Intramolecular hydrogen bond in alkoxyamines. Influence on the C-ON bond homolysis

Enzymatic triggering of C–ON bond homolysis of alkoxyamines

Alkoxyamine 1 is selectively hydrolyzed by chymotrypsin and substilisin A into alkoxyamine 2H+ for which C–ON bond homolysis occurred with a 4-fold increase in rate constants compared to 1 while non-specific proteases had no effect.

How intramolecular hydrogen bonding (IHB) controls the C-ON bond homolysis in alkoxyamines

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_d of alkoxyamine R₁R₂NOR₃ led us to re-investigate the antagonistic effect of intramolecular hydrogen-bonding (IHB) on k_d. 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 k_d, whereas IHB within the fragment (intra IHB) moderately increases k_d. 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.

C-ON Bond Homolysis of Alkoxyamines, Part 11: Activation of the Nitroxyl Fragment

A few years ago, Bagryanskaya and colleagues (J. Org. Chem. 2011) showed that protonation of the nitroxyl fragment deactivated the alkoxyamine C-ON bond. Conversely, our group showed that protonation (Chem. Commun. 2011), as well as other chemical reactions such as oxidation or amine quaternization (Org. Lett. 2012), of the pyridyl moiety carried by the alkyl fragment was suitable to activate the homolysis of the C-ON bond. To pursue our goal of applying alkoxyamines as theranostic agents (Org. Biomol. Chem. 2014 and Mol. Pharmaceutics 2014) by activation of the C-ON bond homolysis, we turned our interest to the chemical activation of the nitroxyl fragment by oxidation/reduction of selected functions. Conversion of a hydroxyl group located close to the nitroxyl moiety successively into aldehyde, then acid, and eventually into ester, led to a successive decrease in kd.

C–ON bond homolysis of alkoxyamines triggered by paramagnetic copper(ii) salts

Pyridine-based alkoxyamines were used as ligands for Cu(hfac)₂ to prepare the first metallic complexes of alkoxyamines. Structures of complexes were determined by X-ray analysis and a 21-fold increase in the C–ON bond homolysis was observed.

On the structure–control relationship of amide-functionalized SG1-based alkoxyamines for nitroxide-mediated polymerization and conjugation

Investigation on the reactivity and the controlling ability of a series of amide-functionalized alkoxyamines based on the SG1 nitroxide is reported.