Chemically triggered C–ON bond homolysis in alkoxyamines. Part 7. Remote polar effect

In a recent work (Org. Lett. 2012, 14, 358–361), we showed that the activation by benzylation of alkoxyamine 1 (diethyl (1‐(tert‐butyl(1‐(pyridin‐4‐yl)ethoxy)amino)‐2,2‐dimethylpropyl)phosphonate) afforded a surprisingly large C–ON bond homolysis rate constant k_d. Taking advantage of the easy preparation of para‐X‐benzyl‐activated alkoxyamines 2 and of the presence of a shielding methylene group between the two aromatic moieties, we investigated the long range (10 bonds between the X group and the C–ON bond) polar effect for X = H, F, OMe, CN, NO₂, NMe₂, ⁺NHMe₂,Br^?. It was observed that the effect was weak (4‐fold) and mainly due to the zwiterionic mesomeric forms generated by the presence of group X on the para position, i.e. k_d increased for CN and NO₂ and decreased for OMe, NMe₂ and ⁺NMe₂H,Br^?. DFT calculations at the B3LYP/6‐31G(d,p) level were performed to determine orbital interactions (natural bond orbital (NBO) analysis), Mulliken and NBO charges which support the reactivity described. Copyright © 2014 John Wiley & Sons, Ltd.