A Synthesis of Furan-2-iminophosphoranes under Appel-Type Reaction Conditions

Abstract

Phenacylmalononitriles reacted with triphenylphosphine and carbon tetrachloride in an Appel-type, cyclization/aromatization reaction to afford 5-aryl-2-[(triphenylphosphoranylidene)amino]-3-furonitriles in yields of 75–92%. The reaction proceeded smoothly in the presence of excess amounts of Ph₃P and CCl₄ without any base or catalyst at room temperature. The structure of one product was confirmed by X-ray crystallographic analysis.

Key words

Appel reaction - iminophosphoranes - triphenylphosphine - furanimines - acylmalononitriles

• References and Notes

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• 14 5-Phenyl-2-[(triphenylphosphoranylidene)amino]-3-furonitrile (2a); Typical Procedure CCl₄ (461 mg, 3 equiv) was added dropwise to a reaction flask containing phenacylmalononitrile (1a; 184 mg, 1 equiv) and Ph₃P (657 mg, 2.5 equiv) in MeCN (6 mL). The flask was sealed and the mixture was stirred at r.t. (25 ℃) until the reaction was complete [~4 h; TLC, hexane–EtOAc (3:1)]. The mixture was then filtered, and the precipitate was washed with EtOH to give a pale-yellow solid; yield: 408 mg (92%); mp 203–205 °C. (In cases where no precipitate was formed, the reaction flask was scratched and placed in a freezer at –18 ℃; after 24 h, crystals of the product were collected by filtration and washed with a sufficient amount of cold MeCN.) IR (KBr): 2207 (CN), 1504 (C=C), 1288 (P=N), 1112 (C–O) cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 6.54 (br s, 1 H, CH), 7.05–7.08 (m, 3 H, Ar), 7.17–7.20 (m, 2 H, Ar), 7.50–7.53 (m, 6 H, PPh₃), 7.59–7.62 (m, 3 H, PPh₃), 7.80 (dd, ³ J _H–H = 8.0 Hz, ³ J _P–H = 12.5 Hz, 6 H, PPh₃). ¹³C NMR (125 MHz, CDCl₃): δ = 77.4 (C–CN), 106.7 (CH of furan), 118.2 (CN), 122.3 (2 CH of Ph), 126.1 (CH of Ph), 128.6 (2 CH of Ph), 128.8 (d, ¹ J _P–C = 103.4 Hz, PPh₃), 129.1 (d, ³ J _P–C = 12.5 Hz, PPh₃), 130.5 (s, CH of Ph), 132.7 (d, ⁴ J _P–C = 2.4 Hz, PPh₃), 132.8 (d, ² J _P–C = 10.6 Hz, PPh₃), 142.9 (C-5 of furan), 164.7 (d, ² J _P–C = 5.5 Hz, C–N=P). ³¹P NMR (202 MHz, CDCl₃): δ = 14.14 (Ph₃P=N). EI-MS: m/z (%) = 445 [M + 1]⁺ (10), 444 [M⁺] (32), 279 (10), 262 (100), 183 (58), 108 (37), 77 (2). Anal. Calcd for C₂₉H₂₁N₂OP (444.47): C, 78.37; H, 4.76; N, 6.30. Found: C, 78.08; H, 4.74; N, 6.32.
• 15 CCDC 1996238 contains the supplementary crystallographic data for compound 2c. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures