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Stereoselective enolizations mediated by magnesium and calcium bisamides: Contrasting aggregation behaviour in solution and in the solid-state

He, X.Y. and Allan, J.F. and Noll, B.C. and Kennedy, A.R. and Henderson, K.W. (2005) Stereoselective enolizations mediated by magnesium and calcium bisamides: Contrasting aggregation behaviour in solution and in the solid-state. Journal of the American Chemical Society, 127 (19). pp. 6920-6921. ISSN 0002-7863

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Abstract

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.