Carbon Adducts Research Articles

1H-1,2,3-Triazol-5-ylidenes as Catalytic Organic Single-Electron Reductants.

Most of the known single-electron reductants are either metal based reagents, used in a stoichiometric amount, or a combination of an organic species and a photocatalyst. Here we report that 1H-1,2,3-triazol-5-ylidenes act not only as stoichiometric one-electron donors but also as catalytic organic reducing agents, without the need of a photocatalyst. As a proof of concept, we studied the reduction of quinones, which are well-known electron conveyors that are involved in various biological and industrial processes. This work also provides experimental evidence for the formation of a bis(triazolium)carbonate adduct, which acts as the resting state of the catalytic cycle and as the carbene reservoir.

Role of mono- and diamines as kinetic promoters in mixed aqueous amine solution for CO2 capture

Tertiary amines are thermodynamically efficient CO2 absorbents but unfortunately suffer from low absorption rates. To overcome this issue the use of formulated absorbents incorporating simple aliphatic amines and di-amines as promoters have been suggested. This work describes the fundamental CO2 capture behaviour in tertiary amine diethylethanolamine (DEEA) based solution with a series of kinetic promoters monoethanolamine (MEA), N,N-dimethyl-1,3-propanediamine (N,N-DM13PDA), N,N-dimethyl-1,2-ethanediamine (N,N-DM12EDA), and 4-amino-1-methylpiperidine (4-A1MPD). The CO2 absorption rate and cyclic capacity was significantly enhanced due to the additional diamines. Using NMR technology, the underlying effect of the additives on the chemical speciation e.g. carbamate, (bi)carbonate was interrogated. The absorbents were found to initially favour carbamate at low CO2 loadings, transitioning to the energetically favoured (bi)carbonate adduct as the total CO2 concentration elevates. Unlike the energy intensive MEA absorbent, rapid and deep CO2 removal was achieved in the new proposed di-amine blend solutions, affirming its promising potential for CO2 capture.

Synthesis of α‐Ketoamides from β‐Ketonitriles and Primary Amines: A Catalyst‐Free Oxidative Decyanation–Amidation Reaction

AN oxidative decyanation–amidation of β‐ketonitriles and primary amines readily occurs using hydrogen peroxide sodium carbonate adduct (Na2CO3·1.5H2O2), K2CO3, and 1,4‐dioxane. This reaction affords α‐ketoamides under mild conditions without the need of a catalyst.

Computational Study of the pH-Dependent Competition between Carbonate and Thymine Addition to the Guanine Radical.

When oligonucleotides are oxidized by carbonate radical, thymine and carbonate can add to guanine radical, yielding either a guanine-thymine cross-link product (G∧T) or 8-oxo-7,8-dehydroguanine (8oxoG) and its further oxidation products such as spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh). The ratio of thymine addition to carbonate addition depends strongly on the pH. Details of the mechanism have been explored by density functional calculations using the ωB97XD/6-31+G(d,p) level of theory with the SMD implicit solvation method, augmented with a few explicit waters. Free energies of intermediates and transition states in aqueous solution have been calculated along the pathways for addition of thymine, CO32-/HCO3- and carbonate radical to guanine radical. The pH dependence was examined by using appropriate explicit proton donors/acceptors as computational models for buffers at pH 2.5, 7, and 10. Deprotonation of thymine is required for nucleophilic addition at C8 of guanine radical, and thus is favored at higher pH. The barrier for carbonate radical addition is lower than for bicarbonate or carbonate dianion addition; however, for low concentrations of carbonate radical, the reaction may proceed by addition of bicarbonate/carbonate dianion to guanine radical. Thymine and bicarbonate/carbonate dianion addition are followed by oxidation by O2, loss of a proton from C8 and decarboxylation of the carbonate adduct. At pH 2.5, guanine radical cation can be formed by oxidization with sulfate radical. Water addition to guanine radical cation is the preferred path for forming 8oxoG at pH 2.5.

UVA-induced carbon-centred radicals in lightly pigmented cells detected using ESR spectroscopy

Ultraviolet-A and melanin are implicated in melanoma, but whether melanin in vivo screens or acts as a UVA photosensitiser is debated. Here, we investigate the effect of UVA-irradiation on non-pigmented, lightly and darkly pigmented melanocytes and melanoma cells using electron spin resonance (ESR) spectroscopy. Using the spin trap 5,5 Dimethyl-1-pyrroline N-oxide (DMPO), carbon adducts were detected in all cells. However, higher levels of carbon adducts were detected in lightly pigmented cells than in non-pigmented or darkly pigmented cells. Nevertheless, when melanin levels were artificially increased in lightly pigmented cells by incubation with L-Tyrosine, the levels of carbon adducts decreased significantly. Carbon adducts were also detected in UVA-irradiated melanin-free cell nuclei, DNA-melanin systems, and the nucleoside 2′-deoxyguanosine combined with melanin, whereas they were only weakly detected in irradiated synthetic melanin and not at all in irradiated 2′-deoxyguanosine. The similarity of these carbon adducts suggests they may be derived from nucleic acid– guanine – radicals. These observations suggest that melanin is not consistently a UVA screen against free-radical formation in pigmented cells, but may also act as a photosensitizer for the formation of nucleic acid radicals in addition to superoxide. The findings are important for our understanding of the mechanism of damage caused by the UVA component of sunlight in non-melanoma and melanoma cells, and hence the causes of skin cancer.

Oxygen Bleach under the Microscope: Microchemical Investigation and Gas-Volumetric Analysis of a Powdered Household Product

A closer look at particles of a solid oxygen-based bleach household product is proposed, both figuratively and literally, to help develop observation and critical thinking skills and to link everyday life to chemistry concepts like acid–base equilibrium, redox reactivity, gas laws, gas volumetry, stoichiometry, and morphology of a heterogeneous solid mixture. In a microchemical approach, students learn to discriminate particles by their morphology under an optical (or low-cost USB) microscope and to investigate how they react with (a) hypochlorite solution (or household chlorine bleach); and (b) acetic acid (or vinegar). Aware of the ingredients listed on the product’s label, and searching for the structure and formula of percarbonate (2Na2CO3·3H2O2), a hydrogen peroxide–sodium carbonate adduct, they end up concluding that the particles showing effervescence with both reagents consist of sodium percarbonate, and those bubbling only in contact with acetic acid are sodium carbonate, while other minor ingred...

How Do Strain and Steric Interactions Affect the Reactions of Aromatic Compounds with Free Radicals? Characterization of the Radicals Formed by Muonium Addition to p-Xylene and [2.2]Paracyclophane by DFT Calculations and Muon Spin Spectroscopy

Muoniated radicals were produced by the addition of muonium (Mu) to the aromatic compound p-xylene (1) in the solid and liquid states and to the strained aromatic compound [2.2]paracyclophane (2) in the solid state. The radicals were characterized by avoided level crossing muon spin resonance spectroscopy and identified by comparing the experimentally determined muon hyperfine coupling constants with values obtained from DFT calculations. Mu was observed to add to both the secondary and tertiary carbons of 1, with the relative yield of the Mu adduct of the tertiary carbons estimated to be ∼10% in the liquid phase. The relative yield of the tertiary adduct is much higher in the solid state although this cannot be calculated exactly due to the overlap of resonances and the apparent nonuniform distribution of the radical orientations. There are three possible addition sites in 2 due to the lower symmetry of the six-membered ring compared with 1. Mu can add to the secondary carbons either from the outside of 2, generating the "exo" adduct, or from the inside, generating the "endo" adduct. The relative yields of the exo, endo, and tertiary carbon adducts are 67.1(1), 21.8(1), and 11.1(1)%, respectively. The barriers to Mu addition at the different sites of isolated molecules were determined from DFT calculations. The barriers for Mu addition to 2 are lower than the barriers for Mu addition to 1, except for addition to the "endo" position, where the unfavorable steric interactions with the second ring of 2 raise the addition barrier considerably. The measured relative yields do not reflect the distribution of products calculated using the activation energies obtained from the DFT calculations due to strong steric interactions with neighboring molecules.

Degradation of matrix glycosaminoglycans by peroxynitrite/peroxynitrous acid: Evidence for a hydroxyl-radical-like mechanism

The oxidant peroxynitrite/peroxynitrous acid (ONOO −/ONOOH) is generated at sites of inflammation via reaction of O 2 − with NO. Previous studies have shown that these species can oxidize cellular targets, but few data are available on damage to extracellular matrix and its components, despite evidence for matrix modification in a number of pathologies. In the current study we show that reaction of ONOO −/ONOOH with glycosaminoglycans results in extensive polymer fragmentation. Bolus authentic ONOO −/ONOOH modifies hyaluronan, heparin, and chondroitin, dermatan, and heparan sulfates, in a concentration-dependent, but O 2-independent, manner. The ONOO −/ONOOH generator 3-(4-morpholinyl)sydnoneimine produces similar time- and concentration-dependent damage. These reactions generate specific polymer fragments via cleavage at disaccharide intervals. Studies at different pH values, and in the presence of bicarbonate, are consistent with ONOOH, rather than the carbonate adduct, CO 3 − or ONOO −, being the source of damage. EPR spin trapping experiments have provided evidence for the formation of carbon-centered radicals on glycosaminoglycans and related monosaccharides; the similarity of these spectra to those obtained with authentic HO is consistent with fragmentation being induced by this oxidant. These data suggest that extracellular matrix fragmentation at sites of inflammation may be due, in part, to the formation and reactions of ONOOH.

Multinuclear NMR studies and ab initio structure calculations of hindered phencyclone diels‐alder adducts from symmetrical cyclic dienophiles: Cyclohexene, vinylene carbonate, vinylene trithiocarbonate and two N‐aryl maleimides

AbstractA series of hindered Diels‐Alder adducts have been prepared from phencyclone, 1, with various unusual symmetrical cyclic dienophiles, including cyclohexene, 2a; vinylene carbonate, 2b; vinylene trithiocarbonate, 2c; and the N‐aryl maleimides: N‐(4‐dimethylamino‐3,5‐dinitrophenyl)maleimide (“Tuppy's maleimide”), 2d; and N‐[3,5‐bis(trifluoromethyl)phenyl]maleimide, 2e. The highly hindered adducts, 3a‐e, respectively, were extensively characterized by one‐ and two‐dimensional NMR methods, observing proton, carbon‐13 and fluorine‐19. High resolution COSY45 spectra permitted rigorous proton NMR assignments. The 2D heteronuclear C‐H chemical shift correlation spectra (HETCOR, XHCORR) were obtained for adducts 3a‐d, allowing specific assignments for protonated carbons. Corrections to earlier proton NMR assignments for the vinylene carbonate adduct are given; results of the gated decoupling 13C NMR experiment for this adduct supported endo adduct stereochemistry. Relative proton chemical shifts for bridgehead phenyls of adduct 3c appeared anomalous relative to other adducts, suggesting possible special anisotropic interactions (with endocyclic sulfur or other anisotropic groups in the product) due to the unusual calculated orientation of the phenyls. The unsubstituted bridgehead phenyls in all adducts were shown to exhibit slow exchange limit (SEL) 1H and 13C spectra on the NMR timescales at ambient temperatures (7 tesla) showing slow rotations about the C(sp3)‐C(aryl sp2) bonds. The rapid rotation of the N‐aryl rings of the maleimide adducts was indicated by fast exchange limit spectra, suggesting that ortho substitution of the N‐aryl ring may be necessary to slow this rotation to the SEL regime. Ab initio geometry optimizations at the Hartree‐Fock level were carried out for each adduct, with the 6‐31G* basis sets. Appreciable geometry differences were seen in calculated structures, and significant NMR chemical shift differences were experimentally observed, depending on the nature of the groups attached to the (Z)‐HC=CH moiety of the dienophiles.

Catalytic activation of C–H bonds of alkanes, ethene and aromatic hydrocarbons by the adducts of active carbons with metallic potassium

It has been shown that the adducts of active carbons with metallic potassium are able to activate C–H bonds of alkanes, ethene and aromatic hydrocarbons and introduce these compounds into the hydrogen–deuterium exchange reactions at room temperature. This paper describes the reactions of the redistribution of hydrogen isotopes in CD 3H, C 6H 5CD 3 and C 6D 5CH 3 catalysed by the potassium–carbon adducts as well as the similar isotope exchange reactions in the systems C 2H 4–C 2D 4, C 6H 6–C 6D 6, n-C 6H 14– n-C 6D 14 and CD 4– n-C 5H 12. A study of the isotope exchange between C 6H 6 and C 6D 6 has shown that the efficiency of the process is strongly dependent on the potassium content in the catalyst passing through a maximum at the K:C molar ratio of 1:7.45.

Structures of Carbon Deposits Formed on a Graphite Electrode during Fullerene Generation

Silver-colored carbon adducts were grown on a graphite electrode connected to the negative lead during fullerene preparation by an electric arc method, and hemispherical carbon adducts were observed with scanning electron microscopy. The carbon adducts were also observed with transmission electron microscopy, which showed spherical carbon particles of 50-300 Å in diameter having a graphitic layered structure. X-ray diffraction measurements suggest that the graphitic stacking of the spherical carbon particles has a turbostatic layered structure. The mechanism of the formation of these carbon adducts as well as fullerenes and nanotubes is discussed on the basis of these observations and by estimation of spin concentrations of each product.

The Reaction of 2,3-Dichloro-5,6-dicyano-p-benzoquinone with Benzofurans and Indoles

Abstract The reaction of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) with 6-methoxy-3-methylbenzofuran gave carbon–oxygen adduct. The reaction in less polar solvents such as benzene and CH2Cl2 proceeds faster than that in more polar solvents such as THF and dioxane. In contrast, the reaction of DDQ with indoles gave carbon–carbon adducts. This reaction proceeds rapidly with increasing solvent polarity.

ChemInform Abstract: OLEFIN CYCLOADDITIONS TO 2′,3′,5′‐TRIACETYLURIDINE

AbstractDie Photocycloaddition von Tetramethyl‐ (IIa), 1,1‐Diäthoxy‐äthylen (IIc), Isopropenylacetat (IIb) und Vinylencarbonat (IId) an das Uridin (I) führt zu isomeren 1:1‐Addukten (III) in meist nahezu quantitativer Ausbeute.

Olefin cycloadditions to 2′,3′,5′-triacetyluridine

Photocycloadditions of 2′,3′,5′-triacetyluridine to tetramethylethylene, isopropenyl acetate, 1,1-diethoxyethylene, and vinylene carbonate are described. The rearrangement of the vinylene carbonate adduct to 2′,3′,5′-triacetyl-5-formylmethyluridine followed by oxidation to the 5-carboxymethyl or decarbonylation to the 5-methyl (ribothymidine) derivative is also described.