Biomimetic Flavin

Bis-hydroxylation. Molecular oxygen or air was used as the terminal oxidant in the osmium-catalyzed oxidation of alkenes to form cis-diols with high conversions at low catalyst amount.1298 In a triple catalytic system using H2O2 as the terminal oxidant, a cinchona alkaloid ligand has a dual function—it provides stereocontrol and acts as reoxidant via its IV-oxide. The formation of the latter is catalyzed by a biomimetic flavin component.1299... 

However, the reverse reaction of alcohol oxidation in this system can be performed with flavin co-factor added (FMNH2 —> FMN), where molecular oxygen is reduced to H202. The entire biomimetic process under consideration is well illustrated by the following diagram [128] ... 

Areas of biomimetic chemistry relating to enzyme systems that function both with and without the benefit of coenzymes are included. Special emphasis has been placed on the following subjects vitamin Bi2 and flavins oxygen binding and activation bioorganic mechanisms and nitrogen and small molecule fixation. 

During the last two decades, the mechanisms of many enzymic processes have been established, and model systems have been developed that effectively mimic their action. In particular, the roles of thiamin, NAD, pyridoxal, flavins, Bl2, ferridoxin, and metals in many enzymic processes now are understood. Model systems have been developed to imitate the action of decarboxylases and esterases, to imitate the action of enzymes in binding their substrates, and to approach the stereospecificity of enzymes. Our laboratory recently has found phosphorylating agents that release monomeric methyl metaphosphate, which in turn carries out phosphorylation reactions, including some at carbonyl oxygen atoms, that suggest the actions of ATP. The ideas of biomimetic chemistry are illustrated briefly in terms of the processes mentioned above. 

Clearly, the chemistry carried out on model systems for both B12-coenzyme dependent and flavin-dependent enzymes have suggested a number of mechanisms by which the natural systems might function. As with all biomimetic chemistry, the information given by the in vitro studies must be compared with and extended to the in vivo system to assess whether or not the model chemistry bears any relationships to the natural systems. 

The emplo5unent of robust aromatic ligand architectures able to introduce the same kind of reactivity in a reversible manner is of course much more desirable. For this purpose, biological redox cofactors such as quinones, porphyrins, flavins, and their functional analogues in biomimetic chemistry contain a built-in butadiene-t5rpe moiety as part of their n-electron system, which can be considered as a minimum functional motif for assisting the reversible uptake of two electrons and two protons. This... 

The possibility that an electron-transfer path is involved in photo-sensitized oxygenation has been considered on several occasions. This is relevant in several fields of application, from the biomimetic oxygenation of indole and flavin derivatives [106] to pollutant control. With reference to latter, it has been suggested that SET occurs in heterogeneous photosensitized oxidation by solid semiconductors, in which the adsorbed substrate donates an electron to the photogenerated hole and... 

With the bisalkaloid ligands, potassium ferricyanide can be used as the stoichiometric oxidant [84, 91]. As with the parent achiral osmium oxidation, NMMO can also be used as the oxidant (see above) [92]. However, rather than using NMMO in stoichiometric amounts, this morpholine component can be used in catalytic amounts by the addition of the biomimetic flavin 4 to set up a triple catalytic system where hydrogen peroxide is the oxidant [93-95], Methyltrioxorhenium can be used in place of the flavin mimic [96], as can tungsten(VI) [97] and carbon dioxide [98]. 

Numerous biomimetic reactions catalyzed by CDs carrying catalytic or reactive functional groups, such as nucleophiles (imidazole, oxime, and amine), enzyme cofactors (pyridoxamine, thiazolium. nicotinamide, cobal-ainine, flavin), metal complexes, etc., were reported. " Typically, enzyme-like kinetics and improved performance of a catalytic group, as compared to a simple analog lacking CD. are observed. 

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