Concomitant Dehydrogenation Research Articles

Spectroscopic characterization and photochemistry of HC3N− and CH3C3N−: implications for ion chemistry in Titan's atmosphere

ABSTRACT Molecular ions are key intermediates in the build-up of chemical complexity in interstellar clouds. Among the more than 300 interstellar molecules, only eight negative ions, i.e. C2n+1N− (n = 0–3) and HC2n− (n = 2–5), have been astronomically observed. Understanding the formation mechanism of these ions under the interstellar conditions is essential for astrochemical modelling and establishing the astrochemical networks. Cyanopolyynes including the parent molecule HC3N are carbon-chain molecules that have been observed in a variety of astronomical objects such as the Titan's atmosphere. Herein, two cyanoacetylene anions HC3N‒ and CH3C3N‒ were generated in solid Ne matrix at 3 K and characterized with matrix-isolation infrared spectroscopy, as aided by isotopic substitutions and the ab initio calculations at the UCCSD(T)-F12a/cc-pVTZ-F12 level using second-order vibrational perturbation theory. Upon red-light irradiation at 625 nm, both ions undergo electron detachment by reformation of the neutral species. Importantly, the concomitant dehydrogenation of HC3N‒ has also been observed in the matrix, providing new insight into the intriguing mechanism for the formation of C3N‒ in the upper atmosphere of Titan.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

Reactivity of an NHC-stabilized pyramidal hydrosilylene with electrophilic boron sources.

Silylenes have become an indispensable tool for molecular bond activation. Their use for the construction of silicon-boron bonds is uncommon in comparison to the numerous studies on silylene-derived silicon-element bond formations. Herein we investigate the reactivity of the pyramidal NHC-coordinated hydrosilylene tBu3SiSi(H)LMe4 (1; NHC = N-heterocyclic carbene, LMe4 = 1,3,4,5-tetramethylimidazolin-2-ylidene) with various boron-centered electrophiles. The reaction of 1 with THF·BH3 or H3N→BH3 afforded the silylene complex 1→BH3 or the product of insertion of the silicon(ii) atom into an N-H bond with concomitant dehydrogenation along the HN-BH moiety (2). The respective conversion of 1 with BPh3 yields 1→BPh3 which readily reacts with excess LMe4 to form the more stable complex LMe4→BPh3 with release of 1. Treatment of 1 with the haloboranes Et2O→BF3, BCl3, BBr3 and Me2S→BBr3 resulted in the formation of the Lewis acid base adducts 1→BX3 (X = F, Cl, Br) and an equilibrium with their auto-ionization products [12BX2]+[BX4]- slowly develops. The ratio of 1→BX3 significantly increases with rising atomic number of the halide, thus 1→BF3 majorly transforms within hours while 1→BBr3 is near-quantitatively retained over time. Accordingly, the complex 1→BPhBr2 was isolated after conversion of 1 with PhBBr2.

Reactive Intermediates Produced from the Metabolism of the Vanilloid Ring of Capsaicinoids by P450 Enzymes

This study characterized electrophilic and radical products derived from the metabolism of capsaicin by cytochrome P450 and peroxidase enzymes. Multiple glutathione and β-mercaptoethanol conjugates (a.k.a., adducts), derived from the trapping of quinone methide and quinone intermediates of capsaicin, its analogue nonivamide, and O-demethylated and aromatic hydroxylated metabolites thereof, were produced by human liver microsomes and individual recombinant human P450 enzymes. Conjugates derived from concomitant dehydrogenation of the alkyl terminus of capsaicin were also characterized. Modifications to the 4-OH substituent of the vanilloid ring of capsaicinoids largely prevented the formation of electrophilic intermediates, consistent with the proposed structures and mechanisms of formation for the various conjugates. 5,5'-Dicapsaicin, presumably arising from the bimolecular coupling of free radical intermediates was also characterized. Finally, the analysis of hepatic glutathione conjugates and urinary N-acetylcysteine conjugates from mice dosed with capsaicin confirmed the formation of glutathione conjugates of O-demethylated quinone methide and 5-OH-capsaicin in vivo. These data demonstrated that capsaicin and structurally similar analogues are converted to reactive intermediates by certain P450 enzymes, which may partially explain conflicting reports related to the cytotoxic, pro-carcinogenic, and chemoprotective effects of capsaicinoids in different cells and/or organ systems.

Hydrogen liberation from the hydrolytic dehydrogenation of dimethylamine–borane at room temperature by using a novel ruthenium nanocatalyst

Herein we report the discovery of an in situ generated, highly active nanocatalyst for the room temperature dehydrogenation of dimethylamine-borane in water. The new catalyst system consisting of ruthenium(0) nanoparticles stabilized by the hydrogenphosphate anion can readily and reproducibly be formed under in situ conditions from the dimethylamine-borane reduction of a ruthenium(III) precatalyst in tetrabutylammonium dihydrogenphosphate solution at 25 ± 0.1 °C. These new water dispersible ruthenium nanoparticles were characterized by using a combination of advanced analytical techniques. The results show the formation of well-dispersed ruthenium(0) nanoparticles of 2.9 ± 0.9 nm size stabilized by the hydrogenphosphate anion in aqueous solution. The resulting ruthenium(0) nanoparticles act as a highly active catalyst in the generation of 3.0 equiv. of H(2) from the hydrolytic dehydrogenation of dimethylamine-borane with an initial TOF value of 500 h(-1) at 25 ± 0.1 °C. Moreover, they provide exceptional catalytic lifetime (TTO = 11,600) in the same reaction at room temperature. The work reported here also includes the following results; (i) monitoring the formation kinetics of the in situ generated ruthenium nanoparticles, by using the hydrogen generation from the hydrolytic dehydrogenation of dimethylamine-borane as a catalytic reporter reaction, shows that sigmoidal kinetics of catalyst formation and concomitant dehydrogenation fits well to the two-step, slow nucleation and then autocatalytic surface growth mechanism, A → B (rate constant k(1)) and A + B → 2B (rate constant k(2)), in which A is RuCl(3)·3H(2)O and B is the growing, catalytically active Ru(0)(n) nanoclusters. (ii) Hg(0) poisoning coupled with activity measurements after solution infiltration demonstrates that the in situ generated ruthenium(0) nanoparticles act as a kinetically competent heterogeneous catalyst in hydrogen generation from the hydrolytic dehydrogenation of dimethylamine-borane. (iii) A compilation of kinetic data depending on the temperature and catalyst concentration is used to determine the dependency of reaction rate on catalyst concentration and the activation energy of the reaction, respectively.

Facile Access to Condensed Arenothiophenes – Preparation of Dihydrophenanthrothiophenes and Dihydrophenanthro[1]Benzothiophenes

4-Bromo-3-formyl-1,2-dihydronaphthalenes undergo Suzuki cross coupling and Wittig olefination reaction in a one pot procedure. The trienes obtained can be cyclised thermally under concomitant dehydrogenation to dihydrophenanthrothiophenes and dihydrophenanthro[1]benzothiophenes. An X-ray crystal structural analysis was carried out for ethyl (E)-3-[1-(1-benzothiophen-3-yl)-3,4-dihydro-2-naphthyl]propenoate.

Lipoxygenase-catalyzed polymerization of phenolic lipids suggests a new mechanism for allergic contact dermatitis induced by urushiol and its analogs

Lipoxygenase was found to catalyze the oxidative polymerization of phenolic lipids containing a ( Z, Z)-pentadiene in the side chain, the model compounds of urushiol and its analog, yielding methanol-soluble and insoluble polymers. The structural analysis of the resulted polymers suggested that the polymerization occurred at both the phenol and the unsaturated side chain. The key step of the polymerization was the generation of the hydroperoxide at the unsaturated side chain by lipoxygenase. The decomposition of hydroperoxide and concomitant dehydrogenation of phenol ring catalyzed by lipoxygenase might produce radicals that could be coupled to form cross-linked polymers. This lipoxygenase-mediated reaction implies a new mechanism for contact allergy of urushiol and its analogs.

Structural properties and heat-induced oxidation-dehydrogenation of manganoan ilvaite from Perda Niedda mine, Sardinia, Italy

An unusually Mn-rich ilvaite sample from the Perda Niedda mine in Sardinia, Italy, was studied in order to clarify the Mn 2 + distribution among the different structural sites, and to observe the structural response of the mineral upon thermally induced oxidation-dehydrogenation. The crystal structure and the chemical composition of one crystal [a = 13.014(5), b = 8.867(3), c = 5.838(4) A, β = 90.02(4)°] were investigated. X-ray crystal-structure refinement, performed in the Pnam space group, and electron microprobe analyses yielded the formula (Ca 0 . 9 8 Mn 2 + 0 . 0 2 )(Fe 3 + Fe 2 + )(Mn 0 . 7 2 Fe 2 + 0 . 2 8 ) (Si 2 O 2 )O(OH). Crystal chemical details, compared to structural data from literature, led to the assumption that Mn 2 + replaces Fe 2 + , mainly at the M2 site. Annealing experiments and structure refinements were performed in the temperature range 400-690 °C. No phase transition was observed over the entire temperature range. Oxidation of Fe 2 + at the M1 site, with concomitant dehydrogenation, was deduced from examination of the structural adjustments occurring as the temperature was increased. A useful model to evaluate a possible OH - ↔ O 2 - substitution in ilvaite was obtained.

Tetra(2,6‐dichlorophenyl)porphyrin — a superior sensitiser for the singlet‐oxygen ene reaction (Schenck reaction)

AbstractThe very useful singlet‐oxygen ene reaction of reluctant alkenes, e.g. 1, 5, 7, 8, and, in particular, the mono(allyl hydroperoxides) 10 and 11, is handicapped by the instability of the sensitiser TPP. A comparative study of TPP, TPFPP, and TDCPP in the singlet‐oxygen ene reaction of these alkenes shows that 1) the persistence increases in the order TPP < TPFPP < TDCPP, rendering TDCPP the sensitiser of choice for the generation of singlet‐oxygen in non‐polar solvents, 2) the persistence of the tetraarylporphyrins decreases in the presence of an alkene, 3) this decrease strongly depends on the nature of the alkene, being most pronounced in the case of cyclohexene. These results are interpreted in terms of unknown substrate‐derived species which induce oxidative destruction of the tetraarylporphyrins. Alternatively, abstraction of allylic hydrogen atoms from the alkenes by the excited sensitisers may give rise to the observed substrate‐dependent photobleaching. – Because the singlet‐oxygen ene reaction is the key step of a 1,2‐carbonyl transposition with concomitant dehydrogenation, the scope and usefulness of this sequence are distinctly improved.

A Mild Procedure for the Preparation of Phenols from Polycyclic Aromatic Ketones

Abstract A mild method is described for the preparation of phenols from polycyclic aromatic ketones. Ketones which have the carbonyl group adjacent to an aromatic ring can be converted to phenyl selenides by treatment with phenylselenenyl chloride in ethyl acetate at room temperature. Oxidation with 30% hydrogen peroxide in tetrahydrofuran at room temperature affords the corresponding phenols in generally high yield. Phenolic derivatives of phenanthrene, fluoranthene, benz[a]anthracene, and benzo[a]pyrene were prepared using this general procedure. Oxidation of the 2-phenylselenenyl derivative of 1,12c-dihydrobenzo[j]fluoranthene with 30% hydrogen peroxide resulted in anomalous formation of the 2,3-quinone instead of 3-hydroxybenzo[j]fluoranthene. The mildness and selectivity of the selenation-oxidation procedure was demonstrated by the aromatization of a cyclic ketone without concomitant dehydrogenation in a partially saturated ring. β-Tetralone reacted with phenylselenenyl chloride at room temperature to form a single compound in high yield identified as 2-naphthol. This reaction occurred in the absence of an added oxidizing agent. Another β-tetralone-type ketone, 1,2-dihydrophenanthren-3(4H)-one, was converted into 3-hydroxyphenanthrene under the same reaction conditions. Similar treatment of 1,10b-dihydrofluoranthen-2(3H)-one failed to yield either 2-hydroxyfluoranthene or the 2,3-quinone.

Reactions of di-isopropylcarbodi-imide with some ruthenium and osmium hydrides, leading to ready dehydrogenation of an isopropyl group; crystal and molecular structure of carbonylchloro(N-isopropenyl-N′-isopropylformamidinato)bis(triphenylphosphine)ruthenium(II)

The reactions of PriNCNPri with the complexes [RuH(X)(CO)(PPh3)3](X = Cl or Br) in boiling benzene involve insertion of the carbodi-imide moiety into the Ru–H bond and concomitant dehydrogenation of an isopropyl group to yield the derivatives [RuX{CH2C(Me)N CH NCHMe2}(CO)(PPh3)2]. A single-crystal X-ray diffraction study of the chloro-complex (X = Cl) has confirmed the presence of an N-isopropenyl-N′-isopropylformamidinate ligand in which the isopropenyl skeleton is essentially coplanar with, and conjugated to, the formamidinate group. The crystals are monoclinic, space group P21/a, with a= 17.836(2), b= 18.443(3), c= 12.607(1)A, β= 104.59(3)°, and Z= 4. The structure has been solved via the heavy-atom method and refined by least squares using 4 381 observed [I > 1.5σ(l)] diffractometer intensities to a final R of 0.086. The octahedral molecule comprises two trans phosphines [Ru–P 2.383(4) and 2.400(4)A], a chelating formamidinate ligand [Ru–N 2.048(10) and 2.242(10)A], a carbonyl group [Ru–C 1.805(12)A], and a chlorine atom [Ru–Cl 2.434(4)A], the latter being trans to the more strongly bound nitrogen atom. The complex [OsH2(CO)(PPh3)3] reacts with PriNCNPri to afford the intermediate product [OsH(Me2HCN CH NCHMe2)(CO)(PPh3)2] which, on further heating, dehydrogenates to [OsH{CH2C(Me)N CH NCHMe2}(CO)(PPh3)2] in good yield.

Dehydrocyclodimerization and concomitant reactions of piperylene on alkali-containing alumina

1. Piperylene on Al2O3 (500°C) is completely converted to cracking and condensation products, n- and iso-pentene, and, to a small extent, to cyclopentane and cyclopentadiene. Addition of alkali to Al2O3 greatly increases the yield of n-pentenes and simultaneously reduces the yield of isoamylenes and of cracking and condensation products. 2. Piperylene on Al2O3.10%K2O (500°C) is mainly converted to n-pentenes, alkenylcyclohexenes, C6-C9 aromatic hydrocarbons, C10 alkenyl- and alkylaromatic hydrocarbons, and also higher-molecular condensation products. We have obtained results that suggest that n-pentenes are formed by hydrogenation of piperylene by hydrogen liberated in concomitant dehydrogenation reactions. 3. We have made a comparison of the reactions of piperylene with those of hexadiene on Al2O3 and alkali-containing Al2O3, as described in the literature. Piperylene undergoes cyclization more sluggishly but is cyclodimerized more reasily than hexadiene. The intermediate cycloolefins are dehydrogenated to form aromatic hydrocarbons (benzene from hexadiene and C10 aromatics from piperylene) while the liberated H2 hydrogenates the starting dienes to the corresponding olefins.