Thermodynamic stereocontrol of the (hexamethyldisilazide)magnesium enolates of propiophenone in THF is reported. The overall stereoselectivity proves to be very sensitive to concentration, since dimeric species with bridging enolates show no stereoselectivity while monomeric enolates show a very strong thermodynamic preference for the Z enolate. Kinetically, interconversion among aggregates is remarkably slow, whereas stereoisomerization of the monomer, even in the absence of a proton source such as ketone or amine, is remarkably fast. Furthermore, stereoisomerization takes place in the absence of a proton source or excess ketone. These observations contrast with accepted views of these fundamentally important processes and have implications for understanding the identity and reactivity of metal enolates.
The alkaline earth metal complex calcium bis(hexamethyldisilazide), Ca(HMDS) 2 , has proven to be a useful reagent to carry out the regio-and stereoselective enolization reactions of ketones. The reactions give almost quantitative conversions to the corresponding silyl enol ethers at 0 °C using THF as solvent medium. Excellent kinetic selectivities (up to >99%) are found in the reactions of Ca(HMDS) 2 with a series of unsymmetrical ketones, and this base system displays high Z-selectivity (up to 96%) for stereoselective enolization reactions. Six new enolate-containing crystal structures have also been elucidated. The amidocalcium enolates adopt monomeric, dimeric, tetranuclear, and charge-separated constitutions. In addition, the unexpected preparation of a hexanuclear complex composed of amide, enolate, and enolized aldolate units was discovered. NMR spectroscopic studies of the amidocalcium enolate systems reveal that a common dynamic equilibrium between multiple species is established in pyridine-d 5 . The solution species have been identified as amidocalcium enolates, bisenolates, bisamides, and charge-separated complexes. These studies demonstrate the structural diversity and complexity underlying these apparently straightforward calcium-mediated deprotonation reactions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.