Base-catalyzed autoxidations

Hydrocarbons are weak C-acids, which can be converted to carbanions by deprotonation with strong bases. Dioxygen reacts with carbanions in two ways  

The factor governing the susceptibility base-catalyzed autoxidation is the acidity of representative pK values are listed in Table II. 

The solvent has a strong influence on acidity and thereby on the oxidation rate. 

a stronger base like benzyltrimethylammonium hydroxide in aqueous pyridine is necessary for oxygenation to occur like in the case of, e.g., fluorene and indene [22]. Carbon acids with of 

30-35 require potassium t-butylate in a non-protic solvent (DMF, HMPT) [24]. 

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Base-Catalyzed Autoxidation of 9, 10-Dihydroanthracene and Related Compounds... 

Mechanism for Base-Catalyzed Autoxidation of 9,10-Dihydroanthracene. The autoxidation of 9,10-dihydroanthracene in pyridine as the solvent and in the presence of benzyltrimethylammonium hydroxide, a strong base, is believed to involve the reaction of a carbanion and molecular oxygen. Indirect evidence of the existence of the carbanion of dihydroanthracene in pyridine solution comes from the color that forms in the presence of the base. When dihydroanthracene is added to a pyridine solution of the base, a deep blood-red color develops immediately. This color is not completely attributable to carbanions since a trace of anthra-quinone alone will produce it. However, under an inert atmosphere (nitrogen) in which no anthraquinone can be formed, a deep red color is also formed. 

Methyl-substituted aromatics may also be converted to the corresponding carboxylic acids. As was mentioned above, a shift in the selectivity of the autoxidation of methyl-substituted arenes is brought about by metal ions eventually producing carboxylic acids. Carboxylic acids may also be formed by the base-catalyzed autoxidation of methyl-substituted arenes.848 Oxidation with tert-BuOK in HMPA affords carboxylic acids in moderate yields with very high selectivities. 

Wallace and Schriesheim dried the solvent by distillation under nitrogen over Linde I3X molecular sieve (calcined under N at 350° for 4 hrs.) and stored it in a dry box equipped with a moisture conductivity cell. They found it superior to DMF or tetramethylurea as solvent for base-catalyzed (KOH or NaOH) autoxidation of thiols and disulfides to sulfonic acids. It is superior also for base-catalyzed autoxidation of alkylthiophenes and toluene, and of 5-0,2 cyclic ketones to the corresponding dibasic acids."... 

Autoxidation [1, 921, after citation of ref. 51 ]. In the base-catalyzed autoxidation of ketones to a-hydroperoxides. the following factors have been shown to favor higher yields short reaction times, temperatures below—8°, polar aprotic solvents (dimethoxyethane or DMF), and a mole ratio of base to ketone greater than 2.51a... 

Base-catalyzed autoxidation of weak carbon acids has been observed using poly(ethylene glycols) as phase-transfer catalysts [26]. Substrates and conversions are given in Table III. Quaternary ammonium salts are ineffective as catalysts owing to immediate Hoffman... 

The base-catalyzed autoxidation of p-nitrotoluene (PNT) yields the corresponding benzoic acid and dimeric products [27]. Remarkably, ultrasonic agitation steers the phase transfer oxidation of PNT, o-nitrotoluene, o-ethylnitrobenzene and p-ethylnitrobenzene toward the carboxylic acid [28]. 

Specific cases of base catalyzed autoxidations under phase-transfer conditions have been reviewed [21]. 

One of the new media with outstanding properties is hexa-methylphosphoramide. It is an aprotic solvent for active metals and an excellent solvent for organometallic compounds . It also has been used for base-catalyzed autoxidation of hydrocarbons ... 

Consequently, as a result of increasing environmental pressure many chlorine and nitric acid based processes for the manufacture of substituted aromatic acids are currently being replaced by cleaner, catalytic autoxidation processes. Benzoic acid is traditionally manufactured (ref. 14) via cobalt-catalyzed autoxidation of toluene in the absence of solvent (Fig. 2). The selectivity is ca. 90% at 30% toluene conversion. As noted earlier, oxidation of p-xylene under these conditions gives p-toluic acid in high yield. For further oxidation to terephthalic acid the stronger bromide/cobalt/manganese cocktail is needed. 

The effects of heteroatoms on autoxidation reactions are reviewed and discussed in terms of six phenomena (1) the effect on reactivity of a-hydrogens in the hydroperoxide chain mechanism in terms of electron supply and withdrawal (2) the effect on a-hydrogen acidity in base-catalyzed oxidation (3) the effect on radical ion stability in base-catalyzed redox chains (4) the possibility of heteroatom hydrogen bond attack and subsequent reactions of the resulting heteroradical (5) the possibility of radical attack on higher row elements via valence expansion (6) the possibility of radical addition to electron-deficient II and III group... 

Autoxidation of indoles under basic conditions leads to the chemiluminescent ring cleavage of the heterocyclic ring (65MI30500, 65MI30501, 66CC522) which, in the case of 3-methylindole, can lead to a subsequent base-catalyzed formation of 2-(2-amino-phenyl)quinoline (77H(8)743>. 

The photooxidized cleavage products of indoles (177) and (179), which are also produced under autoxidation conditions, can undergo, under favourable conditions, a base-catalyzed intramolecular ring closure to give quinolin-2- and -4-ones (178) and (180), respectively (B-70MI30503, 72HC(25-l)l). 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

The oxidation of hydrocarbons by molecular oxygen in the absence of metal complexes has been discussed in Chapter II. The oxidation of hydrocarbons with molecular oxygen, as well as with donors of an oxygen atom (hydrogen peroxide, alkyl hydroperoxides and some other compounds), is a very important field since many industrial processes are based on these reactions [1], In many cases, chain radical non-catalyzed autoxidation of samrated hydrocarbons is not very selective and the yields of valuable products are often low. The use of salts and complexes of transition metals creates great possibilities for solving problems of selective oxidation, as has been demonstrated for a number of important processes. 

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