Scholarly article on topic ' Aerobic Oxidation of Alcohols and the Synthesis of Benzoxazoles Catalyzed by a Cuprocupric Coordination Polymer (Cu + -CP) Assisted by TEMPO '

Aerobic Oxidation of Alcohols and the Synthesis of Benzoxazoles Catalyzed by a Cuprocupric Coordination Polymer (Cu + -CP) Assisted by TEMPO Academic research paper on "Chemical sciences"

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Academic research paper on topic " Aerobic Oxidation of Alcohols and the Synthesis of Benzoxazoles Catalyzed by a Cuprocupric Coordination Polymer (Cu + -CP) Assisted by TEMPO "

Inorganic Chemistry

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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*'t,§

^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

^ Supporting Information

ABSTRACT: A Cu+-CP based on the tetranuclear unit {[(HSQPA)2Cu4(bipy)4]-2H2O}„-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.

The oxidation of alcohols to the corresponding carbonyl compounds 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 complexes 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

alcohol oxidation and the synthesis of CPs10 prompted us to explore the possibility of Cu+-CP as a reusable catalyst in the aerobic oxidation ofalcohols. Herein, we report the synthesis and structural characterization of six-coordinated Cu11 complex 1 and four-coordinated Cu1 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}„-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 Cu11 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 A. These units are further extended by an O atom of the sulfonic group to form a railway-like double-chain aggregate (Figure 1).

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(ll) 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

ACS Publications © XXXX American Chemical Society

HSQPA ligands, and two coordinated and one lattice water molecules (Figure S2b in the SI). Both Cu1 ions exhibit a four-coordinated tetrahedral environment with a N2O2 donor set provided by HSQPA, bipy linker moieties, and the coordinated water. Four adjacent Cu1 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

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 Cu1 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 measurements 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. 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.

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 of Primary and Secondary Alcoholsa

aReaction 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. cIn the absence of TEMPO.

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 (6473%). 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

Table 2. Cu+-CP 2/TEMPO-Catalyzed Aerobic Oxidative Synthesis of 2-Arylbenzoxazolesa

aReaction 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. bIsolated yield.

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 moderate 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 Cun 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.

■ ASSOCIATED CONTENT rh 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.

■ AUTHOR INFORMATION Corresponding Authors

*E-mail: xubohan@163.com. *E-mail: wlya@lynu.edu.cn.

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

Department (14A150049), and Innovation Scientists and Technicians Troop Construction Projects of Henan Province.

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