Clean homogeneous catalysis

Stephan, M. S., De Vries, J. G. Homogeneous catalysis for fine chemicals The Heck reaction as a clean alternative for Friedel-Crafts... 

Base-catalysed reactions such as the Knoevenagel, Michael and aldol reactions continue to be of importance in industrial routes to synthetic chemicals and are often inherently clean, with water (or nothing) as the by-product. Traditional homogeneous methods of catalysis often require upwards of 40 mol% catalyst (such as piperidine) with the attendant difficulties in recovery and reuse of the catalyst. They often offer extremely poor selectivity to the desired products, either due to competing processes (side reactions) or further reaction of the first-formed product. 

The literature of catalysis is so vast that it is impossible to do full justice in one part of a chapter, even to the experimental methods that are in use. Most catalytic reactions are quite complex and a complete reaction mechanism can rarely be determined from a measurement of kinetics alone. It is safe to say that no catalytic gas-solid reaction has yet been fully characterised. Because of the many difficulties, work on catalytic reactions tends to veer towards two opposite poles. One of these can be described as the clean surface approach ", which uses the simplest reactions under high vacuum conditions and attempts to define the catalyst surface completely. The other approach is to study much more complex reactions in order to obtain overall kinetic information which describes the process. Where the mechanism is not understood, it is common to employ expressions similar to those used for homogeneous reactions, or even purely empirical equations. 

The invention of new methods for catalyst recovery appear likely to further increase the attractiveness of SCCO2 as a reaction medium, potentially in partnership with a second phase such as water, ionic liquid, or PEG. Given the high price of chiral homogeneous catalysts and the particularly clean separations that can be obtained using the biphasic catalysis techniques described in Section 3.3, one can expect industrial interest in this aspect in particular. 

In an idealistic sense, a chemical approach which uses a small amount of a chiral catalyst to produce either enantiomer, cleanly and efficiently from a prochiral precursor, is the preferred method. For such asymmetric catalysis the efficiency of chiral multiplication can be infinite. The use of chiral metal complexes as homogeneous catalysts has become one of the most powerful economically and environmentally sound strategies for the preparation of enantiopure compounds. An excellent comprehensive review of asymmetric catalysis in organic synthesis has recently been published by Noyori [30]. 

---