Current Organic Chemistry - Volume 14, Issue 13, 2010

Cyclodextrins have been used in so many fields that we will not even attempt to number them. Their success originates from their ability to entirely or partly include hydrophobic molecules in their cavity, allowing the solubilisation of various organic substrates in water. Of special interest is the use of cyclodextrins in the field of catalysis. Indeed, they show a remarkable ability to solve mass transfer problems in biphasic systems and to generate catalytically active homogeneous species or materials. The ever-growing number of papers dealing with this topic confirms the healthiness of cyclodextrin-based catalytic systems and demonstrates that cyclodextrins can greatly contribute to the development of greener catalytic processes. This edition of Current Organic Chemistry contains nine quite contrasting papers, each of which contains advanced insights into different aspects of catalysis using cyclodextrins. Together these contributions provide a well-rounded sampling of current research in the important field of cyclodextrin and catalysis.

For economic and ecological purposes, the design of stable and reusable noble metal nanoparticles in water has become a concern over the past few years. Among the various water-soluble protective agents, cyclodextrins proved to be attractive candidates to efficiently stabilize catalytically active nanospecies in aqueous media and also to improve the mass transfer in aqueous-organic two-phase systems. Various catalytic reactions such as arenes hydrogenations, olefins reduction or cross-coupling reactions were successfully achieved with good yields and selectivities according to the relevant choice of the cyclodextrins (cavity or functionalization).

This review focuses on the design of catalytic systems based on cyclodextrins via the molecular imprinting technique. The application of CD in the molecular imprinting method for the development of selective macromolecular receptors is discussed and the main features of the CD-based macromolecular receptors synthesized by the imprinting method are considered. A number of catalytic applications of such materials in Wacker-oxidation, aldehyde hydrogenation and oxidative coupling are presented.

During the past fifteen years, the use of chemically modified cyclodextrins (CDs) in aqueous organometallic catalysis has significantly contributed to enlarge the application field of biphasic processes in chemistry. In this paper, we describe how these supramolecular receptors became one of the most efficient solutions to solve mass transfer problems in aqueous organometallic catalysis. The scientific gaps that have been cleared to explain the exact role of the CDs in these biphasic systems are especially emphasized. In particular, the impact of supramolecular interactions between chemically modified CDs and substrates, water soluble ligands or organometallic catalysts is addressed for a better understanding of the recognition processes involved in the catalytic reactions.

The ever-increasing demand for environmentally benign and clean synthetic processes combined with high selectivities led to organic synthesis in water mimicking biochemical selectivity involving host guest complexes. Amongst various possibilities, cyclodextrin (CDs) possessing hydrophobic cavities have acquired significance as they promote organic reactions in water leading to high selectivities. The microenvironment around the substrate in the CD cavity makes it different from the classical reaction medium, protecting the substrate, controlling its orientation and conformation, accommodating only those molecules which fit the size of the particular CD cavity, and promoting the reaction through participation of its functionalities. Some of the applications of CDs to attain high selectivities in a variety of organic reactions such as nucleophilic opening of oxiranes, Strecker & Michael reactions, oxidations, allylations, synthesis of heterocycles, chiral discrimination etc. are discussed.

Cyclodextrins are able to interact in biocatalysis reaction. Their effects as solubilizing agents, inhibitor blocking agents and chiral selectors are well documented and they also interact before the enzyme-catalyzed reaction. Indeed, they have lyo-protectant effect for enzyme in organic medium biocatalysis and they participate in the refolding of inactive enzymes to regain their active form.

Cyclodextrins provide unique opportunities to catalytic applications. The integration of these supramolecular structures and heterogeneous catalysis exhibits potential to fulfill the high demands to the development of sustainable chemical processes. Herein, we have summarized some properties of this important class of catalysts. Selected synthetic applications are discussed. We dedicate this manuscript to the memory of Professor Octavio Augusto Ceva Antunes (1952-2009), a leading figure in the catalysis field in Brazil and a masterful teacher and mentor in the chemical sciences. The development of accessible and efficient green catalysts for as many different applications as organic synthesis, biofuels and nutraceutics, was a major theme of his research. His energy and impressive knowledge was always punctuated with humor and wit, the reason why many students, researchers and colleagues were drawn towards him during his life. Octavio Antunes was much more than a scientist. He enjoyed literature, philosophy, languages and culinary and was deeply concerned with his contribution as scientist to social development. He was thus dedicated to increasing the access to antiviral drugs in Brazil. To those who knew him, he was a genuine scientist-humanist. His untimely and tragic loss deeply affected his colleagues and students but the human and scientific legacies he left through an intense and productive life will remain.

Cyclodextrins are molecular capsules of adequate size to include metallo-organic guests and improve their solubility, shelf-life, and catalytic activity and recyclability, resembling the alchemist's metal-in-a-pot reactions on the molecular scale. This work reports literature examples of solid cyclodextrin inclusion compounds with metal catalysts and metallocyclodextrin catalysts, that find applications in two main and quite distinct fields, chemical reactions of industrial interest - olefin epoxidation, olefin polymerization and silicon curing (cross-linking) reactions - and biochemical reactions that mimic natural enzymes, of which artificial carbonic anhydrase and glutathione peroxidase metal-cyclodextrin compounds were successfully prepared.

This account selectively reviews metal complexes that are covalently linked to β-cyclodextrin and are either designed to act as enzyme mimics or synthetically useful catalysts. The discussion of each supramolecular system is accompanied with a short introduction in order to provide the background of the corresponding application.

The binding properties of cyclodextrins stimulated the interest of many chemists. Cyclodextrins are popular candidates for catalytic reactions, and these are reviewed in this paper. The focus is mainly on the literature on native and modified cyclodextrins as non-metal containing catalysts, after year 1989, since before that year the literature on cyclodextrins as catalysts is very well covered by other comprehensive reviews [1]. This review is structured according to cyclodextrin catalysts, for each catalyst the types of reactions they have been involved in are chronologically summarized.