Polar, steric, and stabilization effects in alkoxyamines C-ON bond homolysis: A multiparameter analysis

We present measurements of the rate constants (k(d)) of the C-ON bond cleavage in new alkoxyamine models containing the N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1) moiety. The homolysis rate constants of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)and SG1-based alkoxyamines are analyzed in terms of polar inductive/field (sigma(U)), steric (nu), and radical stabilization (sigma(RS)) contributions of the leaving alkyl radicals, using a multiparameter equation, i.e., log(k(d)/k(d,0)) = rho(U)sigma(U) + delta nu + rho(RS)sigma(RS). The rate constants increase with increasing electron withdrawing, steric, and stabilization demands of the leaving alkyl radicals. Good correlations are found for TEMPO (log(k(d)/k(d,0)) = 13.6 sigma(U) + 6.6 nu + 13.9 sigma(RS)) and SG1 (log(k(d)/k(d,0)) = 19.5 sigma(U) + 7.0 nu + 15.3 sigma(RS)) derivatives, highlighting the polar sensitivity of the leaving alkyl radical to the nitroxyl moiety. Such correlations should facilitate the design of new alkoxyamines as initiators/regulators and help to improve the tuning of NMP experiments.