Stoichiometric phenol hydrogenation

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

The process based on phenol hydrogenation considered here can be described by the following overall stoichiometric equation ... 

If radicals diffuse from the solvent cage, fragmentation products are formed. Abstraction of hydrogen from the solvent by a phenoxy radical results in phenol, which can almost always be observed among the photoproducts of aryl esters in solution. Chemical evidence for the reaction of phenoxy radical with solvent is the formation of nearly stoichiometric amounts of 4-methyI-phenol and acetone from the irradiation of 4-methylphenyl benzoate (60) in isopropyl alcohol.34... 

The direct oxidation of benzene to phenol is usually affected by a poor selectivity due to the lack of kinetic control. Indeed, phenol is more reactive towards oxidation than benzene itself and consecutive reactions occur, with substantial formation of overoxidized products like catechol, hydroquinone, benzoquinones and tars. This is the usual output of the oxidation of aromatic hydrocarbons by the classical Fenton system, a mixture of hydrogen peroxide and an iron(II) salt, usually ferrous sulfate, most often used in stoichiometric amounts [8]. 

The process would use N2O to hydrojylate benzene to phenol. The phenol would be hydrogenated to cyclohexanone using available technology. The final step is the currently practiced nitric acid oxidation of cyclohexanol and cyclohexanone, which returns N2O for use in the front end of the process. The stoichiometric balance is close however, either some additional on-purpose N2O or KA would likely be required for a stand alone plant. 

Crosslinking with amines can be carried out either catalytically with tertiary amines (e.g., with A,A-dimethylbenzylamine), or especially by equimolar conversion with primary or secondary oligoamines at higher temperatures. This reaction is catalyzed by compounds that are capable of forming hydrogen bonds (water, alcohols, phenols, carboxylic acids, etc.). The most favorable ratio of amine/ epoxide is not necessarily the stoichiometric ratio and therefore must be determined empirically in each case. 

During these catalytic or stoichiometric processes, possible side reactions do occur which are shown in Scheme 4.3. These explain why the use of more than lequiv. of boronic acid is necessary. Furthermore, during the oxidation of copper(ll) to copper(lll), hydrogen peroxide is produced, which can decrease the yields of the reaction as a result of its reaction with the aryl boronic add. This may also explain why more than 1 equiv. of aryl boronic acid provides enhanced yields [10]. Moreover, the aryl boronic acid can form triaryl boroxines [11], and in doing so forms water, which can be removed from the reaction by molecular sieves. Evans and coworkers postulated that phenolic products would be formed as a result of the competitive arylation of water formed during the reaction process. 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

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