σ-Holes promote the concertedness in nucleophilic aromatic substitution reactions of nitroarenes

The mechanism of the SNAr reactions between 1-halo-2,4-dinitrobenzenes and amines was revisited by means of DFT calculations. Remarkably and contrary to the traditional text-book perspective, the dehalogenation of 1-X-2,4-dinitrobenzenes bearing good leaving groups (X = Cl, Br and I) by soft nucleophiles involves a single-step mechanism passing through a barrierless C-X bond cleavage step. Solely the reaction of 1-fluoro-2,4-dinitrobenzene follows the traditional addition-elimination pathway. The analysis of the charge transfer patterns along the reaction path for the whole systems studied suggests that for those nitroarenes exhibiting sigma-holes the dehalogenation mechanism occurs through a single-step. Nucleophile effects on the reaction rates were also discussed.