Dismutational aromaticity

More recently, Scheschkewitz and coworkers reported the synthesis of an exotic tricyclic isomer stabilized with six 2,4,6-triisopropylphenyl (named Tip) substituents, which is an attempt toward production of the first hexa-silabenzene (Chart 13.19). In the crystalline form, the tricyclic ring adopted a cyclohexane-like puckered geometry, which is significantly distorted from a planar benzene-like geometry. The isomer showcased a unique aromaticity, called dismutational aromaticity, supported by three Si resonances and high-level density functional theory calculations. This result can lead to the first isolation of an ideal hexasilabenzene with higher molecular symmetry (six- and three-fold) by designing more elaborate substituents and reaction routes. 

Vaz ADN, S Chakraborty, V Massey (1995) Old yellow enzyme aromatization of cyclic enones and the mechanism of a novel dismutation reaction. Biochemistry 34 4246-4256. 

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-... 

For aromatic aldehydes, 50% alkali (sodium or potassium hydroxide) is most generally used, except for nitrobenzaldehydes, where 15-35% alkali is sufficient to bring about the reaction and, to control it so as to reduce the extent to which side products are formed. The use of less than about 50% alkali usually prolongs the time of the reaction atad, since the products are not sensitive to the action of caustic, offers no advantages. Alcoholic potassium hydroxide (about 25%) has been employed with success for the dismutation of methoxybenzaldehydes but appears to offer no particular advantages since o-methoxybenzalde-hyde is converted into the alcohol in excellent yield by the action of concentrated aqueous potassium hydroxide. Alcoholic alkali may be preferred for difficultly soluble aldehydes. 

As the two one-electron redox potentials involved are close to 1V, many aromatic compounds can be oxidized to a radical cation. This is a common source of free radical production during intoxication with xenobiotics [31]. The large difference in redox potential between the two couples O2/H2O2 and H2O2/H2O provides a thermodynamic driving force for H2O2 dismutation catalyzed by catalase ... 

Certain zeo tes dbplay specific properties for the treatment of aromatics. Mobil Chemcd uses them as catalysts (desisted AF) to conduct the benzene dismut tm reaction in moderate < )etatiag conditioiis, by the Mobil LTD (Mobil Low Temperature Disproportionation) process. Conversion takes place in the liquid phase in the ateence of hydrogen, at a pressure of 45.10 Pa isolate, and a tenq)eratute that is rai progressively with time from 260 to 315 to maintain catalyst activity. 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

The intramolecular condensation of 341 would be expected to give dihydroquinoline 342 (Fig. 133), but a dismutation reaction takes place, affording the tetrahydro-quinoline 343 along with approximately equal amounts of the aromatic derivative 344, as is frequently observed when the reaction is performed in the absenc-e of an oxidizing agent. In polyphosphoric acid medium the proposed mechanism involves migration of the hydride ion from position 2 of 342 toward the cationic site of a protonated dihydroquinoline molecule. "... 

The reaction is most important for the preparation of carbinols from certain aromatic and heterocyclic aldehydes and for the preparation of several aliphatic polyhydroxy compounds. In the normal Cannizzaro reaction the theoretical yield of alcohol is only 50% because half of the aldehyde is converted to the acid. A mixture of an aldehyde with excess formaldehyde, however, results in a dismutation in which most of the higher aldehyde is reduced formaldehyde is oxidized to sodium formate, viz., ... 

A review of chemiluminescent and bioluminescent methods in analytical chemistry has been given by Kricka and Thorpe. A two-phase flow cell for chemiluminescence and bioluminescencc has been designed by Mullin and Seitz. The chemiluminescence mechanisms of cyclic hydrazides, such as luminol, have been extensively analysed. " Fluorescence quantum yields of some phenyl and phenylethynyl aromatic compounds in peroxylate systems have been determined in benzene. Excited triplet states from dismutation of geminate alkoxyl radical pairs are involved in chemiluminescence from hyponitrite esters. Ruorophor-labelled compounds can be determined by a method based on peroxyoxalate-induced chemiluminescence. Fluorescence and phosphorescence spectra of firefly have been used to identify the multiplicity of the emitting species. " The chemiluminescence and e.s.r. of plasma-irradiated saccharides and the relationship between lyoluminescence and radical reaction rate constants have also been investigated. Electroluminescence from poly(vinylcarbazole) films has been reported in a series of four... 

Ruthenium tetroxide was shown to oxidize PCBs in water [20], Water-soluble ruthenium complexes, such as [Ru(H20)2(DMS0)4]2+, are effective catalysts for the KHSO5 deep oxidation of a number of chloroaliphatics, of a-chlorinated al-kenes, polychlorobenzenes, and polychlorophenols. When the reactions are carried out in water in the presence of surfactant agents, degradation of the substrates is definitely faster. Aromatic substrates are mainly converted into HC1 and C02, polychlorophenols being more sensitive to oxidation than substituted benzenes [21]. Replacement of the DMSO- solvated ruthenium by RuPcS results in a definite improvement of the reaction course with hydrogen peroxide, since dismutation of... 

Other reactions of minor relevance are the hydrodealkylation, dismutation, and alkylation of aromatics. 

On treatment with aqueous or alcoholic aLkah solutions aromatic and aliphatic aldehydes 11 lacking a-hydrogens undergo dismutations (Cannizzaro reactions) to give the corresponding carboxylic acid salts 12 and alcohols 13 [Eq. (6)] [146,147]. 

---