Aromatic compounds disproportionation

The hydroxamic acid function in most alicyclic and aromatic compounds is stable to hot dilute acid or alkali, and derivatives cannot undergo normal base-catalyzed Lessen rearrangement. Di Maio and Tardella," however, have shown that some alicyclic hydroxamic acids when treated with polyphosphoric acid (PPA) at 176°-195° undergo loss of CO, CO.2, or H2O, in a series of reactions which must involve earlj fission of the N—0 bond, presumably in a phosphoryl-ated intermediate. Thus, l-hydroxy-2- piperidone(108) gave carbon monoxide, 1-pyrroline (119), and the lactams (120 and 121). The saturated lactam is believed to be derived from disproportionation of the unsaturated lactam. 

Much of the work on the catalytic activation of C—H bonds in alkanes follows from the important observation of Garnett and Hodges (21) that, in the presence of the PtCl42- ion, aromatic compounds exchange hydrogen with deuterium in the D20—CH3C02D solvent. Temperatures in the range 80°-100°C are used and the solvent also contains a mineral acid to stabilize the platinum(II) from disproportionation ... 

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

There is only a small number of reactions known whereby a perfluorinated aromatic compound is formed by thermal disproportionation perfluorotoluene is disproportionated at temperatures above 500 C to form perfluorobenzene and perfluoroxylcnc.42 44 The perfluoroxylene itself can be dismutated at 550°C to yield perfluorotoluene and 1,2.4- and 1,3,5-perfluoro-... 

As expected, the aptitude for disproportionation of the aromatic compound depends upon the nature of the alkyl group, and the order of reactivity is isopropyl > ethyl > methyl. Due to their higher nucleophilicity, polyalkylbenzenes react faster than monoalkylbenzenes. This effect is pronounced in the case of methylbenzenes. Toluene itself shows little reactivity over Nafion-H at 193°C. Diethylbenzenes react much faster than dimethylbenzenes. The rate of conversion of diethylbenzenes over Nafion-H at 193°C is 5 10-5 mol min 1 g-1 of catalyst.269 This is a low rate when compared with that using AICI3 HC1 in the liquid phase at room temperature (10-4 mol min-1 g-1 of catalyst).272 However, one should bear in mind that Nafion-H is a truly insoluble heterogeneous catalyst, whereas in the case of AICI3—HC1 a soluble complex is formed with the hydrocarbon and therefore the rates are not directly comparable. The equilibrium composition of the acid-catalyzed disproportionation of diethylbenzenes depends upon the nature of the catalyst. 

The reactivity of polyfluorinated cyclohexadienes toward antimony or niobium pentafluorides has some peculiarities. The equilibrium mixture of 1,4- and 1,3-cyclohexadienes is formed from pure isomers of each and a catalytic amount of MF5 at room temperature where the non-conjugated diene predominates [167], With an excess of SbF5 both isomers disproportionate to give a mixture of aromatic compounds and cyclohexene polyfluoroaromatics and the corresponding cyclohexene [167,168] ... 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

The molecular sieve zeolites have attained great technological importance for catalysis, gas separation and drying and many other applications. They are now used as industrial catalysts for such reactions as the cracking of paraffins and the isomerization and disproportionation of aromatic compounds (Thomas and Theocharis, 1989 Thomas, 1995, Martens etal., 1997). 

The second thoughtful description was given by Perkins.6 He explained the unusual product distributions of phenylations of aromatic compounds when phenylazotriphenylmethane is used as thermal phenyl radical generator. Scheme 5 provides an example. It had been found7 that benzene solutions yield 1,4-dihydro-4-triphenylmethylbiphenyls as major products besides biphenyl and triphenylmethane, and a particular feature of this scheme is the absence of dimerization and disproportionation of the interme-... 

Homogeneous hydrogenation catalyst. This catalyst is useful for reduction of 1,4-dihydroaromatic compounds. Aromatic compounds are not formed by disproportionation, as is the case when the usual heterogeneous catalyst are used. Moreover it is possible to reduce disubstituted double bonds in the presence of letrasubstituted double bonds. Thus isotetralin (1) or 1,4-dihydrotetralin (2) is reduced with this catalyst to an identical 80 20 mixture of 9,10-octalin (3) and 1,9-octalin (4). The latter octalin evidently arises by isomerization of the double... 

The least satisfactory method involves the addition of an acyl halide to a mixture of the aluminum halide and the aromatic compound (the Bouveault procedure). The problem with this sequence relates to the presence of hydrogen halide, which may be present as a result of the reaction of moisture with the aluminum halide or which will be present in increasing amounts as the reaction proceeds. The hydrogen halide can result in isomerization or disproportionation of, for example, alkylbenzenes. 

The analogies which exist between anodic oxidation and cathodic reduction reactions of aromatic compounds are extensive radicals formed from radical cations by chemical reaction can be further oxidised by other radical cations, i.e. by an overall disproportionation mechanism. Thus the formation of trans 9,10-dihydroxy 9,10-diphenyl anthracene on the addition of water to a solution of 9,10-diphenyl anthracene... 

Laccases (benzenediohoxygen oxidoreductases, EC 1.10.3.2) are a diverse group of multi-copper enzymes, which catalyze oxidation of a variety of aromatic compounds. Laccases oxidize their substrates by a one-electron transfer mechanism. They use molecular oxygen as the electron acceptor. The substrate loses a single electron and usually forms a firee radical. The unstable radical may undergo further laccase-catalysed oxidation or non-enzymatic reactions including hydration, disproportionation, and polymerisation. ... 

Lithium/liquid ammonia conditions can produce 1,4-dihydroquinoline and 3,4-dihydroisoquinoline under certain conditions. Conversely, lithium aluminium hydride reduces generating 1,2-dihydroquinoline and -isoquinoline. These dihydro-heterocycles can be easily oxidised back to the fully aromatic systems, or disproportionate, especially in acid solution, to give a mixture of tetrahydro- and aromatic compounds. 

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