Molecular structure and stability of isoxazolium enolates

Single crystal X-ray diffraction analysis, spontaneous formation and solvolytic stability of the dipolar Schiff base isoxazolium enolate, derived from aldehydes and ketones were studied. The cross conjugated betaine (V) derived from 2,4-dimethoxybenzaldehyde by condensation with 4-phenyl disic acid (II), showed coplanarity and cis configuration. The X-ray results revealed exceptionally long bonds between the two units, that of the anion and that of the cation. Results obtained by omega beta calculations were in line with the experimental observations. Cyclopentanone was the only ketone that gave a stable dipolar condensation product. Derivatives of aromatic aldehydes are generally more stable probably for the reason the coplanarity of the cationic part of the molecule is more feasible. There is evidence for attractive C-H···O interactions which might contribute to stability in aldehyde derivatives (IIIa). Ketisates (IIIb), which were synthesized by an alternative route were found to hydrolyze spontaneously when dissolved in moist solvents. Analogous betaines that have N¹ -substituted pyrazole instead of isoxazole were also studied, leading to a scheme for facile decomposition of oximes and hydrazones.