ORIGIN OF HETERO EFFECT ON NITROGEN INVERSION - COMPARISON OF HYDROXYLAMINES AND AMINOXIDE ANIONS

Rate constants for nitrogen inversion in N-benzyl-N-methylhydroxylamine, NN-diethylhydroxylamine, 1-hydroxy-2,2,4,4-tetramethylpyrrolidine, their conjugate bases, and their O-acetyl derivatives in dimethylformamide-d7 were determined based on the H-1 NMR coalescence temperatures. Relative to -OH, the -O- substituent ought to either raise the barrier to inversion owing to stronger lone-pair repulsions or lower the barrier owing to weaker sigma-inductive effects. Yet nearly identical barriers to inversion, DELTA-G double dagger = 12.0-13.3 kcal/mol, are observed for both the hydroxylamine and its conjugate base. Since the observed barrier is little changed upon deprotonation, it is concluded that the pi-repulsive and sigma-inductive contributions must be nearly equal.