Ionic liquids : synthesis and use in PET reactions

Abstract

Two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), were synthesised, with [bmim]BF4 being used for the study of two photoinduced electron transfer reactions. The previously-reported photosensitised electron transfer reactions between 1,4-dicyanobenzene and 2,3-dimethylbut-2-ene in the presence of phenanthrene were re-examined. In acetonitrile, two isomeric substitution products, 1-( 4-cyanophenyl)-2,3-dimethyl-2-butene and 3-( 4-cyanophenyl)-2,3-dimethyl-l-butene, in the approximate ratio of2:1, were formed. When methanol was introduced as part of the mixture, the NOCAS (nucleophilic olefm combination aromatic substitution) product 3-( 4-cyanophenyl)-2-methoxy-2,3-dimethylbutane was the major product with small amounts of substitution products being formed. In both these reactions, the product ratio was found to be constant as has been shown by previous workers. These two reactions were also studied in [bmim]BF4 as the solvent instead of acetonitrile. The ratio of the substitution reaction products in this case changed from ~2: 1 to~ 1: 1, while the NOCAS reaction afforded more of the substitution isomers when using a 3:1 by volume [bmim]BE4methanol solvent mixture in comparison to when the reaction was carried out in a 3:1 acetonitrile/methanol mixture. Reaction rates in both cases were observed to be slower than in acetonitrile. The results obtained were interpreted as the effect of the ionic nature of the new solvent, its increased basicity over acetonitrile, its polarity and its high viscosity. It was also shown that these photoinduced electron transfer reactions can be carried out in the more environment-friendly ionic liquid solvents, avoiding the use of toxic acetonitrile. In the case of the ionic liquids, these were analysed by NMR and UV spectrometry, while the photosensitised electron transfer reactions were analysed by HPLC, GC-FID, and NMR.