Enantioselective preparation of sec. alcohols from aldehydes and dialkyl zinc compounds, generated in situ from Grignard reagents, using substoichiometric amounts of TADDOL-titanates

Abstract

Using the Schlenk trick (precipitation of MgX 2 from ethereal solutions by the addition of 1,4-dioxane) mixtures of a Grignard reagent RMgX (X = Cl, Br, I) and 0.5 equiv. ZnCl 2 in Et 2 O can be converted to zinc alkyls R 2 Zn which in turn are added with enantio-selectivities of up to 99 : 1 to aliphatic and aromatic aldehydes in the presence of Ti(OCHMe 2 ) 4 and a chiral titanate derived from an α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol (TADDOL). Grignard reagents containing remote double bonds, benzene rings, or acetal groups can also be employed. Different TADDOLs are compared with respect to their usefulness in this kind of enantioselective reaction. Primary alkyl MgX (X=Cl, Br, I) and ZnCl 2 give (RCH 2 ) 2 Zn reagents which can be added to aliphatic and aromatic aldehydes with high enantioselectivities in the presence of chiral titanates.