Derek Feichtinger

Rate Enhancement and Enantioselectivity of the Jacobsen–Katsuki Epoxidation: The Significance of the Sixth Coordination Site

New light on the origin of the high enantioselectivities achieved in the Jacobsen-Katsuki epoxidation is shed by the results of density functional calculations. Axial ligation to the metal center not only enhances the epoxidation rate, but in addition leads to highly nonplanar, bent conformations of the active catalyst.

Coordination chemistry of manganese–salen complexes studied by electrospray tandem mass spectrometry: the significance of axial ligands

Abstract Tandem mass spectrometric techniques in combination with high-level quantum chemical calculations have been employed to study the coordination chemistry of manganese– and oxomanganese–salen complexes in the gas phase. Electrospray ionization was used to transfer the ionic complexes from solution to the gas phase. The formation of five- versus six-coordinate manganese(III) species was subsequently probed by ion–molecule reactions with neutral ligands, e.g. acetonitrile, pyridine, alcohols, etc. The reactivity of the so far elusive oxomanganese(V)–salen complexes, readily accessible by fragmentation of μ-oxomanganese(IV) dimers, and their coordination chemistry was studied in the same way. Hybrid Hartree-Fock/density functional calculations have been performed to assess the geometries and energies of the triplet and quintet states of the manganese complexes in question. The effects of axial ligation on the geometry and reactivity of the oxo complex were found to be quite drastic. Finally, the epoxidation of olefins by oxomanganese(V)–salen was studied intramolecularly by tethering the substrate to the metal center. No indication for precoordination of the substrate as prerequisite for oxidation was found.

Enhanced Reactivity of 2‐Rhodaoxetanes through a Labile Acetonitrile Ligand

New cationic, square-planar, ethene complexes [(Rbpa)RhI(C2H4)]+ [2a]--[2c]+ (Rbpa = N-alkyl-N,N-di(2-pyridylmethyl)amine; [2a]+: alkyl =R=Me; [2b]+: R = Bu; [2c]+: R = Bz) have been selectively oxygenated in acetonitrile by aqueous hydrogen peroxide to 2-rhoda(III)oxetanes with a labile acetonitrile ligand, [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+, [3a]+-[3c]+. The rate of elimination of acetaldehyde from [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+ increases in the order R = Me< R = Bu< R = Bz. Elimination of acetaldehyde from [(Bzbpa)RhIII(kappa2-C,O-CH2CH2O)(MeCN)]+ [3c]+, in the presence of ethene results in regeneration of ethene complex [(Bzbpa)RhI(C2H4)]+ [2c]+, and closes a catalytic cycle. In the presence of Z,Z-1,5-cyclooctadiene (cod) the corresponding cod complex [(Bzbpa)RhI(cod)]+ [6c]+ is formed. Further oxidation of [3c]+ by H2O2 results in the transient formylmethyl-hydroxy complex [(Bzbpa)RhIII(OH)[kappa1-C-CH2C(O)H]]+ [5c]+.