Reactions of Strained Cycloalkanes with Radicals and Diradicaloids: The Roles of Diradical Character and Strain...

Ring-opening reactions of strained cyclic alkanes are powerful methods to construct complex organic molecules. Strain release, quantified by the Bell-Evans-Polanyi (BEP) relationship, leads to rate acceleration, but cyclopropane-containing molecules are even more reactive than predicted from BEP. We report a computational exploration of reactions of 12 strained cycloalkanes and ethane with methyl radical and six unsaturated diradicaloids. The strained cycloalkanes are the same that were studied earlier in reactions with the methyl radical by Sterling et al. We have found that the diradical character of the breaking σ bond of the alkane is a key factor, in addition to strain release, that accelerates ring-opening reactions. Diradical character is quantified by y₀, the occupation number of the σ* orbital from CASSCF calculations of the molecule. For all of these reactions, there is a threshold σ* occupation at the transition state. Molecules with high ground-state diradical character reach this threshold with little geometric distortion, leading to earlier transition states and lower activation barriers. This model demonstrates the importance of intrinsic reactivity factors such as diradical character, bond order, and the Sterling et al. delocalization in addition to thermodynamic strain-release factors. Among these concepts, diradical character directly reveals why strained σ bonds require less geometric distortion to reach transition state geometries, resulting in lower activation barriers.