Ring-Chain and Valence Bond Tautomerism

Ring-chain tautomerism can occur when a substituent group can interact with an NH group or nitrogen atom of a heterocyclic ring. 

An example of the first case is provided by the 1,2,4-triazinone side-chain aldehydes (102a) which exist in equilibrium with the cyclized forms (102b). 

The second possibility is illustrated by a-azidoazines which exist in equilibrium with tetrazole fused systems thus, 3-azido-l,2,4-triazines (103a) exist predominantly as the tetrazolotriazines (103b). 

A 13C NMR study on (104) under strongly protonating conditions (86AQ(C)6l) has determined the equilibrium between the protonated azide (105) and the protonated tetrazolopyridinium salt (106). Comparison of the 13C NMR values of (104) in tiifluoroacetic acid (i.e., those of (105) or (106)) with those of 2-azido-l-ethylpyridinium salt (107) and l-methyltetrazolo[l,5-a]pyridinium salt (108) revealed unambiguously that the bicyclic cation (106) is present under these conditions. 

The 2H-pyran (109) exists in valence bond tautomeric equilibrium with cri-(3-ionone (110). The equilibria are strongly dependent on both the solvent and temperature. In tetrachloroethene, only 9% of the open chain form (110) is present at about 30°C, but this rises to 40% at 113°C (81T1571). The nature of R also plays a significant role (85HCA192). 

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