Recent amazing results (Nkolo et al., Org. Biomol. Chem., 2017, 6167) on the effect of solvents and polarity on the C-ON bond homolysis rate constants k of alkoxyamine RRNOR led us to re-investigate the antagonistic effect of intramolecular hydrogen-bonding (IHB) on k. Here, IHB is investigated both in the nitroxyl fragment RRNO and in the alkyl fragment R, as well as between fragments, that is, the donating group on the alkyl fragment and the accepting group on the nitroxyl fragment, and conversely. It appears that IHB between fragments (inter IHB) strikingly decreases the homolysis rate constant k, whereas IHB within the fragment (intra IHB) moderately increases k. For one alkoxyamine, the simultaneous occurrence of IHB within the nitroxyl fragment and between fragments is reported. The protonation effect is weaker in the presence than in the absence of IHB. A moderate solvent effect is also observed.

Throughout the last decade, the effect of electron withdrawing groups (EWGs) has been known to play a role - minor or moderate depending on the nitroxyl fragment RRNO - in the change in the homolysis rate constant (k) for C-ON bond homolysis in alkoxyamines (RRNOR). It has been shown that the effect of EWGs on k is described by a linear relationship with the electrical Hammett constant σ. Since then, linear multi-parameter relationships f(σ,ν,σ) have been developed to account for the effects involved in the changes in k, which are the stabilization of the released radical (σ) and the bulkiness (ν) and polarity (σ) of the alkyl fragment. Since a decade ago, new results have been published highlighting the limits of such correlations. In this article, previous multi-parameter linear relationships are amended using a parabolic model, i.e. (σ - σ), to describe the effect of EWGs in the alkyl fragment on k. In contrast to previous studies, these improved linear multi-parameter relationships f(σ,ν,Δσ) are able to account for the presence of several EWGs on the alkyl fragment, R. An unexpectedly strong solvent effect - a ca. 1500-fold increase in k - from tert-butylbenzene to the water/methanol mixture is also observed for 3-((2,2,6,6-tetramethylpiperidin-1-yl)oxyl)pentane-2,4-dione 1b in comparison to a ca. 5-fold increase in k that is generally observed.