Catalytic Asymmetric Couplings of Ketenes with Aldehydes To Generate Enol Esters

“…Since enol esters can also serve as a synthon for aldehydes and ketones, much research efforts has been devoted to develop efficient catalytic methods to control both regio- and stereoselectivity in forming substituted enol esters. Notable recent examples on the catalytic synthesis of enol esters include: Zr-catalyzed methylalumination of alkynes,5 Au-catalyzed intramolecular rearrangements of propargylic esters and alcohols,6 Cu-catalyzed oxidative esterification of aldehydes with β-dicarbonyl compounds,7 and asymmetric coupling reaction of ketenes with aldehydes by chiral Fe catalysts 8. From an industrial perspective of increasing synthetic efficiency as well as for reducing waste byproducts, catalytic methods for producing enol esters are highly desired compared to the classical methods that utilize stoichiometric amounts of strong base or toxic Hg salts 9…”

Section: Introductionmentioning

confidence: 99%

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

Gao

2009

Organometallics

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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. 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 the 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 -vinylidenecarboxylate complexes, (PCy3)2(CO)(Cl)Ru(κ 2 -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.

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“…The same catalyst 23 can also achieve the coupling of an aldehyde with a ketene to generate an enol ester that bears an a stereocenter (Eq. (12)) [33]. It is likely that this reaction proceeds through chiral Brønsted-acid catalysis (Scheme 11.3), with the enolate of the aldehyde serving as the O-nucleophile.…”

Section: -80% Eementioning

confidence: 99%

Catalytic Asymmetric Couplings of Ketenes with Aldehydes To Generate Enol Esters

Cited by 86 publications

References 23 publications

Transition metals in organic synthesis: Highlights for the year 2005

Transition metals in organic synthesis: Highlights for the year 2005

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

Applications of Aza‐ and Phosphaferrocenes and Related Compounds in Asymmetric Catalysis

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