Ketenylidenetriphenylphosphorane

Ph3P=C=C=O

[15596-07-3]  · C20H15OP  · Ketenylidenetriphenylphosphorane  · (MW 302.3)

(cumulated phosphonium ylide)

Physical Data: mp 171-172 °C;1 IR;1 31P NMR;1 crystal structure.2

Preparative Methods: prepared most conveniently by a b-elimination from (Methoxycarbonylmethylene)triphenylphosphorane using Sodium Hexamethyldisilazide or Sodium Amide as a base (eq 1).1,3 Other preparations have been reported.4,5

Handling, Storage, and Precautions: absence of moisture and air is necessary.

Reactivity.

In spite of its structural similarity with ketenes, the cumulated ylide displays no tendency to dimerize.6 However, addition of an electrophile X+ from a compound X-Y affords a phosphonium salt which reacts in a manner that is typical for ketenes (eq 2).

Whenever the starting ylide is a stronger nucleophile than Y-, the phosphonium salt undergoes [2 + 2] cycloaddition with a second molecule of unreacted ylide to give 1,3-cyclobutanedione derivatives. If the anion Y- is a stronger nucleophile than the cumulated ylide, then a new carbonyl substituted ylide is formed in which the compound X-Y has been added to the C=C bond of the starting ylide. The cyclic ylide salts formed in the reactions with hydrochloride or halogeno compounds serve as substrates for a series of subsequent reactions (e.g. conversion to the corresponding bis-ylide, Wittig reaction, isomerization, ring opening).4,7 The stabilized phosphoranes resulting from the cumulated ylide and alcohols, thiols, or acidic NH compounds are very useful for the synthesis of a great variety of a,b-unsaturated carboxylic acid derivatives via the Wittig reaction.8 a,b-Unsaturated esters can be synthesized (E) stereoselectively in this way by means of a three-component synthesis, without isolating the intermediate phosphorane.9 Synthesis of heterocycles is possible if the acidic compound contains an additional group capable of cyclization (e.g. in the Wittig reaction) with the initially formed ylide function (eq 3). This synthetic principle has proved to be most useful for the synthesis of five- and six-membered heterocycles,10,11 as well as for the preparation of macrocyclic lactones.12,13

Grignard reagents react with ketenylidenetriphenylphosphorane to give addition compounds which are hydrolyzed to acyl ylides.3 This chain-lengthening difunctionalization of a Grignard reagent together with subsequent carbonyl alkenation allows the synthesis of a wide range of (E)-a,b-unsaturated ketones (eq 4).

Reaction of ketenylidenetriphenylphosphorane with aldehydes and ketones yields 1,3-cyclobutanediones, presumably via [2 + 2] cycloaddition of a second molecule of cumulated ylide to the C=C bond of an intermediate Wittig product or its precursor (eq 5).4

Cycloaddition at the C=C bond of the cumulated ylide also occurs in the reaction with various other compounds. Thus acyl, thioacyl, and imidoyl heterocumulenes as well as a,b-unsaturated carbonyl compounds react to give six-membered heterocycles in formal [4 + 2] cycloadditions (eqs 6 and 7).14,15 Heteroallenes yield four-membered rings in a [2 + 2] cycloaddition or six-membered rings resulting via a [4 + 2] cycloaddition of a second molecule of heteroallene to a dipolar 1:1 intermediate (eq 8).4 The phosphonium ylides which can be synthesized in this way can usually be converted to the parent phosphorus-free derivatives, thus providing a synthetic approach to a series of heterocyclic systems.

The cumulated ylide has also been used as a metal complex ligand.16


1. Bestmann, H. J.; Sandmeier, D. CB 1980, 113, 274.
2. Daly, J. J. JCS(A) 1967, 1913.
3. Bestmann, H. J.; Schmidt, M.; Schobert, R. S 1988, 49.
4. Review: Bestmann, H. J. AG(E) 1977, 16, 349.
5. Review: Bestmann, H. J.; Zimmermann, R. MOC 1982, E1, 759.
6. For a detailed discussion concerning the electronic structure of the cumulated ylide and its reactivity, see Ref. 4.
7. Bestmann, H. J.; Geismann, C. TL 1980, 21, 257.
8. Bestmann, H. J.; Schmid, G.; Sandmeier, D. CB 1980, 113, 912.
9. Bestmann, H. J.; Schobert, R. AG(E) 1985, 24, 790.
10. Bestmann, H. J.; Schmid, G.; Sandmeier, D.; Schade, G.; &OOuml;chsner, H. CB 1985, 118, 1709.
11. Nickisch, K.; Klose, W.; Nordhoff, E.; Bohlmann, F. CB 1980, 113, 3086; Nickisch, K.; Klose, W.; Bohlmann, F. CB 1980, 113, 2038; Klose, W.; Nickisch, K.; Bohlmann, F. CB 1980, 113, 2694.
12. Bestmann, H. J.; Schobert, R. S 1989, 419.
13. Bestmann, H. J.; Kellermann, W.; Pecher, B. S 1993, 149.
14. Bestmann, H. J.; Schmid, G.; Sandmeier, D.; Geismann, C. TL 1980, 21, 2401.
15. Bestmann, H. J.; Schmid, G. TL 1984, 25, 1441.
16. Lindner, E.; Berke, H. CB 1974, 107, 1360.

Hans Jürgen Bestmann & Reiner Zimmermann

University of Erlangen-Nürnberg, Germany



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