Fluoride Ion Catalyzed Aldol Reaction between Enol Silyl Ethers and Carbonyl Compounds

Article abstract of DOI:10.1021/jo00155a003

A new aldol reaction effected by the reaction of enol trimethylsilyl ethers and a quarternary ammonium fluoride is reported.Under the influence of a catalytic amount of tetrabutylammonium fluoride at low temperatures, enol silyl ethers react with various aldehydes to give the corresponding aldol trimethylsilyl ethers in fair to good yields.The silyl group of these products can be smoothly removed under mild conditions.Ketones, epoxides, and esters do not serve as electrophiles in this reaction.The reaction proceeds in a regiospecific manner with respect to the enol silyl ethers; the reactions of two regioisomers of 2-methylcyclohexanone with benzaldehyde cleanly give the respective regioisomeric aldol products.The reaction of 4-tert-butyl-1-methyl-2-(trimethylsiloxy)cyclohexene proceeds exclusively by the axial attack of the electrophile, making a strong contrast with the related cases reported on this cyclohexanone system.The aldol addition of a ketone and an aldehyde can be performed without the isolation of the enol silyl ether of the ketone by effecting both the silylation of the ketone and the aldol reaction with the aid of a fluoride anion.The characteristic behavior of the enolate species in this reaction can be rationalized by considering a mobile equilibrium in which the combination of fluorotrimethylsilane and a quarternary ammonium enolate functions as key controlling factor.