Alkoxyamine C–ON Bond Homolysis: Stereoelectronic Effects

Abstract

AbstractAlkoxyamines 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‐dimethylpropyl)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 nσ→σ* interaction between the nσ lone pair of the oxygen atom of the ester bond and the σ* 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 kd for C–ON bond homolysis led us to question the one‐step mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)