Characterizing ionic liquids as reaction media through a chemical probe.
The triplet N,N-dimethylaminophenyl cation, a highly reactive but chemospecific electrophile, has been used as a probe for characterizing the properties of reaction media for a series of imidazolium ILs. With the N-hexyl-N-methyl imidazolium derivatives (not with the N-butyl analogues), hydrogen transfer leading to the aniline was the main process. Trapping by iodide occurred with an inverse dependence on viscosity. Trapping by pi nucleophiles exhibited a more complex behavior. This was explained by the effect of both the bulk viscosity and the structure of the IL cation on both steps of the reaction, namely, initial electrophilic attack and ensuing cation elimination or nucleophile addition. However, with an excellent nucleophile, such as thiophene, or when the latter step was intramolecular, as with 4-pentenol, the difference was obliterated and trapping became uniform. Incorporation of the probe into the IL cation (through insertion into the C--H bond alpha to the imidazolium ring) was demonstrated, while no addition to the anion tested (including bis(trifluoromethanesulfonimide)) took place.
Dichiarante
Valentina
V
Department of Organic Chemistry, University of Pavia, Via Taramelli 10, 27100 Pavia, Italy.
Betti
Cecilia
C
Fagnoni
Maurizio
M
Maia
Angelamaria
A
Landini
Dario
D
Albini
Angelo
A