Rapid and Efficient Microwave-Assisted Hydrophosphinylation of Unactivated Alkenes with H-Phosphinic Acids without Added Metal or Radical Initiator

Abstract

A microwave-assisted hydrophosphinylation of unactivated alkenes with phosphinic acid and its derivatives under metal-free and initiator-free conditions is reported. Such hydrophosphinylations are operationally simple, use aqueous hypophosphorus acid, H-phenylphosphinic acid, and H-alkylphosphinic acids, and seem to proceed by a radical mechanism. Good isolated yields were obtained using a reasonable excess of the appropriate reagent.

Key words

phosphinic acid - organophosphorus compounds - microwaves - unactivated alkenes - hydrophosphinylation

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• 23 Typical Procedures Procedure A H₃PO₂ (50% aq, 0.21 mL, 2 mmol), 10-undecenoic acid (74 mg, 0.4 mmol), and dioxane (4 mL) were added to a 10 mL microwave vial containing a Teflon-coated stirrer bar and a septum. The vial was sealed and heated under microwave irradiation at 180 °C (100 W) for 1 h. After cooling to r.t. the reaction mixture was partitioned between CH₂Cl₂ (15 mL) and H₂O (10 mL), the organic phase was dried over Na₂SO₄, filtered, and concentrated under reduced pressure. Chromatographic purification on silica gel using CH₂Cl₂–MeOH–AcOH = 9:1:0.5 as eluent afforded 11-(hydroxyhydrophosphoryl)undecanoic acid (3i) as a white solid (40 mg, 45%); mp 82–84 °C.R_f  = 0.41 (CHCl₃–MeOH–AcOH = 7:1:0.5). ¹H NMR (200 MHz, DMSO): δ = 8.23 (br s, 1 H), 5.64 (s, 1 H), 2.18 (t, J = 7.2 Hz, 2 H), 1.51–1.18 (m, 18 H). ¹³C NMR (50 MHz, DMSO): δ = 174.6, 33.7, 29.9 (d, J _PCC = 15.1 Hz), 28.9, 28.9, 28.8, 29.7 (d, J _PC = 95.6 Hz), 28.6, 24.57, 20.8 (d, J _PCC = 2.7 Hz). ³¹P NMR (81 MHz, DMSO): δ = 31.74. MS (ES): m/z calcd for C₁₁H₂₃O₄P [M – H]^–: 249.13; found: 249.25. HRMS (TOF): m/z calcd for C₁₁H₂₃O₄P [M – H]^–: 249.1261; found: 249.1259. Procedure B Phenyphosphinic acid (28 mg, 0.2 mmol), benzyl allyl carbamate (76 mg, 0.4 mmol), and dioxane (4 mL) were added to a 10 mL microwave vial containing a Teflon-coated stirrer bar and a septum. The vial was sealed and heated under microwave irradiation at 160 °C (100 W) for 1 h. After cooling to r.t. the reaction mixture was purified by column chromatography on silica gel using CH₂Cl₂–MeOH–AcOH = 9:0.5:0.5 as eluent to give (3-{[(benzyloxy)carbonyl]amino}propyl)(phenyl)phosphinic acid (6f) as an oil (46 mg, 69%).R_f  = 0.50 (CHCl₃–MeOH–AcOH = 7:1:0.5). ¹H NMR (200 MHz, CDCl₃): δ = 7.84–7.01 (m, Ar, 10 H), 5.04 (s, 2 H), 3.13 (s, 2 H), 1.95–1.48 (m, 4 H). ¹³C NMR (50 MHz, CDCl₃): δ = 156.4, 136.5, 132.1, 131.0, 128.4, 128.0, 66.5, 41.1, 27.5 (d, J _PC = 92.8 Hz), 22.5. ³¹P NMR (81 MHz, CDCl₃): δ = 45.25. MS (ES): m/z calcd for C₁₇H₂₀NO₄P [M – H]^–: 332.11; found: 332.14. HRMS (TOF): m/z calcd for C₁₇H₂₀NO₄P [M – H]^–: 332.1057; found: 332.1068. Procedure C (4-Phenylbutyl)phosphinic acid (3f, 79 mg, 0.4 mmol), benzyl allylcarbamate (229 mg, 1.2 mmol), and dioxane (4 mL) were added to a 10 mL microwave vial containing a Teflon-coated stirrer bar and a septum. The vial was sealed and was heated under microwave irradiation at 180 °C (100 W) for 1 h. After cooling to r.t. the reaction mixture was purified by column chromatography on silica gel using CH₂Cl₂–MeOH–AcOH = 9:0.5:0.5 as eluent to (3-{[(benzyloxy)carbonyl]amino}propyl)(4-phenylbutyl)phosphinic acid (7b) as a white solid (78 mg, 50%); mp 137–139 °C.R_f  = 0.44 (CHCl₃–MeOH–AcOH = 9:0.5:0.5). ¹H NMR (200 MHz, CDCl₃): δ = 7.45–7.06 (m, 10 H), 5.05 (s, 2 H), 3.21 (s, 2 H), 2.58 (s, 2 H), 1.64 (s, 10 H). ¹³C NMR (50 MHz, CDCl₃): δ = 156.5, 141.8, 136.4, 128.5, 128.3, 128.0, 125.8, 66.6, 41.2, 35.4, 32.5 (d, J _PCC = 4.0 Hz), 30.9 (d, J _PC = 140.6 Hz), 30.6 (d, J _PC = 128.8 Hz), 29.6 (d, J _PCC = 4.0 Hz), 22.6, 22.1, 21.3, 14.1. ³¹P NMR (81 MHz, CDCl₃): δ = 59.66. ES-MS (TOF): m/z calcd for C₂₁H₂₈NO₄P [M – H]^–: 388.17; found: 388.22. HRMS: m/z calcd for C₂₁H₂₈NO₄P [M – H]^–: 388.1683; found: 388.1688. Procedure D H₃PO₂ (50% aq, 84 μL, 0.8 mmol), cyclododecene (66 mg, 0.4 mmol), and dioxane (4 mL) were added to a 10 mL microwave vial containing a Teflon-coated stirrer bar and a septum. The vial was sealed and heated under microwave irradiation at 180 °C (100 W) for 1 h. After cooling to r.t. the reaction mixture was partitioned between CH₂Cl₂ (15 mL) and H₂O (4 mL), the organic phase was dried over Na₂SO₄, filtered, and concentrated under reduced pressure. Chromatographic purification on silica gel using CH₂Cl₂–MeOH–AcOH = 9:1:0.5 as eluent afforded cyclododecylphosphinic acid (9e) as a viscous oil (26 mg, 28%).R_f  = 0.35 (CHCl₃–MeOH–AcOH = 9:1:0.5). ¹H NMR (200 MHz, CDCl₃): δ = 9.98 (s, 1 H), 6.91 (d, J = 527.5 Hz, 1 H), 1.45–1.30 (m, 23 H). ¹³C NMR (50 MHz, CDCl₃): δ = 33.7 (d, J _PC = 90.7 Hz), 25.5 (d, J _PCC = 13.1 Hz), 23.6, 23.2, 22.4. ³¹P NMR (81 MHz, CDCl₃): δ = 44.80. MS (ES): m/z calcd for C₁₂H₂₄O₂P [M – H]^–: 231.15; found: 231.27. HRMS (TOF): m/z calcd for C₁₂H₂₄O₂P [M – H]^–: 231.1519; found: 231.1512. Procedure E Phenyphosphinic acid (28 mg, 0.2 mmol), N-carbobenzoxy-3-pyrroline (81 mg, 0.4 mmol), and dioxane (4 mL) were added to a 10 mL microwave vial containing a Teflon-coated stirrer bar and a septum. The vial was sealed and heated under microwave irradiation at 160 °C (100 W) for 1 h. After cooling to r.t. the reaction mixture was purified by column chromatography on silica gel using CH₂Cl₂–MeOH–AcOH = 9:0.5:0.5 as eluent to give {1-[(benzyloxy)carbonyl]pyrrolidin-3-yl}(phenyl)phosphinic acid (10e) as a white solid (23 mg, 33%); mp 121–123 °C R_f  = 0.60 (CH₂Cl₂–MeOH–AcOH = 9:0.5:0.5). ¹H NMR (200 MHz, CDCl₃): δ = 7.64–7.29 (m, 10 H), 5.04 (s, 2 H), 3.33–3.12 (m, 4 H), 2.33 (s, 1 H), 1.86 (s, 2 H). ¹³C NMR (150 MHz, CDCl₃): δ = 154.4, 136.8, 132.0, 131.4, 130.8 (d,J _PCC = 4.3 Hz), 128.4, 127.8 (d,J _PCCC = 9.7 Hz), 66.7, 46.3 (d,J _PCCC = 32.8 Hz), 45.7, 39.3 (d,J _PCC = 93.8 Hz), 25.7 (d,J _PC = 103.2 Hz). ³¹P NMR (81 MHz, CDCl₃): δ = 40.14. MS (ES): m/z calcd for C₁₈H₁₉NO₄P [M – H]^–: 344.11; found: 344.16. HRMS (TOF): m/z calcd C₁₈H₁₉NO₄P [M – H]^–: 344.1057; found: 344.1061.