Benzoquinone methides formation

The anion derived from 2,3-dimethyl-l,4-naphthoquinone behaves as a quinone methide and undergoes a [l,4]-cycloaddition with the benzoquinone (193). The product is the xanthene derivative (194) (70JCS(C)722). There is no indication of the formation of the isomeric xanthene. A [l,3]-cycloaddition occurs simultaneously which leads to the fluorene derivative (195). 

An induction period was observed in the decomposition of cumyl hydroperoxide in chlorobenzene at 70 and 110 °C in the presence of phenolic sulphides CXCVIIa,b262). This was a substantial difference with respect to the behaviour of 4,4 -thio-bis(2,6-di-tert-butylphenol) CLXVIIIb which decomposed ROOH under the same conditions without induction period. The result indicates a mechanistic distinction in the action of both types of phenolic sulphides. In the mechanism of transformations of benzyl sulphide CXCVIIb, there are assumed (Scheme 24) the formation of sulphoxide CXCVIII and the intermediary formation of CIC followed by oxidation and formation of sulphinic acid CC. Further transformation of the acid CC depends on the character of R. If R = 3,5-di-tert-butyl-4-hydroxybenzyl, as it is in the formation of CC from CXCVIIa, the total elimination of the sulphurous part of molecule may occur and the transformation products of phenolic or quinoid character may be formed 3,5-di-tert-butyl-4-hydroxybenzyl alcohol XXXI, the corresponding aldehyde XXXII, and 2,6-di-tert-butyl-l,4-benzoquinone XXII were identified. Another possible sulphurless product is 4,4 -ethylenebis(2,6-di-tert-butyl-phenol) XXVIII, which was isolated in small amounts in its oxidized form as 3,5,3 ,5 -tetra-tert-butyl-4,4 -stilbenequinone (XXIX). Quinone methide XXX formed by thermolysis of sulphoxide CXCVIII, may be also the precursor in formation of XXIX. According to66), XXX is further oxidized by hydroperoxides to XXIX... 

QMs exist in three isomeric forms, namely, o-, m-, and p-quinone methides (also known as o-, m-, and p-QMs) (Figure 7.5). The importance of o- and p-QMs in organic synthesis and their role in biochemistry have been studied in detail. Urdike benzoquinones, o- and p-QM derivatives are highly polarized compounds, usually observed with difficulty or postulated as reactive intermediates because of facile reactions driven by the formation of aromatized phenol derivatives. 

---