SYNTHESIS AND STRUCTURES OF 1-OXA-3,5-DIAZA-1,3,5-HEXATRIENES - THE 1-OXA-3,5-DIAZA-1,3,5-HEXATRIENE/2H-1,3,5-OXADIAZINE TAUTOMERISM

Reaction of N-lithio- or N-trimethylsilyl-substituted imines 1 with N-acylimidic acid derivatives 2 provides 1-oxa-3,5-diaza-1,3,5-hexatrienes 3 in fair to good yields. Depending on the substitution pattern, the acyclic compounds 3 form an equilibrium with their cyclic tautomers, e.g. 2H-1,3,5-oxadiazines 4. Aromatic substituents at C-6 favor the acyclic form 3, whereas aliphatic groups (e.g. tert-butyl) lead to the exclusive formation of the ring tautomer 4. For a typical example (3d/4d) the free enthalpy of activation for the ring/chain tautomerism was determined to be ca. 12.5 kcal/mol. Detailed spectroscopic data (H-1-, C-13-NMR, IR, UV) for both tautomers are given. The X-Ray analysis of the acyclic compound 3a shows a central C = N bond with Z configuration and with s-gauche conformations of the adjacent C = O and C = N systems. According to quantum mechanical ab initio calculations (MP2/6-31G*//6-31G*) for the C3H4N2O model compounds, the ring tautomer 6 is by ca. 6 kcal/mol lower in energy than the acyclic forms 7. Within the series of open-chain tautomers 7, gauche forms are lower in energy than s-trans conformers, indicating that amide resonance is more important for these systems than polyene delocalization. s-cis Forms were found to be transition states. The acyclic conformers 7 are rather flexible; barriers of rotation for both C - N bonds are calculated to 4 - 5 kcal/mol only (RHF/3-21G//3-21G).