Molecular biomimetics catalysis

Crestini, C., Pastorini, A., and Tagliatesta, P., Metalloporphyrins immobilized on motmorillonite as biomimetic catalysts in the oxidation of lignin model compounds. Molecular Catalysis A-Chemical 2004, 208 (1-2), 195-202. 

Biomimetic oxidation catalysis has largely focused on complexes with planar tetradentate ligands such as manganese or iron porphyrins and related macrocyclic trans-chelates[5]. There is considerable interest in the synthesis of multinuclear metal complexes since these molecules might be useful as building block for magnetic molecular materials[6] and model compounds for understanding the properties of metalloproteins[7]. 

First transition metal coordination compounds with bidentate bispidine ligands were described in 1957 (30). The initial report with metal complexes of tetradentate bispidine ligands dates back to 1969 (31). Following these early reports, there have been a number of studies on the complexation properties of several bipidine derivatives (32-35). However, extensive, broad, and thorough studies of the bispidine coordination chemistry began only <10 years ago. These studies will be reviewed here. They include structural and theoretical work, spectroscopy, electron-transfer studies, metal ion selective complexation, and applications in biomimetic chemistry, catalysis, and molecular magnetism. 

As an approach to biomimetic catalysis, Sanders and colleagues [67] synthesized a series of 1,1,2-linked cyclic porphyrin trimers that allow the stereo- and regiochemistry of the Diels-Alder reaction of 84 and 85 within the molecular cavity to be controlled, thereby producing prevalently or exclusively the endo 86 or the exo 87 adduct. Two examples are illustrated in Scheme 4.18. At 30 °C and in the absence of 88, the reaction furnishes a mixture of diastereoisomers, while the addition of one equivalent of trimer 88 accelerates the reaction 1000-fold and the thermodynamically more stable exo adduct 87 is the sole detectable product. 

Das, S., Brudvig, G. W., Crabtree, R. H., Molecular recognition in homogeneous transition metal catalysis a biomimetic strategy for high selectivity. Chem. Commun. 2008,413-424. 

Say R, Erdem M, Ersoz A, Turk H, DenizU A (2005) Biomimetic catalysis of an organophos-phate by molecularly surface imprinted polymers. Appl Catal A Gen 286 221... 

For typical fluorous biphase catalysis the most important aspect is the simple recycling and re-use of the catalyst. Fluorous solvents have one special advantage over hydrocarbon solvents, however. Their very high oxygen dissolving capacity, combined with their extreme resistance to oxidative decomposition makes perfluorocarbons in combination with fluorous catalysts the optimum choice for oxidation reactions. Thus, the biomimetic oxidation of olefins with molecular oxygen and 2-methylpropanal as a co-reductand has been achieved with a fluorous cobalt porphyrin catalyst (22) [23], and also even without catalyst [24] (Scheme 3.7). 

Novel Catalysts. - One other promising option exists that might, in the long run, lead to a viable alternative to the above process concepts. Recent advances in biomimetic catalysis have resulted in the development of molecular catalysts that are selective in liquid-phase oxidation of C,-Cj aliphatic hydrocarbons under mild conditions. The catalysts have now been placed on suitable support materials for vapor-phase oxidation of methane to methanol. Significant laboratory research is still required on the properties and synthesis of these special molecular catalysts before process conditions, products, and yields can be defined, even on a laboratory scale. 

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