Unprecedented Reaction between Ethyl α-Cyanocinnamate and o-Phenylenediamine: Development of an Efficient Method for the Transfer Hydrogenation of Electronically Depleted Olefins

Abstract

A reaction between ethyl α-cyanocinnamate and o-phen­ylenediamine under thermal conditions yielded 2-cyano-3-phenyl-propionic acid ethyl ester, 2-phenyl benzimidazole, and ethyl ­cyanoacetate. The mechanistic revelations led to the development of a simple and efficient transfer-hydrogenation process from the in situ generated benzimidazolines to activated olefins under solventless and catalyst-free conditions.

Key words

organocatalytic reaction - hydrogen transfer - electronically depleted olefins - benzimidazoline - metal-free reduction

References and Notes

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13

General Procedure for Transfer Hydrogenation of Deactivated OlefinsTwo-Component Reaction
In an open vial, ethyl α-cyanocinnamate (1a, 2.0 mmol)and o-phenylenediamine (2, 1.0 mmol) were mixed thoroughly and heated in an oil bath at 100 °C. The reaction mixture liquefied and resolidified within half an hour and the reaction was found to be complete (TLC). After cooling the resultant semi-solid reaction mixture, a 7:3 mixture of EtOAc and PE (15 mL) was added and the insoluble 2-phenylbenzimidazole (4a) was filtered out (92% yield). The filtrate was concentrated, and after column chromatography of the residue using 5% EtOAc-PE as eluent, the reduced product 3a and ethyl cyanoacetate (5a) were isolated in 95% and 60% yield, respectively.
Three-Component Reaction
In an open vial, benzaldehyde, ethyl benzylidenecyano-acetate, and o-phenylenediamine (1:1:1, 1 mmol each) were mixed thoroughly and then heated in an oil bath at 100 °C. The resultant mixture liquefied and then resolidified in about 15 min with completion of the reaction (TLC). The workup was performed as described in the two-component reaction.
Spectral Data of Selected CompoundsEthyl 2-Cyano-3-phenylpropionate (3a) Purified by column chromatography using EtOAc-PE and isolated as a liquid. IR (neat): ν_max = 3043, 2957, 2251 (CºN), 1751 (O-C=O), 1604, 1440, 1275, 1028, 860, 702 cm^-1. ¹H NMR (CDCl₃): δ = 7.31 (5 H, m, ArH), 4.25 (2 H, q, J = 7.2 Hz, OCH ₂CH₃), 3.74 (1 H, dd, J = 8.4, 6.0 Hz), 3.23 (2 H, dABq, J = 14.0, 5.6 Hz), 1.29 (3 H, t, J = 7.2 Hz, OCH₂CH ₃). ¹³C NMR (CDCl₃): δ = 165.9 (C, O-C=O), 135.1, 128.8, 128.7, 127.7, 115.9 (C, CºN), 62.9, 39.4, 35.6, 13.8. LRMS: m/z calcd for C₁₂H₁₃NO₂: 203.0946; found: 202.162 [M - H⁺].

Ethyl 2-Cyano-3-[4-(2-cyano-2-ethoxycarbonyl-ethyl)phenyl]propionate (3m) Purified by column chromatography using EtOAc-hexane and isolated as a liquid. IR (neat): n_max = 2986, 2251 (CN), 1747 (O-C=O), 1518, 1446, 1028, 856, 802 cm^-1. ¹H NMR (CDCl₃): d = 7.24 (4 H, s, ArH), 4.21 (4 H, q, J = 7.2 Hz, 2 × OCH ₂CH₃), 3.71 (2 H, t, J = 8.0 Hz, 2 × HCCNCO₂Et), 3.20 (4 H, dABq, J = 13.6, 5.6 Hz), 1.25 (6 H, t, J = 7.2 Hz, 2 × OCH₂CH ₃). ¹³C NMR (CDCl₃): d = 165.3 (2 × C, O-C=O), 134.7, 129.4 (4 × CH), 115.9 (2 × C, CN), 62.8 (2 × CH₂, OCH₂CH₃), 39.36, 39.34, 35.1 (2 × CH₂), 13.8 (2 × CH₃, OCH₂ CH₃). LRMS: m/z calcd for C₁₈H₂₀N₂O₄: 328.1423; found: 327.10 [M - H⁺]. HRMS: m/z calcd for C₁₈H₂₀N₂O₄Na: 351.1321; found: 351.1315 [M + Na⁺].
2-Phenyl-1 H -benzoimidazole (4a) Mp 289 °C. IR (KBr): n_max = 3051, 1622, 1589, 1462, 1275, 1003, 740, 686, 493 cm^-1. ¹H NMR (DMSO-d ₆): d = 8.09 (2 H, m), 7.68 (2 H, br s), 7.51 (3 H, m), 7.30 (3 H, m). ¹³C NMR (DMSO-d ₆): d = 151.7, 144.0 (C, br), 135.0 (C, br), 130.6 (C), 130.3 (CH), 129.4 (2 × CH), 126.9 (2 × CH), 122.7 (CH, br), 122.3 (CH, br), 119.3 (CH, br), 111.7 (CH, br). ESI-MS: m/z calcd for C₁₃H₁₀N₂: 194.084; found: 195.1 [M + H⁺].
1,4-Di(1 H -benzoimidazol-2-yl)benzene (4j)
Mp >300 °C. IR (KBr): n_max = 3058, 1620, 1583, 1498, 1430, 1274, 1122, 964, 821,747, 545, 492 cm^-1. ¹H NMR (DMSO-d ₆): d = 8.50 (4 H, br s), 7.80 (2 H, d, J = 7.2 Hz), 7.69 (2 H, d, J = 7.2 Hz), 7.4 (4 H, br s). ¹³C NMR (DMSO-d ₆): d = 150.9, 144.2, 135.4, 131.5, 127.3, 123.2, 122.2, 119.3, 111.8. MS: m/z calcd for C₂₀H₁₄N₄: 310.1218; found: 311.3 [M + H⁺].