Abstract

The combined use of a catalytic amount of silver acetate and a stoichiometric amount of DBU efficiently catalyzed the incorporation of CO₂ under mild reaction conditions into a wide range of propargylic alcohols bearing a terminal or an internal triple bond to afford the corresponding cyclic carbonates in high-to-excellent yields. All the cyclic carbonates obtained from the reaction were found to be single isomers. The geometries were determined to be (Z) by X-ray crystal structure analysis and NOE experiments.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The key reactive intermediate of borohydride reduction catalyzed by Schiff base–cobalt complexes is proposed to be the dichloromethylcobalt hydride with a sodium cation, based on experimental and theoretical studies. It was revealed that chloroform is not the solvent but the reactant that activates the cobalt catalyst. The substrate carbonyl compounds are fixed and activated by the alkali cation, which is captured by the oxygen atoms of the planar ligand and the chlorine atom of the axial ligand, and attacked by the hydride on the cobalt atom via a six-membered-like transition state to afford the corresponding alcohol.

Dichtefunktionalrechnungen zu Hetero-Diels-Alder-Reaktionen mit 3-Oxobutylidenaminatocobalt(II) und -cobalt(III)-Komplexen als Katalysatoren belegen, dass die axiale Koordination von Aldehyden als Lewis-Basen einen Spinübergang im Cobalt(III)-Katalysezyklus bewirkt. Die Lewis-Acidität des Komplexes steigt, die Co-O_Aldehyd-Bindung wird kürzer (siehe Bild; Co grün, N blau, O rot) und die Enantioselektivität wird verbessert.

Density functional calculations on the hetero-Diels–Alder reaction catalyzed by 3-oxobutylideneaminatocobalt(II) and -cobalt(III) complexes revealed that axial coordination of an aldehyde as a Lewis base induces a spin transition in the Co^III catalytic cycle. The Lewis acidity of the complex is increased, the CoO_aldehyde distance shortened (see picture; Co green, N blue, O red), and the enantioselectivity improved.

A series of poly(2-alkyl-6-phenylphenylene ether)s were prepared by the oxidative polymerization of 2-alkyl-6-phenylphenols. Their dielectric constants were approximately 2.5, whereas it was found that the phenyl group at the 6-position clearly increased the Q factor of the polymer compared with poly(2,6-dimethylphenylene ether), PPE. These values were qualitatively predicted from the polarizability and the molar volume calculated by the theoretical analysis of the unit structure at the HF/6-31G* level.

The highly enantioselective borohydride reduction of aromatic ketones or imines to the corresponding alcohols was developed in the presence of a catalytic amount of an optically active cobalt(II) complex catalyst. This enantioselective reduction is carried out using a precisely premodified borohydride with alcohols such as tetrahydrofurfuryl alcohol, ethanol and methanol. High optical yields are obtained by choosing the appropriate alcohol as modifiers and a suitable β-ketoiminato ligand of the catalyst. The enantioselective borohydride reduction has been successfully applied to the preparation of optically active 1,3-diols, the stereoselective reduction of diacylferrocenes, and dynamic and/or kinetic resolution of 1,3-dicarbonyl compounds.