Conformational studies by dynamic nuclear magnetic resonance. Part 20. Internal motion in hydrazones and related derivatives

Abstract

Restricted rotation about the N–N bond in unhindered, non-conjugated hydrazones (e.g. Me₂N–NCH₂) has been detected by ¹³C n.m.r. below –150°C. The results indicate that the NC plane is coplanar with the dynamically averaged plane of the sp³ nitrogen, in contrast with hindered hydrazones (containing the 2,2,6,6-tetramethylpiperidinyl ring, TMP) where the NC moiety adopts a perpendicular conformation. Effects due to the slowing of ring reversal were not observed in hydrazones containing the TMP ring, but were detected in other hindered derivatives such as TMP–NH₂ and TMP–NO. A most interesting feature becomes apparent in two hydrazones with the 2,6-cis-dimethylpiperidinyl (DMP) ring, DMP–NCH₂ and DMP–NCHPh. In the same molecule the NC moiety is coplanar with the averaged sp³ nitrogen plane when the methyls are axial, but perpendicular when the methyls are equatorial.