Base-Free Aerobic Oxidation of Alcohols over Copper-Based Complex

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Base-Free Aerobic Oxidation of Alcohols over Copper-Based Complex under ambient condition Qingqing Mei, Huizhen Liu, Youdi Yang, Hangyu Liu, Shaopeng Li, Pei Zhang, and Buxing Han ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03820 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Sustainable Chemistry & Engineering

Base-Free Aerobic Oxidation of Alcohols over Copper-Based Complex under ambient condition †‡

†‡

†‡

†‡

†‡

†

Qingqing Mei , Huizhen Liu* , Youdi Yang , Hangyu Liu , Shaopeng Li , Pei Zhang , Buxing Han*†‡ †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid and

Interface and Thermodynamics, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 (P. R. China). ‡

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing 100049 (P. R. China). *E-mail: [email protected] (Huizhen Liu); [email protected] (Buxing Han).

ABSTRACT: Cu-based complex formed from Cu(OAc)2 and [2,2']-bipyridinyl-5,5'-dicarboxylic acid diethyl ester (BPYDCDE) ligand was synthesized for the first time. It was found that the complex could catalyze aerobic oxidation of alcohols to aldehydes or ketones very efficiently without any external base at ambient temperature and pressure, and the yield of the desired product reached >99% in 2~5 h. Combination of experimental and theoretical studies showed that the ligand enhanced the electron population on the Cu center by a ligand-to-metal charge transfer (LMCT) effect, which made OAc- in the complex have the appropriate alkalinity and be

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a good leaving group, and the Cu center and the OAc- catalyzed the reaction cooperatively. Moreover, the amount of OAc- in the complex was much less than that of the external base added in the catalytic systems reported, suggesting that the basic anion in the complex is more efficient for promoting the reaction than the external base added.

KEYWORDS: Selective oxidation; base-free; ambient condition; Cu (II) complex; enhanced Lewis acid-base pair INTRODUCTION The selective oxidation of alcohols to aldehydes and ketones is among the most important reactions both in the organic synthesis and chemical industry because the products are precursors and intermediates of many drugs, fine chemicals, vitamins and fragrances1-5. O2 or air is very attractive alternatives to traditional stoichiometric oxidants such as chromium salts6, activated DMSO7, peroxides8-9, and hypervalent iodines10. Many heterogeneous11-12 and homogeneous13-14 catalysts have been developed using different metals, such as Ru15-16, Pd17, Co18, Cu19, Pt20, Rh21, V22, Os23, Ce24, Ni25, Mo26, and Au27-28. However, to the best of our knowledge, efficient oxidation of alcohols requires elevated temperature or adding external alkali such as potassium tert-butylate29, carbonate30-31 and organic base32. Generally, metals and alkali co-catalyze alcohol oxidation under mild reaction conditions. Robert and his co-workers reported that little or no activity was observed over Au catalysts without additional alkali33. Even with an alkaline carrier, the use of base was still necessary over Pd catalyst34. The oxidation of alcohols over copper based catalytic system has been studied widely because Cu is a cheap and abundant metal35-39. Masai et al. found that the alkali additives could efficiently promote oxidation of benzyl alcohol to benzaldehyde with Cu-Na-ZSM-5 as the


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catalyst40. (Ligand)Cu/TEMPO/base systems are demonstrated to be efficient for alcohol oxidation29. The synergetic effect of the ligand and base plays a key role on the catalytic efficiency of this kind of catalysts41. Generally, more electron-rich N-based ligands and noncoordination counterions afforded higher activity in the presence of external base42-43. However, this kind of catalytic systems are also suffered from some troublesome issues. Some homogeneous systems (eg. CuBr/BPY/TEMPO/NMI) would form insoluble precipitates during the reaction and failed to reach complete conversion of the reactant29. Some (eg. CuBr2/BPY/TEMPO/KOH) started to be deactivated when the reaction approached completion41. Different external bases also significantly influenced the catalytic reactivity, selectivity and stability41. Stable, recyclable, and highly efficient catalytic systems without any external base are highly desired for the selective aerobic oxidation of alcohols to aldehydes/ketones under room temperature and pressure. It is very interesting but challenging, especially for non-noble metal catalytic systems. Herein, for the first time we designed a Cu-based complex using [2,2']-bipyridinyl-5,5'dicarboxylic acid diethyl ester (BPYDCDE) as the organic ligand and Cu(OAc)2 as the metal precursor, denoted as Cu(BPYDCDE)(OAc)2. It was found that the Cu(BPYDCDE)(OAc)2 was very active and selective for the oxidation of alcohols to aldehydes or ketones without additional base using air as the oxidant at room temperature. Most of the substrates could be completely converted with 100% selectivity to the desired products within 2 h. The Cu (II) complex could be recycled for 5 times without loss of the activity. The reasons for the unique nature of the catalyst were explored by combination of experimental and theoretical studies. EXPERIMENTAL SECTION


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Chemicals. Cu(OAc)2 (98%), Co(OAc)2 (98%), Zn(OAc)2 (99%), Ni(OAc)2 (99%), 2,2'bipyridine-5,5'-dicarboxylic acid (97%), diethyl sulfate (99%), 1,3,5-benzenetricarboxylate (98%), 1,4-benzenedicarboxylic acid (99%), 4,4'-biphenyldicarboxylic acid (99%), and 2,2'bipyridine (98%) were purchased from Beijing InnoChem Science & Technology Co., Ltd. Acetonitrile (AR), n-decane (99%), TEMPO (98%), benzyl alcohol (99%), diethyl sulfate (99%), 3,3',5,5'-tetrakis(trifluoromethyl)benzhydrol (97%), benzhydrol (99%), 3,4-dimethoxybenzyl alcohol (97%), 4-hydroxy-3-methoxybenzyl alcohol (>98%), 4-hydroxybenzyl alcohol (99%), 2hydroxybenzyl alcohol (99%), 3-indolemethanol (97%), 3-phenyl-2-propyn-1-ol (>98%), 1heptyn-3-ol (98%), 4-methylbenzyl alcohol (98%), 2-naphthalenemethanol (98%), 2thiophenemethanol

(98%),

(hydroxymethyl)benzonitrile

2-hydroxyacetophenone (95%),

dihydro

(97%), cuminyl

2-heptyn-1-ol alcohol

(97%),

(85%),

43,5-

bis(trifluoromethyl)benzyl alcohol (98%) and 1, 3-diphenyl-1-propyn-3-ol (90%) were purchased from J&K scientific Ltd. All the chemicals were used as received. Synthesis of [2,2']bipyridinyl-5,5'-dicarboxylic acid diethyl ester (BPYDCDE). Diethyl sulfate (2 mmol), 2,2'-bipyridine-5,5'-dicarboxylic acid (BPYDC, 1 mmol), cesium carbonate (2 mmol) and toluene (20 mL) were charged into a teflon lined stainless steel autoclave (50 mL) equipped with a magnetic stirrer. The autoclave was then loaded to an oil bath at 120 oC and stirred overnight at inner atmosphere. After cooling down, the mixture was filtered, and the solid residue was washed three times by CH2Cl2 (3 × 10 mL). The liquid phase was combined and evaporated under vacuum to obtain crude product, which was purified by recrystallization in ethanol. White needle-like crystal was obtained. 1H NMR (400 MHz, Chloroform-d) δ 9.25 (d, J = 1.5 Hz, 2H), 8.53 (d, J = 8.2 Hz, 2H), 8.39 (dd, J = 8.3, 2.1 Hz, 2H), 4.42 (q, J = 7.1 Hz, 4H), 1.41 (t, J = 7.1 Hz, 6H); 13C NMR (100 MHz, Chloroform-d) δ 165.09, 158.22, 150.54, 137.99,


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126.52, 121.19, 61.47, 14.26. The data is consistent with that reported in literature44, and the NMR spectra are given in Figure S1. Preparation of the catalysts. CuBPYDCDE(OAc)2: Cu(OAc)2 (2 mmol), BPYDCDE (1 mmol) and ethanol (20 mL) were added into a flask (50 mL) and stirred at room temperature for 8 h. The product was obtained after centrifugation, washing with ethanol (3 × 5 mL) and drying at 100 oC in vacuum oven for 24 h. The content of Cu and O were determined by ICP-AES (VISTA-MPX). The contents of C, H and N were obtained from elemental analysis by using the FLASH EA1112 analyzer. Calculated for CuN2C20H22O8, Cu (13.19%), C (49.84%), N (5.81%), O (26.56%), H (4.60%); found Cu (13.20%), C (49.81%), N (5.78%), O (26.62%), H (4.59%). Cu(BPYDCDE)Cl2, Ni(BPYDCDE)(OAc)2, Co(BPYDCDE)(OAc)2 and Zn(BPYDCDE)(OAc)2 were prepared by the similar procedure except that CuCl2, Ni(OAc)2, Co(OAc)2 and Zn(OAc)2 were used instead of Cu(OAc)2 as the metal precursor. Cu(BPYDCDE)(OOCCF3)2, Cu(BPYDCDE)(ClO4)2 and Cu(BPYDCDE)(OTf)2 are soluble in ethanol and MeCN, so the mixtures of the desired amount of precursors in MeCN were directly used for the catalytic reactions. Preparation of the Cu-MOFs. CuBTC was prepared according to the literature45. In a typical experiment, H3BTC (benzene-1,3,5-tricarboxylic acid, 1mmol) was dissolved in ethanol (15 mL), and Cu(NO3)2 (2 mmol) was dissolved in water (15 mL). The two solutions were mixed at ambient temperature for 30 min, and the mixture was transferred into an autoclave. The autoclave was heated at 120 oC under hydrothermal conditions for 18 h. The reaction vessel was cooled to ambient temperature, and blue crystals of CuBTC were isolated by filtration and washed with water (3 × 10 mL) and ethanol (2 × 10 mL). The product was dried in vacuum at 160 oC for 12 h. CuBDC was prepared according to the literature46. In a typical experiment,


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H2BDC (terephthalic acid, 1 mmol) and Cu(NO3)2 (1 mmol) were dissolved in DMF (20 mL) and stirred at room temperature for 30 min. Then the solution was transferred to a sealed container, and heated in an oven at 110 oC for 24 h. The resulting blue powder was collected by filtration, and washed with DMF (3 × 10 mL) and ethanol (2 × 10 mL). The product was dried in vacuum at 160 oC for 12 h. The CuBPDC complex was prepared in a similar procedure with CuBDC. H2BPDC (biphenyldicarboxylic acid, 1 mmol) and Cu(NO3)2 (1 mmol) were dissolved in DMF (20 mL) and stirred at room temperature for 30 min. Then the solution was transferred to a sealed container, and heated in an oven at 120 oC for 24 h. The resulting blue powder was collected by filtration, and washed with DMF (3 × 10 mL) and ethanol (2 × 10 mL). The product was dried in vacuum at 160 oC for 12 h. The CuBPYDC complex was prepared in a similar procedure except that H2BPYDC (2,2'-bipyridine-5,5'-dicarboxylic acid) was used instead of H2BPDC. Reaction procedures. Substrate (2 mmol), catalyst (20 mg), TEMPO (0.2 mmol), acetonitrile (3 mL) was charged to a flask (25 mL) equipped with a condenser. Then the flask was loaded into a water bath at 30 oC and stirred in air for desired time. The product was identified by GCMS, and the yield was quantified by GC using n-decane as an internal standard. Computational details. The structure of the Cu(BPDCOEt)(OAc)2 complex was optimized using M06-2x method47. SDD basis and pseudopotential was applied for Cu, and the TZVP basis was for other atoms. The calculations were performed with Gaussion 09 D01 package48. The electronic density differences were calculated at M06-2x/SDD|TZVP level using Multiwfn code49. The electronic density difference is the diffenece between the electronic density of the complex and the sum of the ligand and the metal salt. It indicates the change of the electronic density due to the formation of the complex. The negative region is where the electronic density


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decreases because of the formation of the complex, and the positive region is where the electronic density increases becasue of the fromation of the complex. The NPA (Natural population analysis) atomic charges50 were calculated at M06-2x/SDD|TZVP level using the NBO 3.1 module51 embedded in Gaussian 09 packge. RESULTS AND DISCUSSION Characterization of the ligand and the catalyst. The prepared Cu(BPYDCDE)(OAc)2 and BPYDCDE were characterized by Fourier transform infrared spectroscopy (FTIR ) and the spectra are shown in Electronic Supplementary Information (Figure S2). The band at 1591 cm−1 was assigned to antisymmetric stretching vibration of C–N–C in the pyridyl rings of the ligand BPYDCDE, which moves to 1629 cm−1 for Cu(BPYDCDE)(OAc)2 because of formation of CuN bond52. The result indicates the formation of coordination bond between Cu and BPYDCDE ligand. Elemental analysis of Cu(BPYDCDE)(OAc)2 was conducted by Inductively coupled plasma atomic emission spectrometry (ICP-AES) and and the results showed that the Cu/N molar ratio in the Cu(BPYDCDE)(OAc)2 was about 1:2, showing that one Cu2+ coordinated with one BPYDCDE ligand (more details were shown in the experimental section). Based on these results, we proposed the structure of Cu(BPYDCDE)(OAc)2 as Figure 1a. We also optimized the proposed structure using DFT method, and the ball stick model was shown in Figure 1b. It can be figured out that the structure of Cu(BPYDCDE)(OAc)2 is not symetric. Detailed structure parameters were listed in the supporting information, and the coorelation of structure with the reaction was discussed in the “Mechanistic insights” section.


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Figure 1. a) proposed structure of Cu(BPYDCDE)(OAc)2 according to FT-IR and elemental analysis; b) the ball stick model of the optimized structure using DFT method. Cu, pink; N, blue; O, red; C, grey; H, light grey.

Catalysis. The activities of different types of copper complexes were checked for the aerobic oxidation of benzyl alcohol without additional alkali at room temperature under atmospheric air. The results are given in Table 1. The reaction did not occur without catalyst (Table 1, entry 1). Cu-based metal organic framework (Cu-MOF) such as CuBTC (BTC = trimesic acid), CuBDC (BDC = terephthalic acid) and CuBPDC (BPDC = 4,4'-biphenyldicarboxylate) were synthesized as described in the literatures. They exhibited very low activity under the reaction conditions and the yield of benzaldehyde was all below 5% (Table 1, entries 2-5). Cu(BPYDCDE)(OAc)2 exhibited the highest activity and the yield of benzaldehyde could reach >99% under the reaction conditions (Table 1, entry 6). However, the other metal-(BPYDCDE) complexes checked were not active for the reaction. Almost no benzaldehyde was detected over Co(BPYDCDE)(OAc)2, Ni(BPYDCDE)(OAc)2 or Zn(BPYDCDE)(OAc)2 complexes (Table 1, entries 7-9), suggesting


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that Cu was the most active metal. Because the oxidation reaction of benzyl alcohol is a continuous reaction, we investigated the effect of reaction time. No benzoic acid was detected when the reaction time extended to 8 hours (Table 1, entry 10). The results indicate that Cu(BPYDCDE)(OAc)2 could catalyze the oxidation of benzyl alcohol to benzaldehyde but could not catalyze the oxidation of benzaldehyde to benzoic acid. Thus, Cu(BPYDCDE)(OAc)2 exhibited very high catalytic selectivity for the reaction as well. In addition, the Cu complex could be recycled for 5 times without loss of the activity (Table 1, entries 11 and 12).

Table 1. Results for the oxidation of benzyl alcohol over different catalytic systemsa

Entry

Catalyst

time (h) Conversion (%) Yield (%) Selectivity (%)

1

None

2

0

0

-

2

CuBTC

2

3

3

100

3

CuBDC

2

4

4

100

4

CuBPYDC

2

5

5

100

5

CuBPDC

2

8

8

100

6

Cu(BPYDCDE)(OAc)2

2

100

>99

100

7

Co(BPYDCDE)(OAc)2

2

1

1

100

8

Ni(BPYDCDE)(OAc)2

2

trace

Trace

100

9

Zn(BPYDCDE)(OAc)2

2

trace

Trace

100

10

Cu(BPYDCDE)(OAc)2

8

100

>99

100

11b

Cu(BPYDCDE)(OAc)2

2

100

>99

100

12c

Cu(BPYDCDE)(OAc)2

2

100

99

100

a). Reaction conditions: benzyl alcohol (2 mmol), catalyst (2 mol%), TEMPO (10 mol%), CH3CN (3 mL), in air and at ambient temperature. TEMPO is 2,2,6,6-tetramethylpiperidine N-oxide. Conversion and yield were analyzed by GC using n-decane as the internal standard; b). the 3rd run; c) the 5th run.


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The activity of copper complexes with different counterions was studied (Table 2, entries 1-5). The pKa value of HOAc is 4.75 and the yield of benzaldehyde was >99% over Cu(BPYDCDE)(OAc)2 (Table 2, entry 1). The pKa of CF3COOH is 0.23 and the yield of benzaldehyde was 80% over Cu(BPYDCDE)(CF3COO)2 in 2 h (Table 2, entry 2). For Cu(BPYDCDE)Cl2, the pKa of HCl is -3.0 and the yield of benzaldehyde was 35% (Table 2, entry 3). The alkalinity of ClO4- and OTf- is very weak since the pKa values of the corresponding acids are -8 and -14.7, respectively. The yields of benzaldehyde over Cu(BPYDCDE)(ClO4)2 and Cu(BPYDCDE)(OTf)2 were only 11% and 8%, respectively (Table 2, entries 4 and 5). These results indicate that the activity of the copper complexes was related with the alkalinity of counterion. The activity of the complexes also depended strongly on the structures of the ligands. The yield of benzaldehyde over Cu(BPY)(OAc)2 (BPY = 2, 2'-bipyridine, Table 2, entry 6) was 49%, which was lower than that over Cu(BPYDCDE)(OAc)2. The yield of benzaldehyde was only 1.3% over Cu(BPY)Cl2 (Table 1, entry 7), which was also much lower than Cu(BPYDCDE)Cl2.

Table 2. The effect of counterions and organic ligands Entry

Catalyst Cu(ligand)X2

pKa of HXa

1

Cu(BPYDCDE)(OAc)2

4.75

100

>99

2

Cu(BPYDCDE)(CF3COO)2

0.23

80

80

3

Cu(BPYDCDE)Cl2

-3

35

35

4

Cu(BPYDCDE)(ClO4)2

-8

11

11

5

Cu(BPYDCDE)(OTf)2

-14.7

8

8

6

Cu(BPY)(OAc)2

4.75

49

49

Conversion (%) Yield (%)


10

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7

Cu(BPY)Cl2

-3

1.3

1.3

Reaction conditions: benzyl alcohol (2 mmol), catalyst (2 mol%), TEMPO (10 mol%), CH3CN (3 mL), in air and at ambient temperature, 2 h. aHX stands for the corresponding conjugated acid of the counterion X-. Conversion and yield were analyzed by GC using n-decane as the internal standard.

Mechanistic insights. The structure of Cu(BPYDCDE)(OAc)2, was optimized using DFT as shown in Figure 2a, which is consistent with the results of ICP-AES elemental analysis, i.e., the molar ratio of Cu and the ligand is 1:1. The detailed structure parameters were shown in Electronic Supplementary Information. According to DFT calculation, the four Cu-O bonds in the Cu(BPYDCDE)(OAc)2 complex are not equivalent, as shown in Figure 2a, the bond length of two of the Cu-O bonds was 2.0 Å (Cu-O1 and Cu-O2), while the other two Cu-O bonds were longer, and the length was 2.1 Å (Cu-O3) and 2.4 Å (Cu-O4), respectively. The NPA atomic charge29 of the O2 atom is -0.747, which is the most negative among the four O atoms of the two acetate anions (O1, -0.711; O3, -0.694; O4, -0.705). It indicates the basicity of the O2 site is strengthened. Thus, the Cu and O2 formed an enhanced Lewis acid-base pair. Longer Cu-O4 bond indicates a weaker interaction between Cu and O4, which provides a site favourable for the coordination of benzyl alcohol. Meanwhile, the O2 atom which has stronger basicity is a good accepter of the proton from benzyl alcohol. As a result, the benzyl alcohol is activated. The electron density difference isosurface map (Figure 2b and 2c) shows that the coordination of the BPYDCDE ligand to Cu(OAc)2 lead to the increase of the electron density around the OAcanion, which is in favour of it to accept the proton from the benzyl alcohol and leave out as HOAc. Figure 2c shows that the electron density between N and Cu atom increased, suggesting that the BPYDCDE ligand is an electron donor. Taken together figure 2c and 2d, the electron density around Cu atom increases in some region while decreases in some other region. Thus, the


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formation of the complex also provides sites around Cu in favour to accept the lone pair electrons of the hydroxyl in benzyl alcohol.

Figure 2. a) The structure of Cu(BPYDCDE)(OAc)2 optimized by DFT method. b) The isosurface map of the electron density difference. Isovalue of 0.0004 is used. Purple and blue colours represent the electron increase and decrease region after the formation of the complex, respectively. c) isosurface map of the enhanced electron density region. d) isosurface map of the decreased electron density region.

The electronic state of Cu(BPYDCDE)(OAc)2, Cu(BPYDCDE)Cl2, Cu(BPY)(OAc)2 and Cu(OAc)2 were characterized by XPS (Figure 3). The binding energy of N1s of the ligand BPYDCDE is 398.8 eV and it moves to 399.43 eV and 399.52 eV for Cu(BPYDCDE)(OAc)2 and Cu(BPYDCDE)Cl2, respectively. The Cu binding energy of Cu(OAc)2 is 934.75 eV and it


12

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moves to 933.86 eV for Cu(BPYDCDE)(OAc)2. The results further indicate the electron interaction between ligand and Cu salts. The Cu binding energy of Cu(BPYDCDE)(OAc)2 is lower than that of Cu(BPYDCDE)Cl2 and Cu(BPY)(OAc)2, which are 934.15 eV and 934.22 eV, respectively, demonstrating that the counterions of Cu salt also greatly affect the electronic state of the Cu complex and BPYDCDE combined with OAc- enriched electrons on Cu more efficiently.

Figure 3. XPS spectra of a) N1s in BPYDCDE, Cu(BPYDCDE)Cl2 and Cu(BPYDCDE)(OAc)2 and b) Cu2p in Cu(OAc)2, Cu(BPYDCDE)Cl2, Cu(BPYDCDE)(OAc)2 and Cu(BPY)(OAc)2

As discussed above, the reported catalysts needed external base for effective reaction at ambient condition. In this work, the OAc- in the complex acts as base and the catalysts was very active and selective. It is very interesting that the amount of OAc- in the complex was much less than that of the external base added in the catalytic systems reported, indicating that the basic anion in the complex is more efficient for promoting the reaction than the external bases.


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Scheme 1. The possible reaction mechanism of aerobic oxidation of alcohol over Cu(BPYDCDE)(OAc)2.

A possible reaction mechanism was proposed (Scheme 1) for the oxidation reaction of alcohol over the Cu(BPYDCDE)(OAc)2 catalytic system based on the experimental results and the related knowledge in literature32, 43. The alcohol first coordinates to the Cu centre, and break the Cu-O4 bond, which has a bond length of 2.4 Å and is much weaker than the other three Cu-O bonds. Meanwhile, the proton of the alcohol forms hydrogen bond with the O2 atom, which has the most negative atomic charge among the four O atoms of the two OAc- anions. As shown in structure II, copper center and OAc- cooperatively activate the alcohol. Then one HOAc molecule leaves and forms complex III. As referred in previous paragraphs, BPYDCDE enhances the alkalinity of O2 atom and increases the electron density around OAc-, which are beneficial to the breaking of the H-O bond of the alcohol and the leaving of HOAc. As there


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leaves a vacant coordination site in complex III, TEMPO (2,2,6,6-tetramethylpiperidine Noxide) molecule coordinates to the Cu centre of III and forms complex IV. Then the Cα–H of the alcoholate is abstracted by the TEMPO molecule, meanwhile TEMPOH and RCHO leave and result in complex V, where the Cu(II) has been reduced to Cu(I). In the subsequent steps, Cu(II) is regenerated by oxidation of Cu(I) with oxygen, producing water as a byproduct and completing the catalytic cycle, which are similar to those reported. Scale-up experiment. Exploring catalytic system to scale up syntheses is also very interesting29. It should be noted that when the substrate scaled up to 5.0 g in our catalytic system, benzaldehyde could be produced with a yield of 100% (Scheme 2), which further indicates the potential application of the catalytic system for the direct aerobic oxidation of alcohols.

Scheme 2. The oxidation of benzyl alcohol to benzaldehyde on the gram-scale catalyzed by Cu(BPYDCDE)(OAc)2.

Substrates

scope.

The

substrate

scope

of

the

reaction

was

explored

using

Cu(BPYDCDE)(OAc)2 as the catalyst and air as the oxidant at room temperature and atmospheric pressure. As shown in Table 2, derivatives of benzyl alcohol, bearing electronwithdrawing or electron-donating groups, could all be oxidized to the corresponding desired products with yields of >99% in 2 h (1a-1h). Cu(BPYDCDE)(OAc)2 could catalyze complete conversion of 4-methylbenzyl alcohol (1a) to 4-methyl-benzaldehyde and 4-methyoxylbenzyl alcohol (1b) to 4-methoxy-benzaldehyde. The reactivity of 2-hydroxybenzyl alcohol (1c) and 4hydroxybenzyl alcohol (1d) were the same and the yield of the corresponding aldehyde could


15


Page 16 of 31

reach >99% in 2 h. The result indicates that steric hindrance had little effect on the catalytic reaction. 1, 4-Phenylenedimethanol (1e) which has two hydroxyl groups was smoothly converted into the corresponding aldehyde and the yield of terephthalaldehyde also could reach >99% in 2 h. The oxidations of polysubstituted benzyl alcohols were also studied, and the yield of the desired product for isovanillin and 3,4-dimethoxy-benzaldehyde could all approach >99% in 2 h (1f and 1g). Cu(BPYDCDE)(OAc)2 was also very active for the oxidation of 3,5bis(trifluoromethyl)benzenemethanol

(1h)

with

>99%

yield

of

3,5-

bis(trifluoromethyl)benzaldehyde in 2 h. However, 2-naphthalenemethanol (1i) was less reactive and it took 4 h for its completely oxidation to β-naphthaldehyde over the catalyst. Heterocyclic compounds were also reactive and indole-3-carbinol, pyridine-4-methanol, 2-thiophenemethanol could be converted completely to the corresponding aldehyde in 2 h (1j-1l). It took 4 h for the complete oxidation of furfuryl alcohol (1m) to furfural. Interestingly, 1-phenyl-2hydroxyethanone (1n) could be completely converted to 1,2-dicarbonyl compound 2-oxo-2phenylacetaldehyde in 2 h. The conjugated allylic alcohols were smoothly transformed into the corresponding α,β-unsaturated aldehydes (1o and 1p). The time for complete oxidation of cinnamic alcohol (1o) to cinnamaldehyde was 2 h. It took 5 h for the complete conversion of perillol (1p) to perillaldehyde. The catalyst was also active for the oxidation of alkynol. Hept-2yn-1-ol could be completely oxidized to hept-2-ynal in 2 h (1q). The catalyst was also active for secondary alcohols. For example, 1, 3-Diphenyl-1-propyn-3-ol (1r) was completely oxidized to 1,3-diphenylpropynone in 2 h. For the oxidation of 3,3’,5,5’-tetrakis(trifluoromethyl)benzhydrol (1s)

catalyzed

by

the

Cu(BPYDCDE)(OAc)2,

the

yield

of

3,3’,5,5’-tetrakis

(trifluoromethyl)benzophenone reached >99% in 2 h. For all the substrates, the selectivity of the corresponding aldehydes or ketones were all 100%.


16

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Table 3. Oxidation of various substrates to aldehydes/ketones over Cu(BPYDCDE)(OAc)2

Substrates

t(h)

Yield

2

>99%

1a

Substrates

t(h) Yield 2

>99%

1b

2

>99%

1d

2

>99%

t(h)

Yield

2

>99%

2

>99%

4

>99%

2

>99%

2

>99%

2

>99%

1c

>99%

1e

2

Substrates

1f

2

>99% 1i

1g 1h

2

>99%

2

>99%

1k

1l

1j

4

>99%

2

>99%

1n

1m

5

>99%

1o

2

>99%

1q

1p

1r

2

>99%

1s

Reaction conditions: alcohol (2 mmol), TEMPO (10 mol%), CH3CN (3 ml), stirred under air at room temperature for desired time. The conversion of alcohols and the yield of aldehydes and ketones were analysed by GC using n-decane as the internal standard except that the product of 1n is analysed by 1H NMR using s-trioxane as the internal standard. The selectivity to aldehydes for all the substrates were 100%.

CONCLUSIONS


17


Page 18 of 31

In summary, the Cu(BPYDCDE)(OAc)2 is very active and selective for the aerobic oxidation of alcohols to aldehydes or ketones without external base at ambient temperature and pressure, and the yield of desired product can approach >99% in short time. The catalytic performance of a complex depends strongly on its metal, ligand, and counterion. In addition to that Cu in the complex is a suitable metal for catalyzing the reaction, the main reason for the outstanding activity and selectivity of Cu(BPYDCDE)(OAc)2 is that the coordination of ligand and OAcenriches the electron population on the Cu center and makes OAc- in the complex be a proper alkalinity and leaving group, and the Cu center and OAc- cooperate very well for accelerating the reaction. The results of this work also indicate that the basic anion in the complex is more efficient than the external base added for promoting the reaction. We believe that the simple and efficient catalyst has a great potential of application and the strategy of this work can also be used to design some other effective catalysts without need of additives.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. 1

H NMR and 13C NMR spectra of the BPDCDE ligand; FTIR spectra of BPDCDE; structure

parameters of BPDCDE determined by DFT calculations (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Huizhen Liu); [email protected] (Buxing Han). Tel: +86 10 62562821, fax: +86 10 62562821.


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Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (21603235), the Recruitment Program of Global Youth Experts of China, and Chinese Academy of Sciences (QYZDY-SSW-SLH013) for financial supports. REFERENCES 1.

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Synopsis: A new Cu complex was designed for highly selective aerobic oxidation of alcohols at ambient condition without external base. 46x17mm (300 x 300 DPI)


Page 28 of 31

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Figure 1. a) proposed structure of Cu(BPYDCDE)(OAc)2 according to FT-IR and elemental analysis; b) the ball stick model of the optimized structure using DFT method. Cu, pink; N, blue; O, red; C, grey; H, light grey. 207x256mm (96 x 96 DPI)



Figure 2. a) The structure of Cu(BPYDCDE)(OAc)2 optimized by DFT method. b) The isosurface map of the electron density difference. Isovalue of 0.0004 is used. Purple and blue colours represent the electron increase and decrease region after the formation of the complex, respectively. c) isosurface map of the enhanced electron density region. d) isosurface map of the decreased electron density region. 374x305mm (96 x 96 DPI)


Page 30 of 31

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Figure 3. XPS spectra of a) N1s in BPYDCDE, Cu(BPYDCDE)Cl2 and Cu(BPYDCDE)(OAc)2 and b) Cu2p in Cu(OAc)2, Cu(BPYDCDE)Cl2, Cu(BPYDCDE)(OAc)2 and Cu(BPY)(OAc)2 162x53mm (220 x 220 DPI)


Base-Free Aerobic Oxidation of Alcohols over Copper-Based Complex

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