Search

DFT calculations show that square-planar LCo^IR complexes of a diiminopyridine ligand are best regarded as containing low-spin Co^II antiferromagnetically coupled to a ligand radical anion. The lowest triplet state, corresponding to a 3dz²&rlarr2;π* excitation, is calculated to be only a few kcal/mol above the ground state, and is thermally accessible. The anomalous ¹H NMR chemical shifts of the LCoR complexes are suggested to be due to thermal population of the triplet state at room temperature. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

A number of cobalt(II) chloride complexes of pyridine/amine N3 and N4 donors have been prepared and structurally characterised. Even though they are paramagnetic, assignment of the ¹H NMR signals was possible in several cases. With the exception of tpa, which formed a cationic monochloride complex, all new complexes have cis-coordinated chlorides. Nevertheless, only the relatively rigid pyp ligands gave rise to (low) ethene polymerisation activity on activation with MAO. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

A variety of allylic O,O- and N,O-acetals were synthesized using a mild palladium-catalyzed coupling of an alcohol or sulfonamide with an alkyl or aryl 1,2-propadienyl ether. The resulting linear acetals were used for the synthesis of unsaturated rings via ring-closing metathesis, in which the acetal carbon–a precursor for oxycarbenium or N-sulfonyliminium ions, respectively–served as a reactive center for further introduction of functional groups. The products–unsaturated oxygen and nitrogen heterocyclic scaffolds–offer multiple opportunities for derivatization as illustrated with the synthesis of substituted dihydropyrans, chromenes, enantiopure tetrahydropyridines and an enantiomerically pure quinolizidine amino acid.

No Abstract

Activated by reduction: On treatment with methylaluminoxane (MAO), the Brookhart/Gibson cobalt‐based polymerization precatalyst [LCoIICl2] is first reduced to [LCoICl], subsequently alkylated to [LCoIMe], and finally converted into the (as yet unknown) active species (see scheme, PE=polyethene).

No Abstract