Aryl azides formation under mild conditions: a kinetic study in some ionic liquid solutions

The kinetics of nucleophilic aromatic substitution of three nitrothiophene derivatives in different [1-butyl-3-methylimidazolium][N3]/ionic liquid binary mixtures was studied spectrophotometrically at 298 K. Ionic liquids differing for cation structure (imidazolium or pyrrolidinium) and for size, shape, and coordination ability of the anion ([BF4 -], [PF6 -], [SbF6 -], and [NTf2 -]) were used. Furthermore, in order to have a comparison with conventional organic solvents, the target reaction was also carried out in DMF solution at increasing concentration of NaN3 or [bmim][N3]. Data collected show that the reaction occurs faster inDMFthan in ionic liquid solution. Furthermore, as a consequence of the ability of all solvent media to favor the leaving group departure, a simple linear dependence of the pseudo-first-order kinetic constant from nucleophile concentration was detected. The results of this kinetic investigation once more underline that ionic liquids are able to exert peculiar effects that can be understood also considering their three-dimensional organization.