[16721-45-2]  · C25H21P  · Benzylidenetriphenylphosphorane  · (MW 352.41)

(Wittig reagent for the conversion of aldehydes into alkenes1)

Form Supplied in: usually, Wittig reagents are prepared in situ from the appropriate phosphonium salt. As such, benzyltriphenylphosphonium chloride is available as a white powder or as a mixture with sodium amide, NaNH2.

Preparative Methods: benzylidenetriphenylphosphorane is prepared from benzyltriphenylphosphonium chloride by deprotonation with base.1,2 Formation of the ylide from this salt has been accomplished with many different base and solvent combinations. In more recent preparations the salt typically is suspended in dry THF under an inert atmosphere and a solution of the base, either BuLi or KHMDS, is added by syringe.3a,b Alternatively, the salt and NaNH2 are transferred to a flask in a dry box and then benzene is added.4a The ylide can also be generated with alkoxide bases in protic solvents5 (such as NaOEt/EtOH) or under phase transfer conditions (CH2Cl2/H2O with NaOH).6

Purification: benzyltriphenylphosphonium chloride can be recrystallized from absolute ethanol or CHCl3/petroleum ether.

Handling, Storage, and Precautions: benzyltriphenylphosphonium chloride is a slightly hygroscopic solid. It can be dried efficiently under vacuum at elevated temperature (<10 Torr and 40 °C) overnight. Benzylidenetriphenylphosphorane is destroyed rapidly by exposure to oxygen or to moisture, and for this reason is usually prepared immediately before use. When protected from these materials, however, solutions of the ylide are stable for extended periods of time.7

Wittig Reagent for the Conversion of Aldehydes into Alkenes.

The reaction between benzylidenetriphenylphosphorane and an aldehyde to form an alkene and triphenylphosphine oxide is experimentally easy to carry out and proceeds under mild conditions at room temperature.1a A valuable feature of the Wittig procedure is that, in contrast to many elimination and pyrolytic reactions, it gives rise to alkenes in which the position of the double bond is established unambiguously.1b The aldehyde component may contain a wide variety of other functional groups such as hydroxyl, ether, ester, halogen, and terminal alkyne which generally do not interfere with the reaction.1a,b The utility of the Wittig reaction is shown by the fact that it has already been used in the synthesis of many alkenes, including a considerable number of natural products.1b,c,d Often, excellent selectivity for either the (Z)- (with a nonstabilized ylide) or (E)-alkene (with a stabilized ylide) can be obtained. In the case of benzylidenetriphenylphosphorane (a moderated ylide), (Z):(E) selectivity is often poor,1b and apparently quite sensitive to experimental conditions (see Table 1).

This ylide is historically important because its reactions with aromatic aldehydes were explored extensively during the first attempts to understand the mechanism of the Wittig reaction.1a,c Regrettably, the key reaction Ph3P=CHPh + PhCHO has proved to be the most difficult to control among all of the known Wittig systems. An inspection of Table 1 reveals inconsistencies in a number of the Z:E ratios for this reaction when the work of different groups is compared.4-6,8-12 For example, the reported (Z) selectivity among the lithium-free entries varies from 25%12 (entry 9) to 74%11 (entry 8). Dramatic variations such as these were sometimes cited to support claims of reversal, cation effects, anion effects, electronic effects, or solvent effects in the mechanism of the Wittig reaction.14 More recent studies are beginning to clarify these mechanistic questions and to explain the origin of selectivity in the Wittig reaction.15

Examples of Wittig reactions of benzylidenetriphenylphosphorane with aliphatic aldehydes are uncommon in the literature, and, as with aromatic aldehydes, the (Z):(E) selectivity is poor. However, if the structure of the substituents at phosphorus in the ylide are varied, then dramatic changes in the selectivity of the reaction are observed. A number of examples of this effect have been published. Two of the most striking examples are shown in eqs 1 and 2.16

1. (a) Maryanoff, B. E.; Reitz, A. B. CRV 1989, 89, 863. (b) Gosney, I.; Rowley, A. G. In Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.; Academic: New York, 1979. (c) Schlosser, M. Top. Stereochem. 1970, 5, 1. (d) Reucroft, J.; Sammes, P. G. QR 1971, 25, 135.
2. Johnson, A. W. Ylid Chemistry; Academic: New York, 1966.
3. References with detailed experimental procedures: (a) BuLi: Ward, W. J., Jr.; McEwen, W. E. JOC 1990, 55, 493. (b) KHMDS: Vedejs, E.; Marth, C. F.; Ruggeri, R. JACS 1988, 110, 3940.
4. (a) Schlosser, M.; Christmann, K. F. LA 1967, 708, 1. (b) House, H. O.; Jones, V. K.; Frank, G. A. JOC 1964, 29, 3327.
5. Allen, D. W. JCR(S) 1980, 384.
6. Märkl, G.; Merz, A. S 1973, 295.
7. A moderated ylide, Ph2MeP=CHPh, has been isolated as a crystalline solid. See Ref. 3a. A detailed procedure for the isolation of several crystalline ylides is found in: Vedejs, E.; Meier, G. P.; Snoble, K. A. J. JACS 1981, 103, 2823.
8. (a) Wittig, G.; Schöllkopf, U. CB 1954, 87, 1318. (b) Bergelson, L. D.; Shemyakin, M. M. T 1963, 19, 149.
9. Bergelson, L. D.; Barsukov, L. I.; Shemyakin, M. M. T 1967, 23, 2709.
10. Wittig, G.; Haag, W. CB 1955, 88, 1654.
11. Bergelson, L. D.; Shemyakin, M. M. T 1963, 19, 149.
12. Wheeler, O. H.; Battle de Pabon, H. N. JOC 1965, 30, 1473.
13. Bestmann, H. J.; Stransky, W.; Vostrowsky, O. CB 1976, 109, 1694.
14. See the review papers in Ref. 1.
15. Leading references only: (a) Vedejs, E.; Marth, C. F. JACS 1988, 110, 3948. (b) Schlosser, M.; Schaub, B. JACS 1982, 104, 5821. (c) Bestmann, H. J.; Vostrowsky, O. Top. Curr. Chem. 1983, 109, 85.
16. (a) (E) selectivity: Bandmann, H.; Bartik, T.; Bauckloh, S.; Behler, A.; Brille, F.; Heimback, P.; Louven, J.-W.; Ndalut, P.; Preis, H.-G.; Zeppenfeld, E. ZC 1990, 30, 193. (b) (Z) selectivity: Tsukamoto, M.; Schlosser, M. SL 1990, 605. Jeganathan, S.; Tsukamoto, M.; Schlosser, M. S 1990, 109.

Matthew J. Peterson

Abbott Laboratories, North Chicago, IL, USA

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