Mild and Simple Preparation of Ketophosphine-boranes through Uncatalysed Hydrophosphination of Enones

Abstract

An efficient and simple method for the synthesis of γ-ketophosphines is reported. This method is based on the atom efficient hydrophosphination reaction using secondary phosphine-boranes as precursors. Different examples with various substrates and phosphine precursors are described. The reaction occurs without any ­catalyst upon simple thermal activation (50-80 °C). Reduction of a γ-ketophosphine yields in good yield and with an excellent dia­stereoselectivity a new hydroxyphosphine.

Key words

enones - phosphorus - boron - γ-ketophosphines - hydrophosphination

References

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14

All compounds gave satisfactory NMR spectra and HRMS analyses. Solid compounds were also characterised by elemental analysis.

16

Typical Procedure for Microwave Reaction.
A quartz reactor was charged with phosphine-borane 4 (50 mg, 0.24 mmol) and an excess of methylvinylketone (97 µL, 1.20 mmol). The reactor was placed into the microwaves device and the mixture was stirred for 30 min at 80 °C. Then the solution was cooled to r.t. The crude product was directly purified on silica gel chromatography (CH₂Cl₂). The product was dried under oil pump vacuum, affording phosphine-borane 8 as a white powder in 53% yield (36 mg). Mp 71 °C. ³¹P NMR (101.25 MHz, CDCl₃): δ = 26.68 (qm, ¹ J _PB = 73.9 Hz). ¹H NMR (250.13 MHz, CDCl₃): δ = 2.65 (dt, 2 H, ³ J _HH = 7.5 Hz, ³ J _HP = 8.2 Hz), 2.11 (s, 3 H), 1.90-1.65 (m, 14 H), 1.40-1.15 (m, 10 H), 0.24 (q, 3 H, ¹ J _BH = 72.5 Hz). ¹³C NMR (100.61 MHz, CDCl₃): δ = 207.31 (d, ³ J _CP = 10.9 Hz), 38.30 (s), 32.30 (d, ¹ J _CP = 33.0 Hz), 30.20 (s), 27.19 (d, ³ J _CP = 10.9 Hz), 27.17 (s), 26.87 (d, ⁴ J _CP = 2.3 Hz), 26.30 (s), 12.42 (d, ¹ J _CP = 33.9 Hz). ¹¹B NMR (128.38 MHz, C₆D₆): δ = -39.8 (dq, ¹ J _BP = 73.9 Hz, ¹ J _BH = 72.5 Hz). HRMS: m/z calcd for [M - H]⁺ = 281.22056; found: 281.2220; m/z calcd for [M - BH₃]⁺ = 268.19560; found: 268.1967. Anal. Calcd (%) for C₁₆H₃₂BOP: C, 68.10; H, 11.43. Found: C, 67.88; H, 11.77.

17

Typical Procedure for Oil Bath Reaction.
An oven-dried schlenk tube was charged with phosphine-borane precursor 1 (1 g, 5 mmol). After three cycles of oil pump vacuum/nitrogen, cyclopent-2-enone (504 µL,
6 mmol) was added under nitrogen flow. At last, toluene (1 mL) was added to the solution. The mixture was stirred for 24 h at 50 °C, then cooled to r.t. and directly purified on silica gel chromatography (CH₂Cl₂-EtOAc, 95:5). The product was dried under oil pump vacuum, affording phosphine-borane 11 as a white solid in 74% yield (1.044 g). Mp 97 °C. ³¹P NMR (161.98 MHz, CDCl₃): δ = 22.9 (qm, ¹ J _PB = 65.9 Hz). ¹H NMR (400.13 MHz, CDCl₃): δ = 7.83-7.66 (m, 4 H), 7.60-7.40 (m, 6 H), 3.20-3.14 (m, 1 H), 2.60-2.00 (m, 6 H), 0.93 (q, 3 H, ¹ J _HB = 91.8 Hz). ¹³C NMR (62.90 MHz, CDCl₃): δ = 215.77 (d, ³ J _CP = 12.6 Hz), 132.48 (d, ² J _CP = 8.8 Hz), 132.28 (d, ² J _CP = 8.8 Hz), 131.61 (d, ⁴ J _CP = 1.9 Hz), 129.02 (d, ³ J _CP = 10.1 Hz), 128.22 (d, ¹ J _CP = 54.7 Hz), 127.82 (d, ¹ J _CP = 54.7 Hz), 39.77 (d, ² J _CP = 2.5 Hz), 38.05 (d, ³ J _CP = 5.7 Hz), 31.64 (d, ¹ J _CP = 40.2 Hz), 24.21 (d, ² J _CP = 2.5 Hz). ¹¹B NMR (128.38 MHz, CDCl₃): δ = -39.59 (dq, ¹ J _BP = 69.5 Hz, ¹ J _BH = 91.8 Hz). HRMS: m/z calcd for [M - H]⁺ = 281.12666; found: 281.1246; m/z calcd for [M - BH₃]⁺ = 268.1017; found: 268.1001. Anal. Calcd (%) for C₁₇H₂₀BOP: C, 72.37; H, 7.17. Found: C, 72.19; H, 7.17.