A hard-soft acid-base and DFT analysis of singlet-triplet gaps and the addition of singlet carbenes to alkenes

The electronic structures of sixteen carbenes and four alkenes calculated a t the B3LYP/6-31G* level were used to carry out a hard-soft acid-base inves tigation of carbene singlet-triplet gaps and of the addition of carbenes to alkenes. The carbenes chosen include examples where the ground state varie s from triplet to singlet state and examples that have been characterized a s nucleophilic, electrophilic, or ambiphilic based on their reactivity with alkenes having varying electron demands. We confirmed that singlet-triplet gaps calculated by DFT methods with the B3LYP functional correlate well wi th experimentally known values of carbenes spanning over 70 kcal/mol. Corre lations between electron-density-based parameters and singlet-triplet gaps, or with carbene-alkene reactivity, were analyzed to gain insight into the electronic structure of the carbenes and their acid-base tendencies. A good correlation was found between the singlet-triplet energy gap and the conde nsed softness for electrophilic attack at the triplet carbene carbon [s(C)( -)(T)], which reflects the level of carbene stability upon electron donatio n. Trends in electron donation between carbenes and alkenes with different electrophilicities and nucleophilicities analyzed by the electronegativity equalization principle correlate well with Moss' reactivity indices. We als o found that interaction energies calculated between alkenes and carbenes a re most favorable with parameters that reflect mutual electron donation, re flecting the simultaneous acidity and basicity of carbenes and alkenes.