Reaction of cyclic nitroxides with nitrogen dioxide: the intermediacy of the oxoammonium cations

Piperidine and pyrrolidine nitroxides, such as 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 3-carbamoylproxyl (3-CP), respectively, are cell-permeable stable radicals, which effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The kinetics and mechanism of their reactions with .OH, superoxide, and carbon-centered radicals have been extensively studied, but not with .NO2, although the latter is a key intermediate in cellular nitrosative stress. In this research, .NO2 was generated by pulse radiolysis, and its reactions with TPO, 4-OH-TPO, 4-oxo-TPO, and 3-CP were studied by fast kinetic spectroscopy, either directly or by using ferrocyanide or 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), which effectively scavenge the product of this reaction, the oxoammonium cation. The rate constants for the reactions of .NO2 with these nitroxides were determined to be (7-8) x 10(8) M(-)(1) s(-)(1), independent of the pH over the range 3.9-10.2. These are among the highest rate constants measured for .NO2 and are close to that of the reaction of .NO2 with .NO, that is, 1.1 x 10(9) M(-1) s(-1). The hydroxylamines TPO-H and 4-OH-TPO-H are less reactive toward .NO2, and an upper limit for the rate constant for these reactions was estimated to be 1 x 10(5) M(-1) s(-1). The kinetics results demonstrate that the reaction of nitroxides with .NO2 proceeds via an inner-sphere electron-transfer mechanism to form the respective oxoammonium cation, which is reduced back to the nitroxide through the oxidation of nitrite to .NO2. Hence, the nitroxide slows down the decomposition of .NO2 into nitrite and nitrate and could serve as a reservoir of .NO2 unless the respective oxoammonium is rapidly scavenged by other reductant. This mechanism can contribute toward the protective effect of nitroxides against reactive nitrogen-derived species, although the oxoammonium cations themselves might oxidize essential cellular targets if they are not scavenged by common biological reductants, such as thiols.     

Structure-activity relationship of defective electrocatalysts for nitrogen fixation

Ammonia (NH₃), as an important chemical substance and clean energy carrier, plays an indispensable role in industrial and agricultural production. The electrocatalytic synthesis of NH₃ under mild conditions has attracted worldwide attention in the energy field due to its environmental friendliness and cost efficiency, but unsatisfactory NH₃ yields and Faradaic efficiencies are restricting its development. The introduction of defect has been demonstrated as a feasible way to overcome the disadvantages of electrochemistry, as it can regulate the electronic structure and modulate coordination environment of electrocatalysts, which further create active sites and enhance nitrogen adsorption. In this regard, it is necessary to understand the effects of various types of defects on electrocatalysts based on the latest progress in the defect engineering for nitrogen reduction reaction (NRR). In this review, the concept, classifications, and characterization of defects as well as the approaches to create them in electrocatalysts are firstly discussed. Then, certain types of defects (vacancy, dopant, amorphism, edge/corner, and porousness) affecting the performances of various electrocatalysts are further described. Finally, the summary and challenges of electrocatalytic ammonia synthesis are proposed to design advanced electrocatalysts with high efficiency.     

The reaction of methoxy radicals with nitric oxide and nitrogen dioxide

By allowing dimethyl peroxide (10^?4M) to decompose in the presence of nitric oxide (4.5 × 10^?5M), nitrogen dioxide (6.5 × 10^?5M) and carbon tetrafluoride (500 Torr), it has been shown that the ratio k₂/k_2′ = 2.03 ± 0.47: CH₃O + NO → CH₃ONO (reaction 2) and CH₃O + NO₂ → CH₃ONO₂ (reaction 2′). Deviations from this value in this and previous work is ascribed to the pressure dependence of both these reactions and heterogeneity in reaction (2). In contrast no heterogeneous effects were found for reaction (2′) making it an ideal reference reaction for studying other reactions of the methoxy radical. We conclude that the ratio k₂/k_2′ is independent of temperature and from k₁ = 10^10.2±0.4M^?1 sec^?1 we calculate that k_2′ = 10^9.9±0.4M^?1 sec^?1. Both k₂ and k_2′ are pressure dependent but have reached their limiting high-pressure values in the presence of 500 Torr of carbon tetrafluoride. Preliminary results show that k₄ = 10.^9.0±0.6 10^?4.5±1.1/ΘM^?1 sec^?1 (Θ = 2.303RT kcal mole^?1) and by k₄ = 10^8.6±0.6 10^?2.4±1.1/ΘM^?1 sec^?1: CH₃O + O₂ → CH₂O + HO₂ (reaction 4) and CH₃O + t-BuH → CH₃OH + (t-Bu) (reaction 4′).     

Heterogeneous catalysts for cyclic carbonate synthesis from carbon dioxide and epoxides

1. Download : Download high-res image (249KB)
2. Download : Download full-size image

     

乙酸盐上二氧化碳和二醇合成环状碳酸酯

在乙腈体系中,以不同的乙酸盐作催化剂,研究了CO2与二醇合成环状碳酸酯的反应.乙腈在反应过程中不仅是溶剂,而且还起到了脱水剂的作用,促进了反应的进行.以1,2-丙二醇为反应物对催化剂进行筛选,发现无水乙酸锌具有最高的催化活性.在无水乙酸锌上考察了二氧化碳和不同二醇的反应,结果表明,五元环碳酸酯的产率明显高于六元环碳酸酯,其中碳酸丙烯酯的产率最高.以1,2-丙二醇为反应基质,无水乙酸锌为催化剂,确定了最佳反应条件,1,2-丙二醇100 mmol,乙睛10 mL,催化剂2.5 mmol,反应压力10 MPa,温度170 ℃,反应12 h.在此条件下,碳酸丙烯酯的产率达到了24.2%,1,2-丙二醇的转化率为38.9%.     

Sodium carbonate as carbon dioxide source for the synthesis of cyclic carbonates containing the trifluoromethyl group

Trifluoromethylated cyclic carbonates were prepared through the palladium-promoted reaction of tertiary trifluoromethylated propargylic alcohols and sodium carbonate.     

Catalytic utilization of carbon dioxide to polymer blends via cyclic carbonate

In the present study, the synthesis of bis(cyclic carbonate) from carbon dioxide and bisphenol A (or bisphenol S)-diglycidyl ether was investigated using quaternary ammonium salts as catalyst. Among the salts tested, the one having a larger alkyl group and more nucleophilic counter anion exhibited a better catalytic activity. Poly(hydroxyurethane)s were prepared by the polyaddition reaction of bis(cyclic carbonate) and diamine. The poly(hydroxyurethane) has shown higher thermal stability than conventional polyurethane, and is expected as novel reactive polyurethane. The miscibility of blends containing poly(hydroxyurethena) and poly(styrene-co-acrylonitrile)(SAN) has been also studied by the optical clarity method and DSC.     

Computational design of cyclic nitroxides as efficient redox mediators for dye-sensitized solar cells

Cyclic nitroxide radicals represent promising alternatives to the iodine-based redox mediator commonly used in dye-sensitized solar cells (DSSCs). To date DSSCs with nitroxide-based redox mediators have achieved energy conversion efficiencies of just over 5?% but efficiencies of over 15?% might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one-electron oxidation and it must possess an oxidation potential in a range of 0.600-0.850?V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25?°C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N-containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl-, methoxy-, amino-, carboxy-, etc.) to the ring. Standard oxidation potentials were calculated using high-level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16?mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired E(ox)°, and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs.     

Design and synthesis of all diastereomers of cyclic pseudo-dipeptides as mimics of cyclic CXCR4 pentapeptide antagonists

The four diastereomers of 2,5-bis[(3-guanidino)propyl]-1-[3-(4-hydroxyphenyl)propionyl]-7-(2-naphthylacetyl)-1,4,7-triazacycloundec-9-en-3-one (-) and of 2,5-bis[(3-guanidino)propyl]-1-(4-hydroxyphenylacetyl)-7-(2-naphthylacetyl)-1,4,7-triazacycloundec-9-en-3-one (-) were synthesized by a divergent methodology from l- and D-glutamic acids. The 11-membered ring core was made by ring closing metathesis of linear bis(allylamines), and the guanidyl functions were introduced by a simultaneous double Mitsunobu reaction using bis(Boc)guanidine. These compounds were designed to mimic cyclic pentapeptide FC131 (c[Gly-D-Tyr-Arg-Arg-Nal]).     

Bifunctional organocatalyst for activation of carbon dioxide and epoxide to produce cyclic carbonate: betaine as a new catalytic motif

Bifunctional organocatalysts bearing an ammonium betaine framework have been developed as a new catalytic motif for the activation of carbon dioxide and epoxides to produce cyclic carbonates.     

Combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine and guanine radicals in DNA: oxidation and nitration end-products

The oxidation and nitration reactions in DNA associated with the combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine (8-oxoGua) and guanine radicals were explored by kinetic laser spectroscopy and mass spectrometry methods. The oxidation/nitration processes were triggered by photoexcitation of 2-aminopurine (2AP) residues site-specifically positioned in the 2'-deoxyribooligonucleotide 5'-d(CC[2AP]TC[X]CTACC) sequences (X = 8-oxoGua or G), by intense 308 nm excimer laser pulses. The photoionization products, 2AP radicals, rapidly oxidize either 8-oxoGua or G residues positioned within the same oligonucleotide but separated by a TC dinucleotide step on the 3'-side of 2AP. The two-photon ionization of the 2AP residue also generates hydrated electrons that are trapped by nitrate anions thus forming nitrogen dioxide radicals. The combination of nitrogen dioxide radicals with the 8-oxoGua and G radicals occurs with similar rate constants (approximately 4.3 x 10(8) M(-1) s(-1)) in both single- and double-stranded DNA. In the case of 8-oxoGua, the major end-products of this bimolecular radical-radical addition are spiroiminodihydantoin lesions, the products of 8-oxoGua oxidation. Oxygen-18 isotope labeling experiments reveal that the O-atom in the spiroiminodihydantoin lesion originates from water molecules, not from nitrogen dioxide radicals. In contrast, combination of nitrogen dioxide and guanine neutral radicals generated under the same conditions results in the formation of the nitro products, 5-guanidino-4-nitroimidazole and 8-nitroguanine adducts. The mechanistic aspects of the oxidation/nitration processes and their biological implications are discussed.     

Development in the green synthesis of cyclic carbonate from carbon dioxide using ionic liquids

This brief review presents the recent development in the synthesis of cyclic carbonate from carbon dioxide (CO₂) using ionic liquids as catalyst and/or reaction medium. The synthesis of cyclic carbonate includes three aspects: catalytic reaction of CO₂ and epoxide, electrochemical reaction of CO₂ and epoxide, and oxidative carboxylation of olefin. Some ionic liquids are suitable catalysts and/or solvents to the CO₂ fixation to produce cyclic carbonate. The activity of ionic liquid is greatly enhanced by the addition of Lewis acidic compounds of metal halides or metal complexes that have no or low activity by themselves. Using ionic liquids for the electrochemical synthesis of the cyclic carbonate can avoid harmful organic solvents, supporting electrolytes and catalysts, which are necessary for conventional electrochemical reaction systems. Although the ionic liquid is better for the oxidative carboxylation of olefin than the ordinary catalysts reported previously, this reaction system is at a preliminary stage. Using the ionic liquids, the synthesis process will become greener and simpler because of easy product separation and catalyst recycling and unnecessary use of volatile and harmful organic solvents.     

Kinetics of the reaction between nitroxide and thiyl radicals: nitroxides as antioxidants in the presence of thiols

Cyclic nitroxides effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The present study focuses on the kinetics and mechanisms of the reactions of piperidine and pyrrolidine nitroxides with thiyl radicals, which are involved in free radical "repair" equilibria, but being strong oxidants can also produce cell damage. Thiyl radicals derived from glutathione, cysteine, and penicillamine were generated in water by pulse radiolysis, and the rate constants of their reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO), 4-OH-TPO, and 3-carbamoyl-proxyl were determined to be (5-7) x 10 (8) M (-1) s (-1) at pH 5-7, independent of the structure of the nitroxide and the thiyl radical. It is suggested that the reaction of nitroxide (>NO (*)) with thiyl radical (RS (*)) yields an unstable adduct (>NOSR). The deprotonated form of this adduct decomposes via heterolysis of the N-O bond, yielding the respective amine (>NH) and sulfinic acid (RS(O)OH). The protonated form of the adduct decomposes via homolysis of the N-O bond, forming the aminium radical (>NH (*+)) and sulfinyl radical (RSO (*)), which by subsequent reactions involving thiol and nitroxide produce the respective amine and sulfonic acid (RS(O) 2OH). Nitroxides that are oxidized to the respective oxoammonium cations (>N (+)O) are recovered in the presence of NADH but not in the presence of thiols. This suggests that the reaction of >N (+)O with thiols yields the respective amine. Two alternative mechanisms are suggested, where >N (+)O reacts with thiolate (RS (-)) directly generating the adduct >NOSR or indirectly forming >NO (*) and RS (*), which subsequently together yield the adduct >NOSR. Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals. The proposed mechanism can account for the protective effect of nitroxides against reactive oxygen- and nitrogen-derived species in the presence of thiols.     

Experimental and theoretical studies of the redox potentials of cyclic nitroxides

The redox potentials of 25 cyclic nitroxides from four different structural classes (pyrrolidine, piperidine, isoindoline, and azaphenalene) were determined experimentally by cyclic voltammetry in acetonitrile, and also via high-level ab initio molecular orbital calculations. It is shown that the potentials are influenced by the type of ring system, ring substituents and/or groups surrounding the radical moiety. For the pyrrolidine, piperidine, and isoindolines there is excellent agreement (mean absolute deviation of 0.05 V) between the calculated and experimental oxidation potentials; for the azaphenalenes, however, there is an extraordinary discrepancy (mean absolute deviation of 0.60 V), implying that their one-electron oxidation might involve additional processes not considered in the theoretical calculations. This recently developed azaphenalene class of nitroxide represents a new variant of a nitroxide ring fused to an aromatic system and details of the synthesis of five derivatives involving differing aryl substitution are also presented.     

Investigation of short lived radicals in aromatic nucleophilic substitution by use of nitroso compounds as scavengers

Radical intermediates formed in reaction of substituted nitrobenzene compounds with nucleophilic reagents have been trapped by t-nitrosobutane to yield nitroxide radicals which have been characterized in situ by ESR. The formation of short-lived radicals originating from reagents and solvents has been shown.     

Structure-activity relationship of new octaethylporphyrin-based benzochlorins as photosensitizers for photodynamic therapy

An in vitro and in vivo structure-activity relationship study was carried out on a series of benzochlorins with variable lipophilicity. The structural features evaluated in this study include the length of the alkyl or fluoroalkyl groups attached to the six-member exocyclic ring either by an ether or by a carbon-carbon bond. In preliminary in vitro (radiation-induced fibrosarcoma [RIF] cells) and in vivo screening (C3H mice, bearing RIF tumors), all Zn (II) benzochlorins were found to be effective. However, benzochlorins bearing alkyl groups with carbon-carbon bonds showed enhanced efficacy compared with the related alkyl ether analogs. A comparative intracellular localization study of the newly synthesized benzochlorins with Rhodamine-123 indicated that the effective photosensitizers localize in mitochondria, and a displacement study with PK11195 showed their partial affinity for the peripheral benzodiazepine receptor (PBR). Interestingly, compared with the Zn(II) benzochlorin that was found to be quite effective in vivo, the corresponding free-base analog produced less photosensitizing activity and was found to localize in lysosomes. A comparative study with dansyl-proline confirmed the binding of the effective benzochlorins to Site II of human serum albumin (HSA). However, no direct correlation was observed between the binding constant values (to HSA or to PBR) of benzochlorins and their photosensitizing ability.     

Water as an efficient medium for the synthesis of cyclic carbonate

Herein, we report a novel method for the synthesis of cyclic carbonate in water. By tuning the amount of water, cycloaddition of CO₂ to epoxide in aqueous medium leads to cyclic carbonates with moderate to excellent yields and high selectivities. In addition, the presence of water could remarkably improve the activity of ionic liquids by which the turnover frequency of the reaction is about 4-5 times higher in the presence than in the absence of water. The relationship between the higher catalytic reactivity and water was proposed.     

Polymer‐supported pyridinium catalysts for synthesis of cyclic carbonate by reaction of carbon dioxide and oxirane

Polymer‐supported pyridinium salts, prepared by quaternarization of crosslinked poly(4‐vinylpyridine) with alkyl halides, effectively catalyze the reaction of carbon dioxide (1 atm) and glycidyl phenyl ether (GPE) to afford the corresponding five‐membered cyclic carbonate (4‐phenoxymethyl‐1,3‐dioxolan‐2‐one). Poly(4‐vinylpyridine) quarternarized with alkyl bromides show high catalytic activities, and the reaction of carbon dioxide (1 atm) and GPE at 100 °C affords 4‐phenoxymethyl‐1,3‐dioxolan‐2‐one quantitatively in 6 h. The rate constant in the reaction of GPE and carbon dioxide in N‐methyl pyrrolidinone using poly(4‐vinylpyridine) quarternarized with n‐butyl bromide (k_obs = 102 min^?1) is almost comparable with those for homogeneous catalysts with good activities (e.g., LiI), and the rate of the reaction obeys the first‐order kinetics. A used catalyst may be recovered by centrifugation, and the recycled catalyst also promotes the reaction of GPE and carbon dioxide. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5673–5678, 2007