1341
Consequently, reactions exhibit cleaner products and more facile work-up procedures. Although few
papers have shown the application of microwave heating with a limited scope to the synthesis of
the carbon–carbon double bond6 using organophosphorus reagents, none of the work has examined
the formation of phosphonium salts, the precursors to the ylides utilized in the Wittig reaction. This
communication describes a general and facile preparation of phosphonium salts using microwave heating
to accelerate the reaction.
The desired phosphonium salts were synthesized by microwave-heated quaternization of triphenyl-
phosphine with an organic halide (Table 1).7 Several representative organic halides (2a–g) were treated
with triphenylphosphine (1), either neat or with xylene as the solvent, to afford the phosphonium
salts (3a–g) in excellent yields.8 In each case, the reaction times were reduced to 4 min or less from
conventional heating times ranging from 30 min to 14 days.3
In conclusion, we have demonstrated a rapid and general synthesis of phosphonium salts accomplished
using a variety of organic halides with heating in a domestic microwave oven. In addition to the
phosphonium salts being produced in excellent yields, the short reaction times should make this method
applicable to more sensitive organic halides used in the synthesis of Wittig reagents.
Acknowledgements
The author wishes to thank the North Carolina Biotechnology Center Institutional Development Grant
Program (9610-IDG-1009) for the purchase of the Bruker 400 MHz NMR used in this study.
References
1. For Part 1, see: Kiddle, J. J.; Gurley, A. F. Phosphorus, Sulfur Silicon Relat. Elem. 1999, accepted.
2. Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863–927.
3. Lawrence, N. J. In Preparation of Alkenes, a Practical Approach; Williams, J. M. J., Ed.; Oxford University Press: London,
1996; pp. 19–58, and references cited therein.
4. (a) Gedye, R.; Smith, F.; Westaway, K.; Humera, A.; Baldisera, L.; Laberge, L.; Roussel, J. Tetrahedron Lett. 1986, 27,
279–282. (b) Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetrahedron Lett. 1986, 27, 4945–4948.
5. For reviews, see: (a) Majetich, G.; Wheless, K. In Microwave-Enhanced Chemistry Fundamentals, Sample Preparation, and
Applications; Kingston, H. M.; Haswell, S. J., Eds.; American Chemical Society: Washington DC, 1997, pp. 455–505. (b)
Caddick, S. Tetrahedron 1995, 51, 10 403–10 432.
6. (a) Xu, C. D.; Chen, G. Y.; Huang, X. Chinese Chem. Lett. 1995, 6, 467–468. (b) Xu, C. D.; Chen, G. Y.; Huang, X. Org.
Prep. Proc. 1995, 27, 559–561. (c) Xu, C. D.; Chen, G. Y.; Huang, X. Synth. Commun. 1995, 25, 2229–2233. (d) Spinella,
A.; Fortuanti, T.; Soriente, A. Synlett 1997, 93–94.
7. Caution! It is hazardous to rapidly heat organic reactions in closed vessels by either traditional means or with microwave
irradiation. Therefore, caution should be exercised when conducting reactions of this type. A typical procedure for the
synthesis of the phosphonium salts follows: Organic halide (1 equiv.) and triphenylphosphine (1 equiv.) are added either
neat or in 5 ml of xylene to a pressure tube with a threaded Teflon cap (Ace Glass). The threaded cap on the pressure tube is
sealed finger tight, and the tube placed in a beaker surrounded by glass wool. The glass wool not only serves to evenly heat
the tube, but to absorb the reaction mixture if the tube were to shatter. The tube and beaker are then placed in the microwave
oven (Panasonic) and heated for the appropriate time on high power (1100 watts). Once the heating cycle is complete and the
beaker and tube have cooled to ambient temperature, the tube is opened and the phosphonium salts are removed and purified
by recrystallization from an appropriate solvent.
8. All compounds showed satisfactory melting points, 1H NMR, 13C NMR and 31P NMR data consistent with those previously
reported. See Ref. 3 and those sited therein.
9. Albright, T. A.; Freeman, W. J.; Schweizer, E. E. J. Am. Chem. Soc. 1975, 97, 2946–2950. Johnson, A. W.; Kyllingstad, V.
L. J. Org. Chem. 1966, 31, 334–336.