Co-Metal-Free Enantioselective Conjugate Addition Reactions of Zinc Reagents

Asymmetric conjugate addition of diethylzinc to cinnamaldehyde in a co‐metal‐free fashion by using N,O‐ligands with planar and central chirality is described. Different modulations of the ligand structure, including several combinations of the chiral units, indicate that a [2.2]paracyclophane backbone is essential for the activity and the enantioselectivity of the generated active catalyst. By using the optimized ligand, an isolated yield of 90 % was obtained with up to 99 % ee. Zinc, but almost no copper: Ligand optimization led to an increased 1,4‐selectivity in the reaction of diethylzinc with cinnamaldehyde (see scheme). We have shown that the zinc‐based catalytic system used in this reaction is concurrently affected by copper salts and that copper‐free conditions are required to obtain the desired 1,4‐addition product selectively.