Abstract
Mass spectrometry is used to evaluate the occurrence of the nitrile-ketenimine tautomerism. Mass spectra of two differently substituted nitriles, ethyl-4,4-dicyano-3-methyl-3-butenoate and diethyl-2-cyano-3-methyl-2-pentenodiate are examined looking for common mass spectral behaviors. Ion fragmentation assignments for specific tautomers allow to predict the presence of the corresponding structures. Additionally, the mass spectrum and nuclear magnetic resonance spectra of ethyl-4,4-dicyano-2,2-diethyl-3-methyl-3-butenoate and that of the corresponding amination product support the occurrence of the ketenimine tautomer in the equilibrium.
Highlights
Few reports have been found on the occurrence of nitriles in equilibrium with the corresponding tautomers, the ketenimines
The energy barrier to form tautomers II, V and, VI indicates that they are likely to occur in some extent while tautomerism involving the methyl moiety (IV) and the double tautomerization process (VII and VIII) seem to be less likely
The reported evidences found by mass spectrometry in regard to the occurrence of the nitrile-ketenimine tautomerism have been supported through isotopic exchange, MS2 and reactivity experiments
Summary
Few reports have been found on the occurrence of nitriles in equilibrium with the corresponding tautomers, the ketenimines. Some studies where enolization of nitriles takes place have been found [1,2,3]. Kasturi et al [2] have carried out the study of the UV absorption spectra of several 1,2dicyano esters and 1,1,2-tricyano compounds with a view to demonstrate the presence of nitrile-ketenimine tautomerism. They have synthesized several condensation products of β-ketoesters with malononitrile and ethyl-cyanoacetate in connection with the synthesis of heterocyclic compounds. From the analysis of the UV spectra in ethanol and in ethanol/sodium hydroxide solution there was observed a hyperchromic effect on the band around 355 nm (detectable only in polar hydroxylic solvents) that could be assigned to the presence of the ketenimine structure [3]
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