Immobilized Catalysts

Preface Over the last decade the environmental setup has changed for synthetic organic chemists to a considerable degree. So far synthetic organic chemistry had focussed on methodology development which mainly deals with the development of new reactions as well as new reagents and catalysts. These ought to be able to perform preferentially highly selective (chemo-, regio- and stereoselective) synthetic transformations, often applied in the context of complex and highly functionalized molecules. Except for the synthesis of peptides and oligonucleic acids, little attention has been spent on the question of how synthesis can be carried out in an environment of sophisticated technologies which includes improved hardware. While peptides and oligonucleotides are conveniently prepared by Merrifield’s solid phase technique, solution phase synthesis of most other synthetic targets have not been substantially replaced by this solid phase approach. Without discussing this aspect in detail it is obvious that today a renaissance of sophisticated solution phase synthesis can be noted. Immobilization of reagents and particularly catalysts, an old concept indeed, recently returned back onto the stage and this is addressed in this volume of Topics in Current Chemistry in a broader sense. The volume consists of eight chapters and is intended to introduce the reader to various aspects of immobilizing catalysts. Particular focus was spent on the concept that immobilization is only one out of many new enabling technologies introduced to organic synthesis and that the future of intelligent use of enabling technologies is associated with the clever and successful combination and integration of these techniques. As enabling techniques ionic liquids or perfluorinated solvents need to be listed, as well as the use of microwave assistance and continuous flow devices, and these aspects have been incorporated into this volume. In fact, immobilization of homogeneous catalysts not only requires detailed knowledge on the performance and properties of homogeneous catalysts. Successful applications of homogeneous catalysts in an heterogeneous environment must take the second phase and the mode of immobilization into account, often underscored by synthetic organic chemists. Thus, this volume tries to tell a story. Definitely, it is not the only story to be told on immobilized catalysts. Our story starts with the phase to which the catalyst is attached to and further proceeds to the question of how homogeneous VIII Preface catalysts can best be fixed to a second phase. From there the journey proceeds to actual synthetic applications of catalytic processes with particular focus on Pd-catalyzed transformations before two contributions will show how immobilized catalysts can conveniently be combined with other enabling technologies, namely microwave assistance and continuous flow reactors. Finally, the breakthrough of new technologies can clearly be spotted when they have paved their way into industrial applications. Then such technologies are versatile and economical enough to create products or improve processes on larger scales.