Homo- and Heterochiral Alkylzinc Fencholates: Linear or Nonlinear Effects in Dialkylzinc Additions to Benzaldehyde

Scalemic mixtures of chiral anisyl fenchols with different ortho‐substituents (X) in the anisyl moieties [X=H (1), Me (2), SiMe3 (3) and tBu (4)] are employed as pre‐catalysts in enantioselective additions of diethylzinc to benzaldehyde. While a remarkable asymmetric depletion is apparent for X=H and Me, a linear relationship between the enantiomeric purity of the chiral source and the product 1‐phenylpropanol is observed for X=SiMe3 and tBu. X‐ray single crystal analyses show that racemic methylzinc fencholates obtained from 1 (X=H) and 2 (X=Me) yield homochiral dimeric complexes, while for 3 (X=SiMe3) and 4 (X=tBu) the heterochiral dimeric alkylzinc structures are formed. The enantiopure fenchols 1–4 all yield homochiral dimeric methylzinc complexes. Computed relative energies of homo‐ and heterochiral fencholate dimers with X=H and Me reveal an intrinsic preference for the formation of the homochiral dimers, consistent with the observed negative NLE. In contrast, similar stabilities are computed for homo‐ and heterochiral complexes of ligands 3 (X=SiMe3) and 4 (X=tBu), in agreement with the absence of a nonlinear effect for bulky ortho‐substituents. The nature of the ortho‐substituents (X) in anisyl moieties of alkylzinc fencholate catalysts controls the appearance of nonlinear effects (asymmetric depletions) in diethylzinc additions to benzaldehyde and is also crucial for the formation of homo‐ or heterochiral dimeric alkylzinc fencholates. A heterochiral dimer with X=SiMe3 is shown.