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Issue 22, 2008
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Constrained geometry aminooxazolinate ligands giving chiral zirconium guanidinates; catalytic cyclohydroamination

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Abstract

DFT calculations indicate that contrary to a prior report, N-heterocycle-augmented constrained geometry ligands e.g.Cp′-SiMe2-NR (Cp′ = Me4C5, R = 2-pyridine) should be capable of binding both atoms of the diazaallyl fragment at zirconium. This was confirmed by a molecular structure of a previously reported complex. Similar R = 2-oxazoline complexes were also shown to be feasible, although an additional N,O binding mode was accessible. The proligands HCp′-SiMe2-NHR (R = chiral non-racemic 2-oxazoline) were readily synthesised in high yield via base mediated reaction of 2-aminooxazolines and Cp′-SiMe2Cl. Subsequent reaction with Zr(NMe2)4 gave, rather than the desired complexes, configurationally stable chiral-at-zirconum guanidinate/alkoxide chelate products; the aminooxazolinate units had undergone ring-opening and migratory insertion of –NMe2. Trends in the level diastereoselection follow the steric demand of the oxazoline substituent, with the larger groups (tBu, iPr) giving single diastereomers. The modest performance of these guanidinate compounds in enantioslective catalytic cyclohydroamination of aminoalkenes follows the expected trends for metal accessibility in a σ-amido insertative mechanism.

Graphical abstract: Constrained geometry aminooxazolinate ligands giving chiral zirconium guanidinates; catalytic cyclohydroamination

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Publication details

The article was received on 05 Mar 2008, accepted on 08 Apr 2008 and first published on 01 May 2008


Article type: Paper
DOI: 10.1039/B803831G
Dalton Trans., 2008, 2983-2990

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    Constrained geometry aminooxazolinate ligands giving chiral zirconium guanidinates; catalytic cyclohydroamination

    A. L. Gott, G. J. Clarkson, R. J. Deeth, Max. L. Hammond, C. Morton and P. Scott, Dalton Trans., 2008, 2983
    DOI: 10.1039/B803831G

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