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

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posted on 2021-07-12, 15:39 authored by Henri Brunner, Masahiro Ikeshita, Takashi Tsuno
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.

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