Rh‐Catalyzed Oxidation of Anthracenes with tert‐Butyl Hydroperoxide. Kinetic Aspects

Abstract

The polar effects of substituents on reactivity in oxidation of 2-substituted anthracenes with terr-butyl hydroperoxide (TBHP)/Rh(PPh,),CI have been investigated and compared with those obtained with TBHP/ VO(acac)* and chromic acid. The anthracene reactivities obtained from competition experiments are correlated with Hummett's op-constants. The p-values are -2.60 for chromic acid and 0.72 for TBHP/VO(acac), A poor correlation withp = -0.17 (r = 0.756) was obtained for TBHP/Rh(PPh,),CI. It is concluded that the Rh-catalyzed reaction does not consist in electrophilic oxygen transfer to the anthracene.Recently, we discovered that the combination of tert -butyl hydroperoxide (TBHP) with Rh(PPh,),Cl converts anthracenes to anthraquinones in high yields [ 11. We considered the reaction of some general interest, since the large majority of the recently developed catalytic systems involve transformations of easily oxidizable functional groups such as alcohols, double bonds, while progress in reactions involving aromatic compounds is slow [2]. Although oxidation of anthracene may be of limited practical value, the compound has some favorable features for mechanistic investigations. It is quite reactive and the attack occurs almost exclusively at C(9) and C(10). In addition, the lateral benzene rings prevent further degradation of the final product, anthraquinone, so that only a limited amount of side-products are to be expected. We hoped that mechanistic studies of this model system would provide some insight allowing a more rational approach towards oxidation of other aromatic substrates.After the discovery of the system Rh(PPh,),Cl, some screening experiments were carried out with other systems (Table). The Wilkinson catalyst (Rh(PPh,),Cl) was the most efficient among the compounds studied for reaction with anthracene. RuCl,(PPh,), was less suitable, but this complex in conjunction with TBHP oxidizes alcohols and catechols [3]. A kinetic study of anthracene with Mo(CO),/TBHP has been published, but no yields are indicated [4]. The reaction is accompanied by extensive decomposition of starting material, and we could not increase the yields over 40%. Among the solvents tested, benzene gave best results, followed by chlorobenzene, but no appreciable transformation took place in tetrachloroethylene.In the Mo-or V-catalyzed epoxidation of alkenes the oxygen transfer takes place from coordinated TBHP to free alkene [5] or, alternatively, via peroxymetalation involving coordinated TBHP and n -coordinated alkene [6]. The p -value of -1.4 for epoxidation of styrene with Mo(VI)/TBHP is characteristic for an electrophilic oxygen transfer [7]. By analogy we proposed an electrophilic attack at C(9) and C(10) of anthracene for the system Rh(PPh,),Cl [ 11 (Scheme). This hypothesis is however questionable. For electro-

Oxidation kinetics of anthracene with nitrogen dioxide in acetic acid

“…Due to the advantages involved in carrying out anthracene oxidation at low temperature and under generally moderate conditions, various systems in the liquid phase have been studied. Thus, proposals have been made for the use of oxidants such as oxygen and/or nitric acid (Olivier and Gothlich, 1968; Das and Das, 1982); ammonium dichromate Bobillier, 1983 and1985); cerium (IV) ammonium nitrate (Rindone and Scolastico, 1973; Mittal, 1983 and1985); oxygen in the presence of copper (11) bromide (Brossard et al, 1977;Cepeda y Diaz, 1987), tertbutylhydroperoxide (Muller and Bobillier, 198 1 and 1985) or copper (11) chloro complexes (Koshitani et al, 1982); potassium bromate (Banerjee et al, 1983); and ammonium persulfate catalyzed by cerium ammonium nitrate, silver nitrate, and sodium dodecyl sulfate (Skarzewski, 1984).…”