Computational study of inversion‐topomerization pathways in 1,3‐dimethylcyclohexane and 1,4‐dimethylcyclohexane: Ab initio conformational analysis

Abstract

This work concerns the special conformational behaviors for di‐substituted cyclohexanes that inherently depend on spatial orientations of side chains in flexible cyclic ring. The 1,3‐dimethylcyclohexane and 1,4‐dimethylcyclohexane in both cis‐ and trans‐configurations were focused here to unravel their inversion‐topomerization mechanisms. Full geometry optimizations were separately performed at B3LYP/6‐311++G(d,p) and MP2/6‐311++G(d,p) levels to explicitly identify all distinguishable molecular structures, and thereby the complete interconversion routes were carefully explored. Additional quantum calculations were carried out by G4 and CCSD(T)/6‐311++G(d,p) methods to acquire high‐level single point energies. With respect to quantum results, conformational analysis was conducted to study determinations, thermodynamic stabilities, and relative energies of distinct conformers. Temperature‐dependent populations of local minima for each dimethylcyclohexane were computed by Boltzmann distribution within 300–2500 K. Moreover, their unique inversion and topomerization processes were fully investigated, and potential energy surfaces were illustrated with the rigorous descriptions in two or three‐dimensional schemes for clarify.

The Inversion Process of 1,3-cyclohexanedione

“…The so-called ground state destabilization energy that would decrease the barrier magnitude is not observed in this case. [35] The conformational process of 1,3-cyclohanedione shows important differences with respect to other studied systems, [3,4,20,23] such as the existence of only two trajectories for the inversion of twisted boats conformers, the inversion barrier has a smaller value with respect to the barrier with which the twisted-boats are exchanged, and also that it has not been possible to locate the transition state related to the interchange of twisted-boats B and B'. Finally, the magnitude of the determined barriers makes it impossible to use NMR to study the conformational equilibrium at temperatures below -160 °C.…”

“…[16,17] In these studies it was possible to stablish the stability of conformers [18] and study reaction mechanism, for example, the combustion reaction, specifically in the determination of the migratory aptitude of different types of hydrogen atoms. [19][20][21][22] Some work has focused on the study of the rigorous mechanism of inversion-topomerization of substituted cyclohexanoanes, [23] both processes by the introduction of heteroatoms into the ring and by the incorporation of substituents. [24] Finally, this type of study allows us to establish how conformation at enzyme recognition-catalytic centers defines the absolute stereochemistry of natural products.…”

Structure‐dependence in initial decomposition of trans‐1,2‐dimethylcyclohexyl isomers: Kinetic exploration and conformational analysis

“…Such process is responsible for ring flip to accomplish the exchange of equatorial and axial positions for branched chains. Whereas, the transformation circuit contributed by pseudorotation in ring allows side chains altering their relative orientations in accordance with the rule: pseudo‐equatorial ↔ isoclinal ↔ pseudo‐axial [38]. It should be mentioned that the whole inversion‐topomerization process would trigger or inhibit different types of H‐transfers by altering ring forms or positions of side chains during the low temperature oxidation [8, 9] and pyrolysis [14] of cycloalkanes.…”