Evaluation of Zn²⁺ Coordination Structures in Chiral Zn²⁺ Complexes Based on Shape Measurement Factors: Relationships between Activity and the Coordination Structure

We previously reported on enantioselective aldol reactions of acetone and cyclic ketones with benzaldehyde derivatives catalyzed by chiral Zn²⁺ complexes that mimic class II aldolases. The results and mechanistic studies indicated that the catalytic activity of Zn²⁺ complexes is dependent not only on the coordination numbers of Zn²⁺ but also on its coordination structure. In this study, we report on shape measures (abbreviated as S values in this manuscript) of such Zn²⁺ complexes with respect to the 5-coordination structure based on calculations with continuous shape measures (CShM) using their crystal structures. The S values of Zn²⁺ ions in Zn²⁺ complexes for trigonal bipyramidal (D_3h) or square pyramidal (C_4v) structures suggest that Zn²⁺ ions in the catalytically active Zn²⁺ complexes possess D_3h and C_4v structures, while Zn²⁺ ions in the unreactive Zn²⁺ complex has a C_4v structure. Moreover, the S values of Zn²⁺ were calculated with respect to 6-coordination geometry, based on the assumption that aldol reactions proceed via 6-membered transition states, and it was implied that 6-coordinated Zn²⁺ ions in aldol-active Zn²⁺ complexes prefer an octahedron structure (O_h). These results for the S values of Zn²⁺ ions in the active Zn²⁺ complexes and those for complexes reported by other research groups suggest that the transition of 5-coordinated D_3h to the 6-coordinated O_h structure is essential for achieving catalytic stereoselective aldol reactions.