Fuson-type enols

Molecules that contain two or three bulky aryl groups. An example is 2,2-dimesitylethenol (112). In this case, the keto content at equilibrium is only In cases such as this steric hindrance (p. 189) destabilizes the keto form. In 112 the two aryl groups are 120° apart, but in 113 they must move closer together (w 109.5°). Such compounds are often called Fuson-type enols ° There is one example of an amide with a bulky aryl group, A-methyl l w(2,4,6-triisopropylphenyl)acetamide, that has a measurable enol content, in sharp contrast to most amides. 

As a general problem, early studies were always obscured in terms of interpretation by the simultaneous presence of both enol and keto tautomers. Only recently, a direct study of the chemistry of enol radical cations was undertaken using stable simple enols of the Fuson type [63,64]. Extensive work originally by Fuson [115-117] and later by Rappoport [118] showed for a series of sterically hindered, mesityl-substituted systems that both the kinetically stabilized enols and the corresponding ketones can be prepared in tautomeri-cally pure form. 

Even in the presence of nucleophiles like methanol, these sterically hindered enols only afforded benzofurans under oxidative conditions. Importantly, when stable enols of the Fuson type, which are less sterically hindered in the P-position, were oxidized with one-electron oxidants a-substituted carbonyl compounds 31 and 34 were obtained with water and various alcohols [64]. With acetonitrile, formation of the oxazole 32 was observed. A related example of this chemistry can be found in the context of Fuson s oxidation studies using Pb(OAc)4 [123] that provided the a-acetoxy aldehyde 31c from 30. To explain the products 31 and 34, three different mechanistic hypotheses were advanced (Scheme 3) [64] similar to the ones rationalizing the benzofuran formation. 

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