Phenylcyclopropane Energetics and Characterization of Its Conjugate Base: Phenyl Substituent Effects and the C-H Bond Dissociation Energy of Cyclopropane

The α-C-H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally determined using Hess' law. An equilibrium acidity determination of 1 afforded ΔH°_acid = 389.1 ± 0.8 kcal mol^-1, and isotopic labeling established that the α-position of the three-membered ring is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor in correctly determining the proper ionization site (i.e., secondary amide ions are needed, and primary ones and OH^- lead to incorrect conclusions since they scramble the deuterium label). An experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ± 2.8 kcal mol^-1) was combined with the ionization energy of hydrogen (313.6 kcal mol^-1) to afford BDE = 93.0 ± 2.9 kcal mol^-1. This enabled the effect of the phenyl substituent to be evaluated and compared to other situations where it is attached to an sp³- or sp²-hybridized carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out, and a revised C-H BDE for cyclopropane of 108.9 ± 1.0 kcal mol^-1 is recommended.