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

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Xuyang He,^† John F. Allan,^† Bruce C. Noll,^† Alan R. Kennedy,^‡ and Kenneth W. Henderson*^†

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL, U.K.

J. Am. Chem. Soc., 2005, 127 (19), pp 6920–6921

DOI: 10.1021/ja051050c

Publication Date (Web): April 21, 2005

†

University of Notre Dame.

‡

University of Strathclyde.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, khenders@nd.edu

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.

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Published In Issue May 18, 2005
Received February 18, 2005

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Communication

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

Abstract

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Related Content

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