The synthesis of chiral tridentate ligands from l -proline and their application in the copper(II)-catalyzed enantioselective Henry reaction

Tetrahedron: Asymmetry 28 (2017) 954–963

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The synthesis of chiral tridentate ligands from L-proline and their application in the copper(II)-catalyzed enantioselective Henry reaction Daqian Xu a,b, Qiangsheng Sun b, Zhengjun Quan a, Wei Sun b,⇑, Xicun Wang a,⇑ a

College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu, China State Key Laboratory for Oxo Synthesis and Selective Oxidation, and Suzhou Research Center of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, Gansu, China b

a r t i c l e

i n f o

Article history: Received 7 April 2017 Revised 15 May 2017 Accepted 23 May 2017 Available online 19 June 2017

a b s t r a c t A series of chiral tridentate ligands derived from readily available enantiopure L-proline were designed and synthesized. The ligands together with Cu(OAc)2 were successfully used in asymmetric Henry reactions. Various structurally divergent aldehydes and nitromethane were converted into versatile b-nitro alcohols in MeOH at room temperature with very good yields (up to 85%) and enantioselectivities (up to 86%). Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction The Henry reaction is one of the most important CAC bond forming reactions producing b-nitro alcohols.1 Since it is highly atom economic and the resultant products can be conveniently transformed into aminoalcohol, diamines by reduction or carbonyl compounds by Nef reaction,2,3 the Henry reaction has received much attention due to its diverse applications in synthetic chemistry.4 It is not surprising that considerable effort has been devoted to the development of the catalytic asymmetric Henry reaction over the past few decades. Shibasaki et al. developed a series of bimetallic catalyst for the enantioselective Henry reaction.5–8 Since then, a great number of metal9,10 based catalysts as well as organocatalysts11,12 have been successfully established in the development of asymmetric Henry reactions. Various chiral ligands, such as Salen,13 iminopyridine,14 oxazoline,15 amino alcohol,16 Schiff,17 sulfonylamide,18 pyrrole,19 imidazole,20 and nitric oxide21 are involved. In spite of these successful achievements, the development of highly effective catalysts for catalytic asymmetric Henry reactions is still of great interest. Cocurrent activation is usually employed combining Lewis acid and Brønsted base in Henry reactions,22 in which the metal center of the complex activates the aldehyde and the chiral ligand facilitates the deprotonation of nitroalkane. Copper has exhibited effective activities in Henry reaction as a Lewis acid on the basis of

⇑ Corresponding authors. E-mail address: [email protected] (W. Sun). http://dx.doi.org/10.1016/j.tetasy.2017.05.013 0957-4166/Ó 2017 Elsevier Ltd. All rights reserved.

previous works.23 Herein we have design and prepared a series of chiral tridentate ligands derived from readily available L-proline and evaluated these ligands in copper-catalyzed asymmetric Henry reactions. Our group has focused on the development of chiral metal complexes derived from an inexpensive rigid skeleton over the past decade and these catalysts have been successfully used in many reactions, such as epoxidation.24 Encouraged by these results, we envisioned that these in situ generated chiral complexes bearing Lewis acid and Brønsted base would promote the Henry reaction efficiently under mild conditions. The chiral ligands were synthesized by introduction of benzimidazole into L-proline followed by alkylation or reductive amination as shown in Scheme 1. Different large groups were modified through reductive amination using salicylaldehydes for the stereoselectivity of the asymmetric Henry reaction. 2. Results and discussion Initially, we studied the reactivities and selectivities of the tridentate chiral ligands in Henry reactions using p-nitrobenzaldehyde and nitromethane as model substrates in the presence of anhydrous Cu(OAc)2 (5 mol %) at room temperature in ethanol. It was found that the complex generated in situ from 5 mol % L2 and Cu(OAc)2 catalyzed the Henry reaction efficiently, giving 84% yield and 76% ee (Table 1, entry 2). Although L7 proved to be effective in furnishing the desired b-nitro alcohol with 85% yield, the enantioselectivity of the reaction was extremely low (Table 1, entry 7, racemic). Other analogues of the tridentate ligand L1,

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R3

O N

OH

Boc +

R1

1. NMM, isopropyl chloroformate, THF, 0 oC, 12h

R1 N

o C,

2h 2. HOAc, 60 3.CH 3COCl, methanol, o 60 C, 2h

N H

N

R4

N

R2

O

N

NaBH(OAc)3 , 12h, methanol

TEA, 12h methanol

R1

L1: L2: L3: L4: L5: L6:

N N N

R2

R3

Cl

N

NH2

N OH

R4

NH R2

R1 OH

N

R2

L7: R 1=Me, R 2=H

R1 = Me, R 2 = H, R3 = t-Bu, R 4 = t-Bu R1 = Me, R 2 = H, R3 = Ph, R4 = H R1 = Me, R 2 = t-Bu, R3 = Ph, R4 = H R1 = Me, R 2 = Ph, R 3 = Ph, R4 = H R1 = Me, R 2 = t-Bu, R3 = H, R 4 = H R1 = Bn, R2 = H, R 3 = Ph, R4 = H

Scheme 1. Preparation of chiral tridentate ligands derived from L-proline.

Table 1 Optimization of the ligands and copper salts for the asymmetric Henry reactiona O

OH H

O2 N

a b c

1m

+

Ln, Cu II/CuI

CH 3NO2

(S)

NO2

EtOH, rt, 24h O 2N

2

Entry

Lewis acid (%)

Ligand (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) Cu(OAc)2 (5 mol %) CuCl2 (5 mol %) CuBr2 (5 mol %) Cu(OTf)2 (5 mol %) CuOTf (5 mol %) CuI (5 mol %) Cu(OAc)2 (2 mol %) Cu(OAc)2 (1 mol %) Cu(OAc)2 (10 mol %)

L1 L2 L3 L4 L5 L6 L7 L2 L2 L2 L2 L2 L2 L2 L2

(5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (5 mol %) (2 mol %) (1 mol %) (10 mol %)

3m

Yield (%)b 75 84 65 66 76 68 85 — — — — 69 82 56 88

ee (%)c 54 76 63 69 56 71 rac — — — — 70 81 79 79

Reactions were performed with p-nitrobenzaldehyde 1 (0.5 mmol), Ln (5 mol %), copper salt (5 mol %), and nitromethane 2 (5 mmol) in EtOH (1 mL) at room temperature. Isolated yields after column chromatography. Determined by HPLC on a chiral stationary phase with a Daicel OD-H column.

L3–L6 were also examined, however, the yields and enantioselectivities of the reactions were not improved (Table 1, entries 1, 3– 6). Different copper salts were tested using 5 mol % L2 as the chiral ligand but the results did not improve (Table 1, entries 8–12). Additionally, the amount of Lewis acid Cu(OAc)2 and Brønsted base L2 were screened (Table 1, entries 13–15). It was shown that 2 mol % of the catalyst proved to be the best loading for the asymmetric Henry reaction giving 82% yield with 81% ee at room temperature (Table 1, entry 13). Furthermore, the effect of solvents and temperature on Henry reaction was evaluated carefully. The results revealed that the polar protic solvents (Table 2, entries 1–2) were better than aprotic solvents (Table 2, entries 3–7). When the Henry reaction was carried out in MeOH at room temperature, 85% yield and 86% ee were obtained (Table 2, entry 2). Since the Henry reaction was reversible, we also studied the Henry reaction using substrate 2 as the solvent to improve the yield. The yield and enantioselectivity of the reaction in nitromethane were better than those reactions in other

aprotic solvents (Table 2, entry 8, 80% yield and 76% ee). However, the results obtained in MeNO2 were not as good as in MeOH (Table 2, entry 2 vs 8). We then studied the temperature effect on the Henry reaction in MeOH. Unfortunately, lowering the reaction temperature to 0 °C and
10 °C did not improve the enantioselectivities of the Henry reaction while the yield decreased (Table 2, entries 9–10). Therefore, the best result was achieved in the presence of 2 mol % anhydrous Cu(OAc)2 and L2 in MeOH at room temperature, which gave b-nitro alcohol in 85% yield and with 86% ee after 24 h. With the optimized conditions in hand, we next explored the scope of the Henry reaction with respect to aldehydes. In general, both aliphatic and aromatic aldehydes were well tolerated and very good yields (up to 85%) and enantioselectivities (up to 86%) were obtained under mild conditions as shown in Table 3. First, benzaldehyde 1a gave the desired product in 76% yield and with 77% ee under the standard reaction condition (Table 3, entry 1). Benzaldehydes 1b–1g bearing electron-donating groups on the

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D. Xu et al. / Tetrahedron: Asymmetry 28 (2017) 954–963

Table 2 The effect of solvents and temperature on the asymmetric Henry reactiona

Me O H + CH 3NO2 O2 N

1m

(S)

solvent, T, 24h

N

NO 2

N OH

O2 N

2

N

OH

L2 (2 mol%), Cu(OAc)2 (2 mol%)

L2

3m Ph

a b c

Entry

Solvent

T (°C)

1 2 3 4 5 6 7 8 9 10

EtOH MeOH PhCH3 MeCN THF Et2O CH2Cl2 CH3NO2 MeOH MeOH

25 25 25 25 25 25 25 25 0
10

Yield (%)b

ee (%)c

82 85 72 70 66 68 78 80 60 17

81 86 60 45 61 60 65 76 86 85

Reactions were performed with p-nitrobenzaldehyde 1 (0.5 mmol), L2 (2 mol %), anhydrous Cu(OAc)2 (2 mol %), and nitromethane (5 mmol) in 1 mL solvent. Isolated yields after column chromatography. Determined by HPLC on a chiral stationary phase with a Daicel OD-H column.

Table 3 Substrate scope of the asymmetric Henry reaction with respect to the aldehydesa

Me

O + R

CH 3NO2

H

MeOH, rt

1

N

L2 (2 mol%), Cu(OAc) 2 (2 mol%)

N

OH R

NO 2

(S)

2

N OH

3

L2 Ph

a b c

Entry

R

Time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

C6H5 1a 4-Me-C6H4 1b 3-Me-C6H4 1c 2-Me-C6H4 1d 4-MeO-C6H4 1e 3-MeO-C6H4 1f 4-t-Bu-C6H4 1g 4-Cl-C6H4 1h 3-Cl-C6H4 1i 2,4-DiCl-C6H3 1j 4-Br-C6H4 1k 3-Br-C6H4 1l 4-NO2-C6H4 1m 3-NO2-C6H4 1n 4-CF3-C6H4 1o Heliotropin 1p 1-Naphthaldehyde 1q Furfural 1r CH3(CH2)6 1s C6H5(CH2)2 1t

48 48 48 48 48 24 48 24 48 48 48 48 24 24 24 48 48 48 48 48

Yield (%)b 76 78 73 65 79 76 81 76 74 67 80 78 85 84 83 72 70 62 71 69

ee (%)c 77 85 86 63 83 75 84 70 82 48 80 81 86 84 76 75 64 77 82 67

Reactions were performed with aldehyde (0.5 mmol) and nitromethane (5 mmol) in MeOH (1 mL) at room temperature. Isolated yields after column chromatography. Determined by HPLC on chiral stationary phase.

benzene ring were investigated (Table 3, entries 2–7). It was found that Me-, MeO- or t-Bu substituted substrates underwent the Henry reaction smoothly with good yields (up to 81%) and enantioselectivities (up to 86%). 2-Me-benzaldehyde 1d was also transformed successfully although the yield and enantioselectivity were

moderate (Table 3, entry 4). We evaluated the electron-withdrawing groups on benzene ring of benzaldehydes (Table 3, entries 8–15). Halogenated benzaldehydes 1h–1l and benzaldehydes bearing strongly electron-withdrawing groups 1m–1o, such as ANO2, ACF3, generated the products with good yields (up to 85%) and

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enantioselectivities (up to 86%). Similarly, it was observed that benzaldehyde 1j bearing a substituent at the ortho-position (Table 3, entry 10) only afforded moderate yield (67%) and low enantioselectivity (48%). Furthermore, piperonal 1p, 1-naphthaldehyde 1q and furfural 1r were also suitable for the Henry reaction and formed the corresponding b-nitro alcohols with moderate yields and enantioselectivities (Table 3, entries 16–18). It is noteworthy that aliphatic aldehydes 1s and 1t were converted successfully with moderate yields and good enantioselectivities (Table 3, entries 19–20). The absolute configuration of the products was assigned as (S) by comparison of HPLC chromatogram and the specific rotation with those reported in the literature.22 In order to rationalize the stereoselective outcome of the asymmetric Henry reaction, we propose that the steric hindrance of the benzene ring at the ortho-position of salicylaldehyde repulses the benzene ring of the substrates as shown in Figure 1. Nitromethane coordinates with copper complex at the weak coordination site to release the acetate ion, which facilitates the following deprotonation, while the aldehydes coordinate with the copper center at the rest of strong coordination site.22b Thus, nitromethane attacks the aldehydes through the Re face to afford (S)-enriched products.

R1

NH NH 2

O +

N

OH

Boc

4. Experimental 4.1. General All solvents and commercial reagents were used as supplied without further purification. Room temperature (rt) refers to 20– 25 °C. NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer operating at 400 MHz for 1H NMR and 100 MHz for 13C NMR in deuterated solvent. The chemical shifts (d) are quoted in ppm and coupling constants (J) in Hz. All coupling constants were determined by analysis using MestReNova v6.1.0 software. High resolution mass spectra (HRMS) were acquired by Bruker microTOF-QII spectrometer (ESI) at 296(2)K. GC-MS was acquired by an Agilent 7890A/5975C. TLC inspections were on silica gel GF254 plates and TLC visualization was carried out with ultraviolet light (254 nm). Flash column chromatography was generally performed on silica gel (200–300 mesh). 4.2. Synthesis of ligands 4.2.1. Synthesis of (S)-2-(pyrrolidin-2-yl)-1H-benzimidazole

R1

1. NMM, isopropyl chloroformate, THF, 0 oC, 12h

N

oC,

2h 2. HOAc, 60 3. CH 3COCl, methanol, 60 oC, 2h

N H

N

R2

2

R

R 1 = Me, Bn R 2 = t-Bu, Ph, H

3. Conclusion In conclusion, a series of chiral tridentate ligands were prepared successfully from readily available L-proline. The resulting in situ chiral copper complexes in the presence of anhydrous Cu(OAc)2 delivered the asymmetric Henry reaction smoothly to provide (S)-enriched b-nitro alcohols with very good yields (up to 85%) and enantioselectivities (up to 86%) under mild conditions in MeOH. A range of aldehydes were well tolerated in the asymmetric Henry reaction without any additives or bases. In particular, aliphatic aldehydes gave the desired products in good yield (71%) and enantioselectivity (82%). Further investigations into the modification of chiral tridentate ligands and the mechanism of the stereoselective outcome are underway in our laboratory.

O O

H

Cu II OAc R

O

N

N O

A solution of L-Boc-proline (2.15 g, 10 mmol) and N-methyl morpholine (NMM, 1 mL, 10 mmol) in THF (20 mL) was treated at 0 °C with isobutyl chloroformate (1.2 mL, 10 mmol). After 10 min at 0 °C, 1,2-phenylenediamine (10 mmol) was added. The reaction mixture was allowed to stir while slowly warming to room temperature (1 h) and then stirred for 24 h. The solvent was evaporated, and the residue was extracted with EtOAc from H2O. The EtOAc layer was washed with 5% NaHCO3, brine and dried over Na2SO4. The solution was filtered, the solvent was evaporated, and the residual solid was dissolved in glacial AcOH (10 mL). The solution was heated at 65 °C for 1 h. After the solvent was evaporated, the residue was purified using column chromatography (petroleum ether/

H Me N N

O N O

O

Si-attack H

disfavored

H N

C u II OAc H

H Me N N O

Re-attack R

favored

Figure 1. The proposed mechanism of the asymmetric Henry reaction.

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ethyl acetate = 3:1) to give the corresponding benzimidazole as a white solid. A solution of the derived benzimidazole was re-dissolved in methanol (20 mL) and cooled to 0 °C, after which 1 mL acetyl chloride was added slowly. After 10 min at 0 °C, the system was heated to 60 °C for 2 h, after which the solvent was evaporated, and the residue was dissolved in CHCl3 (10 mL). Next 3 M NaOH was added and the pH was adjusted to 9, extracted with another two portions of CHCl3 (2  10 mL). The combined the organic layers were washed with NaHCO3, brine and dried over Na2SO4. After the solvent was evaporated, the residue was purified using column chromatography (methanol/ ethyl acetate 1:2) to give the (S)-2(pyrrolidin-2-yl)-1H-benzimidazole as a white solid (90–95% yield).

N N OH

L1, White solid, 60% yield, ½a20 D =
32.0 (c 0.1, CHCl3), mp 151– 153 °C, 1H NMR (400 MHz, CDCl3) d 10.58 (s, 1H), 7.85–7.71 (m, 1H), 7.24–7.25 (m, 3H), 7.16 (s, 1H), 6.56 (s, 1H), 4.10 (d, J = 13.1 Hz, 2H), 3.49 (s, 2H), 3.41 (d, J = 13.2 Hz, 1H), 3.34–3.21

R3

R1

OH R1

N H

N

O

N

R4

N

NaBH(OAc)3 , 12h, methanol

R2

N

N

R2

OH R4

4.2.2. Synthesis of L1–L7 A solution of (S)-2-(pyrrolidin-2-yl)-1H-benzimidazole was dissolved in methanol (20 mL) under an argon atmosphere at room temperature, after which salicylaldehyde (1.1 equiv) was added and stirred for 2 h until the solvent changed to yellow. Next NaBH(OAc)3 (1.5 equiv) was added in two portions, stirred for another 10 h when the raw materials were consumed completely detected by TLC. The solvent was evaporated and quenched with NH4Cl aq., then extracted with CHCl3 (3  10 mL). The organic layers were combined and washed with NaHCO3, brine and dried over Na2SO4. After the solvent was evaporated, the residue was purified using column chromatography (methanol/ethyl acetate 1:10) to give the target product L1–L6 as a white solid, 45–65% yield.

N

R3

(m, 1H), 2.71 (d, J = 7.8 Hz, 1H), 2.51–2.35 (m, 1H), 2.28–2.13 (m, 2H), 2.01–2.17 (m, 1H), 1.40 (s, 9H), 1.18 (s, 9H). 13C NMR (101 MHz, CDCl3) d 154.2, 154.0, 142.2, 140.2, 135.7, 135.4, 123.2, 122.9, 122.5, 122.1, 121.7, 119.7, 109.1, 57.1, 53.4, 34.9, 34.0, 31.6, 31.1, 29.6, 23.5. HRMS (ESI) calcd for C27H37N3ONa [M +Na]+: 442.2829, found: 442.2824.

N N

N OH Ph

A solution of (S)-2-(pyrrolidin-2-yl)-1H-benzimidazole (1 mmol) was dissolved in methanol (20 mL) under an argon atmosphere at room temperature. Next 2-chloromethyl-pyridine (1.1 equiv) was added with TEA (1.5 equiv) and stirred for 10 h. The solvent was evaporated and then water added, and extracted with CHCl3 (3  10 mL). The organic layers were combined and washed with NaHCO3, brine and dried over Na2SO4. After the solvent was evaporated, the residue was purified using column chromatography (methanol/ethyl acetate 1:10) to give the target product L7 as a brown oil (248 mg), 85% yield.

L2, White solid, 65% yield, ½a20 D = +70.0 (c 0.1, CHCl3), mp 173– 175 °C, 1H NMR (400 MHz, CDCl3) d 11.01 (s, 1H), 7.76 (ddd, J = 3.8, 2.3, 0.6 Hz, 1H), 7.69–7.56 (m, 2H), 7.41 (dd, J = 10.4, 4.7 Hz, 2H), 7.35–7.17 (m, 5H), 6.89 (dd, J = 7.4, 1.4 Hz, 1H), 6.78 (t, J = 7.5 Hz, 1H), 4.26 (d, J = 13.1 Hz, 1H), 4.09–3.91 (m, 1H), 3.67 (s, 3H), 3.37 (d, J = 13.2 Hz, 1H), 3.31–3.20 (m, 1H), 2.56 (dd, J = 16.9, 7.9 Hz, 1H), 2.46–2.31 (m, 1H), 2.16–1.99 (m, 2H), 1.99– 1.84 (m, 1H). 13C NMR (101 MHz, CDCl3) d 154.6, 154.5, 138.9, 136.0, 130.1, 129.6, 128.9, 128.3, 127.9, 126.6, 123.4, 122.6, 122.2, 119.8, 118.7, 109.0, 60.5, 56.8, 53.2, 30.7, 30.0, 23.1. HRMS (ESI) calcd for C25H25N3O2Na [M+Na]+: 406.1889, found:406.1894.

N N

N OH Ph

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D. Xu et al. / Tetrahedron: Asymmetry 28 (2017) 954–963

L3, White solid, 55% yield, ½a20 D = +46.0 (c 0.1, CHCl3), mp 210– 212 °C, 1H NMR (400 MHz, CDCl3) d 7.79 (d, J = 1.4 Hz, 1H), 7.69– 7.56 (m, 2H), 7.49–7.33 (m, 3H), 7.31 (dt, J = 9.2, 4.3 Hz, 1H), 7.24 (dd, J = 9.3, 3.3 Hz, 2H), 6.88 (dd, J = 7.3, 1.3 Hz, 1H), 6.78 (t, J = 7.5 Hz, 1H), 4.24 (d, J = 13.2 Hz, 1H), 4.15–4.03 (m, 1H), 3.65 (s, 3H), 3.44 (s, 1H), 3.28 (dd, J = 6.3, 2.6 Hz, 1H), 2.60 (dd, J = 16.5, 7.7 Hz, 1H), 2.48–2.34 (m, 1H), 2.23–2.06 (m, 2H), 1.96 (ddd, J = 11.8, 9.5, 3.0 Hz, 1H), 1.39 (d, J = 4.1 Hz, 9H). 13C NMR (101 MHz, CDCl3) d 154.6, 145.7, 142.0, 138.7, 133.9, 130.0, 129.5, 128.8, 128.1, 127.9, 126.6, 123.2, 120.7, 118.7, 116.0, 108.4, 100.0, 77.3, 77.2, 77.0, 76.7, 60.3, 56.7, 53.2, 34.8, 31.8, 30.7, 30.0, 23.1. HRMS (ESI) calcd for C29H33N3ONa [M+Na]+: 462.2516, found: 462.2512.

L6, White solid, 45% yield, ½a20 D = +42.0 (c 0.1, CHCl3), mp 112– 114 °C, 1H NMR (400 MHz, CDCl3) d 8.45 (dd, J = 4.8, 0.8 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.40 (td, J = 7.7, 1.8 Hz, 1H), 7.26 (ddd, J = 7.2, 6.0, 3.2 Hz, 4H), 7.19–7.06 (m, 3H), 7.03–6.92 (m, 3H), 5.95 (d, J = 16.9 Hz, 1H), 5.48 (d, J = 16.9 Hz, 1H), 4.11 (t, J = 8.2 Hz, 1H), 3.80 (dd, J = 115.1, 13.6 Hz, 2H), 3.22 (dd, J = 11.8, 4.9 Hz, 1H), 2.43 (dd, J = 17.2, 8.6 Hz, 1H), 2.19 (ddd, J = 15.8, 7.8, 5.0 Hz, 1H), 2.03–1.72 (m, 3H). 13C NMR (101 MHz, CDCl3) d 158.0, 154.4, 148.7, 142.2, 136.5, 136.1, 136.1, 128.8, 127.5, 125.9, 123.3, 122.7, 122.1, 121.8, 119.5, 109.9, 63.2, 60.2, 53.9, 47.1, 30.6, 22.9. HRMS (ESI) calcd for C31H29N3ONa [M+Na]+: 482.2203, found: 482.2207.

N N N

N

N

N

Ph N

OH Ph

L4, White solid, 61% yield, ½a20 D = +40.0 (c 0.1, CHCl3), mp 81– 83 °C, 1H NMR (400 MHz, CDCl3) d 7.98 (d, J = 1.2 Hz, 1H), 7.63 (dd, J = 12.4, 4.8 Hz, 4H), 7.53 (d, J = 1.6 Hz, 1H), 7.42 (dt, J = 17.0, 7.6 Hz, 4H), 7.37–7.27 (m, 3H), 7.22 (dt, J = 12.3, 6.4 Hz, 1H), 6.90 (dd, J = 13.0, 6.8 Hz, 1H), 6.78 (dd, J = 9.4, 5.6 Hz, 1H), 4.24 (d, J = 13.3 Hz, 1H), 4.14–4.02 (m, 1H), 3.68 (s, 3H), 3.44 (d, J = 13.3 Hz, 1H), 3.36–3.23 (m, 1H), 2.58 (m, 1H), 2.42 (m, 1H), 2.25–2.10 (m, 3H), 1.96 (m, 1H). 13C NMR (101 MHz, CDCl3) d 154.9, 154.5, 142.3, 141.8, 138.7, 136.1, 135.5, 130.1, 129.5, 128.8, 128.7, 128.1, 127.9, 127.5, 126.7, 126.6, 123.2, 122.5, 118.8, 118.1, 109.3, 60.5, 56.9, 53.4, 30.7, 30.1, 23.2. HRMS (ESI) calcd for C31H29N3ONa [M+Na]+: 482.2203, found: 482.2200.

N N

N OH

L5, White solid, 60% yield, ½a20 D = +17.0 (c 0.1, CHCl3), mp 178– 180 °C, 1H NMR (400 MHz, CDCl3) d 7.84 (d, J = 1.4 Hz, 1H), 7.38 (dd, J = 8.5, 1.8 Hz, 1H), 7.29–7.22 (m, 1H), 7.17 (dd, J = 11.0, 4.3 Hz, 1H), 6.95 (dd, J = 12.6, 7.0 Hz, 2H), 6.73 (td, J = 7.3, 0.9 Hz, 1H), 4.30 (d, J = 12.9 Hz, 1H), 4.02–3.92 (m, 1H), 3.71 (s, 3H), 3.21 (dd, J = 15.6, 8.1 Hz, 2H), 2.51–2.36 (m, 2H), 2.08–1.87 (m, 3H), 1.39 (s, 9H). 13C NMR (101 MHz, CDCl3) d 157.6, 155.3, 133.9, 129.1, 128.8, 123.6, 120.6, 118.5, 116.6, 116.0, 108.2, 56.4, 53.0, 34.8, 31.9, 30.8, 29.9, 23.0. HRMS (ESI) calcd for C23H29N3ONa [M +Na]+: 386.2203, found: 386.2198.

1 L7, Brown oil, 85% yield, ½a20 D = +36.0 (c 0.1, CHCl3), H NMR (400 MHz, CDCl3) d 8.45 (dd, J = 4.8, 0.8 Hz, 1H), 7.78–7.64 (m, 1H), 7.47 (td, J = 7.7, 1.8 Hz, 1H), 7.37–7.13 (m, 4H), 7.06–6.91 (m, 1H), 4.11 (t, J = 8.3 Hz, 1H), 3.95 (s, 3H), 3.95–3.58 (m, 2H), 3.35–3.18 (m, 1H), 2.47 (dd, J = 17.1, 8.9 Hz, 1H), 2.40–2.31 (m, 1H), 2.20–2.05 (m, 2H), 1.96 (ddd, J = 7.5, 6.0, 2.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 158.5, 154.1, 148.8, 136.7, 136.1, 123.1, 122.3, 121.8, 119.4, 108.9, 63.6, 60.3, 54.0, 30.4, 30.3, 23.0. HRMS (ESI) calcd for C18H20N4Na [M+Na]+: 315.1580, found: 315.1577.

4.3. General procedure for asymmetric Henry reaction Ligand L2 (2 mol %) and anhydrous Cu(OAc)2 were added to MeOH (1 mL) in a 10 mL flask at room temperature under air condition, after which the substrate (0.5 mmol) and CH3NO2 were added after stirring for 1 h and the mixture was stirred for a certain time. After the reaction was completed, the mixture was concentrated under vacuum. The crude residue was purified directly by flash column chromatography with silica gel to give pure products. The absolute configuration of the products was assigned as (S) by comparison of HPLC chromatogram and optical rotation to the literature. 4.3.1. (S)-2-Nitro-1-phenylethanol 3a16e

OH NO 2

Ph N N

N OH Ph

Colorless oil (76% yield, 77% ee). Chiral HPLC analysis with Chiralcel OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 18.4 min, tR(major) = 22.8 min; ½a20 D = +17.0 (c 0.2, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.55 – 7.27 (m, 5H), 5.40 (dd, J = 9.6, 3.1 Hz, 1H), 4.56 (dd, J = 13.3, 9.6 Hz, 1H), 4.46 (dd, J = 13.3, 3.1 Hz, 1H), 3.10 (s, 1H).13C NMR (101 MHz, CDCl3) d 138.1, 129.0, 128.9, 126.0, 81.2, 71.0.

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4.3.2. (S)-1-(4-Methyphenyl)-2-nitroethanol 3b25

OH NO2

Colorless oil (78% yield, 85% ee). Chiral HPLC analysis with Chiralcel AD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 15.3 min, tR(major) = 14.3 min; ½a20 D = +26.0 (c 1.6, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.19 (m, 4H), 5.32 (dt, J = 9.4, 3.3 Hz, 1H), 4.52 (dd, J = 13.1, 9.6 Hz, 1H), 4.41 (dd, J = 13.1, 3.3 Hz, 1H), 3.16 (d, J = 3.8 Hz, 1H), 2.33 (s, 3H). 13C NMR (101 MHz, CDCl3) d 138.8, 135.3, 129.6, 125.9, 81.2, 70.8, 21.1.

Colorless oil (76% yield, 83% ee). Chiral HPLC analysis with Chiralcel OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C);); tR(minor) = 14.7 min, tR(major) = 18.9 min; 1 ½a20 D = +15.5 (c 0.6, CHCl3). H NMR (400 MHz, CDCl3) d 7.28 (d, J = 8.6 Hz, 2H), 6.95 – 6.85 (m, 2H), 5.35 (dd, J = 9.6, 3.1 Hz, 1H), 4.56 (dd, J = 13.1, 9.6 Hz, 1H), 4.44 (dd, J = 13.1, 3.2 Hz, 1H), 3.79 (s, 3H), 3.09 (s, 1H). 13C NMR (101 MHz, CDCl3) d 159.9, 130.3, 127.3, 114.3, 81.2, 70.6, 55.3. 4.3.6. (S)-1-(3-Methoxyphenyl)-2-nitroethanol 3f25

OH NO 2

OMe

4.3.3. (S)-1-(3-Methyphenyl)-2-nitroethanol 3c25

OH NO 2

Colorless oil (73% yield, 86% ee). Chiral HPLC analysis with Chiralcel AD-H column (95/5 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 12.8 min, tR(major) = 12.1 min; ½a20 D = +27.0 (c 0.8, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.26 (t, J = 7.5 Hz, 1H), 7.21 – 7.10 (m, 3H), 5.34 (dd, J = 9.6, 3.1 Hz, 1H), 4.53 (dd, J = 13.2, 9.6 Hz, 1H), 4.43 (dd, J = 13.2, 3.1 Hz, 1H),3.13 (s, 1H), 2.35 (s, 3H). 13C NMR (101 MHz, CDCl3) d 138.8, 138.2, 129.6, 128.9, 126.6, 123.0, 81.2, 71.0, 21.4.

Colorless oil (76% yield, 75% ee). Chiral HPLC analysis with Chiralcel OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 24.5 min, tR(major) = 32.6 min; ½a20 D = +14.5 (c 0.6, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.44 (dd, J = 7.5, 1.4 Hz, 1H), 7.33 (ddt, J = 8.0, 5.1, 2.6 Hz, 1H), 7.01 (td, J = 7.5, 0.8 Hz, 1H), 6.91 (d, J = 8.2 Hz, 1H), 5.63 (ddd, J = 9.3, 6.2, 3.3 Hz, 1H), 4.65 (dd, J = 13.1, 3.3 Hz, 1H), 4.57 (dd, J = 13.1, 9.2 Hz, 1H), 3.88 (s, 3H), 3.16 (d, J = 6.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 136.3, 134.5, 130.8, 128.7, 126.7, 125.6, 80.2, 67.9, 18.8. 4.3.7. (S)-1-(4-tBu-phenyl)-2-nitroethanol 3g

OH NO2

4.3.4. (S)-1-(2-Methyphenyl)-2-nitroethanol 3d25

OH NO 2

Colorless oil (65% yield, 63% ee). Chiral HPLC analysis with Chiralcel OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 21.6 min, tR(major) = 19.8 min; ½a20 D = +9.0 (c 0.2, CHCl3) 1H NMR (400 MHz, CDCl3) d 7.52 – 7.40 (m, 1H), 7.28 – 7.19 (m, 2H), 7.18 – 7.08 (m, 1H), 5.60 (dt, J = 9.6, 3.1 Hz, 1H), 4.47 (dd, J = 13.3, 9.7 Hz, 1H), 4.36 (dd, J = 13.3, 3.1 Hz, 1H). 3.09 (d, J = 3.6 Hz, 1H), 2.34 (s, 3H). 13C NMR (101 MHz, CDCl3) d 136.3, 134.5, 130.8, 128.7, 126.7, 125.6, 80.2, 67.9, 18.8. 4.3.5. (S)-1-(4-Methoxyphenyl)-2-nitroethanol 3e16e

OH NO2 MeO

White solid (81% yield, 84% ee), mp 66–68 °C. Chiral HPLC analysis with Chiralcel OD-H column (90/10 hexanes:isopropanol, 0.8 mL/min, 215 nm, 25 °C); tR(minor) = 16.9 min, tR(major) 1 = 24.7 min; ½a20 D = +11.6 (c 1.2, EtOH). H NMR (400 MHz, CDCl3) d 7.46 – 7.37 (d, J = 8.2 Hz, 2H), 7.32 (d, J = 8.3 Hz, 2H), 5.41 (dd, J = 9.7, 3.0 Hz, 1H), 4.59 (dd, J = 13.3, 9.7 Hz, 1H), 4.48 (dd, J = 13.3, 3.0 Hz, 1H), 2.81 (s, 1H), 1.31 (s, 9H). 13C NMR (101 MHz, CDCl3) d 152.1, 135.1, 125.9, 125.7, 81.2, 70.8, 34.6, 31.2. 4.3.8. (S)-1-(4-Chlorophenyl)-2-nitroethanol 3h16e

OH NO 2 Cl

Colorless oil (76% yield, 70% ee). Chiral HPLC analysis with AD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 18.3 min, tR(major) = 23.7 min; ½a20 D = +24.0 (c 0.2, EtOH). 1H NMR (400 MHz, CDCl3) d 7.53–7.26 (m, 4H), 5.41 (dt,

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J = 9.3, 3.4 Hz, 1H), 4.55 (dd, J = 13.3, 9.4 Hz, 1H), 4.47 (dd, J = 13.3, 3.2 Hz, 1H), 3.25 (d, J = 3.9 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 136.6, 134.7, 129.2, 127.3, 81.0, 70.3. 4.3.9. (S)-1-(3-Chlorophenyl)-2-nitroethanol 3i26

Colorless oil (78% yield, 81% ee). Chiral HPLC analysis with AD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 10.5 min, tR(major) = 13.6 min; ½a20 D = +19.8 (c 0.4, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.53 (d, J = 1.7 Hz, 1H), 7.46 (m, 1H), 7.31 – 7.19 (m, 2H), 5.38 (dt, J = 8.8, 3.2 Hz, 1H), 4.64 – 4.29 (m, 2H), 3.45 (d, J = 3.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 140.4, 132.0, 130.6, 129.1, 124.6, 123.0, 80.9, 70.2.

OH

4.3.13. (S)-2-Nitro-1-(4-nitrophenyl)ethanol 3m16e

NO 2

OH

Cl

Colorless oil (74% yield, 82% ee). Chiral HPLC analysis with AD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 11.1 min, tR(major) = 13.8 min; ½a20 D = +28.3 (c 2.6, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.41 (s, 1H), 7.36 – 7.31 (m, 2H), 7.31 – 7.21 (m, 1H), 5.42 (dd, J = 9.2, 3.3 Hz, 1H), 4.64 – 4.35 (m, 2H), 3.15 (s, 1H). 13C NMR (101 MHz, CDCl3) d 140.1, 134.9, 130.3, 129.1, 126.2, 124.1, 80.9, 70.3 4.3.10. (S)-1-(2,4-Dichlorophenyl)-2-nitroethanol 3j25

OH NO 2 Cl

NO 2 O2 N

Yellow oil (85% yield, 86% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 15.8 min, tR(major) = 19.4 min; ½a20 D = +16.5 (c 0.2, CHCl3). 1H NMR (400 MHz, CDCl3) d 8.23 (d, J = 8.7 Hz, 2H), 7.63 (d, J = 8.7 Hz, 2H), 5.63 (dd, J = 7.4, 4.8 Hz, 1H), 4.67–4.55 (m, 2H), 3.49 (s, 1H). 13C NMR (101 MHz, CDCl3) d 148.0, 145.2, 127.0, 124.1, 80.6, 70.0. 4.3.14. (S)-2-Nitro-1-(3-nitrophenyl)ethanol 3n25

Cl

White solid (67% yield, 48% ee), mp 68–70 °C. Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 6.6 min, tR(major) = 6.8 min; ½a20 D = +8.0 (c 0.4, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.61 (d, J = 8.4 Hz, 1H), 7.41 (d, J = 2.0 Hz, 1H), 7.34 (dd, J = 8.4, 2.0 Hz, 1H), 5.79 (ddd, J = 9.4, 3.8, 2.4 Hz, 1H), 4.65 (dd, J = 13.7, 2.3 Hz, 1H), 4.42 (dd, J = 13.7, 9.5 Hz, 1H), 3.15 (d, J = 4.2 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 135.2, 134.1, 132.0, 129.5, 128.5, 128.00, 79.0, 67.4. 16e

4.3.11. (S)-1-(4-Bromophenyl)-2-nitroethanol 3k

OH NO 2

OH NO 2

NO2

White solid (84% yield, 84% ee), mp 74–75 °C. Chiral HPLC analysis with AD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 15.8 min, tR(major) = 17.9 min; ½a20 D = +12.9 (c 1.0, CHCl3). 1H NMR (400 MHz, CDCl3) d 8.32 (dd, J = 2.6, 1.2 Hz, 1H), 8.21 (ddd, J = 8.2, 2.2, 0.9 Hz, 1H), 7.79 (d, J = 7.7 Hz, 1H), 7.62 (t, J = 8.0 Hz, 1H), 5.63 (dd, J = 8.2, 4.2 Hz, 1H), 4.77 – 4.51 (m, 2H), 3.31 (s, 1H).13C NMR (101 MHz, CDCl3) d 148.5, 140.2, 132.0, 130.1, 123.8, 121.1, 80.6, 69.8.

Br

Colorless oil (80% yield, 80% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 10.9 min, tR(major) = 14.0 min; ½a20 (c 1, D = +13.5 CHCl3). 1H NMR (400 MHz, CDCl3) d 7.51 (d, J = 8.4 Hz, 2H), 7.26 (d, J = 8.4 Hz, 2H), 5.40 (dd, J = 9.3, 3.1 Hz, 1H), 4.51 (qd, J = 16.6, 13.3, 6.3 Hz, 2H), 3.25 (s, 1H). 13C NMR (101 MHz, CDCl3) d 137.1, 132.1, 127.6, 122.9, 80.9, 70.3.

4.3.15. (S)-2-Nitro-1-(4-trifluoromethylphenyl)ethanol 3o27

OH NO 2 F3 C

4.3.12. (S)-1-(3-Bromophenyl)-2-nitroethanol 3l25

OH NO 2

Br

Colorless oil (83% yield, 76% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 0.8 mL/min, 215 nm, 25 °C); tR(minor) = 16.9 min, tR(major) = 24.7 min; ½a20 D = +11.6 (c 1.2, EtOH). 1H NMR (400 MHz, CDCl3) d 7.66 (d, J = 8.2 Hz, 2H), 7.54 (d, J = 8.2 Hz, 2H), 5.53 (dd, J = 9.1, 3.3 Hz, 1H), 4.56 (qd, J = 13.5, 6.3 Hz, 2H), 3.23 (s, 1H). 13C NMR (101 MHz, CDCl3) d 141.9, 130.6, 127.8, 126.3, 119.7, 80.8, 70.3.

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4.3.16. (S)-1-(Benzo[d][1,3]dioxol-5-yl)-2-nitroethan-1-ol 3p

OH NO2

O

(s, 1H), 1.63–1.42 (m, 3H), 1.29 (m, 9H), 0.88 (m, 3H). 13C NMR (101 MHz, CDCl3) d 80.71, 6.7, 33.7, 31.7, 29.2, 29.0, 25.17, 22.6, 14.0. 4.3.20. (S)-1-Nitro-4-phenylbutan-2-ol 3t16e

O

White solid (72% yield, 75% ee), mp 87–89 °C. Chiral HPLC analysis with IC column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 15.3 min, tR(major) = 12.7 min; ½a20 D = +28.0 (c 0.5, EtOH). 1H NMR (400 MHz, CDCl3) d 7.00 – 6.72 (m, 3H), 5.37 (dd, J = 9.5, 3.1 Hz, 1H), 4.57 (dd, J = 13.3, 9.5 Hz, 2H), 4.47 (dd, J = 13.3, 3.1 Hz, 2H), 2.73 (s, 1H).13C NMR (101 MHz, CDCl3) d 148.2, 148.1, 131.9, 119.6, 108.6, 106.3, 101.4, 81.2, 70.8. 4.3.17. (S)-2-Nitro-1-(naphthyl)ethanol 3q16e

OH NO2

NO2 OH

Colorless oil (69% yield, 67% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 17.1 min, tR(major) = 18.5 min; ½a20 (c 0.2, D = +6.0 CHCl3). 1H NMR (400 MHz, CDCl3) d 7.30 (m, 2H), 7.25–7.14 (m, 3H), 4.42–4.35 (m, 2H), 4.35–4.24 (m, 1H), 2.85 (m, J = 14.4, 9.0, 5.7 Hz, 1H), 2.79 – 2.66 (m, 2H), 2.09 – 1.55 (m, 2H). 13C NMR (101 MHz, CDCl3) d 140.6, 128.6, 128.4, 126.3, 80.6, 67.8, 35.1, 31.3. Acknowledgments We thank the National Natural Science Foundation of China (21473226, 21362031) for financial support.

White solid (70% yield, 64% ee), mp 51–53 °C. Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 12.5 min, tR(major) = 19.1 min; ½a20 D = +5.0 (c 1.0, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.99 (d, J = 8.2 Hz, 1H), 7.90–7.86 (m, 1H), 7.82 (d, J = 8.2 Hz, 1H), 7.71 (d, J = 7.2 Hz, 1H), 7.60 – 7.41 (m, 3H), 6.25 – 6.13 (m, 1H), 4.69 – 4.54 (m, 2H), 3.03 (s, 1H). 13C NMR (101 MHz, CDCl3) d 133.7, 133.6, 129.5, 129.4, 129.3, 127.1, 126.1, 125.5, 123.8, 121.8, 80.8, 68.3. 4.3.18. (S)-2-Nitro-1-(furanyl)ethanol 3r28

OH O

NO2

Colorless oil (62% yield, 77% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 14.4 min, tR(major) = 15.5 min; ½a20 D = +11.0 (c 0.2, CHCl3). 1H NMR (400 MHz, CDCl3) d 7.42 (d, J = 12.2 Hz, 1H), 6.37 (t, J = 2.1 Hz, 2H), 5.43 (dd, J = 9.1, 3.6 Hz, 1H), 4.75 (dd, J = 13.4, 9.1 Hz, 1H), 4.64 (dd, J = 13.4, 3.6 Hz, 1H), 3.37 (s, 1H). 13C NMR (101 MHz, CDCl3) d 150.7, 143.1, 110.6, 108.2, 78.3, 64.7. 4.3.19. (S)-1-Nitrononan-2-ol 3s23l

OH NO 2

Colorless oil (71% yield, 82% ee). Chiral HPLC analysis with OD-H column (90/10 hexanes:isopropanol, 1.0 mL/min, 215 nm, 25 °C); tR(minor) = 28.5 min, tR(major) = 42.5 min; ½a20 D = +14.0 (c 0.2, CHCl3). 1H NMR (400 MHz, CDCl3) d 4.52 – 4.34 (m, 3H), 2.91

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