Recent Developments in Ring-Chain Tautomerism II. Intramolecular Reversible Addition Reactions to the C=N, C=N, C=C, and C=C Groups

The chapter discusses intramolecular reversible addition reactions to the C═N group and other groups. The chapter offer an introduction to and general considerations of the ring-chain tautomeric process. Despite the lower reactivity of the azomethine group as compared with the carbonyl group, the intramolecular reversible nucleophilic additions of OH, NH, or SH groups to the C=N bond often proceed. The intramolecular addition of nucleophilic groups to the C=N bond can occur along two pathways—endo and exo)—depending on the orientation of the azomethine group in the molecule. The intramolecular reversible addition reactions to C≡N group are described in the chapter. Many examples of the intramolecular addition of hydroxy, amino, or mercapto groups to C═C or C≡C bonds are known to lead to the formation of a heterocycle. In many cases, an increase in the sensitivity of the methods used in the investigation of ring-chain equlibria allow the detection of more than one open-chain and/or cyclic tautomer in equilibrium. The regularities governing simple two-component ring-chain equilibria are of rather limited value for such complicated tautomeric systems.