Stable equivalents of enolate ions

The nitroso compound is unstable because it can tautomerize with the transfer of a proton from carbon to the oxygen of the nitroso group. This process is exactly like enolization but with N=0 in the place of the C=0 group. It gives an oxime as the stable enoT. The oxime s O-H can form an intramolecular hydrogen bond with the ketone carbonyl group. Hydrolysis of the oxime reveals the second ketone. 

If the ketone is unsymmetrical, this reaction will occur on the more substituted side, for the same reason that acid-catalysed enol bromination gives the more substituted a-bromocarbonyl compound (see the box on p. 463). 

Before we move on to any more reactions, we want you to take away this message from the reactions of enols and enolates with Br2 and with NO  

Even with fairly strong bases such as hydroxides or alkoxides, most carbonyl compounds are converted to their enolates only to a very small extent. A typical value for the pK, of protons adjacent (a) to a carbonyl group is 20-25, while the pK of methanol is around 16, so we can only hope for about 1 part enolate in 10 parts carbonyl compound. With a much stronger base this all changes, and the enolate is formed quantitatively from the carbonyl compound. This is a very important result that we shall capitalize on in Chapters 25 and 26. The base usually used is EDA (lithium diisopropylamide), and it works like this. 

Interactive mechanism for lithium enolate formation with LDA 

---