Application of Halogen-Bonding Catalysis for Markovnikov-Type Hydrothiolation of Alkenes

Xue Zhang; Nuoyu Liang; Ruining Li; Zhankui Sun

doi:10.1055/a-1984-9105

Synlett, Table of Contents

Synlett 2023; 34(04): 379-387
DOI: 10.1055/a-1984-9105

letter

Application of Halogen-Bonding Catalysis for Markovnikov-Type Hydrothiolation of Alkenes

Xue Zhang

,

Nuoyu Liang

,

Ruining Li

,

Zhankui Sun ∗

Abstract

All articles of this category

Abstract

Carbon–sulfur bond-formation reactions are applied widely in organic synthesis and chemical biology. Hydrothiolation of alkenes provides a direct way to build carbon–sulfur bonds. Most known methods proceed via radical processes and result in anti-Markovnikov-type products. Herein, we demonstrate that I₂ catalyzes the hydrothiolation of alkenes and provides Markovnikov-type products in good to excellent yields. Titration studies revealed that thiol was activated by I₂ through halogen bonding. This metal-free reaction has advantages such as green and mild conditions, high functionality tolerance, broad substrate scope, and atom economy. Its application was further demonstrated in peptide syntheses.

Key words

thiols - halogen bonding - hydrothiolation - Markovnikov reaction - alkenes - addition

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References

References and Notes

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7 General Procedure: To a 25 mL single-necked bottle were added thiol compound (2.00 mmol, 2.0 equiv), styrene derivative (1.00 mmol, 1.0 equiv), iodine (0.20 mmol, 0.2 equiv), and EtOAc (6 mL). The mixture was stirred at room temperature for 7 hours. The mixture was concentrated and the residue was purified by column chromatography on silica with PE/EtOAc mixture as the eluent. Ethyl 2-((2-Phenylpropan-2-yl)thio)acetate (3a) The product was obtained as colorless oil (224 mg, 94% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.56–7.50 (m, 2 H), 7.32 (t, J = 7.7 Hz, 2 H), 7.21 (t, J = 7.3 Hz, 1 H), 4.02 (q, J = 7.1 Hz, 2 H), 2.97 (s, 2 H), 1.73 (s, 6 H), 1.20 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 145.5, 128.3, 126.8, 126.7, 61.3, 48.5, 32.59, 30.1, 14.1. HRMS (ESI): m/z calcd for C₁₃H₁₈NaO₂S [M + Na]⁺: 261.0920; found: 261.0921. IR (film): 2966, 2929, 1736, 1447, 1366, 1264, 1130, 1074, 1030, 699 cm^–1. Ethyl 2-((2-(Naphthalen-1-yl)propan-2-yl)thio)acetate (3b) The product was obtained as colorless oil (245 mg, 85% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 9.13 (d, J = 8.6 Hz, 1 H), 7.87 (d, J = 7.9 Hz, 1 H), 7.79 (d, J = 8.1 Hz, 1 H), 7.57–7.46 (m, 3 H), 7.41 (t, J = 7.7 Hz, 1 H), 3.80 (q, J = 7.1 Hz, 2 H), 2.92 (s, 2 H), 1.98 (s, 6 H), 1.06 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 139.4, 135.0, 131.1, 129.2, 129.1, 127.3, 125.3, 124.8, 124.5, 61.0, 48.8, 32.8, 31.6, 13.8. HRMS (ESI): m/z calcd for C₁₇H₂₀NaO₂S [M + Na]⁺: 311.1076; found: 311.1082. IR (film): 2966, 2361, 1735, 1509, 1388, 1263, 1119, 1073, 1031, 802, 778 cm^–1. Ethyl 2-((2-(Naphthalen-2-yl)propan-2-yl)thio)acetate (3c) The product was obtained as colorless oil (242 mg, 84% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.83 (dt, J = 12.2, 4.2 Hz, 5 H), 7.52–7.45 (m, 2 H), 3.94 (q, J = 7.1 Hz, 2 H), 2.96 (s, 2 H), 1.85 (s, 6 H), 1.12 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.6, 142.8, 132.9, 132.3, 128.2, 128.2, 127.4, 126.2, 126.1, 125.7, 124.7, 61.2, 48.8, 32.7, 29.9, 14.0. HRMS (ESI): m/z calcd for C₁₇H₂₀NaO₂S [M + Na]⁺: 311.1076; found: 311.1079. IR (film): 2977, 2931, 2361, 1735, 1274, 1124, 1030, 817, 749, 477 cm^–1. Ethyl 2-((2-(Anthracen-2-yl)propan-2-yl)thio)acetate (3d) The product was obtained as colorless oil (315 mg, 93% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 9.78 (s, 1 H), 8.45 (s, 1 H), 8.18–8.11 (m, 1 H), 8.02–7.93 (m, 2 H), 7.50 (dd, J = 8.0, 3.9 Hz, 3 H), 7.40 (t, J = 7.8 Hz, 1 H), 3.77 (q, J = 7.1 Hz, 2 H), 2.95 (s, 2 H), 2.07 (s, 6 H), 1.01 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.6, 139.2, 133.3, 131.0, 130.8, 129.7, 129.4, 128.9, 127.5, 127.4, 126.9, 125.9, 125.5, 125.2, 124.1, 61.1, 49.3, 33.0, 31.7, 13.9. HRMS (ESI): m/z calcd for C₂₁H₂₂NaO₂S [M + Na]⁺: 361.1233; found: 361.1262. IR (film): 2976, 2928, 2361, 1734, 1265, 1121, 1031, 880, 735, 473 cm^–1. Ethyl 2-((2-(4-Methoxyphenyl)propan-2-yl)thio)acetate (3e) The product was obtained as colorless oil (254 mg, 95% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.42 (d, J = 8.8 Hz, 2 H), 6.82 (d, J = 8.8 Hz, 2 H), 4.02 (q, J = 7.1 Hz, 2 H), 3.76 (s, 3 H), 2.94 (s, 2 H), 1.68 (s, 6 H), 1.18 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 158.2, 137.4, 127.8, 113.4, 61.1, 55.2, 48.1, 32.5, 30.1, 14.0. HRMS (ESI): m/z calcd for C₁₄H₂₀KO₃S [M + K]⁺: 307.0765; found: 307.0712. IR (film): 2967, 2935, 1735, 1610, 1512, 1254, 1184, 1130, 1033, 832 cm^–1. Ethyl 2-((2-(p-Tolyl)propan-2-yl)thio)acetate (3f) The product was obtained as colorless oil (246 mg, 97% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.41 (d, J = 8.2 Hz, 2 H), 7.12 (d, J = 8.0 Hz, 2 H), 4.03 (q, J = 7.1 Hz, 2 H), 2.96 (s, 2 H), 2.32 (s, 3 H), 1.71 (s, 6 H), 1.20 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.6, 142.4, 136.2, 128.8, 126.5, 61.1, 48.3, 32.5, 30.0, 20.9, 14.0. HRMS (ESI): m/z calcd for C₁₄H₂₀NaO₂S [M + Na]⁺: 275.1076; found: 275.1079. IR (film): 2966, 2924, 1735, 1512, 1365, 1264, 1129, 1094, 1032, 818 cm^–1. Ethyl 2-((2-(4-Fluorophenyl)propan-2-yl)thio)acetate (3g) The product was obtained as colorless oil (230 mg, 90% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.46 (dd, J = 8.6, 5.5 Hz, 2 H), 6.96 (t, J = 8.7 Hz, 2 H), 4.00 (q, J = 7.1 Hz, 2 H), 2.93 (s, 2 H), 1.67 (s, 6 H), 1.17 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 161.5 (d, J = 246 Hz), 141.3 (d, J = 3.0 Hz), 128.4 (d, J = 8.1 Hz), 114.8 (d, J = 21.2 Hz), 61.2, 48.0, 32.5, 30.2, 14.0. ¹⁹F NMR (376 MHz, CDCl₃): δ = –116.11. HRMS (ESI): m/z calcd for C₁₃H₁₇FNaO₂S [M + Na]⁺: 279.0825; found: 279.0817. IR (film): 2965, 2361, 2344, 1735, 1509, 1262, 1092, 1031, 836, 800 cm^–1. Ethyl 2-((2-(4-Chlorophenyl)propan-2-yl)thio)acetate (3h) The product was obtained as colorless oil (240 mg, 88% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.25 (d, J = 8.6 Hz, 2 H), 7.06 (d, J = 8.6 Hz, 2 H), 3.81 (q, J = 7.1 Hz, 2 H), 2.75 (s, 2 H), 1.49 (s, 6 H), 0.99 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 144.2, 132.5, 128.2, 128.2, 61.3, 48.0, 32.4, 30.0, 14.0. HRMS (ESI): m/z calcd for C₁₃H₁₇ClNaO₂S [M + Na]⁺: 295.0530; found: 295.0530. IR (film): 2979, 2361, 1735, 1491, 1266, 1131, 1097, 1031, 1013, 829 cm^–1. Ethyl 2-((2-(4-Bromophenyl)propan-2-yl)thio)acetate (3i) The product was obtained as colorless oil (298 mg, 94% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.47–7.33 (m, 4 H), 3.99 (q, J = 7.1 Hz, 2 H), 2.93 (s, 2 H), 1.66 (s, 6 H), 1.17 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.3, 144.7, 131.2, 128.5, 120.7, 61.3, 48.1, 32.5, 29.9, 14.0. HRMS (ESI): m/z calcd for C₁₃H₁₇BrKO₂S [M + K]⁺: 354.9764; found: 354.9767. IR (film): 2979, 2931, 1735, 1489, 1396, 1269, 1130, 1094, 1008, 825 cm^–1. Ethyl 2-((1,1-Diphenylethyl)thio)acetate (3j) The product was obtained as colorless oil (215 mg, 72% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.45–7.40 (m, 4 H), 7.34–7.28 (m, 4 H), 7.27–7.22 (m, 2 H), 4.05 (q, J = 7.1 Hz, 2 H), 3.06 (s, 2 H), 2.10 (s, 3 H), 1.21 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 145.4, 128.3, 128.1, 127.0, 61.5, 57.1, 33.3, 30.6, 14.2. HRMS (ESI): m/z calcd for C₁₈H₂₀NaO₂S [M + Na]⁺: 323.1076; found: 323.1079. IR (film): 2979, 1735, 1492, 1444, 1264, 1134, 1029, 762, 742, 699 cm^–1. Ethyl 2-((2-(4-Ethylphenyl)butan-2-yl)thio)acetate (3k) The product was obtained as colorless oil (239 mg, 85% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.39 (d, J = 7.7 Hz, 2 H), 7.14 (d, J = 7.9 Hz, 2 H), 4.01 (q, J = 7.1 Hz, 2 H), 2.94 (s, 2 H), 2.62 (q, J = 7.6 Hz, 2 H), 2.06 (dq, J = 14.5, 7.4 Hz, 1 H), 1.96–1.83 (m, 1 H), 1.69 (s, 3 H), 1.22 (t, J = 4.0 Hz, 3 H), 1.19 (t, J = 3.0 Hz, 3 H), 0.82 (t, J = 7.4 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 142.5, 141.1, 127.6, 127.2, 61.1, 52.6, 35.2, 32.0, 28.3, 25.3, 15.4, 14.1, 9.1. HRMS (ESI): m/z calcd for C₁₆H₂₄NaO₂S [M + Na]⁺: 303.1389; found: 303.1390. IR (film): 2967, 2935, 2361, 2344, 1736, 1263, 1131, 1092, 1032, 801 cm^–1. Ethyl 2-((2-Phenylpentan-2-yl)thio)acetate (3l) The product was obtained as colorless oil (244 mg, 92% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.46 (d, J = 7.6 Hz, 2 H), 7.28 (t, J = 7.7 Hz, 2 H), 7.16 (t, J = 7.3 Hz, 1 H), 3.98 (qd, J = 7.1, 1.9 Hz, 2 H), 2.91 (s, 2 H), 2.02–1.93 (m, 1 H), 1.88–1.77 (m, 1 H), 1.71 (s, 3 H), 1.40–1.28 (m, 1 H), 1.16 (t, J = 7.1 Hz, 3 H), 1.11–1.00 (m, 1 H), 0.83 (t, J = 7.3 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.5, 144.2, 128.0, 127.1, 126.5, 61.0, 52.2, 45.0, 31.8, 25.7, 17.9, 14.3, 14.0. HRMS (ESI): m/z calcd for C₁₅H₂₂NaO₂S [M + Na]⁺: 289.1233; found: 289.1244. IR (film): 2961, 2935, 2361, 2344, 1735, 1446, 1265, 1133, 1030, 699 cm^–1. Ethyl 2-((1-(4-Methoxyphenyl)ethyl)thio)acetate (3m) The product was obtained as colorless oil (180 mg, 71% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.25 (d, J = 8.6 Hz, 2 H), 6.83 (d, J = 8.6 Hz, 2 H), 4.15–4.08 (m, 3 H), 3.77 (s, 3 H), 3.00 (d, J = 14.9 Hz, 1 H), 2.88 (d, J = 14.9 Hz, 1 H), 1.53 (d, J = 7.1 Hz, 3 H), 1.24 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.6, 158.8, 134.7, 128.6, 113.8, 61.2, 55.2, 43.6, 33.0, 22.0, 14.2. HRMS (ESI): m/z calcd for C₁₃H₁₈NaO₃S [M + Na]⁺: 277.0869; found: 277.0811. IR (film): 2965, 2930, 2836, 1734, 1611, 1512, 1464, 1249, 1032, 833 cm^–1. Ethyl 2-((1-(Naphthalen-2-yl)ethyl)thio)acetate (3n) The product was obtained as colorless oil (218 mg, 80% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.88–7.80 (m, 3 H), 7.75 (d, J = 1.3 Hz, 1 H), 7.57 (dd, J = 8.5, 1.8 Hz, 1 H), 7.53–7.45 (m, 2 H), 4.36 (q, J = 7.1 Hz, 1 H), 4.18–4.05 (m, 2 H), 3.05 (d, J = 15.0 Hz, 1 H), 2.92 (d, J = 15.0 Hz, 1 H), 1.69 (d, J = 7.1 Hz, 3 H), 1.25 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.6, 140.0, 133.2, 132.9, 128.7, 127.8, 127.7, 126.5, 126.3, 126.0, 125.2, 61.3, 44.5, 33.0, 21.9, 14.2. HRMS (ESI): m/z calcd for C₁₆H₁₈KO₂S [M + K]⁺: 313.0659; found: 313.0676. IR (film): 2968, 2928, 1735, 1264, 1125, 1030, 818, 800, 749, 478 cm^–1. Ethyl 2-((1-([1,1′-Biphenyl]-4-yl)ethyl)thio)acetate (3o) The product was obtained as colorless oil (245 mg, 82% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.66 - 7.57 (m, 4 H), 7.46 (t, J = 7.8 Hz, 4 H), 7.37 (t, J = 7.3 Hz, 1 H), 4.26 (q, J = 7.0 Hz, 1 H), 4.18 (qd, J = 7.1, 3.0 Hz, 2 H), 3.11 (d, J = 14.9 Hz, 1 H), 3.00 (d, J = 14.9 Hz, 1 H), 1.66 (d, J = 7.1 Hz, 3 H), 1.30 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 141.9, 140.6, 140.2, 128.7, 127.9, 127.3, 127.2, 126.9, 61.2, 43.9, 33.0, 21.9, 14.1. HRMS (ESI): m/z calcd for C₁₈H₂₀NaO₂S [M + Na]⁺: 323.1076; found: 323.1073. IR (film): 2967, 1735, 1486, 1263, 1130, 1030, 798, 767, 734, 698 cm^–1. Diethyl 2,2′-((1,4-Phenylenebis(propane-2,2-diyl))bis(sulfanediyl))diacetate (3p) The product was obtained as colorless oil (179 mg, 90% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.46 (s, 4 H), 4.05 (q, J = 7.1 Hz, 4 H), 2.98 (s, 4 H), 1.71 (s, 12 H), 1.21 (t, J = 7.1 Hz, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.5, 143.9, 126.4, 61.2, 48.1, 32.5, 29.9, 14.0. HRMS (ESI): m/z calcd for C₂₀H₃₀NaO₄S₂ [M + Na]⁺: 421.1478; found: 421.1475. IR (film): 2964, 2361, 2344, 1735, 1458, 126, 1097, 1028, 800, 669 cm^–1. Ethyl 2-((2-Phenylpropan-2-yl)thio)acetate (4a) The product was obtained as colorless oil (224 mg, 94% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.56–7.50 (m, 2 H), 7.32 (t, J = 7.7 Hz, 2 H), 7.21 (t, J = 7.3 Hz, 1 H), 4.02 (q, J = 7.1 Hz, 2 H), 2.97 (s, 2 H), 1.73 (s, 6 H), 1.20 (t, J = 7.1 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 145.5, 128.3, 126.8, 126.7, 61.3, 48.5, 32.59, 30.1, 14.1. HRMS (ESI): m/z calcd for C₁₃H₁₈NaO₂S [M + Na]⁺: 261.0920; found: 261.0921. IR (film): 2966, 2929, 1736, 1447, 1366, 1264, 1130, 1074, 1030, 699 cm^–1. Ethyl 2-((2-Phenylpropan-2-yl)thio)propanoate (4b) The product was obtained as colorless oil (224 mg, 89% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.56–7.51 (m, 2 H), 7.34–7.28 (m, 2 H), 7.23–7.18 (m, 1 H), 4.02–3.89 (m, 2 H), 3.00 (q, J = 7.3 Hz, 1 H), 1.74 (s, 3 H), 1.71 (s, 3 H), 1.23–1.15 (m, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 174.2, 145.8, 128.1, 126.7, 126.0, 60.9, 49.4, 40.6, 30.5, 30.2, 18.9, 14.0. HRMS (ESI): m/z calcd for C₁₄H₂₀NaO₂S [M + Na]⁺: 275.1076; found: 275.1070. IR (film): 2967, 2930, 1733, 1447, 1259, 1132, 1098, 1029, 802, 699 cm^–1. Methyl 3-((2-Phenylpropan-2-yl)thio)propanoate (4c) The product was obtained as colorless oil (217 mg, 91% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.55–7.51 (m, 2 H), 7.32 (t, J = 7.8 Hz, 2 H), 7.23–7.18 (m, 1 H), 3.62 (s, 3 H), 2.49 (t, J = 7.5 Hz, 2 H), 2.32 (t, J = 7.5 Hz, 2 H), 1.71 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 172.4, 146.4, 128.2, 126.6, 126.5, 51.7, 47.9, 34.2, 30.2, 24.5. HRMS (ESI): m/z calcd for C₁₃H₁₈NaO₂S [M + Na]⁺: 261.0920; found: 261.0925. IR (film): 2965, 1741, 1437, 1364, 1244, 1133, 1099, 1029, 769, 699 cm^–1. 3-((2-Phenylpropan-2-yl)thio)propanoic Acid (4d) The product was obtained as colorless oil (198 mg, 89% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.55–7.52 (m, 2 H), 7.35–7.30 (m, 2 H), 7.24–7.20 (m, 1 H), 2.50–2.45 (m, 2 H), 2.36–2.31 (m, 2 H), 1.72 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 178.3, 146.3, 128.3, 126.8, 126.6, 48.1, 34.3, 30.3, 24.2. HRMS (ESI): m/z calcd for C₁₂H₁₆NaO₂S [M + Na]⁺: 247.0763; found: 247.0766. IR (film): 2966, 1711, 1419, 1446, 1261, 1132, 1098, 1029, 768, 698 cm^–1. 3-((2-Phenylpropan-2-yl)thio)propan-1-ol (4e) The product was obtained as colorless oil (197 mg, 74% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.56 - 7.51 (m, 2 H), 7.32 (t, J = 7.7 Hz, 2 H), 7.21 (t, J = 7.3 Hz, 1 H), 3.55 (t, J = 6.1 Hz, 2 H), 2.32 (t, J = 7.1 Hz, 2 H), 1.72 (s, 6 H), 1.64–1.57 (m, 2 H). ¹³C NMR (101 MHz, CDCl₃): δ = 146.7, 128.2, 126.6, 126.6, 61.8, 47.7, 31.8, 30.3, 26.1. HRMS (ESI): m/z calcd for C₁₂H₁₈NaOS [M + Na]⁺: 233.0971; found: 233.0971. IR (film): 3384, 2964, 2927, 2870, 1494, 1446, 1133, 1030, 768, 698 cm^–1. Cyclohexyl(2-phenylpropan-2-yl)sulfane (4f) The product was obtained as colorless oil (218 mg, 93% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.57 (dd, J = 8.3, 1.0 Hz, 2 H), 7.31 (t, J = 7.7 Hz, 2 H), 7.20 (t, J = 7.3 Hz, 1 H), 2.35–2.27 (m, 1 H), 1.72 (s, 6 H), 1.66–1.55 (m, 4 H), 1.44 (s, 1 H), 1.29–1.12 (m, 5 H). ¹³C NMR (101 MHz, CDCl₃): δ = 147.4, 128.0, 126.6, 126.4, 48.3, 42.6, 35.4, 31.1, 26.2, 25.7. HRMS (ESI): m/z calcd for C₁₅H₂₂NaS [M + Na]⁺: 257.1334; found: 257.1361. IR (film): 2968, 1588, 1489, 1222, 1155, 1129, 1088, 823, 769, 697 cm^–1. Benzyl(2-phenylpropan-2-yl)sulfane (4g) The product was obtained as colorless oil (210 mg, 87% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.64–7.60 (m, 2 H), 7.38 (t, J = 7.8 Hz, 2 H), 7.29–7.23 (m, 3 H), 7.20 (d, J = 6.9 Hz, 1 H), 7.18–7.14 (m, 2 H), 3.43 (s, 2 H), 1.75 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 146.4, 138.3, 129.0, 128.5, 128.3, 126.8, 126.7, 126.7, 48.7, 34.7, 30.4. GC-MS (EI): m/z calcd for C₁₆H₁₈S 242.1; found: 242.1. IR (film): 2964, 2924, 1494, 1453, 1261, 1132, 1098, 1029, 803, 697. (4-(tert-Butyl)benzyl)(2-phenylpropan-2-yl)sulfane (4h) The product was obtained as colorless oil (250 mg, 84% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.66 (d, J = 7.5 Hz, 2 H), 7.40 (t, J = 7.7 Hz, 2 H), 7.33–7.28 (m, 3 H), 7.13 (d, J = 8.1 Hz, 2 H), 3.45 (s, 2 H), 1.80 (s, 6 H), 1.34 (s, 9 H). ¹³C NMR (101 MHz, CDCl₃): δ = 149.7, 146.5, 135.1, 128.7, 128.2, 126.7, 126.6, 125.4, 48.6, 34.5, 34.1, 31.5, 30.4. HRMS (ESI): m/z calcd for C₂₀H₂₆NaS [M + Na]⁺: 321.1647; found: 321.1636. IR (film): 2963, 2867, 1494, 1447, 1364, 1133, 1098, 1029, 768, 698 cm^–1. (4-Methoxybenzyl)(2-phenylpropan-2-yl)sulfane (4i) The product was obtained as colorless oil (242 mg, 89% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.61–7.55 (m, 2 H), 7.36 (t, J = 7.7 Hz, 2 H), 7.23 (d, J = 7.3 Hz, 1 H), 7.05 (d, J = 8.6 Hz, 2 H), 6.76 (d, J = 8.5 Hz, 2 H), 3.76 (s, 3 H), 3.36 (s, 2 H), 1.72 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 158.6, 146.5, 130.2, 130.1, 128.3, 126.7, 126.6, 113.9, 55.4, 48.6, 34.0, 30.4. HRMS (ESI): m/z calcd for C₁₇H₂₀KOS [M + K]⁺: 311.0866; found: 311.0890. IR (film): 3508, 2962, 2926, 1610, 1511, 1249, 1097, 1032, 804, 698 cm^–1. (4-Fluorobenzyl)(2-phenylpropan-2-yl)sulfane (4j) The product was obtained as colorless oil (219 mg, 84% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.68–7.60 (m, 2 H), 7.41 (t, J = 7.7 Hz, 2 H), 7.31 (d, J = 7.2 Hz, 1 H), 7.12 (dd, J = 8.3, 5.6 Hz, 2 H), 6.95 (t, J = 8.6 Hz, 2 H), 3.43 (s, 2 H), 1.78 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 161.8 (d, J = 246 Hz), 146.2, 134.0 (d, J = 4.0 Hz), 130.5 (d, J = 8.1 Hz), 128.2, 126.7, 115.2 (d, J = 21.2 Hz), 48.6, 33.8, 30.3. ¹⁹F NMR (376 MHz, CDCl₃): δ = –115.90. HRMS (ESI): m/z calcd for C₁₆H₁₈FS [M + H]⁺: 261.1108; found: 261.1103. IR (film): 2964, 2360, 2344, 1508, 1261, 1223, 1098, 1030, 803, 698 cm^–1. (4-Chlorobenzyl)(2-phenylpropan-2-yl)sulfane (4k) The product was obtained as colorless oil (237 mg, 86% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.60–7.56 (m, 2 H), 7.36 (t, J = 7.7 Hz, 2 H), 7.26 (t, J = 7.3 Hz, 1 H), 7.19 (d, J = 8.4 Hz, 2 H), 7.05 (d, J = 8.3 Hz, 2 H), 3.37 (s, 2 H), 1.73 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 146.2, 136.9, 132.5, 130.3, 128.5, 128.3, 126.7, 126.7, 48.8, 34.0, 30.3. HRMS (ESI): m/z calcd for C₁₆H₁₇ClKS [M + K]⁺: 315.0371; found: 315.0393. IR (film): 2965, 2925, 1490, 1446, 1132, 1094, 1015, 807, 768, 698 cm^–1. (2-Phenylpropan-2-yl)(4-(trifluoromethyl)benzyl)sulfane (4l) The product was obtained as colorless oil (272 mg, 88% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.59–7.55 (m, 2 H), 7.48 (d, J = 8.1 Hz, 2 H), 7.36 (t, J = 7.7 Hz, 2 H), 7.28 (s, 1 H), 7.22 (d, J = 8.0 Hz, 2 H), 3.45 (s, 2 H), 1.74 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 146.1, 142.7 (d, J = 2.0 Hz), 129.0 (q, J = 97.0 Hz), 129.3, 128.3, 126.8, 126.7, 124.3 (d, J = 274 Hz), 125.3 (q, J = 12.1 Hz), 48.9, 34.3, 30.3. ¹⁹F NMR (376 MHz, CDCl₃): δ = –62.50. HRMS (ESI): m/z calcd for C₁₇H₁₈F₃S [M + H]⁺: 311.1076; found: 311.1077. IR (film): 2967, 1618, 1492, 1324, 1164, 1124, 1067, 1019, 847, 698 cm^–1. (4-Fluorophenyl)(2-phenylpropan-2-yl)sulfane (4m) The product was obtained as colorless oil (236 mg, 96% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 10:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.47 (d, J = 7.6 Hz, 2 H), 7.35 (t, J = 7.3 Hz, 2 H), 7.29 (d, J = 6.8 Hz, 1 H), 7.21–7.14 (m, 2 H), 6.93 (t, J = 8.4 Hz, 2 H), 1.77 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 163.3 (d, J = 249 Hz), 146.2, 138.6 (d, J = 8.1 Hz), 128.0, 126.7, 126.6, 115.4 (d, J = 22.2 Hz), 51.1, 29.6. HRMS (ESI): m/z calcd for C₁₅H₁₆FS [M + H]⁺: 247.0951; found: 247.0951. IR (film): 2968, 2924, 1588, 1489, 1222, 1155, 1088, 832, 767, 697 cm^–1. S-(2-Phenylpropan-2-yl) Ethanethioate (4n) The product was obtained as colorless oil (140 mg, 72% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.55 (d, J = 7.5 Hz, 2 H), 7.34 (t, J = 7.7 Hz, 2 H), 7.27–7.21 (m, 1 H), 2.19 (s, 3 H), 1.87 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 195.4, 145.8, 128.2, 126.9, 126.3, 52.5, 31.1, 28.9. HRMS (ESI): m/z calcd for C₁₁H₁₄NaOS [M + Na]⁺: 217.0658; found: 217.0620. IR (film): 2965, 2926, 2360, 2343, 1735, 1263, 1129, 1029, 766, 698 cm^–1. S-(2-Phenylpropan-2-yl) Benzothioate (4o) The product was obtained as colorless oil (184 mg, 72% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.90 (d, J = 7.5 Hz, 2 H), 7.65 (d, J = 7.6 Hz, 2 H), 7.52 (t, J = 7.4 Hz, 1 H), 7.38 (dt, J = 12.3, 7.9 Hz, 4 H), 7.26 (t, J = 7.3 Hz, 1 H), 2.00 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 191.3, 146.1, 137.9, 133.1, 128.5, 128.2, 127.1, 126.9, 126.3, 52.6, 29.3. HRMS (ESI): m/z calcd for C₁₆H₁₆NaOS [M + Na]⁺: 279.0814; found: 279.0829. IR (film): 2968, 2352, 1660, 1580, 1447, 1204, 1175, 1132, 907, 690 cm^–1. Ethane-1,2-diyl Bis(2-((2-phenylpropan-2-yl)thio)acetate) (4p) The product was obtained as colorless oil (189 mg, 85% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.55–7.50 (m, 4 H), 7.32 (t, J = 7.7 Hz, 4 H), 7.21 (t, J = 7.3 Hz, 2 H), 4.07 (s, 4 H), 2.99 (s, 4 H), 1.72 (s, 12 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.4, 145.4, 128.3, 126.9, 126.7, 62.7, 48.6, 32.3, 30.1. HRMS (ESI): m/z calcd for C₂₄H₃₀NaO₄S₂ [M + Na]⁺: 469.1478; found: 469.1481. IR (film): 2966, 2360, 1735, 1447, 1261, 1126, 1099, 1030, 769, 699 cm^–1. Methyl N-((Benzyloxy)carbonyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (4q) The product was obtained as colorless oil (280 mg, 73% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 20:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.51 (d, J = 7.7 Hz, 2 H), 7.42–7.27 (m, 7 H), 7.21 (t, J = 7.3 Hz, 1 H), 5.55 (d, J = 8.0 Hz, 1 H), 5.10 (s, 2 H), 4.45 (dt, J = 7.8, 5.2 Hz, 1 H), 3.66 (s, 3 H), 2.75–2.58 (m, 2 H), 1.70 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.9, 155.5, 145.7, 136.1, 128.3, 128.1, 128.0, 128.0, 126.6, 126.3, 66.8, 53.2, 52.3, 47.9, 31.7, 30.0. HRMS (ESI): m/z calcd for C₂₁H₂₅NNaO₄S [M + Na]⁺: 10.1397; found: 410.1336. IR (film): 3345, 3033, 2965, 1726, 1507, 1438, 1212, 1057, 770, 699 cm^–1. Methyl N-(((Benzyloxy)carbonyl)-l-alanyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5a) The product was obtained as yellow oil (133 mg, 58% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 3:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.48 (d, J = 7.7 Hz, 2 H), 7.40–7.27 (m, 7 H), 7.19 (t, J = 7.3 Hz, 1 H), 6.48 (d, J = 6.1 Hz, 1 H), 5.43 (d, J = 6.7 Hz, 1 H), 5.13 (s, 2 H), 4.59 (dt, J = 7.5, 5.1 Hz, 1 H), 4.27–4.15 (m, 1 H), 3.66 (s, 3 H), 2.71–2.66 (m, 2 H), 1.67 (s, 6 H), 1.34 (d, J = 7.0 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 172.0, 170.7, 155.8, 146.0, 136.4, 128.6, 128.3, 128.3, 128.1, 126.9, 126.5, 67.1, 52.7, 51.7, 50.4, 48.2, 31.4, 30.2, 30.0, 19.0. HRMS (ESI): m/z calcd for C₂₄H₃₀N₂NaO₅S [M + Na]⁺: 481.1768; found: 481.1768. IR (film): 3309, 2954, 1719, 1671, 1523, 1454, 1256, 1070, 745, 699 cm^–1. Methyl N-(((Benzyloxy)carbonyl)-l-methionyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5b) The product was obtained as yellow oil (160 mg, 62% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 2:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.48 (d, J = 7.8 Hz, 2 H), 7.33 (m, 7 H), 7.19 (t, J = 7.3 Hz, 1 H), 6.69 (d, J = 7.3 Hz, 1 H), 5.66 (d, J = 8.0 Hz, 1 H), 5.13 (s, 2 H), 4.58 (dd, J = 12.4, 5.2 Hz, 1 H), 4.36 (dd, J = 13.5, 6.6 Hz, 1 H), 3.66 (s, 3 H), 2.67 (ddd, J = 17.5, 12.9, 5.1 Hz, 2 H), 2.58 (t, J = 7.1 Hz, 2 H), 2.12 (s, 3 H), 1.96 (dd, J = 15.0, 7.9 Hz, 2 H), 1.67 (s, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 170.5, 155.8, 145.8, 136.3, 128.6, 128.3, 128.2, 128.1, 127., 126.5, 67.1, 53.5, 52.6, 51.6, 48.2, 32.1, 31.3, 30.2, 30.0, 29.8, 15.2. HRMS (ESI): m/z calcd for C₂₆H₃₄N₂NaO₅S₂ [M + Na]⁺: 541.1801; found: 541.1801. IR (film): 3308, 2963, 1745, 1668, 1522, 1438, 1214, 1178, 1049, 699. Methyl N-(((Benzyloxy)carbonyl)-l-seryl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5c) The product was obtained as yellow oil (143 mg, 48% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 1:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.59 (s, 1 H), 7.35 (d, J = 11.3 Hz, 8 H), 7.28 - 7.20 (m, 2 H), 5.73 (s, 1 H), 5.09 (s, 2 H), 4.90 (s, 1 H), 4.36 (s, 1 H), 3.75 (s, 3 H), 3.57 (s, 1 H), 3.29 (t, J = 8.1 Hz, 1 H), 3.19 (d, J = 4.6 Hz, 2 H), 1.56 (d, J = 6.0 Hz, 6 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.5, 170.3, 156.2, 145.1, 136.3, 128.7, 128.5, 128.3, 128.2, 127.3, 125.8, 67.2, 62.7, 54.7, 52.9, 52.0, 40.7, 29.8, 28.2. HRMS (ESI): m/z calcd for C₂₄H₃₀N₂NaO₆S [M + Na]⁺: 497.1717; found: 497.1716. IR (film): 2923, 2852, 2360, 1735, 1654, 1508, 1211, 1083, 970, 697 cm^–1. Methyl N-(((Benzyloxy)carbonyl)-l-threonyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5d) The product was obtained as yellow oil (134 mg, 55% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 2:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.48 (d, J = 7.6 Hz, 2 H), 7.34 (m, 7 H), 7.21 (t, J = 7.2 Hz, 1 H), 6.94 (d, J = 7.4 Hz, 1 H), 5.74 (d, J = 7.7 Hz, 1 H), 5.15 (s, 2 H), 4.57 (dd, J = 12.9, 5.6 Hz, 1 H), 4.30 (s, 1 H), 4.17 (d, J = 7.5 Hz, 1 H), 3.67 (s, 3 H), 3.09 (s, 1 H), 2.73–2.62 (m, 2 H), 1.67 (s, 6 H), 1.17 (d, J = 6.4 Hz, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.7, 170.3, 156.6, 145.7, 136.1, 128.5, 128.2, 128.2, 128.0, 126.8, 126.4, 67.3, 67.2, 58.7, 52.6, 51.7, 48.1, 31.2, 30.1, 30.0, 17.8. HRMS (ESI): m/z calcd for C₂₅H₃₃N₂O₆S [M + H]⁺: 489.2054; found: 489.2054. IR (film): 2962, 2924, 2361, 2344, 1735, 1522, 1458, 1260, 1021, 799 cm^–1. Methyl N-(((Benzyloxy)carbonyl)-l-phenylalanyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5e) The product was obtained as yellow oil (151 mg, 56% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 2:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.43 (d, J = 7.7 Hz, 2 H), 7.39–7.23 (m, 10 H), 7.20 (d, J = 6.9 Hz, 3 H), 6.41 (d, J = 7.0 Hz, 1 H), 5.46 (d, J = 7.7 Hz, 1 H), 5.10 (s, 2 H), 4.57 (dd, J = 12.3, 5.1 Hz, 1 H), 4.45 (d, J = 7.0 Hz, 1 H), 3.65 (s, 3 H), 3.05 (d, J = 6.3 Hz, 2 H), 2.68–2.56 (m, 2 H), 1.65 (s, 3 H), 1.63 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.5, 170.4, 155.8, 145.9, 136.3, 129.5, 128.7, 128.6, 128.3, 128.2, 128.1, 127.0, 126.9, 126.5, 67.0, 55.9, 52.6, 51.7, 48.1, 38.8, 31.4, 30.2, 30.0. HRMS (ESI): m/z calcd for C₃₀H₃₄N₂NaO₅S [M + Na]⁺: 557.2081; found: 557.2081. IR (film): 3309, 2955, 1744, 1665, 1522, 1497, 1438, 1213, 1029, 698 cm^–1. Methyl N-(((Benzyloxy)carbonyl)-l-tyrosyl)-S-(2-phenylpropan-2-yl)-l-cysteinate (5f) The product was obtained as yellow oil (144 mg, 52% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 2:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.43 (d, J = 7.8 Hz, 2 H), 7.39–7.25 (m, 7 H), 7.18 (t, J = 7.1 Hz, 1 H), 7.00 (d, J = 7.5 Hz, 2 H), 6.68 (d, J = 8.0 Hz, 2 H), 6.45 (s, 1 H), 6.34 (d, J = 7.0 Hz, 1 H), 5.43 (d, J = 7.4 Hz, 1 H), 5.10 (s, 2 H), 4.54 (dd, J = 12.3, 5.1 Hz, 1 H), 4.35 (d, J = 6.3 Hz, 1 H), 3.64 (s, 3 H), 2.95 (d, J = 6.2 Hz, 2 H), 2.62 (qd, J = 12.9, 5.1 Hz, 2 H), 1.65 (s, 3 H), 1.63 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 170.9, 170.5, 156.0, 155.3, 145.9, 136.2, 130.6, 128.7, 128.4, 128.3, 128.1, 127.6, 127.0, 126.5, 115.8, 67.3, 56.2, 52.7, 51.8, 48.2, 38.1, 31.4, 30.2, 30.0. HRMS (ESI): m/z calcd for C₃₀H₃₄N₂NaO₆S [M + Na]⁺: 573.2030; found: 573.2022. IR (film): 3324, 2962, 1665, 1516, 1439, 1365, 1218, 1174, 1028, 699 cm^–1. Methyl N-(N2,N6-Bis((benzyloxy)carbonyl)-l-lysyl)-S-(2-phe-nylpropan-2-yl)-l-cysteinate (5g) The product was obtained as yellow oil (173 mg, 53% yield) after purification by flash column chromatography on silica gel (PE/EtOAc = 1:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.48 (d, J = 7.7 Hz, 2 H), 7.41–7.24 (m, 12 H), 7.20 (t, J = 7.1 Hz, 1 H), 6.51 (d, J = 7.2 Hz, 1 H), 5.61 (d, J = 7.6 Hz, 1 H), 5.14 (s, 1 H), 5.11 (s, 2 H), 5.06 (s, 2 H), 4.58 (dd, J = 11.9, 5.0 Hz, 1 H), 4.17 (d, J = 6.0 Hz, 1 H), 3.60 (s, 3 H), 3.17 (d, J = 4.9 Hz, 2 H), 2.67 (d, J = 4.7 Hz, 2 H), 1.79 (s, 1 H), 1.66 (s, 6 H), 1.55 (s, 1 H), 1.47 (dd, J = 16.7, 9.9 Hz, 2 H), 1.34 (s, 2 H). ¹³C NMR (101 MHz, CDCl₃): δ = 171.4, 170.8, 156.6, 156.1, 145.9, 136.7, 136.4, 128.6, 128.5, 128.3, 128.2, 128.2, 128.1, 128.1, 126.9, 126.5, 67.1, 66.7, 54.5, 52.6, 51.7, 48.2, 40.4, 32.4, 31.3, 30.2, 30.0, 29.4, 22.0. HRMS (ESI): m/z calcd for C₃₅H₄₃N₃NaO₇S [M + H]⁺: 672.2714; found: 672.2719. IR (film): 3325, 2952, 1718, 1519, 1455, 1240, 1132, 1028, 740, 698 cm^–1.

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