Scope and Mechanistic Investigations on the Solvent-Controlled Regio- and Stereoselective Formation of Enol Esters from the Ruthenium-Catalyzed Coupling Reaction of Terminal Alkynes and Carboxylic Acids

Authors

Chae S. Yi, Marquette UniversityFollow
Ruili Gao, Marquette UniversityFollow

Document Type

Article

Language

eng

Publication Date

10-30-2009

Publisher

American Chemical Society

Source Publication

Organometallics

Source ISSN

0276-7333

Original Item ID

DOI: 10.1021/om9007357

Abstract

The ruthenium-hydride complex (PCy₃)₂(CO)RuHCl was found to be a highly effective catalyst for the alkyne-to-carboxylic acid coupling reaction to give synthetically useful enol ester products. A strong solvent effect was observed for the ruthenium catalyst in modulating the activity and selectivity; the coupling reaction in CH₂Cl₂ led to the regioselective formation of gem-enol ester products, while the stereoselective formation of (Z)-enol esters was obtained in THF. The coupling reaction was found to be strongly inhibited by PCy₃. The coupling reaction of both PhCO₂H/PhC≡CD and PhCO₂D/PhC≡CH led to extensive deuterium incorporation on the vinyl positions of the enol ester products. An opposite Hammett value was observed when the correlation of a series of para-substituted p-X-C₆H₄CO₂H (X = OMe, CH₃, H, CF₃, CN) with phenylacetylene was examined in CDCl₃ (ρ = +0.30) and THF (ρ = −0.68). Catalytically relevant Ru-carboxylate and -vinylidene-carboxylate complexes, (PCy₃)₂(CO)(Cl)Ru(κ²-O₂CC₆H₄-p-OMe) and (PCy₃)₂(CO)(Cl)RuC(═CHPh)O₂CC₆H₄-p-OMe, were isolated, and the structure of both complexes was completely established by X-ray crystallography. A detailed mechanism of the coupling reaction involving a rate-limiting C−O bond formation step was proposed on the basis of these kinetic and structural studies. The regioselective formation of the gem-enol ester products in CH₂Cl₂ was rationalized by a direct migratory insertion of the terminal alkyne via a Ru-carboxylate species, whereas the stereoselective formation of (Z)-enol ester products in THF was explained by invoking a Ru-vinylidene species.

Comments

Accepted version. Organometallics, Vol. 28, No. 22 (October 30, 2009): 6585-6592. DOI. © 2009 American Chemical Society. Used with permission.

Recommended Citation

Yi, Chae S. and Gao, Ruili, "Scope and Mechanistic Investigations on the Solvent-Controlled Regio- and Stereoselective Formation of Enol Esters from the Ruthenium-Catalyzed Coupling Reaction of Terminal Alkynes and Carboxylic Acids" (2009). Chemistry Faculty Research and Publications. 367.
https://epublications.marquette.edu/chem_fac/367