Solvent-Free Synthesis of α-Cyanophosphonates from β-Nitro­styrenes by Using a Deep-Eutectic Solvent Catalyst

Abstract

α-Cyanophosphonates, which are useful reagents for the Horner–Wittig reaction, were synthesized under solvent-free conditions by using a choline chloride–zinc chloride deep-eutectic solvent (DES) as a catalyst. This is only the second report on the synthesis of these compounds. In the previous report, diethyl trimethylsilyl phosphite was used as a reagent and TiCl₄ as a catalyst, whereas in this study, both the reagent (triphenylphosphine) and the catalyst (choline chloride–zinc chloride DES) are cheaper, more readily available, and less harmful than those used in the previous work. Moreover, the process involves an interesting cascade reaction between a β-nitrostyrene and two equivalents of triphenyl phosphite, leading to the desired product by a new synthetic route. The products can be used in the pharmaceutical and agricultural industries, in addition to their synthetic applications in the preparation of α,β-unsaturated nitriles. The reactions were completed on using 20 mol% of DES at 80 °C in six hours. Ten different β-nitrostyrenes were synthesized in yields of 55–87% after purification. β-Nitrostyrenes containing electron-donating groups showed higher yields. The reaction failed when aliphatic or heteroaromatic nitroalkenes or β-nitrostyrenes with electron-withdrawing substituents were employed. Finally, three plausible mechanistic routes are proposed for the reaction, starting with the nucleophilic addition of triphenyl phosphite to the carbon, nitrogen, or oxygen atom in the α-position.

Key words

cyanophosphonates - choline chloride - deep-eutectic solvent - nitrostyrenes - triphenyl phosphite - organocatalysis

Publication History

Received: 09 December 2023

Accepted after revision: 05 January 2024

Accepted Manuscript online:
05 January 2024

Article published online:
31 January 2024

• References and Notes

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• 52 α-Cyanophosphonates 3a–j; General Procedure A 10 mL flask was charged with P(OPh)₃ (2; 620.6 mg), the appropriate nitrostyrene 1 (1 mmol), and a 1:2 choline chloride–ZnCl₂ mixture (82.4 mg, 0.2 mmol, 20 mol%), and the flask was immersed in an oil bath on a magnetic heater–stirrer. The mixture was then stirred under an inert atmosphere at 80 °C for about 6 h while the progress of the reaction was monitored by TLC (EtOAc–hexane, 7:3). When the reaction was complete, H₂O (5 mL) was added to the mixture and the organic phase was extracted with EtOAc (2 × 10 mL). The combined organic phase was washed with sat. brine (3 × 15 mL), then dried (Na₂SO₄) and concentrated. The crude product was purified by column chromatography [silica gel, EtOAc–hexane (7:3)]. Diphenyl [Cyano(phenyl)methyl]phosphonate (3a; MW = 349.32) Colorless oil; yield: 283 mg (81%). FTIR (KBr): 3067, 2879, 2254, 1588, 1485, 1183, 951, 937, 771, 592 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.59 (m, 2 H), 7.45 (m, 3 H), 7.35 (t, J = 7.9 Hz, 2 H), 7.25 (m, 3 H), 7.20–7.12 (m, 3 H), 6.94 (d, J = 8.5 Hz, 2 H), 4.65 (d, J = 26.7 Hz, 1 H). ¹³C NMR (101 MHz, CDCl₃): δ = 149.9 (d, J = 9.9 Hz), 149.7 (d, J = 9.3 Hz), 130.0 (m), 129.9 (d, J = 0.9 Hz), 129.4 (d, J = 3.0 Hz), 129.3 (d, J = 3.4 Hz), 129.1 (d, J = 5.3 Hz), 126.5 (d, J = 8.6 Hz), 126.0 (d, J = 1.2 Hz), 125.8 (d, J = 1.2 Hz), 120.4 (d, J = 4.4 Hz), 120.2 (d, J = 4.5 Hz), 114.7 (d, J = 10.5 Hz), 36.8 (d, J = 142.8 Hz).< Diphenyl [Cyano(3-methoxyphenyl)methyl]phosphonate (3b; MW = 379.35) Colorless oil; yield: 269 mg (77%). FTIR (KBr): 2964, 2892, 2248, 1584, 1509, 1482, 1284, 1263, 1207, 1183, 963, 939 cm^–1. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.49–7.38 (m, 5 H), 7.27 (m, 2 H), 7.18 (m, 1 H), 7.11–7.02 (m, 6 H), 6.11 (d, J = 27.7 Hz, 1 H), 3.78 (s, 3 H). ¹³C NMR (76 MHz, DMSO-d ₆): δ = 160.0 (d, J = 3.0 Hz), 150.0 (d, J = 2.8 Hz), 149.9 (d, J = 2.5 Hz), 131.0 (d, J = 2.9 Hz), 130.6, 129.2 (d, J = 8.5 Hz), 126.4, 121.6 (d, J = 5.4 Hz), 120.6 (d, J = 3.3 Hz), 116.2 (d, J = 9.9 Hz), 115.5 (d, J = 5.4 Hz), 114.9 (d, J = 3.2 Hz), 55.8, 35.9 (d, J = 141.8 Hz). Diphenyl [Cyano(4-methoxyphenyl)methyl]phosphonate (3c; MW = 379.35) Colorless oil; yield: 304 mg (87%). FTIR (KBr): 2964, 2892, 2249, 1586, 1484, 1284, 1263, 1183, 1160, 963, 939, 770 cm^–1. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.54–7.34 (m, 6 H), 7.26 (m, 2 H), 7.15–6.95 (m, 6 H), 6.04 (d, J = 27.6 Hz, 1 H), 3.80 (s, 3 H). ¹³C NMR (76 MHz, DMSO-d ₆): δ = 160.2 (d, J = 3.2 Hz), 150.0 (d, J = 2.6 Hz), 149.9 (d, J = 2.3 Hz), 130.9 (d, J = 5.4 Hz), 130.6, 126.39, 120.7 (d, J = 4.3 Hz), 119.3 (d, J = 8.9 Hz), 116.4 (d, J = 9.3 Hz), 115.2 (d, J = 2.7 Hz), 55.8, 36.0.