Paul Brémond

Chemically Triggered C–ON Bond Homolysis of Alkoxyamines. Quaternization of the Alkyl Fragment

The C-ON bond homolysis in alkoxyamine 2a can be chemically triggered by the protonation of the 4-pyridylalkyl fragment. The resulting 15-fold increase in k(d) (Chem. Commun. 2011, 47, 4291-4293) was investigated experimentally and theoretically by quaternization of the pyridyl moiety using methylating (MeOTs), acylating (AcCl), and benzylating (PhCH(2)Br) agents as well as by oxidation of the pyridyl moiety into N-oxide and by the formation of a dative bond with BH(3) as a Lewis acid.

Chemically Triggered C–ON Bond Homolysis in Alkoxyamines. Part 2: DFT Investigation and Application of the pH Effect on NMP

In recent work, a 15-fold increase in the C-ON bond homolysis rate constant kd of 4-pyridylethyl-SG1-based alkoxyamine was observed upon protonation of the pyridyl moiety in organic solvent. In this report, the pH dependence of kd (pKa = 4.7) is investigated in D2O/CD3OD (v/v 1:1). A 64-fold increase in kd is observed at acidic pH. Calculations show that the increase in kd upon protonation is due to both an increase in the stabilization of the protonated 4-pyridylethyl radical and an increase of the destabilization of the starting materials through an increase in the polarity of the alkyl fragment. This new alkoxyamine is applied to NMP of styrene and sodium styrene sulfonate.

Enzymatically Shifting Nitroxides for EPR Spectroscopy and Overhauser‐Enhanced Magnetic Resonance Imaging

In vivo investigations of enzymatic processes using non-invasive approaches are a long-lasting challenge. Recently, we showed that Overhauser-enhanced MRI is suitable to such a purpose. A β-phosphorylated nitroxide substrate prototype exhibiting keto-enol equilibrium upon enzymatic activity has been prepared. Upon enzymatic hydrolysis, a large variation of the phosphorus hyperfine coupling constant (Δa(P)=4 G) was observed. The enzymatic activities of several enzymes were conveniently monitored by electronic paramagnetic resonance (EPR). Using a 0.2 T MRI machine, in vitro and in vivo OMRI experiments were successfully performed, affording a 1200% enhanced MRI signal in vitro, and a 600% enhanced signal in vivo. These results highlight the enhanced imaging potential of these nitroxides upon specific enzymatic substrate-to-product conversion.

Alkoxyamines: toward a new family of theranostic agents against cancer.

Theranostics combines therapeutic and diagnostic or drug deposition monitoring abilities of suitable molecules. Here we describe the first steps of building an alkoxyamine-based theranostic agent against cancer. The labile alkoxyamine ALK-1 (t(1/2) = 50 min at 37 °C) cleaves spontaneously to generate (1) a highly reactive free alkyl radical used as therapeutic agents to induce cell damages leading to cell death and (2) a stable nitroxide used as contrast agent for Overhauser-enhanced magnetic resonance imaging (OMRI). The ALK-1 toxicity was studied extensively in vitro on the glioblastoma cell line U87-MG. Cell viability appeared to be dependent on ALK-1 concentration and on the time of the observation following alkoxyamine treatment. For instance, the LC50 at 72 h was 250 μM. Data showed that cell toxicity was specifically due to the in situ released alkyl radical. This radical induced oxidative stress, mitochondrial changes, and ultimately the U87 cell apoptosis. The nitroxide production, during the alkoxyamine homolysis, was monitored by OMRI, showing a progressive MRI signal enhancement to 6-fold concomitant to the ALK-1 homolysis. In conclusion, we have demonstrated for the first time that the alkoxyamines are promising molecules to build theranostic tools against solid tumors.

Chemically Triggered C–ON Bond Homolysis in Alkoxyamines. 6. Effect of the Counteranion

We showed (J. Org. Chem. 2012, 77, 9634) that the activation by methylation of pyridyl-based alkoxyamine 1 increased with the hydrogen bond donor properties of solvents. In this paper, activation of 1 by protonation with acids, CF3COOH and CSA, in tert-butylbenzene (t-BuPh) and in H2O/MeOH afforded, with CF3COOH, k(d) 28-fold larger in H2O/MeOH than in t-BuPh, whereas it was only 4-fold larger when CSA was used. This puzzling observation was ascribed to the dissociation of the intimate ion pair.

Trityl-based alkoxyamines as NMP controllers and spin-labels

Recently, new applications of trityl-nitroxide biradicals were proposed. In the present study, attachment of a trityl radical to alkoxyamines was performed for the first time. The rate constants kd of C-ON bond homolysis in these alkoxyamines were measured and found to be equal to those for alkoxyamines without trityl. The electron paramagnetic resonance (EPR) spectra of the products of alkoxyamine homolysis (trityl-TEMPO and trityl-SG1 biradicals) were recorded, and the corresponding exchange interactions were estimated. The decomposition of trityl-alkoxyamine showed more than an 80% yield of biradicals, meaning that the C-ON bond homolysis is the main reaction. The suitability of these labelled initiators/controllers for polymerisation was exemplified by means of successful nitroxide-mediated polymerisation (NMP) of styrene. Thus, this is the first report of a spin-labelled alkoxyamine suitable for NMP.

Intramolecular Hydrogen Bond in Alkoxyamines. Influence on the C–ON Bond Homolysis

The C-ON bond homolysis in alkoxyamines can be influenced by the presence of an intramolecular hydrogen bond (IHB) between the alkyl and the nitroxyl fragments, which leads to an 8-fold decrease in the homolysis rate constant k(d). When the IHB is disrupted by the solvent or by substitution of the hydrogen involved in the IHB by a protecting group (OMe, OAc, OBz, OBn, or OTBDMS), a higher homolysis rate constant k(d) is observed, as expected from the correlations developed by Marque (Bertin, D.; Gigmes, D.; Marque, S.; Tordo, P. Macromolecules 2005, 38, 2638-2650). Results were confirmed by DFT calculations at the B3LYP/6-31G(d,p) level.

Chemically triggered C-ON bond homolysis of alkoxyamines. 8. Quaternization and steric effects.

The C-ON bond homolysis in alkoxyamine 2a was chemically triggered by quaternization of the 1-(pyridin-2-yl)ethyl fragment using protonation, acylation, and oxidation into the N-oxide. The solvent effect was also investigated, and DFT calculations were performed to explore this chemical activation. Alkoxyamines 2a-d were also compared to the 1-(pyridin-4-yl)ethyl analogues 3a-d.

β-Phosphorus hyperfine coupling constant in nitroxides: 5. Solvent effect

Recently, we published the titration of water in organic solvents and conversely using this family of nitroxides (Org. Biomol. Chem. 2015 ASAP). In this article, we show that the aβ,P of persistent cyclic β-phosphorylated nitroxides decreases with the normalized polarity Reichardts constant ENT. Koppel–Palm and Kalmet–Abboud–Taft relationships were applied to get a deeper insight into the effects influencing aN and aβ,P: polarity/polarizability, hydrogen bond donor property, and structuredness of the cybotactic region.

Solvent effect in β-phosphorylated nitroxides. Part 4: detection of traces of water by electron paramagnetic resonance.

For decades, the nitrogene hyperfine coupling constant aN of nitroxides has been applied to probe their environment using EPR. However, the small changes observed (≈2 G from n-pentane to water) with the solvent polarity allow only a qualitative discussion. A stable β-phosphorylated nitroxide exhibiting a small change in aN (≈3 G from n-pentane to water) and a striking change (≈25 G from n-pentane to water) in phosphorus hyperfine coupling constant aP with the polarity of solvent was prepared and used to develop the first procedure for the titration of water in THF by EPR, down to 0.1% v/v.