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NMR and EPR Study of Homolysis of Diastereomeric Alkoxyamines

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NMR and EPR Study of Homolysis of Diastereomeric Alkoxyamines molecules Article NMR and EPR Study of Homolysis of Diastereomeric Alkoxyamines Sergey Cherkasov 1,2, Dmitriy Parkhomenko 1 , Alexander Genaev 1, Georgii Salnikov 1, Mariya Edeleva 1, Denis Morozov 1 , Tatyana Rybalova 1 , Igor Kirilyuk 1 , Sylvain R. A. Marque 3 and Elena Bagryanskaya 1,* 1 N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Pr. Lavrentjeva 9, 630090 Novosibirsk, Russia; [email protected] (S.C.); [email protected] (D.P.); [email protected] (A.G.); [email protected] (G.S.); [email protected] (M.E.); [email protected] (D.M.); [email protected] (T.R.); [email protected] (I.K.) 2 National Research University—Novosibirsk State University, 630090 Novosibirsk, Russia 3 Aix Marseille Univ, CNRS, ICR, UMR 7273, case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille CEDEX 20, France; [email protected] * Correspondence: [email protected] Academic Editor: Derek J. McPhee Received: 10 October 2020; Accepted: 30 October 2020; Published: 1 November 2020 Abstract: Three alkoxyamines based on imidazoline radicals with a pyridine functional group—potential initiators of nitroxide-mediated, controlled radical polymerization—were synthesized. Electron Paramagnetic Resonance (EPR) measurements reveal biexponential kinetics for the thermolysis for diastereomeric alkoxyamines and monoexponential kinetics for an achiral alkoxyamine. For comparison, the thermolysis of all three alkoxyamines was studied by NMR in the presence of three different scavengers, namely tetramethylpiperidine-N-oxyl (TEMPO), thiophenol (PhSH), and β-mercaptoethanol (BME), and detailed analysis of products was performed. NMR differentiates between N-inversion, epimerization, and homolysis reactions. The choice of scavenger is crucial for making a reliable and accurate estimate of the true homolysis rate constant. Keywords: nitroxide; alkoxyamine; nitroxide mediated polymerization; kinetics; scavenger; imidazoline radical; homolysis; nitrogen inversion; chirality; stereoisomerization 1. Introduction Alkoxyamines have been used for three decades [1] as initiators in Nitroxide-Mediated Polymerization (NMP) [2–15]. The main parameters determining the applicability of alkoxyamines for NMP are the rate constants of their homolysis, kd, the rate of recombination of the corresponding alkyl and nitroxide radicals, kc (Scheme1), and the absence of substantial side reactions. Each of the kd and kc rate constants depend on the structure of both the nitroxide and the alkyl radicals. The equilibrium constants vary over a wide range depending on the steric and/or electronic effects of substituents in both the nitroxyl [16–19] and the alkyl parts [19–22]. In addition, the alkoxyamine homolysis parameters can be varied via the introduction of functional groups capable of reversible protonation [23–25], hydrogen bonding [26] or coordination with metals [21,27–29] (for example, pyridine or OH groups) in so-called “switchable” alkoxyamines [30–32]; another such factor is light in photo-cleavable alkoxyamines [33]. For example, the introduction of a pyridine group into the alkoxyamine produced up to a 30-fold increase or decrease of kd upon complexation [21,28–30]. In the present work, we investigate the homolysis of alkoxyamines composed of imidazoline radicals with a pyridine functional group capable of complexation with metals (Chart1). Sterically hindered imidazoline radicals have shown their applicability to NMPs [34]. However, Molecules 2020, 25, 5080; doi:10.3390/molecules25215080 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 20 Molecules 2020, 25, 5080 2 of 20 the introduction of bulky groups often leads to chiral alkoxyamines, which raises the question of whether the rate kd and kc constants can be different for different stereoisomers. In particular, Ananchenko et al. studiedScheme tetramethylpiperidine-N-oxyl 1. C–ON bond homolysis in (TEMPO) alkoxyamines. and N-(2-methylpropan-2-yl)- N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl)-based alkoxyamines by 1H and 31P NMR spectroscopyIn the present [35–37 work,]. They we showed investigate that the the diastereomeric homolysis of alkoxyamines excess upon homolysis composed and of reformationimidazoline radicalsof the diastereomeric with a pyridine alkoxyamines functional group depends capable strongly of complexation on the structure with of metals both the(Chart nitroxyl 1). Sterically and the hinderedreleased alkyl imidazoline part. radicals have shown their applicability to NMPs [34]. However, the Molecules 2020, 25, x FOR PEER REVIEW 2 of 20 introduction of bulky groups often leads to chiral alkoxyamines, which raises the question of whether the rate kd and kc constants can be different for different stereoisomers. In particular, Ananchenko et al. studied tetramethylpiperidine-N-oxyl (TEMPO) and N-(2-methylpropan-2-yl)-N-(1- diethylphosphono-2,2-dimethylpropyl)-aminoxyl)-based alkoxyamines by 1H and 31P NMR spectroscopy [35–37]. They showed that the diastereomeric excess upon homolysis and reformation of the diastereomeric alkoxyamines depends strongly on the structure of both the nitroxyl and the released alkyl part. SchemeScheme 1.1. C–ONC–ON bondbond homolysishomolysis inin alkoxyamines.alkoxyamines. In the present work, we investigate the homolysis of alkoxyamines composed of imidazoline radicals with a pyridine functional group capable of complexation with metals (Chart 1). Sterically hindered imidazoline radicals have shown their applicability to NMPs [34]. However, the introduction of bulky groups often leads to chiral alkoxyamines, which raises the question of whether the rate kd and kc constants can be different for different stereoisomers. In particular, Ananchenko et al. studied tetramethylpiperidine-N-oxyl (TEMPO) and N-(2-methylpropan-2-yl)-N-(1- diethylphosphono-2,2-dimethylpropyl)-aminoxyl)-based alkoxyamines by 1H and 31P NMR spectroscopy [35–37]. They showed that the diastereomeric excess upon homolysis and reformation of the diastereomeric alkoxyamines depends strongly on the structure of both the nitroxyl and the released alkyl part. • RRRR/SS/SS RSRS/RS/RS Chart 1.1. StructuresStructures ofof thethenitroxide nitroxide ( 1(1•),), alkoxyamines alkoxyamines ( 2(2 , 22 , ,3),3), and and scavengers scavengers (TEMPO, (TEMPO, thiophenol (PhSH), β-mercaptoethanol-mercaptoethanol (BME)). The coupling of a chiral nitroxyl radical with a prochiral alkyl radical produces an alkoxyamine exhibiting two chiral centers: one on the the alkyl alkyl fragment fragment and and one one on on the the nitroxyl nitroxyl fragment. fragment. In In addition, addition, the NN-atom-atom could could be be considered considered as a as third a third pseudo-chiral pseudo-c centerhiral becausecenter because of its three of diitsfferent three substituent different substituentgroups, with groups, its electron with its lone electron pair as lone the fourthpair as group.the fourth The group. chiral nitrogenThe chiral implies nitrogen the implies possibility the possibilityof configuration of configuration inversion at inversion the N-atom at the through N-atom a flip-flop through or a inversionflip-flop or process. inversion These process. three These chiral threecenters chiral could centers undergo could two independentundergo two processes: independent (a) nitrogen processes: inversion (a) nitrogen and (b) stereoisomerizationinversion and (b) stereoisomerizationof the alkyl part following of the alkoxyaminealkyl part following homolysis alkoxyamine and recombination homolysis (Scheme and recombination2). Hereafter, ( weScheme will 2refer). Hereafter, to this second we will process refer to as this epimerization. second process as epimerization. TheChart homolysis 1. Structures of of alkoxyaminesthe nitroxide (1• commonly), alkoxyamines has (2RR/SS a monoexponential, 2RS/RS, 3), and scavengers time (TEMPO, dependence, and consequently,thiophenol (PhSH), EPR β-mercaptoethanol is often used to (BME)). study the kinetics of the homolysis reaction. In such experiments, the oxygen dissolved in the sample is used as a scavenger for the alkyl radicals, so that the increasingThe coupling nitroxyl of radical a chiral concentration nitroxyl radical reveals with the a kineticsprochiral of alkyl the homolysis radical produces reaction an [38 alkoxyamine]. However, thisexhibiting approach two is chiral not useful centers: for one the on more the complicated,alkyl fragment non-monoexponential and one on the nitroxyl kinetics fragment. caused In addition, by side reactions.the N-atom In suchcould cases, be considered NMR spectroscopy as a third is usedpseudo-c to detecthiral and center monitor because the reactionof its three intermediates different andsubstituent final products groups, [ 39with–41 ].its Here, electron we lone used pair three as di thefferent fourth scavengers, group. The namely chiral TEMPO,nitrogen thiophenolimplies the (PhSH),possibility and ofβ -mercaptoethanolconfiguration inversion (BME) at (Chart the N1),-atom to show through the importance a flip-flop ofor theinversion selection process. of a radical These trapthree for chiral NMR centers measurements could undergo of different two alkoxyamines.independent processes: We examine (a) hownitrogen various inversion processes and can(b) stereoisomerization of the alkyl part following alkoxyamine homolysis and recombination (Scheme 2). Hereafter, we will refer to this second process as epimerization. Molecules 2020, 25, 5080 3 of 20 manifest themselves and show that only a careful analysis
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