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

X.Y. He, J.F. Allan, B.C. Noll, A.R. Kennedy, K.W. Henderson

Research output: Contribution to journalArticlepeer-review

36 Citations (Scopus)

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.
Original languageEnglish
Pages (from-to)6920-6921
Number of pages1
JournalJournal of the American Chemical Society
Volume127
Issue number19
DOIs
Publication statusPublished - 21 Apr 2005

Keywords

  • calcium
  • ion pair complex
  • monomeric amidocalcium enolate
  • heteroleptic amidocalcium enolate

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