Coupling Dimer Research Articles

Substrate free radicals are intermediates in ligninase catalysis.

The H2O2-requiring ligninase of the basidiomycete Phanerochaete chrysosporium oxidatively cleaves both lignin and lignin model compounds between C alpha and C beta (C-1 and C-2) of their aliphatic side chains. Previous work has demonstrated a reaction mechanism by which ligninase oxidizes aromatic substrates to their cation radicals, which then undergo side chain cleavage to yield carbon-centered free radicals. These carbon-centered radicals add O2 to give substrate peroxyl radicals that react further to yield the hydroxylated and carbonylated end products usually seen in experiments with ligninase. To investigate this radical mechanism, we have now designed three dimeric lignin models: 1-(3,4-dimethoxyphenyl)-2-phenylethanol (I), 1-(3,4-dimethoxyphenyl)-2-phenylpropanol (II), and 1-(3,4-dimethoxyphenyl)-2-methyl-2-phenylpropanol (III). The following results were obtained when these models were oxidized by ligninase: methyl groups at C beta of the substrate favored C alpha-C beta cleavage versus C alpha oxidation to the ketone. GC/MS and HPLC analysis showed that II gave a radical coupling dimer, 2,3-diphenylbutane, as a major (26% yield) reaction product under anaerobic conditions. The anaerobic oxidation of III yielded 2,3-dimethyl-2,3-diphenylbutane. Spin-trapping experiments with nitrosobenzene showed that model II oxidation produced alpha-methylbenzyl radicals, whereas model III oxidation gave alpha, alpha-dimethylbenzyl radicals. TLC and iodometric determinations showed that III gave cumene hydroperoxide as a major (21% yield) reaction product in air. These findings demonstrate that carbon-centered and peroxyl radicals at C beta are major products of C alpha-C beta cleavage by ligninase.

Oxidative dimerization of substituted o-methylphenyl isocyanides; synthesis of bidentate bis-isonitrile and cyclic tetradentate tetrakis-isonitrile ligands

Substituted o-methylphenyl isocyanides ( 1) were oxidatively dimerized via the corresponding o-lithiomethylphenyl isocyanides ( 2) by means of 1,2-dibromoethane to give 1,2-bis(2-isocyanophenyl)ethane derivatives ( 3). 1,2-Bis(2-isocyano-3-methyl-phenyl)ethane ( 3e), a coupling dimer of 2,6-xylyl isocyanide ( 1e), was further dimerized via its dilithiated intermediate to give a cyclic tetramer of 2,6-xylyl isocyanide ( 1e), which may be a cyclic tetradentate ligand for transition metal compounds.

Effects of activated carbon on the reactions of free chlorine with phenols

The use of prechlorination in drinking water treatment results in contact of free chlorine with activated carbon which has been added to remove organic compounds from water. The chlorine then reacts with the carbon and adsorbed compounds. Free chlorine reacts readily with a group of phenolic compounds (phenol, guaiacol, catechol, 2,6-dimethoxyphenol, and p-chlorophenol) in dilute aqueous solutions (10/sup -5/ M) to produce mono-, di-, or trichloro derivatives, but when it reacts with phenols adsorbed on granular activated carbon (GAC), many additional products are formed. GAC exposed to chlorine becomes capable of promoting reactions such as hydroxylations of the aromatic ring, oxidation to quinones, chlorine substitution, carboxylation, and oxidative coupling (dimer formation). The formation of chlorohydroxybiphenyls (hydroxylated PCBs) (in vivo metabolites of PCBs) is particularly important because of their potential toxicity. Such compounds are the main reaction products from chlorophenols, but they are also formed in smaller amounts from nonchlorinated phenols (phenol and guaiacol).

Mechanistic action of a new p‐hydroxybenzoate light stabiliser in polypropylene film: A spectroscopic study

AbstractThe photostabilising action of a new aliphatic p‐hydroxybenzoate light stabiliser has been examined in polypropylene film together with the aid of 3,5‐di‐tert‐butyl‐4‐hydroxybenzoic acid by both normal and derivative UV absorption and IR spectroscopy, and hydroperoxide analysis. The stabiliser has also been examined in solution together with a number of related compounds using micro‐second flash photolysis. During processing the stabiliser operates as an effective chain breaking donor by terminating macroalkyl radicals and inhibiting the formation of hydroperoxides. Under both monochromatic (365 nm) and polychromatic (λ > 320 nm) irradiation conditions the decomposition of the stabiliser shows a direct dependence on initial hydroperoxide concentration in the film, indicating that it operates as an effective light stable alkoxy and hydroxy radical scavenger. The protective effect of a peroxide decomposer confirms this mechanism. Evidence is also presented to show that under photosensitised radical attack the presence of the long aliphatic hydrocarbon chain has a powerful protective effect on the molecule. On flash photolysis of all the compounds studied end‐of‐pulse transient absorption spectra were assigned to phenoxy radicals. The radicals decayed by a second order process to give dimeric coupling products. The nature of these products are discussed and identified for the compounds studied. The technique of flash photolysis is concluded to be a useful method for assessing anti‐oxidant activity.

Competitive pathways in the electrochemical reduction of activated olefins Hydrogenation vs, dimerization of fumarodinitrile in water

The competition between electrochemical dimerization and hydrogenation of the double bond in the reduction of activated olefins in water has been systematically investigated on the example of fumarodinitrile. The addition of a surfactant, here tetrabutylammonium paratoluenesulphonate, to the solution, suppresses the surface character of the protonation of the initially formed anion radical. The kinetic data were obtained using classical polarography in buffered medium as a function of the pH and of the substrate and buffer concentration. Analysis of the experimental data in terms of several theoretical models allows the following conclusions to be drawn as to the mechanism of the overall electrochemical reaction. The p K A of the protonated anion radical-anion radical couple appears to be >7.2. Protonation thus proceeds irreversibly. The dimerization reaction occurs essentially through the radical-radical coupling of two anion radicals. The interference of protonated anion radicals in the dimerization process, through self-coupling or coupling with anion radicals, if any, appears as a minor pathway. The protonated anion radicals tend to be reduced faster than they can dimerize. Their reduction occurs both by electrode-electron transfer and by homogeneous electron transfer, the reductant then being the anion radical.

Competitive pathways in the electrochemical reduction of activated olefins: Hydrogenation vs. dimerization of fumarodinitrile in water

The competition between electrochemical dimerization and hydrogenation of the double bond in the reduction of activated olefins in water has been systematically investigated on the example of fumarodinitrile. The addition of a surfactant, here tetrabutylammonium paratoluenesulphonate, to the solution, suppresses the surface character of the protonation of the initially formed anion radical. The kinetic data were obtained using classical polarography in buffered medium as a function of the pH and of the substrate and buffer concentration. Analysis of the experimental data in terms of several theoretical models allows the following conclusions to be drawn as to the mechanism of the overall electrochemical reaction. The p K A of the protonated anion radical-anion radical couple appears to be >7.2. Protonation thus proceeds irreversibly. The dimerization reaction occurs essentially through the radical-radical coupling of two anion radicals. The interference of protonated anion radicals in the dimerization process, through self-coupling or coupling with anion radicals, if any, appears as a minor pathway. The protonated anion radicals tend to be reduced faster than they can dimerize. Their reduction occurs both by electrode-electron transfer and by homogeneous electron transfer, the reductant then being the anion radical.

Heat capacity of a five-coordinated iron(III) complex, iodobis (N, N-Dimethyldithiocarbamato) iron(III), between 0.4 and 300 K

The heat capacity of iodobis ( N, N-dimethyldithiocarbamato)iron(III) has been measured between 0.4 and 300 K. Based on the heat capacity and entropy at low temperatures it was found that the present sample consists of a mixture of monomer ( ca. 40%) and dinier ( ca. 60%); the former brings about a λ-type phase transition from an antiferromagnetic to a paramagnetic state at T N = (1.65 ±0.04) K while the latter exhibits a Schottky-ype anomaly due to antiferromagnetic dimeric coupling, the effect of which becomes dominant below ca. 0.7 K. The zero-field splitting parameter of a single ferric ion was estimated to be D k = D D k = 14 K for the monometer and the dimer, while the dimeric coupling constant was J D k = −0.15 K . The entropy at low temperatures cannot be accounted for solely by the spin manifold. Additional contribution from a tunnel-splitting of the rotational levels of four constituent methyl-groups has been discussed. In this case, the level splitting of the ground state is 2.5 J mol −1 and the barrier height of hindering potential is 2.3 kJ mol −1.

Solid phase synthesis of polynucleotides. VIII. Synthesis of mixed oligodeoxyribonucleotides by the phosphotriester solid phase method.

A solid phase method for the simultaneous synthesis of mixed oligonucleotides using a phosphotriester approach has been developed. For this synthesis, a mixture of mono or dimeric coupling units is used, and a slight difference in the reactivity of those units is found. However, this difference does not hamper the simultaneous, mixed oligonucleotide synthesis, and the sequence analysis of a product demonstrates the existence of all desired sequences in the final mixture.

Heat capacity of a five-coordinated iron(III) complex with spin [formula omitted], bromobis( N, N-diethyldithiocarbamato)iron(III), between 0.4 and 300 K

Heat capacity measurements have been made for six kinds of specimens prepared by different methods. Among them, Sample A exhibited a A-type ferromagnetic pahse transition at 1.347 K and a Schottky-type anomaly due to the zero-field splitting around 9K. The total entropy and enthalpy were (11.05 ± 0.04) J K −1mol −1 and (97.0 ± 0.4) J mol −1, respectively. Sample B exhibited a Sehottky-type anomaly around 0.4 K due to the ferro-magnetic dimeric coupling with J D k = + 0.30 K as well as the Schottky-type anomaly at 9K. The total magnetic entropy and enthalpy were (11.45 ± 0.03) JK −1 mol −1 and (93.8 ± 0.8) J mol −1, respectively. The remaining samples are simple mixtures of the λ-type modification and the dimeric modification. Irrespective of the magnetic behavior at low temperatures, all the samples showed a non-magnetic first-order phase transition around 270 K. The heat capacity and entropy of this phase transition have been accounted for in terms of the Frenkel theory of heterophase fluctuation. Construction of an adiabatic-type calorimeter workable between 1.5 and 393 K has been also presented.

Mecanisme de la reduction electrochimique des carboxy-3 et -4 coumarines et de leurs esters ethyliques

Mechanisms of the electroduction of the title compounds are settled from polarographic and voltammetric studies in protic and aprotic media. Ethyl 4-coumarin carboxylate is reduced according to the sequences H + eeH + in acidic media and eeH +H + in basic media; for 4-coumarin carboxylic acid, a DISP 2 mechanism is revealed. These results explain the different behaviour of the two compounds with respect to asymetric synthesis of substituted dihydrocoumarins. If substituent is in position −3, the reduction leads to hydrodimers. They may result from radical—anion substrate coupling or radical anion dimerization. Reaction of the dianion with the substrate clearly appears in DMF solutions.

ChemInform Abstract: MONOTERPENOID SYNTHESIS BY TRANSITION METAL CATALYZED COUPLING OF ENEDIYLMAGNESIUM WITH C5 ORGANIC HALIDES

AbstractReaktion der Endiylmagnesiumverbindung (I) mit dem Chlorisopenten (II) liefert ‐ mit oder ohne Katalyse durch Übergangsmetalle ‐ die Isodecadiene (III) und (IV) in wechselnden Ausbeuten.

Monoterpenoid Synthesis by Transition Metal Catalyzed Coupling of Enediylmagnesium with C5-Organic Halides

Abstract A series of isoprene coupling dimers bonded at 1-2, 1-3, 1-4, 2-4, 3-4, or 4-4 position was prepared by regiocontrolled catalysis of trasition metals or without catalysts in the reaction of 2-methyl-2-butene-1,4-diylmagnesium or 3-methyl-2-butenylmagnesium chloride with C5-alkenyl halides.

Nuclear-magnetic-resonance study of the conformation of a dinucleotide in solution using the lanthanide probe method.

The conformation of the dinucleotide adenylyl(3'-5')adenosine 2'-phosphate (ApA2'p) in aqueous solution at different pH values and temperatures has been studied using the lanthanide(III) ethylenodiaminetetraacetate(EDTA) 1:1 complexes as shift and relaxation probes. The conformational analysis, based on mixing different conformations in fast exchange within the nuclear magnetic resonance time scale, agrees well with the results from vicinal coupling constants and dimerization shifts obtained for the metal-free system. The dinucleotide exists in a temperature-dependent and pH-dependent conformational equilibrium between unstacked and base-stacked forms. At neutral pH and low temperature, the stacked form predominates, and it is a predominantly right helical structure, characterized as an anti, 3E, g-, g- g-, g'g', gg, 3E, anti conformation. This situation contrasts with adenylyl(3'-5')adenosine (ApA), where both right and left helices contribute to the stacked form. The nucleotidyl units of the unstacked form of ApA2'p have average conformations which are very similar to those of the corresponding mononucleotides in similar conditions.

Cationic heterocyclisation One‐step synthesis of condensed heterocyclics

AbstractAcid treatment of N‐aryl‐5‐hydroxy‐2‐pyrrolidinones and their ethers obtained from N‐arylsuccinimides by means of NaBH4/H+ reduction leads to mixtures of dimeric coupling products which may give additional ring closure with the N‐aryl substituent.

Short range antiferromagnetic coupling in bis(N,N-dialkyldithiocarbamato) ironIII halides

Several bis(N,N-dialkyldithiocarbamato) ironIII have been studied by Mössbauer spectroscopy in the presence of external magnetic fields. The spectra have shown strong evidence of dimeric antiferromagnetic coupling between the molecular units in the crystal lattice. The dimeric coupling occurs through superexchange interacton between different molecules. The magnitude of the coupling constant JD, which was estimated from the magnetically perturbed Mössbauer spectra, was found to be of the order of 0.25–0.50 K for the complexes studied here.

Stable carbocations. Part 16. Properties of 2-substituted exo-5,6-trimethylenenorbornan-2-ylium ions

In formic acid solution, the 2-methyl-exo-5,6-trimethylenenorbornan-2-ylium ion gives products resulting from Wagner–Meerwein rearrangement, 3,2-hydride shift, and cation–alkene coupling. Under similar conditions, the 2-phenyl analogue also gives dimeric coupling products, whereas the 2-ferrocenyl analogue is stable. The last-mentioned cation has been obtained in stereoisomeric forms which interconvert in trifluoroacetic acid solution.

The Magnetic Susceptibility of Two Coupled Dimers Consisting of S=1/2 Spins

Abstract Tetramers consisting of two coupled dimers were classified as (a) linear, (b) cyclic, (c) rhombic, or (d) tetrahedral. The magnetic susceptibility for each tetramer model consisting of S=1/2 spins was obtained as an analytical equation. For the linear tetramer, the susceptibility equation was calculated numerically and compared with the observed values.

ChemInform Abstract: PALLADIUM‐CATALYZED COUPLING REACTION OF AROMATIC COMPOUNDS

AbstractPd‐diacetat in Gegenwart einer äquivalenten Menge Acetylaceton stellt einen guten Katalysator für die Kopplung (bei 150°C / 50 at) von Toluol zu einem Gemisch der isomeren Tolyltoluole (Hauptprodukte 3,4′‐ und 3,3′‐Dimethylbiphenyl) dar.

Palladium-Catalyzed Coupling Reaction of Aromatic Compounds

Abstract The oxidative coupling reaction of aromatic compounds proceeds catalytically with palladium salt under oxygen pressure. Yields and isomer distributions are attributed to the reaction variables such as temperature, additives, and the nature of the substituent on a benzene ring. Naphthalene was converted mainly into 1-substituted products and o-xylene into 4-substituted products. The coupling of diphenyl ether afforded diphenylene oxide with coupling dimers. An intramolecular two stage dehydrogenation process is proposed.

Nitrate as an Oxidant in the Reaction of Silver(I) Salts with Grignard Reagents

Abstract The reaction between various Grignard reagents and silver(I) salts in the presence of nitrogen-containing oxidants such as lithium nitrate, nitrogen dioxide, and methyl nitrate has been examined in tetrahydrofuran solutions. Coupling dimer is formed in high yields almost independently of the structure of the Grignard reagent. From the stoichiometry, it is concluded that these oxidants oxidize two equivalents of metallic silver formed in situ to silver(I) species under proper reaction conditions. Other transition metal halides are also oxidized by these oxidants in a similar manner. Soluble forms of reduced silver complexes formed in the reaction of silver(I) nitrate and Grignard reagent are also described.