Hydroquinone, synthesis using metal oxide

The Scholl reaction is one of the oldest and most useful C-C coupling reactions, and is often used in the synthesis of polycyclic aromatic hydrocarbons (PAHs). In general, a large amount of metal oxidant (such as FeCls) is employed in this reaction, and chlorinated by-products are produced in many cases. Rathore found that DDQ and protic acid effectively promote the Scholl reaction of various 1,2-diarylbenzenes (86) to give triphenylenes (87) in excellent yields (Scheme 8.40). The advantages of this reaction over typical metal oxidant promoted Scholl reactions are (l) an excess of DDQ is not required (2) no chlorinated by-products are produced and (3) recovery and re-use of reduced hydroquinone DDQ-H2 is possible. The reaction of other diaiyls tethered with methylene and propylene, and the intermolecular reaction also proceeds effectively. Furthermore, the treatment of hexaatylbenzene (88) with DDQ quantitatively affords collonene (89). 

The two oxygen-activating complexes [Co(L)j [L = salophen, tetra-tert-butylsalo-phen (55)] have been prepared and were also synthesized within dehydrated zeolite NaY using the intrazeolite ligand synthesis method [164]. These encapsulated metal complexes were shown to be capable of oxidizing hydroquinone and so were then used in a triple catalytic system to mediate the palladium-catalyzed aerobic 1,4-diacetoxylation of 1,3-dienes (Figure 5.28) [165]. The catalytic system involved [Pd(OAc)2], hydroquinone and the [Co(salophen)] complex in acetic acid (Co Pd diene hydroquinone LiOAc = 1 2.23 50 8.3 690, acetic acid, 25 °C,... 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

As with shikimic acid, considerable improvements in the microbe-cat yzed synthesis of quinic acid from glucose need to be and likely can be achieved. The subsequent oxidation of quinic acid to hydroquinone is another area of research in which fundamental advances in catalysis have to be achieved. The current oxidation of quinic acid to benzoquinone using Mn02 is a stoichiometric oxidation. This runs counter to the current trend toward development of metal-catiyzed oxidations that are catalytic in the amounts of metal required. Although a variety of cooxidants can be employed, elaboration of a metal-catalyzed oxidation in which O2 is the cooxidant would be particularly desirable. Sufficiently mild conditions for oxidation of quinic acid to hydroquinone may be identified whereby overoxidation to benzoquinone is avoided and hydroquinone is obtained directly from quinic acid. Metal-catalyzed oxidations have to be compatible with use of water as the reaction solvent. F referably, elaborated catalysts should be sufficiently robust to mediate the oxidation of quinic acid in clarified, crude fermenter broth. 

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