title: Stereoselective enolizations mediated by magnesium and calcium bisamides: contrasting aggregation behavior in solution and in the solid state.
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.
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA.
authors.forename: Xuyang
authors.initials: X
authors.lastname: He
authors.forename: John F
authors.initials: JF
authors.lastname: Allan
authors.forename: Bruce C
authors.initials: BC
authors.lastname: Noll
authors.forename: Alan R
authors.initials: AR
authors.lastname: Kennedy
authors.forename: Kenneth W
authors.initials: KW
authors.lastname: Henderson