Equilibrium Configurations of Azines of Esters

Article abstract of DOI:10.1021/jo00193a006

The equilibrium configuration of hydrazonates (ester azines) such as R²O(R¹)C=NN=C(R³)2 (2) and 2O(R¹)C=N>2 (3) is to found to be that in which the alkoxy group is anti (E configuration) even if the alkyl group (R¹) is larger than the alkoxy group.This is the same as the configuration of imidates ( CH3O(R)C=NCH3 ) < formula > and opposite to the configuration observed for azines, hydrazones, and semicarbazones of aldehydes and ketones, where the more bulky group occupies the anti position.With R²=CH3 or CH2CH3, R¹ groups smaller than tert-butyl do not force the hydrazonates into the Z configuration and even in the case R¹=tert-butyl (R²=CH3), the E isomer predominates in the two-component system (2) at equilibrium.Similarly, in the three component system (3) (R²=CH3, R¹=C(CH3)3) the order of predominance at equilibrium is E,E,>E,Z,>Z,Z.It is concluded that dipole alignment is an important factor stabilizing the E configuration and that there may also be some destabilization of the Z configuration resulting from repulsions between nonbonded electrons at nitrogen and at oxygen.Both factors are illustrated above with the isomers of 2.