Abstract: The rate constants kd 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 kd from 1 to 3 was due to the compression (buttressing effect) of the reactive center by the second styryl moiety. The difference in kd for the diastereoisomer was clearly an activation entropy effect DS≠ 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 DS≠, and thus lowers kd.
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Blachon, A.; Marque, S.R.A.; Roubaud, V.; Siri, D. Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO. Polymers 2010, 2, 353-363.
Blachon A, Marque SRA, Roubaud V, Siri D. Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO. Polymers. 2010; 2(3):353-363.
Blachon, Alexandra; Marque, Sylvain R. A.; Roubaud, Valérie; Siri, Didier. 2010. "Diasteromeric Effect on the Homolysis of the C–ON Bond in Alkoxyamines: A DFT Investigation of 1,3-Diphenylbutyl-TEMPO." Polymers 2, no. 3: 353-363.