S-Block Metal Amide Complexes and Enolization Reactions

Enolates are highly useful synthetic tools for generating new carbon-carbon bonds. Many decades of work have been devoted to their formation and application in organic reactions. This dissertation focuses on the use of s-block metal amides in enolization reactions. The first section includes the synthesis and structural characterization of two new s-block metal complexes. The first is a mixed lithium-magnesium amidoenolate hexameric cage. This structure was isolated as a result of the enolization of propiophenone by a mixed lithium-magnesium amide base. The second structure is a lithium 2-aza-allyl complex in which the metal cation is completely separated from the anion by two crown ether molecules. This structure was the surprising result of attempts to induce charge separation through addition of lithium-sequestering 12-crown-4 to a mixed lithium-magnesium amide. These two complexes are examples of the diverse range of structures that can be obtained from mixed metal systems.The second section is dedicated to the use of one magnesium bisamide base, Mg(HMDS)2, and its performance in regio- and stereoselective enolizations. The results outlined illustrate the high levels of selectivity that are achievable for this base. The product ratios for stereoselective enolizations are shown to be dependent on the choice of solvent for the deprotonation reaction. Enolization carried out in THF demonstrates excellent levels of (Z)-selectivity, while those in toluene favor the (E)-isomer. The utility of the generated enolates is presented through their application in aldol addition reactions. These reactions are able to reach high conversion, and show some moderate diastereomer selectivity.The third section explores the mechanism of enolization mediated by Mg(HMDS)2 in THF. Previous work in our group determined the mechanism in toluene, and this work aims to illuminate mechanistic differences in THF that may lead to the observed reversal in stereoselectivity. The most significant change found in the mechanism is THF solvation of magnesium. This results in a four-coordinate metal center in the transition state, as opposed to three-coordinate in the case of toluene. This section also includes a detailed study of the magnesium amidoenolate species in solution. Several NMR techniques were utilized to identify each species. Observation of the magnesium amidoenolate species over time revealed the presence of an equilibrium between monomeric and dimeric aggregates. In addition, an unequal distribution of the stereoisomers across the aggregation states results in an E/Z ratio that is concentration, temperature and time dependent.