Recently, we examplified the activation of the C-ON bond homolysis by protonation, alkylation, benzylation, acylation, oxidation and complexation with a Lewis acid of the nitrogen atom of the 1-(pyridin-4-yl)ethyl fragment (Chem. Commun., 2011, 4291 and Org. Lett., 2012, 358) and of the 1-(pyridin-2-yl)ethyl fragment (J. Org. Chem. ASAP Doi:10.1021/jo401674v) of (N-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl) SG1-based alkoxyamines. The quaternization of the 1-(pyridin-3-yl)ethyl fragment by the aforementioned reactions was investigated for the corresponding SG1-based alkoxyamines. In sharp contrast to the quaternization at ortho and para positions of the pyridyl moiety, the effect of the quaternization at the meta position was weak. The effects of quaternization at ortho, meta and para positions were investigated through natural bond orbital and Mulliken charges, HOMO-LUMO interactions in the starting materials and the radical stabilization energy of the released 1-puridylmethyl radicals using DFT calculations with the B3LYP/6-31G(d) and UBMK/6-311+G(3df,2p)//R(O)B3LYP/6-31G(d) methods, respectively.

WOS:000334301600003International audienceIn 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)phosphon ate) 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 (c) 2014 John Wiley & Sons, Ltd