Other Carbonylic and Transformation Products

The versatility of the phenolic molecule towards oxidation is demonstrated by the formation of other oxidation derivatves than the coupling, quinone and cyclohexadienone series. 2,4,6-Trimethylphenol has been converted in 98% yield to 3,5-dimethyl-4-hydroxybenzaldehyde by aerial oxidation in dimethylformamide/methanol solution (1 5) containing cuprous chloride by passage of air through the solution during 8 hours (ref.62). 

Formation of a trimethyltropone has been described by the transformation of the cyclohexadienone intermediate formed in the Reimer-Tiemann reaction in 59% yield from 2,4,6-trimethylphenol, chloroform and a little cetyltrimethylammonium bromide by treatment dropwise at 50°C over 10 mins, with aqueous sodium hydroxide. Refluxing of the cyclohexadienone for 4 hours with tributyltin hydride in benzene containing azoisobutyronitrile (AIBN) afforded 2,4,7-trimethyltropone in quantitative yield (ref.63). 

Although outside the scope of the present chapter, another transformation of interest is the conversion of the fully hydrogenated product from phenol, namely cyclohexanol, to cyclohexanone in 100% yield by addition of a dichloromethane solution to bis(quinuclidine)bromine fluoroborate and silver fluoroborate in dichloromethane followed by reaction for 30 mins.at ambient temperature (ref.65). 

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The oxidation of phenols has many different aspects. In this account, oxidative coupling, the formation of cyclohexadienones, conversion to quinones and other carbonylic and transformation products (other than those obtained by electrophilic substitution or rearrangement of phenyl esters) are considered. Particular attention has been paid to work carried out in the last decade. An early review has described initial work (ref.1) and that carried out later is available in the same source (ref.1, Ch.2). To a certain extent this present chapter inevitably impinges on the chemistry of alkylphenols (Ch.6), although oxidation has been excluded in that section except in the case of hindered phenols which are only briefly referred to in the following examples. 

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