Benzene oxides polysubstituted

Figure 6. Structures of synthesized di- and polysubstituted benzene oxides. References to each synthesis are superscripted.

Toluene (methylbenzene) is similar to benzene as a mononuclear aromatic, but it is more active due to presence of tbe electron-donating metbyl group. However, toluene is much less useful than benzene because it produces more polysubstituted products. Most of tbe toluene extracted for cbemical use is converted to benzene via dealkylation or disproportionation. Tbe rest is used to produce a limited number of petro-cbemicals. Tbe main reactions related to tbe cbemical use of toluene (other than conversion to benzene) are the oxidation of the methyl substituent and the hydrogenation of the phenyl group. Electrophilic substitution is limited to the nitration of toluene for producing mono-nitrotoluene and dinitrotoluenes. These compounds are important synthetic intermediates. 

Finally, benzenepolycarboxylates were obtained by ruthenium-catalyzed cross-benzannulation of acetylenedicarboxylates with allylic compounds [52] (Eq. 39). A ruthenacyclopentene is postulated to occur via oxidative coupling of one molecule of alkyne with allylic alcohol. Subsequent insertion of another molecule of alkyne gives the corresponding polysubstituted benzene derivatives. 

The ethylene moiety of zircona- and hafnacyclopentane can be replaced with more strongly coordinating ligands. Trimethylphosphine induces retro-oxidative cyclization of a zirconacyclopentane and replaces one ethylene ligand to afford a monoethylene trimethylphosphine complex (Scheme 1.63) [82]. Alkynes, nitriles, and aldehydes also substitute the olefin moiety through retro-oxidative cyclization and re-oxidative cyclization to afford thermodynamically more stable zirconacy-cles (Scheme 1.64) [83]. The replacement reaction has been applied to the selective synthesis of polysubstituted benzenes and pyridines [84]. 

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