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Journal of Structural Chemistry

2008 year, number 2


V. B. Kobychev, N. M. Vitkovskaya, B. A. Trofimov
Keywords: imines, enamines, oximes, O-methyloximes, O-vinyloximes, hydroxylamines, internal rotation, isomerism, ab initio calculations
Pages: 230-237


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.