Propargyltriphenylphosphonium Bromide

[2091-46-5]  · C21H18BrP  · Propargyltriphenylphosphonium Bromide  · (MW 381.26)

(Wittig reagent; synthon for b-amino- and b-iminopropenyltriphenylphosphonium bromides, precursors of heterocyclic systems)

Physical Data: mp 156-158 °C,1 179 °C;2 IR;3 1H NMR;2 13C NMR;4 pKa.5

Preparative Method: readily available from Triphenylphosphine and propargyl bromide (see Propargyl Chloride) (eq 1).1,2 The presence of hydrogen bromide is absolutely necessary, otherwise the base Ph3P causes the formation of tarry products.1

Wittig Reactions.

Deprotonation of propargyltriphenylphosphonium bromide with liquid Ammonia affords the corresponding phosphonium ylide which, in the presence of aldehydes, undergoes Wittig reaction to give enynes (eq 2).1 Other bases which are usually used for the generation of phosphonium ylides from their salts give rise to the formation of dark, resinous products. Furthermore, only conjugated aldehydes are suitable substrates for the Wittig reactions under these conditions, leading to stereoisomeric mixtures.1,6 Formation of an isomeric cumulene instead of the expected enyne, presumably as a consequence of an isomerization of the propargylidenephosphorane to the corresponding allylidene species, has been observed when n-Butyllithium is used as base.7


Isomerization of propargyltriphenylphosphonium bromide to give the corresponding propadienyl and prop-1-ynyl salts can be accomplished by treatment with a catalytic amount of Potassium t-Butoxide or by heating with phenol, respectively (eq 3).2

Addition of Nucleophiles.

Propargyltriphenylphosphonium bromide adds oxygen, sulfur, and nitrogen nucleophiles, yielding b-substituted propenyltriphenylphosphonium bromides (eq 4),2,8-18 obviously via intermediate formation of propadienyltriphenylphosphonium bromide.2

The addition of various amino compounds has been investigated in most detail (eq 5).2,8-15,18

The resulting b-aminovinylphosphonium salts, which can be in equilibrium with the isomeric b-iminopropyl compounds,2 are very useful reagents for Wittig reactions. Deprotonation affords the corresponding phosphoranes, which can be used in typical ylide reactions. Ylide generation followed by a Wittig reaction represents an efficient synthesis of 2-amino-1,3-butadienes (eq 6).18

A variety of heterocycles has been synthesized starting from propargyltriphenylphosphonium bromide and amino compounds (eq 7). This can be realized if the intermediate phosphonium salt is substituted in such a manner to allow intramolecular Wittig reaction2,8 or intramolecular attack at the b-C-atom of the vinyl group followed by extrusion of methylenetriphenylphosphorane.2,9,12

Heterocycles may also result from thermal rearrangement of primarily formed open-chain Wittig products which are available from b-hydrazonopropylphosphonium bromides.11,14,15 The phosphonium salts resulting from azirines and propargyltriphenylphosphonium bromide also undergo rearrangements affording phosphorus-substituted heterocycles.13

Reaction with Iminophosphoranes.

Reactions of propargyltriphenylphosphonium bromide with iminophosphoranes or phosphazines give 2-imino-3-phosphoranylidene phosphonium salts (eq 8), which can formally be rationalized as the result of a [2 + 2] cycloaddition followed by an electrocyclic ring opening.8,11,19,20

The resulting ylide salts are versatile precursors for the synthesis of b-enamino phosphonium salts, divinylic ketimines, and the corresponding ketones.19,20 In some cases the resulting Wittig products undergo cyclization, affording heterocyclic compounds.11

1. Eiter, K., Oediger, H. LA 1965, 682, 62.
2. Schweizer, E. E.; Goff, S. D.; Murray, W. P. JOC 1977, 42, 200.
3. Appleyard, G. D.; Stirling, C. J. M. JCS(C) 1969, 1904.
4. Albright, T. A.; Freeman, W. J.; Schweizer, E. E. JACS 1975, 97, 2946.
5. Ling-Chung, S.; Sales, K. D.; Utley, H. P. CC 1990, 662.
6. Ogawa, H.; Mukae, J. TL 1978, 4929.
7. Corey, E. J.; Ruden, R. A. TL 1973, 1495.
8. Schweizer, E. E.; Kim, C. S.; Labaw, C. S.; Murray, W. P. CC 1973, 7.
9. Schweizer, E. E.; DeVoe, S. V. JOC 1975, 40, 144.
10. Schweizer, E. E.; Calcagno, M. A. JOC 1977, 42, 2641.
11. Albright, T. A.; Evans, S.; Kim, C. S.; Labaw, C. S.; Russiello, A. B.; Schweizer, E. E. JOC 1977, 42, 3691.
12. Schweizer, E. E.; Goff, S. D. JOC 1978, 43, 2972.
13. Calcagno, M. A.; Schweizer, E. E. JOC 1978, 43, 4207.
14. Schweizer, E. E.; Hsueh, W.; Rheingold A. L.; Durney, R. L. JOC 1983, 48, 3889.
15. Schweizer, E. E.; Hayes, J. E.; Rheingold, A.; Wei, X. JOC 1987, 52, 1810.
16. Khandker, M. N. I.; Ahmad, A.; Hossain, M. G. IJC(B) 1987, 26B, 773 (CA 109, 109 968s).
17. Khandker, M. N. I.; Ahmad, A. J. Bangladesh Acad. Sci. 1989, 13, 5 (CA 1989, 111, 174 253k).
18. Barluenga, J.; Merino, I.; Palacios, F. TL 1990, 31, 6713.
19. Barluenga, J.; Merino, I.; Palacios, F. TL 1989, 30, 5493.
20. Barluenga, J.; Merino, I.; Palacios, F. JCS(P1) 1991, 341.

Hans Jürgen Bestmann & Reiner Zimmermann

University Erlangen-Nürnberg, Germany

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