(Perfluoro)sulfonic acids having an imidazolium tag as homogeneous and reusable ionophilic Brönsted acid catalysts for carboxylic acid esterification

The concept of adding a peripheral imidazolium tag to obtain ionophilic sulfonic acids has been applied to the synthesis of four Brönsted acids having perfluoroalkyl and propyl linkers. The resulting sulfonic acids were tested as homogeneous and reusable Brönsted acid for the esterification of octanoic acid by n-propanol. It was found that while perfluoroalkyl sulfonic acids exhibit low catalytic activity due to miscibility problems, propylsulfonic acids were highly active and reusable. It was concluded that no strong Brönsted sites are necessary to promote this carboxylic esterification and that the fluorous properties of the perfluroalkyl chain (>30% fluorine in the imidazolium cation) play an adverse role on the catalytic activity under the conditions used.

Four ionophilic Brönsted sulfonic acids having hydrocarbon or perfluorinated (see the structure) have been synthesised and their catalytic activity tested for the esterification of octanoic acid with n-propanol. It was found that imidazolium having a propylsulfonic acid is a highly active and reusable catalyst, while the perfluoro analogs exhibit considerably less activity.Figure optionsDownload as PowerPoint slide