Highly enantioselective cyclopropanation of styrene derivatives and diazoacetates was effectively catalyzed by ketoiminatocobalt(II) complexes. Addition of a catalytic amount of n-methylimidazole significantly accelerated the reaction and enhanced the enantioselectivity due to its coordination to the center cobalt atom of the complex as an axial ligand. Analysis of the transition states by the PM 3(tm) method indicated that the olefin approached parallel to the cobalt-carbene bond with bisecting an O-Co-O angle. The reaction pathway of the cyclopropanation was analyzed by the density functional method to reveal that the axial donor ligand produced two prominent effects. One is that the activation energy for the formation of the cobalt carbene complex was reduced and that the activation energy for the cyclopropanation step was increased. The other is that the distance of the carbene carbon above the ligand plane was shortened during the cyclopropanation step. It was revealed that the carbene-carbon bond of the ketoiminatocobalt-carbene complexes is characterized as an extraordinary single bond based on the theoretical and FT-IR analysis. The key reactive intermediate of borohydride reduction catalyzed by Schiff base-cobalt complexes is proposed to be the dichloromethyl-cobalt 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. It was found that a catalytic amount of chloroform effectively activated the present catalytic system to convert various ketones into the corresponding reduced product with a high ee in the THF solvent. Furthermore, the theoretical simulation of various axial groups in cobalt complex catalysts predicted that the cobalt-carbene complexes could be employed as efficient catalysts. The newly designed complexes generated from cobalt complex and methyl diazoacetate made it possible to catalyze the enantioselective borohydride reduction in a halogen-free solvent.
|ジャーナル||Yuki Gosei Kagaku Kyokaishi/Journal of Synthetic Organic Chemistry|
|出版ステータス||Published - 2008 2|
ASJC Scopus subject areas
- Organic Chemistry