Ionic liquids as recyclable reaction media for the tetrahydropyranylation of alcohols

A comparative study of the catalysed tetrahydropyranylation of 1-phenylethanol by p-toluenesulphonic acid (TsOH), pyridinium p-toluenesulphonate (PPTS) and triphenylphosphine hydrobromide (TPP.HBr) using 3,4-dihydro-2H-pyran in dichloromethane or in the ionic liquids 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) or 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) showed that the reaction occurs readily in [bmim][PF6] for PPTS or TPP.HBr by a non-reversible process. These reaction media were recycled on 22 occasions without loss of activity and their scope of application to other alcohols was also studied.