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Synlett 2004(2): 0311-0315  
DOI: 10.1055/s-2004-815411
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Marek Stankeviča, Grzegorz Andrijewskib, K. Michał Pietrusiewicz*a,c
a Institute of Organic Chemistry, the Polish Academy of Scacid-boraneiences, Kasprzaka 44/52, 01-224 Warsaw, Poland
b University of Łódź, Department of Chemistry and Physics, Narutowicza 68, 90-136 Łódź, Poland
c Maria Curie-Skodowska University, Department of Organic Chemistry, Gliniana 33, 20-614 Lublin, Poland
Fax: +48(22)6326681; e-Mail: kmp@icho.edu.pl;
Further Information

Publication History

Received 31 October 2003
Publication Date:
12 January 2004 (online)

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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.

Key words

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

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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.

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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.