Keto-enolic compounds, distinction

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

Macralstonine is the water-addition product of (225). This addition cannot be at the 8-keto-enol ether system of the alstophylline half since macralstonine shows the same u.v. absorption as the anhydro-compound (225), bands at 1646 and 1615 cm in the i.r. spectrum, and the low-field singlet of the C(21 )-proton in its n.m.r. spectrum. Hydration must therefore be at the double bond of the macroline component, resulting in structure (220) for macralstonine. In chloroform both the ring-open and ring-closed forms are present [singlets for the — C(18)H3 grouping at 1.67 and 1.49 p.p.m. (total 3H) and a medium ketone band at 1706 cm ]. The presence of two distinct species causes other n.m.r. signals to be doubled, a fact which does not facilitate interpretation. 

It becomes clear that in all these compounds it is the conjugate base that takes part in the substitution proper. For mono- and particularly 1,3-dicarbonyl compounds this result actually removes the problem of whether it is the keto or the enol form which enters into an electrophilic substitution by diazonium ions, halogenating agents, and many other reagents. The keto and the enol form are distinct species, but they have one (common) conjugate base This was made clear quite early, but even today there are many chemists who seem not to be aware of it. 

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