Methylphenols oxidation, ortho

It was shown that complexes 19 of the zwitterionic precursors of ortho-quinone methides and a bis(sulfonium ylide) derived from 2,5-di hydroxyl 1,4 benzoquinone46 were even more stable than those with amine N-oxides. The bis(sulfonium ylide) complexes were formed in a strict 2 1 ratio (o-QM/ylide) and were unaltered at —78 °C for 10 h and stable at room temperature under inert conditions for as long as 15—30 min (Fig. 6.18).47 The o-QM precursor was produced from a-tocopherol (1), its truncated model compound (la), or a respective ortho-methylphenol in general by Ag20 oxidation in a solution containing 0.50-0.55 equivalents of bis(sulfonium ylide) at —78 °C. Although the species interacting with the ylide was actually the zwitterionic oxidation intermediate 3a and not the o-QM itself, the term stabilized o-QM was introduced for the complexes, since these reacted similar to the o-QMs themselves but in a well defined way without dimerization reactions. 

FIGURE 6.18 Oxidation of ortAo-methylphenols to the corresponding ortho-quinone methide via transient zwitterionic intermediates that are stabilized by forming a complex 19 with the 2,5-dihydroxy[l,4]benzoquinone-derived bis(sulfonium ylide). 

Oxidation of phenols. Treatment of phenols with this reagent in CH2CI2 at 25 results in ortho- and para-hydroxylation. Thus oxidation of 2,4,6-tri-methylphenol (1) results in formation of (2) and (3), the dimer of the o-hydroxycyclohexadienone. 

Oxidation of isoeugenol with (BPA)Cu(II)Clzin ethanol gave the ortho-ortho coupling dimer and the aldehyde derivatives, On the other hands, oxidation of isoeugenol in acetic acid Hz O gave the adduct of two molar isoeugenol and one molar acetic acid. When 2,6-dibutyl-4-methylphenol was oxidized in the system of copper(II) acetate-pyridine quinone methide derivatives were produced. 

Toluene reacts at a similar rate to that of benzene and prodnces ring-hydroxylated compounds (56% 4-methyI- and 41% 2-methylphenol) as the sole produets [99], In contrast, mesoporons titaninm-silicates favor oxidation of the tolnene side-chain [100], The rate of hydroxylation of xylene isomers over TS-1 was sterically controlled, orthopores zeolite Ti-MOR, it inereased as expected for an electrophilic attack, parasterically hindered aromaties beeause of pore size limitation. The development of an efficient heterogeneons catalyst for the seleetive oxidation of alkylarenes remains a great challenge [100],... 

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