Regio- and Stereoselective Enolizations Using Calcium Bis(hexamethyldisilazide) as a Base: Synthetic, Solid-State, and Solution Studies

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-d5. 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.