Decomposition of model alkoxyamines in simple and polymerizing systems. I. 2,2,6,6‐tetramethylpiperidinyl‐ N‐oxyl‐based compounds

The thermal decomposition of five alkoxyamines labeled TEMPO–R, where TEMPO was 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl and R was cumyl (Cum), 2‐tert‐butoxy‐carbonyl‐2‐propyl (PEst), phenylethyl (PhEt), 1‐tert‐butoxy‐carbonylethyl (EEst), or 1‐methoxycarbonyl‐3‐methyl‐3‐phenylbutyl (Acrylate‐Cum), was studied with ¹H NMR in the absence and presence of styrene and methyl methacrylate. The major products were alkenes and the hydroxylamine 1‐hydroxy‐2,2,6,6‐tetramethyl‐ piperidine (TEMPOH), and in monomer‐containing solutions, unimeric and polymeric alkoxyamines and alkenes were also found. Furthermore, the reactions between TEMPO and the radicals EEst and PEst were studied with chemically induced dynamic nuclear polarization. In comparison with coupling, TEMPO reacted with the radicals Cum, PEst, PhEt, and EEst and their unimeric styrene adducts by disproportionation to alkenes and TEMPOH only to a minor extent (0.6–3%) but with the radical adducts to methyl methacrylate to a considerable degree (≥20%). Parallel to the radical cleavage, TEMPO–EEst (but not the other alkoxyamines or TEMPO–Acrylate‐Cum) underwent substantial nonradical decay. The consequences for TEMPO‐mediated living radical polymerizations are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3604–3621, 2001     

Hydrolysis of CpTiCl2(TEMPO) and its application on one-pot syntheses of CpTiCl(OR)2 complexes

The eta1-hydroxylamido half-titanocene complex, CpTiCl2(TEMPO) 1, hydrolyzes extremely efficiently to generate (CpTiClO)4 and the protonated hydroxylamine. The efficient hydrolysis chemistry provides selective syntheses of CpTi dialkoxide and diaryloxide complexes.     

Reduction-induced micellization of a diblock copolymer containing stable nitroxyl radicals

A novel micelle formation induced by reduction was attained using a diblock copolymer supporting 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Poly(4-vinylbenzyloxy-TEMPO)-block-polystyrene (PVTEMPO-b-PSt) showed ultraviolet (UV) absorption at 467 nm as λ _max based on the TEMPO radicals. As the phenylhydrazine was added to the copolymer solution in benzene, the UV absorbance decreased. The decrease in the absorbance suggested that the TEMPO radicals were reduced to the colorless hydroxylamine by phenylhydrazine. The PVTEMPO-b-PSt copolymer showed no self-assembly in benzene due to the nonselective solvent. A light scattering study demonstrated that the scattering intensity of the copolymer increased with a decrease in the UV absorbance. The hydrodynamic diameter of the copolymer rapidly increased with the addition of phenylhydrazine and became almost steady over the molar ratio of phenylhydrazine to the VTEMPO unit of 0.2. It was found that the hydroxylamine in the micelles reverted to the TEMPO radicals by oxidation with oxygen.     

Simultaneous evaluation of one‐electron reducing systems and radical reactions in cells by nitroxyl biradical as probe

In the present study, a novel probe for the simultaneous evaluation of one‐electron reducing systems (electron transport chain) and one‐electron oxidizing systems (free radical reactions) in cells by electron chemical detection was developed. Six‐membered cyclic nitroxyl radicals (2,2,6,6‐tetramethylpiperidine‐1‐oxyl; TEMPO series) are sensitive to one‐electron redox systems, generating the hydroxylamine form [TEMPO(H)] via one‐electron reduction, and the secondary amine form [TEMPO(N)] via one‐electron oxidation in the presence of thiols. In contrast, the sensitivities of five‐membered cyclic nitroxyl radicals (2,2,5,5‐tetramethylpyrrolidine‐1‐oxyl; PROXYL series) to the one‐electron redox systems are comparatively low. The electron chemical detector can detect 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), TEMPO(H) and PROXYL but not TEMPO(N). Therefore, nitroxyl biradical, TEMPO‐PROXYL, as a probe for the evaluation of one‐electron redox systems was employed. TEMPO‐PROXYL was synthesized by the conjunction of 4‐amino‐TEMPO with 3‐carboxyl‐PROXYL via the conventional dicyclohexyl carbodiimide reaction. TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were simultaneously quantified by HPLC with Coularray detection. Calibration curves for the quantification of TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were linear in the range from 80 nm to 80 μm , and the lowest quantification limit of each molecule was estimated to be <80 nm . The relative standard deviations at 0.8 and 80 μm were within 10% (n = 5). Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrogen atom transfer reactions of iron-porphyrin-imidazole complexes as models for histidine-ligated heme reactivity

Hydrogen atom transfer (HAT) reactions of the bis(histidine) cytochrome active site models (TPP)FeII(ImH)2 (FeIIImH) and (TPP)Fe(Im)(ImH) (FeIIIIm) have been examined in acetonitrile solvent (TPP = tetraphenylporphyrin, ImH = 4-methylimidazole). The ascorbate derivative 5,6-isopropylidine ascorbate, hydroquinone, and the hydroxylamine TEMPOH all rapidly add H* to FeIIIIm to give FeIIImH. Similarly, the phenoxyl radical 2,4,6-tBu3C6H2O* and excess TEMPO* each oxidize FeIIImH to give FeIIIIm. On the basis of redox potential, pKa, and equilibrium measurements, the N-H bond in FeIIImH was found to have a bond dissociation free energy (BDFE) of 70 +/- 2 kcal mol(-1). A hydrogen atom transfer mechanism (concerted transfer of e- and H+) is indicated based on data for the ascorbate and TEMPO* reactions.     

Supramolecular stabilization of hydroxylamine TEMPOH by complexation with an amphiphilic calixarene

In an ethanol/water mixture, the nitroxyl radical TEMPO abstracts a hydrogen atom from a phenolic OH group of the amphiphilic para-hexanoylcalix[4]arene, and the hydroxylamine TEMPOH formed yields a stable inclusion complex with another molecule of the calixarene.     

Investigations of thermal polymerization in the stable free‐radical polymerization of 2‐vinylnaphthalene

The feasibility of utilizing stable free‐radical polymerization (SFRP) in the synthesis of well‐defined poly(2‐vinylnaphthalene) homopolymers has been investigated. Efforts to control molecular weight by manipulating initiator concentration while maintaining a 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO):benzoyl peroxide (BPO) molar ratio of 1.2:1 proved unsuccessful. In addition, systematic variations of the TEMPO: BPO molar ratio did not result in narrow molecular weight distributions. In situ Fourier transform infrared spectroscopy (FTIR) indicated that the rate of monomer disappearance under SFRP and thermal conditions were identical. This observation indicated a lack of control in the presence of the stable free radical, TEMPO. The similarities in chemical structure between styrene and 2‐vinylnaphthalene suggested thermally initiated polymerization occurred via the Mayo mechanism. A kinetic analysis of the thermal polymerization of styrene and 2‐vinylnaphthalene suggested that the additional fused ring in 2‐vinylnaphthalene increased the propensity for thermal polymerization. The observed rate constant for thermal polymerization of 2‐vinylnaphthalene was determined using in situ FTIR spectroscopy and was one order of magnitude greater than styrene, assuming pseudo‐first‐order kinetics. Also, an Arrhenius analysis indicated that the activation energy for the thermal polymerization of 2‐vinylnaphthalene was 30 kJ/mol less than styrene. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 583–590, 2002; DOI 10.1002/pola.10131     

A Tr Esr study of the quenching of photoexcited dioxouranium (VI) salts by stable nitroxyl free radicals

TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U₂ ^2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO₂₊ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity.     

Generation of Threonine- and Azathreonine N-Carboxy Anhydrides from alpha-Hydroxy beta-Lactams Promoted by 2,2,6,6-Tetramethylpiperidinyl-1-oxyl (TEMPO) in Combination with Sodium Hypochlorite

A versatile one-pot oxidation-Baeyer-Villiger reaction sequence applied to alpha-hydroxy beta-lactams and promoted by 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) leads to alpha-amino acid N-carboxy anhydrides. The examples reported constitute the first application of TEMPO in a Baeyer-Villiger reaction and provide a way for peptide coupling from non alpha-amino acid precursors.     

Coordination of nitroxide and nitronyl-nitroxide organic radicals to trimeric perfluoro-o-phenylene mercury

The interaction of trimeric perfluoro-o-phenylene mercury (1) with TEMPO (1,1,5,5-tetramethylpentamethylene nitroxide) in CH2Cl2 leads to the formation of the 1:1 adduct [1.TEMPO] (2). The same reaction carried out with NIT-Ph (2-(phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) leads to the formation of either [1.NIT-Ph.1] (3) or [1*NIT-Ph]n (4), depending on the amount of NIT-Ph present in solution. Adducts 2, 3, and 4 have been fully characterized and their crystal structures determined. The solid-state structure of 2 contains molecules of [1*TEMPO] in which the nitroxide oxygen atom is triply coordinated to the mercury centers of 1. A similar situation is encountered in the structure of 3 where each oxygen atom of the NIT-Ph molecule interacts with the mercury centers of an adjacent molecule of 1. The structure of 4 consists of extended helical polymeric chains that contain alternating molecules of 1 and NIT-Ph. As in 2 and 3, the interactions responsible for the formation of these chains involve the triple coordination of the oxygen atoms of the NIT-Ph molecule to the mercury centers of 1. DFT calculations suggest that the bonding in adducts such as 2, 3, and 4 is most likely dominated by electrostatic rather than covalent interactions. In agreement with this view, magnetic susceptibility measurements carried out on these adducts indicate that 1 does not mediate significant coupling between organic radicals coordinated on either side of the trinuclear core.     

Reactions of Ethyl 3-(4-Oxo-4H-chromen-3-yl)prop-2-enoates with 1,2-Binucleophiles

Russian Journal of General Chemistry - Reactions of ethyl 3-(4-oxo-4H-chromen-3-yl)prop-2-enoates with 1,2-binucleophilic agents (hydrazine, phenylhydrazine, hydroxylamine) leads to the formation...     

Radical reactions of a stable N-heterocyclic silylene: EPR study and DFT calculation

Reaction of the stable silylene, 1,3-di-tert-butyl-1,3,2-diazasilol-2-ylidene, with the free radical sources TEMPO, Hg[P(O)(OPri)2]2, (CO)3CpM-MCp(CO)3 (M = W, Mo), (CO)5Re-Re(CO)5, and toluene leads to radical adducts. The EPR spectra of these radicals indicate that the unpaired electron is delocalized over the silicon-containing five-membered ring.     

Coordination chemistry of stable radicals: homolysis of a titanium-oxygen bond

Thermolysis of Cp2TiCl(TEMPO) (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl) at 60 degrees C in a benzene/CCl4 mixture generates Cp2TiCl2. Kinetic studies implicate a mechanism involving the reversible cleavage of a Ti-O bond to generate the TEMPO radical and Cp2TiCl, which is trapped by CCl4 to give Cp2TiCl2. The rate of this reaction is strongly inhibited by added TEMPO and increases with increasing CCl4 concentration, indicating that the coupling of TEMPO to Cp2TiCl is faster than chloride atom abstraction from CCl4.     

Large ground-state entropy changes for hydrogen atom transfer reactions of iron complexes

Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.     

Addition of stable nitroxide radical to stable divalent compounds of heavier group 14 elements

The reactions of stable cyclic dialkylgermylene 2 and dialkylstannylene 3 with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical (2 equiv) gave the corresponding 1:2 adducts 4 and 5, respectively, which were characterized by NMR, MS, and X-ray analyses. The kinetics of the stepwise addition of two TEMPO molecules to germylene 2 revealed that the initial addition of TEMPO to 2 was 1010 times slower than the second TEMPO addition to the resulting germyl radical. The origin of the rate difference was discussed on the basis of the qualitative perturbation theory. In contrast to the reactions of 2 and 3, the reaction of dialkylsilylene 1 with TEMPO gave an interesting 1,3-dioxadisiletane derivative.     

Controlled radical polymerization of styrene in the presence of nitronyl nitroxides

Bulk radical polymerization of styrene in the presence of nitronyl nitroxides (2-(4-substituted phenyl)-4,4,5,5-tetramethyl-4,5-dihydroimidazolyl-1-oxyl 3-oxide) was studied. All nitronyl nitroxides, like other nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine 1-oxyl radical (TEMPO), act as reversible radical scavengers. The efficiency of controlling the polymerization is affected by the substituent at the 4′-position. The efficiency increases with electron donating strength of 4′-substituents, at least at the beginning of the reaction. However, the thermal stability of nitronyl nitroxides decreases in the same order. Thus, TEMPO is more suitable than nitronyl nitroxides for controlled/“living” radical polymerization of styrene.     

2D sp2 Carbon-Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp² carbon-conjugated porphyrin-based covalent organic framework (Por-sp²c-COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light-emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π-conjugation of porphyrin linker leads to extensive absorption of red light; the sp² −C=C− double bonds linkage ensures the stability of Por-sp²c-COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por-sp²c-COF is pivotal for cooperative photocatalysis with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.     

牛血清白蛋白修饰的近红外荧光纳米金顺磁标记分子探针

以光稳定性良好、 Stokes位移大且可近红外发射的谷胱甘肽包裹纳米金(GSH-AuNPs)为发光载体, 以4-氨基-2,2,6,6-四甲基哌啶氮氧自由基(4-NH₂-TEMPO)作为顺磁标记基团, 对构建发光-顺磁双模式传感分子探针进行了研究; 以牛血清白蛋白(BSA)为表面修饰剂, 通过调节荧光纳米金的表面状态, 改善顺磁标记微环境, 获得了基于顺磁基团识别诱导信号传导的荧光-顺磁双模式响应型分子探针. 顺磁标记BSA修饰GSH-AuNPs形成弱荧光-强顺磁复合物(GSH-AuNPs@BSA-TEMPO), 复合物中顺磁基团TEMPO经抗坏血酸还原后呈现出荧光增强和顺磁信号减弱现象, 表现出对抗坏血酸浓度相关的荧光Off-on与顺磁On-off的双模式响应.     

Water‐proton relaxivity of hyperbranched polymers carrying TEMPO radicals

High water‐soluble hyperbranched poly(styrene) (HPS) polymers carrying stable 2, 2, 6, 6‐tetramethylpiperidine‐1‐oxyl (TEMPO) radicals, HPS‐N‐TEMPO, HPS‐Im‐TEMPO, and HPS‐Im‐(TEMPO)₂, were prepared in ca. 60% introducing yield. HPS‐N‐TEMPO and HPS‐Im‐TEMPO were determined to be nearly spherical shapes of the diameter of 2.4 ± 0.6 and 2.2 ± 0.6 nm, respectively, by transmission electron microscope (TEM) images. The values of water‐proton relaxivity, r₁, at 25 MHz, 0.59 T, and 25 °C were 6.0, 5.2, and 14 mM^?1 sec^?1 for HPS‐N‐TEMPO, HPS‐Im‐TEMPO, and HPS‐Im‐(TEMPO)₂, respectively. The spin‐lattice relaxation time (T₁)‐weighted images in phantom were also observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Ammonia Oxide as a Building Block for High-Performance and Insensitive Energetic Materials

3,5-Dinitrimino-1,2,4-triazole ( 2 ) with three protons has the potential of deprotonation to form energetic salts. Neutralization of 2 with 50 % hydroxylamine in varying molar ratios leads to the formation of the corresponding mono/dihydroxylammonium energetic salts. Additionally compound 5 , an ammonia oxide adduct of dihydroxylammonium 3,5-dinitramino-1,2,4-triazolate, was prepared when excess hydroxylamine was used. The structures of 3 – 5 are supported by single-crystal X-ray diffraction. The energetic properties of the new materials are competitive. Utilization of ammonia oxide adducts in hydroxylammonium energetic salts could lead to future practical applications as energetic materials.