Pyridine/carboxylic acid systems, hydrogen

This enzyme [EC 1.2.3.1] catalyzes the reaction of an aldehyde with water and dioxygen to produce a carboxylic acid and hydrogen peroxide. The enzyme uses both heme and molybdenum as cofactors. In addition, the enzyme can also catalyze the oxidation of quinoline and pyridine derivatives. In some systems this enzyme may be identical with xanthine oxidase. 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

The two-phase epoxidation of alkenes by hydrogen peroxide in water-dichloromethane system, catalysed by manganese(III)-porphyrin, is strongly accelerated by addition of catalytic amounts of a carboxylic acid and lipophilic imidazole or pyridine axial ligand365,366. Manganese(III)-porphyrin bound to colloidal anion-exchange particles is more active in the selective epoxidation of styrene by aqueous hypochlorite than the same catalyst in aqueous solution367. 

The proper choice of the electrolysis medium may provide significant product selectivity in each reaction. For instance, in the electrolysis of cyclopropane carboxylic acids (I) [Eq. (9)], a dramatic change in products is observed [52] in a pyridine-H20-Et3N-(Pt) system the Kolbe dimer (II) is mainly obtained, but in an MeOH-MeONa-(Pt) system predominantly cyclopropane (III) is formed via hydrogen atom abstraction. The cyclopropane carboxylic acid (IV), however, undergoes decarboxylative coupling even in an MeOH-MeONa-(Pt) system to afford the dimer (V) as a major product along with the ester (VI) [Eq. (10)] [53] ... 

Using FeS04 (1.67 x 10 M) in conjunction with equimolar amounts of methyl-pyrazine-5-carboxylic acid N-oxide and trifluoroacetic acid, in a water-acetonitrile-benzene (5 5 1 v/v/v) biphasic system, with benzene-H202-FeS04 = 620 60 1, a benzene conversion of 8.6% is achieved (35 °C 4h). Hydrogen peroxide conversion is almost complete (95%) and selectivities to phenol are 97% (based on benzene) and 88% (based on H2O2) [13]. These values are definitely higher than those described in the literature for the classical Fenton system [14], whereas iron complexes with pyridine-2-carboxylic acid derivatives are reported to be completely ineffective in the oxidation of benzene under the well-knovm Gif reaction conditions [15]. 

---