Estimating Ring Strain Energies of Highly Substituted Cyclohexanes with the Semi-homodesmotic Approach: Why Substantial Ring Strain Exists for Nominally Tetrahedral Ring Carbon Atoms

Abstract

Estimation of ring strain energies (RSEs) of substituted cyclohexanes c-C6H(x)R(12-x) (R = F, Cl, Me; x = 0, 2, 4, 8, 10, 12) using homodesmotic reaction methods gives implausible results for highly substituted cases, particularly, c-C6R12. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We apply here our semi-homodesmotic approach that minimizes use of acyclic references and ensures cancellation of intramolecular substituent interactions. The approach provides RSEs that are more consistent with chemical intuition, although they are higher than expected for "strain-free" cyclohexanes. The RSE for c-C6Me12 is predicted to be 11.9 kcal mol(-1). RSEs for halogenated rings rise significantly from 8-9 kcal mol(-1) for c-1,1,2,2-C6H8R4 to 44-50 kcal mol(-1) for c-C6R12 (R = F, Cl). The increase, and accompanying observation of larger RSEs for "adjacent CR2" systems, can be tied to increased bond distances in the rings upon progressive substitution. The sizable RSE for perchlorocyclohexane suggests that it may be susceptible to ring-opening reactions, a facet of its chemistry that is currently unexplored.

Switching from Controlled Ring-Opening Polymerization (cROP) to Controlled Ring-Closing Depolymerization (cRCDP) by Adjusting the Reaction Parameters That Determine the Ceiling Temperature

“…48 This result likely originates from the higher ring strain in a six-membered cyclic carbonate monomer than in a lactone with the same ring size. 56,57 …”

Thermodynamic Presynthetic Considerations for Ring-Opening Polymerization

“…One possible explanation is that the added substituent of αMe-PDX reduces the strain of the ring by placing the ether bond at a more preferable angle. This same logic would also explain the reversed strain-substituent relation of the substituted δ-lactones . In this case, the bond angles of the unsubstituted δ-lactones are already at a somewhat preferred angle, meaning that the addition of a substituent implies more strain on the ring.…”

“…This same logic would also explain the reversed strain-substituent relation of the substituted δ-lactones. 81 In this case, the bond angles of the unsubstituted δ-lactones are already at a somewhat preferred angle, meaning that the addition of a substituent implies more strain on the ring.…”

Isoprene Soya Diels–Alder Adduct and Epoxidation for Photopolymerization

“…[ 32 ] An E RSE value of 0.8 kcal mol −1 is reported instead of the previously assumed value of 0. [ 33 ] In our case, ECSO has a methylcyclohexane ring attached to the chain and reported literature suggests an E RSE value of 1.0–2.0 kcal mol −1 . [ 34 ] It varies from a low value of 1.0 kcal mol −1 when the methyl substituent is attached in the equatorial position to the cyclohexane and a higher value of 1.8–2.0 kcal mol −1 if the same methyl substituent is attached in the axial position.…”

“…Several techniques are employed to calculate this parameter, including ab initio calculations, computational group equivalents, homodesmotic methods, etc. [ 30–33 ] However, there are no reported values for E RSE for the pertinent moieties (methylcyclohexane epoxide in ESO and epoxy norbornane soybean oil in ENSO) discussed in this research work. The E RSE values of the epoxy group have been reported in the literature, ranging from 26 [ 30 ] to 27 kcal mol −1 .…”