Novel Effect of Zinc Nitrate/Vanadyl Oxalate for Selective Catalytic Oxidation of α-Hydroxy Esters to α-Keto Esters with Molecular Oxygen: An In Situ ATR-IR Study

Selective oxidation of α-hydroxy esters is one of the most important methods to prepare high value-added α-keto esters. An efficient catalytic system consisting of Zn(NO3)2/VOC2O4 is reported for catalytic oxidation of α-hydroxy esters with molecular oxygen. Up to 99% conversion of methyl DL-mandelate or methyl lactate could be facilely obtained with high selectivity for its corresponding α-keto ester under mild reaction conditions. Zn(NO3)2 exhibited higher catalytic activity in combination with VOC2O4 compared with Fe(NO3)3 and different nitric oxidative gases were detected by situ attenuated total reflection infrared (ATR-IR) spectroscopy. UV-vis and ATR-IR results indicated that coordination complex formed in Zn(NO3)2 in CH3CN solution was quite different from Fe(NO3)3; it is proposed that the charge-transfer from Zn2+ to coordinated nitrate groups might account for the generation of different nitric oxidative gases. The XPS result indicate that nitric oxidative gas derived from the interaction of Zn(NO3)2 with VOC2O4 could be in favor of oxidizing VOC2O4 to generate active vanadium (V) species. It might account for different catalytic activity of Zn(NO3)2 or Fe(NO3)3 combined with VOC2O4. This work contributes to further development of efficient aerobic oxidation under mild reaction conditions.

General procedure for synthesis of benzaldehyde: Benzyl alcohol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After 3 the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 4 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of 4-nitrobenzaldehyde: 4-Nitrobenzyl alcohol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 4 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of 2-thenaldehyde: 2-Thiophenemethanol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 1.5 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of 2-pyridinecarboxaldehyde: 2-Pyridinemethanol (5 mmol), , and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 1.5 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of acetophenone: Phenylethyl alcohol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 12 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of 3-pyridinecarboxaldehyde: 3-Pyridinemethanol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 6 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of 2-pentanone: 2-Amyl alcohol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the 4 autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 12 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples.
General procedure for synthesis of cyclohexanone: Cyclohexanol (5 mmol), VOC 2 O 4 ·2H 2 O (0.25 mmol), Zn(NO 3 ) 2 ·6H 2 O (0.25 mmol), and CH 3 CN (5 mL) were placed in autoclave (25 mL). After the autoclave was closed, oxygen was added (0.2 MPa). The mixture was stirred at 80 °C for 12 h. After the reaction was completed, the autoclave was cooled to room temperature. The resulting mixture was determined by GC and comparison with the authentic samples. Figure S1. The FT-IR spectrum of VOC 2 O 4 ·2H 2 O. Figure S2. UV-vis spectra of Zn(NO 3 ) 2 or Fe(NO 3 ) 3 in CH 3 CN solvent. Figure S3. ATR-IR spectra in the C-N region of Zn(NO 3 ) 2 /Fe(NO 3 ) 3 in CH 3 CN solvent

The effect of catalyst loading and Zn(NO3)2 loading for catalytic oxidation of methyl DLmandelate
The results for catalytic oxidation of methyl DL-mandelate under different catalyst loading and Zn(NO 3 ) 2 loading are shown in Table S1. Firstly, catalytic oxidation of methyl DL-mandelate with different catalyst loading was performed under the same reaction conditions (80 °C, 0.2 MPa O 2 , 1.5 h).
It was found that the conversion of methyl DL-mandelate increased with an increase of the catalyst loading (Table S1, Entries 1-5). The reaction in the presence of 1 mol% catalyst afforded 6% conversion of methyl DL-mandelate (Table S1, Entry 1), and the conversion of methyl DL-mandelate increased slightly even by prolonging the reaction time from 1.5 h to 7.5 h (Table S1, Entry 2). In contrast, up to 99% methyl DL-mandelate conversion with above 99% methyl phenylglyoxylate selectivity was obtained using 5 mol% catalyst (Table S1, Entry 4), and high conversion and selectivity could maintain in the presence of 10 mol% catalyst under the same reaction conditions (Table S1, Entry 5). Therefore, the optimization of catalyst loading is 5 mol%. Subsequently, the effect of Zn(NO 3 ) 2 loading for catalytic oxidation of methyl DL-mandelate was investigated under the same reaction conditions. It was found that the conversion of methyl DL-mandelate firstly increased from 6% to 99% and then decreased to 89% with an increase of the molar ratio of Zn(NO 3 ) 2 /VOC 2 O 4 from 0 to 4, and the selectivity toward for methyl phenylglyoxylate remained basically unchanged (Table S1, Entries 6-11). Hence, Zn(NO 3 ) 2 loading has an important effect for catalytic oxidation of methyl DL-mandelate, the optimization of molar 6 ratio of Zn(NO 3 ) 2 /VOC 2 O 4 is 1, and at this point the conversion of methyl DL-mandelate and selectivity for methyl phenylglyoxylate were more than 99% (Table S1, Entry 9). Selectivity toward for methyl phenylglyoxylate.

Catalytic oxidation of various alcohols
A wide variety of alcohol substrates such as benzylic and aliphatic alcohols were tested using Zn(NO 3 ) 2 /VOC 2 O 4 as catalyst under mild reaction conditions (80 °C, 0.2 MPa O 2 ). The summarized results for catalytic oxidation of alcohols are shown in Table S2. We are pleased to find that various benzylic alcohols such as benzyl alcohol, 4-nitrobenzyl alcohol, 2-thiophenemethanol, 2pyridinemethanol undergo selective oxidation to afford the corresponding aldehydes with excellent yields using Zn(NO 3 ) 2 /VOC 2 O 4 as catalyst (Table S2, Entries 1-4). Up to 99% conversion of benzyl alcohol and 4-nitrobenzyl alcohol was transformed within 4 h, and the selectivity of benzaldehyde and 4-nitrobenzaldehyde was more than 99% (Table S2, Entries 1-2). Furthermore, nearly quantitative conversion of 2-thiophenemethanol and 2-pyridinemethanol could be facilely obtained within 1.5 h (Table S2, Entries 3-4), 84% conversion of phenylethyl alcohol with 80% selectivity of acetophenone 7 was obtained by prolonging the reaction time to 12 h (Table S2, Entry 5), whereas only 20% of the 3pyridinemethanol was oxidized after 6 h (Table S2, Entry 6). Subsequently, the aliphatic alcohols are difficult to be oxidized, for the cyclohexanol and 2-amyl alcohol, 6% and 20% conversion was obtained even by prolonging the reaction time to 12 h, respectively (Table S2, Entries 7-8).