Factors influencing the C-O bond homolysis of alkoxyamines: effects of H-bonding and polar substituents.

The synthesis of various new trialkylhydroxylamines is described. The rate constant of the C-O bond cleavage of these new alkoxyamines has been measured. For example, C-O bond homolysis rates in a series of para-substituted TEMPO-styryl compounds TEMPO-CH(CH3)C6H5X 1a (p-MeO), 1b (p-Me), 1d (p-H), 1e (p-Br), and 1f (p-MeO2C) are presented. Furthermore, rate constants for the C-O bond cleavage of alpha-heteroaryl-substituted secondary alkoxyamines are discussed. A correlation by which the rate constant for the C-O bond cleavage of TEMPO-derived alkoxyamines can be predicted from the C-H BDEs of the corresponding alkanes is presented. Solvent effects as well as the effect of camphorsulfonic acid on the rate of the C-O bond homolysis are discussed. Finally, EPR and kinetic evidence show that alkoxyamines derived from nitroxides which are capable of intramolecular H-bonding undergo C-O bond cleavage faster than the corresponding non-H-bond-forming analogues.