Stereocontrolled Copper Iodide Catalyzed Phosphorus-Carbon Bond Formation: An Efficient Synthesis of Scalemic Tertiary Phosphineboranes

 

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Abstract

A mild and efficient protocol for P-C bond formation was developed with sub-stoichiometric amounts of copper iodide as a catalyst. A series of sterically and electronically divergent phosphineboranes were subjected to coupling reactions and the corresponding products were obtained in high yields with retention of configuration. The reaction proceeded under non-basic conditions thus avoiding base-catalyzed racemization.

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The reaction was conducted in a wide range of solvents and the isolated yields of product (±)-2 were: CH₂Cl₂, 15%; CHCl₃, 10%; DMF, 25%; THF-DMS, 5%; and HMPA, 12%.

Other than the phosphineborane, we have also tested diethyl phosphate and ethyl diazoester under similar conditions but no trace of coupled product was detected; only starting material was recovered.

Absolute stereochemistry was assigned by analogy with compound 10a.

[( R ) - (Ethoxycarbonylmethyl)phenylmethylphosphino]-borane ( 2a); Typical Procedure A mixture of 1 (138 mg, 1.0 mmol), CuI (9.5 mg, 0.05 mmol, 5 mol%), and diazo compound (136 mg, 1.2 mmol) was dissolved in MeCN and the resulting solution was stirred at ambient temperature. After 3 h, the reaction mixture was quenched with a sat. solution of NH₄Cl, and the aqueous layer was extracted with EtOAc. The combined extracts were washed with a sat. solution of NaCl and dried over anhyd Na₂SO₄. The solvent was evaporated under reduced pressure and the resulting residue was subjected to column chromatography on silica gel (EtOAc-hexane, 2:8) to give (R_p )-2a (212 mg) in 95% yield as a colorless oil; [α]_D ²⁵ -5.9 (c 1.0, benzene). ¹H NMR (200 MHz, CDCl₃): δ = 1.15 (t, J = 6.77 Hz, 3 H), 1.80 (d, ³ J _P-H = 10.16 Hz, 3 H), 2.80-2.95 (m, 2 H), 4.00 (q, J = 4.23, 7.6 Hz, 2 H), 7.40-7.55 (m, 3 H), 7.65-7.80 (m, 2 H). MS (70 eV): m/z (%) = 224 (M⁺, 24).