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Coordinated experimental and orthogonal space random walk (OSRW) molecular dynamics simulation studies were performed to understand the exceptionally low temperature required for thermal conversion of a poly(phenylenevinylene) (PPV) precursor containing the macromolecular counterion poly(ethylene glycol)-4-nonylphenyl-3-sulfopropyl ether (PEGNOPS). Simulations of this solvent-free system converged for starting points where the backbone was artificially expanded or compressed. In this example of a polymer organized by a polymer, the simulations revealed the ability of PEGNOPS to partially preorder the precursor chain in a conformation that favors the E2 elimination pathway. [ABSTRACT FROM AUTHOR]

The results are reported of mass-spectrometric studies of the nucleobases adenine 1h (1, R = H), guanine 2h, and cytosine 3h. The protonated nucleobases are generated by electrospray ionization of adenosine 1r (1, R = ribose), guanosine 2r, and deoxycytidine 3d (3, R = deoxyribose) and their fragmentations were studied with tandem mass spectrometry. In contrast to previous EI-MS studies of the nucleobases, NH3 elimination does present a major path for the fragmentations of the ions [1h + H]+, [2h + H]+, and [3h + H]+. The ion [2h + H − NH3]+ also was generated from the acyclic precursor 5-cyanoamino-4-oxomethylene-dihydroimidazole 13h and from the thioether derivative 14h of 2h (NH2 replaced by MeS). The analyses of the modes of initial fragmentation is supported by density functional theoretical studies. Conjugate acids 15–55 were studied to determine site preferences for the protonations of 1h, 2h, 3h, 13h, and 14h. The proton affinity of the amino group hardly ever is the substrate’s best protonation site, and possible mechanisms for NH3 elimination are discussed in which the amino group serves as the dissociative protonation site. The results provide semi-direct experimental evidence for the existence of the pyrimidine ring-opened cations that we had proposed on the basis of theoretical studies as intermediates in nitrosative nucleobase deamination. [Copyright &y& Elsevier]

Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O(3P) H2O(X 1A1) → HO2(2A‘) H(2S). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50−80 kcal mol-1) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol-1to a peak at ∼115 kcal mol-1and then decreases at higher energies as secondary dissociation of the internally excited HO2product becomes dominant. The calculations show that the cross section for H-atom elimination (O H2O → HO2H) is about 10−25% that of the H-atom abstraction (O H2O → OH OH) cross section for collision energies in the 70−160 kcal mol-1range. [ABSTRACT FROM AUTHOR]

Heck-type 4-exo-trigcyclization of linear 2-enol triflate-1,5-hexadienes provides functionalized methylene cyclobutanes. Intramolecular palladium coordination can initiate -hydride elimination leading to 1,2-dimethylene cyclobutane derivatives, which are obtained with high selectivity if substrates having a geminal diphenyl group at Care used. In parallel, formal 5-endo-trigcyclization and -hydride elimination form 1-methylene cyclopent-2-en derivatives. [ABSTRACT FROM AUTHOR]

The transition state for -hydride elimination in alkyl groups on the Pt(111) surface has been probed by studying the effects of fluorine substitution on the barriers to -hydride elimination, %@mt;sys@%%@ital@%E%@rsf@%%@sx@%H%@be@%‡%@sxx@%%@mx@%. Four different fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of four fluoroalkyl iodides:  RCH2CH2−I (R CF3, CF3CH2, and CF3CF2) and (CF3)2CHCH2−I. In the absence of preadsorbed hydrogen, fluoroalkyl groups on the Pt(111) surface dehydrogenate via -hydride elimination to form unsaturated fluorocarbons and deposit hydrogen atoms onto the surface. Those hydrogen atoms then hydrogenate the remaining fluoroalkyl groups to produce fluoroalkanes that desorb rapidly from the surface. The kinetics of hydrogenation and fluoroalkane desorption are rate limited by the -hydride elimination step and thus serve as measures of the kinetics of -hydride elimination. The field effects of the fluorinated substituents increase the barriers to -hydride elimination with a reaction constant of F 19 ± 2 kJ/mol. The interpretation of this effect is that the -carbon atom in the transition state is cationic, RC···H‡, with respect to the reactant. The field effect of the fluorinated substituent energetically destabilizes the electron deficient -carbon atom in the transition state. This is consistent with observations made on the Cu(111) surface; however, the substituent effect is significantly smaller on the Pt(111) surface. On the Pt(111) surface, the transition state for -hydride elimination is less polarized with respect to the initial state alkyl group than on the Cu(111) surface. [ABSTRACT FROM AUTHOR]

Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C6H3F2 and C6H2F3 from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made. [ABSTRACT FROM AUTHOR]

A five-step preparation of carbohydrate-based oxepines from hept-l-enitols is presented. The hept-l-enitols were subjected to silyl protection and hydroboration/oxidation to give the protected heptan-1-itols. Swern oxidation of the homologated alcohols followed by sequential acetal formation/ cyclization provided methyl 2-deoxyseptanosides that underwent elimination reactions to give the carbohydratebased oxepines. The new sequence is an alternative to the ring-closing metathesis for the synthesis of carbohydratebased oxepines from protected pyranose sugars. The product oxepines can serve as glycosyl donors in the synthesis of novel septanose carbohydrates. In addition, C-methylenealdehydo arabinofuranoside 16 was formed from 2-deoxyseptanoside 10 as the only product during protic acid mediated elimination reactions. This novel ring contraction complements other reported preparations of C-methylenaldehydo furanosides and underscores the acid-mediated reactivity introduced by competing eliminations between the C-1/C-2 and C-2/C-3 bonds. [ABSTRACT FROM AUTHOR]

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe3)Ir(H)[C(O)CF3] (1) leads to the quantitative formation of Cp*(PMe3)Ir(CO) (2) and CF3H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF-3 to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3] and Cp*(PMe3)Ir(Cl)[C(O)CF3]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF3D in CD3OD. Three additional acyl hydrides, Cp*(PMe3)Ir(H)[C(O)RF] (where RF = CF2CF3, CF2CF2CF3, or CF2(CF2)6CF3) undergo RF--H elimination to give 2 at a faster rate than CF3H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the β-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium. [ABSTRACT FROM AUTHOR]

Multiwalled carbon nanotubes (MWCNTs) were used as the active elements for the first time for affinity-based elimination of ionic dyes. MWCNTs were encapsulated it cross-linked alginate (ALG) microvesicles using Ba2+ as the bridging ion. The Ba2+-alginate matrix constitutes a cage which holds the physically trapped MWCNTs. The cage carries negative charges on its surface. The cage restricts the access of anions of large molecular weight. such as humic acids, because of electrostatic repulsion. U cage also restricts the access of colloids of large size, because of size exclusion. Ionic dyes partition into the cage and then are captured by MWCNTs probably on the basis of van der Waals interactions occurring between the hexagonally arrayed carbon atoms in the graphite sheet of MWCNTs and the aromatic backbones of the dyes. As a result of these interactions the target species, namely, the ionic dyes, are eliminated efficiently by the MWCNTs of Ba2+ -ALG/MWCNT composite adsorbents. The adsorptive capacities for elimination of acridine orange, ethidium bromide, eosin bluish, and orange B (the model species used for this study) were found as high as 0.44, 0.43, 0.33, and 0.31 μmol, respectively, for 1.0 mg of the caged MWCNTs. Adsorptive experiments with carbon nanofibers and activated carbons as the adsorbents were also performed. The MWCNT-based adsorbents provided the best capability for the affinity-based elimination of these targeted species. Biocompatibility experiments performed in vitro and in vivo provided promising results, suggesting potential applications of the caged MWCNTs in in situ environmental remediation. [ABSTRACT FROM AUTHOR]

Addition of BF3·OEt2 to ethereal solutions of the Ni(ll) β-diketiminates (Me2NN]Ni(F1)(2,4-lutidine) (R = Et (1), Pr (2)) allows the isolation of the neutral β-H agostic monoalkyls (Me2NN]Ni(R) (R = Et (3), Pr (4)). X-ray studies of primary alkyls 3 and 4a reveal acute Ni-Cα-Cβ angles with short Ni-Cβ distances, indicating structures along the β-H elimination pathway. Positional disorder of the alkyl group in the X-ray structure of 4 corresponds to partial (22%) occupancy by the secondary alkyl [Me2NN)Ni(CHMe2) (4b). Variable-temperature NMR spectra of 3 and 4 reveal fluxional behavior that result from β-H elimination, in-plane rotation of the β-CH3 group, and a tetrahedral triplet structure for 3 that were investigated by density functional theory calculations at the Becke3LYP/6-31 G* level of theory without simplifications on the β-diketiminate ancillary ligand. Calculations support low temperature NMR studies that identify the linear β-H agostic propyl isomer 4a as the ground state with the branched β-H agostic isomer 4b slightly higher in energy. NMR studies and calculations show that the β-agostic 3 reluctantly coordinates ethene and that 3 is the ground state for this ethylene oligomerization catalyst. The thermodynamic isotope effect KH/KD = 1.3(2) measured for the loss of 2,4-lutidine from 1 to form β-agostic 3 was also examined by DFT calculations. [ABSTRACT FROM AUTHOR]

The unimolecular fragmentation of metastable anisole dications is studied by tandem-mass spectrometry. Permutations of OH, OCH3, and NH2 substituents as well as extensive labeling experiments are used as probes to unravel the mechanisms of the various fragmentation pathways observed. The main channel corresponds to methyl ion elimination, which leads to an energetically favorable charge separation (“Coulomb explosion”). Loss of a neutral CO molecule forms another important branch, and the expelled CO has incorporated a carbon atom from the phenyl ring of anisole. Substitution of the anisole dication with OH, OCH3, and NH2 groups results in more specific fragmentations. Ortho- and para-substitutions lead to preferential methyl ion elimination, whereas meta-substitution favors CO loss. The substituent effects are also reflected in pronounced changes in the apparent kinetic isotope effects upon substitution of OCH3 by OCD3. On the basis of the various experimental findings and some explorative DFT calculations, possible mechanisms for the skeletal rearrangements prior to CO elimination are proposed. [ABSTRACT FROM AUTHOR]

A direct ab initio dynamics study is carried out for three channels of the unimolecular reaction of fluoromethoxy, CH2FO. The minimum energy paths (MEPs) of the three channels are computed with the MPW1K/6-31+G(d,p) method. The geometries, harmonic vibrational frequencies, energies, and enthalpies of all stationary points are calculated at the MPW1K and QCISD levels of theory in conjunction with the 6-31+G(d,p) basis set. The energies of stationary points and the points along MEP for each channel are further refined by using single-point multilevel energy calculations. The rate constants are evaluated by means of the conventional transition state theory (TST), the canonical varitional transition state theory (CVT), and the canonical varitional transition state theory with small-curvature tunneling correction (CVT/SCT) in the temperature range of 200-2500 K. The fitted Arrhenius expressions of the calculated CVT/SCT rate constants are k1CVT/SCT(T) = (1.26 × 106)T2.19e-7150.6/T s-1, k2CVT/SCT(T) = (3.68 × 102)T3.23e-12327.7/T s-1, k3CVT/SCT(T) = (9.97 × 10-26)T10.90e-3873.3/T s-1 for the H elimination, HF elimination, and isomerization channels, respectively. [ABSTRACT FROM AUTHOR]

The article presents a study on the competitive reductive elimination (RE) from a [RhIII(CH3)(CN)][I] complex to give CH3CN in protic olvents and CH3I in aprotic solvents. It states that the reductively eliminated CHI undergoes selective electrophilic attack on the cyano ligand of the RhI complex in an aprotic solvent to give the corresponding alkyl isonitrile complex.

Pd(II) metallacyclic complexes are key intermediates in sequential reactions in which three new C−C bonds are formed in an aromatic ring. The Pd(II) metallacyclic complex 10with phenanthroline as ligand undergoes oxidative addition to benzyl bromide or chloride in DMF to generate benzyl-Pd(IV) complexes. The rate constant of the oxidative addition is determined by means of electrochemical techniques, with the expected reactivity order PhCH 2Br ( k1Br= 3.6 M −1s −1) > PhCH 2Cl ( k1Cl= 6 × 10 −3M −1s −1) at 29 °C. When the benzyl-Pd(IV) complex is generated from PhCH 2Br, the oxidative addition is followed by a slow C−C reductive elimination, which gives a Pd(II) complex. The rate constant of the reductive elimination has been determined in DMF ( k2Br= 6 × 10 −4s −1) at 29 °C. [ABSTRACT FROM AUTHOR]

Exposure of a series of zirconocene amido and hydrazido hydride complexes, (η 5-C 5Me 4H) 2Zr(NHR)H (R = tBu, NMe 2, Me, H), to 4 atm of D 2gas at 56 °C produced isotopic exchange in both the N−H and Zr−H positions. In general, the relative rates of 1,2-elimination can be rationalized on the basis of ground-state effects, whereby amido compounds with the strongest N−H bonds, as judged by the corresponding free amine, undergo the slowest isotopic exchange. For the compound with the strongest N−H bond in the series, (η 5-C 5Me 4H) 2Zr(NH 2)H, the barrier for 1,2-elimination is sufficiently high such that σ-bond metathesis becomes the dominant intermolecular exchange pathway. For the other amido zirconocene hydrides, the rate constants for deuterium exchange into the N−H position are faster than for the Zr−H position. This behavior is a result of a faster intramolecular isomerization process driven by an equilibrium isotope effect favoring N−D over Zr−D bond formation. Computational studies on a related model compound, (η 5-C 5H 5) 2Zr(NH tBu)H, successfully reproduce these observations and support a pathway involving the formation of rare d 0dihydrogen complexes. [ABSTRACT FROM AUTHOR]

Tyr 64, hydrogen-bonded to coenzyme phosphate in Treponema denticola cystalysin, was changed to alanine by site-directed mutagenesis. Spectroscopic and kinetic properties of the Tyr 64 mutant were investigated in an effort to explore the differences in coenzyme structure and kinetic mechanism relative to those of the wild-type enzyme. The wild type displays coenzyme absorbance bands at 418 and 320 nm, previously attributed to ketoenamine and substituted aldamine, respectively. The Tyr 64 mutant exhibits absorption maxima at 412 and 325 nm. However, the fluorescence characteristics of the latter band are consistent with its assignment to the enolimine form of the Schiff base. pKspec values of ∼8.3 and ∼6.5 were observed in a pH titration of the wild-type and mutant coenzyme absorbances, respectively. Thus, Tyr 64 is probably the residue involved in the nucleophilic attack on C4' of pyridoxal 5'-phosphate (PLP) in the internal aldimine. Although the Tyr 64 mutant exhibits a lower affinity for PLP and lower turnover numbers for α,β-elimination and racemization than the wild type, the pH profiles for their Kd(PLP) and kinetic parameters are very similar. Rapid scanning stopped-flow and chemical quench experiments suggest that, in contrast to the wild type, for which the rate-determining step of α,β-elimination of β-chloro-L-alanine is the release of pyruvate, the rate-determining step for the mutant in the same reaction is the formation of α-aminoacrylate. Altogether, these results provide new insights into the catalytic mechanism of cystalysin and highlight the functional role of Tyr 64. [ABSTRACT FROM AUTHOR]

The synthesis of substituted styrenes was achieved by ring-closing enyne metathesis (RCEM)/elimination of enyne substrates 12. The synthetic approach was also effective for a different type of enyne substrate 14, yielding corresponding styrene 15. [ABSTRACT FROM AUTHOR]

New hafnocene triarylstannyl complexes were prepared and were shown to undergo clean thermal decompositions via α-aryl-elimination to produce the corresponding stannylene and a hafnocene aryl complex. The rate of the decomposition is highly dependent on the nature of the ancillary ligand, with the stabilities of the CpCp*Hf(SnPh3)X compounds following the order X = NMe2 > Np (α-agostic) > OMe > Cl > Me. Mechanistic information suggests that α-aryl-elimination may be viewed as a concerted process involving nucleophilic attack of the migrating aryl group onto the electrophilic metal center. [ABSTRACT FROM AUTHOR]

(CpMoμ-S)2S2CH2, 2, and related derivatives serve as electrocatalysts for the reduction of protons with current efficiencies near 100%. The kinetics of the electrochemical reduction process has been studied, and the effects of varying the proton source, the solvent, the cyclopentadienyl substituents, and the sulfur substituents on the catalyst have been examined. The reduction of excess p-cyanoanilinium tetrafluoroborate under a hydrogen atmosphere in 0.3 M Et4NBF4/acetonitrile buffered at pH 7.6 is catalyzed by 2 at -0.64 V versus ferrocene, with an overpotential of 120 mV. Protonation of the sulfido ligand in 2 is an initial step in the catalytic process, and the rate-determining step at high acid concentrations appears to be the elimination of dihydrogen. The elimination may occur either from adjacent hydrosulfido sites or from a hydrosulfido-molybdenum hydride intermediate. [ABSTRACT FROM AUTHOR]

This article describes an efficient synthesis of a potent trehalase inhibitor, 1,1'-N-linked pseudodisaccharide 1 (consisting of two valienamines), in 14 steps with an overall yield of 12% and a first synthesis of 2 (consisting of two 2-epi-valienamines) in 15 steps with an overall yield of 24% from (-)-quinic acid. The synthesis involves a stereospecific palladium-catalyzed coupling reaction between an allylic amine and an allylic chloride as the crucial step. The acetonide blodking groups were shown to be the best hydroxyl protecting groups, compatible with the palladium-catalyzed allylic amination reaction that afforded high yields of the 1,1'-N-linked pseudodisaccharicles with a minimum amount of an elimination diene side product. [ABSTRACT FROM AUTHOR]

In enzymes, multiple structural effects cooperatively lead to the high catalytic activity, while individually these effects can be small. The design of artificial enzymes requires the understanding and ability to manipulate such subtle effects. The 34E4 catalytic antibody, catalyzing Kemp elimination of 5-nitrobenzisoxazole, and its Glu50Asp (E50D) variant are the subject of the present investigation. This removal of only a methylene group yields an approximately 30-fold reduction in the rate for the catalyzed Kemp elimination. Here, the aim is to understand this difference in the catalytic performance. The mechanism of Kemp elimination catalyzed by 34E4 and the E50D mutant is elucidated using QM/MM Monte Carlo simulations and free energy perturbation theory. In both proteins, the reaction is shown to follow a single-step, concerted mechanism. In the mutant, the activation barrier rises by 2.4 kcal/mol, which corresponds to a 62-fold rate deceleration, which is in good agreement with the experimental data. The positions and functionality of the residues in the active site are monitored throughout the reaction. It is concluded that the looser contact with the base, shorter base-Asn58 contact, less favorable π-stacking with Trp91 in the transition state of the reaction, and different solvation pattern all contribute to the reduction of the reaction rate in the E50D variant of 34E4. [ABSTRACT FROM AUTHOR]

The radical reaction of tributyltin hydride with o-iodo-N- methylanilides derived from α-azido acids provides an excellent access to α-(aminocarbonyl)iminyl radicals through 1,5-hydrogen transfer reaction of initially formed aryl radicals followed by β-elimination of dinitrogen from ensuing α-azido-α-(aminocarbonyl)alkyl radicals. The outcoming iminyls display a peculiar tendency to form corresponding nitriles by β-elimination of aminocarbonyl radicals. [ABSTRACT FROM AUTHOR]

The mechanism of the spontaneous decomposition of 3-bromo-2,2-bis(bromomethyl)propanol (TBNPA) and the kinetics of the reaction of the parent compound and two subsequent products were determined in aqueous solution at temperatures from 30 to 70 °C and pH from 7.0 to 9.5. TBNPA is decomposed by a sequence of reactions that form 33-bis(bromomethyl)oxetane (B B MO), 3-bromomethyl-3- hydroxymethyloxetane (B MHMO), and 2,6-dioxaspiro[3.3]- heptane (DOH), releasing one bromide ion at each stage. The pseudo-first-order rate constant of the decomposition of TBNPA increases linearly with the pH. The apparent activation energy of this transformation (98 ± 2 KJ/mol) was calculated from the change of the effective second-order rate constant with temperature. The pseudoactivation energies of BBMO and BMHMO were estimated to be 109 and 151 KJ/mol, respectively. Good agreement was found between the rate coefficients derived from changes in the organic molecules concentrations and those determined from the changes in the Br- concentrations. TBNPA is the most abundant semivolatile organic pollutant in the aquitard studied, and together with its byproducts they posess an environmental hazard. TBNPA half-life is estimated to be about 100 years. This implies that high concentrations of TBNPA will persist in the aquifer long after the elimination of all its sources. [ABSTRACT FROM AUTHOR]

The application of photoactivated periodate (UV/I04-) to the degradation of 4-chlorophenol (4-CP) was explored in this study. Under low irradiation intensities (23 μW/ cm2), wavelength of 266 nm, and pH 3, 4-CR was observed to degrade by pseudo-first-order reaction kinetics. The small reduction in 4-CP degradation in the presence of tert-butyl alcohol (t-BuOH) suggests that degradation of 4-CP by UV/lO4- was not dominated by an OH pathway. 03 production was suppressed in the presence of t-BuOH under O2-limited environments and in the presence of 4-CP. Faster degradation of 4-CP in the presence of an OW scavenger under nitrogen purging as compared to air- saturated conditions indicates that O(3P) is an important reactive species with 4-CR in this system. When 4-CP is added to the system, 103 production is enhanced. On the basis of the elimination of potential reaction pathways, O(3P) and lO3- are suspected reactive species with 4-CP in this system. [ABSTRACT FROM AUTHOR]