Alkylphenols and Alkoxyarenes

The presence of alkyl or alkoxy substituents in the phenol molecule facilitates the oxidation process and stabilizes the resulting quinones with regard to further oxidation, which enables high selectivity at high substrate conversions. 

1 Oxidation with Molecular Oxygen Co(II)-Schiff base complexes have long been used as catalysts for selective oxidation of substituted phenols, and this subject has been comprehensively reviewed [15,113]. Electron-releasing groups in the salen ligand favor the formation of BQ. Oxidation of 2,3,6-trimethylphenol (TMP) to TMBQ was accomplished with a 88% yield in DMF 

SCHEME 14.13 Oxidation of syringyl alcohol with O, catalyzed by (pyr)Co(salen). 

A drawback of Schiff base complexes is their low productivity in oxidation catalysis because the salen ligand is highly prone to oxidative destruction. Kinetically more stable phthalocyanine complexes have been widely explored as catalysts for oxidation of alkylphenols. This subject has been recently reviewed by Sorokin [98]. 

Substituted phenols undergo a variety of oxidative transformations in the presence of copper compounds [15, 117-119]. The nature of both phenol and ligands and the reaction conditions strongly affect the reaction selectivity. Hay and coworkers discovered the oxidative C—O coupling of 2,6-dimethylphenol to polyphenylene ether, an important industrial polymer, in the presence of a homogeneous Cu(I) catalyst and amines [117a]. The presence of large substituents (j-Pr or r-Bu) at ortho positions of phenol and elevated reaction temperature shift the oxidation 

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