A DFT study of the domino reactions between imidazole NHC, ketenimines and DMAD or MP acetylene derivatives yielding spiro-pyrroles

The domino reactions of imidazole carbene 9, ketenimine 10 and DMAD 5 or MP 7 to yield the formal [3+2] cycloadducts 12 or 14 have been investigated using DFT methods at the B3LYP/6-31G* level. These domino reactions are initialized by the nucleophilic attack of imidazole 9 on ketenimine 10 to yield the zwitterionic intermediate 11. The second reaction is a formal [3+2] cycloaddition (32CA) reaction between the zwitterionic intermediate 11 and acetylene carboxylates DMAD 5 or MP 7 yielding spiro-pyrroles 12 and 14. These formal 32CA reactions are initialized by the nucleophilic attack of the N3 nitrogen of intermediate 11 on the most electrophilic centres of these acetylene derivatives yielding zwitterionic intermediates, which by a fast ring-closure afford spiro-pyrroles 12 and 14. The first step of these formal 32CA reactions is completely chemoselective, while the reaction with MP 7 is completely regioselective. Analysis of DFT reactivity indices allows one to explain the behaviour of these domino reactions. The nucleophilic addition of imidazole 9 to ketenimine 10 generates a strong nucleophilic intermediate 11 able to attack electrophilic acetylenes DMAD 5 or MP 7 in the subsequent reaction.