Palladium-catalyzed regio- and enantioselective hydroesterification of aryl olefins with CO gas

Jingfu Li a, Wenlong Ren a, Jie Dai a and Yian Shi *ab
aState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
bDepartment of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA. E-mail: Yian.Shi@colostate.edu

Received 23rd July 2017 , Accepted 17th September 2017

First published on 18th September 2017


An effective Pd-catalyzed regio- and enantioselective hydroesterification of aryl olefins with CO gas is described. A variety of phenyl 2-arylpropanoates can be obtained in good yields with high b/l ratios and ees.


Introduction

Carboxylic acids and esters are a class of important compounds for pharmaceuticals, fine chemicals, and organic synthesis. For example, 2-arylpropanoic acids such as ibuprofen, naproxen, ketoprofen, and fenoprofen are highly effective nonsteroidal anti-inflammatory agents (Fig. 1).1 Asymmetric hydrocarboxylation and hydroesterification of olefins would provide attractive approaches to optically active acids and esters (Scheme 1). During the last few decades, a variety of such processes have been reported with various degrees of success.2–4 In general, the main challenge is to achieve high regio- and enantioselectivity simultaneously. Recently, we reported an asymmetric hydroesterification of aryl olefins with phenyl formate to give optically active phenyl 2-arylpropanoates in generally high regio- and enantioselectivity.5 During our mechanistic studies, it was found that such a process was also feasible with CO gas. While various CO surrogates have been developed to avoid handling toxic CO gas,6 in many cases, the use of CO gas is still more cost-effective particularly for a large scale reaction process, which prompted us to further investigate and optimize the aforementioned CO-based asymmetric hydroesterification (Scheme 2). Herein, we wish to report our studies on this subject.
image file: c7qo00622e-f1.tif
Fig. 1 Examples of anti-inflammatory 2-arylpropanoic acids.

image file: c7qo00622e-s1.tif
Scheme 1 Hydrocarbonylation of olefins.

image file: c7qo00622e-s2.tif
Scheme 2 Regio- and enantioselective hydroesterification.

Results and discussion

Styrene (1a) was used as the test substrate for the initial studies. The asymmetric hydroesterification was first investigated with different pressures of CO in the presence of 5 mol% Pd(OAc)2, 10 mol% (R)-(−)-DTBM-SEGPHOS,7 and PhOH (1.2 equiv.) in n-hexane at 50 °C (Table 1, entries 1–9). The reaction yield increased significantly (from 43 to 87%) as the CO pressure was reduced from 4.0 to 0.1 MPa. Generally, a high b/l ratio and enantioselectivity were obtained at different pressures of CO with 3.0 to 0.3 MPa being optimal. Among the solvents examined (Table 1, entries 5 and 10–16), n-hexane gave the best yield, b/l ratio, and enantioselectivity. It was notable that ester 2a was also formed in a reasonable yield (53%) with a high b/l ratio and ee when the reaction was carried out in water (Table 1, entry 16). The b/l ratio and ee decreased as the reaction temperature increased while the yield might increase in some cases (Table 1, entries 17–19 vs. entry 5).
Table 1 Studies on the reaction conditionsa

image file: c7qo00622e-u1.tif

Entry CO (MPa) Solvent Temp (°C) Yieldb (%) (2a[thin space (1/6-em)]:[thin space (1/6-em)]3a)c ee (%) (2a)
a The reactions were carried out with styrene (1a) (0.50 mmol), Pd(OAc)2 (0.025 mmol), (R)-(−)-DTBM-SEGPHOS (0.050 mmol), CO, PhOH (0.60 mmol), and solvent (0.50 mL) for 48 h. b The yield was determined from a crude reaction mixture by 1H NMR with 1-methoxy-4-methylbenzene as an internal standard. c The ratio of 2a[thin space (1/6-em)]:[thin space (1/6-em)]3a was determined by 1H NMR analysis of the crude reaction mixture.
1 4.0 n-Hex 50 43 (16[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
2 3.0 n-Hex 50 45 (22[thin space (1/6-em)]:[thin space (1/6-em)]1) 96
3 2.0 n-Hex 50 53 (26[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
4 1.0 n-Hex 50 71 (23[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
5 0.5 n-Hex 50 77 (25[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
6 0.4 n-Hex 50 76 (24[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
7 0.3 n-Hex 50 79 (25[thin space (1/6-em)]:[thin space (1/6-em)]1) 96
8 0.2 n-Hex 50 81 (13[thin space (1/6-em)]:[thin space (1/6-em)]1) 96
9 0.1 n-Hex 50 87 (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 94
10 0.5 None 50 37 (5[thin space (1/6-em)]:[thin space (1/6-em)]1) 93
11 0.5 Tol 50 35 (8[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
12 0.5 MEK 50 Trace
13 0.5 THF 50
14 0.5 EtOAc 50 Trace
15 0.5 DMF 50
16 0.5 H2O 50 53 (20[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
17 0.5 n-Hex 60 95 (15[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
18 0.5 n-Hex 70 91 (9[thin space (1/6-em)]:[thin space (1/6-em)]1) 92
19 0.5 n-Hex 80 71 (5[thin space (1/6-em)]:[thin space (1/6-em)]1) 85


The substrate scope for the asymmetric hydroesterification reaction was investigated with the optimized reaction conditions [5 mol% Pd(OAc)2, 10 mol% (R)-(−)-DTBM-SEGPHOS, CO (0.50 MPa), and PhOH (1.2 equiv.) in n-hexane (0.50 mL) at 50 °C]. As shown in Table 2, the reaction process can be extended to a variety of aryl olefins. Ester 2a was isolated in 72% yield with >20[thin space (1/6-em)]:[thin space (1/6-em)]1 b/l ratio and 97% ee (Table 2, entry 1). With para-substituted styrenes, the corresponding 2-arylpropanoates were obtained in 72–88% yield and 92–96% ee with the b/l ratio ranging from 16[thin space (1/6-em)]:[thin space (1/6-em)]1 to >20[thin space (1/6-em)]:[thin space (1/6-em)]1 (Table 2, entries 2–8). The substrates can bear various substituents including OMe, alkyl, phenyl, Cl, and F groups. meta-Substituted styrenes were also effective for the reaction process. The branched esters were isolated in 84–95% yield and 89–96% ee with >20[thin space (1/6-em)]:[thin space (1/6-em)]1 b/l ratio (Table 2, entries 9–11). Poorer results were obtained with ortho-methylstyrene, likely due to the steric effect of the Me group (Table 2, entry 12). The asymmetric hydroesterification process was also effective for di-substituted styrenes, giving the corresponding esters in 50–89% yield and 87–93% ee with 7[thin space (1/6-em)]:[thin space (1/6-em)]1 to >20[thin space (1/6-em)]:[thin space (1/6-em)]1 b/l ratio (Table 2, entries 13–15). Heteroaryl olefins such as 2-vinylpyridine, 1-vinylimidazole and aliphatic alkenes such as allyltrimethylsilane and 4-pentenyl acetate were not effective substrates under the current reaction conditions.

Table 2 Hydroesterification of olefinsa

image file: c7qo00622e-u2.tif

Entry Substrate (1) Yieldb (%) (b[thin space (1/6-em)]:[thin space (1/6-em)]l)c eed (%)
a The reactions were carried out with olefin (1) (0.50 mmol), Pd(OAc)2 (0.025 mmol), (R)-(−)-DTBM-SEGPHOS (0.050 mmol), CO (0.50 MPa), PhOH (0.60 mmol), and n-hexane (0.50 mL) at 50 °C for 48 h. b Isolated yield. c The b/l ratio was determined by 1H NMR analysis of the crude reaction mixture. d For entries 1, 3, and 7, the absolute configurations were determined by comparing the optical rotations with the reported values.5,8 For entries 2, 4–6, and 8–15, the absolute configurations were tentatively assigned by analogy.
image file: c7qo00622e-u3.tif
1 X = H, 1a 72 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 97
2 X = OMe, 1b 84 (16[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
3 X = iBu, 1c 73 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
4 X = tBu, 1d 80 (20[thin space (1/6-em)]:[thin space (1/6-em)]1) 92
5 X = Me, 1e 72 (16[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
6 X = Ph, 1f 84 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 96
7 X = Cl, 1g 88 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
8 X = F, 1h 82 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
image file: c7qo00622e-u4.tif
9 X = OMe, 1i 95 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 95
10 X = Me, 1j 84 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 96
11 X = Cl, 1k 88 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 89
12 image file: c7qo00622e-u5.tif 49 (5[thin space (1/6-em)]:[thin space (1/6-em)]1) 89
13 image file: c7qo00622e-u6.tif 87 (14[thin space (1/6-em)]:[thin space (1/6-em)]1) 91
14 image file: c7qo00622e-u7.tif 89 (>20[thin space (1/6-em)]:[thin space (1/6-em)]1) 93
15 image file: c7qo00622e-u8.tif 50 (7[thin space (1/6-em)]:[thin space (1/6-em)]1) 87


A precise reaction mechanism is not fully understood at this moment and requires further study. A plausible hydroesterification catalytic cycle is proposed in Scheme 3. Complex 4 generated from Pd(0), CO, and PhOH hydropalladated the olefin to give complexes 5 and 6, which were converted into acylpalladium complexes 7 and 8 after migratory insertion. Upon reductive elimination, 7 and 8 were transformed to the esters with 2 being favoured.


image file: c7qo00622e-s3.tif
Scheme 3 Proposed hydroesterification catalytic cycle.

Conclusions

In summary, we have developed an efficient Pd-catalyzed regio- and enantioselective hydroesterification of aryl olefins with CO gas under mild reaction conditions, giving a wide variety of phenyl 2-arylpropanoates in good yield with a high b/l ratio and ee. The reaction process is operationally simple and provides a potentially useful method for the synthesis of optically active 2-arylpropanoates and their derivatives. The development of more effective catalytic systems with a broad substrate scope is currently being pursued.

Experimental

General methods

All commercially available reagents were used without further purification unless otherwise stated. All solvents used for the reaction were purified with a solvent purification system. Column chromatography was performed on silica gel (200–300 mesh). 1H NMR spectra were recorded on a 400 MHz NMR spectrometer and 13C NMR spectra were recorded on a 100 MHz NMR spectrometer. IR spectra were recorded on a FT-IR spectrometer. Melting points were uncorrected. (R)-(−)-DTBM-SEGPHOS was purchased from a commercial supplier. Olefins 1a, 1b, 1d–l, and 1o were purchased from commercial suppliers. Olefins 1c, 1m, and 1n were prepared from the corresponding aldehydes via the Wittig reaction.9a

Representative procedure for hydroesterification (Table 2, entry 1)

To a mixture of Pd(OAc)2 (0.0056 g, 0.025 mmol), (R)-(−)-DTBM-SEGPHOS (0.059 g, 0.050 mmol), PhOH (0.0564 g, 0.60 mmol), and n-hexane (0.50 mL) in a high pressure reactor (12.0 mL) was added styrene (1a) (0.0521 g, 0.50 mmol) via a syringe. The reactor was purged with CO three times to remove the air, filled with CO gas (0.5 MPa), and tightly sealed. The reaction mixture was stirred at 50 °C for 48 h, cooled to rt, and purified by flash chromatography (silica gel, eluent: petroleum ether/ethyl acetate = 100/1) to give compound 2a as a colorless oil (0.0815 g, 72% yield, 97% ee).

2a.5,8a,9b Colorless oil; [α]D20 = −93.7 (c 0.50, CHCl3) (97% ee); IR (film) 1749, 1487 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.45–7.27 (m, 7H), 7.23–7.16 (m, 1H), 7.03–6.96 (m, 2 H), 3.97 (q, J = 7.2 Hz, 1H), 1.62 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.3, 151.0, 140.3, 129.6, 129.0, 127.8, 127.6, 126.0, 121.6, 45.9, 18.8; HRMS (ESI) calcd for C15H15O2 (M + H): 227.1067; found: 227.1066.

2b.5,9 Colorless oil; [α]D20 = −83.2 (c 0.54, CHCl3) (95% ee); IR (film) 1749, 1507 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.37–7.29 (m, 4H), 7.23–7.16 (m, 1H), 7.02–6.96 (m, 2H), 6.94–6.87 (m, 2H), 3.91 (q, J = 7.2 Hz, 1H), 3.82 (s, 3H), 1.59 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.5, 159.1, 151.1, 132.3, 129.5, 128.8, 125.9, 121.6, 114.4, 55.5, 45.0, 18.8; HRMS (ESI) calcd for C16H16NaO3 (M + Na): 279.0992; found: 279.0990.

2c.5,9b Colorless oil; [α]D20 = −76.0 (c 0.51, CHCl3) (95% ee); IR (film) 1753, 1491 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.39–7.27 (m, 4H), 7.23–7.17 (m, 1H), 7.17–7.11 (m, 2H), 7.03–6.96 (m, 2H), 3.95 (q, J = 7.1 Hz, 1H), 2.48 (d, J = 7.2 Hz, 2H), 1.95–1.81 (m, 1H), 1.61 (d, J = 7.2 Hz, 3H), 0.93 (s, 3H), 0.91 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 173.5, 151.1, 141.0, 137.5, 129.7, 129.5, 127.4, 125.9, 121.6, 45.5, 45.3, 30.4, 22.6, 18.7; HRMS (ESI) calcd for C19H23O2 (M + H): 283.1693; found: 283.1690.

2d. White solid; mp. 49–51 °C; [α]D20 = −72.2 (c 0.92, CHCl3) (92% ee); IR (film) 1745, 1487 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.43–7.36 (m, 2H), 7.36–7.31 (m, 4H), 7.24–7.17 (m, 1H), 7.06–7.00 (m, 2H), 3.96 (q, J = 7.2 Hz, 1H), 1.62 (d, J = 7.2 Hz, 3H), 1.35 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 173.4, 151.1, 150.4, 137.2, 129.5, 127.4, 125.9, 121.6, 45.4, 34.7, 31.6, 18.8; HRMS (ESI) calcd for C19H23O2 (M + H): 283.1693; found: 283.1691.

2e.5 Colorless oil; [α]D20 = −89.9 (c 1.05, CHCl3) (95% ee); IR (film) 1753, 1487 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.28 (m, 4H), 7.23–7.16 (m, 3H), 7.04–6.97 (m, 2H), 3.94 (q, J = 7.2 Hz, 1H), 2.37 (s, 3H), 1.61 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.4, 151.1, 137.3, 137.2, 129.7, 129.5, 127.6, 125.9, 121.6, 45.4, 21.3, 18.8; HRMS (ESI) calcd for C16H16NaO2 (M + Na): 263.1043; found: 263.1037.

2f.5,9b White solid; mp. 74–76 °C; [α]D20 = −84.2 (c 0.68, CHCl3) (96% ee); IR (film) 1740, 1478 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.63–7.56 (m, 4H), 7.50–7.39 (m, 4H), 7.38–7.29 (m, 3H), 7.24–7.15 (m, 1H), 7.05–6.98 (m, 2H), 4.00 (q, J = 7.1 Hz, 1H), 1.65 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.2, 151.0, 140.9, 140.5, 139.3, 129.5, 129.0, 128.2, 127.7, 127.5, 127.3, 126.0, 121.6, 45.5, 18.7; HRMS (ESI) calcd for C21H19O2 (M + H): 303.1380; found: 303.1376.

2g.5,9b Colorless oil; [α]D20 = −79.4 (c 0.63, CHCl3) (95% ee); IR (film) 1752, 1490 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.31 (m, 6H), 7.24–7.17 (m, 1H), 7.01–6.95 (m, 2H), 3.94 (q, J = 7.2 Hz, 1H), 1.61 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 172.9, 150.9, 138.7, 133.5, 129.6, 129.2, 126.1, 121.5, 45.3, 18.7; HRMS (ESI) calcd for C15H1335ClNaO2 (M + Na): 283.0496; found: 283.0494.

2h.5 Colorless oil; [α]D20 = −79.3 (c 0.59, CHCl3) (95% ee); IR (film) 1753, 1507 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.42–7.31 (m, 4H), 7.24–7.17 (m, 1H), 7.10–7.02 (m, 2H), 7.02–6.95 (m, 2H), 3.96 (q, J = 7.2 Hz, 1H), 1.61 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.1, 162.3 (d, J = 244.0 Hz), 151.0, 136.0 (d, J = 4.0 Hz), 129.6, 129.4 (d, J = 8.0 Hz), 126.1, 121.5, 115.9 (d, J = 21.0 Hz), 45.1, 18.8; HRMS (ESI) calcd for C15H13FNaO2 (M + Na): 267.0792; found: 267.0793.

2i.5,9b Colorless oil; [α]D20 = −75.7 (c 0.61, CHCl3) (95% ee); IR (film) 1753, 1489 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.24 (m, 3H), 7.23–7.16 (m, 1H), 7.03–6.93 (m, 4H), 6.84 (dd, J = 8.2, 2.0 Hz, 1H), 3.94 (q, J = 7.2 Hz, 1H), 3.82 (s, 3H), 1.61 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.1, 160.1, 151.0, 141.8, 130.0, 129.5, 126.0, 121.6, 120.1, 113.5, 112.9, 55.5, 45.9, 18.7; HRMS (ESI) calcd for C16H16NaO3 (M + Na): 279.0992; found: 279.0994.

2j.5,9b Colorless oil; [α]D20 = −87.8 (c 0.54, CHCl3) (96% ee); IR (film) 1752, 1486 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.37–7.29 (m, 2H), 7.29–7.23 (m, 1H), 7.22–7.15 (m, 3H), 7.14–7.08 (m, 1H), 7.02–6.96 (m, 2H), 3.92 (q, J = 7.1 Hz, 1H), 2.37 (s, 3H), 1.60 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.4, 151.0, 140.2, 138.7, 129.5, 128.9, 128.5, 128.3, 125.9, 124.7, 121.6, 45.8, 21.7, 18.8; HRMS (ESI) calcd for C16H17O2 (M + H): 241.1223; found: 241.1219.

2k.5 Colorless oil; [α]D20 = −69.2 (c 0.58, CHCl3) (89% ee); IR (film) 1749, 1487 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.43–7.39 (m, 1H), 7.38–7.31 (m, 2H), 7.31–7.27 (m, 3H), 7.24–7.18 (m, 1H), 7.03–6.97 (m, 2H), 3.94 (q, J = 7.2 Hz, 1H), 1.62 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 172.7, 150.9, 142.1, 134.8, 130.3, 129.6, 128.1, 127.9, 126.1, 126.0, 121.5, 45.6, 18.6; HRMS (ESI) calcd for C15H1335ClNaO2 (M + Na): 283.0496; found: 283.0498.

2l.5,9b Colorless oil; [α]D20 = −92.7 (c 0.22, CHCl3) (89% ee); IR (film) 1752, 1486 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.29 (m, 3H), 7.26–7.16 (m, 4H), 7.01–6.96 (m, 2H), 4.20 (q, J = 7.1 Hz, 1H), 2.45 (s, 3H), 1.58 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.6, 151.0, 138.9, 135.9, 130.9, 129.5, 127.4, 126.8, 126.7, 125.9, 121.6, 41.7, 19.9, 18.1; HRMS (ESI) calcd for C16H17O2 (M + H): 241.1223; found: 241.1218.

2m.5,9b Colorless oil; [α]D20 = −86.3 (c 0.48, CHCl3) (91% ee); IR (film) 1752, 1490 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.31 (m, 2H), 7.23–7.17 (m, 2H), 7.17–7.13 (m, 2H), 7.04–6.99 (m, 2H), 3.91 (q, J = 7.1 Hz, 1H), 2.29 (s, 3H), 2.27 (s, 3H), 1.60 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.5, 151.1, 137.8, 137.2, 135.9, 130.2, 129.5, 129.0, 125.9, 125.0, 121.6, 45.4, 20.1, 19.6, 18.9; HRMS (ESI) calcd for C17H18NaO2 (M + Na): 277.1199; found: 277.1194.

2n. Colorless oil; [α]D20 = −65.8 (c 0.61, CHCl3) (93% ee); IR (film) 1753, 1454 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.38–7.31 (m, 2H), 7.23–7.17 (m, 1H), 7.04–6.98 (m, 2H), 6.56 (d, J = 2.2 Hz, 2H), 6.40 (t, J = 2.3 Hz, 1H), 3.89 (q, J = 7.1 Hz, 1H), 3.81 (s, 6H), 1.60 (d, J = 7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.0, 161.2, 151.0, 142.5, 129.5, 126.0, 121.6, 105.9, 99.5, 55.6, 46.0, 18.7; HRMS (ESI) calcd for C17H18NaO4 (M + Na): 309.1097; found: 309.1093.

2o.9b White solid; mp. 60–61 °C; [α]D20 = −89.4 (c 0.90, CHCl3) (87% ee); IR (film) 1745, 1491 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.90–7.82 (m, 4H), 7.55 (dd, J = 8.5, 1.8 Hz, 1H), 7.53–7.45 (m, 2H), 7.37–7.29 (m, 2H), 7.22–7.16 (m, 1H), 7.02–6.96 (m, 2H), 4.14 (q, J = 7.1 Hz, 1H), 1.71 (d, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 173.3, 151.0, 137.7, 133.7, 132.9, 129.6, 128.8, 128.1, 127.9, 126.54, 126.48, 126.2, 126.0, 125.8, 121.6, 46.0, 18.7; HRMS (ESI) calcd for C19H16NaO2 (M + Na): 299.1043; found: 299.1045.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (21632005, 21472083) and Nanjing University for the financial support.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/C7QO00622E

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