Carbon-nitrogen bonds oxime isomerism

Owing to the relative rigidity of the carbon-nitrogen double bond, oximes can exist in two discrete geometrically isomeric forms the E or anti isomer anti-E-217) and the (Z) or syn isomer (syn-Z-217) (equation 70). In solid state, both oximes show high configurational stability and discrete existence. In solution, equilibrium between both isomers is rapidly established, favouring the thermodynamically most stable isomer " ... 

In the case of the carbon-nitrogen double bond, the reaction of isomerization can occur via rotation about the double bond, and by nitrogen inversion (XLVII). The latter process as a rule is strongly favored over rotation, resulting in a lowered barrier of overall isomerization as compared to ethylenes. For many imines (XLVII, c = H), this barrier is in the range 20-30 kcal/mol. Electronegative substituents on the nitrogen atom increase stability toward inversion, as evidenced by the relative stability of oximes (XLVII, c = OH) and hydrazones (XLVII, c = NRR ) [56]. 

We have previously seen how geometric isomerism can exist around both the carbon-carbon double bond in an alkene and the carbon-nitrogen double bond in, for example, an oxime (Chapter 4), and we have now seen how an alkylidenecyclohexane such as 6 can be chiral without the need for a stereogenic carbon. Can one therefore obtain a chiral molecule in which the alkene part of 6 is replaced by, say, an oxime Oxime 8 has four different groups in the key positions. The nature of the oxime group, in particular the lone pair on N, makes it impossible to construct a stable chiral oxime with only two different groups. 

Ketoximes.—This name is given to the oximes derived from ketones Benzophenone yields but one oxime, but certain derivatives of benzophenone of the general formula R.CO.R in which R and R are different radicals, yield oximes which exist in two forms. The isomerism appears to be stereochemical and to result from the fact that the oximes contain a carbon atom linked by a double bond to a nitrogen atom. The formulas of the two oximes of tolyphenyl ketone are as follows —... 

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