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 (D3h) or square pyramidal (C4v) structures suggest that Zn2+ ions in the catalytically active Zn2+ complexes possess D3h and C4v structures, while Zn2+ ions in the unreactive Zn2+ complex has a C4v 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 (Oh). 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 D3h to the 6-coordinated Oh structure is essential for achieving catalytic stereoselective aldol reactions.
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