Epoxidation benzene

In the case of benzene, a number of epoxides are known, e.g., mono-, di-, and triepoxides. All undergo interesting electrocyclic reactions. Benzene epoxide itself exists as a valence tautomer of 86 and 96.8 The enthalpy difference between the two forms is 1.7 kcal/mol. The activation energies for forward and reverse reactions are 9.1 and 7.2 kcal/mol, respectively. At room temperature the rate of exchange is so fast that the NMR spectrum is the average of chemical shifts that are due to protons represented by structures 86 and 96. 

A number of variously substituted benzene epoxides have been reported. Their valence tautomeric behavior cannot be described with certainty because of inadequate information. 

In the cases of benzene epoxide and its congeners, cooling the sample to — 113°C brings about a separation of the peaks of the two tautomers as a result of the slowing down of the rapid transformation that ordinarily takes place at ambient temperature. Thus in the case of 86 96 the NMR spectrum... 

In the framework of these ideas, the phenol hydroxylation mechanism is implemented by oxygen transfer from the oxaziridine intermediate to phenol giving different intermediate compounds—benzene epoxide ... 

Hydroxylation can follow epoxidation, as shown by the following rearrangement reaction for benzene epoxide ... 

Reduction of Epoxides Some epoxides, including the arene oxide, benzene epoxide, can be reduced back to their alkenes (Kato et ah, 1976 Yamazoe et ah, 1978) ... 

To be carcinogenic, benzene must first be metabolized in the liver, mainly via cytochrome P4502E1. The major product is phenol (19), which is either conjugated—primarily to phenyl sulfate in humans—or further hydroxylated by P450 2E1 to hydroquinone. Other major metabolites include catechol (34) and trans-trans-mucomc acid (189 1,6-hexadienedioic acid). The latter is presumed to be formed from the ring opening of benzene epoxide (190 benzene oxepin), or perhaps benzene dihydrodiol (191 3,5-cyclohexene-l,2-diol). ... 

Reduction of hexamethyl-Dewar-benzene epoxide with lithium aluminium hydride affords a mixture of the tricyelie alcohol (424) and the ketone (425) in equal proportions. Use of alane as the reducing agent, followed by an aqueous basic work-up... 

Interestingly, transfer of oxygen from the oxaziridine reagent leads to a benzene epoxide intermediate. 

Jerina e( al. (1968) have shown that an epoxide hydrase of rabbit liver microsomes, or the. soluble fraction of liver homogenate, convert benzene epoxide to the dihydrodiol (see Fig. 4). The reaction was also demonstrated with styrene epoxide, indene epoxide, and cyclohexene epoxide. A soluble dehydrogenase from the liver preparation oxidized dihydrodiols to catecliols. The conversion of the epoxide to phenol was considered to be a nonenzymic isomerization. In the presence of glutathione and glutathione-iS-epoxide transferase, a glutathione conjugate was formed. 

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