Thermal stability of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and related N-alkoxyamines

The carbon-oxygen bond dissociation enthalpies, BDE(C-O), in several N-alko xyamine derivatives based on 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) h ave been determined in the liquid phase by photoacoustic calorimetry. The B DE(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 electrone gative element adjacent to the C-O bond is present, as in tetrahydrofuryl, a substantial deviation from the correlation is noticed. Due to the anomeri c interaction the (THF) N-alkoxyamine is stabilized by an additional 14 kca l mol(-1). The Arrhenius expression for the homolytic decomposition of meth yl-TEMPO in the gas phase obeys k/s(-1) = 10(15.3) 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-dihydroanthrac ene, has been observed. Above 380 K, TEMPO is converted into the hydroxyami ne 2,2,6,6-tetramethyl-1-piperidinol (TEMPOH) and the amine 2,2,6,6-tetrame thylpiperidine (TEMPH). An acid-catalyzed mechanism for TEMPO deoxygenation is proposed.