Pyridine dioxygenation

Figure 14.5 (a) Reaction of Al,Al -ethylenebis(3-Bu -salicylideniminato)cobalt(II) with dioxygen and pyridine to form the superoxo complex [Co(3-Bu Salen)2(02)py] the py ligand is almost coplanar with the Co-O-O plane, the angle between the two being 18°.< (b) Reversible formation of the peroxo complex [Ir(C0)Cl(02)(PPh3)2]. The more densely shaded part of the complex is accurately coplanar. ... 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

Both 2-hydroxy- and 3-hydroxypyridine are hydroxylated to 2,5-dihydroxypyridine by strains of Achromobacter sp. (Houghton and Cain 1972). These metabolites are probably, however, formed by different reactions whereas 3-hydroxypyridine behaves as a true pyridine, addition of H2O across the Cg Nj bond would produce the 2,5-dihydroxy compound 2-hydroxypyridine is a cyclic amide and hydroxylation apparently occurs at the diagonal position. The degradation of 4-hydroxypyridine is also initiated by hydroxylation and is followed by dioxygenation before ring fission (Figure 10.12) (Watson et al. 1974). 

It has been observed (52) that Fe(DMG)2 readily takes up dioxygen in the presence of ligands such as pyridine, ammonia, histidine or imidazole bubbling nitrogen through the solution reverses the process... 

We have seen, in the previous section (and section III.A), that cobalt (Salen) and its active derivatives normally form diamagnetic peroxo type I dioxygen adducts. However, a pyridine solution of Co(3-methoxy Salen) has been shown 137) to take up dioxygen with 1 1 (Co O2) stoichiometry it was found 137) that there was no significant IR absorption band, attributable to the 0—0 stretch, for the oxygenated complex, and this suggested that the dioxygen is symmetrically bonded in an unidentate manner,... 

In a typical experiment, the Fe(n) derivative of (314) rapidly binds dioxygen in pyridine subsequent deoxygenation may be effected by freeze-thawing. Several such cycles can be performed with little deterioration of the system. Moreover, the Oz adduct in pyridine has a half-life of about 20 hours. Following this initial success, the 02-binding properties of a number of other related capped porphyrin derivatives have been investigated (Baldwin Perlmutter, 1984). 

Diphenylmethane reacts with dioxygen in the presence of potassium 1,1-dimethylethoxide in various solvents (dimethylformamide [DMF], hexamethylphosphoramide [HMPA], pyridine) to produce nearly 100% yields of benzophenone [284]. The adduct of benzophenone with dimethylsulfoxide (DMSO) [l,l-diphenyl-2-(methylsulfinyl)ethanol] is formed as the final product of the reaction. The stoichiometry of the reaction and the initial rate depends on the solvent (conditions 300 K, [Ph2CH2] = 0.1mol L [Me3COK] = 0.2mol L 1,p02 = 97kPa). 

On the other hand, pyridine stabilizes the dioxygen molecule less well than the 1-alkyl imidazoles, because it is itself a weak 7r-acceptor and not a 7t-donor. The displacement reaction (7) occuring in excess pyridine may illustrate this argument (102). 

Other aspects of solvation have included the use of surfactants (SDS, CTAB, Triton X-100), sometimes in pyridine-containing solution, to solubilize and de-aggregate hemes, i.e., to dissolve them in water (see porphyrin complexes, Section 5.4.3.7.2). An example is provided by the solubilization of an iron-copper diporphyrin to permit a study of its reactions with dioxygen and with carbon monoxide in an aqueous environment. Iron complexes have provided the lipophilic and hydrophilic components in the bifunctional phase transfer catalysts [Fe(diimine)2Cl2]Cl and [EtsBzNJpeCU], respectively. 

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. 

Table 4 Substrate scope for direct dioxygen-coupled heterocyclization using the Pd/pyridine catalyst system...

It has long been known that, when bound to cobalt(II), the pyridine-based chelate ligands 2,2 -bipyridine (bipy), 1,10-phenanthroline (phen), and 2,2 6, 2"-terpyridine (terpy) form complexes that react with dioxygen in aqueous solution (32-34). The mixed-ligand complexes [Co(terpy)(bipy)]2+ and [Co(terpy)(phen)]2+ can act as oxygen carriers in aqueous solutions at pH values as low as 3.0 (34b), and the superoxo species thus formed are apparently dinuclear. In addition, the dinuclear bipyridine complex [(bipy)2Coin(/ 2-0 )(/ 2-02 )CoIn(bipy)2 ]3+ has been shown to catalyze the oxidation of 2,6-di-ter -butylphenol to the feri-butyl-substituted diphenoquinone and quinone (35). 

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

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