Ionic Tags in Catalyst Optimization: Beyond Catalyst Recycling

In the pursuit of sustainability and efficiency in catalysis, the recourse to liquid-liquid biphasic homogeneous conditions is rapidly growing. In this context, the efficient entrapment of a catalyst in a solvent such as water or an ionic liquid can be achieved by installing an ionic substituent on the catalyst structure, the so called ion-tag strategy. Little attention has been devoted in the literature to the fact that ion tags often improve catalytic performances compared to the parent tag-free catalyst. From statistical scrutiny of literature data, a working hypothesis has been developed as a basis for further investigation and testing; a decrease of the free-energy barrier is frequently observed as the result of an interplay of coulombic and steric interactions between developing charges in a polar transition state and the tag's ion pair. If this is true, a way to develop more efficient catalytic protocols will entail a rational installation of an ionic group onto the structure of a known catalyst, complying with simple empirical guidelines that are discussed herein.