
3d
TBHP or air as oxidant by our group.
With bipyridine as
ligand, the reaction was accelerated significantly and gave good
to excellent yields to a variety of propargylic alcohols. The
nanoparticals could be recovered conveniently by centrifugation
from the reaction mixture, but it is hard to be reused effectively
without additional fresh ligand supplied. Thus, this reaction was
selected as prototype to examine the catalytic activity and
recyclability of the dual-immobilized copper catalyst.
6
Recently, Vinu and co-workers reported the synthesis of
mesoporous carbon nitride and demonstrated that the Au
nanoparticles embedded in mesoporous carbon nitride were a
highly efficient and recyclable catalyst in a three-component
coupling reaction of benzaldehyde, piperidine, and
6a
phenylacetylene to synthesize propargylamine.
However, it
———
∗ Corresponding author. Tel.: +86-25-52112902; fax: +86-25-52112626; e-mail: yaoxq@nuaa.edu.cn

2
Tetrahedron Letters
.
Table 1. Copper nanoparticles-catalyzed oxidation of 1k.a
OH
O
Catalyst (10 mol%)
Oxidant (10 eq.)
solvent
1k
2k
Entry Catalyst
Oxidant
Solvent
Temperature (oC)
Time (h)
Yield (%)b
93
1
CuNPs
TBHP (10equiv)
TBHP (10equiv)
TBHP (10equiv)
TBHP (10equiv)
TBHP (10equiv)
TBHP (5equiv)
TBHP (2equiv)
TBHP (2equiv)
TBHP (10equiv)
H2O2 (10equiv)
H2O2 (10equiv)
H2O2 (10equiv)
O2
DCM
40
40
40
40
40
40
rt
6
2
CuNPs/bipyridine
CuNPs-CN
CN
DCM
2
95
3
DCM
2
>99
NDc
83
4
DCM
24
24
24
6d
6d
2
5
CuNPs-AC
CuNPs-CN
CuNPs
DCM
6
DCM
99
7
DCM
46e
8
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
CuNPs-CN
DCM
rt
82f
9
Toluene
DCM
80
40
80
80
40
60
80
80
93
10
11
12
13
14
15
16
24
24
24
24
24
24
24
46
Toluene
t-BuOH
DCM
51
55
trace
trace
50
O2
THF
O2
Toluene
Dioxane
O2
trace
a Reaction condition: 1k (0.1mmol), copper catalyst (10 mol%), TBHP (70% in water, 10 equiv.), dichloromethane (DCM, 1.5 mL).
b Isolated yields.
c Not detected.
d The reaction was interrupted at 6h.
e ca.50% of starting material was recovered.
f ca. 14% of starting material was recovered.
In the present work, utilizing the nitrogen-containing carbon
material, carbon nitride, as functionalized support or immobilized
ligand, a novel, ligand and metal dual-immobilized copper
nanoparticle catalyst were prepared and evaluated with the
oxidation of propargylic alcohol. Significant ligand/support-
effect was observed in the reaction as well as good to excellent
yields were achieved. Furthermore, the dual-immobilized
nanoparticle catalyst could be recycled for 3 times at least
without obvious decrease in catalytic activities and no additional
fresh ligands were required. To best of our knowledge, it is also
the first report on the ligand/support-effect of carbon nitride in
catalytic organic reactions.
(entry 3). These results above suggested that the carbon nitride
might show some effect similar to the free bpy ligand in current
reaction. Further more, in case the carbon nitride could catalyze
the oxidation along, a blanket reaction was also carried out (entry
4). It is obviously that the copper nanoparticals should be the real
catalyst in the supported metal catalyst.
To further understand the ligand/support effect of the
nitrogen-containing carbon support material,
a
control
experiment was also carried out with CuNPs-AC as catalyst, but
the result was even worse than that of CuNPs (entries 1 and 3 vs.
entry 5). It is obviously that the nitrogen-containing structure of
carbon nitride improves the catalytic activity of CuNps similar to
an N-donor ligand.
The oxidation of 1-(naphthalene-1-yl)-3-phenyl prop-2-yn-1-
ol (1k) was selected as the prototype to start our investigation
and screen for the optimized reaction conditions. With 10 mol%
of copper nanoparticles as catalyst, 10 equiv. of TBHP as oxidant
With the decrease of TBHP from 10 to 5 equivalents, 24h was
required to entire the reaction (entry 6). In order to study the
ligand/support effect of carbon nitride detailedly, we also
decreased TBHP to 2 equiv. of 1k, and carried the reactions at
room temperature with CuNPs and CuNPs-CN as catalysts,
respectively. It can be seen from entries 7 and 8, that under the
milder condition, CuNPs-CN catalyst gave almost double yield
than that of CuNPs during the same reaction time. This result
provided another possible evidence that the carbon nitride
support did improve the catalytic activity of CuNps under present
conditions.
o
in DCM solution at 40 C, we compared the catalytic characters
of the carbon nitride-supported copper nanoparticles (CuNPs-
CN)10 with pure copper nanoparticals (CuNPs), CuNPs &
bipyridine ligand and active carbon supported CuNPs (CuNPs-
AC).11
It can be seen from Table 1, with CuNPs as catalyst, the
desired product, 1-(naphthalen-2-yl)-3-phenylprop-2-yn-1-one
(2k), was afforded successfully with 93% yield after 6 hours
reaction (Table 1, entry 1). An obvious ligand effect was
observed when 10 mol% bipyridine was added to the reaction
mixture, and the reaction time was shorten from 6 hours to 2
hours with slightly increase in yield (entry 2). When CuNPs-CN
was utilized as catalyst, a >99% of yield was achieved in 2 hours
Toluene was also used as the solvent instead of DCM. The
reaction was carried out at 80oC and 2k was afforded with 93%
of yield (entry 9). On the other hand, other oxidants were also
scanned. H2O2 gave moderate yields from 46%-55% (Table 1,
entries 10-12). When oxygen was utilized as oxidant, only 50%

3
o
of 2k was obtaind in toluene solution at 80 C after 24 hours
reaction. (entries 13-16).
15
16
17
18
19c
8
6
8
8
6
92
89
82
60
35
With the optimized reaction conditions in hand, we studied
12
substrate scope of the reaction. Typical results are listed in
Table 2. It can be seen that the substrates derived from aromatic
aldehydes and alkynes, no matter it is electron-rich or electron –
deficient, gave the corresponding products in good to excellent
yields (Table 2, entries 1-17). It was worthy to note that the
substrates derived from aliphatic aldehydes and alkynes also gave
excellent yields (entries13–15). Even with compound 1r as
substrate which was derived from salicyldehyde and usually
sensitive to oxidation conditions, 60% of product was also
achieved (entry 18). However, with 3-phenylprop-2-yn-1-ol (1s)
as the starting material, a complicated reaction mixture was
obtained and only 35% of 2s was given even at room temperature
(entry 19).
a Reaction condition: propargylic alcohol (0.1 mmol), CuNPs-CN (10 mol%),
TBHP (70% in water, 10 equiv.), dichloromethane (DCM, 1.5 mL), 40 oC.
b Isolated yields.
Table 2. Carbon nitride supported copper nanoparticles
catalyzed oxidation of propargylic alcoholsa
c The reaction was carried out at room temperature.
OH
In order to understand the nature of the CuNPs-CN catalyst
further more, we also selected 1a, 1m and 1n to detected more
conditions, and the results were summarized in Table 3.
O
Cu NPs-CN (10 mol%)
TBHP (10eq.)
DCM,40oC
R1
R1
R2
R2
1
2
Entry Substrates
Products
Time (h)
Yield (%)b
>99
Table 3. Copper nanoparticles-catalyzed oxidation of 1a, 1m
and 1n.a
1
2
2
2
2
6
8
8
8
8
2
2
8
2
8
Entry Sub.
Conditions
result b
6h, 94%
6h, 70%c
84%d
1
1a
10 mol% CuNPs, 2 equiv. of TBHP, rt
10 mol% CuNPs, 10 equiv. of TBHP, 40 oC,
10 mol% CuNPs-CN, 2 equiv. of TBHP, rt
2
3
96
97
99
86
95
81
85
90
>99
>99
89
97
92
2
1a
3
1a
4
1a
10 mol % CuNPs-CN, 10 equiv. of TBHP, 40 oC 2h, 99%
5
1m
1m
1m
1m
1n
1n
1n
1n
10 mol % CuNPs, 2 equiv. of TBHP, rt
10 mol % CuNPs, 10equiv. of TBHP, 40 oC,
10 mol % CuNPs-CN, 2 equiv. of TBHP, rt
38%c,d
OMe OH
6
8h, 56% c
65%d
4
7
OMe
1d
8
10 mol % CuNPs-CN, 10 equiv. of TBHP, 40 oC 2h, 97%
5
9
10 mol % CuNPs, 2 equiv. of TBHP, rt
10% CuNPs, 10equiv. of TBHP, 40 oC,
10 mol % CuNPs-CN, 2 equiv. of TBHP, rt
10%d
15% c,d
59%d
10
11
12
6
10 mol % CuNPs-CN, 10 equiv. of TBHP, 40 oC 8h, 92%
7
a Reaction condition: propargylic alcohol (0.1 mmol), Cu catalyst (10 mol%),
TBHP (70% in water, 10 equiv.), dichloromethane (DCM, 1.5 mL).
b Isolated yields.
8
c A complicated reaction mixture was obtained.
d The total conversion could not be reached even after overnight reaction.
9
As shown in Table 3, for substrate 1a, the CuNPs seems show
a little better catalytic activity than CuNPs-CN at room
temperature (entry 1 vs entry 3). However, much worse result
10
11
12
13
14
o
was obtained by using CuNPs as catalyst at 40 C with 10 equiv
of TBHP, and a complicated reaction mixture was obtained
(entry 2 vs entry 4). On the other hand, when less reactive
substrate 1m and 1n were examined, CuNPs-CN showed much
better catalytic activity than CuNPs no matter at room
o
temperature or 40 C (entries 5-6 vs entries 7-8, entries 9-10 vs
entries 11-12). These results above might indicate that the most
suitable activation temperature for CuNPs-CN catalyst was 40 oC,
and also provided evidence that the carbon nitride materials do
modify the catalytic characters of CuNPs.
As a metal and ligand dual-immobilized catalyst, our original
intention is to look for a novel strategy to recycle the catalyst and
ligand together in the ligand-promoted, nanoparticle-catalyzed

4
Tetrahedron
P.; Qu, X. M.; Wang, L. J.; Zhao, C. J.; Xu, J. Q. Electroanalysis
2008, 20, 1981-1986; d) Jurewicz, K.; Pietrzak, R.; Nowicki, P.;
Wachowska, H. Electrochim. Acta 2008, 53, 5469-5475; e) Lota,
G.; Lota, K.; Frackowiak, E. Electrochem. Commun. 2007, 9,
1828-1832; f) Kim, Y. J.; Abe, Y.; Yanagiura, T.; Park, K. C.;
Shimizu, M.; Iwazaki, T.; Nakagawa, S.; Endo, M.; Dresselhaus,
M. S. Carbon 2007, 45, 2116-2125; g) Li, W. R.; Chen, D. H.; Li,
Z.; Shi, Y. F.; Wan, Y.; Huang, J. J.; Yang, J. J.; Zhao, D. Y.;
Jiang, Z. Y. Electrochem. Commun. 2007, 9, 569-573, and the
references therein; (h) Giraudet, S.; Zhu, Z.; Yao, X.; Lu, G.; J.
Phy. Chem. C 2010, 114, 8639-8645.
reactions to achieve a greener approach. Thus, we recovered the
supported catalyst conveniently by centrifugation from the
reaction mixture, and reused it directly with fresh solvent and
substrate to examine the recyclability of the catalyst. 13 It can be
seen from Table 3 that the catalyst could be reused for 3 times at
least without appreciable loss in its catalytic activities (Table 4).14
This result above showed the excellent potential for the
recyclability of the dual-immobilized catalyst.
Table 4. The experiments of catalyst recycle. a
5. As metal-free catalysts: (a) Jin, X.; Balasubramanian, V. V.;
Selvan, S. T.; Sawant, D. P.; Chari, M. A.; Lu, G. Q.; Vinu, A.
Angew. Chem. 2009, 121, 8024-8027; (b) Goettmann, F.; Fischer,
A.; Antonietti, M.; Thomas, A. Angew. Chem. Int. Ed. 2006, 45,
4467-4471; (c) Molmann, L.; Baar, M.; Rieβ, J.; Antonietti, M.;
Wang, X.; Blecherta, S. Adv. Synth. Catal. 2012, 354, 1909-1913.
6. Examples as the support of metal catalysts: (a) Datta, K. K. R.;
Reddy, B. V. S.; Ariga, K.; Vinu, A. Angew. Chem. Int. Ed. 2010,
49, 5961-5965; (b) Wang, Y.; Yao, J.; Li, H.; Su, D.; Antonietti,
M. J. Am. Chem. Soc. 2011, 133, 2362–2365.
OH
O
CuNPs-CN (10 mol%)
TBHP (10 eq.)
DCM, 40 oC, 2 h
2k
1k
Run
Yield/%
1
2
3
4
>99
97
7. In fact, gold nanoparticle could catalyze the A3 addition
effectively without any ligand promotion. The reported examples,
see: (a) Elie, B. T.; Levine, C.; Ubarretxena-Belandia, I.; Varela-
Ramirez, A.; Contel, M. Eur. J. Inorg. Chem. 2009, 3421-3430.
(b) Zhang, X.; Corma, A. Angew. Chem. Int. Ed. 2008, 47, 4358-
4361.
94
92
8. In this example, the graphitic carbon nitride was believed as a
basic support to enhance the chemical absorb of phenol.
a Reaction condition: 1j (0.1 mmol), CuNPs-CN (10 mol%), DCM (1.5 mL),
stirred at 40 oC for 2h.
9. Preparation of carbon nitride: Carbon nitride was synthesized
following the reported method from polypyrrole: Sevilla, M.;
Valle-Vigón, P.; Fuertes, A. B. Adv. Funct. Mater. 2011, 21,
2781-2787. The copper nanoparticles were also prepared based on
a reported procedure: Sarkar, A.; Mukherjee, T.; Kapoor, S. J.
Phy. Chem. C, 2008, 112, 3334. In a typical synthesis, 3 g of
pyrrole was added to a solution of FeCl3 (0.5 M, 200 mL) and the
mixture was magnetically stirred for 12h. The polypyrrole was
then separated by filtration and washed with abundant distilled
water and dried. The polypyrrole was chemically activated by
heating a PPy-KOH mixture (KOH/PPy at a weight ratio of 2)
under N2 up to a temperature of 650 °C (heating rate: 3 °C/min.,
holding time: 2h). The activated samples were then thoroughly
washed several times with HCl (10 wt%) to remove any inorganic
salts and then washed with distilled water until neutral pH.
Finally, the activated carbon nitride was dried in an oven at 120
°C.
b Isolated yields.
In conclusion, with carbon nitride as a functionalized support
instead of bpy ligand, a novel ligand and metal dual-immobilized
copper nanoparticle catalyst were prepared and evaluated with
the oxidation of propargyl alcohol. Significant ligand/support
effect was observed in the reaction as well as good to excellent
yields were achieved. The carbon nitride-supported nanoparticle
catalyst could be effectively recovered and reused for 3 times
without obvious decrease in catalytic activities. The detailed
mechanism, the effect of particle size, hole size and nitrogen
content of carbon nitride as well as the scope of the reaction are
currently under further investigation.
10. Carbon nitride supported copper nanoparticles: Under
nitrogen atmosphere, in a 250 mL three round-bottom flask
equipped with a stir bar, 20 mL absolute alcohol was mixed with
40 mL aqueous solution of Cu(OAc)2 (1 mmol),
polyvinylpyrrolidone (PVP, average MW: 40,000, 200 mg) and
carbon nitride (0.32 g) preheated to 40 oC. Then, 80 mL of
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (21172107 and 21472092 to X.
Yao). We also sincerely thank Prof. Xiaogang Zhang’s help in
carbon materials synthesis.
aqueous solution containing sodium borohydride (NaBH4,
4
mmol) and potassium hydroxide (KOH, 4 mmol), was added
dropwise into the solution over a period of 30 minutes, and black
precipitates were observed in the mixture. The mixture was kept to
stir vigorously for 2 h at 40 °C. Then, the particles were separated
from the solution by high-speed centrifugation at 6500 ppm for 2
min. The precipitates were washed by distilled water, absolute
alcohol and diethyl ether for 3 times, respectively. The final
References and notes
1. For recent reviews, see: (a) Shiju, N. R.; Guliants V. V. Appl.
Catal. A: General 2009, 356, 1-17; (b) Somorjai, G. A.; Park, J.
Y. Angew. Chem. Int. Ed. 2008, 47, 9212-9228; (c) Gu, Y.; Li, G.
Adv. Synth. Catal. 2009, 351, 817-847; (d) Chng, L. L.;
Erathodiyil, N.; Ying, J. Y. Acc. Chem. Res. 2013, 46, 1825-1837.
2. For recent reviews: (a) Astruc, D.; Lu, F.; Aranzaes, J. M. Angew.
Chem. Int. Ed. 2005, 44, 7852-7872. (b) Corma, A.; Garcia, H.
Chem. Soc. Rev. 2008 37, 2096-2126, also see Ref. 1c and 1d.
o
product was dried in vacuum at 80 C overnight and stored under
nitrogen atmosphere.
11. The detailed characterization of carbon nitride and its supported
copper nanoparticles were described in supporting information.
The TEM image of active carbon supported CuNPs was also
provided in supporting information.
3.
A
phosphine ligand-stablized Au(0) nanoparticle-catalyzed
diboration, see: (a) Ramirez, J.; Sanau, M.; Fernandez, E. Angew.
Chem. Int. Ed. 2008, 47, 5194-5197; Recently, we reported two
examples about ligand-promoted, silver nanoparticals-catalyzed
reaction. See: (b) Yu, M.; Lin, M.; Han, C.; Li, Z.; Li, C.-J.; Yao,
X. Tetrahedron Lett. 2010, 51, 6722-6725. (c) Yu, M.; Wang, Y.;
Sun, W.; Yao, X. Adv. Synth. Catal. 2012, 354, 71-76. We also
reported a copper nanoparticle-catayzed, bpy-promoted oxidation
of propargylic alcohols to ynones: (d) Han, C.; Yu, M.; Sun, W.;
Yao, X. Synlett 2011, 16, 2363-2368.
12. Typical procedure for carbon nitride supported copper
nanoparticles catalyzed oxidation of propargylic alcohols
(Entry 1, Table 2): 1-(Naphthalene-1-yl)-3-phenylprop-2-yn-1-ol
(1k) (0.1 mmol), CuNPs-CN (10 mol%), TBHP (10 eq.) and DCM
(1mL) were added into a 10-mL sealed tube in air. The mixture
o
was stirred at 40 C for 2 hours. Then, the reaction mixture was
purified by flash column chromatography on silica gel
(hexanes/EtOAc 15:1). Compound 2k was obtained in a >99 % of
yield
4. Some examples as electrode materials (a) Yang, M.; Cheng, B.;
Song, H.; Chen, X. Electrochimica Acta. 2010, 55, 7021-7027; b)
Jurcakova, D. H.; Kodama, M.; Shiraishi, S.; Hatori, H.; Zhu, Z.
H.; Lu, G. Q. Adv. Funct.Mater. 2009, 19, 1800-1809; c) Dong, J.
13. Typical procedure for recycling of the catalyst: The oxidation
of 1k (0.1 mmol scale) was carried out under the optimized
reaction conditions for 2 hours. When the reaction was stopped,
the solid catalysts were separated through the centrifugal method

5
from the reaction mixture. The collected catalyst was washed with
diethyl ether and dried under vacuum at room temperature. Then,
fresh substrate 1k (0.1 mmol), TBHP (10 equiv.) and DCM (1.5
mL) was added to run the next reaction for another 2 hours.
14. As a control experiment, the bpy-Cu NPs catalyst was also reused
with 1k as the substrate for one more time: without additional bpy
ligand, 84% of yield was obtained after 14 h reaction; with fresh
bpy ligand, the reaction finished in 8 h, and gave 91% of 2k.
Supplementary Material
Supplementary data associated with this article can be found
in the online version.