Catalyst-philic liquid phases can be used to promote the catalytic activity of heterogeneous catalysts, and to facilitate product-catalyst separation. A variety of different constituents of such catalyst-philic phases can be used, the most attractive being quaternary ammonium and phosphonium salts, PEGs, as well as water and other kinds of low-temperature molten salts. In each system, the catalyst-philic phase is characterized as being separate from the remainder of the reaction mixture, and the catalyst should reside within this phase.
The types of reactions that can be run under multiphasic conditions range from substitution reactions, to metal catalyzed transformations (such as reductions, oxidations, hydrodehalogenations, hydrogenolyses, isomerizations, hydroformyla-tions, C-C bond forming), to Friedel-Crafts and Wittig reactions. These multiphasic systems represent a tool—available to process and organic chemists—that can tune a catalytic reaction and provide a built-in method for product-catalyst separation. In principle, the latter can be done either in batch reactions by decanting and physically separating phases, or by operating in CF. An added advantage is that the volume of the catalyst-philic phase can be reduced until only catalytic amounts are used, a benefit in cases where cost and availability are an issue (e.g., many kinds of ionic liquids).
We hope we have managed to highlight the common underlying idea on how multiphasic systems can be formed and on how they can be used. We are nonetheless aware that it is difficult to determine how one system can be better than another, and how to choose the right one for a particular application. This negative impression may be increased by the many different possible multiphasic systems, and by the overwhelming number of different names attributed even to similar systems.
The point we would like to make is that, while it is of crucial importance to advertise new chemistry and new applications, also through catchy words and acronyms, one should be careful to maintain a clear record of what has already been done in the field. This is sometimes overlooked due to the legitimate aspiration of authors to name their discoveries, and may lead them to deem old things new and to mislead less experienced and young readers. For example, the thin line that divides PTC from the use of ammonium-based molten salts should be kept in mind when describing rate enhancements or phase separations in their presence. The origins of PTC and molten salts are different, since PTC started as a new method for catalytic anion activation, while molten salts attracted attention as new reaction media. Nonetheless, the latter field seems to parallel the development of PTC in some instances, and this should be remembered when putting one's results in context. The future of multiphasic systems is in its infancy, and many other kinds of multiphasic systems that have not been addressed here (supercritical fluids, fluorous fluids, etc.) are gathering a great deal of attention, and also should be considered when designing new chemical reactions and/or processes. In addition, combinations of the possibilities outlined herein should be considered; as was the case of a recently published article where imidazolium salts were supported on PEGs ([PEGmim][Cl]) and used as a reaction medium for the Heck reaction.65
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