Synthesis of Symmetrical and Unsymmetrical Functionalized Arylphosphines from Chlorophosphines and Organozinc Reagents

Abstract

A stepwise procedure allowing the formation of symmetrical arylphosphines is described. It relies on the use of preformed functionalized aromatic organozinc reagents to perform arylations of chlorophosphines. Some preliminary results concerning the synthesis of unsymmetrical diarylphenylphosphines through sequential coupling of organozinc species with dichlorophenylphosphine are also reported.

Key words

phosphorus - arylation - organometallic reagents - chlorophosphines - functionalized arylphosphines

References and Notes

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• 9 The original procedure: Imamoto T. Kusumoto T. Suzuki N. Sato K. J. Am. Chem. Soc.  1985,  107:  5301.  This procedure, which reported the concomitant reduction-protection of phosphine oxides, was simplified according to the fact that no reduction was necessary. Consequently LiAlH₄ has been removed from the procedure

7

In a typical procedure, about 10-12 mmol of the organozinc reagent were prepared from 15 mmol of an aryl bromide according to the method described in ref. 6. To the filtered solution containing the organozinc reagent was added chlorodiphenylphosphine (10 mmol) at r.t. Stirring was continued for additional 2 h at r.t. The reaction was quenched using a 5% HCl solution and the resulting mixture was extracted with CH₂Cl₂. After evaporation to dryness, the chromatographic purification of the crude oil over silica gel using a pentane-Et₂O mixture as an eluent afforded the analytically pure phosphine derivative.

8

The protection of phosphines could be verified using ³¹P NMR (80 MHz, CDCl₃). Indeed, the δ value shifts from ca. -12 ppm (R₃P) to ca. 13 ppm (R₃PBH₃).

10

In order to add exactly the required amount of the organozinc compound, a titration was realized as follows: aliquots of the solution were exposed to iodine and analyzed using GC. The amount of aryl iodide so obtained was compared to the amount of the starting aryl bromide using dodecane as internal standard.

11

Coupling products were characterized using ¹H NMR (200 MHz, CDCl₃), ¹³C NMR (50 MHz, CDCl₃) when required, ³¹P NMR (80 MHz, CDCl₃), mass spectroscopy and, when useful, ¹⁹F NMR (188 MHz, CDCl₃) and FT-IR spectroscopy.
Some Data for Selected Compounds.
Diphenyl[3-(trifluoromethyl)phenyl]phosphine (1d): ¹H NMR: δ = 7.80-7.10 (m, 14 H) ppm. ³¹P NMR: δ = -9.99 ppm. ¹⁹F NMR: δ = -62.38 ppm. MS: m/z (rel. intensity) = 330 (100) [M], 251 (19) [M - 79], 203 (18) [M - 127], 183 (48) [M - 147], 108 (22) [M - 222].
Phenylbis[3-(trifluoromethyl)phenyl]phosphine (2c): ¹H NMR: δ = 7.84-7.23 (m, 13 H) ppm. ³¹P NMR: δ = -9.97 ppm. ¹⁹F NMR: δ = -62.71 ppm. MS: m/z (rel. intensity) = 399 (24) [M + 1], 398 (100) [M], 397 (21) 9M - 1], 251 (40) [M - 147], 203 (44) [M - 195], 183 (22) [M - 215].
Tris(3-methylphenyl)phosphine (3g): ¹H NMR: δ = 7.15-6.94 (m, 12 H), 2.19 (s, 9 H) ppm. ³¹P NMR: δ = -10.22 ppm. MS: m/z (rel. intensity) = 304 (100) [M], 303 (37) [M - 1], 211 (36) [M - 93], 197 (20) [M - 107]. (4-Methoxyphenyl)(phenyl)(o-tolyl)phosphine oxide (4c): ¹H NMR: δ = 7.83-6.92 (m, 13 H), 3.81 (s, 3 H), 2.43 (s, 3 H) ppm. ³¹P NMR: δ = 29.08 ppm. MS: m/z (rel. intensity) = 322 (40) [M], 321 (100) [M - 1], 213 (21) [M - 109].