Thermal Stability of 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) and Related <i>N</i>-Alkoxyamines

Abstract

The carbon−oxygen bond dissociation enthalpies, BDE(C−O), in several N-alkoxyamine derivatives based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) have been determined in the liquid phase by photoacoustic calorimetry. The BDE(C−O) and the BDE(C−H) in the corresponding hydrocarbons follow a linear correlation: BDE(C−O) = 1.04BDE(C−H) − 62.1 kcal mol-1. When an electronegative element adjacent to the C−O bond is present, as in tetrahydrofuryl, a substantial deviation from the correlation is noticed. Due to the anomeric interaction the (THF) N-alkoxyamine is stabilized by an additional 14 kcal mol-1. The Arrhenius expression for the homolytic decomposition of methyl-TEMPO in the gas phase obeys k/s-1 = 1015.3 exp(−45.3/RT) to yield a BDE(C−O) of 47 ± 1 kcal mol-1 at 298 K. Furthermore, a high reactivity of TEMPO toward hydrogen donors, 1,4-cyclohexadiene or 9,10-dihydroanthracene, has been observed. Above 380 K, TEMPO is converted into the hydroxyamine 2,2,6,6-tetramethyl-1-piperidinol (TEMPOH) and the amine 2,2,6,6-tetramethylpiperidine (TEMPH). An acid-catalyzed mechanism for TEMPO deoxygenation is proposed.