Tautomerism in Substituted Nitroethylenes

Nitronic acids are stabilized by inter- and intramolecular hydrogen bonding. Kang et al. [6b] showed that nitro compound 4 is slowly, but spontaneously converted to nitronic acid tautomer 4t due to the stabilization that results from 

5 and related via a 1,3-H shift (keto-enol) and to 5-tl via a symmetry allowed 1,5-H shift, is also stabilized by an intramolecular H-bond with a strength of 7 kcal/mol. The barrier for the 1,5-H shift amounts to only 5.1 kcal/mol and illustrates that ucz-nitro derivatives are easily accessible in systems enabling intramolecular H-bonding. 

Energy differences between tautomers and the influence of the N02, CHO, CHNH and vinyl substituent (in kcal/mol) at G2MP2 level.  

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