Arenes oxidative cleavage

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

Abstract This chapter covers one of the most important areas of Ru-catalysed oxidative chemistry. First, alkene oxidations are covered in which the double bond is not cleaved (3.1) epoxidation, cis-dihydroxylation, ketohydroxylation and miscellaneous non-cleavage reactions follow. The second section (3.2) concerns reactions in which C=C bond cleavage does occur (oxidation of alkenes to aldehydes, ketones or carboxylic acids), followed by a short survey of other alkene cleavage oxidations. Section 3.3 covers arene oxidations, and finally, in section 3.4, the corresponding topics for aUcyne oxidations are considered, most being cleavage reactions. 

Arenes can also be cleaved with peroxyacetic acid831 and permanganate.636,832 Ru04 effectively oxidizes arenes to cleavage products with the use of a cooxidant such as NaT04. Violent reaction with benzene and immediate oxidation of several other aromatics such as triphenylmethane and tetralin834 take place, but oxidation products could not be isolated. Since alkanes are resistant to Ru04,... 

PV2M010O40 has usually been used in the acid form. H5PV2M010O40 catalyzes aerobic oxidative cleavage of cycloalkanes, 1-phenylalkanes, and ketones. For example, the oxidation of 2,4-dimethyl cyclopentanone and 2-methylcyclo-hexanone gives 5-oxo-3-methylhexanoic acid and 6-oxoheptanoic acid, respectively, in yields higher than 90% [285, 286). Bromination of arenes with HBr [287), oxidative dehydrogenation of cyclohexadiene [288, 289) and a-terpinene [290), oxidation of 2,4-dimethylphenol [291) and sulfides [292) are other examples. 

Reactions of diaryl ditellurides with [bis(acyloxy)iodo]arenes, on the other hand, lead to oxidative cleavage of the Te-Te bond, and afford aryltellurinic mixed anhydrides 15 (Scheme 8) [26]. When such oxidations are conducted in a medium containing aqueous sodium hydroxide, aryltellurinic anhydrides 16 are obtained [16]. 

Significant characteristics of the porphyrin iron monoxide are seen in the chemical reactivity. Naphthalene is converted initially to the corresponding arene oxide on treatment with P 450 (19), consistent with a molecular mechanism of oxygen transfer from an iron monoxide to the aromatic nucleus. Retention of stereochemistry in the P-450 catalyzed hydroxylation of d ethylbenzene also supports the molecular mechanism. The unusually large kinetic isotope effect observed for the P-450 oxidation of dideutero 1,3-diphenylpropane, kJkD = 11, demonstrates that C—H cleavage is involved in the rate determining step (20), probably in a very unusual environment, not incompatible with a molecular mechanism. 

Radical cations are probably involved in the photooxidation of olefins [1-7, 24, 50-54, 56, 61, 66] (Eqs. 2 and 5). Many oxidizable arenes and dienes are, similarly, oxidatively cleaved or rearranged by routes consistent with initial formation of a radical cation. As we saw above, the observed oxidative cleavage product probably derives from the combination of a surface-bound radical cation with superoxide or adsorbed oxygen. With alkanes or simply substituted alkenes, however, the capture of a photogenerated hole is often thermodynamically forbidden. Thus, instead of radical cations, radicals formed by hydrogen atom abstraction by an activated oxygen species dominate the observed chemistry. With alkanes, oxygenation at sites... 

The cobalt(III) initiation and catalysis pathways are very effective in many oxidations but suffer some limitations, e.g., Co " is strongly inhibited by cobalt(II) ions, which seem to form dimers with Co ". Such dimers are only weak catalysts in arene oxidations. As a result the rate of oxidations is inversely dependent on the concentration of Co " in the reaction mixture thus the cleavage of such dimers by addition of small amounts of co-catalysts will attain the reaction rate [11c, 12]. Additionally in the case of deactivated, electron-poor systems such as toluic acid or p-nitrotoluene, cobalt(III) alone is not an efficient catalyst - synergistic co-catalysts are necessary to achieve good results. 

Calixquinones can be easily reduced to the corresponding calixhydroquinones (Zn/HCl or Na2S204) . The calix[8]hydroquinone 57b, however, was prepared from the octabenzyl ether 57a obtained by one-pot condensation in a mixture with the analogous calix[6]- and -[7]arene. Oxidation of 57b to the respective octaquinone was not reported, but the ewrfo-ether 57c was obtained by exhaustive 0-propylation prior to the cleavage of the benzyl ether groups . Inherently chiral derivatives of a calix[4]arene monoquinone have been obtained by 1,4-addition of various nucleophiles to the quinoid system. 

In some cases an insertion of the oxygen atom into a substrate leads to the formation of an unstable intermediate which stabilizes by electrophilic attack at molecules of the surrounding media e. g. semi-acetals formed by an insertion into the C—H bonds of OK rbons at heteroatoms undergo heterolytic cleavage yielding the corresponding aldehydes. If with epoxides, arene oxides or hydroxylamines the sta-... 

Thus, N-H bond cleavage in this system provides a foundation for the development of mild catalytic transformation of ammonia as in arene-oxidative amination and olefin hydroamination. 

Nonetheless, since the EIE for interconversion of the aryl hydride and the arene complex is known for the Cp Rh(PMe3) system, and the KIE for oxidative cleavage of the arene C-H bond is determined, the KIE for reductive coupling of an arene C-H bond can be calculated via ircn/ rcD = ( ocn/ ocoIAeq. The values give a rcH/ rcD = 0.52, an inverse KIE. In light of the previous discussion, this result represents an unprecendented inverse KIE for a single step. 

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