Theoretical study of the [1,3]-prototropic rearrangements of oximes and their ethers

In the framework of the MP2/6-311++G**//RHF/6-31G* ab initio approach we investigated the structure and relative stability of the imine (-CHR-CH=N-) and enamine (-CR=CH-NH-) forms of the simplest imines, oximes, and their ethers. Although the enamine form is unstable, double bond migration R₂CH-CH=N-→ R₂C=CH-NH-is often regarded as one of the stages of a series of reactions that take place in superbasic media, in particular, synthesis of pyrroles from ketoximes and acetylene. For isomerization of E-ethaneimine CH₃-CH=NH to vinylamine CH₂=CH-NH₂, calculations predict an increase of 4.3 kcal/mol in energy. A close value (4.8 kcal/mol) was obtained for the energy of isomerization of ketimine (CH₃)₂C=NH to 2-aminopropene. The methyl group in CH₃-CH=CH-NH₂ stabilizes the neighboring double bond, and the transformation of E-propane-1-imine into E-and Z-aminoprop-1-ene is accompanied by an increase of 2.8 kcal/mol in energy. After the transition from imines to oximes, the enamine form is drastically destabilized. The highly endothermal character of the CH₃-CH=NOH → CH₂=CH-NHOH rearrangement (16.4 kcal/mol) is retained from acetaldoxime to its methyl ether and decreases by only 1.0 kcal/mol for the isomerization reaction of the vinyl ether of acetaldoxime to N,O-divinylhydroxylamine. These rearrangements are thermodynamically unfavorable probably because of the increased negative charge on the nitrogen atom and, as a consequence, destabilization of the N-O bond.