Quinones dehydrogenation

Most examples of quinone dehydrogenations adjacoit to have been earned out on steroidal ketones and are essentially limited to readily enolizable species. Reactions on esters and amides (Table 8) are far less common and, because of their relatively low ease of enolization, require hanh conditions. Thus, unless stabilization of the intermediate carbonium ion is possible, - elevated temperatures and prolonged reaction times are required (Table 8), which increases the incidence of unwanted side reactions. Frequent by-products are those arising as a result of Diels-Alder reactions or Michael addition to the quinone." Allylic alcohols may be rapidly oxidized to aldehydes or ketones under these conditions and requite prior protection. 

Dehydrogenation.1 Treatment of the Diels-Alder adducts of 1,3-dienes and citracononitrile and mesacononitrile (1) with potassium f-butoxide and anthraquinone (3 eq.) in benzene at room temperature gives the substituted benzonitrile (2). This is apparently the first recorded instance of quinone dehydrogenation of carbanions. 

Scheme 2.30. Mechanistic aiternatives of quinone dehydrogenations of hydroaromatic compounds. (1) Hydrogen atom transfer, (2) direct hydride transfer, (3) singie electron transfer, and (4) pericyclic hydrogen transfer.

Phenanthrene ring, A soln. of 17/ -hydroxy-17a-methylandrosta-l,4,9(ll)-trien-3-one, 2 equivalents didilorodicyanoquinone, and p-toluenesulfonic acid in dioxane refluxed 72 hrs. 3-hydroxy-l,17,17-trimethylgona-l,3,5(10),6,8,ll,13-heptaene. Y 48-52%. - p-Toluenesulfonic acid is required as catalyst to promote enoliza-tion of l 4-3-ketones in quinone dehydrogenation to Z i 4,6-3-ketones. F. e., also isolation of intermediates, s. W. Brown and A. B. Turner, Soc. (C) 1971, 2566. 

N. Trachtenberg, 1969). products or even as the major 1958). The reaction sequence with SeOj, except that the C—< the dehydrogenation of C—C other reagents have also been quinones. 

Dehydrogenation. The oldest and stiH important synthetic use of quinones is in the removal of hydrogen, especially for aromati2ation. This... 

A reaction time of one hour at —7° to — 10°C was found to give maximum yields of 7a-methyl compounds. In some cases it is necessary to subject the reaction mixture to chloranil dehydrogenation this transforms (32) to the A -compound, thereby facilitating separation of the 7a-methyl isomer (31). The latter isomer is not attacked by the quinone since it lacks an axial hydrogen at C-7. 

Oxidations of pyridopyrimidines are rare, but the covalent hydrates of the parent compounds undergo oxidation with hydrogen peroxide to yield the corresponding pyridopyrimidin-4(3 T)-ones. Dehydrogenation of dihydropyrido[2,3-(i]pyrimidines by means of palladized charcoal, rhodium on alumina, or 2,3-diehloro-5,6-dicyano-p-benzo-quinone (DDQ) to yield the aromatic derivatives have been reported. Thus, 7-amino-5,6-dihydro-1,3-diethylpyrido[2,3-d]-pyri-midine-2,4(lif,3f/)-dione (177) is aromatized (178) when treated with palladized charcoal in refluxing toluene for 24 hours. 

Quinones, which become reduced to the corresponding hydroquinones. Two important quinones often used for aromatizations are chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone) and DDQ (2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone). The latter is more reactive and can be used in cases where the substrate is difficult to dehydrogenate. It is likely that the mechanism involves a transfer of hydride to the quinone oxygen, followed by the transfer of a proton to the phenolate ion °... 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Clearly, our effort toward understanding the course of the dehydrogenation reaction was key in achieving optimal performance for production. The understanding of this reaction, however, goes beyond the finasteride process changing the way that we think about quinone oxidations. 

Manganese dioxide on bentonite clay has also been used for oxidation of phenols to quinones (30-100%) [97] and Mn02 on silica effects the dehydrogenation of pyrrolidines (58-96%) [98]. 

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

As an example the reaction with aniline may be selected. In accordance with the scheme formulated, the first reaction product obtained is anilinoquinol (R=NH.C8H5). The reaction does not stop at this stage, however. Between this first reaction product and unchanged quinone still present there promptly occurs a reciprocal hydrogenation and dehydrogenation characteristic of very many of the reactions of quinone. 

According to a general rule, not only dihydric phenols, but also those diamines of the p-series which still contain one hydrogen atom attached to each nitrogen, are dehydrogenated to quinone or quinonediimine with great ease. Hence in the oxidation solution emeraldine is also immediately converted into the doubly quinonoid chain... 

If this cleavage principle is applied to the molecule of aniline black it will be found that from it four molecules of quinone, three molecules of p-phenylenediamine, and one molecule each of aniline and ammonia arise. Since an excess of chromic acid is present, the p-phenylene-diamine is readily dehydrogenated to quinonediimine, which is hydrolytically decomposed into quinone and ammonia. The single molecule of aniline begins the cycle anew. 

Fig. 15.8 Redox-active polymers for heterogeneous dehydrogenation catalysis, (a) trimerized phenanthrene quinone [42], (b) benzoquinone biphenyl copolymer [41], (c) polynaphthoquinone [40], (d) polyaniline [43], (e) pyrolyzed polyacrylonitrile [44].

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