Biomimetic catalytic oxidation

Biomimetic Catalytic Oxidation of Lignin Model Compounds... 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

A breakthrough was reported by Stack and co-workers in 1998 (212) who reported the first biomimetic catalytic system for the oxidation of primary alcohols by air. Independently, in the same year Chaudhuri, et al. (216) reported efficient aerobic oxidation of primary and secondary alcohols by the dinuclear catalyst [Cu2 2(Ls )2]C12 (216). Next, we will briefly review the salient features of these two systems. 

Although the preceding catalase biomimetic reactions are important, the true biological role of the OEC is to evolve 02 from H20. Only a few complexes evolve 02 from H20 and even fewer contain Mn (see complex 67). Perhaps the best known case is the catalytic oxidation of water by the dinuclear ruthenium complex [(bpy)2(H20)Ru0Ru(H20)(bpy)2]4+ 68, which has been well documented [9,160] and will not be discussed here. 

This biomimetic catalytic system displayed high activity in hydrocarbon oxidation. For cyclohexane oxidation to cyclohexanol and cyclohexanone, the number of catalytic cycles... 

Copper would seem to be an appropriate choice of metal for the catalytic oxidation of alcohols with dioxygen since it comprises the catalytic centre in a variety of enzymes, e.g. galactose oxidase, which catalyze this conversion in vivo [188, 189]. Several catalytically active biomimetic models for these enzymes have been designed which are seminal examples in this area [190-193]. A complete overview of this field can be found in a review [194]. 

C. Floriani, Chem. Commun. 1996,1257 (artificial porphyrins 21 references) D. Dolphin, ed., The Porphyrins, Academic Press, New York (reference work in 7 volumes) B. Frank and A. Nonn, Angew. Chem. Int Ed. Engl. 1995,34, 1795 (biomimetic syntheses of porphy-rinoids) R. Sheldon and Y. Naruta, Metalloporphyrins in Catalytic Oxidations, Dekker, New York, 1994 F. Montanari and L. Casella, eds., Metalloporphyrin Catalysed Oxidations, Kluwer, Dordrecht, 1994. 

A reoxidation of the catalytic amounts of hydroquinone (HQ) to benzoquinone (BQ) in Scheme 8-11 by molecular dioxygen was realized by the use of an oxygen-activating macrocyclic metal complex as cocatalyst [53,62-65]. This leads to a mild biomimetic aerobic oxidation which is now based on a triple catalytic system (Scheme 8-12). With this system cyclohexa-1,3-diene is oxidized to frans-l,4-diacetoxycyclohex-2-ene at room temperature in 85-89% (>91% tmns) [62]. With the use of 2-phenylsulfonyl-l,4-benzoquinone as quinone, the trans selectivity of this process was >97% [53]. 

Catalytic Oxidations with Biomimetic Vanadium Systems... 

There are many reviews that cover various aspects of oxidation. These include ones on alkane activation,166 catalytic selective oxidation,167 metal complexes of dioxygen,168 metal-catalyzed oxidation,169 biomimetic oxidations,170 oxidation with peroxides,171 catalytic oxidations with peroxides,172 catalytic oxidations with oxygen,173 oxidations with dioxiranes,174 and oxidation of pollutants.175... 

Much of the research devoted to catalytic oxidations mediated by metal complexes of water-soluble ligands falls into the category of biomimetic oxidations. Hence, water-soluble porphyrins and the structurally related phthalocyanines have been... 

The present review has outlined the efforts to develop biomimetic non-heme iron and manganese catalysts for alkane hydroxylation, olefin epoxidation, and cis-dihydroxylation reactions. However, the examples reviewed here are mostly presented as reported in the literature, since the various reaction conditions involved in the catalytic oxidations hamper a direct comparison and critical evaluation of the data. The survey has not only illustrated a rich variety of iron and manganese complexes that lead to the successful structural modeling of important non-heme iron and manganese enzymes, but also significant features of the oxidation reactions catalyzed by these complexes in combination with dihydrogen peroxide. 

Key words Catalytic oxidation, homogeneous catalysis, dioxygen activation, dioxygen conplexes, biomimetic oxidation, functional metaUoenzyme models, oxidation mechanisms, oxidative dehydrogenation, oxygen insertion, aUcene epoxidation, catecholase reaction... 

His research interests include catalysis by cobalt, iron and manganese, complexes (biomimetic dioxygen activation, catalytic oxidation and carbonylation), as well as kinetics and mechanisms of inorganic reactions in solution (fast redox and electron transfer). 

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