Current Organic Chemistry - Volume 12, Issue 2, 2008

Part I of the second special issue of Current Organic Chemistry dedicated to the topic of the use of heterogeneous catalysts in organic synthesis, again, brings together experts of a few selected, important fields to highlight recent major achievements. I am pleased that by these contributions we can maintain the diversity established in the previous volume. The opening account by Srinivasan Palaniappan and Amalraj John (Indian Institute of Chemical Technology, Hyderabad, India) covers recent developments of polymer-based solid acid catalysts in organic synthesis. These inexpensive, stable materials or their salts, complexes and metal composites may be used as efficient and versatile catalysts. The main emphasis is on the characterization and use of polyaniline-based catalytic materials. The second review has been provided by Benjaram M. Reddy and Meghshyam K. Patil of the Indian Institute of Chemical Technology (Hyderabad, India) on the topic of the use of promoted zirconia solid acid catalysts for organic synthesis. At the time of its discovery, promoted zirconia was considered to be a new generation of acid catalysts catalyzing transformations of industrial importance and as such elicited extreme interest in this type of catalysts. Although these expectations have not been materialized, this review shows that promoted zirconia catalysts proved to be highly versatile in synthetic organic chemistry. The third chapter written by Basudeb Basu and his colleagues (Pralay Das and Sajal Das) from North Bengal University (Darjeeling, India) is devoted to developments of the last five years in KF/alumina promoted organic reactions. KF/alumina is a remarkably useful heterogeneous catalyst to promote many base-catalyzed organic transformations. Various applications, including its particularly attractive use in metal-catalyzed coupling reactions, have been treated accordingly. The final contribution to this issue (A. Molnar, University of Szeged) is a review about Nafion-silica nanocomposites. The use in organic synthesis of Nafion resins as useful catalysts was pioneered by Professor G.A. Olah in the 1970s and 1980s. An important discovery in 1996 by M.A. Harmer transforming Nafion beads into a new type of strongly acidic catalyst of high specific activity opened up new application possibilities. This timely account summarizes a decade-long vigorous and productive research activity. It is the sincere hope of both the contributing authors and myself that the readers will find the first part of this second special issue about heterogeneous catalysis in organic synthesis interesting and informative.

In this review, we have focused our attention on the application of conjugated polymers as heterogeneous catalyst in organic transformations, which appeared in the literature. Conjugated polymers have attracted interest as promising materials for catalyst support for a decade. Conjugated polymers having heteroatom in their backbone can be easily doped with acids like Bronsted, organic, Lewis and heteropoly acids. These conjugated polymers containing acids can act as polymer based solid acid catalyst in organic synthesis. Use of conjugated polymer salts, complexes and metal composites in organic transformations is discussed and the advantages of conjugated polymers in catalysis field are brought out in this review.

This review deals with the catalytic performance of sulfate, molybdate and tungstate ion promoted zirconia solid acid catalysts for various acid-catalyzed organic synthesis and transformation reactions in the liquid phase. These promoted zirconia catalysts exhibit superacidity which mainly depends on the preparation conditions. In particular, the sulfated zirconia catalyst exhibits very strong solid acidity and excellent catalytic activity not only for simple acylation, condensation and esterification reactions but also for other important reactions such as synthesis of aromatic gem-dihalides, stereocontrolled glycosidation, regioselective ring opening of aziridines, production of diaryl sulfoxides and so on. Molybdate and tungstate promoted zirconia catalysts that also exhibit good catalytic activity for various organic reactions of practical importance.

Organic reactions promoted on a solid heterogeneous phase attract widespread interest within synthetic chemists from industries and academia. Potassium fluoride impregnated over aluminum oxide (KF/alumina) has been recognized as remarkably useful heterogeneous surface to promote many base-catalyzed organic transformations. The basic sites on KF/alumina may be associated to a very hard anion (F- anion), which possibly augments the surface to be active as a potential base and differentiates from other alkaline earth metal oxides as base catalysts. The source of the basicity has, however, been the subject of some debate in the literature. Besides classical organic reactions, such as alkylation, condensation and elimination reactions, KF/alumina has been successfully utilized as heterogeneous basic surface in several metal- catalyzed coupling reactions. Palladium-catalyzed C-C bond-forming reactions (e.g. Heck, Stille, Suzuki, Trost-Tsuji reactions), C-N bond-forming Buchwald-Hartwig reactions, and C-O bond-forming Baylis-Hillman type reactions have been reported with the aid of KF/alumina as the heterogeneous base. The KF/alumina catalyzed reactions are in many cases carried out without using any solvent, thus providing a clean and “green” reaction set up followed by easy product isolation. The solid surface has also been employed for preparing combinatorial libraries of different small molecules. In many cases, application of microwave irradiation has shown dramatic acceleration of the rate of transformations. This article will review the applications of KF/alumina in various organic reactions focusing on achievements during the period from 2001 to 2005.

The methods to prepare Nafion materials with significantly improved specific activities, their characterization and applications as useful and efficient catalysts are summarized. The sol-gel technique used most widely, covalent anchoring of the sulfonic acid group to the surface via a perfluoroalkane tether, impregnation of high-porosity spinodal silica, and mechanochemistry all allow the formation of Nafion materials with enhanced accessibility of the active sites and, consequently, improved catalytic properties can be experienced. The transformations induced by these solid acids including Friedel-Crafts and related reactions, various transformations of alkenes and alcohols, protective group chemistry, chemistry of heterocycles, oxidations, and a few examples of non-catalytic applications are treated in details.