The relative stability of mesoionic and N-heterocyclic carbene tautomers: a computational study of substituent effects

Abstract

The influence of exocyclic substituents on the relative energies of mesoionic and N-heterocyclic carbene (NHC) tautomers has been investigated. Calculated Gibbs free energy differences between mesoionic 1,2,4-triazolium derivatives and NHC isomers vary considerably depending upon the substituents in position 3. Substituent effect stabilisation energies (SESE) for 3-substituents on each ring have been estimated using homodesmotic equations. For the mesoionic rings the SESE values are in accord with their known chemistry and stabilisation is in the order O › NPh ›› NMe. 3-Amino substituents stabilise the NHC ring in the order NHMe › NHPh › NH2. However, OH, OMe and F substituents are destabilising but anionic O− and PhN− substituents are particularly favourable. Knowledge of the 3-substituent SESE values for each ring system provides insight into the origins of the free energy differences between the tautomers and suggests the possibility that nitron may react via an anionic NHC.