A Semi-homodesmotic Approach for Estimating Ring Strain Energies (RSEs) of Highly Substituted Cyclopropanes That Minimizes Use of Acyclic References and Cancels Steric Interactions: RSEs for c‑C3R6 that Make Sense

Estimation of ring strain energies (RSEs) of substituted cyclopropanes c-C3H x R6–x (R = F, Cl, Me; x = 0, 2, 4) using homodesmotic reaction methods has been plagued by implausible results. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We report a semi-homodesmotic approach that minimizes use of acyclic references, focusing instead on canceling substituent interactions. The method requires employing homodesmotic group equivalent reactions only for disubstituted cyclopropanes and relies solely on absolute energy calculations for more substituted rings. This provides RSEs consistent with chemical intuition regardless of the degree of substitution. We find that RSEs increase with substitution regardless of the electronic nature of R, although the increase is more dramatic when R is electron-withdrawing. The RSEs determined are consistent with QTAIM data, which show that progressive substitution always increases critical path angles. Overall, the semi-homodesmotic approach is simpler than hyperhomodesmotic reaction methods, and gives more trustworthy results.