On the importance of the novel RIRC technique to highlight the Hidden
Intermediate of Reaction in the SNAr Reaction of piperidine on
2-bromo-3,5-dinitrothiophene
Abstract
DFT/M062X/6-311+G(2d,2p) theoretical calculations have been applied to
the reaction mechanism of the SNAr describing the action of the
piperidine C5H10NH on the 2-bromo-3,5-dinitrothiophene. In a first
predictive approach, the role of the ligand X in position 5 (X = NO2, CN
and H) of the 2-bromo-3-nitrothiophene motif has been studied. Our
results show that the dinitro compound represents the best element
playing the role of electrophile. In a second part focused on this
latter compound, we elucidate the aromatic substitution of bromide by
piperidine mechanism. To do this, we first use the traditional tools
such as the energy, force and internal coordinate profiles based on the
intrinsic reaction coordinate. Then, we strengthen our study by tracing
the atomic charge and Wiberg bond indice profiles. All the results
demonstrate that we were in the presence of a concerted but asynchronous
reaction. This excluded the zwitterion to act as a reaction
intermediate. However, by using our new technique, called Reactive
Internal Reaction Coordinate (RIRC), which gives access to the reaction
path according to the selected active internal coordinates, we were able
to locate without ambiguity a singular point associated with the Hidden
Reaction Intermediate (HRI), compatible with the zwitterion structure.
The latter compound corresponded to a long-lived transient species.