| Research Abstract |
Steric and electronic nature of ligand greatly influences to reactivity of the metal complexes. While a number of ligand-effects have been proposed, the ‘cis-effect', is considered as small influence to the reactivity. We focused on the ‘cis-effect' of ‘hard' Lewis bases and utilized this effect to realize efficient organic transformations catalyzed by cobalt complexes and enantioselective synthesis of novel cobalt complexes.
The dicobalt octacarbonyl-mediated [2+2+1]-cocyclization of an alkyne, an alkene, and a carbon monoxide to produce cyclopentenones is known as the Pauson-Khand reaction and has been utilized for construction of five-membered rings including natural products. While a number of catalytic systems have been developed, the catalytic activity was usually low to moderate. To improve the catalytic activity, we started to investigate a new catalyst based on the use of ‘cis-effect' of ‘hard' Lewis bases. Various diarylethynehexacarbonyldicobalt complexes with ‘hard' Lewis ba
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se substituents at the orttho position of one of aryl groups were synthesized and surveyed their catalytic activity in the Pauson-Khand reaction. Among them, [η^2, μ^2-1-(2-aminophenyl)-2-phenylethyne]hexacarbonyldicobalt showed the highest catalytic activity, which was the world record in the turnover number of the reaction. The 'cis-effect' of the amine inside of the complex dramatically increased the reactivity.
Benzylidynetricobalt nonacarbonyl, which had similar structure with (η^2,μ^2-alkyne)hexacarbonyldicobalt, was known to be inert in reaction of alkynes and/or alkenes. Once hydride coordinated to the cobalt, the ‘cis-effect' of the hydride changed the electronic nature of the cobalt, and therefore, the cluster turned to show higher catalytic activity in cyclization of 1-hepten-6-ynes.
Since (η^2,μ^2-diarylethyne)hexacarbonyldicobalt complexes has similar structure with substituted stilbenes, some of the complex might have axial chirality. However, there has been no report about the chiral complex since it was discovered. We found that chiral [η^2,μ^2-1-(2,5-dimethylphenyl)-2-phenylethyne]hexacarbonyldicobalt was produced when the complexation was carried out in the presence of a catalytic amount of (2R,3R)-2,3-O-cyclohexylidene-1,1,4,4-tetraphenylthreitol. Less
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