Showing papers on "Bond cleavage published in 1986"
204 citations
TL;DR: The mechanism of ethanol decomposition on the Ni(111) surface has been investigated between 155 and 500 K as mentioned in this paper, and the sequence of bond scission steps which occur as ethanol undergoes dissociative reactions on this surface was deduced using deuterium and 13 C isotopic labels.
158 citations
TL;DR: In this paper, the authors describe a C-O bond cleavage reaction at elevated temperatures to afford the oxo alkyl derivative, Cp*/sub 2/Ta(CH/sub 3/)(=O).
Abstract: Transition-metal complexes of the general class L/sub n/M(H)(=X) (X = CH/sub 2/; C = CH/sub 2/, O) are of interest as models for reactive intermediates in a variety of catalytic processes. Hydrido alkylidene complexes, generated by a 1,2-hydrogen shift from metal alkyls, have been postulated to play an important role in the metathesis and polymerization of olefins, while surface-bound vinyldene species have been detected and recently proposed to account for linear/branched hydrocarbon distributions in Fischer-Tropsch synthesis. The third member of the class described here, L/sub n/M(H)(=O), is of fundamental importance in reactions involving H/sub 2/O, O/sub 2/, or peroxides. Although an example of an oxo hydride has not previously been reported, several oxo alkyl species have been characterized recently. The authors have described the preparation of a number of permethyltantalocene derivatives and commented on the remarkable stability conferred by two pentamethylcyclopentadienyl ligands. This important facet and a general reluctance for permethyl-metallocene derivatives to undergo oligomerization processes have allowed us to prepare and investigate the reaction chemistry of a number of monomeric hydrido derivatives of general formula Cp*/sub 2/ Ta(H)(=X) (Cp* = eta/sup 5/-C/sub 5/Me/sub 5/; X = CH/sub 2/ (1), C = CH/sub 2/. We have also isolated an eta/supmore » 2/-formaldehyde adduct CP*/sub 2/Ta(H)(eta/sup 2/-CH/sub 2/O). Such species continue to attract interest as models for possible intermediates in CO hydrogenation, and they describe herein a novel C-O bond cleavage reaction at elevated temperatures to afford the oxo alkyl derivative, Cp*/sub 2/ Ta(CH/sub 3/)(=O).« less
111 citations
TL;DR: As the first well-characterized enzymatic reaction of an organometallic substrate and the first example of an enzyme-mediated SE2 reaction the organomercurial lyase catalyzed carbon-mercury bond cleavage provides an arena for investigating novel enzyme structure-function relationships.
Abstract: Mechanistic studies of the protonolytic carbon-mercury bond cleavage by organomercurial lyase from Escherichia coli (R831) suggest that the reaction proceeds via an SE2 pathway. Studies with stereochemically defined substrates cis-2-butenyl-2-mercuric chloride (1) and endo-norbornyl-2-mercuric bromide (2) reveal that a high degree of configurational retention occurs during the bond cleavage, while studies with exo-3-acetoxynortricyclyl-5-mercuric bromide (3) and cis-exo-2-acetoxy-bicyclo[2.2.1]hept-5-enyl-3-mercuric bromide (4) show that the protonolysis proceeds without accompanying skeletal rearrangement. Kinetic data for the enzymatic reactions of cis-2-butenyl-2-mercuric chloride (1) and trans-1-propenyl-1-mercuric chloride (6) indicate that these substrates show enhanced reaction rates of ca. 10-200-fold over alkylvinylmercurials and unsubstituted vinylmercurials, suggesting that the olefinic methyl substituent may stabilize an intermediate bearing some positive charge. Enzymatic reaction of 2-butenyl-1-mercuric bromide (5) yields a 72/23/5 mixture of 1-butene/trans-2-butene/cis-2-butene, indicative of intervening SE2' cleavage. The observation of significant solvent deuterium isotope effects at pH 7.4 of Vmax (H2O)/Vmax(D2O) = 2.1 for cis-2-butenyl-2-mercuric chloride (1) turnover and Vmax(H2O)/Vmax(D2O) = 4.9 for ethylmercuric chloride turnover provides additional support for a kinetically important proton delivery. Finally, the stoichiometric formation of butene and Hg(II) from 1 and methane and Hg(II) from methylmercuric chloride eliminates the possibility of an SN1 solvolytic mechanism. As the first well-characterized enzymatic reaction of an organometallic substrate and the first example of an enzyme-mediated SE2 reaction the organomercurial lyase catalyzed carbon-mercury bond cleavage provides an arena for investigating novel enzyme structure-function relationships.
104 citations
TL;DR: Analysis of the mechanism of porphyrin-mediated strand breakage in terms of the DNA cleavage mechanism of methidium-propyl-iron-EDTA and Fe-bleomycin, the potential of the cationic metalloporphyrins as footprinting probes and as new "reporter ligands" for DNA is presented and discussed.
Abstract: The ability of a group of water-soluble metalloporphyrins to cleave DNA has been investigated. Incubation of Mn3+, Fe3+, or Co3+ complexes of meso-tetrakis(N-methyl-4-pyridiniumyl)porphine (H2T4MPyP) with DNA in the presence of ascorbate, superoxide ion, or iodosobenzene results in DNA breakage. Comparisons between the rates of porphyrin autodestruction with the rates of strand scission of covalently closed circular PM2 DNA indicate that the porphyrins remain intact during the cleavage process. Analysis of the porphyrin-mediated strand scissions on a 139-base-pair restriction fragment of pBR322 DNA using gel electrophoresis/autoradiography/microdensitometry reveals that the minimum porphyrin cleavage site is (A X T)3. The cleavage pattern within a given site was found to be asymmetric, indicating that porphyrin binding and the strand scission process are highly directional in nature. In addition to an analysis of the mechanism of porphyrin-mediated strand breakage in terms of the DNA cleavage mechanism of methidium-propyl-iron-EDTA and Fe-bleomycin, the potential of the cationic metalloporphyrins as footprinting probes and as new "reporter ligands" for DNA is presented and discussed.
98 citations
98 citations
TL;DR: In this article, the preparation of 2,2-bipyridylchromium peroxide, pyridinechromium permoxide, and chromium peroxy etherate is described.
81 citations
TL;DR: EtAlCl2-promoted additions of organocopper reagents to camphor-derived, conjugated N-enoyl-sultams gave saturated and olefinic β-silylcarboxyl derivatives with high diastereodifferentiation as mentioned in this paper.
Abstract: EtAlCl2-promoted additions of organocopper reagents to camphor-derived, conjugated N-enoyl-sultams gave saturated and olefinic β-silylcarboxyl derivatives with high diastereodifferentiation. Nondestructive removal of the chiral auxiliary followed by oxidative Si-C bond cleavage furnished enantiomerically pure acetate-derived aldols and propionate-derived ‘anti’ -aldols (via silyl-directed α-methylation).
79 citations
75 citations
TL;DR: A moderately stable intermediate consisting of cleaved DNA bearing a base propenal precursor is formed after the action of iron-bleomycin and O2 in cleaving DNA has been resolved into two kinetic events following the initial attack on DNA by the kinetically competent drug species.
70 citations
TL;DR: In this article, photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR) was used to study the conversion of C1-C4 alcohols to hydrocarbons over 12-tungstophosphoric acid, H3PW12O40.
TL;DR: In this paper, the photochemical behavior of the two isomeric endoperoxides (9,10-PMO2 and 1,4-PMOMO2) of 1,4-dimethyl-9,lO-diphenylanthracene was found to differ in their kinetics and reaction efficiencies, and it was shown that the generation of IO2 is wavelength dependent, occurring from upper excited singlet states of the endoperoxide, whereas bond cleavage of the 0-0 end-operoxide bond occurs principally from the lowest excited
Abstract: The photochemical behavior of the two isomeric endoperoxides (9,10-PMO2 and 1 ,4-PMO2) of 1,4-dimethyl-9,lO-diphenylanthracene was found to differ in their kinetics and reaction efficiencies. Consistent with the work of Rigaudy et al. and Brauer et al. on a related endoperoxide we find that the generation of IO2 is wavelength dependent, occurring from upper excited singlet states of the endoperoxide, whereas bond cleavage of the 0-0 endoperoxide bond occurs principally from the lowest excited singlet and triplet states. Results of picosecond kinetics and absolute quantum yield measurements are discussed in terms of various concerted and nonconcerted mechanisms for the formation of IO2 and the anthracene fragment.
TL;DR: In this paper, an approach to the regio-controlled synthesis of 1,2- and/or 1,3-diols was proposed. But this approach was not suitable for allyl and homoallyl alcohols and the subsequent oxidative cleavage of the resultant carbon-silicon bond.
TL;DR: The gas-phase reactions of CrO/sup +/ with alkanes have been studied by using ion beam reactive scattering techniques, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr/sup +/-O bond as an initial step.
Abstract: The gas-phase reactions of CrO/sup +/ with alkanes have been studied by using ion beam reactive scattering techniques. CrO/sup +/ undergoes facile reactions with alkanes larger than methane. CrO/sup +/ selectively oxidizes ethane to form ethanol. In addition to the possibility of alcohol formation, reactions with larger alkanes are more complex, yielding products in which dehydrogenation and loss of alkenes and alkanes occur. In reactions with cyclic alkanes, cyclopropane and cyclobutane yield products characteristic of C-C bond cleavage. In contrast, reactions with cyclopentane and cyclohexane mainly involve dehydrogenation and elimination of H/sub 2/O. A series of hydrogen abstraction reactions are examined to determine the bond dissociation energy D/sup 0/ (CrO/sup +/-H) = 89 +/- 5 kcal mol/sup -1/. This bond energy has implications for the reaction mechanisms of CrO/sup +/ with alkanes, leading to the suggestion of a multicenter reaction intermediate, in which alkyl C-H bonds add across the Cr/sup +/-O bond as an initial step. This is supported by an examination of the reactions of Cr/sup +/ with alcohols.
TL;DR: In this article, the scission se fait a l'aide du triethylsilane en presence de Nafion-H dans le dichloromethane.
Abstract: La scission se fait a l'aide du triethylsilane en presence de Nafion-H dans le dichloromethane
TL;DR: The determination of the sites of G-specific strand cleavage in a total of 0.5 kbp of DNA by sequencing gel electrophoresis showed that scission at dG residues is sequence specific and may permit prediction of the Sites of strand scission in DNA molecules of known sequence that have been modified by diol-epoxides of polycyclic hydrocarbons.
Abstract: Three 5'-end-labelled double-stranded linear DNA fragments of defined sequence were treated with r-7, t-8-dihydroxy-t-9, 10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) The DNA samples were then examined by gel electrophoresis both before and after denaturation and treatment with alkali The extent of modification of deoxyguanosine (dG) residues was estimated from changes in electrophoretic mobility: at saturation less than 25% of the dG residues appeared to be modified by reaction with anti-BPDE The determination of the sites of G-specific strand cleavage in a total of 05 kbp of DNA by sequencing gel electrophoresis showed that scission at dG residues is sequence specific and that whilst, for example, cleavage occurred at the central dG residues of all 5'-CGG-3' (21/21), of all 5'-TGG-3' (14/14), of all 5'-TGT-3' (7/7) and of all 5'-CGT-3' (5/5) sequences examined, it did not occur in any of the 5'-GGA-3' (0/12) or 5'-GGC-3' (0/15) sequences and only occurred rarely in the 5'-GGG-3' (1/48) and 5'-GGT-3' (2/11) sequences No cleavage was found at internal dG residues within poly(dG)9 or poly(dG)18 sequences The data may permit prediction of the sites of strand scission in DNA molecules of known sequence that have been modified by diol-epoxides of polycyclic hydrocarbons
TL;DR: In this article, deuterium labeling has been used to investigate processes occurring during the conversion of methyl formate to methanol over silica supported copper at 393 to 419 K. Control experiments show that this process, and a similarly rapid CH3OH + D2 CH3OD + HD equilibration, are catalyzed by copper and not the silica support.
TL;DR: In this paper, the electron-transfer photochemistry of substituted tertiary amines that contain potentially labile C-C bonds adjacent to the amine was investigated and it was shown that photo-induced electron transfer can lead to reactions in which the net process involves use of visible light to mediate selective cleavage of C -C bonds to produce radicals.
Abstract: This paper reports the electron-transfer photochemistry of some substituted tertiary amines that contain potentially labile C-C bonds adjacent to the amine. Results are presented which indicate that photoinduced electron transfer can lead to reactions in which the net process involves use of visible light to mediate selective cleavage of C-C bonds to produce radicals. 34 references.
TL;DR: Comparison of the Kassoc values for binding to calf thymus DNA suggests that the enhanced binding observed with the linker -NH(CH2)3 NH(CH 2)3- contributes to the efficiency of sequence neutral DNA scission and may be a factor in the relative anticancer activities of these agents.
Abstract: A series of metalloporphyrins linked through basic chains to certain DNA interactive groups has been synthesized. Several of these agents reproduce the characteristic properties of the antitumor glycopeptide bleomycin, including the oxygen-mediated scission of DNA in the presence of thiols, antibiobic activity under aerobic conditions, and activity against human and animal tumor models. Initial screening by scission of PM2-CCC-DNA identified six of the compounds, including those bearing acridine and acodazole intercalating groups, as the most active. The specificity of the oxygen-mediated scission of a 139 base pair HindIII/NciI restriction fragment of pBR322 by these six selected agents was then determined and compared with the action of pancreatic DNase by densitometric scans. All six of these compounds produce uniform base and sequence neutral cleavage of the restriction fragment at each base site. The six active compounds bear either of two types of intercalators, 6-chloro-2-methoxyacridine or acodazole, and with linkages to the ferric binding domain of -NH(CH2)2-, -NH(CH2)3-, -NH(CH2)4-, or -NH(CH2)3NH(CH2)3- and either porphyrin or deuteroporphyrin moieties. Comparison of the Kassoc values for binding to calf thymus DNA suggests that the enhanced binding observed with the linker -NH(CH2)3NH(CH2)3- contributes to the efficiency of sequence neutral DNA scission and may be a factor in the relative anticancer activities of these agents. The iron porphyrins give no evidence of the production of base propenals in DNA degradation, and the autoradiograms clearly indicate that a phosphate group is attached to the 5' end of the oligomer. The scission is partially suppressible by catalase and superoxide dismutase.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: No C-N bond scission products were observed on W(100)-(2 x 1)-O, consistent with the predominance of reversible molecular adsorption without significant decomposition.
Abstract: The adsorption and reaction of mono- and trimethylamine have been investigated on W(100), W(100)-(5 x 1)-C, and W(100)-(2 x 1)-O, using temperature-programmed reaction spectroscopy. Reactivity and selectivity are dependent on the presence of the oxide or carbide overlayers and the degree of substitution of the amine. The activation energy for irreversible C-H bond cleavage in trimethyl- and methylamine was significantly greater on the -(2 x 1)-O and -(5 x 1)-C surfaces compared to W(100). Most notable was the elevated temperature up to where molecular amine desorption is observed on the -(2 x 1)-O: 650 and 600 K for methyl- and trimethylamine, respectively, corresponding to approximate desorption energies of 39 and 35 kcal/mol. The extent of amine substitution also qualitatively alters the product distribution, with CH/sub 4/ produced only from (CH/sub 3/)/sub 3/N and NH/sub 3/ only from (CH/sub 3/)NH/sub 2/ on W(100) and W(100)-(5 x 1)-C> No C-N bond scission products were observed on W(100)-(2 x 1)-O, consistent with the predominance of reversible molecular adsorption without significant decomposition. Possible reaction mechanisms are discussed in terms of competing C-H, N-H, and C-N bond activation.
TL;DR: In this article, the yields of the photochemical and thermal rearrangement reaction have been determined for 13 endoperoxides of the anthracene and tetracene series, and it was shown that a strong substituent effect operates on Qdec(S1) and Adec, which are closely correlated.