Aerobic oxidation of alcohols and the synthesis of benzoxazoles catalyzed by a cuprocupric coordination polymer (Cu+-CP) assisted by TEMPO

Article abstract of DOI:10.1021/ic502884z

A Cu⁺-CP based on the tetranuclear unit {[(HSQPA)₂Cu₄(bipy)₄]·2H₂O}_n·2nH₂O has been constructed through Cu²⁺ salt, 2-(sulfonylquinlium-8-yloxy)phthalic acid (H₃SQPA), and 4,4′-bipyridine (bipy). This Cu⁺-CP combined with 2,2,6,6-tetramethylpiperidine-1-oxyl as the cocatalyst is an effective catalyst for aerobic oxidation of alcohols and the synthesis of benzoxazoles and can be recycled at least four times without losing its catalytic activity.

Full text of DOI:10.1021/ic502884z

Communication
pubs.acs.org/IC
Aerobic Oxidation of Alcohols and the Synthesis of Benzoxazoles
Catalyzed by a Cuprocupric Coordination Polymer (Cu+‑CP) Assisted
by TEMPO
Xun Feng,† Chen Xu,*,† Zhi-Qiang Wang,† Si-Fu Tang,‡ Wei-Jun Fu,† Bao-Ming Ji,† and Li-Ya Wang*,†,§
†College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, China
‡Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
§College of Chemistry and Pharmacy Engineering, Nanyang Normal University, Nanyang 473601, China
S
* Supporting Information
alcohol oxidation and the synthesis of CPs10 prompted us to
ABSTRACT: A Cu+-CP based on the tetranuclear unit
{[(HSQPA)2Cu4(bipy)4]·2H2O}n·2nH2O has been con-
structed through Cu2+ salt, 2-(sulfonylquinlium-8-yloxy)-
phthalic acid (H3SQPA), and 4,4′-bipyridine (bipy). This
Cu+-CP combined with 2,2,6,6-tetramethylpiperidine-1-
oxyl as the cocatalyst is an effective catalyst for aerobic
oxidation of alcohols and the synthesis of benzoxazoles
and can be recycled at least four times without losing its
catalytic activity.
explore the possibility of Cu+-CP as a reusable catalyst in the
aerobic oxidation of alcohols. Herein, we report the synthesis and
structural characterization of six-coordinated CuII complex 1 and
four-coordinated CuI complex 2 and present an effective
procedure for the aerobic oxidation of primary and secondary
alcohols to the aldehydes, ketones, and benzoxazoles using a
reusable heterogeneous Cu+-CP 2/TEMPO catalytic system
under an air atmosphere.
The Cu2+-CP 1, {[Cu2(SQPA)2]·2H2O}n·2nH2O, was
obtained by mixing the ligand 2-(sulfonylquinlium-8-yloxy)-
phthalic acid (H3SQPA) and copper(II) acetate under
solvothermal conditions. The asymmetric unit of 1 is simply
composed of one CuII center supported by HSQPA ligands and
two coordination molecules, as well as one lattice water molecule,
as shown in Figure S2a in the Supporting Information (SI). The
two head-to-tail arrangements of the HSQPA ligands adopt the
same chelating modes, which link the CuII centers to generate a
dimer unit with a metal−metal distance of 8.49 Å. These units are
further extended by an O atom of the sulfonic group to form a
railway-like double-chain aggregate (Figure 1).
he oxidation of alcohols to the corresponding carbonyl
Tcompounds is one of the most important reactions in
organic synthesis, and extensive efforts have been focused on the
development of metal-catalyzed aerobic oxidation methods.1 It
traditionally requires numerous stoichiometric oxidants and
expensive metal catalysts such as palladium and ruthenium.2
Copper compounds are well established among the known
alcohol oxidation catalysts;3 those employing 2,2,6,6-tetrame-
thylpiperidine-1-oxyl (TEMPO) as a redox-active cocatalyst
were particularly effective for the aerobic oxidation of alcohols
and exhibited broad utilities in both academe and industry.4 In
contrast to the great development of homogeneous copper-
based catalysts, only two supported copper catalysts have been
reported,5 and heterogeneous copper catalyst systems are still
largely unexplored.
On the other hand, coordination polymers and metal−organic
frameworks (MOFs) have emerged as a new class of attractive
functional materials with considerable promise in catalysis.6
These heterogeneous catalysts can be readily recovered and
reused while possessing spatially separated single catalytic active
sites in their frameworks, which is highly desirable for cost and
sustainability considerations. Some monounclear copper com-
plexes with N,N and N,O ligands in conjunction with TEMPO
were found to be efficient catalysts for the aerobic oxidation of
alcohols.7 However, only four examples of Cu2+-MOF derived
from tricarboxylic ligands8 and 4,7-bis(4-pyridyl)-1,1,3,3-tetra-
methylisoindolin-2-yloxy9 have been reported to be effective
catalysts for the aerobic oxidation of alcohols, and these catalyst
systems require an oxygen atmosphere or tert-butyl nitrite. To
our best of knowledge, there are presently no reports concerning
a Cu+-MOF/CP catalyst system for the aerobic oxidation of
alcohols. Our recent works on Pd/Cu-cocatalyzed aerobic
Figure 1. Ball-and-stick view of the 1D packing array in 1.
The Cu+-CP 2 was prepared in high yield by mixing ligands
H3SQPA and bipy and copper(II) acetate under solvothermal
conditions. Interestingly, the CuII ions were reduced to CuI ions
by in situ reaction owing to the reducibility of the bipy moiety at
high temperature.11 Single-crystal X-ray diffraction (XRD)
analysis reveals that 2 crystallizes in a triclinic system. The
asymmetric unit of 2 is composed of two types of CuI ions, two
bipy molecules, two deprotonated (−SO3H and −COOH)
Received: December 4, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ic502884z
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Inorganic Chemistry
Communication
HSQPA ligands, and two coordinated and one lattice water
molecules (Figure S2b in the SI). Both CuI ions exhibit a four-
coordinated tetrahedral environment with a N2O2 donor set
provided by HSQPA, bipy linker moieties, and the coordinated
water. Four adjacent CuI ions form a tetranuclear cluster through
a O/N linker. These units are further interconnected by bipy
ligands, resulting in a 2D layer, as shown in Figure 2a. The Cu2
The reusability of Cu+-CP 2 was investigated for the oxidations
of benzyl alcohol and 4-methylbenzyl alcohol under the
optimized conditions as described in Table S4 in the SI. It was
observed that the Cu+-CP catalyst could be recycled and reused
four times without any loss of activity. The XRD pattern of the
recovered CP further confirmed its recyclability (Figure S7 in the
SI). As can be seen, there are only minor changes in the relative
intensities of some peaks. This may be because TEMPO will
coordinate with the active sites of CP.8a Additionally, the catalyst
could retain the integrity of its molecular structure, as evidenced
by Fourier transform infrared (Figure S8 in the SI).
In order to extend the scope of the aerobic oxidation by Cu+-
CP assisted by TEMPO, we performed aerobic oxidation of
various alcohols (Table 1). The electronic nature of the
Table 1. Cu+-CP 2/TEMPO Catalyzed the Aerobic Oxidation
a
of Primary and Secondary Alcohols
Figure 2. (a) 2D packing array in 2. (b) Schematic view of the (3,4)-
connected (4.6.8)(4.64.8)(42.62.82) topology. A 6-membered short
circuit is highlighted in red, and the rod passing through it is highlighted
in green (orange nodes are for CuI centers and blue nodes for H3SQPA
ligands).
ion is coordinated to one HSQPA and two bipy ligands through
Cu−O/N bonds, while the Cu1 and HSQPA ligand link four
moieties. Cu1, Cu2, and HSQPA ligand can be regarded as 3 and
4-connected nodes, topologically, and the layer can be reduced to
a (3,4)-connected network with a Schlafli symbol of (4.6.8)-
̈
(4.64.8)(42.62.82) (Figure 2b).
Thermogravimetric/differential thermal analysis (Figure S3 in
the SI) indicates that a dehydrate of 2 can be kept intact beyond
300 °C, followed by expulsion of organic components. In order
to confirm the oxidation states of the Cu ion, magnetic
susceptibility and X-ray photoelectron spectroscopy measure-
ments were performed. The former indicates that 2 is just a
diamagnetic material (Figure S4 in the SI). The spin−orbit
components (2p3/2 and 2p1/2) of the Cu 2p peak were well
deconvoluted by two curves at approximately 932.2 and 952.1 eV
with a spin−orbit separation of 19.9 eV (Figure S5 in the SI),
confirming the presence of a monovalent oxidation state of CuI in
2.12 The pure 2 was confirmed by powder XRD measurement, in
which diffraction peaks of experimental data are in agreement
with the simulated data from single-crystal X-ray crystallography
(Figure S6 in the SI).
Initially, the oxidation of benzylic alcohol was chosen as a
model reaction to optimize the reaction conditions under an air
atmosphere (Table S3 in the SI). A blank reaction was carried out
with KOH as the base in CH3CN; negligible product
benzaldehyde was attained. An additional blank experiment
was performed under similar conditions using 2 mol % Cu+-CP;
the isolated yield of benzaldehyde was only 28% (entry 1).
However, the yield was greatly improved by the addition of
TEMPO (70%, entry 2). The Cu2+-CP 1 generated the product
only in 16% yield under the same conditions (entry 3). After a
variety of solvents were screened, CH3CN was found to be the
most effective solvent and toluene showed a comparable result
(entry 6). Then, in a quick survey of the bases, CsOH was found
to give the best result (entry 12) and Cs2CO3 also displayed good
efficiency (entry 10). We also investigated the above reaction at
room temperature; the yield of the product was only 30% (entry
13). These obtained results suggested that the Cu+-CP, TEMPO,
and a strong base medium are required to promote the oxidation
reaction.
a
Reaction conditions: alcohol (1 mmol), CsOH (1 mmol), Cu+-CP 2
(0.02 mmol), TEMPO (0.1 mmol), CH3CN (5 mL), air atmosphere,
80 °C, 12 h. Isolated yield. In the absence of TEMPO.
b
c
substituents on the benzylic alcohol did have an effect on the
reaction. Electron-donating substrates reacted to give the
products 3a and 3b, and the yields (93 and 88%) are obviously
higher than the yields of electron-withdrawing substrates. The
oxidation of heterocyclic alcohols proceeded smoothly to
provide the desired products 3g−3i in moderate yield (64−
73%). In the case of aliphatic alcohol, the product 3j−3l yields
(60−76%) were lower than those of the benzylic alcohols. It was
noteworthy that the aerobic oxidation of secondary alcohols also
gave the expected products 3m−3o in good yield (83−89%)
only using Cu+-CP in the absence of TEMPO, while the
oxidation of 1-phenylethanol catalyzed by Cu3(BTC)3/TEMPO
gave the product 3n with only a very low yield (14%).8a
Prompted by the good results obtained in the aerobic
oxidation of alcohols catalyzed by Cu+-CP, we explored whether
this catalytic system would allow the preparation of 2-
arylbenzoxazoles from benzyl alcohols. The results obtained
are summarized in Table 2. To our delight, under the same
conditions, benzyl alcohol and 2-aminophenol were efficiently
converted to the 2-phenylbenzoxazole 4a (82%). Subsequently,
the reactions of 2-aminophenol with benzylic alcohols bearing
methyl and methoxy groups were investigated, and good yields
were obtained (85 and 86%). In contrast to 2-monoarylbenzox-
azoles, only the two examples of 2-biarylbenzoxazoles 4d and 4e
have been reported.13 To broaden the substrate scope, we further
investigated 2-aminophenol with a variety of electronic and
B
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Communication
Table 2. Cu+-CP 2/TEMPO-Catalyzed Aerobic Oxidative
Synthesis of 2-Arylbenzoxazoles
Department (14A150049), and Innovation Scientists and
Technicians Troop Construction Projects of Henan Province.
a
REFERENCES
■
(1) (a) Backvall, J. E. Modern Oxidation Methods; Wiley-VCH: Berlin,
̈
2004. (b) Tojo, G.; Fernan
́
dez, M. Oxidation of Alcohols to Aldehydes and
Ketones; Basic Reactions in Organic Synthesis; Springer: New York,
2010. (c) Parmeggiani, C.; Cardona, F. Green Chem. 2012, 14, 547−564.
(2) (a) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400−3420.
(b) Mallat, T.; Baiker, A. Chem. Rev. 2004, 104, 3037−3058.
(c) Gligorich, K. M.; Sigman, M. S. Chem. Commun. 2009, 26, 3854−
3856.
(3) (a) Punniyamurthy, T.; Rout, L. Coord. Chem. Rev. 2008, 252,
134−154. (b) Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.;
Kozlowski, M. C. Chem. Rev. 2013, 113, 6234−6458.
(4) (a) Mase, N.; Mizumon, T.; Tatemoto, Y. Chem. Commun. 2011, 7,
2086−2088. (b) Hoover, J. M.; Stahl, S. S. J. Am. Chem. Soc. 2011, 133,
16901−16910. (c) Hoover, J. M.; Steves, J. E.; Stahl, S. S. Nat. Protoc.
2012, 7, 1161−1166. (d) Hoover, J. M.; Ryland, B. L.; Stahl, S. S. J. Am.
Chem. Soc. 2013, 135, 2357−2367. (e) Steves, J. E.; Stahl, S. S. J. Am.
Chem. Soc. 2013, 135, 15742−15745. (f) Cao, Q.; Dornan, L. M.; Rogan,
L.; Hughes, N. L.; Muldoon, M. J. Chem. Commun. 2014, 35, 4524−
4526. (g) Ryland, B. L.; Stahl, S. S. Angew. Chem., Int. Ed. 2014, 53,
8824−8836.
(5) (a) Yang, G. Y.; Zhu, W. M.; Zhang, P. K.; Xue, H. Z.; Wang, W.;
Tian, J. S.; Song, M. P. Adv. Synth. Catal. 2008, 350, 542−546.
(b) Herbert, M.; Montilla, F.; Galindo, A. Dalton Trans. 2010, 3, 900−
907.
a
Reaction conditions: 2-aminophenol (1 mmol), alcohol (1.2 mmol),
CsOH (1 mmol), Cu+-CP (0.02 mmol), TEMPO (0.1 mmol),
CH3CN (5 mL), air atmosphere, 80 °C, 12 h. Isolated yield.
b
structural biarylmethanols. The corresponding 2-biarylbenzox-
azoles 4f−4k were isolated in moderate-to-good yields (61−
81%). Finally, this protocol was found also to proceed
successfully with 4-heteroarylbenzyl alcohols, furnishing moder-
ate yields (64−76%). The detailed structure of 4h was confirmed
by single-crystal X-ray crystallography (Figure S2c in the SI).
In summary, we have synthesized two coordination polymers
containing the dinuclear CuII and tetranuclear CuI units. The
latter cuprocupric CP combined with TEMPO shows surprising
catalytic performnace in the aerobic oxidation of primary and
secondary alcohols under an air atmosphere. It is easy to
synthesize and can be recycled and reused for four runs without
any significant loss of catalytic activity. Moreover, this catalytic
system has been successfully applied to the synthesis of 2-
arylbenzoxazoles from benzyl alcohols and 2-aminophenol.
(6) (a) Wang, Z.; Chen, G.; Ding, K. L. Chem. Rev. 2009, 109, 322−
359. (b) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112,
1196−1231. (c) Liu, J. W.; Chen, L. F.; Cui, H.; Zhang, J. Y.; Zhang, L.;
Su, C. Y. Chem. Soc. Rev. 2014, 43, 6011−6061.
(7) (a) Thomas, F.; Gellon, G.; Gautier-Luneau, I.; Saint-Aman, E.;
Pierre, J. L. Angew. Chem., Int. Ed. 2002, 41, 3047−3050. (b) Figiel, P. J.;
Leskela, M.; Repo, T. Adv. Synth. Catal. 2007, 349, 1173−1179.
̈
(c) Mahmudov, K. T.; Kopylovich, M. N.; Silva, M. F. C. G.; Figiel, P. J.;
Karabach, Y. Y.; Pombeiro, A. J. L. J. Mol. Catal. A: Chem. 2010, 318, 44−
50. (d) Ma, Z.; Wei, L. J.; Alegria, E. C. B. A.; Martins, L. M. D. R. S.;
Silva, M. F. C. G.; Pombeiro, A. J. L. Dalton Trans. 2014, 43, 4048−4050.
(8) (a) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. ACS Catal. 2011,
1, 48−53. (b) Paul, M.; Adarsh, N. N.; Dastidar, P. Cryst. Growth Des.
2014, 14, 1331−1337. (c) Qi, Y.; Luan, Y.; Wu, J.; Peng, X.; Wang, G.
Chem.Eur. J. 2014, 21, 1589−1597.
(9) Li, L. C.; Matsuda, R.; Tanaka, I.; Sato, H.; Kanoo, P.; Jeon, H. J.;
Foo, M. L.; Wakamiya, A.; Murata, Y.; Kitagawa, S. J. Am. Chem. Soc.
2014, 136, 7543−7546.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic details in CIF format, experimental details,
spectral data, and additional figures and tables. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
(10) (a) Xu, C.; Li, H. M.; Xiao, Z. Q.; Wang, Z. Q.; Tang, S. F.; Ji, B.
M.; Hao, X. Q.; Song, M. P. Dalton Trans. 2014, 43, 10235−10247.
(b) Xu, C.; Li, H. M.; Yuan, X. E.; Xiao, Z. Q.; Wang, Z. Q.; Fu, W. J.; Ji,
B. M.; Hao, X. Q.; Song, M. P. Org. Biomol. Chem. 2014, 12, 3114−3122.
(c) Feng, X.; Feng, Y. Q.; Liu, L.; Wang, L. Y.; Song, H. L.; Ng, S. W.
Dalton Trans. 2013, 42, 7741−7754.
S
(11) (a) Zhan, S. Z.; Li, M.; Zhou, X. P.; Ni, J.; Huang, X. C.; Li, D.
Inorg. Chem. 2011, 50, 8879−8892. (b) Chen, Y. Q.; Li, G. R.; Chang, Z.;
Qu, Y. K.; Zhang, Y. H.; Bu, X. H. Chem. Sci. 2013, 4, 3678−3682.
(c) Baruah, B.; Gabriel, G. J.; Akbashev, M. J.; Booher, M. E. Langmuir
2013, 29, 4225−4234.
(12) (a) Cheng, J. K.; Chen, Y. B.; Wu, L.; Zhang, J.; Wen, Y. H.; Li, Z.
J.; Yao, Y. G. Inorg. Chem. 2005, 44, 3386−3388. (b) Chen, Y.; Li, J. L.;
Tong, G. S. M.; Lu, W.; Fu, W. F.; Lai, S. W.; Che, C. M. Chem. Sci. 2011,
2, 1509−1514.
(13) (a) Geigy, C. Patent DE2309614, 1973. (b) Teo, Y. C.; Riduan, S.
N.; Zhang, Y. G. Green Chem. 2013, 15, 2365−2387.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: xubohan@163.com.
*E-mail: wlya@lynu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 21272110, 21273101, and 21171173), the
Foundation of the Program for Backbone Teachers in
Universities of Henan Province (Grants 2013GGJS151 and
2012GGJS158) and Tackle Key Problem of Science and
Technology Project of Henan Province, China (Grant
142102310483), Science Foundation of Henan Education
C
DOI: 10.1021/ic502884z
Inorg. Chem. XXXX, XXX, XXX−XXX