Stereoselective Enolizations Mediated by Magnesium and Calcium Bisamides: Contrasting Aggregation Behavior in Solution and in the Solid State

• Published in: Journal of the American Chemical Society Vol. 127; no. 19; pp. 6920 - 6921
• Main Authors: He, Xuyang, Allan, John F, Noll, Bruce C, Kennedy, Alan R, Henderson, Kenneth W
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 18.05.2005
• Subjects: Chemical reactivity; Chemistry; Exact sciences and technology; Organic chemistry; Reactivity and mechanisms; Stereoselectivity; Kinetic control; Catalytic reaction; Catalyst selectivity; Enolization; Benzenic compound; Experimental study; Catalyst activity; Alkaline earth metal Compounds; Catalyst; Deprotonation
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja051050c

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-d 8 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-d 5 reveal the co-existence of the heteroleptic amidocalcium enolate, the bisamide, the bisenolate and the ion pair complex.