An overview of the wide range of nanoreactors that have been constructed from synthetic and biological building blocks using both covalent and noncovalent approaches is given. Focus is on self-assembled systems, varying in size from a few nanometers to tens of micrometers. The review is divided into several sections that cover the development of tailor-made nanoreactors, starting from small organic molecular containers expanding to large compartment-containing assemblies. First, the construction of capsules from low molecular weight compounds by means of covalent synthesis and self-assembly by highly directive and pre-designed interactions is discussed. Second, nanocapsules based on micellar and vesicular assemblies that are built up frow low molecular weight molecules are described. Finally, the construction of nanoreactors from macromolecular building blocks, as well as recent developments in the use of viruses as nanocontainers and reactors are outlined.

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Self-Assembled Nanoreactors

Cyclodextrins are extremely attractive components of artificial enzymes and other biomimetic materials. They are readily available, they bind hydrophobic substrates into their cavities in water solution, and they have two rims of hydroxyl groups (Figure 1) that can either react with substrates themselves or be used to attach other catalytic and functional groups. Of course, they have disadvantages. For one, unless they are extensively modified their complexes with substrates can be rather flexible and, perhaps, with unpredictable preferred geometry. They are also unstable to strong acid. Thus for some purposes such synthetic cavity species as calixerenes1 or synthetic macrocycles2-4 may have advantages. However, one of the chief advantages of cyclodextrins is highly attractivesthey are readily available, so it is possible to avoid the synthesis of a binding group and go directly to studies of what can be achieved with their use. Afterward, the lessons learned may be applied to other systems with advantage. This review will cover all the literature on reactions in which cyclodextrins bind substrates and then either catalyze their reactions or mimic a step in an enzymatic catalytic sequence. However, it will not describe work in which cyclodextrins simply change the course of a reaction without playing an obvious catalytic role involving substrate binding. For example, there are systems in which the main function of the cyclodextrin seems to be to complex a metal ion and keep it in solution.5-11 There are other studies in which binding into a cyclodextrin simply alters the selectivity of attack by an external reagent in some way12-24 or causes solubilization to facilitate phase transfer catalysis.12,25,26 Presumably such other areas are described elsewhere in this volume. While much work on artificial enzymes using cyclodextrins has been done in the author’s laboratory, and will be described, every effort is made to describe all the relevant work in the field. Several reviews of this subject already exist and should be consulted for further information.2,27-70 The readily Ronald Breslow, born in 1931 in Rahway, NJ, completed his B.A. in chemistry in1952, his M.A. in medical science in 1953, and his Ph.D. in chemistry in 1955 with R. B. Woodward, all at Harvard University. After a postdoctoral year with Alexander Todd in Cambridge, he came to Columbia University where he is now University Professor and Professor of Chemistry. His work on enzyme models, on novel conjugated aromatic and antiaromatic molecules, on electrochemical and hydrophobic methods in mechanistic chemistry, and on anticancer cytodifferentiating agents has been recognized by a number of awards, including the U.S. National Medal of Science. In 1996, he served as President of the American Chemical Society.

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives.

New Mechanistic and Synthetic Aspects of Singlet Oxygen Chemistry

Cyclodextrine werden gerne als Bausteine eingesetzt, da sie sich gezielt sowohl kovalent, als auch nichtkovalent verknupfen lassen. So wurden an ein β-Cyclodextrinmolekul regioselektiv ein, zwei, drei, sieben, vierzehn, achtzehn oder zwanzig Substituenten kovalent gebunden. Cyclodextrine sind zudem organische Wirtmolekule. In ihrem Innenraum finden ein oder zwei Gastmolekule Platz. Umgekehrt konnen ein, zwei oder viele (hundert und mehr) Cyclodextrinmolekule auf ein passendes Gastmolekul aufgefadelt werden. Solche supramolekularen Strukturen entstehen meist in Losung, so das sie durch hochauflosende spektroskopische Methoden charakterisiert werden konnen. Sie lassen sich weiter in sonst sehr schwer zugangliche molekulare Architekturen wie Catenane, Rotaxane, Polyrotaxane und Rohren effizient umwandeln. Cyclodextrine konnen auch recht interessante Funktionen ausuben. So wird ihre Fahigkeit zur molekularen Erkennung zur chromatographischen Trennung von komplexen Stoffgemischen, insbesondere von Racematen, genutzt. Auch konnen Cyclodextrine oder deren Derivate bemerkenswerte katalytische Aktivitaten aufweisen. Schlieslich kann man mit Cyclodextrinen die Verfugbarkeit von Wirkstoffen gunstig beeinflussen, was sicherlich in nachster Zeit zu vielen Anwendungen fuhren wird.

Cyclodextrine als Bausteine supramolekularer Strukturen und Funktionseinheiten

The design of biomimetic complexes for the modeling of metallo-enzyme active sites is a fruitful strategy for obtaining fundamental information and a better understanding of the molecular mechanisms at work in Nature's chemistry. The classical strategy for modeling metallo-sites relies on the synthesis of metal complexes with polydentate ligands that mimic the coordination environment encountered in the natural systems. However, it is well recognized that metal ion embedment in the proteic cavity has key roles not only in the recognition events but also in generating transient species and directing their reactivity. Hence, this review focuses on an important aspect common to enzymes, which is the presence of a pocket surrounding the metal ion reactive sites. Through selected examples, the following points are stressed: (i) the design of biomimetic cavity-based complexes, (ii) their corresponding host–guest chemistry, with a special focus on problems related to orientation and exchange mechanisms of the ligand within the host, (iii) cavity effects on the metal ion binding properties, including 1st, 2nd, and 3rd coordination spheres and hydrophobic effects and finally (iv) the impact these factors have on the reactivity of embedded metal ions. Important perspectives lie in the use of this knowledge for the development of selective and sensitive probes, new reactions, and green and efficient catalysts with bio-inspired systems.

Biomimetic cavity-based metal complexes

Cyclodextrin-Sandwiched Porphyrin

Cyclodextrin sandwiched porphyrin catalysed epoxidation of hydrophobic alkenes in an aqueous buffer solution.

Epoxidation reaction catalysed by cyclodextrin sandwiched porphyrin in aqueous buffer solution

Cyclodextrin sandwiched porphyrin catalysed epoxidation of hydrophobic alkenes in an aqueous buffer solution

Takashi Hiroshige

Papers

Cyclodextrin-Sandwiched Porphyrin

Epoxidation reaction catalysed by cyclodextrin sandwiched porphyrin in aqueous buffer solution

Epoxidation reaction catalysed by cyclodextrin sandwiched prophyrin in aqueous buffer solution

Direct synthesis of a sandwich-type molecule involving porphyrin and two molecules of cyclomaltoheptaose

Epoxidation Reaction Catalyzed by Cyclodextrin-Sandwiched Porphyrin in Aqueous Buffer Solution.