Methyltriphenylphosphonium Bromide1

Ph3+PMe Br-

[1779-49-3]  · C19H18BrP  · Methyltriphenylphosphonium Bromide  · (MW 357.24)

(forms methylenetriphenylphosphorane upon treatment with butyllithium,2 sodamide,3 dimsylsodium,4 or potassium t-butoxide;5 also used as a phase-transfer catalyst6)

Physical Data: mp 234-235 °C.

Solubility: sol alcohols, DMSO; insol Et2O, THF, benzene, toluene, hexane.

Form Supplied in: white solid; widely available. Drying: when used to prepare methylenetriphenylphosphorane, methyltriphenylphosphonium bromide must be strictly dried immediately prior to use for best results. Drying can be achieved effectively by heating the flask containing the phosphonium salt at 110 °C/0.2 mmHg for several hours, or by repetitive (three times or more) azeotropic removal of water with benzene or toluene.

Preparation of Methylenetriphenylphosphorane.

Methyltriphenylphosphonium bromide can be readily deprotonated with a strong base to give the corresponding methylide (eq 1),2 -5,7,8 the most versatile reagent for Wittig methylenation (see also Methylenetriphenylphosphorane). n-Butyllithium is the most widely preferred base because of its ready availability. The solvents chosen in this case are invariably Et2O,2a,b THF,2c,d or DME.2e A typical procedure involves suspension of the dry phosphonium salt in THF and subsequent addition of 1 equiv of 1.6 M solution of butyllithium in hexane at rt followed by stirring at rt for 3 h.2c Other lithium bases, although less frequently used, include Phenyllithium, Methyllithium, and Lithium Diisopropylamide.7

Sodium Amide3 is generally used to produce a salt-free ylide. The ylide is generated by stirring a mixture of methyltriphenylphosphonium bromide and NaNH2 in anhydrous liquid ammonia.2d,3a Upon completion, ammonia is allowed to evaporate, a dry solvent (Et2O, THF, or toluene) is added, and the resultant ylide solution is now ready to use. NaNH2 in refluxing THF also can be used instead of liquid ammonia.3b,c Dimsylsodium (Sodium Methylsulfinylmethylide) is also widely used to provide the ylide.4 The required dimsylsodium solution is prepared by stirring a mixture of NaH in dry DMSO at 75-80 °C for ca. 45 min. The resultant solution is cooled to rt followed by addition of methyltriphenylphosphonium bromide in DMSO. In some applications the base used is Potassium t-Butoxide.5 The reaction is usually run in THF or benzene at rt. Potassium t-pentylate is also used occasionally.8

Phase-Transfer Catalysis.

Due to the higher thermal stability of phosphonium salts as compared to that of ammonium salts, methyltriphenylphosphonium bromide has been used as a phase-transfer catalyst in a number of reactions.6 4,4-Biphenyldicarboxylic acid has been efficiently converted to the corresponding diacid dichloride with Thionyl Chloride in the presence of a catalytic amount of methyltriphenylphosphonium bromide (eq 2).6a Likewise, the yields of thiocyanatoalkyl methacrylic esters have been increased under phase-transfer conditions (eq 3).6b In most cases, however, ammonium salts exhibit far better phase-transfer catalytic activities than phosphonium salts.6c

1. (a) Maercker, A. OR 1965, 14, 270. (b) House, H. O. Modern Synthetic Reactions; Benjamin: Menlo Park, CA, 1972; pp 682-709.
2. (a) Wittig, G.; Schoellkopf, U. OSC 1973, 5, 751. (b) Paquette, L. A.; Teleha, C. A.; Taylor, R. T.; Maynard, G. D.; Rogers, R. D.; Gallucci, J. C.; Springer, J. P. JACS 1990, 112, 265. (c) Stevens, R. V.; Albizati, K. F. JOC 1985, 50, 632. (d) Vedejs, E.; Meier, G. P.; Snoble, K. A. J. JACS 1981, 103, 2823. (e) Danishefsky, S.; Schuda, P. F.; Kitahara, T.; Etheredge, S. J. JACS 1977, 99, 6066.
3. (a) Seyferth, D.; Singh, G. JACS 1965, 87, 4156. (b) Koster, R.; Simic, D.; Grassberger, M. A. LA 1970, 739, 211. (c) Kirchmeyer, S.; De Meijere, A. HCA 1990, 73, 1182.
4. Greenwald, R.; Chaykovsky, M.; Corey, E. J. JOC 1963, 28, 1128.
5. (a) Levy, L. A.; Sashikumar, V. P. JOC 1985, 50, 1760. (b) Paquette, L. A.; Stevens, K. E. CJC 1984, 62, 2415.
6. (a) Burdett, K. A. S 1991, 441. (b) Berthe, M.-C.; Fort, Y.; Caubere, P. SC 1992, 22, 617. (c) Herriott, A. W.; Picker, D. JACS 1975, 97, 2345.
7. Shen, Y.; Qiu, W.; Xin, Y.; Huang, Y. S 1984, 924. (b) Barton, D. H. R.; Hansen, P. E.; Picker, K. JCS(P1) 1977, 1723. (c) Marino, J. P.; Abe, H. S 1980, 872.
8. (a) Shibuya, H.; Kawashima, K.; Narita, N.; Kitagawa, I. CPB 1992, 40, 1166. (b) Smith, A. B., III; Jerris, P. J. JOC 1982, 47, 1845.

Kevin C. Lee

DuPont Agricultural Products, Newark, DE, USA

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