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.     

Deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of styrene: A comparative study with the 2,2,6,6‐tetramethyl‐1‐piperidinyloxy/acrylonitrile system

The competitiveness of the combination and disproportionation reactions between a 1‐phenylpropyl radical, standing for a growing polystyryl macroradical, and a 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) radical in the nitroxide‐mediated free‐radical polymerization of styrene was quantitatively evaluated by the study of the transition geometry and the potential energy profiles for the competing reactions with the use of quantum‐mechanical calculations at the density functional theory (DFT) UB3‐LYP/6‐311+G(3df, 2p)//(unrestricted) Austin Model 1 level of theory. The search for transition geometries resulted in six and two transition structures for the radical combination and disproportionation reactions, respectively. The former transition structures, mainly differing in the out‐of‐plane angle of the N? O bond in the transition structure TEMPO molecule, were correlated with the activation energy, which was determined to be in the range of 8.4–19.4 kcal mol^?1 from a single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//unrestricted Austin Model 1 level. The calculated activation energy for the disproportionation reaction was less favorable by a value of more than 30 kcal mol^?1 in comparison with that for the combination reaction. The approximate barrier difference for the TEMPO addition and disproportionation reaction was slightly smaller for the styrene polymerization system than for the acrylonitrile polymerization system, thus indicating that a β‐proton abstraction through a TEMPO radical from the polymer backbone could diminish control over the radical polymerization of styrene with the nitroxide even more than in the latter system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 232–241, 2007     

Bis(ketenyl)benzenes: preparation,observation, and reaction with tetramethylpiperidin-1-yloxyl

1,2- and 1,3-Bis(ketenyl)benzenes formed by double dehydrochlorination and by double Wolff rearrangement, respectively, gave ketenyl IR absorption at 2115, and 2122, and 2116 cm^–1, respectively. Reaction of these bisketenes with the aminoxyl radical tetramethylpiperidin-1-yloxyl (TEMPO) gave the corresponding tetraadducts as mixtures of meso- and d,l-isomers. The kinetics of the reaction of 1,3-bis(ketenyl)benzene with TEMPO gave a rate constant comparable to that of the monoketene PhCH=C=O. The reactions proceed by the initial attack of TEMPO on the carbonyl carbon of one ketenyl group followed by fast capture of the intermediate radical by a second TEMPO, and then reaction of the remaining ketene.     

Rapid Dihydrogen Cleavage by Persistent Nitroxide Radicals under Frustrated Lewis Pair Conditions

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C₆F₅)₃ boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H₂ splitting reaction path.     

Thermal self‐initiation of styrene in the presence of TEMPO radicals: Bulk and miniemulsion

In TEMPO (2,2,6,6,‐tetramethyl‐1‐piperidinyloxy) controlled styrene radical polymerizations, the thermal self‐initiation reaction of styrene monomer is one of the main sources for the deviations from ideal living polymerization. However, it is also important because it continuously generates radicals to compensate for the loss of radicals caused by the termination reactions and thereby maintains a reasonable reaction rate. In this report, different initial TEMPO concentrations were used in styrene miniemulsion polymerizations without any added initiator. The consumption rate of TEMPO or radical generation rate was calculated from the length of the induction period and the increasing total number of polymer chains. It was found that there is little difference between the miniemulsions and the corresponding bulk systems in terms of the length of the induction period, which increases linearly with initial TEMPO concentration. After the induction period, the consumption rate of TEMPO or radical generation rate was reduced to a lower level, and a faster initial polymerization rate was found in the bulk system compared to the corresponding miniemulsion system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4921–4932, 2004     

Optimization of cellouronic acid synthesis by TEMPO-mediated oxidation of cellulose III from sugar beet pulp

Microfibrillated cellulose from purified sugar beet pulp was converted into cellulose III by immersion in liquid ammonia. When freed from ammonia, this product was oxidized in water at pH-10 using NaBr, NaOCl and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) under various conditions and concentrations. The resulting water-soluble cellouronic acid—i.e. cellulose oxidized at the C6 position- was analyzed by high performance size exclusion chromatography (HPSEC) together with ¹³C NMR spectroscopy. The oxidation parameters, namely reaction time, temperature, NaBr and TEMPO concentrations were varied to determine the optimum reaction conditions. A low TEMPO concentration, a rather fast reaction time and the conducting of the oxidation at 0 °C were critical to obtain pure cellouronic acid in high yield, high purity and high DP.     

Reaction of several resveratrol glycoside derivatives with hypochlorites in various media

The reactions of pterostilbenoside (trans-3,5-dimethoxystilben-4′-O-β-D-glucoside) and Ar–O–Tr derivatives of resveratroloside (3,5-dihydroxystilben-4′-O-β-D-glucoside) and pinostilbenoside (3-methoxy-5-hydroxystilben-4′-O-β-D-glucoside) with NaOCl and t-BuOCl in the presence of the stable nitroxyl radical TEMPO were studied in various media. It was found that the principal product of pterostilbenoside transformation was its 2,6-dichloroderivative, a part of which was oxidized to form 2,6-dichloropterostilbene glucuronide. Trityl ethers of resveratroloside and pinostilbenoside reacted with the hypochlorites to form mixtures of products.     

Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds--based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)--with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONE via an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.     

Photochromism of dihydroindolizines. Part XV: Synthesis and photophysical properties of dihydroindolizine photoswitches bearing a conjugated aryleneethynylene tripodal linker system

Abstract  

Different photochromic dihydroindolizines (DHIs) bearing conjugated aryleneethynylene tripodal linker systems were synthesized using several Sonogashira coupling reactions. The spirocyclopropene precursors incorporating different acetylenic bridge moieties at the 2-position of the fluorene moiety were synthesized via chemical and photochemical routes. Multiaddressable photochromic properties of the DHI derivatives substituted in the fluorene (region A) and pyridazine (region C) parts were studied. Optimization of the formation of the DHIs was also done by applying different palladium-mediated Sonogashira coupling reactions. Irradiation of the photochromic DHIs with polychromatic light led to colored betaines which undergo thermal 1,5-electrocyclization. The kinetics of the thermal 1,5-electrocyclization were studied by using a multichannel FT–UV–Vis spectrophotometer. A pronounced effect on the kinetic behavior of the 1,5-electrocyclization process of the betaines was observed by changing substitution from non-substituted to dimethyl-substituted pyridazines. Photodegradation experiments and the bleaching and fading cycles revealed high photostability of the betaines under investigation. These properties of betaines of tripodal linker conjugates will help these materials to find applications.     

Laccase-catalyzed mediated oxidation of benzyl alcohol: the role of TEMPO and formation of products including benzonitrile studied by nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Substituted benzyl alcohol was oxidized enzymatically with a laccase-mediator system and the products were investigated as a function of time by nanoelectrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (nanoESI-FTICRMS). With Trametes versicolor laccase (TVL), the mediator, 2,2',6,6'-tetramethylpiperidine-N-oxyl radical (TEMPO), undergoes oxidation and forms oxoammonium ion. Oxidized TEMPO oxidizes the alcohol and is simultaneously reduced to the N-OH form. The laccase then restores TEMPO back to the normal radical form and the oxidation cycle starts again. The role of TEMPO and the structures of its oxidized and reduced forms in the enzymatic oxidation process were clarified in collision-induced dissociation experiments and gas-phase hydrogen/deuterium (H/D) exchange reactions. The amounts of enzyme and mediator were significant for product formation: with greater amounts overoxidation products, the corresponding benzoic acid and benzonitrile were formed. Smaller amounts of laccase and mediator generated benzaldehyde in high yield. The reaction pathway for benzonitrile formation is discussed and it is suggested to start from benzaldehyde and the ammonia in the ammonium acetate buffer.     

Reactive oxygen and nitrogen species (RONS) scavenging reactions of o-vanillin: Pulse radiolysis and stopped flow studies

Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action. Trichloromethylperoxyl radicals (CCl₃O ₂ ^· ) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 10⁵ M⁻¹s⁻¹, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate constant of 3 × 10³ M⁻¹s⁻¹. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s⁻¹. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other phenolic compounds.     

Quenching dynamics study on photoinduced excited triplet duroquinone by TEMPO in 1,2-propandiol

Chemically induced dynamic electron polarization (CIDEP) spectrum and transient absorptive spectrum are recorded in photolysis of duroquinone (DQ) in 1,2-propanodiol (PG). Durosemiquinone neutral radical DQH^• and PG ketyl radical \textCH₃[(\textC)\dot]\textOHCH₂\textOH{\text{CH}}_{3}{{{\dot{\text{C}}}\text{OHCH}}}_{2}{\text{OH}} are produced through hydrogen transfer reaction from PG to ³DQ*. When stable radical TEMPO is added to DQ/PG solution, photolysis results in CIDEP on TEMPO, which can be interpreted as a quartet precursor radical-triplet pair mechanism (QP-RTPM). There is competition between PG and TEMPO to quench ³DQ*. The CIDEP intensity of DQH^• decreases with the increase of TEMPO concentration. The quenching dynamics in photolysis of DQ/TEMPO/PG system is analyzed in detail. Based on the dynamics analysis and the measurement of the lifetime of ³DQ* by its transient absorbance decay, the quenching rate constant of ³DQ* by TEMPO in PG is obtained as 1.34 × 10⁷ L mol⁻¹ s⁻¹. This quenching rate constant is closely diffusion-controlled.     

A polythiophene derivative bearing TEMPO as a cathode material for rechargeable batteries

A polythiophene derivative bearing TEMPO radical was synthesized by oxidative chemical polymerization of its monomer. The polymer had a high spin density (2.05 × 10²¹ spins/g of polymer). CV studies of the polymer showed that the electrochemical redox reaction of the TEMPO radicals were completely reversible. We demonstrated, for the first time, construction and charge/discharge characteristics of an organic radical battery utilizing a TEMPO bearing polythiophene based cathode material. The battery had an initial specific discharge capacity of 79 A h/kg (87% of the theoretical capacity) and an average output voltage of 3.6 V. The specific energy capacity initially discharged was 268 W h/kg.     

Kinetics of the reaction of the TEMPO radical with alkylarenes

The kinetics of the reaction of the stable radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with a series of alkylarenes containing primary and secondary benzyl C—H bonds was studied by ESR, and the reaction rate constants were determined. The scheme of the process under study was examined, and the applicability boundaries of the simplification during analysis were shown. The selectivities of TEMPO and the more reactive cumylperoxyl radical were compared.     

Cu(bipy)2+/TEMPO-catalyzed oxidation of alcohols: radical or nonradical mechanism?

In the oxidation of alcohols with TEMPO as catalyst, the substrate has alternatively been postulated to be oxidized but uncoordinated TEMPO(+) (Semmelhack) or Cu-coordinated TEMPO(?) radical (Sheldon). The reaction with the Cu(bipy)(2+)/TEMPO cocatalyst system has recently been claimed, on the basis of DFT calculations, to not be a radical reaction but to be best viewed as electrophilic attack on the alcohol C-H(α) bond by coordinated TEMPO(+). This mechanism combines elements of the Semmelhack mechanism (oxidation of TEMPO to TEMPO(+)) and the Sheldon proposal ("in the coordination sphere of Cu"). The recent proposal has been challenged on the basis of DFT calculations with a different functional, which were reported to lead to a radical mechanism. We carefully examine the results for the two functionals and conclude from both the calculated energetics and from an electronic structure analysis that the results of the two DFT functionals are consistent and that both lead to the proposed mechanism with TEMPO not acting as radical but as (coordinated) positive ion.     

Atomic radical—molecule reactions F + CH<Subscript>3</Subscript>C≡CH: mechanistic study

The reaction of atomic radical F with propyne has been studied theoretically using ab initio quantum chemistry methods and transition state theory. The potential energy surface was calculated at the CCSD(T)/aug-cc-pVDZ (single-point) level using the UMP2/6-311++G(d,p) optimized structures. Two reaction mechanisms including the addition–isomerization–elimination reaction mechanism and the directed hydrogen abstraction reaction mechanism are considered. For the hydrogen abstraction reactions, i.e., the most probable evolution pathway in the title reaction, the HF formation occurs via direct abstraction mechanism dominantly and the H atom picked up by the atomic radical F should come mostly from the methyl group of normal propyne. On the other hand, for the addition–isomerization–elimination mechanism, the most feasible pathway should be the atomic radical F attacking on the C≡C triple bond in propyne (CH₃C≡CH) to form a weakly-bound adduct A1 with no barrier, followed by F addition to the C≡C triple bond to form the low-lying intermediate isomer 5. Subsequently, isomer 5 directly dissociates to P3 H₂CCCHF + H via transition state TS₅/P3. The other reaction pathways on the doublet PES are less competitive due to thermodynamical or kinetic factors. Furthermore, based on the analysis of the kinetics of all channels through which the addition and abstraction reaction proceed, we expect that the competitive power of reaction channels may vary with experimental conditions for the title reaction. The present work will provide useful information for understanding the processes of atomic radical F reaction with other unsaturated hydrocarbons. This material is available from author via E-mail.     

Transition states for deactivation reactions in the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy‐mediated free‐radical polymerization of acrylonitrile

The geometries and energetics of transition states (TS) for radical deactivation reactions, including competitive combination and disproportionation reactions, have been studied for the modeled 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO)‐mediated free‐radical polymerization of acrylonitrile with quantum mechanical calculations at the DFT/UB3‐LYP/6‐311+G(3df,2p)//(U)AM1 level of theory (where DFT is density functional theory, AM1 is Austin model 1, and UAM1 is unrestricted Austin model 1). A method providing reasonable starting geometries for an effective search for TS between the TEMPO radical and 1‐cyanopropyl radical mimicking the growing polyacrylonitrile macroradical is shown. For the hydrogen atom abstraction reaction by the TEMPO radical from the 1‐cyanopropyl radical, practically one TS has been found, whereas for the combination reaction of the radicals, several TS have been found, mainly differing in out‐of‐plane angle α of the N? O bond in the TEMPO structure. α in the TS is correlated with the activation energy, ΔE, determined from the single‐point calculation at the DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 level for the combination reaction of CH₃AN· with the TEMPO radical. The theoretical activation energy for the coupling reaction from DFT UB3‐LYP/6‐311+G(3df, 2p)//UAM1 calculations has been estimated to be 11.6 kcal mol^?1, that is, only about 4.5 times smaller than ΔE for the disproportionation reaction obtained with the DFT UB3‐LYP/6‐311+G(3df, 2p)//(U)AM1 approach. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 914–927, 2006     

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.     

Structure and Reactivity of the <Emphasis Type="BoldItalic">N</Emphasis>-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of N-acetyl-cysteine followed by the homolytic cleavage of the S–NO bond in the gas phase. IRMPD spectroscopy coupled with DFT calculations revealed that for the radical cation the radical migrates from its initial position on the sulfur atom to the α-carbon position, which is 2.5 kJ mol^–1 lower in energy. The radical migration was confirmed by time-resolved ion-molecule reactions. These results are in contrast with our previous study on cysteine methyl ester radical cation (Osburn et al., Chem. Eur. J. 2011 , 17, 873–879) and the study by Sinha et al. for cysteine radical cation (Phys. Chem. Chem. Phys. 2010 , 12, 9794–9800) where the radical was found to stay on the sulfur atom as formed. A similar approach allowed us to form a hydrogen-deficient radical anion of N-acetyl-cysteine, (M – 2H) ^•– . IRMPD studies and ion-molecule reactions performed on the radical anion showed that the radical remains on the sulfur, which is the initial and more stable (by 63.6 kJ mol^–1) position, and does not rearrange.     

Nitroxyl radical reactions with 4-pentenyl- and cyclopropylketenes: new routes to 5-hexenyl- and cyclopropylmethyl radicals

4-Pentenylketenes 4a and 9 and cyclopropylketenes 3a, 13, 14 (RCH=C=O) are generated by photochemical Wolff rearrangements and observed by IR as relatively long-lived species at room temperature in hydrocarbon solvents. The reactions of these ketenes with the nitroxyl radicals tetramethylpiperidinyloxyl (TEMPO, TO*) and tetramethylisoindoline-2-oxyl (TMIO, IO*) form carboxy substituted 5-hexenyl and cyclopropylmethyl radicals which are either trapped by a second nitroxyl radical or undergo rearrangements followed by trapping. The rate constant of the reaction of 4a with TEMPO was similar to that of n-BuCH=C=O (1b), while 3a was 4.3 times more reactive, indicating cyclopropyl stabilization of the incipient radical.