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.