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Sustainable Synthesis of 2-Arylbenzoxazoles Over a Cobalt-based Nanocomposite Catalyst Jian He, Fu Lin, Xufang Yang, Di Wang, Xiaohua Tan, and Shujun Zhang Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00168 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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Organic Process Research & Development

Sustainable Synthesis of 2-Arylbenzoxazoles over a Cobalt-based Nanocomposite Catalyst Jian He, a* Fu Lin,b Xufang Yang, c Di Wang,d Xiaohua Tan,b Shujun Zhang e* a.

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,

China b.

College of Pharmacy, Wenzhou Medical University, Wenzhou 325035, China

c.

Mudanjiang Medical University, Mudanjiang 157011, China

d.

School of Life Sciences, Jilin University, Changchun 130012, China

e.

Harbin Institute of Chemical Engineering, Harbin 150030, China

1

ACS Paragon Plus Environment


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Table of Contents:

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ABSTRACT: A new cobalt-based heterogeneous catalyst was prepared by adsorbing in situ produced nitrogen-ligated cobalt (II) acetate complexes on commercially available SiO2 and subsequent pyrolysis at 800 °C for 2 h under N2 atmosphere. By applying this catalyst under the O2 balloon, the aerobic oxidation of phenolic imines proceeded smoothly and gave various 2-arylbenzoxazoles in good yields. Meanwhile, on the basis of the experimental results, a plausible reaction pathway was described to elucidate the reaction mechanism. KEYWORDS: heterogeneous catalysis; cobalt nanoparticles; oxidative cyclization; benzoxazoles.

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INTRODUCTION The interaction between central metal with ligand or support is a key technology in the design of catalyst. The facile control of their interactions has been of long-standing interest for chemists to exploit catalytic performance. In this context, many versatile heterogeneous catalysts were mainly developed by controlling the size and shape of the active metal species, and modifying the composition of the supports. Despite numerous inherent advantages of heterogeneous catalysts over homogeneous ones, for example, their reusability and durability, the fine-tuning of metal-ligand interactions in heterogeneous catalysts is much more difficult than in homogeneous catalysts, and still a challenging task. Thus, the application of organmetallic complexes as precursors for the synthesis of stable heterogeneous catalysts is still need to be well established. On the other hand, in the community of fused heterocycles, benzoxazole is common and has been referred as “core structure” in drug discovery. Benzoxazole derivatives displayed extensive biological activities such as anti-inflammatory, antitumor, antidepressant, antibiotic and tranquilizer (Scheme 1).2 In past decades, many strategies for their construction have been reported.3 Among them, the catalytic oxidative cyclization of phenolic imines has attracted sizeable attention for its unique features. Various homogeneous catalysts such as Ruthenium/Iridium complexes, Pd(OAc)2,

RuCl3,

Ortho-oiodoxybenzoic

acid

(IBX),

4-methoxy-TEMPO

(2,2,6,6-tetramethyl-piperidyl-1-oxy TEMPO), FeCl3, CuCl2, NaCN were employed.4 More recently, the development of easily recoverable and recyclable heterogeneous

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catalysts that can solve the problem of the homogeneous systems has captured special attention, including activated carbon, copper nanoparticles, platinum nanoclusters, ZnO nanoparticles and organic photocatalyst eosin Y bis (tetrabutylammonium salt) (TBA-eosin Y).5 However, in most cases, tedious procedure for the catalyst synthesis, high reaction temperatures, noble metals and/or additives are involved. Therefore, a new sustainable catalytic system to synthesis these compounds under a mild condition is highly desirable. Scheme 1. Prevalent benzoxazole functionality in pharmaceuticals O

COOH F

N COOH

Cl O

O HOOC

N

N Cl

HO

Flunoxaprofen

Caboxamycin

Tafamidis

In 2013, Beller and co-workers carried out pioneering studies on heterogenized cobalt oxide catalysts for the selective hydrogenation of nitroarenes by pyrolysis of organometallic complexes.21 Subsequently, they designed a series of Co- and Fe-oxide supported catalysts for the selective reduction and oxidative reactions.6 Inspired by these findings, we became interested in applying these catalysts in the aerobic oxidation of phenolic imines. Herein, we wish to report a protocol to prepare a CoOx-NGr/SiO2 nanocomposite catalyst through pyrolysis of organometallic complexes. The aerobic oxidation of phenolic imines proceeded smoothly promoted by the catalyst and the corresponding benzoxazoles were obtained in good to excellent yields. Depending on ligands we used, we modified the catalyst structure and notable differences in catalyst activity were observed. Therefore, the ligand indirectly controls

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the selectivity and activity of the title reaction. Furthermore, silica supported catalysts are hopeful for academic and industrial applications due to their high mechanical and thermal stabilities, noncorrosive and easy separation feature of the catalyst from the reaction mixture.

RESULTS AND DISCUSSION We began our studies by preparing various cobalt-based catalysts from different cobalt salts and nitrogenous ligands (Table 1) by adsorbing a solution of the respective cobalt complex on SiO2 and subsequent pyrolysis under N2, and then we evaluated their performance in catalyzing the oxidative cyclization of phenolic imine 1a. Cobalt oxide particles were expected to form which were surrounded by nitrogen-doped graphene layers (NGr) loaded on SiO2 surface. Table 1 summarized some results obtained during the optimization of the reaction. Among the different candidates nitrogenous ligands, 1, 10-phenanthroline (L5) was considered to be the most active catalytic system, and 2-phenylbenzoxazole 2a was formed in a yield of 87% (Table 1, entry 6). The other similar nitrogenous ligands such as 2,2'-bipyridine, pyridine etc. gave no product at all (Table 1, entries 1-5). Therefore, comparably small changes in the organic ligand caused wide loss of selectivity and reactivity. The title reaction did not occur in the presence of pyrolyzed SiO2-supported Co(OAc)2, and L5 (Table 1, entries 7-8). Next, the pyrolysis of Co(OAc)2-L5 on Al2O3, TiO2, and activated

carbon

supports

gave

also

active

catalysts

which

delivered

2-phenylbenzoxazole in 45-79% yields(Table 1, entries 9-11). All these results showed that the Co-L5 supported on SiO2 will be the most active catalyst. It is

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demonstrated in the Table 1 (Table 1, entry 12) that O2 is crucial for the oxidative process. Table 1. Optimization of the Aerobic Oxidation of Phenolic Imine 1a

entrya support

ligand

pyrolysis

yield

[oC, h, gas]

[%]b

-

trace

cobalt salt

1

none

none

none

2

SiO2

pyridine

Co(OAc)2•4H2O 800, 2, N2

N.R.

3

SiO2

quinoline

Co(OAc)2•4H2O 800, 2, N2

N.R.

4

SiO2

quinolin-8-ol

Co(OAc)2•4H2O 800, 2, N2

N.R.

5

SiO2

2,2’-bipyridine

Co(OAc)2•4H2O 800, 2, N2

N.R.

6

SiO2

1,10-phenanthroline

Co(OAc)2•4H2O 800, 2, N2

87

7

SiO2

none

Co(OAc)2•4H2O 800, 2, N2

trace

8

SiO2

1,10-phenanthroline

none

N.R.c

9

Al2O3

1,10-phenanthroline

Co(OAc)2•4H2O 800, 2, N2

45

10

TiO2

1,10-phenanthroline

Co(OAc)2•4H2O 800, 2, N2

53

11

C

1,10-phenanthroline

Co(OAc)2•4H2O 800, 2, N2

79

12d

SiO2

1,10-phenanthroline

Co(OAc)2•4H2O 800, 2, N2

trace

a

800, 2, N2

Unless otherwise noted, reactions were carried out with 1a (0.5 mmol), cobalt

catalyst (3 mol% Co) in 1,4-dioxane (1.0 mL) at 65 oC for 24 h under O2 balloon. b

Isolated yields.

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c

N.R. = no reaction

d

In the presence of N2 balloon.

Characterization of the metal species in the active catalyst (CoOx-N-Gr/SiO2) by XRD, HRTEM, XPS demonstrated the preferential formation of smallCoOx particles of 20-80 nm in size on a carbon-nitrogen-based support surface. On the other hand, a few larger particles (20-80 nm) are also present. XRD patterns (Figure 1(a)) of samples pyrolysed at 800 oC showed that they contained Co(0), CoOx and CoNx species. As shown in Fig. 1b), the survey spectra of Co-N-Gr/SiO2 samples reveal the presence of Si, O, C, N as well as Co without any other impurities, confirming that nitrogen and cobalt were successfully incorporated into the SiO2. Figure 1. Catalyst Characterization

Based on the optimized catalytic system already in hand (Table 1, entry 6), we

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explored the scope of the present aerobic oxidation of phenolic imines. As shown in Table 2, the results revealed that electronic nature and steric bulk of the substituents on the aromatic imines had a slight influence on the catalytic performance (Table 2, entries 1-9). Furthermore, important functional groups, for example

nitro, chloro,

bromo, allyl, were tolerable under mild reaction conditions, thus giving the functionalized benzoxazole derivatives with 87-99% yields (Table 2, entries 6-10). In the case of imines derived from 2-naphthaldehyde, 92% yield could also be obtained (Table 2, entry 11). Meanwhile, imines derived from heteroaromatic aldehydes (2-pyridinyl and 2-furyl) were also suitable substrates to afford the desired adducts with 86-95% yields (Table 2, entries 12-13). However, by using aliphatic aldehyde, the corresponding product was produced in a lower yield (Table 2, entry 14). Because imines bearing aliphatic groups are less unstable than aryl ones, and these imines may undergo undesirable side-reactions under our catalytic conditions. In order to show the synthetic utility of the catalyst system, the gram-scaled reaction was carried out with a gram of imine 1a. The reaction was carried out smoothly and we obtained the desired product 2a with 93% yield. Scheme 2. The Substrate Scope of 2-Arylbenzoxazoles

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The cobalt catalyst was filtered off from the reaction mixture after 36% conversion of imine 1a. Continued stirring of the filtrate under identical reaction conditions for 12 h did not promote the reaction to proceed any more. ICP-AES analysis of the filtrate showed only negligible amount of Co (0.28 ppm corresponding to 1.7 wt % of initial charge) was leached into the solution. We also carried out recovery and reuse studies with CoOx-NGr/SiO2 catalyst for the oxidative cyclization reaction of Schiff base 1a and we found that the catalyst could be reused five cycles without noticeable loss of catalytic activity (for details see Supporting Information). These results confirmed that the oxidative coupling reaction was indeed catalyzed by the heterogeneous

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catalyst. A possible oxidative cyclization mechanism for this reaction has been depicted in Scheme 3. Initially, the intramolecular nucleophilic addition of hydroxyl group to the C=N double bond provides hemiaminal A. Notably, this intermediate is unstable and immediately undergoes Co-catalyzed aerobic cyclization to generate the desired product 2. Conversely, 1 could be hydrolyzed by H2O to regenerate 2-aminophenol and aldehydes, these reactants could be oxidized to their corresponding byproducts ortho-quinones and acids. Scheme 3. The mechanism for the CoOx-NGr/SiO2 catalyzed aerobic oxidative synthesis 2-aryl benzoxazole

CONCLUSIONS In summary, we have designed a CoOx-NGr/SiO2 nanocomposite catalyst for the aerobic oxidation of phenolic imines to give various benzoxazoles in good to excellent yields. The cobalt catalytic system could be easily recovered from the reaction mixture and reused five times without any loss of catalytic activity. By altering the ligands, several heterogeneous cobalt catalysts were produced and led to the development of high efficiency catalysts. We believe that this strategy is attractive for the design of other heterogeneous metal catalysts. Further extension application of this catalytic system for the preparation of some other kind of useful heterocyclic 11



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compounds is underway in our laboratory.

EXPERIMENTAL Materials. Unless otherwise stated, all the reagents and reactants were purchased from commercial suppliers without further purification. General part including instruments, synthesis and characterization of various substrates are provided in the Supporting Information. Preparation of CoOx-NGr/SiO2 catalyst. Cobalt (II) acetate tetrahydrate (249 mg, 1.0 mmol) and ligand (L) (2.0 mmol) were combined in EtOH (100 mL). The mixture was stirred for 1 h at 25 oC. Then, SiO2 (200-300 mesh, 1.40 g) was added and the whole reaction mixture was refluxed for 4 h. The reaction mixture was cooled to room temperature. The ethanol was removed in vacuum. The solid sample obtained was dried at 80 °C for 12 h. Then, the grinded powder was transferred into a ceramic crucible and placed in the oven. In order to exclude air, N2 was constantly passed through the oven for 30 min. Then, the oven was heated at 10 °C/min until 800 °C, and was held at 800 °C for 2 h under N2 (Scheme 4). After the heating was finished, the oven cooled back to room temperature under N2. Accordingly, the catalyst was obtained. Elemental analysis of catalyst Co-L5 (wt%): Si = 28.40, C = 4.51, N = 2.10, Co = 3.42, H = 0.36, O = 61.21. Scheme 4. General Procedure for Preparation of CoOx-NGr/SiO2 Catalyst

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General procedure for aerobic oxidation of phenolic imines. In a glass test tube was added CoOx-NGr/SiO2 (25.0 mg), imine 2 (0.5 mmol), and 1, 4-dioxine (1.0 mL). The reaction mixture was stirred for 24 h at 65 oC under a O2 balloon. The crude product was directly purified by column chromatography (EA: PE = 1:10) to provide the corresponding 2-substituted benzoxazoles 3. The identity of the product was confirmed by 13C and 1H

NMR spectroscopic analysis.

ASSOCIATED CONTENT Supporting Information General procedures and spectral characterization of compounds 2a-2m are included. This material is available on http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 13



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This work was supported by the scholarship from China Scholarship Council (No 2010617090), Wenzhou Science and Technology Bureau Research Foundation (Y20140740) and National Science Foundation of China (81500519) and Wenzhou Medical University Research Foundation (QTJ14014).

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