| Abstract
| - Isodesmic and homodesmic equations at the B3LYP/6-311+G(d,p)+ZPVE level of theory have beenused to estimate strain for the homologous series of cyclic allenes and cyclic butatrienes. A simple fragmentdeformation approach also has been applied and appears to work better for the larger rings. For thecyclic 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. Newlypredicted 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.
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