Direct Conversion of sec-Phosphine Oxides into Phosphinous Acid-Boranes

Abstract

Two straightforward and general procedures for the synthesis of phosphinous acid-boranes directly from sec-phosphine oxides and borane reagents under basic and under neutral conditions have been developed. The conversion is stereoselective and occurs with predominant inversion of configuration at P. The protonation constants of a representative series of phosphinous acid-phosphinous acid-boranes have been determined.

phosphinous acid-boranes - acidity - stereoselectivity - sec-phosphine oxides - P-stereogenic

References

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For alkoxy analogues (R′O)(R′′O)P(O)·BH₃ ^- see:

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8

Typical Procedure for Preparation of Phosphinous Acid-Boranes by Reaction of Secondary Phosphine Oxide Anions and BH ₃ ·THF complex:
In a reaction flask equipped with magnetic stirrer and dry argon inlet was placed secondary phosphine oxide (0.5 mmol) in 15 mL of dry THF. Next, an equimolar amount of a base (NaH or butyllithium) (0.5 mmol) was added under argon atmosphere. After 15 min at r.t. 1 M BH₃·THF complex (0.75 mmol) in THF was added. The reaction mixture was then stirred at r.t. for two h. Aq HCl (1 mL) was added and the reaction mixture was extracted several times with CH₂Cl₂, organic layers were collected, dried over anhyd MgSO₄, and evaporated. The crude product was purified by flash chromatography using hexane:EtOAc (2:1) as eluent.
t-Butylphenylphosphinous Acid-Borane (2a). Yield 98% (74%). ¹H NMR (CDCl₃): δ = 0.00-1.70 (br m, 3 H), 1.14 (d, J _P-C = 14.66 Hz, 9 H), 4.52 (br s, 1 H), 7.41-7.89 (m, 3 H), 7.69-7.85 (m, 2 H) ppm. ¹³C NMR (CDCl₃): δ = 23.85, 23.92, 31.31, 32.13, 127.82, 128.03, 131.20, 131.26, 131.46 ppm. ³¹P NMR (CDCl₃): δ = 114.39 ppm (m). Anal. Calcd for C₁₀H₁₈BOP: C, 61.27; H, 9.26. Found: C, 61.26; H, 9.08.
Benzylphenylphosphinous Acid-Borane (2b). Yield 63%. ¹H NMR (CDCl₃): δ = 0.07-1.65 (br m, 3 H), 3.38 (d,
J _P-H = 10.22 Hz, 2 H), 4.90 (br s, 1 H), 7.01-7.13 (m, 2 H), 7.21-7.34 (m, 3 H), 7.40-7.75 (m, 5 H) ppm. ¹³C NMR (CDCl₃): δ = 39.41, 40.14, 126.68, 126.74, 127.98, 128.03, 128.08, 128.28, 129.95, 130.04, 130.19, 130.41, 131.06, 131.13, 131.18, 131.51, 131.56, 132.32 ppm. ³¹P NMR (CDCl₃): δ = 101.64 ppm (m). Anal. Calcd for C₁₃H₁₆BOP: C, 67.87; H, 7.01. Found: C, 67.96; H, 7.18.
(2-Naphthylmethyl)phenylphosphinous Acid-Borane (2c). Yield 63% (67%). ¹H NMR (CDCl₃): δ = 0.05-1.72 (br m, 3 H), 3.52 (d, J _P-H = 9.98 Hz, 2 H), 3.60 (br s, 1 H), 7.11-7.21 (m, 1 H), 7.36-7.56 (m, 5 H), 7.56-7.89 (m, 6 H) ppm. ¹³C NMR (CDCl₃): δ = 39.64, 40.36, 125.71, 126.02, 127.39, 127.51, 127.68, 128.12, 128.25, 128.46, 128.64, 128.77, 128.89, 130.26, 130.48, 131.73, 131.77, 132.22 ppm. ³¹P NMR (CDCl₃): δ = 103.16 ppm (m). Anal. Calcd for C₁₇H₁₈BOP: C, 72.99; H, 6.48. Found: C, 73.04, H, 6.53.
o-Anisylphenylphosphinous Acid-Borane (2d). Yield 43%. ¹H NMR (CDCl₃): δ = 0.20-1.96 (br m, 3 H), 3.81 (s, 3 H), 5.11 (br s, 1 H), 6.91-7.04 (m, 1 H), 7.10-7.22 (m, 1 H), 7.38-7.76 (m, 6 H), 7.83-8.00 (m, 1 H) ppm. ¹³C NMR (CDCl₃): δ = 55.98, 111.37, 121.49, 121.75, 128.13, 128.34, 130.04, 130.27, 131.10, 131.15, 134.11, 134.33 ppm. ³¹P NMR (CDCl₃): δ = 98.00 ppm (m). Anal. Calcd for C₁₃H₁₆BO₂P: C, 63.46; H, 6.55. Found: C, 63.44; H, 6.57.

9 Reetz T. J. Am. Chem. Soc.  1960,  82:  5039 

10

General Procedure for the Synthesis of Phosphinous Acid-Boranes by the Reaction of Secondary Phosphine Oxides with NaBH ₄ /BF ₃ :
To the solution of secondary phosphine oxide (0.5 mmol) in 15 mL of dry THF ethereal solution of BF₃ (2 mmol) was added under argon atmosphere. Promptly after, NaBH₄ (1.5 mmol) was added to the reaction mixture. Then, the reaction flask was fitted with a reflux condenser and the reaction mixture was heated to reflux for 3 h. The reaction mixture was then allowed to cool to r.t. and aq HCl (1 mL) was added to quench the reaction. The reaction mixture was extracted several times with CH₂Cl₂, organic layers were collected, dried over anhyd MgSO₄, and evaporated. The crude product was purified by flash chromatography using hexane/EtOAc (2:1) as eluent.
[(2-Methyl)-1-naphthyl]phenylphosphinous Acid-Borane (2f). Yield 86%. ¹H NMR (CDCl₃): δ = 0.30-1.96 (br m, 3 H), 2,77 (s, 3 H), 5.65 (br s, 1 H), 7.32-7.54 (m, 6 H), 7.54-7.72 (m, 2 H), 7.78-7.98 (m, 2 H), 8.40-8.58 (m, 1 H) ppm. ¹³C NMR (CDCl₃): δ = 24.14, 24.27, 112.20, 125.27, 126.54, 126.89, 127.03, 128.44, 128.57, 128.66, 129.94, 130.03, 130.17, 130.24, 131.16, 131.20, 132.88, 132.93 ppm. ³¹P NMR (CDCl₃): δ = 99.14 ppm (m). Anal. Calcd for C₁₇H₁₈BOP: C, 72.99; H, 6.48. Found: C, 73.11; H, 6.59.
Phenyl-o-tolylphosphinous Acid-Borane (2g). Yield 82%. ¹H NMR (CDCl₃): δ = 0.30-2.00 (br m, 3 H), 2.28 (s, 3 H), 4.45 (br s, 1 H), 7.18-7.60 (m, 6 H), 7.60-7.78 (m, 2 H), 7.92-8.10 (m, 1 H) ppm. ¹³C NMR (CDCl₃): δ = 21.28, 21.38, 125.58, 125.81, 128.43, 128.64, 130.68, 130.92, 131.35, 131.52, 131.61, 132.04, 132.64, 132.93 ppm. ³¹P NMR (CDCl₃): δ = 98.57 ppm (m). Anal. Calcd for C₁₃H₁₆BOP: C, 67.87; H, 7.01. Found: C, 67.94; H, 7.16.
Di-c-hexylphosphinous Acid-Borane (2h). Yield 91%. ¹H NMR (CDCl₃): δ = -0.45-1.35 (br m, 3 H), 1.15-1.62 (m, 10 H), 1.66-1.97 (m, 12 H), 4.13 (br s, 1 H) ppm. ¹³C NMR (CDCl₃): δ = 24.79, 24.84, 25.70, 25.94, 26.35, 26.54, 26.61, 34.20, 34.95 ppm. ³¹P NMR (CDCl₃): δ = 119.36 ppm (m). Anal. Calcd for C₁₂H₂₆BOP: C, 63.18; H, 11.49. Found: C, 63.04; H, 11.40.
i-Propylphenylphosphinous Acid-Borane (2i). Yield 82%. ¹H NMR (CDCl₃): δ = -0.09-1.5 (br m, 3 H), 1.02-1.23 (m, 6 H), 2.02-2.24 (m, 1 H), 4.65 (br s, 1 H), 7.43-7.61 (m, 3 H), 7.71-7.87 (m, 2 H) ppm. ¹³C NMR (CDCl₃): δ = 15.31, 15.49, 28.91, 29.77, 128.13, 128.33, 128.66, 128.86, 130.56, 130.77, 131.31, 131.36 ppm. ³¹P NMR (CDCl₃): δ = 110.09 ppm (m). Anal. Calcd for C₉H₁₆BOP: C, 59.39; H, 8.86. Found: C, 59.28; H, 8.85.
c-Hexylphenylphosphinous Acid-Borane (2j). Yield 90%. ¹H NMR (CDCl₃): δ = -0.09-1.5 (br m, 3 H), 1.11-1.64 (m, 6 H), 1.64-2.02 (m, 5 H), 4.25 (br s, 1 H), 7.42-7.61 (m, 3 H), 7.70-7.85 (m, 2 H) ppm. ¹³C NMR (CDCl₃): δ = 25.05, 25.16, 25.79, 26.10, 26.36, 38.80, 39.65, 128.11, 128.31, 130.55, 130.76, 131.20, 131.24 ppm. ³¹P NMR (CDCl₃): δ = 107.13 ppm (m). Anal. Calcd for C₁₂H₂₀BOP: C, 64.90; H, 9.08. Found: C, 64.83; H, 9.18.
Di-n-hexylphosphinous Acid-Borane (2k). Yield 90%. ¹H NMR (CDCl₃): δ = -0.30-1.40 (br m, 3 H), 0.84-1.04 (m, 6 H), 1.24-1.52 (m, 12 H), 1.52-1.92 (m, 8 H), 4.08 (br s, 1 H) ppm. ¹³C NMR (CDCl₃): δ = 14.06, 21.79, 22.48, 28.68, 29.47, 30.54, 30.797, 31.35, ³¹P NMR (CDCl₃): δ = 116.06 ppm (m). Anal. Calcd for C₁₂H₃₀BOP: C, 62.08; H, 13.03. Found: C, 61.99; H, 12.97.
Phenyl-p-tolylphosphinous Acid-Borane (2l). Yield 65%. ¹H NMR (CDCl₃): δ = 0.20-1.95 (br m, 3 H), 2.41 (s, 3H), 5.08 (br s, 1 H), 7.21-7.32 (m, 2 H), 7.36-7.56(m, 3 H), 7.56-7.81 (m, 4 H) ppm. ¹³C NMR (CDCl₃): δ = 21.54, 128.30, 128.52, 129.12, 129.34, 130.67, 130.83, 130.90, 131.07, 131.34 ppm. ³¹P NMR (CDCl₃): δ = 93.65 ppm (m). Anal. Calcd for C₁₃H₁₆BOP: C, 67.87; H, 7.01. Found: C, 67.66; H, 7.17.

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Stankeviè, M.; Pietrusiewicz, K. M. to be published.

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