Diasteromeric Effect on the Homolysis of the C-ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO

International audience; The rate constants k(d) of the homolysis of the C-ON bond in styryl dyads TEMPO-based alkoxyamines have recently been published (Li et al. Macromolecules 2006, 39, 9201). The diastereoisomers exhibited different values which were higher than for the unimer TEMPO-styryl alkoxyamine 1. At a first glance, the localization of the steric strain was not obvious. To decipher this problem, diastereoisomer models 2 (RR/SS) and 3 (RS/SR), as well as the released alkyl radicals, were calculated at the \\B3LYP/6-31G(d) level. It was revealed that the increase in k(d) from 1 to 3 was due to the compression (buttressing effect) of the reactive center by the second styryl moiety. The difference in k(d) for the diastereoisomer was clearly an activation entropy effect Delta S-not equal. because the alkyl fragment of the RS/SR diastereoismer exhibited the same conformation as the released radical whereas the conformation for the RR/SS diastereoisomer was quite different and thus required the rotation of several bonds to reach the correct TS, which cost Delta S-not equal, and thus lowers k(d).