Strain estimates for small-ring cyclic allenes and butatrienes

Isodesmic and homodesmic equations at the B3LYP/6-311+G(d, p)+ZPVE level of theory have been used to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragment deformation approach also has been applied and appears to work better for the larger rings. For the cyclic allene series, estimates for allene functional group strain (kcal/mol) include: 1,2-cyclobutadiene, 65; 1,2-cyclopentadiene, 51; 1,2-cyclohexadiene, 32; 1,2-cycloheptadiene, 14; 1,2-cyclooctadiene, 5; 1,2-cyclononadiene, 2; 1,2,4-cyclohexatriene, 34; and bicyclo [3.2.1] octa-2,3-diene, 39. For cyclic butatrienes, functional group strain estimates include: 1,2,3-cyclobutatriene, > 100; 1,2,3-cyclopentatriene, 80; 1,2,3-cyclohexatriene, 50; 1,2,3-cycloheptatriene, 26; 1,2,3-cyclooctatriene, 17; and 1,2,3-cyclononatriene, 4. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Newly predicted values include: 1,2-cyclopentadiene < 1 kcal/mol and bicyclo[ 3.2.1]octa-2,3-diene, 7.4 kcal/ mol. Estimated levels of strain parallel the known reactivity of these substances.