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Bisoxazolines with one and two sidearms: stereodirecting ligands for copper-catalysed asymmetric allylic oxidations of alkenes
No full textA series of sidearm functionalized bisoxazoline ligands has been synthesized by reaction of the monolithiated methyl{bis(oxazolinyl)} methane with the appropriate electrophiles, and tested in the copper catalyzed asymmetric allylic oxidation of cyclohexene ("Kharasch - Sosnovski" reaction). The observed enantioselectivities were higher ( up to 85% ee) than for the unfunctionalized bisoxazoline ("BOX") derivatives (ca. 60% ee). Regardless of the functional groups incorporated into the sidearm unit, the ee's obtained for the different derivatives were essentially indistinguishable. This implies that the sidearms do not interfere directly in this reaction and only play an indirect role by virtue of their steric demand. Three of the copper complexes have been characterized by X-ray diffraction, establishing a distorted octahedral coordination geometry around the copper atom in all three cases. In the elongated distorted CuN2O4 octahedra, the two nitrogen atoms of the oxazolines and one oxygen atom of each acetate ligand occupy the 'equatorial' positions whereas the sidearms do not interact with the metal centres
ChemInform Abstract: N‐Heterocyclic Carbenes: Great Ligands for Organometallic Chemistry
No full textChemInform Abstract: Group 1 and 2 and Early Transition Metal Complexes Bearing N‐Heterocyclic Carbene Ligands: Coordination Chemistry, Reactivity, and Applications
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ChemInform Abstract: Exploiting Threefold Symmetry in Asymmetric Catalysis: The Case of Tris(oxazolinyl)ethanes (“Trisox”)
No full textCatalyst or Catalyst System? Nonlinear Behaviour and the Limits of Mechanistic Understanding in Proline-Based Asymmetric Catalysis.
No full textAsymmetric catalysis has expanded the range of chiral products readily accessible through increasingly efficient synthetic catalysts. The development of these catalysts often starts with a result obtained by systematic screening of known privileged chiral structures and is based on the assumption that the active species would be an isolated monomolecular species. Here, we have studied the activity of three proline-derived ligands that differ in minor chemical modifications. In the zinc-catalysed alkylation of benzaldehyde, we found that they exhibit completely different systems-level behaviours, characterized by multiple aggregation levels that are catalytically active simultaneously. Notably, we were able to establish the possibility of at least trimeric active species in equilibrium with less aggregated active species. These results were obtained through a combination of nonlinear effect studies and other related studies such as product ee versus catalyst loading and temperature as well as in situ NMR studies. Simulations using a mathematical model have confirmed the possibility of such systems-level behaviour. This indicates that the chiral structure alone does not necessarily correlate with systems-level behaviour that could alter the outcome of a given catalytic reaction
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