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

Abstract

The correlation of the rotation about the bond C′−C α and the pyramidalization of the sp 2 hybridized carbon atom C′ and its three bonding partners to a flattened tetrahedron was investigated for the substructure C β −C α H−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′−C α −C β versus the pyramidalization angle θ = O(MeO)C′C α and the ψ/θ av curve show an unusual undulating pattern with three maxima and minima for a 360°rotation about the bond C′−C α . There is no net chiral induction from the (M ψ )/(P ψ ) conformations of the bond C′−C α to the (R θ )/(S θ ) configurations of the flattened tetrahedron because the contributions of the three maxima and minima cancel each other. The wavelike ψ/θ av curve demonstrates that the rotation about the bond C′−C α and the pyramidalization of the group C′(O)(OMe)C α are not independent processes. They are coupled with each other in one common molecular motion. The ψ/θ av curve appears as the third harmonic of a sinusoidal fundamental frequency. DFT calculations of the propanoate anion, methyl propanoate, and methyl 2methylpropanoate confirm the results of the CSD search.

Twofold and Threefold Sinusoidal Patterns in Coupled Molecular Motions of 184,025 Structures of Phenylethane, Nitroethane, and Carboxylate Derivatives

“…In refs , , we had presented our results concerning the pyramidalization of sp 2 center C′ in half a million structures, containing the substructure X-C α H-C′(Y)­Z, during rotation of X about the bond C α –C′. However, we did not explain the strange three-max/min curves, obtained in the analysis of rotation angle ψ versus pyramidalization angle θ with their six zero-crossings.…”

“…The coupled molecular motion of rotation about the bond C α –C i and pyramidalization of the group C α C i C o C o′ , discussed above for phenylethane C β -C α H 2 -Ph, was described for the first time in refs , for carboxylates, carboxamides, ketones, etc. Dealing with 14,169 phenyls C β -C α H 2 -Ph, we came across other molecular motions, which are intimately coupled with each other.…”

“…These symmetry planes are absent in groups in which the sp 2 center is surrounded by different substituents. Such a substructure is C β -C α HX-C′(O)­OMe, depicted on the cover picture of J. Org. Chem.…”

“…In substituted acetates of the substructure C β -C α HX-C′(O)­O, a coupled molecular motion of rotation about the bond C α –C′ and pyramidalization of the sp 2 hybridized atom C′ had been observed. , With three open positions for substituents at C β and one open position at C α and the singly bonded oxygen atom, respectively, the substructure C β -C α HX-C′(O)­O included 84,572 structures retrieved from the Cambridge Structural Database (CSD).…”

“…In our previous studies, , we had included ortho -substituted phenyls C β -C α HX-C i (C o -Y)­(C 4 H 4 ) to keep the group C α C i C o C o′ chiral on pyramidalization. In the present paper, we expand our studies to 14,169 phenyls C β -C α H 2 -Ph with three open coordination positions at C β and 150,568 phenyls C β -C α HX-Ph with an additional open coordination position X at C α .…”