Hydrogen peroxide reaction with alkyl benzenes

TiJraleic anhydride is known to form two classes of adducts with alkyl-benzenes, depending on the conditions of excitation. At reflux temperatures and in the presence of catalytic amounts of peroxides adducts of Type I are formed (3, 4). A free radical chain reaction is assumed involving abstraction of a benzylic hydrogen. The chain length, based on the ratio of product to added peroxide, is 20 to 100. 

CU2O could catalyze oxidative alkylarylation of acrylamides with simple alkanes to alkyl-substituted oxindoles using dicumyl peroxide (DCP) as radical initiator. Alkyl radical could be formed via hydrogen abstraction reaction with cumyloxyl radical which is generated from the homolysis of DCP. Then alkyl radical reacts with C=C bond followed by cyclization to benzene core and oxidation to give the alkyl-substituted oxindoles. Selective activation of unactivated C(sp )-H and C(sp2)-H bonds by one single step is achieved in this transformation [70] (Scheme 8.33). 

Benzene may be oxidised to phenol in an ionic liquid-aqueous biphasic system employing dodecanesulfonate salts of Fe3+, Fe2+, Cu2+, Co2+ and Ni2+ as catalyst and hydrogen peroxide as oxidant.[85] Ionic liquids with long alkyl chains like [C8Ciim][PF6] form one phase with benzene and at the same time dissolve the catalyst. As the phenol product is preferably soluble in the aqueous phase product isolation was facile. Yet, reaction rates were only moderate and after six hours at 50°C conversion of benzene was in the range of 35-55%. 

Another metallocene, namely, decamethylosmocene, (Mc5C5)20s (catalyst 1.2), turned out to be a good precatalyst in a very efficient oxidation of alkanes with hydrogen peroxide in acetonitrile at 20 — 60 °C [9]. The reaction proceeds with a substantial lag period that can be reduced by the addition of pyridine in a small concentration. Alkanes, RH, are oxidized primarily to the corresponding alkyl hydroperoxides, ROOH. TONs attain 51,000 in the case of cyclohexane (maximum turnover frequency was 6000 h ) and 3600 in the case of ethane. The oxidation of benzene and styrene afforded phenol and benzaldehyde, respectively. A kinetic study of cyclohexane oxidation catalyzed by 1.2 and selectivity parameters (measured in the oxidation of n-heptane, methylcyclohexane, isooctane, c -dimethylcyclohexane, and trans-dimethylcyclohexane) indicated that the oxidation of saturated, olefinic, and aromatic hydrocarbons proceeds with the participation of hydroxyl radicals. 

We have already seen in this chapter that we can substitute bromine and chlorine for hydrogen atoms on the benzene ring of toluene and other alkylaromatic compounds using electrophilic aromatic substitution reactions. We can also substitute bromine and chlorine for hydrogen atoms on the benzylic carbons of alkyl side chains by radical reactions in the presence of heat, light, or a radical initiator like a peroxide, as we first saw in Chapter 10, (Section 10.9). This is made possible by the special stability of the benzylic radical intermediate (Section 15.12A). For example, benzylic chlorination of toluene takes place in the gas phase at 400-600 °C or in the presence of UV light, as shown here. Multiple substitutions occur with an excess of chlorine. 

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