Control of Enantioselectivity with Flexible Biaryl Axes: Terpene-Based Alkylzinc Catalysts in Enantioselective Dialkylzinc Additions

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 alkyl-zinc 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.