Chirality of the Conformation Attacks the Planarity of the sp2 Carbon Atom in a Covalent Bond.

The correlation of the rotation about the bond C'-Cα and the pyramidalization of the sp2 hybridized carbon atom C' and its three bonding partners to a flattened tetrahedron was investigated for the substructure Cβ-CαH-C'(═O)-OMe. A search in the Cambridge Structural Database (CSD) gave 15,295 structures with a substituted methyl acetate group at the end of the molecules. The scatter plot of the rotation angle ψ = O═C'-Cα-Cβ versus the pyramidalization angle θ = O(MeO)C'Cα and the ψ/θav curve show an unusual undulating pattern with three maxima and minima for a 360° rotation about the bond C'-Cα. There is no net chiral induction from the (Mψ)/(Pψ) conformations of the bond C'-Cα to the (Rθ)/(Sθ) configurations of the flattened tetrahedron because the contributions of the three maxima and minima cancel each other. The wavelike ψ/θav curve demonstrates that the rotation about the bond C'-Cα and the pyramidalization of the group C'(═O)(OMe)Cα are not independent processes. They are coupled with each other in one common molecular motion. The ψ/θav curve appears as the third harmonic of a sinusoidal fundamental frequency. DFT calculations of the propanoate anion, methyl propanoate, and methyl 2-methylpropanoate confirm the results of the CSD search.