Potassium, calcium, and mixed potassium-calcium amide combinations have been shown to be efficient reagents in enolization reactions, and a set of representative intermediate mono- and heterobimetallic enolates have been successfully isolated and crystallographically characterized.

The reactions of magnesium and calcium bis(hexamethyldisilazide) with propiophenone have been studied with a view to determine the utility of these bases in the stereoselective enolization of ketones and to uncover the nature of the metal enolate intermediates produced. Both base systems are highly Z-selective when the reactions are conducted in the presence of polar solvents. However, in situ monitoring of the magnesium system in arene solution revealed a preference for E-enolate formation, which was confirmed by silyl enol ether trapping studies. Solution NMR studies of the magnesium system in toluene-d8 show the presence of a monomer-dimer equilibrium for the intermediate amidomagnesium enolates. This assignment is supported by the characterization of a disolvated amidomagnesium enolate dimer by crystallographic analysis. Comparative studies of the calcium system show distinctly different behavior. This is exemplified by the characterization of a novel solvent-separated ion pair complex and a monomeric amidocalcium enolate in the solid state. Solution NMR studies of the calcium system in pyridine-d5 reveal the co-existence of the heteroleptic amidocalcium enolate, the bisamide, the bisenolate and the ion pair complex.

The heterodimetallic compound [(Me3Si)2NMg{μ-N(SiMe3)2}2CaN(SiMe3)2], 3, can be prepared as single-component crystals through manipulation of the equilibrium established on mixing the homometallic derivatives Mg[N(SiMe3)2]2, 1, and Ca[N(SiMe3)2]2, 2, in toluene/hexane solution. Specifically, doping solutions with a 30% molar excess of 1 is required in order to avoid cocrystallization with 2. Complex 3 has been characterized by single-crystal X-ray diffraction and multinuclear NMR spectroscopy and is found to adopt a heterodimetallic Mg(μ2-N)2Ca ring structure in the solid state and in arene solutions. DFT computational studies confirm the experimentally observed variations in the metrical parameters between the homo- and heterodimetallic complexes. The calculations also indicate that the reaction to form 3 is close to being thermoneutral. Addition of pyridine to 3 results in asymmetric cleavage of the mixed-metal ring structure, in turn forming a kinetically stable, charge-separated “ate” species of the type [(Me3Si)2NCa·(pyr) n ]+[Mg{N(SiMe3)2}3]-, 4. The three bases 1, 2, and 4 react with propiophenone in pyridine media at ambient temperature to give high Z stereoselectivities of ≥98%.