Propenoidic phenols

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

The propenoidic phenols, E-methyl ferulate (1), E-4-hydroxycinnamic acid me yl ester (4) and E-3-chloro-4-hydroxycinnamic acid methyl ester (7), can be catalytically oxidized with dioxygen in the presence of [Co(salen)] (Figure 8) . The yields depend on the solvent and the phenyl substituents. EPR and electronic spectra suggest the involvement of a coordinated o-benzosemiquinone type radical as the active intermediate. 

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