Showing papers on "Bond cleavage published in 2000"
1,157 citations
TL;DR: In this article, the effect of Co or Ni on the HDS activity of Mo/alumina was studied by using dibenzothiophene (DBT) and 4,6-dimethyldibenzethiophene(46DMDBT), and the reaction was carried out at 340°C under a 4 MPa total pressure.
396 citations
TL;DR: In this paper, various iron(III) porphyrin complexes containing electron-withdrawing and-donating substituents on phenyl groups at the meso position of the porphrin ring were employed to study the electronic effect of POR ligands on the heterolytic versus homolytic O−O bond cleavage of the hydroperoxides.
Abstract: The mechanisms of heterolytic versus homolytic O−O bond cleavage of H2O2, tert-butyl hydroperoxide (t-BuOOH), 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), and m-chloroperoxybenzoic acid (m-CPBA) by iron(III) porphyrin complexes have been studied by carrying out catalytic epoxidations of cyclohexene in protic solvent. In these reactions, various iron(III) porphyrin complexes containing electron-withdrawing and -donating substituents on phenyl groups at the meso position of the porphyrin ring were employed to study the electronic effect of porphyrin ligands on the heterolytic versus homolytic O−O bond cleavage of the hydroperoxides. In addition, various imidazoles were introduced as axial ligands to investigate the electronic effect of axial ligands on the pathways of hydroperoxide O−O bond cleavage. Unlike the previous suggestions by Traylor, Bruice, and co-workers, the hydroperoxide O−O bonds were found to be cleaved both heterolytically and homolytically and partitioning between heterolysis and homol...
219 citations
TL;DR: Highly efficient catalytic oxidation of alcohols with molecular oxygen by N-hydroxyphthalimide (NHPI) combined with a Co species was developed and 2-octanol in the presence of catalytic amounts of NHPI and Co(OAc)2 under atmospheric dioxygen in AcOEt at 70 degrees C gave 2- Octanol in 93% yield.
Abstract: Highly efficient catalytic oxidation of alcohols with molecular oxygen by N-hydroxyphthalimide (NHPI) combined with a Co species was developed. The oxidation of 2-octanol in the presence of catalytic amounts of NHPI and Co(OAc)2 under atmospheric dioxygen in AcOEt at 70 °C gave 2-octanone in 93% yield. The oxidation was significantly enhanced by adding a small amount of benzoic acid to proceed smoothly even at room temperature. Primary alcohols were oxidized by NHPI in the absence of any metal catalyst to form the corresponding carboxylic acids in good yields. In the oxidation of terminal vic-diols such as 1,2-butanediol, carbon−carbon bond cleavage was induced to give one carbon less carboxylic acids such as propionic acid, while internal vic-diols were selectively oxidized to 1,2-diketones.
196 citations
Book•
15 Dec 2000
TL;DR: In this paper, the authors present a general overview of electrochemical Oxidation and Reduction of Organic Compounds: General Technique Anode and Cathode Materials Kinetics of Electron Transfer Analytical and Spectroscopic Techniques Pulse Radiolysis.
Abstract: 1 Electrochemical Oxidation and Reduction of Organic Compounds: General Technique Anode and Cathode Materials Kinetics of Electron Transfer Analytical and Spectroscopic Techniques Pulse Radiolysis. 2 Oxidation of Alkanes, Haloalkanes and Alkenes: Radical-Cations Oxidation of Alkanes Oxidation of Haloalkanes Oxidation of Alkenes Intramolecular Cyclization Electrochemically Generated Reagents for Alkenes. 3 Alkenes and Phenylalkenes: Alkene Radical-Anions as Nucleophiles Activated Alkenes - Introduction Activated Alkenes - Dimerization in Aprotic Media Activated Alkenes - Dimerization in Protic Media B-Unsaturated Aldehydes and Ketones Intramolecular Cyclization Mixed Hydrocoupling Reactions Asymmetric Reduction Electrocatalytic Hydrogenation. 4. Reductive Bond Cleavage Processes - I: Dissociative Electron Transfer Reduction of Alkyl and Benzyl Halides Reactions of the Trichloremethyl Group Reductive Cyclization of B-Dihaloalkanes Alkenes from 1,2-Dibromides and Related Compounds Reduction of Aryl Halides Electrochemically Induced SRN1 Reaction Electrochemically Induced Radical Cyclization Reactions of Aryl Halides Carbon-Halogen Bond Reactions Catalysed by the Anode Material Carbon-Halogen Bond Reactions Catalysed by Ni, Co and Pd Complexes Reactions with Carbon Dioxide.
191 citations
TL;DR: The production of OH* from O2 by HRP in the presence of NADH or DHF was confirmed by EPR spectroscopy using 5,5-dimethyl-1-pyrroline-N-oxide as a spin trap, confirming the involvement OH* as the causative agent for macromolecule cleavage.
167 citations
165 citations
TL;DR: The mesolytic cleavage of a β-C−X bond (ArCR2−X•+ → ArCR2•/+ + X+/•) is one of the most important reactions of alkylaromatic radical cations as discussed by the authors.
Abstract: The mesolytic cleavage of a β-C−X bond (ArCR2−X•+ → ArCR2•/+ + X+/•) is one of the most important reactions of alkylaromatic radical cations. In this Account, our group's results concerning some fundamental aspects of this process (cleavage mode, structural and stereoelectronic effects, competitive breaking of different β-bonds, nucleophilic assistance, possible stereochemistry, carbon vs oxygen acidity in arylalkanol radical cations) are presented and critically discussed for reactions where X = H, CR3, SR, and SiR3. Several examples illustrating how this information was exploited as a tool to detect electron-transfer mechanisms in chemical and enzymatic oxidations are also reported.
153 citations
TL;DR: In this article, an extension of the theory of dissociative electron transfer is proposed to rationalize the kinetic results and estimate the magnitude of the interaction energy, which explains how a relatively small interaction energy results in a substantial acceleration of the reaction, caused by both an increase of the driving force and a decrease of the intrinsic barrier.
Abstract: The electrochemical reduction of carbon tetrachloride in N,N‘-dimethylformamide follows a mechanism in which electron transfer and bond cleavage are concerted, at least at low and moderate driving forces. A detailed analysis of the kinetics of the reductive cleavage reveals that a small but significant interaction between the Cl- and Cl3C• fragments exists in the product state and is responsible for a strong acceleration of the reaction. An extension of the theory of dissociative electron transfer is proposed to rationalize the kinetic results and estimate the magnitude of the interaction energy. The model explains how a relatively small interaction energy results in a substantial acceleration of the reaction, caused by both an increase of the driving force and a decrease of the intrinsic barrier. Due to the strong polarization of the CCl3 radical, the reaction is a particularly clear example of the possibility that attractive interactions between fragments survive in a polar solvent. Another attractive f...
139 citations
127 citations
TL;DR: The bacterial PTE is able to catalyze the hydrolysis of a wide range of organophosphate nerve agents and has been shown to consist of a unique binuclear metal center that has evolved to deliver hydroxide to the site of bond cleavage.
Abstract: The bacterial PTE is able to catalyze the hydrolysis of a wide range of organophosphate nerve agents. The active site has been shown to consist of a unique binuclear metal center that has evolved to deliver hydroxide to the site of bond cleavage. The reaction rate for the hydrolysis of activated substrates such as paraoxon is limited by product release or an associated protein conformational change.
TL;DR: In this article, the reaction of heterotetrameric sarcosine oxidase (TSOX) of Arthrobactor sp. 1-IN has been studied by stopping-flow spectroscopy, with particular emphasis on the reduction of the enzyme by sarcosines.
Abstract: The reaction of heterotetrameric sarcosine oxidase (TSOX) of Arthrobactor sp. 1-IN has been studied by stopped-flow spectroscopy, with particular emphasis on the reduction of the enzyme by sarcosine. Expression of the cloned gene encoding TSOX in Escherichia coli enables the production of TSOX on a scale suitable for stopped-flow studies. Treatment of the enzyme with sulfite provides the means for selective formation of a flavin-sulfite adduct with the covalent 8alpha-(N(3)-histidyl)-FMN. Formation of the sulfite-flavin adduct suppresses internal electron transfer between the noncovalent FAD (site of sarcosine oxidation) and the covalent FMN (site of enzyme oxidation) and thus enables detailed characterization of the kinetics of FAD reduction by sarcosine using stopped-flow methods. The rate of FAD reduction displays a simple hyperbolic dependence on sarcosine concentration. Studies in the pH range 6.5-10 indicate there are no kinetically influential ionizations in the enzyme-substrate complex. A plot of the limiting rate of flavin reduction/the enzyme-substrate dissociation constant (k(lim)/K(d)) versus pH is bell-shaped and characterized by two macroscopic pK(a) values of 7.4 +/- 0.1 and 10.4 +/- 0.2: potential candidates for the two ionizable groups are discussed with reference to the structure of monomeric sarcosine oxidase (MSOX). The kinetic data are discussed with reference to potential mechanisms for the oxidation of amine molecules by flavoenzymes. Additionally, kinetic isotope effect studies of the rate of C-H bond breakage suggest that a ground-state quantum tunneling mechanism for H-transfer, facilitated by the low-frequency thermal motions of the protein molecule, accounts for C-H bond cleavage by TSOX. TSOX thus provides another example of C-H bond breakage by ground-state quantum tunneling, driven by protein dynamics [vibrationally enhanced ground-state quantum tunneling (VEGST)], for the oxidation of amines by enzymes.
TL;DR: It is proposed, on the basis of the results of mechanistic studies, that the role of the imidazole is to decelerate the O-O bond cleavage of an iron(III) hydroperoxide porphyrin (or H2O2-iron(II) p Morphyrin adduct) and that the intermediate transfers its oxygen to olefins prior to the O -O bond Cleavage.
TL;DR: In this paper, a potential energy surface for the reaction step from compound A, with the O2 molecule coordinated to heme a3, to the bond-cleaved compound P is constructed.
Abstract: The mechanism of O−O bond cleavage at the binuclear center in cytochrome oxidase has been investigated by using hybrid density functional theory (B3LYP). A potential energy surface for the reaction step from compound A, with the O2 molecule coordinated to heme a3, to the bond-cleaved compound P is constructed. The features of the calculated potential surface agree well with experimental information on this reaction step. First, a free energy of activation of 15 kcal/mol is obtained, reasonably close to the value of 12 kcal/mol corresponding to the observed lifetime of compound A. Second, the calculations give a large entropy effect on the reaction rate, which explains the weak temperature dependence observed for the P formation reaction. Third, the calculated potential surface has no stable intermediate between A and P, in agreement with the experimental observation that compound A decays with the same rate as compound P forms. Fourth, the calculations show that the oxo−ferryl compound P together with a t...
TL;DR: In this article, four possible decomposition reaction pathways of HMX in the gas phase were investigated: N−NO2 bond dissociation, HONO elimination, C−N bond scission of the ring, and the concerted ring fission.
Abstract: Using the BLYP and B3LYP level of density functional theory, four possible decomposition reaction pathways of HMX in the gas phase were investigated: N−NO2 bond dissociation, HONO elimination, C−N bond scission of the ring, and the concerted ring fission. The energetics of each of these four mechanisms are reported. Dissociation of the N−NO2 bond is putatively the initial mechanism of nitramine decomposition in the gas phase. Our results find the dissociation energy of this mechanism to be 41.8 kcal/mol at the BLYP level and 40.5 kcal/mol at the B3LYP level, which is comparable to experimental results. Three other mechanisms are calculated and found at the BLYP level to be energetically competitive to the nitrogen−nitrogen bond dissociation; however, at the B3LYP level these three other mechanisms are energetically less favorable. It is proposed that the HONO elimination and C−N bond scission reaction of the ring would be favorable in the condensed phase.
TL;DR: In this article, the authors investigated various competing mechanisms involved in the early stages of the decomposition of a "push−pull" aromatic (1,3,5-triamino-2,4,6-trinitrobeznene, TATB), and particularly how hydrogen transfer affects the competition between ring closure and single bond scission.
Abstract: Using gradient-corrected density functional theory, we have investigated various competing mechanisms involved in the early stages of the decomposition of a “push−pull” (containing both electron-donating and -withdrawing groups) aromatic (1,3,5-triamino-2,4,6-trinitrobeznene, TATB), and particularly how hydrogen transfer affects the competition between ring closure and single bond scission. On the basis of the obtained energetics, we found several previously suggested mechanisms to be energetically disfavored. These mechanisms include direct N−H, N−O, C−NH2 bond dissociation, carbon ring cleavage, and the production of NO2-1 anion. Our results indicate that the rate-limiting step is ring closure mediated by intramolecular hydrogen transfer. This mechanism is predicted to have a barrier height of 47.5 kcal/mol. The hydrogen motion forms a biradical state which we suggest is the precursor to further decomposition products, such as benzofurazans.
TL;DR: In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)2]+, 6a+, which has been isolated as the diamagnetic PF6- salt; 6b+PF6- has been similarly prepared.
Abstract: The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap•-)(Br)(CO)(PPh3)2], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)2]Br, 7, as a minor byproduct. Upon replacing pap by the better π-acceptor azo-2,2‘-bipyridine (abp) in the above synthesis, the radical complex [Os(abp•-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os−H bond in 5; in the formation of 7, the Os−Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7·CH2Cl2 have been determined. In 6b, the N−N length is 1.35(2) A, consistent with the anion radical description; in 7·CH2Cl2 the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be ∼70% azo-π* in character associated with a small metal contribution. An electronic band observed in the range 600−700 nm in solutions of 6 is assigned to the HOMO → LUMO transition, the LUMO being ...
TL;DR: A mechanism is suggested for the transformation from a species with a molecularly coordinated O2, to an O-O cleaved species with an oxo-ferryl group, and the overall reaction is close to thermoneutral, in line with the requirement that the energy wasted as heat should be minimized.
TL;DR: The proof of 4beta-stereochemistry in 3b is described by oxidative degradation of the derivative 3d bearing differential (O-benzyl and O-methyl) protecting groups in its "top" and "bottom" epicatechin moieties, to (R)-(-)-2,4-diphenylbutyric acid.
Abstract: The assignment of interflavan bond regio- and stereochemistry in oligomeric proanthocyanidins has in the past relied on empirical spectroscopic techniques which are influenced by the conformation of the C rings. Only recently was the 4,8-regiochemistry of procyanidin B2 (3b) firmly established by 2-dimensional NMR methods. We describe herein the proof of 4beta-stereochemistry in 3b by oxidative degradation of the derivative 3d bearing differential (O-benzyl and O-methyl) protecting groups in its "top" and "bottom" epicatechin moieties, to (R)-(-)-2,4-diphenylbutyric acid. The key elements of the degradative process are (1) removal of the C-3 alcohol functions through a modified Barton deoxygenation employing hypophosphorous acid as the reducing agent; (2) deprotection of the "top" unit by hydrogenolysis, followed by exhaustive aryl triflate formation with N,N-bis(trifluoromethanesulfonyl)aniline and DBU in DMF; (3) hydrogenolytic deoxygenation of the "top" unit over Pearlman's catalyst with concomitant scission of the O-C2 bond; (4) selective oxidation of the "bottom" unit with NaIO4/RuCl3. The hitherto unreported absolute configuration of (-)-2,4-diphenylbutyric acid was established as R by X-ray crystal structure analysis of the (R)-(+)-alpha-methylbenzylamine salt. As a corollary, the selectivity of hydrogenolytic and solvolytic reactions of epicatechin-derived tetrasulfonates has been investigated.
TL;DR: In this paper, 3,3-Difluoroserine and cysteine derivatives were synthesized via Mg(0)-promoted defluorination of trifluorsomethylimines as a key step, followed by addition of alcohols and sulfenyl chloride, respectively.
TL;DR: In this article, a reaction of Ru(OAc)2(1 or 2) with HBF4 gave P−C bond cleavage with the formation of compounds that contain the new phosphinite anion, 6 (C12H11BF2O2P), derived from hydrolysis of the BF4- anion.
TL;DR: In this article, aryliminoiminyl radical was implicated in the NO bond cleavage of the intermediate ester in acetic anhydride and the observed regioselectivity of the reaction was also rationalized.
TL;DR: A highly nucleophilic phosphine, tris(2,4,6-trimethoxyphenyl)phosphine (TTMPP), catalyzes the aldol reaction between ketene silyl acetals and aldehydes in good to high yields as discussed by the authors.
TL;DR: In this article, the activation energies of the C−C bond scission steps using the unity bond index−quadratic exponential potential method, known as the bond-order conservation−Morse potential method was investigated.
Abstract: C−C bond scission steps are often considered rate-determining in ethane hydrogenolysis. This paper is devoted to the calculations of the activation energies of these steps using the unity bond index−quadratic exponential potential method, formerly known as the bond-order conservation−Morse potential method. Binding energies of atomic carbon with groups VIII and IB metals Ni(111), Pd(111), Pt(111), Rh(111), Ru(0001), Ir(111), Fe(110), Cu(111), and Au(111) are estimated from experimental data on the chemisorption of various species on these surfaces. The resulting estimates are corrected taking into account DFT data on CHx binding energies. The strengths of carbon binding to the surfaces allow arranging the metals into the following series: Cu(111) < Au(111) < Pd(111) < Ru(0001) < Pt(111) ≈ Ni(111) < Rh(111) < Ir(111) < Fe(110). The values of carbon binding energies range from 122.9 kcal/mol for Cu(111) to 192.5 kcal/mol for Fe(110). The activities of these surfaces toward C−C bond scission increase in the...
TL;DR: This is the first report of peroxidases catalyzing both epoxidation reactions and carbon-carbon bond cleavage and the results are discussed in terms of mechanisms involving ferryl oxygen transfer and electron transfer.
TL;DR: In this paper, the surface chemistry of iodobenzene (C6H5I) on Pt(111) has been studied by temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), high resolution electron energy loss spectrum (HREELS) and isotopic labeling.
TL;DR: The heterolytic scission is only observed in contact ionic pairs with unsaturated cations (electrophilic catalysis), and it is especially efficient when the pair has a controlled topology with a tetralkylammonium cation (saturated cation) near the oxygen atom.
Abstract: Electrophilic and electrostatic catalysis have been identified as distinct contributions that affect the reactivity of radical anions in the reductive cleavage of alkyl aryl ethers. Two modes of mesolytic scission of these radical anions are possible: homolytic (dealkylation, a thermodynamically favored but kinetically forbidden process) and heterolytic (dealkoxylation). From our studies (alkali metal reductions, electrochemical studies, use of substrates with a preformed positive charge in certain positions of their structure) it can be concluded that the heterolytic scission is very much dependent on the electrophilic assistance by the counterion and it is only observed in contact ionic pairs with unsaturated cations (electrophilic catalysis). On the other hand, the homolytic scission is observed in solvent-separated ionic pairs, and it is especially efficient when the pair has a controlled topology with a tetralkylammonium cation (saturated cation) near the oxygen atom. The effect of the cation has, in this case, electrostatic origin (electrostatic catalysis), probably lowering the barrier of the intramolecular pi-sigma electron transfer process and thus reducing the kinetic control of the reaction in such a way that the thermodynamically more favorable process is produced.