Structure, conformation, anomeric effects and rotational barriers in the HERON amides, N,N ′-diacyl-N,N ′-dialkoxyhydrazines

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Abstract
N,N′-Diacyl-N,N′-dialkoxyhydrazines are HERON amides that exhibit theoretical and physical properties of bisheteroatom-substituted amides. Amide nitrogens are pyramidal and they adopt a preferential conformation which permits an anomeric interaction in which one nitrogen overlaps strongly with the adjacent N–O σ* orbital. A crystal structure of N,N′-di(p-chlorobenzoyl)-N,N′-diethoxyhydrazine 4d has been obtained which confirms these properties in the solid state. Infrared data for ten such hydrazines indicate unusually high carbonyl stretch frequencies in accord with pyramidality at nitrogen. Diastereotopic resonances and dynamic 1H NMR studies indicate both a significant N–N rotational barrier of between 65 and 72 kJ mol–1, which is consistent with a strong anomeric interaction, as well as a much smaller than usual amide rotation barrier of 54 kJ mol–1, a direct consequence of pyramidality at nitrogen.