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Synfacts 2021; 17(11): 1245
DOI: 10.1055/s-0041-1737069
Metals in Synthesis

Copper-Mediated Homologation of Alkynes to Allenes: The Crabbé Reaction

Contributor(s):
Mark Lautens
,
Joachim Loup
Crabbé P, *, Fillion H, André D, Luche J.-L. Université Scientifique et Médicale, Grenoble, France
Efficient Homologation of Acetylenes to Allenes.

J. Chem. Soc., Chem. Commun. 1979; 859-860
DOI: 10.1039/C39790000859.
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Key words

Crabbé reaction - homologation - allenes - copper catalysis

Significance

Crabbé and co-workers reported a copper-mediated homologation of terminal alkynes to allenes. Copper(I) bromide gave the best results among the metal salts that were investigated, and diisopropylamine (which serves multiple roles as a base, an iminium-forming carbonyl activator and a hydride donor) was found to be the optimal amine in terms of yields and reaction rates. The mechanism of the reaction was discussed in subsequent reports (see S. Searles et al. J. Chem. Soc., Perkin Trans. 1 1984, 747), and relies on the addition of a copper acetylide to an iminium ion, followed by a 1,5-hydride shift.


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Comment

The Crabbé reaction remains a method of choice for the preparation of allenes. While the original conditions were limited to the synthesis of simple monosubstituted allenes, later contributions by Ma and others allowed for the preparation of multiply substituted allenes, as well as enantioselective variants (see Review below).


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Review

X. Huang, S. Ma Acc. Chem. Res. 2019, 52, 1301–1312.


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Publication History

Article published online:
19 October 2021

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