Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin–spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1′-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents

Imidazoline and imidazolidine nitroxides as controlling agents in nitroxide-mediated pseudoliving radical polymerization

WOS:000431973300002Controlled, or pseudoliving, radical polymerization provides unique opportunities for the synthesis of structurally diverse polymers with a narrow molecular -weight distribution. These reactions occur under relatively mild conditions with broad tolerance to functional groups in the monomers. The nitroxide-mediated pseudoliving radical polymerization is of particular interest for the synthesis of polymers for biomedical applications. This review briefly describes one of the mechanisms of controlled radical polymerization. The studies dealing with the use of imidazoline and imidazolidine nitroxides as controlling agents for nitroxide-mediated pseudoliving radical polymerization of various monomers are summarized and analyzed. The publications addressing the key steps of the controlled radical polymerization in the presence of imidazoline and imidazolidine nitroxides and new approaches to nitroxide-mediated polymerization based on protonation of both nitroxides a nd monomers are considered

Alkoxyamine Re-Formation Reaction. Effects of the Nitroxide Fragment: A Multiparameter Analysis

International audienceA few years ago, Studer and co-workers (Macromolecules 2006, 39, 1347-1352) reported the dramatic effect of the reaction of re-formation of alkoxyamines on the fate of the nitroxide-mediated polymerization (NMP) of styrene. This prompted us to investigate more carefully the effects of the nitroxide structure on the re-formation rate constant k(c). Ten new values of k(c) were obtained for the reaction of imidozalidine nitroxide and the phenethyl radical. These values were combined with the 21 values of k(c) reported in the literature for a multiparameter analysis (log(k(c)/M-1 s(-1)) = (10.22 +/- 0.10) + (0.46 +/- 0.02)E-s + (0.41 +/- 0.17)sigma(1)) using the electrical Hammett constant sigma(1) to describe both the stabilization and polar effects as well as the modified Taft steric constant E-s of the nitroxide. The same analysis was performed for the k(c) values of the cross-coupling reaction of nitroxides with tert-butoxylcarbonyl-2-prop-2-yl radical (log(k(c)/M-1 s(-1)) = (11.10 +/- 0.25) + (0.57 +/- 0.05)E-s + (1.42 +/- 0.18)sigma(1)) and tert-butoxycarbonylethyl radical (log(k(c)/M-1 s(-1)) = (10.23 +/- 0.16) + (0.35 +/- 0.03)E-s + (0.93 +/- 0.25)sigma(1)). These correlations were applied for the analysis of the NMP of styrene controlled by 6 pi(.), 6 theta(.), and 6p(.) using a Fischer phase diagram

Intramolecular proton transfer (IPT) in alkoxyamine: a theoretical investigation

WOS:000322517800018International audienceThe Intramolecular Proton Transfer (IPT) in alkoxyamines is one of the main factors determining the process of Nitroxide Mediated Polymerization (NMP). Recently, we proposed an experimental approach to study IPT and applied it to a series of alkoxyamines. It was found that IPT dramatically depends on the structure of the alkoxyamine, but it was not clear which factors are significant for IPT (M. V. Edeleva et al., J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 6579-6595). To understand the mechanism and the factors determining the IPT process, in this article we investigate the geometrical parameters and thermokinetics of this reaction using the BMK/6-311++G(3df,3pd)//B3LYP/6-31+G(d,p) method. It was found that the thermokinetics and geometrical parameters of the transition state (TS) for IPT do not depend on the alkoxyamine structure. The only factor which determines the occurrence of IPT is the position of the TS energy level of the C-ON bond homolysis

Scavenging of Organic C‑Centered Radicals by Nitroxides

Scavenging of Organic C‑Centered Radicals by Nitroxide

The effect of the oxophilic Tb(III) cation on C-ON bond homolysis in alkoxyamines

WOS:000430904400002Recently, we reported on the activation of the C-ON bond homolysis in alkoxyamines R1R2NOR3 using the coordination of the alkyl fragment R-3 with metal cations Cu(II) and Zn(II). Here, we report the selective coordination of the diethylphosphoryl group carried by the nitroxyl fragment by the oxophilic metal cation Tb(III). Coordination on the nitroxyl fragment afford a slight 2-fold decrease in the C-ON bond homolysis rate constant kd. (C) 2018 Elsevier B.V. All rights reserved

Activity of phenoxy-imine titanium catalysts in ethylene polymerization : A quantum chemical approach

The mechanism of ethylene polymerization on phenoxy-imine (FI) titanium catalysts was studied theoretically to identify the major factors affecting the catalytic activity. Geometry optimizations of FI ligands, octahedral titanium dichloride complexes, active cationic species, and their π‐complexes with ethylene as well as calculations of the energy profile of chain propagation were performed at the BP86-D3 level. We found that the calculated energy gaps between frontier orbitals (HOMO and LUMO) in the active cations of the catalysts correlate with the experimental activity values. High activities of FI catalysts with α‐Cumyl groups were attributed to smaller HOMO-LUMO gaps due to hyperconjugation between π-systems of α‐Cumyl and (N‐aryl)salicylaldimine moieties in the active cations. The correlation provides a qualitative estimate of the catalytic activity for further design of new FI titanium complexes

Hydrogen-Bonding Effects for the C-ON Bond Homolysis and Reformation Reactions of Alkoxyamines

International audienceN-(2-methylpropyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-O-(2-carboxyprop-2-yl) hydroxylamine (BlocBuilder MA) is, among the commercially available alkoxyamines, one of the most efficient for nitroxide-mediated polymerization (NMP). However, recent results have shown that it does not perform well for the NMP of isoprene. The occurrence of intramolecular hydrogen bonding (IHB) between the carboxylic function and the diethoxyphosphoryl group has been proposed as the reason for its low efficiency. In this article, the presence of this IHB is confirmed using IR, P-31 NMR, P-31-H-1 HOESY, and DFT calculation results. The solvent effect on this IHB and consequently on k(d) values is also investigated. However, combining kinetic analysis and rate measurements in various solvents, the influence of this IHB on the C-ON bond homolysis and reformation in alkoxyamine is shown to be very weak