Conformational studies by dynamic nuclear magnetic resonance. Part 8. Effect of aromatic substituents on the stereodynamics of hindered hydrazones

Abstract

¹ H and ¹³C n.m.r. measurements show that hydrazones containing the 2,2,6,6-tetra methylpiperidin-1-yl (TMP) ring (i.e. TMP–NCR¹R²) have the–NC plane 90° twisted with respect to the dynamically averaged plane of the TMP ring itself. Restricted rotation occurs around the N–N bond, allowing the determination of the torsional barriers by means of line shape analysis of the variable temperature n.m.r. spectra. Steric and conjugative effects upon the barrier when R² is an aromatic substituent (phenyl, naphthyl, pyridyl, thienyl, fury) are discussed.