Catalyst artificial homogeneous

In addition to the mimicking of natural enzymes, a fascinating goal in supramolecular chemistry is the development of entirely artificial catalysts. While both heterogeneous and homogeneous catalysts based in particular on coordination of ligands to transition metals are common, supramolecular catalysts use either exclusively, or in addition, some aspect of molecular recognition and non-covalent... 

Mother nature has resolved the various limitations involved in multi-electron processes. Unique assemblies composed of cofactors and enzymes provide the microscopic catalytic environments capable of activating the substrates, acting as multi-electron relay systems and inducing selectivity and specificity. Artificially tailored heterogeneous and homogeneous catalysts as well as biocatalysts (enzymes and cofactors) are, thus, essential ingredients of artificial photosynthetic devices. 

A variety of transition-metal complexes have been applied as homogeneous catalysts for C02-fixation in artificial photosynthetic assemblies. Typical photo-... 

After getting somewhat acquainted with PET in natural and artificial systems, the reader may become aware of its tremendous implications by studying the next article. There, heterogeneous and homogeneous catalysts as well as biocatalysts are described for artificial photosynthetic applications dealing with H2 evolution, COz fixation, hydrogenation, and hydroformylation processes. 

The use of biomacromolecules and artificial variants thereof as hosts for homogeneous catalysts, linked by noncovalent bonds, was first implemented in the 1970s recently, this field has attracted enormous attention. The biomacromolecules are used specifically to induce chirality into catalysts for well-known reactions. Linking by noncovalent interactions is more easily achieved than linking by covalent bonding—and these interactions need not imply that the link is weak the strengths of the bonds vary enormously. For example, the avidin-biotin bond is much stronger than most supramolecular bonds considered in this context. As yet, the interest in this field is merely scientific, and applications are not anticipated in the near future. 

Visible light irradiation of solutions of CO2 in (HOCH2CH2)3N-DMF containing [Ru(bipy)3] -Co has been found to lead to CO and H2 formation, whereas if Re(bipy)(CO)3X is used as homogeneous catalyst, only CO is obtained. Both systems represent new processes of artificial photosynthesis and light energy storage. 

Monoterpenes are widely used in the pharmaceutical, cosmetic and food industry as active components of drugs and ingredients of artificial flavours and fragrances [1]. Camphene is converted to isobomeol and bomeol that are used in formulation of soaps, cosmetic perfumes and medicines, as well as in the industrial production of camphor [2], which is used as an odorant/flavorant in pharmaceutical, household and industrial products [7]. Traditionally, homogeneous catalysts, e.g sulphuric acid, are used, but the effluent disposal leads to environmental problems and economical inconveniences. These problems can be overcome by the use of solid acid catalysts. USY zeolite [3], heteropolyacids [4,5] and sulfonic acid surface-functionalised silica [6] have also been used for the camphene hydration. 

E. Homogeneous catalysts mimicking cofactor-enzyme functionalities in artificial photosynthetic systems... 

E. Homogeneous Catalysts Mimicking Cofactor-Enzyme Functionalities in Artificial Photosynthetic Systems... 

Recent advances in the development of artificial photosynthetic systems, where native enzymes are coupled to photoinduced ET products, have been reviewed. Chemical means to modify bioactive proteins and to establish electrical communication with photoexcited entities have been addressed. Such functionalized proteins can substitute complex cofactor-enzyme assemblies. Finally, the use of heterogeneous and homogeneous catalysts, and their functions as cofactor-enzyme models in activation of the substrates and accumulation of electrical charges, have been discussed. 

Kaiser and Whitesides suggested the possibility of creating artificial metalloenzymes as long ago as the late 1970s. However, there was a widespread belief that proteins and organometaUic catalysts were incompatible with each other. This severely hampered research in this area at the interface between homogeneous and enzymatic catalysis. Since 2000, however, there has been a growing interest in the field of artificial metalloenzymes for enantioselective catalysis. 

As summarized here, artificial metalloenzymes based on the biotin-avidin technology have developed into a versatile approach to enantioselective catalysis. In many cases, selectivities exceeding 92% ee could be obtained, relying on a chemogenetic optimization strategy. Such hybrid catalysts display features which are reminiscent of both homogeneous and enzymatic catalysis. 

Although the reactions implemented so far are limited to model systems, we believe that artificial metalloenzymes will reveal their full potential for transformations for which there exists to date no good homogeneous catalyst. In this context, onr current efforts are directed towards the hydroxylation of alkanes (where overoxidation is difficult to prevent with homogeneous systems) as well as highly sequence-specific DNA hydrolysis. 

Colloidal dispersions of metal nanoclusters usually work at rather low temperatures as homogeneous catalysts. On this point, metal nanoclusters are similar to enzymes and are often regarded as artificial enzymes. From the viewpoint of green chemistry (less energy, fewer by-products, more efficiency, more selectivity, etc.), enzymes are model industrial catalysts. Thus, metal nanoclusters may provide the means for industrial catalysts to step up from the present practical heterogeneous catalysts to more ideal and enzyme-like ones. 

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