Heck reactions of aryl halides with styrene using 1-butyl-3-methylimidazolium bromide ionic liquid as a green solvent under microwave irradiation

Abstract

Easily accessible ionic liquids, 1-butyl-3-methylimidazolium bromide, 1-hexyl-3-methylimidazolium bromide, and 1-octyl-3-methylimidazolium bromide, respectively, were synthesized under microwave irradiation condition, and characterized by ¹H and ¹³C NMR, and MS. The three ionic liquids were demonstrated to be an efficient and recyclable solvent for the Heck cross-coupling reaction between several aryl halides and styrene under microwave irradiation to form trans-stilbenes as the principal products. Interestingly, it was found that increasing the length of the alkyl chain in the ionic liquid caused a significant drop in the reaction rate. Higher reaction rate was observed for the Heck reaction using 1-butyl-3-methylimidazolium bromide, as compared to that of the reaction using 1-hexyl-3-methylimidazolium bromide, and 1-octyl-3-methylimidazolium bromide, respectively. Using the ionic liquid as the reaction solvent in conjunction with microwave irradiation, the reaction rate was dramatically enhanced, with 99% conversion being achieved within 1.5 minutes for the butyl-based ionic liquid, compared to conversions obtained after 2 hours under conventional conditions. Furthermore, the ionic liquid – Pd²⁺ system could be reused in subsequent reaction without significant degradation in activity.