| Popis: |
New enantiopure pyridyl alcohols are efficiently accessible through few synthetic steps from commercially available terpenes, that is, (+)-fenchone, (-)-menthone and (—)— verbenone as well as 2,6-diphenylpyridine. These chelating pyridyl alcohols exhibit flexible pyridyl-phenylene axes, which give rise to P and M conformers. Alkylzincation of the hydroxy groups eliminates equilibria of the conformers and generates alkylzinc complexes with adjusted biaryl axes, as it is demonstrated by NMR studies. These alkylzinc catalysts perform well in the addition of dimethylzinc or diethylzinc to benzaldehyde with yields up to 99% and ee's up to 95 %. The adjusted pyridylphenylene conformations in the ligands now control enantioselectivities of the catalysts, which were also analysed by computations at the DFT level. |
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