Novel superacidic ionic liquid catalysts for arene functionalization

There is a continuing interest in the subject of arene carbonylation, especially in strong acids and environmentally-benign alternatives are sought to HF/BF3 and to AlCl3 as conversion agents. Ionic liquids offer a powerful solvent for useful conversion agents such as aluminum chloride. The IL's permit AlCl3 to be used at lower HCl partial pressures than with other solvents. The superior reactivity demonstrated by acidic, chloroaluminate IL's is probably due to their enhanced solvation power for HCl and CO. Addition of HCl gas increased reactivity of the system by forming Bronsted acids, and toluene carbonylation is a Bronsted demanding reaction. It was found that reaction is stoichiometric in Al species and only intrinsically acidic IL's are active for toluene carbonylation, therefore it was possible to correlate observed conversion with predicted amounts of Lewis + Bronsted acids. Molecular modeling provided information about the different species present in these IL's and predicted 1H NMR, and 27Al NMR spectrum. Predictions suggested that three types of HCI species are present; and these predictions were confirmed using data of 13C-labeled acetone and its 13C-NMR spectra. These data showed that only one of the three types of HCl in the IL were super acidic. Reactivity towards arene formylation can be tuned by adjusting the ligands R and R' in the organic cation and by changing the anion. This reactivity tuning can be exploited in a process where high acidity is required for the conversion of substrate but where separation of product from IL is facilitated by low acidity.