Acid-Base Combined Catalytic System for Enantioselective Reactions

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This project was focused on the combined catalytic systems of base catalysts and the chiral Lewis acid of the optically active ketoiminatocobalt complexes. It was reported that screening of the central metal of the ketoiminato complex catalysts for the enantioselective hetero Diels-Alder reaction revealed that cobalt was the most effective for affording the corresponding pyranone derivatives. The corresponding cationic cobalt(III) complexes were developed as an efficient Lewis acid catalyst and subjected to the hetero Diels-Alder reaction and the carbonyl-ene reaction. It was found that their counter anions significantly influenced the reactivity and the enantioselectivity in the carbonyl-ene reaction. The cationic cobalt(III) complexes could be employed as the efficient catalysts for the enantioselective 1,3-dipolar cycloaddition reaction. Because of the competitive coordination of the nitrone, the 1,3-dipolar cycloaddition with the monodentate α,β-unsaturated aldehyde has been limited to several catalyses, although these cationic cobalt(III) complexes effectively catalyzed the reaction to afford the isoxazoline derivative with high-to-excellent stereoselectivities. These types of cobalt complex catalysts could be applied to the enantioselective Henry reaction even in the presence of amine bases, and the enantioselective chemical fixation of carbon dioxide to obtain the optically active cyclic carbonate. Density functional calculations on the hetero Diels-Alder reaction catalyzed by 3-oxobutylideneaminatocobalt complexes revealed that axial coordination of an aldehyde as a Lewis base induce a spin transition in the cobalt(III) catalytic cycle. The Lewis acidity of the complex was increased and the enantioselectivity was improved.