Abstract
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 kd of alkoxyamine R1R2NOR3 led us to re-investigate the antagonistic effect of intramolecular hydrogen-bonding (IHB) on kd. Here, IHB is investigated both in the nitroxyl fragment R1R2NO and in the alkyl fragment R3, 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 kd, whereas IHB within the fragment (intra IHB) moderately increases kd. 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.
Highlights
Ence of intramolecular hydrogen-bonding (IHB) on kd was investigated in tert-butylbenzene (t-BuPh) as a non-polar solvent and water as a polar/hydrogen bond acceptor (HBA) solvent, known to suppress IHB.[30] increase in kd (Fig. 4a)[24,25,26,27] and IHB from the alkyl fragment to the nitroxyl fragment affords a decrease in kd spanning from weak (2–3 kJ mol−1)[28] to moderate[29]
The same trends are observed for Δδ (Fig. 7 and Table 1) and 1H NMR signals of EtO protons for RS/SR diastereoisomers of 4 and 5 as those for diastereoisomers RR/SS and are ascribed to the occurrence of IHB between the diethylphosphoryl and the hydroxyl groups (Fig. 10)
The IHB observed between the hydroxyl group and the N atom of the nitroxyl moiety in the conformer C is not strong enough to balance the steric strain in C, which is less stable than A by 19 kJ mol−1 (Fig. 14)
Summary
Dation,[20] alkylation,[20] or metal cation coordination.[21,22] in our view, activation processes based on physicochemical events cannot be disregarded. Intramolecular hydrogen-bonding (IHB) displays very different trends, that is, IHB within the nitroxyl fragment affords an. Ence of IHB on kd was investigated in tert-butylbenzene (t-BuPh) as a non-polar solvent and water as a polar/hydrogen bond acceptor (HBA) solvent, known to suppress IHB.[30] increase in kd (Fig. 4a)[24,25,26,27] and IHB from the alkyl fragment to the nitroxyl fragment (interR, Fig. 4c) affords a decrease in kd spanning from weak (2–3 kJ mol−1)[28] to moderate[29] Other types of IHBs have not yet been investigated – IHB from the nitroxyl fragment to the alkyl fragment (interN, Fig. 4d) and IHB within the alkyl fragment (Fig. 4b) – and are the focus of this article.
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