Aromatic rings stoichiometric oxidations

An intere.sting example in the context of waste minimization is the manufacture of the vitamin K intermediate, menadione. Traditionally it was produced by stoichiometric oxidation of 2-methylnaphthalene with chromium trioxide (Eqn. (8)), which generates 18 kg of solid, chromium containing waste per kg of menadione. Catalytic alternatives have been reported, but selectivities tend to be rather low owing to competing oxidation of the second aromatic ring (the. selectivity in the classical process is only 50-60%). The best results were obtained with a heteropolyanion as catalyst and O2 as the oxidant (Kozhevnikov, 1993). 

Another approach uses the coupling reaction of p-anisidine. In the presence of H202 and peroxidase (16), an oxidation product that contains two aromatic rings, benzoquinone-4-methoxyaniline, is formed stoichiometri-cally (92). Equations 14-16 indicate that an electron donor or hydrogen donor is required for peroxidase-mediated decomposition of H202. In two natural waters and one soil suspension, peroxidatic activity was identified by the stoichiometric removal of p-anisidine by the addition of H202 (in the dark) (16). This procedure provides an independent corroboration of the results obtained by Moffett and Zafiriou (1). However, this method does not quantify the relative importance of peroxidases versus catalases in the decomposition of H202. 

Support for the intermediacy of the carbonyl oxide mechanism stems mainly from the observation of stoichiometric 5-amino group expulsion from the pyrimidine cofactors during PAH turnover regardless of the extent of coupling [106]. However, an attempt to demonstrate PAH-catalyzed cyclization of 25 to 26 proved unsuccessful, despite the requirements for such a process if the tetrahydropterin follows a similar reaction course [102]. Thus the case for their intermediacy is flawed. Carbonyl oxides are rather poor electrophilic reagents so that the hydroxylation of an aromatic ring probably proceeds via a radical species [115]. 

Oxidative cyclization by stoichiometric amounts of Pd(OAc) has been shown to be an effective means of bringing about formation of new fused aromatic rings from properly constructed Indole derivatives. For example, the diindolylsuccinimide 84 was cyclized to 85, a precursor of staurosporine in 75% yield. <93TL8361>... 

An important route for the C-H activation of arenes and heteroarenes is through electrophilic metallation of an aromatic ring, followed by reaction with an alkene. There are numerous simple examples with arenes and heteroarenes reacting with alkenes under palladium catalysis." This has been referred to as the dehydro-genative Heck reaction and the oxidative Heck reaction, as well as the Fujiwara reaction, or Fujiwara-Heck reaction. Both benzene 3.4 and its derivatives (Schemes 3.6 and 3.7) and heteroarenes (Schemes 3.8 and 3.9) can be used. While the reaction has been carried out with a stoichiometric amount of palladium, catalytic processes, with an added oxidant are widespread. 

Increase in the ruthenium concentration increases the stoichiometric factor, n in Eq. (2), from about 6 up to about 20, and in these more concentrated solutions rates of ruthenium(III) reduction are no longer first order in ruthenium(III). Under these conditions reaction products depend on the hydroxide concentration and include hydroxy-aromatic ligands [cf. Eq. (3)], carbonate, and trace amounts of dioxygen. Ruthenium complexes of ligands in which one pyridine ring had been completely oxidized were also characterized (2). This accounts for the carbonate, and the minor dioxygen yields could originate from complexes oxidized to ruthenium(IV) (8). Unlike the iron(III) system, neither free 2,2 -bipyridine nor the N-oxide was detected. 

Another example is represented by the oxidative decarboxylation of oc-ketoacids in the presence of the S2082 /Ag+ redox system, which leads to the formation of acyl radicals by means of the intermediate Ag2+ (Equations 14.5 and 14.6) [10]. In this case, the re-aromatization of the ring can occur according to two parallel paths oxidation by persulfate (Scheme 14.1a) and by Ag(II) (Scheme 14.1b). Thus, this system needs more than the stoichiometric quantity of persulfate, as it both reacts... 

Only one ring of condensed aromatic systems is cleaved with either of the two mixtures of oxidizing agents shown in Figure 14.23—stoichiometric NalO catalytic NaOCl... 

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