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 controlled reaction of molecular oxygen with hydrocarbon solutions of tBuLi results in the unexpected formation of the octameric species [(tBuOLi)8], 18. Single-crystal X-ray diffraction studies indicate that the connectivity within 18 corresponds with the fusion of two face-opened tetrameric cubanes. Cryoscopic studies establish the stability of the octamer in benzene solution, but heating results in aggregate rearrangement to the commonly found prismatic hexameric form, 16. Theoretical calculations confirm that the hexameric form is the thermodynamically favored aggregate.

Bis(hexamethyldisilazido)magnesium has successfully been used to mediate aldol additions of selected ketones and aldehydes in hydrocarbon media, and the structure of an intermediate amido(aldolate), [{(Me 3 Si) 2 NMg[m-OC(Me)-Bu t CH 2 C(Bu t ) = O]} 2 ] 17, produced by the self-coupled reaction of pinacolone has been characterised by X-ray crystallography.

No abstract