An Unconventional Synthesis of Dibromophosphines

Abstract

Dibromophosphines, RPBr₂, are obtained by reaction of tetrabromomethane with 7-substituted 7-phosphanorbornenes in toluene at ca. 100 °C. The phosphanorbornenes are obtained in situ by cycloaddition of N-phenylmaleimide with 1-substituted 3,4-dimethylphospholes. The overall reaction sequence shows a good compatibility with functional groups.

Key words

phospholes - cycloreversion - dihalophosphines

• References and Notes

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• 8 Synthesis of 6: A solution of 3,4-dimethyl-1-phenyl-phosphole (0.5 mL, 2.65 mmol) and N-phenylmaleimide (0.45 g, 2.65 mmol) in toluene was stirred at 100 °C for 3 h (³¹P NMR: δ = 47.3 ppm). Then CBr₄ (0.44 g, 1.3 mmol) was added to the solution and heated at 100 °C for 1 h (³¹P NMR: δ = 150.6 ppm). The solution was added to W(CO)₅(MeCN) (2.65 mmol) in THF and the mixture was stirred at 50 °C for 10 h. After evaporation of the solvents, the residue was extracted with hexane and purified by chromatography on silica gel at –10 °C with petroleum ether as the eluent to yield 6 as a yellow liquid (0.23 g, yield: 15%). ¹H NMR (CDCl₃): δ = 7.49–7.63 (m, 3 H), 7.97–8.04 (m, 2 H). ¹³C NMR (CDCl₃): δ = 128.69 (d, ³ J _C–P = 11.1 Hz, 2 × CH), 129.06 (d, ² J _C–P = 17.1 Hz, 2 × CH), 132.58 (d, ⁴ J _C–P = 1.7 Hz, CH), 141.44 (d, J _C–P = 18.8 Hz, P–C), 195.60 (td, J _C–P = 7.5 Hz, cis CO), 198.37 (d, J _C–P = 45.7 Hz, trans CO). ³¹P NMR (CDCl₃): δ = 79.4 (t, J _P–W = 329.1 Hz).
• 9 General Procedure for 5: CBr₄ was added to the solution of 7-phosphanorbornenes in toluene and heated at 90–120 °C for 10 min to 4 h. The reactions were monitored by ³¹P NMR spectroscopy. 4a: ³¹P NMR (toluene): δ = 150.6, 4b: ³¹P NMR (toluene): δ = 152.1, 4c: ³¹P NMR (toluene): δ = 129.0, 4d: ³¹P NMR (toluene): δ = 151.1, 4e: ³¹P NMR (toluene): δ = 172.4. The solution was cooled to 0 °C, and Et₂NH was added. After the mixture was warmed to r.t. and stirred for 30 min, S₈ was added and reacted for 1 h. The mixture was filtered through silica to remove salts and concentrated. Purification was performed via column chromatography on silica gel. 5a: ¹H NMR (CDCl₃): δ = 1.04 (t, J _H–H = 7.1 Hz, 12 H, Me), 3.07–3.20 (m, 8 H, CH₂), 7.38–7.45 (m, 3 H), 7.92–8.00 (m, 2 H). ¹³C NMR (CDCl₃): δ = 13.31 (d, ³ J _C–P = 4.0 Hz, Me), 39.19 (d, ² J _C–P = 4.2 Hz, CH₂), 128.01 (d, ² J _C–P = 13.2 Hz, o-Ph), 130.81 (d, ⁴ J _C–P = 2.9 Hz, p-Ph), 131.44 (d, ³ J _C–P = 10.6 Hz, m-Ph), 135.45 (d, J _C–P = 124.3 Hz, P–C). ³¹P NMR (CDCl₃): δ = 77.6. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₄H₂₅N₂PSNa: 307.1374; found: 307.1375. 5b: ¹H NMR (CDCl₃): δ = 1.04 (t, J _H–H = 7.2 Hz, 12 H, Me), 3.07–3.19 (m, 8 H, CH₂), 3.84 (s, 3 H, OMe), 6.93 (dd, ⁴ J _H–P = 2.7 Hz, J _H–H = 9.0 Hz, 2 H, m-Ph), 7.88 (dd, ³ J _H–P = 12.6 Hz, J _H–H = 9.0 Hz, 2 H, o-Ph). ¹³C NMR (CDCl₃): δ = 13.37 (d, ³ J _C–P = 4.1 Hz, Me), 39.20 (d, ² J _C–P = 4.3 Hz, CH₂), 55.28 (s, OMe), 113.41 (d, ² J _C–P = 14.4 Hz, o-Ph), 126.68 (d, J _C–P = 130.4 Hz, P–C), 133.39 (d, ³ J _C–P = 12.0 Hz, m-Ph), 161.70 (d, ⁴ J _C–P = 3.2 Hz, p-Ph). ³¹P NMR (CDCl₃): δ = 77.0. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₅H₂₇N₂OPSNa: 337.1479; found: 337.1483. Anal. Calcd for C₁₅H₂₇N₂OPS: C, 57.30; H, 8.66; N, 8.91. Found: C, 58.12; H, 8.65; N, 8.01. 5c: ¹H NMR (CDCl₃): δ = 1.09 (t, J _H–H = 7.1 Hz, 12 H, Me), 3.14–3.25 (m, 8 H, CH₂), 7.01 (dd, J _H–P = 5.1 Hz, J _H–H = 3.6 Hz, 1 H, Th-CH), 7.15 (dd, J _H–P = 2.7 Hz, J _H–H = 3.8 Hz, 1 H, Th-CH), 7.24 (m, 2 H, Th-CH), 7.46 (dd, J _H–P = 8.0 Hz, J _H–H = 3.6 Hz, 1 H, Th-CH). ¹³C NMR (CDCl₃): δ = 13.34 (d, ³ J _C–P = 4.2 Hz, Me), 39.14 (d, ² J _C–P = 4.5 Hz, CH₂), 124.07 (d, ² J _C–P = 14.2 Hz, Th-CH), 124.60 (s, Th-CH), 125.42 (s, Th-CH), 128.01 (s, Th-CH), 135.34 (d, ³ J _C–P = 9.4 Hz, Th-CH), 136.45 (s, Th-CH), 137.18 (d, J _C–P = 134.0 Hz, P–C), 144.21 (d, ⁴ J _C–P = 6.0 Hz, Th-C). ³¹P NMR (CDCl₃): δ = 65.8. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₆H₂₅N₂PS₃Na: 395.0815; found: 395.0817. Anal. Calcd for C₁₆H₂₅N₂PS₃: C, 51.58; H, 6.76; N, 7.52. Found: C, 51.11; H, 6.71; N, 6.57. 5d: ¹H NMR (CDCl₃): δ = 1.16 (t, J _H–H = 7.1 Hz, 12 H, Me), 3.15–3.41 (m, 8 H, CH₂), 7.41–7.60 (m, 3 H), 7.70 (ddd, J = 1.2, 7.2, 16.5 Hz, 1 H), 7.84 (d, J _H–H = 7.8 Hz, 1 H), 7.92 (d, J _H–H = 8.4 Hz, 1 H), 9.00 (d, J _H–H = 8.4 Hz, 1 H). ¹³C NMR (CDCl₃): δ = 14.15 (d, ³ J _C–P = 2.4 Hz, Me), 40.64 (d, ² J _C–P = 4.8 Hz, CH₂), 124.31 (d, J _C–P = 14.8 Hz), 126.21 (s), 126.44 (s), 127.77 (d, ⁴ J _C–P = 5.4 Hz), 128.67 (d, ⁴ J _C–P = 1.7 Hz), 130.35 (d, ³ J _C–P = 9.4 Hz), 131.30 (d, J _C–P = 124.6 Hz, P–C), 132.35 (d, ³ J _C–P = 3.4 Hz), 132.70 (d, ² J _C–P = 10.9 Hz), 134.39 (d, ³ J _C–P = 10.4 Hz). ³¹P NMR (CDCl₃): δ = 73.4. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₈H₂₇N₂PSNa: 357.1530; found: 357.1528. Anal. Calcd for C₁₈H₂₇N₂PS: C, 64.64; H, 8.14; N, 8.38. Found: C, 64.61; H, 8.37; N, 7.79. 5e: ¹H NMR (CDCl₃): δ = 1.00 (t, J _H–H = 7.2 Hz, 12 H, Me), 2.96–3.18 (m, 8 H, Et), 3.41 (d, J _H–P = 14.7 Hz, 2 H, P–CH₂), 7.21–7.31 (m, 3 H), 7.40–7.43 (m, 2 H). ¹³C NMR (CDCl₃): δ = 13.93 (d, ³ J _C–P = 3.6 Hz, Me), 39.61 (d, ² J _C–P = 3.4 Hz, CH₂), 39.66 (d, J _C–P = 84.7 Hz, P–CH₂), 126.82 (d, ⁵ J _C–P = 3.7 Hz, p-Ph), 127.92 (d, ⁴ J _C–P = 3.2 Hz, 2 × C, m-Ph), 130.67 (d, ³ J _C–P = 5.7 Hz, 2 × C, o-Ph), 132.49 (d, ² J _C–P = 6.1 Hz). ³¹P NMR (CDCl₃): δ = 81.9. HRMS (ESI): m/z [M + Na]⁺ calcd for C₁₅H₂₇N₂PSNa: 321.1530; found: 321.1526.
• 10 4f: ¹H NMR (CDCl₃): δ = 7.13–7.69 (m, 7 H), 8.36 (ddd, J = 1.5, 3.3, 7.8 Hz, 1 H). ¹³C NMR (CDCl₃): δ = 124.23 (d, ⁴ J _C–P = 5.0 Hz, C–Br), 127.43 (s), 129.32 (d, J _C–P = 3.2 Hz), 129.53 (s), 130.25 (s), 131.65 (d, J _C–P = 5.0 Hz), 132.29 (s), 132.75 (s), 133.74 (d, J _C–P = 1.5 Hz), 137.50 (d, J _C–P = 62.1 Hz, P–C), 139.41 (d, ³ J _C–P = 8.3 Hz), 143.15 (d, ² J _C–P = 43.9 Hz). ³¹P NMR (CDCl₃): δ = 150.2.
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• 13 5f: ¹H NMR (CDCl₃): δ = 0.96 (t, J _H–H = 7.2 Hz, 6 H, Me), 1.09 (t, J _H–H = 7.2 Hz, 6 H, Me), 2.78–3.09 (m, 6 H, CH₂), 3.21–3.31 (m, 2 H, CH₂), 7.16–7.30 (m, 4 H), 7.40–7.50 (m, 2 H), 7.58–7.60 (m, 1 H), 7.90 (dd, J = 12.4, 7.6 Hz, 1 H). ¹³C NMR (CDCl₃): δ = 13.6 (d, ³ J _C–P = 5.2 Hz, Me), 13.7 (d, ³ J _C–P = 2.0 Hz, Me), 39.7 (d, ² J _C–P = 4.5 Hz, CH₂), 39.9 (d ² J _C–P = 3.8 Hz, CH₂), 124.4 (s, C–Br), 126.0 (s), 127.1 (d, J _C–P = 11.5 Hz), 128.7 (s), 130.0 (d, J _C–P = 2.8 Hz), 131.4 (s), 131.6 (d, J _C–P = 7.2 Hz), 132.2 (s), 133.0 (d, J _C–P = 125.5 Hz, P–C), 133.4 (d, J _C–P = 11.9 Hz), 142.3 (d, ³ J _C–P = 3.9 Hz), 144.9 (d, ² J _C–P = 11.8 Hz). ³¹P NMR (CDCl₃): δ = 73.1. HRMS (ESI): m/z [M + Na]⁺ calcd for C₂₀H₂₈N₂PSBrNa: 461.0792; found: 461.0792. Anal. Calcd for C₂₀H₂₈N₂PSBr: C, 54.67; H, 6.42. Found: C, 55.45; H, 6.34.