Alkoxyamine C-ON bond homolysis: Stereoelectronic effects

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP) reactions. Because polymerization times decrease with an increase in the rate constant for the homolysis of the C-ON bond between the polymer chain and the nitroxyl moiety, the factors influencing the cleavage rate constant are of considerable interest. Therefore, it was interesting to check whether the methyl effect (+17 kJ/mol for each methyl added onto the carbon atom of the C-ON bond) observed in the TEMPO-based alkoxyamine series was also observed in the SG1 [N-tert-butyl-N-(1-diethoxyphosphoryl-2,2-dimethyl-propyl)aminoxyl] series. Moreover, we extended the incremental substituent scale proposed earlier and confirmed the versatility of the multiparameter analysis developed in previous work. X-ray and natural bond orbital (NBO) analyses of several SG1-CHMeCOOR alkoxyamines showed that the difference in reactivity between the (RR/SS) and (RS/SR) diastereoisomers is caused by a n(sigma)->sigma* interaction between the n(sigma) lone pair of the oxygen atom of the ester bond and the sigma* orbital of the cleaved O-C bond. Furthermore, a compilation of the effects - steric, polar, stabilizing, long-distance polar and long-distance reverse steric - of the leaving alkyl radical on the value of k(d) for C-ON bond homolysis led us to question the one-step mechanism. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.