Diphenylphosphine

Ph2PH

[829-85-6]  · C12H11P  · Diphenylphosphine  · (MW 186.20)

(readily deprotonated to give diphenylphosphide;1a useful for the synthesis of phosphorus-based alkenation reagents;2 source of phosphorus-centered radical3)

Physical Data: bp 280 °C; d 1.070 g cm-3.

Solubility: sol ethanol, ether, benzene, concd HCl; insol H2O.

Form Supplied in: colorless liquid.

Analysis of Reagent Purity: 1H NMR (P-H) d 5.14; 31P NMR (P-H, 85% H3PO4) d +40.7.4

Preparative Methods: from Ph3P,1b Ph2PCl3,5 or Ph2POCl.5

Purification: distillation.1b

Handling, Storage, and Precautions: foul-smelling liquid. Very easily oxidized in air; pyrophoric; light-sensitive.

Alkylation.

The alkylation of diphenylphosphine has been used extensively for the preparation of novel phosphine ligands, Wittig-Horner reagents, and phosphonium salts. Deprotonation of diphenylphosphine with n-Butyllithium gives the characteristically orange solution of lithium diphenylphosphide,1 providing access to the desired tertiary phosphines via an SN2 or SN2 displacement.2 Phosphonium salts are also readily available by direct reaction of an appropriate alkylating agent and diphenylphosphine.6 Optically active bis(diphenylphosphino) ligands can be prepared through tosylate displacement/epoxide ring opening. In some cases, complete regio- and stereoselectivity can be achieved (eq 1).7 Elaborate diphenylphosphine oxide-based alkenation reagents can be synthesized by alkylation, followed by direct oxidation to provide advanced precursors of complex natural products (eq 2).2

During the preparation of many phosphines, due care must be taken to avoid phosphorus oxidation. The use of the diphenylphosphine-borane complex circumvents the problem of phosphine oxidation. The diphenylphosphine-borane complex is not only protected from oxidation, but is also activated so that it can be reacted under mild conditions (methanolic KOH, 0 °C) with a variety of electrophiles (eq 3). The desired tertiary phosphine can be obtained by decomplexation with a large excess of a secondary amine.8

Addition to Alkenes and Alkynes.

The addition of diphenylphosphine to unsaturated systems under radical conditions provides access to a variety of functionalized phosphines.9 Phosphorus-centered radicals can be formed under standard (Azobisisobutyronitrile, refluxing benzene) or photochemical conditions in a manner similar to silicon-, tin-, and selenium-based radicals. Highly functionalized phosphines are obtained upon radical cyclization of dienes or enynes (eq 4).3 Tin- and silicon-substituted phosphines are available from the corresponding vinylstannane10 or -silane.11 Phosphine radical addition to diketene provides access to phosphine-substituted lactones.12

Michael addition of diphenylphosphine to activated alkenes provides products useful for the construction of phosphine ligands. These reactions can be carried out under neutral,13 acidic,14 or basic conditions,15 or via the preformed diphenylphosphide.16 Diastereoselective addition of lithium diphenylphosphide to lactone (1) provides a good yield of the trans adduct (eq 5).16 Double addition to bromomaleic anhydride provides useful yields of the bis(diphenylphosphino) adduct as the trans isomer (eq 6).14

Addition to C=X Bonds.

The nucleophilic nature of diphenylphosphine (and diphenylphosphide) can be utilized in the addition to carbon-heteroatom double bonds. The 1,2-addition to aldehydes,17 ketones,18 imines,19 isocyanates, isothiocyanates, carbodiimides,20 cyclic carbonates,21 and 2-halo heterocycles22 provide a host of interesting phosphines. Addition of diphenylphosphine to aromatic aldehydes in alcoholic media or dilute acid gives a-alkoxy17 or a-hydroxy18a benzylphosphines, respectively. Alternatively, employment of a typical Lewis acid in aprotic media also leads to a-hydroxy phosphines, which air oxidize to the a-hydroxy phosphine oxides. However, addition to aldehydes in the presence of a Lewis acid and a catalytic amount of niobium(V) chloride provides phosphine oxides (eq 7).18b,c The use of concentrated protic acid media also provides a simple, high-yielding synthesis of unsymmetrical phosphine oxides (eq 8).18a

Other Applications.

Lithium diphenylphosphide can be used to effect a number of functional group interconversions, such as the dealkylation of alkyl aryl ethers,23,1b or the dehydroxylation of a-hydroxy ketones.24 In some cases, 1,2-dibromoalkenes can be converted into alkynes by treatment with lithium diphenylphosphide in refluxing THF.25 Treatment of epoxides with lithium diphenylphosphide followed by phosphine oxidation,26 or conversion into the phosphonium salt27 ultimately provides a method for alkene inversion (eq 9).


1. (a) Wittenberg, D.; Gilman, H. JOC 1958, 23, 1063. (b) Ireland, R. E.; Walba, D. M. OSC 1988, 6, 567.
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9. (a) Kirby, A. J.; Warren, S. G. The Organic Chemistry of Phosphorus; Elsevier: Amsterdam, 1967; pp 158-183; (b) Bentrude, W. G. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, pp 595-663; (c) Stacey, F. W.; Harris, J. F., Jr. OR 1963, 13, 150; (d) Emsley, J.; Hall, D. The Chemistry of Phosphorus; Harper and Row: London, 1976; pp 352-378.
10. Mitchell, T. N.; Belt, H.-J. JOM 1988, 345, C28.
11. (a) Iyer, S. R.; Tueting, D. R.; Schore, N. E. JOM 1987, 320, 339. (b) Holmes-Smith, R. D.; Osei, R. D.; Stobart, S. R. JCS(P1) 1983, 861.
12. Dingwall, J. G.; Tuck, B. JCS(P1) 1986, 2081.
13. Mann, F. G.; Millar, I. T. JCS 1952, 4453.
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19. (a) Barluenga, J.; Campos, P. J.; Canal, G.; Asensio, G. SL 1990, 261. (b) Grim, S. O.; Matienzo, L. J. TL 1973, 2951. (c) Pudovik, A. N.; Romanov, G. V.; Pozhidaev, V. M. ZOB 1978, 48, 1008.
20. Thewissen, D. H. M. W.; Ambrosius, H. P. M. M. RTC 1980, 99, 344.
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24. Leone-Bay, A. JOC 1986, 51, 2378.
25. Gillespie, D. G.; Walker, B. J. TL 1975, 4709.
26. (a) Bridges, A. J.; Whitham, G. H. CC 1974, 142. (b) Boeckh, D.; Huisgen, R.; Nöth, H. JACS 1987, 109, 1248.
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David W. Piotrowski

DuPont Agricultural Products, Newark, DE, USA



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