The stereochemistry and reactivity of metal-Schiff base complexes. V. The solvent effect on the stereoselectivity of cobalt(III) complexes containing a sal2en-type Schiff base and L-amino acid.

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The solvent effect on the stereoselectivity between Λ- and Δ-β2-diastereoisomers of cobalt(III) complexes with the general formula of [Co(Schiff base)(L-aa)], where Schiff base represents sal2en2−, α-Me-sal2en2−, and 5,6-benzo-sal2en2−, and where L-aa denotes L-alaninate, L-valinate, L-methioninate, and L-phenylalaninate, was studied. Generally, the Λ-β2-isomer was more stable than the Δ-β2-isomer in solution, and the stereoselectivity of all the L-ala-complexes and of the L-met-complex with 5,6-benzo-sal2en increased in the following order of solvents : acetone=DMF≤t-BuOH<s-BuOH<i-PrOH<n-BuOH<n-PrOH<EtOH<MeOH. On the other hand, the stereoselectivity of L-val- and L-phe-complexes with sal2en and 5,6-benzo-sal2en increased in the following order : acetone=DMF<MeOH<EtOH<n-PrOH<n-BuOH<i-PrOH<s-BuOH<t-BuOH. In the case of 5,6-benzo-sal2en complexes with L-val and L-ile, the Δ-β2-isomer was preferred rather than the Λ-β2-isomer in acetone and DMF with 8–13% stereoselectivity. From the spectroscopic evidence, a hydrogen bonding between the OH group of alcoholic solvents and phenolic oxygen atoms of a Schiff base ligand was assummed, and the above phenomena were explained in terms of the steric difference between the Λ- and Δ-β2-isomers for the solvation with the alcoholic solvents by means of the hydrogen bonding.
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