Showing papers on "Hydrogen atom abstraction published in 1988"
TL;DR: In this paper, a fixed-bed flow reactor, operating at atmospheric pressure, achieved a 45% C 2 (sum of ethane and ethylene) selectivity at a 33% methane conversion over 2.0 g of 15 wt% Na CaO catalyst at 725 °C with a gas mixture of CH 4 O 2 = 2.
149 citations
TL;DR: In this article, the reaction of OH radicals with dimethyl sulfoxide (DMSO) in aqueous solutions was investigated and the maximum yield in the radiolysis experiments was 86%.
Abstract: The reaction of OH radicals with dimethyl sulfoxide (DMSO) in aqueous solutions was investigated. The OH radicals were produced via radiolysis of N/sub 2/O saturated aqueous solutions and via Fenton's reagent. With Fenton's reagent the authors observed a quantitative conversion of OH radicals to CH/sub 3/ radicals, giving CH/sub 4/ and C/sub 2/H/sub 6/. The maximum yield in the radiolysis experiments was 86%. In both cases C/sub 2/H/sub 6/ was a major product with the C/sub 2/H/sub 6//CH/sub 4/ ratio as high as 45. The C/sub 2/H/sub 6//CH/sub 4/ ratio depends on the steady-state concentration of CH/sub 3/ radicals and on the DMSO concentration. Results show that CH/sub 3/ radicals in addition to abstracting hydrogen from methanesulfinic acid also abstract hydrogen from DMSO to give CH/sub 4/. In the radiolysis of deuteriated DMSO they observed a much higher ethane/methane ratio than in the nondeuteriated DMSO. The isotope effect supports the hydrogen abstraction from DMSO by CH/sub 3/ radicals. 9 references, 6 tables.
125 citations
TL;DR: In the MM2(87) program as mentioned in this paper, the van der Waals radius of the hydrogen involved in hydrogen bonding was reduced by about 3 kcal/mol, depending on the particular atoms involved.
Abstract: Hydrogen bonding is qualitatively accounted for in the early versions of the MM2 program, but not quantitatively. Experimentally, the hydrogen bonds are somewhat shorter and stronger than calculated by MM2. This has been corrected now in MM2(87), by reducing the van der Waals radius of the hydrogen involved in hydrogen bonding for that interaction only, and by increasing the van der Waals' attraction between the hydrogen and the various electronegative atoms to which it can hydrogen bond by about 1–3 kcal/mol, depending on the particular atoms involved. It is now possible to reproduce reasonably well ab initio calculations on simple amides and the methanol dimer, and experimental data on compounds in which a hydroxyl hydrogen is hydrogen bonded to an alcohol, an alkyl chloride, or to a carbon–carbon double bond.
111 citations
89 citations
TL;DR: Lithium-doped ZnO has been found to be a relatively good catalyst for the oxidative dimerization of methane to ethane and ethylene as discussed by the authors, and maximum C2 yields of approximately 15% have been attained and are comparable with those previously observed over Li doped MgO catalysts.
80 citations
80 citations
72 citations
TL;DR: In this paper, the cis-[RuII(L)2(OH2)2]2+ complexes (L = substituted 2,2′-bipyridines or 1,10-phenanthrolines) catalyse oxidation of saturated hydrocarbons to alcohols and ketones by t-butylhydroperoxide; the low values of kinetic isotope effect (kH/kD= 35 for cyclohexane) and tertiary of secondary C-H bond relative reactivities (ktert/ksecca 65 for adamantane
Abstract: The cis-[RuII(L)2(OH2)2]2+ complexes (L = substituted 2,2′-bipyridines or 1,10-phenanthrolines) catalyse oxidation of saturated hydrocarbons to alcohols and ketones by t-butylhydroperoxide; the low values of kinetic isotope effect (kH/kD= 35 for cyclohexane) and tertiary of secondary C–H bond relative reactivities (ktert/ksecca 65 for adamantane) are in a range expected for hydrogen atom abstraction by radical species
62 citations
TL;DR: The use of 16α,19-hydroxytestosterone, labelled with 3H predominantly in the HSi position, led to the conclusion that in oestriol biosynthesis the step, –CH2OH→–CHO, is accompanied by the loss of HRe and in the overall process the C-19 is ejected as HCOOH.
Abstract: Methods for the synthesis of 19-hydroxy and 19-oxo derivatives of 16α-hydroxytestosterone [(11 b) and (12b) respectively] have been developed These compounds were labelled with 18O, 2H, and 3H at C-19 and also with 3H at C-17 The conversion of [17α-3H]-16α,19-dihydroxytestosterone (11c) into oestriol (4c), using human placental aromatase was demonstrated in good yield and it was shown that in this process the 19-oxo compound (3c) is involved as an intermediate The use of 16α,19-hydroxytestosterone, labelled with 3H predominantly in the HSi position, led to the conclusion that in oestriol biosynthesis the step, –CH2OH→–CHO, is accompanied by the loss of HRe and in the overall process the C-19 is ejected as HCOOH On conducting experiments with either 18O2 or substrate containing 18O at C-19 it was shown that, in the conversion of 16α-hydroxy-19-oxotestosterone into oestriol, an atom of oxygen from O2 is incorporated into the formate These features are similar to those already established for the corresponding biosynthesis of oestrone/oestradiol from androstenedione/testosterone Our previous postulate that in oestrogen biosynthesis the same enzyme is involved in the hydroxylation reaction, –CH3→–CH2OH, and in the conversion, –CH2OH→–CHO, as well as in the final cleavage of the C-10–C-19 bond is further developed Attention is drawn to the fact that, if cytochrome P-450 dependent reactions are viewed to occur via a radical mechanism, then a concept can be developed which unifies the wide variety of transformations catalysed by this group of enzymes The diversity of reactions would then arise from the alternative mode of decomposition of radical species by one or a combination of the following processes: (a) hydrogen abstraction; (b) disproportionation; (c) fragmentation; and (d) association of radicals
62 citations
TL;DR: In this article, it was shown that only odd-electron ions undergo reactive ion/surface collisions, in line with the thermochemical advantage of their acquisition of the more stable evenelectron configuration.
51 citations
TL;DR: In this paper, the gas phase reactions between hydroxyl radicals and Cn alcohols (n⩾2) are described and the fraction of OH-alcohol reactivity that proceeds through this catalytic channel depends on the detailed molecular structure of the alcohol.
TL;DR: In this article, the conjugate addition via LSA followed by enolate trapping with electrophiles produces the corresponding α-substituted β-amino esters, which are, in turn, converted into β-lactams and α, β-unsaturated esters.
TL;DR: In this paper, the authors studied the isomerization and dissociation reactions of a family of distonic amine ions, in order to discover the characteristic reactions of these species and examine the difference between these reactions and those of the corresponding conventional molecular ions.
Abstract: The characteristic reactions of a family of distonic radical cations show that the gas-phase chemistry of these species differs considerably from that of the isomeric conventional molecular ions. Fifteen distonic radical cations, derived from 'CH2CHZNH3+ by introduction of alkyl substituents at C or N, have been demonstrated to be stable species whose reactions are characteristic of their structure. The predominant fragmentation and isomerization processes are the following: (i) simple cleavage of a C-N bond with formation of alkene ions or alkene molecules, (ii) 1,2-migration of protonated amino groups, and (iii) formation of ammonium ions by elimination of alkenyl radicals. Chain length permitting, these reactions may be preceded or acconpanied by specific intramolecular hydrogen abstraction reactions, which lead to isomeric distonic ions with characteristically different reactions. The hydrogen abstraction reactions are generally reversible; in the presence of long alkyl groups isomerization to amine molecular ions (et vice versa) can occur. Correspondingly, the reactions of many low-energy amine molecular ions require isomerization to distonic intermediates prior to fragmentation. A considerable number of recent papers have demonstrated the existence in the gas phase of stable distonic radical cations,'-I4 and distonic ions are with increasing frequency being suggested as key intermediates in unimolecular fragmentation The collision-induced reactions of these species have often been used to establish their structural integrity, but the question of how the particular properties of distonic ions influence their reactions has not been addressed in the literature. We have studied the isomerization and dissociation reactions of a family of distonic amine ions, in order to discover the characteristic reactions of these species and to examine the difference between these reactions and those of the corresponding conventional molecular ions. The term distonic ion was coined by Radom and co-workersZ3 to describe radical cations arising (formally) by ionization of zwitterions or diradicals. The difference between the properties of stable distonic ions and those of conventional molecular ions generated by direct ionization of neutral molecules can be con- siderable, and Radom's neologism has rapidly gained acceptance among gas-phase ion chemists. However, the properties of distonic ions often vary with the number of heavy atoms separating the charge and radical bearing sites. In the present paper we employ the adjectives a-distonic, P-distonic, y-distonic, etc., to denote respectively species with charge and radical at adjacent atoms (a-distonic ions or ylidions, e.g., l), with charge and radical separated by one heavy atom (P-distonic ions, e.g. 2), by two heavy atoms (y-distonic ions), etc. Recent experimental and ab initio st~dies~'-~~~~~~ ~~~~~ of the various CHSN'+ and C2H7N'+ isomers have shown that a- and
TL;DR: In this article, the authors describe a photolysis of several hydroxy compounds in the presence of diphenylselenuranes (1) and iodine, which undergo intramolecular hydrogen abstraction to afford cyclic ethers in good yield.
TL;DR: Etude du mecanisme de reduction des α-nitrosulfones par la (dimethyl-1,3 phenyl-2) benzimidazoline.
Abstract: Etude du mecanisme de reduction des α-nitrosulfones par la (dimethyl-1,3 phenyl-2) benzimidazoline
TL;DR: In this paper, the results of ab initio molecular orbital calculations support the proposal that hyperconjugative delocalisation onto the phosphorus ligand competes with conjugative delocation onto the ring in the complexed arylboryl radicals.
Abstract: The ligated arylboryl radicals L→ḂHAr [L = Me3N, Et3P, or (MeO)3P; Ar = Ph or p-ButC6H4] have been generated in oxirane solvent by hydrogen atom abstraction from L→BH2Ar using t-butoxyl radicals produced by u.v. photolysis of di-t-butyl peroxide. The e.s.r. spectra of the phosphine- or phosphite-ligated radicals show that there is substantial conjugative delocalisation of the unpaired electron from boron onto the aromatic rings, although this delocalisation is less extensive than in comparable benzylic carbon-centred radicals. The results of ab initio molecular orbital calculations support the proposal that hyperconjugative delocalisation onto the phosphorus ligand competes with conjugative delocalisation onto the ring in the complexed arylboryl radicals. The e.s.r. spectra of the amine–arylboryl radicals were too weak to detect, although these radicals and Et3P→ḂHAr abstract halogen atoms readily from alkyl bromides to afford spectra of the corresponding alkyl radicals. The ligated arylboryl radicals are less reactive and more selective in bromine atom abstraction than homoleptic ligated alkylboryl radicals, presumably because the former are appreciably stabilised by conjugative delocalisation of the unpaired electron onto the aromatic rings.
TL;DR: Not only does the activated NCS-chrom product have the same structure as the inactivated drug without DNA, but two of the incorporated deuterium atoms have been substituted by hydrogen.
Abstract: The antitumor antibiotic neocarzinostatin exhibits its main drug action by abstracting hydrogen from DNA deoxyribose with consequent strand breakage or related lesions. All biological activities of the drug derive solely from a nonprotein chromophoric substance (NCS-chrom) consisting of a novel epoxy-bicyclo-diyne-ene system. Thiol or sodium borohydride activates NCS-chrom into a labile, reactive species that induces DNA damage but causes inactivation of the drug in the absence of the target DNA. Since three deuterium atoms can be incorporated into the drug by treatment with sodium borodeuteride without DNA, adding an unlabeled DNA under parallel conditions permitted the ready identification of the activated NCS-chrom product that abstracted hydrogen from the DNA. Not only does the activated NCS-chrom product have the same structure as the inactivated drug without DNA, but two of the incorporated deuterium atoms have been substituted by hydrogen. With the aid of NMR spectrometry, the two replaced hydrogen atoms are found to be incorporated into the C-2 and C-6 positions of the bicyclo-diyne-ene ring of NCS-chrom and are derived neither from borodeuteride nor from the hydroxyl functions of the solvents. In accord with current proposals, the two hydrogens incorporated into the drug may come from closely opposed sites on themore » complementary strands of the DNA at which the drug is bound. These findings are consistent with a diradical mechanism in which carbon-centered radicals at C-2 and C-6 of the activated drug result in abstraction of hydrogen atoms either from the deoxyribose moiety of DNA or, in the absence of DNA, from some other source, but not from the hydrogen attached to the sulfur of thiols under the conditions described.« less
TL;DR: In this paper, the authors used an usual merry-go-round apparatus to estimate the yields for photocydization of 2,4,6-triisopropybenzophenones in the solid state.
TL;DR: The complex (5, 10, 15, 20, 20tetraphenylporphinato)bis(tri-n-butylphosphine)iron(II), 1 has been isolated in a triclinic crystal form as discussed by the authors.
Abstract: The complex (5, 10, 15, 20-tetraphenylporphinato)bis(tri-n-butylphosphine)iron(II), 1, has been isolated in a triclinic crystal form, a = 12499(3), b = 12528(2), c = 12039(2) A, α = 11639(1), β = 10979(1), γ = 9813(1)°, Z = 1, space group The structure was solved by conventional heavy atom methods and was refined by full-matrix least-squares procedures to R = 0060 and Rw = 0070 for 3551 reflections with I ≥ 3σ(I) The molecule, which has crystallographic , symmetry, displays a relatively undistorted pseudo-octahedral coordination geometry with Fe—P = 2346(1) and Fe—N = 1998(3) and 1993(3) A In CH2Cl2 solution, 1 reacts with CO and aldehydes to generate the carbonyl(phosphine) derivative, and decarbonylation of phenylacetaldehyde to toluene becomes catalytic under an Ar stream The aldehyde decarbonylations involve radical pathways via carbonyl loss from The hydrogen abstraction/initiation reaction probably utilizes trace O2 (and possibly trace (water); speculative mechanisms are discussed
TL;DR: In this article, it was shown that the underlying mechanism for the formation of this species is based on pH-dependent conformational changes of glutathione followed by ˙OH addition to the cysteine sulphur, or hydrogen abstraction from the CH2 group in the α-position to the sulphur leading to the forming of a reducing carbon-centred radical.
Abstract: The ˙OH-induced oxidation of glutathione in basic aqueous solution has been found to produce the sulphur-centred radical GS˙(GSSG–˙) and two reducing radicals. Decarboxylation, considered to be initiated by ˙OH radical addition to the nitrogen of the amino group on the γ-glutamyl unit, was found to be effective for methylated glutathione (GSMe) and opthalmic acid (GMe) but of minor importance for glutathione. The G-values for the strongly reducing α-amino radicals formed on decarboxylation were found to be 3.3, 3.3, and 0.5 for GSMe, GMe, and GSH, respectively, at pH 10.5. The formation of an additional strongly reducing radical with G= 2.2 at pH 10.5 was demonstrated by measuring the electron-transfer to p-nitroacetophenone (PNAP). This radical is not formed from GSMe or GMe. We suggest that the underlying mechanism for the formation of this species is based on pH-dependent conformational changes of glutathione followed by ˙OH addition to the cysteine sulphur, or hydrogen abstraction from the CH2 group in the α-position to the sulphur leading to the formation of a reducing carbon-centred radical.
TL;DR: In this paper, the effects of vibrational excitation of the ν2 mode of ND+3 (v) and NH+3(v) on the three channels of their reaction with NH3 and ND3, respectively, are investigated up to v=12 in the center-of-mass kinetic energy range from 0.9 to 4.5 eV by use of the TESICO technique.
Abstract: The effects of the vibrational excitation of the ν2 mode of ND+3 (v) and NH+3 (v) on the three channels of their reaction with NH3 and ND3 , respectively, are investigated up to v=12 in the center‐of‐mass kinetic energy range from 0.9 to 4.5 eV by use of the TESICO technique. The ratio (γ) of the hydrogen/deuterium atom abstraction cross section over the competing deuteron/proton transfer cross section has a maximum systematically at a vibrational level (vmax ) slightly higher than that giving maximum Franck–Condon factor (vFC ) for the neutralization of ND+3 (v)/NH+3 (v). A new reaction model based on a nonadiabatic transition theory and the potential energy surface calculated for the ammonia dimer cation is proposed to explain the experimental results. The hydrogen/deuterium atom abstraction reaction is interpreted as a near resonant electron jump at larger intermolecular separation followed by proton/deuteron transfer which proceeds on the ‘‘electron (charge)–transferred’’ potential surface.
TL;DR: In this paper, the photochemistry of five novel water-soluble para-substituted benzophenone derivatives is examined in water and 2-propanol using absorption, luminescence and conventional flash photolysis techniques.
Abstract: The photochemistry of five novel water-soluble para-substituted benzophenone derivatives is examined in water and 2-propanol using absorption, luminescence and conventional flash photolysis techniques. The data are related to their ability to photoinduce the polymerization of 2-hydroxyethyl methacrylate in aqueous media. Photopolymerization of the latter is found to occur effectively only in the absence of oxygen and the presence of a secondary or tertiary amine. In the latter case, the lower the ionization potential of the amine the more efficient is the photopolymerization. These results are consistent with the spectroscopic and flash photolysis data which show a correlation between the photoinitiation activities and the measured phosphorescence quantum yields of the initiators. On flash photolysis, the lowest excited triplet state of the initiator is abstracting an electron from the amine via an excited triplet exciplex followed by hydrogen abstraction to give the ketyl radical. This is confirmed through a detailed study on the effect of oxygen, pH and the ionization potential of the amine on transient formation. The significance in the behaviour of these initiators in contrast to the water-soluble thioxanthones is discussed.
Journal Article•
TL;DR: In this article, the intramolecular hydrogen abstraction of nifedipine was performed from the triplet excited sitate, which was shown to yield 2,6-dimethyl-3,5-dicarbomethoxy-4-(2'-nitrosophen yl)-pyridine with the quantum yield 0.37.
Abstract: Irradiaton of nifedipine in methylene chloride at 366 nm yielded 2,6-dimethyl-3,5-dicarbomethoxy-4-(2'-nitrosophen yl)-pyridine with the quantum yield 0.37, while irradiation at 254 nm initially gave nitroso compound which in turn is photooxidized to 2,6-dimethyl-3,5-dicarbomethoxy-4-(2'-nitrophenyl) -pyridine with the quantum efficiency of 0.014 on further irradiation in the presence of oxygen. The intramolecular hydrogen abstraction of nifedipine proceeded from the triplet excited sitate.
TL;DR: For instance, reaction d'alkyl-1 tripyrrolidino benzenes avec le nitrate d'argent: obtention des bicyclohexadienylium correspondants (complexes sigma dimeres); transformation de ces complexes en biphenyles ou en radical libre cationique; andude RX de 2 of 2 of the complexes (alkyl H ou ethyl).
Abstract: Reaction d'alkyl-1 tripyrrolidino benzenes avec le nitrate d'argent: obtention des bicyclohexadienylium correspondants (complexes sigma dimeres); transformation de ces complexes en biphenyles ou en radical libre cationique; etude RX de 2 de ces complexes (alkyl-H ou ethyl). Reaction des trimorpholino- et tripiperidino-1,3,5 benzenes avec Br 2 ou XCN: obtention des produits de dimerisation (biphenyles) et des produits de monobromation sur le benzene
TL;DR: Ionized ketones are shown to eliminate alkanes by processes in addition to the previously characterized 1,2-eliminations of alkyl moieties and an opposite α-hydrogen.
TL;DR: In this article, the authors investigated factors affecting the production of amidocarboxy-substituted free radicals and showed that hydrogen-atom transfer reactions may afford the less stable products if electrophilic radicals are involved in the hydrogen atom abstraction.
Abstract: Factors affecting the production of amidocarboxy-substituted free radicals have been investigated by examining reactions of derivatives of valine and sarcosine. Variations in the regioselectivity of reactions of these compounds are exemplified by the reactions of N-benzoylvaline methyl ester (4a) and N-benzoylsarcosine methyl ester (12a) with sulphuryl chloride and N-bromosuccinimide. Whereas the reaction of (4a) with sulphuryl chloride involves hydrogen-atom transfer from the β-position of (4a) with subsequent chlorine incorporation to give (4g), in direct contrast the reaction with N-bromosuccinimide proceeds via hydrogen-atom abstraction from the α-position of (4a) and yields the dibromide (4d). N-Benzoylsarcosine methyl ester (12a) reacts with N-bromosuccinimide to give the α-bromosarcosine derivative (12b), whereas with sulphuryl chloride the product is the N-chloromethylglycine derivative (12c). These studies indicate that amidocarboxy-substituted radicals such as (3a) and (13) are considerably more stable than the tertiary alkyl radical (1a) and the amidosubstituted radical (14), respectively, but hydrogen-atom transfer reactions may afford the less stable products if electrophilic radicals are involved in the hydrogen-atom abstraction and if there is little development of radical character in the reaction transition state.