C-ON bond homolysis of alkoxyamines: when too high polarity is detrimental

A novel method of alkoxyamine homolysis activation via photochemical rearrangement

We proposed the nitrone-oxaziridine rearrangement as a novel method for photochemical activation for the homolysis of alkoxyamine in nitroxide-mediated polymerization. The photoisomerization of the aldo-/ketonitrone-group into the oxaziridine one in 2,5-dihydroimidazole 3-oxide-based alkoxyamines was studied; the products of photolysis have been identified, and quantum yields were measured. Conversion of the nitrone group into the oxaziridine one was found to decrease the activation energy of alkoxyamine homolysis by ca. 10 kJ mol-1.

Smart Alkoxyamines: A New Tool for Smart Applications

In 1986, Rizzardo et al. discovered the nitroxide-mediated polymerization which relies on the reversibility of homolysis of the C-ON bond of alkoxyamine R¹R²NOR³, a unique property of these molecules. This discovery has generated a tremendous endeavor in the field of polymer chemistry. Alkoxyamines have been used as initiators/controllers for nitroxide-mediated polymerization. Moreover, photoexcitable alkoxyamines that dissociate under light at different wavelengths have also been developed for polymer chemistry. Over the past few years, alkoxyamines have started to be used in materials sciences. In many cases (e.g., self-healing polymers), the development of smart materials requires the use of smart building blocks, that is, molecules or systems whose properties and/or structures change upon external stimuli. Alkoxyamines exhibit a unique property: reversible homolysis (i.e., homolysis of the C-ON bond into alkyl R³• and nitroxyl R¹R²NO• radicals and reformation via the coupling of these two species). Until now, this property has been controlled only by changes in temperatures or by light irradiation. Chemical and/or biochemical control of the homolysis event would open new gates for the application of these molecules in different fields such as biology and medicine. Thus, the concept of smart alkoxyamines is discussed and exemplified via the activation of alkoxyamines using chemical or/and biochemical changes amplifying the polar, steric, and stabilization effects. In situ activation is also discussed. It is shown that (i) increasing the electron-withdrawing properties of the alkyl fragment weakens the C-ON bond and thus favors homolysis but is opposite for the nitroxyl fragment; (ii) increasing the steric hindrance on the nonactive site affords dramatic conformation changes which weaken the C-ON bond; and (iii) increasing the stabilization of the released alkyl radical weakens the C-ON bond. Solvent effects and intramolecular hydrogen bonding are also discussed. Reactions used to highlight our purpose are either reversible or nonreversible and used under conditions that are as mild as possible (temperatures below 40 °C and atmospheric pressure). For example, a several (thousands of millions of) millions of orders of magnitude enhancement of the homolysis rate constant is observed upon enzymatic hydrolysis at 37 °C, meaning that a shift from a stable alkoxyamine (t_1/2 = 42 000 milleniums) to a highly labile alkoxyamine (t_max = 1500 s for 35% conversion) is achieved. Applications of this concept are discussed for safe NMP initiators and for theranostic agents.

Alkoxyamines Designed as Potential Drugs against Plasmodium and Schistosoma Parasites

Malaria and schistosomiasis are major infectious causes of morbidity and mortality in the tropical and sub-tropical areas. Due to the widespread drug resistance of the parasites, the availability of new efficient and affordable drugs for these endemic pathologies is now a critical public health issue. In this study, we report the design, the synthesis and the preliminary biological evaluation of a series of alkoxyamine derivatives as potential drugs against Plasmodium and Schistosoma parasites. The compounds (RS/SR)-2F, (RR/SS)-2F, and 8F, having IC₅₀ values in nanomolar range against drug-resistant P. falciparum strains, but also five other alkoxyamines, inducing the death of all adult worms of S. mansoni in only 1 h, can be considered as interesting chemical starting points of the series for improvement of the activity, and further structure activity, relationship studies. Moreover, investigation of the mode of action and the rate constants k_d for C-ON bond homolysis of new alkoxyamines is reported, showing a possible alkyl radical mediated biological activity. A theoretical chemistry study allowed us to design new structures of alkoxyamines in order to improve the selectivity index of these drugs.

2-Butyl-2-tert-butyl-5,5-diethylpyrrolidine-1-oxyls: Synthesis and Properties

Nitroxides are broadly used as molecular probes and labels in biophysics, structural biology, and biomedical research. Resistance of a nitroxide group bearing an unpaired electron to chemical reduction with low-molecular-weight antioxidants and enzymatic systems is of critical importance for these applications. The redox properties of nitroxides are known to depend on the ring size (for cyclic nitroxides) and electronic and steric effects of the substituents. Here, two highly strained nitroxides, 5-(tert-butyl)-5-butyl-2,2-diethyl-3-hydroxypyrrolidin-1-oxyl (4) and 2-(tert-butyl)-2-butyl-5,5-diethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (5), were prepared via a reaction of the corresponding 2-tert-butyl-1-pyrroline 1-oxides with butyllithium. Thermal stability and kinetics of reduction of the new nitroxides by ascorbic acid were studied. Nitroxide 5 showed the highest resistance to reduction.

An enzymatic acetal/hemiacetal conversion for the physiological temperature activation of the alkoxyamine C–ON bond homolysis

Enzymatic trigger. Upon enzymatic hydrolysis by Subtilisin A, highly stable alkoxyamines are transformed into highly labile alkoxyamines able to homolyze spontaneously in less than 500 seconds, at 37 °C.

Smart Control of Nitroxide-Mediated Polymerization Initiators' Reactivity by pH, Complexation with Metals, and Chemical Transformations

Because alkoxyamines are employed in a number of important applications, such as nitroxide-mediated polymerization, radical chemistry, redox chemistry, and catalysis, research into their reactivity is especially important. Typically, the rate of alkoxyamine homolysis is strongly dependent on temperature. Nonetheless, thermal regulation of such reactions is not always optimal. This review describes various ways to reversibly change the rate of C–ON bond homolysis of alkoxyamines at constant temperature. The major methods influencing C–ON bond homolysis without alteration of temperature are protonation of functional groups in an alkoxyamine, formation of metal–alkoxyamine complexes, and chemical transformation of alkoxyamines. Depending on the structure of an alkoxyamine, these approaches can have a significant effect on the homolysis rate constant, by a factor of up to 30, and can shorten the half-lifetime from days to seconds. These methods open new prospects for the application of alkoxyamines in biology and increase the safety of (and control over) the nitroxide-mediated polymerization method.

Why can Dispolreg 007 control the nitroxide mediated polymerization of methacrylates?

The alkoxyamine 3-(((2-cyanopropan-2-yl)oxy)(cyclohexyl)amino)-2,2-dimethyl-3-phenylpropanenitrile (Dispolreg 007) has recently emerged as a robust regulator for the controlled (co)polymerization of methacrylates and styrene by nitroxide mediated polymerization.

How intramolecular coordination bonding (ICB) controls the homolysis of the C–ON bond in alkoxyamines

Because the C–ON bond homolysis rate constant k_d is an essential parameter of alkoxyamine reactivity, it is especially important to tune k_d without a major alteration of the structure of the molecule. Recently, several approaches have become known, e.g., protonation of functional groups and formation of metal complexes. In this paper, coordination reactions of [Zn(hfac)₂(H₂O)₂] with a series of new SG1-based alkoxyamines affording complexes with different structures are presented. The k_d values of the complexed forms of the alkoxyamines were compared to those of free and protonated ones to reveal the contribution of the electron-withdrawing property and structure stabilization. Together with previously published data, this work provides clues to the design of alkoxyamines that can be effectively activated upon coordination with metal ions. Furthermore, our results provide insight into the mechanism underlying the influence of complexation on the reactivity of alkoxyamines. This led us to describe different types of coordination: intramolecular in nitroxyl fragment, intramolecular in alkyl fragment, intramolecular between alkyl and nitroxyl fragment, and intermolecular one. All of them exhibit different trends which are dramatically altered by changes in conformation.

Because the C–ON bond homolysis rate constant k_d is an essential parameter of alkoxyamine reactivity, it is especially important to tune k_d without a major alteration of the structure of the molecule.

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.