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Effective Modulation of the Donor Properties of N-Heterocyclic Carbene Ligands by "Through-Space" Communication within a Planar Chiral Scaffold

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Fürstner,  Alois
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Alcarazo,  Manuel
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Krause,  Helga
Research Department Fürstner, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Lehmann,  Christian W.
Service Department Lehmann (EMR), Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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ja076028tsi20070919_055908.pdf
(Supplementary material), 992KB

ja076028tsi20070810_084045.cif
(Supplementary material), 24KB

ja076028tsi20070810_084105.cif
(Supplementary material), 19KB

ja076028tsi20070810_084124.cif
(Supplementary material), 23KB

Citation

Fürstner, A., Alcarazo, M., Krause, H., & Lehmann, C. W. (2007). Effective Modulation of the Donor Properties of N-Heterocyclic Carbene Ligands by "Through-Space" Communication within a Planar Chiral Scaffold. Journal of the American Chemical Society, 129(42), 12676-12677. doi:10.1021/ja076028t.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0025-7401-9
Abstract
A high yielding approach to planar chiral carbene ligands is described, in which an imidazo[1,5-a]pyridine-3-ylidene unit is embedded into a cyclophane scaffold. As evident from the IR data of the corresponding rhodium complex 18 as the parent member of this family, these new ligands turned out to be exceptionally strong electron donors, rivaling or even outperforming the other diamino-stabilized five-membered N-heterocyclic carbenes (NHC) known to date. If the remote ring of the cyclophane is substituted by four fluorine atoms, however, the donor capacity is significantly reduced by the through-space interaction of the tetrafluorobenzene moiety with the underneath carbene entity. Since X-ray data suggest that the steric demand of the fluorinated and the nonfluorinated cyclophane-2-ylidenes are virtually identical, these results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes. The synthesis route leading to the planar chiral imidazopyridinium salts serving as the immediate carbene precursors involves a previously unrecognized 6π-electrocyclic ring opening reaction upon exposure of the pyridine-N-oxide 12 to N,N-dimethyl-carbamoyl chloride and TMSCN, as evident from the crystal structure analysis of the resulting azatriene adduct 13.