A DFT study on the mechanisms for the cycloaddition reactions between 1‐Aza‐2‐azoniaallene cations and carbodiimides

The mechanisms of the title reactions between 1‐aza‐2‐azoniaallene cations and carbodiimides in the gas phase have been examined using the Becke‐3‐parameter‐Lee‐Yang‐Parr (B3LYP) at 6‐31++G** level. The theoretical results revealed that the reaction is a domino reaction that comprises two consecutive reactions: an ionic [3+2] cycloaddition reaction between 1‐aza‐2‐azoniallene cation and carbodiimide to yield the cycloadduct 3 and then a [1,2]‐shift to yield the thermodynamically more stable adduct 4 . Both stepwise and concerted pathways are accessible in the first cycloaddition in the model reaction. The activation barriers of them are almost equivalent. For the [1,2]‐shift reactions, both of the electron‐withdrawing chlorine substituent and the electron‐releasing methyl substituent on the 1‐aza‐2‐azoniaallene cation can facilitate the reaction but have little effects when substituted in the carbodiimide moiety. The model reaction has also been investigated at the QCISD (quadratic configuration interaction using single and double substitutions)/6‐31++G** and CCSD(T) (coupled cluster calculations with single and double excitations and a perturbative estimate of triple contributions calculations)/6‐31++G** levels as well as by the density functional theory. In addition, solvent effects with the isodensity‐surface polarized continuum model are also reported for all the reactions. In solvent dichloromethane, the cycloadducts of all the reactions, except model reaction and reaction d, were obtained from reactants directly as the result of the solvent effect. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011