Anion-Catalyzed Reactions of Silyl Ester Polyenolates with Electrophiles

Article abstract of DOI:10.1021/jo00250a022

The stereochemical assignments for the trimethylsilyl ester polyenolates 1, 2, and 3 have been made.Unlike simple silyl ketene acetals, 1, 2, and 3 are obtained predominantly as the thermodynamic Z isomers.Bifluoride-catalyzed equilibration studies showed the equilibrium compositions of 1, 2, and 3 to be 89percent Z, 67percent Z, and 95percent 1-Z,3-E, respectively.The regiochemistry of the bifluoride-catalyzed Reformatsky-type reaction of benzaldehyde with 1, 2, and 4 was determined.Regiochemical control of the condensation of 1 with benzaldehyde by reaction temperature was demonstrated, with 95percent α reaction at -95 deg C and 99percent γ-reaction at +34 deg C.Mechanism studies are consistent with a dissociative process in which a dienolate reacts with benzaldehyde at the α position to give the kinetic-controlled product, and as the reaction temperature increases, increasing amounts of the thermodynamic γ product are formed.A nondissociative cyclic process leading to α product may occur at low temperatures.The naked ester dienolate prepared by reaction of 1 with TASF and removal of the resulting fluorotrimethylsilane was alkylated by benzyl bromide exclusively at the α position at -100 deg C.The bifluoride-catalyzed reverse reaction of 1 with benzaldehyde is much slower than the processes that determine the regiochemistry in the forward reaction.