Catalyst supramolecular host-guest

The closed-structural complex [H3Ru3(C6H5)(C6Me6)2(0)] was first thought to efficiently catalyze the hydrogenation of benzene to cyclohexane under biphasic conditions, but it has been proved that this complex was not the true catalyst.Nevertheless, supramolecular, host-guest complexation of benzene by this complex was... 

These descriptions, while helpful, are by their nature noncomprehensive and there are many exceptions if such definitions are taken too literally. The problem may be linked to the definition of organometallic chemistry as the chemistry of compounds with metal-to-carbon bonds . This immediately rules out Wilkinson s compound, RhCl(PPh3)3, for example, which is one of the most important industrial catalysts for organometallic transformations known in the field. Indeed, it is often the objectives and thought processes of the chemist undertaking the work, as much as the work itself, which determine its field. Work in modern supramolecular chemistry encompasses not just host-guest systems but also molecular devices and machines, molecular recognition, so called self-processes ... 

There are thousands of discoveries in molecular science reported every year but very few of these are destined to promote a new generation of research activity. The serendipitous preparation of di-benzo-18-crown-6 1 by Pedersen in 1967 [1] and the subsequent discovery [1,2] that 1 and other crown ethers selectively complex biologically relevant alkali and alkaline earth cations was, however, the catalyst for a huge explosion of activity in the field of host-guest or supramolecular chemistry. The resulting inspired and innovative work by Lehn [3,4] on, in particular, the 3-dimensional bicyclic cryptands (e.g., 2) and by Cram [5] on chiral crown ethers and rigid spherands (e.g., 3) was recognised by the award to Pedersen... 

The primary properties of supramolecular systems are to alter the reactivity of one or more of its components, or to perform a specific function. For example, host-guest systems have been designed to function as sensors [1-3], as catalysts [4], or for preferential solubilization [5,6]. Self-assemblies, such as micelles, vesicles, and biological membranes, have been explored to perform functions, such as preferential solubilization [7], delivery of drugs [8,9], or photoinduced charge separation [10-12]. 

Encapsulation of enzyme active sites in the intracrystalline pore volume of zeolites and molecular sieves is possible and sterically induces selectivity in oxygenation as well as in hydrogenation reactions. We denote the new catalysts, generated as result of a host-guest interaction between the zeolite (host) and the complex (guest) as ZEOZYMES, a family of materials which can function as supramolecular catalysts. 

There are two directions in the development of supramolecular catalytic compositions, that is, (1) creation of systans based on macrocyclic compounds as host molecules that bind substrates with their hydrophobic cavity and (2) development of the systems that bind substrates using aggregates formed by am-phiphihc compounds. Compounds that form host-guest complexes like modified cahxarenes are able to aid transport of substrates into the aqueous phase. This approach has been implemented in the Wacker oxidation [40,41], oxidation of alkylaromatic compounds [42], hydroxylation of aromatic compounds [43], hydrogenation [44,45], hydroformylation [45-48], and carbonylation [49]. In this case, the substrate is transported into the aqueous phase in the form of the corresponding inclusion complex. This not only affects the activity of the catalyst, but also provides selectivity of the process. Thus, in the Wacker oxidation of 1-alkenes the maximum yield of methyl ketone was achieved when 1-hexene is used, and for systems based on calix[6]arene with 1-octene among catalytic systems with modified calix[4]arenes [50]. 

Most of these reactions are accelerated cooperatively by the formation of host-guest (host-substrate) complexes in which the energy of the transition state is lowered, thus accelerating rates of reaction. Inhibitor molecules that bind to active sites more strongly than substrates can significantly reduce reaction rates, often to a level similar to that of a reaction without a host catalyst. This chapter focuses on the development of novel supramolecular catalysts that utilize host-guest interactions that mimic certain aspects of enzymatic catalysts. 

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