142-29-0Relevant academic research and scientific papers
Probing Ensemble Effects in Surface Reactions. 1. Site-Size Requirements for the Dehydrogenation of Cyclic Hydrocarbons on Pt(111) Revealed by Bismuth Site Blocking
Campbell, C. T.,Campbell, J. M.,Dalton, P. J.,Henn, F. C.,Rodriguez, J. A.,Seimanides, S. G.
, p. 806 - 814 (1989)
Catalytic reactions on bimetallic surfaces are often thought to be controlled by ensemble effects, whereby a side reaction requiring a large ensemble of active sites can be selectively suppressed by diluting the active metal with a second, inert metal.Unfortunately, the lack of knowledge of surface structure and the complications due to coexisting electronic effects have, until now, precluded accurate determinations of ensemble requirements for surface reactions.We analyze here applications of a new method for determining ensemble sizes that partially overcomes these obstacles and allows for semiquantitative assessment of ensemble effects.The method involves the controlled blocking of sites on a well-defined transition-metal surface with a dispersed overlayer of inert bismuth adatoms.The interactions of five cyclic hydrocarbones (cyclopentane, cyclohexane, cyclopentene, cyclohexene, and benzene) with Pt(111) have been studied in this way in an accompanying series of papers.In particular, the influence of Bi upon the competing dehydrogenation and desorption kinetics of these adsorbed molecules has been qualitatively measured.This present paper correlates the results for those five molecules and fits them with a simple kinetic model to extract the absolute ensemble requirements for the surface dehydrogenation reactions.The method and model may have applicability to a broad range of surface reactions.In addition, an effective ensemble requirement is defined, whose value is useful in predicting ensemble effects in catalysis.Trends in the value of kinetic parameters and the ensemble requirements with hydrocarbon character are discussed. The absolute ensemble requirements for the dehydrogenation of these adsorbed hydrocarbons are surprisingly large and indicate in some cases that at least six additional free Pt atoms are necessary for dehydrogenation (beyond those required for adsorption).Mechanistic implications of these results are discussed.
Structural distortions and dynamic behavior of the elusive "L"-shaped cis-bicyclo[3.3.0]octenyllithium: X-ray crystallographic and NMR studies
Fraenkel, Gideon,Chen, Xiao,Gallucci, Judith,Lu, Yan
, p. 4961 - 4965 (2007)
(Chemical Equation Presented) Substituted cis-bicyclo[3.3.0]octenyllithium prepared by addition of t-BuLi to 3-methylene-1,4-cyclooctadiene in the presence of TMEDA crystallizes as a dimer with one unsolvated Li+ sandwiched between the external faces of two allyl anions in a triple ion, and external to it the second Li+ is bidentately complexed to TMEDA, 8. Within each allyl unit, the allyl bonds have different lengths, and all four rings deviate from coplanarity which relieves strain in the rings despite introducing partial localization of the allyl anions. A similar structure prevails in solution as shown by 7Li NMR and the results of 7Li{1H} HOESY and 1H, 1H NOESY experiments. Carbon-13 NMR line shape changes indicate that the system undergoes a fast allyl bond shift concerted with conformation shifts of the out of plane carbons, ca. ΔG? = 9 kcal·mol-1. Cyclopentyllithium prepared by CH3Li cleavage of the trimethylstannyl derivative slowly undergoes an allowed ring opening to pentadienyllithium as well as deprotonating the solvent. The different behavior of dienylic lithium species is attributed to the relative separation of their termini.
Ring opening and hydrogen atom transfer trapping of the bicyclo[2.1.0]pent-2-yl radical
Newcomb, Martin,Manek, M. Beata,Glenn, Anne G.
, p. 949 - 958 (1991)
Relative rate constants (kr/kH) for ring opening of the bicyclo[2.1.0]pent-2-yl radical (1) to the cyclopent-3-enyl radical and trapping of radical 1 with t-BuSH, PhSH, and PhSeH in solvent THF were measured at temperatures between -78 and 50 °C. The hydrogen atom donors reacted more rapidly with radical 1 than with the cyclopropylcarbinyl radical (6). Rate constants for ring opening of 1 (kr) could be obtained by estimating the values of kH via Marcus theory. From initial kH values for reactions with radical 6, new kH values were calculated for increasingly exergonic reactions until the derived kr values from the three trapping agents agreed with one another and an extrapolated value of kr from Tempo trapping of 1. The results suggest that hydrogen atom transfer reactions with 1 were about 3 kcal/mol more exergonic than reactions with 6. Arrhenius functions for ring opening of 1 averaged log (ks/s-1) = 13.0 - 5.2/2.3RT; the value of kr at 25 °C is 1.5 × 109 s-1. Trapping studies of 1 and 6 with 2,6-dimethylthiophenol indicated that no special steric effects were present in hydrogen atom transfers to 1. However, highly stereoselective trapping of 1 was observed in reactions with ArSD with endo-bicyclo[2.1.0]pentane-2-d predominating, and the rate constant for decarboxylation of the endo-bicyclo[2.1.0]pentane-2-carboxy radical (endo-3) at -78 °C apparently was greater than that for decarboxylation of exo-3. The stereochemical results are ascribed to a stereoelectronic effect between the C1-C4 bond and endo-C2-X bonds of bicyclo[2.1.0]pentanes that weakens endo-C2-X bonds.
Transition-metal-promoted chemoselective photoreactions at the cucurbituril rim
Koner, Apurba L.,Marquez, Cesar,Dickman, Michael H.,Nau, Werner M.
, p. 545 - 548 (2011)
When included in a supramolecular barrel with transition-metal ions as lids, bicyclic azoalkanes undergo phase-selective photolysis to afford new photoproducts and photoproduct distributions. In the presence of the macrocycle cucurbit[7]uril and Ag+ ions, 2,3-diazabicyclo[2.2.1]hept-2-ene forms a ternary host-guest inclusion complex in which the cations are coordinated to the carbonyl rims of the host. Direct photolysis of this ternary complex provides cyclopentene as a new photoproduct.
Kinetics of the Gas-phase Addition Reactions of Trichlorosilyl Radicals. Part 3.-Additions to 2-Olefins
Dohmaru, Takaaki,Nagata, Yoshio
, p. 1141 - 1148 (1982)
The following Arrhenius parameters for the forward and reverse steps of trichlorosilyl radical additions to trans-but-2-ene, cis-but-2-ene, cis-pent-2-ene, 2-methyl-but-2-ene and cyclopentene have been obtained by a competitive method.The relevant elementary reactions are .SiCl3 + CH3COCH3 --> (CH3)2C.OSiCl3 (3) .SiCl3 + >C=C >C.-C-SiCl3 (5,-5) and >C.-C-SiCl3 + HSiCl3 --> HC-C-SiCl3 + .SiCl3 (6). The rate parameters of reaction (5) are expressed per reaction site; an asterisk indicates the site of addition in an unsymmetrical olefin.Evaluted values of A-5 and A5 imply a fairly 'loose' transition state in reaction (5).The Si-C bond energy has been estimated. .SiCl3 radicals have been revealed to be electrophilic and susceptible to steric hindrance.
Benzophenone-sensitized two-photon chemistry of azoalkanes in the "laser/ liquid jet": Evidence for photoinduced hydrogen 1,2-shift in 1,3-cyclopentadiyl triplet diradicals
Adam, Waldemar,Finzel, Ralf,Kita, Fumio
, p. 2211 - 2214 (1991)
While the triplet-sensitized photolysis of the bicyclicazoalkanes 1 led on denitrogenation to the corresponding bicycloalkanes 2, the laser/liquid jet photochemistry gave additionally also the cyclopentenes 3 by hydrogen 1,2-shift; the amount of the latter increased with increasing lifetime of the intermediary cyclopentadiyl triplet diradicals 4, which suggests that two-photon chemistry may operate.
SELECTIVE HYDROGENATION OF CYCLOPENTADIENE TO CYCLOPENTENE USING COLLOIDAL PALLADIUM SUPPORTED ON CHELATE RESIN.
Hirai,Komatsuzaki,Toshima
, p. 488 - 494 (1984)
Cyclopentadiene was hydrogenated to cyclopentene selectively by using colloidal palladium supported on chelate resin with iminodiacetic acid moieties as a catalyst. The hydrogenation rate was correlated to the polarity parameter, E//T(30) values, of the solvents used in the reaction, except in the case of dimethyl sulfoxide. The equilibrium constant for complex formation between cyclopentadiene and the catalyst, K//D, was estimated to be over 400 times larger than that between cyclopentene and the catalyst, K//E. A mechanism, including the coordination of olefins to the catalyst and the subsequent hydrogenation of the coordinated complexes, was proposed.
Infrared Multiphoton Photochemistry of Vinylcyclopropane. Variation of Yield and Branching Ratio with Experimental Parameters
Farneth, William E.,Thomsen, Marcus W.,Schultz, Nancy L.,Davies, Mark A.
, p. 4001 - 4006 (1981)
The infrared photochemistry of vinylcyclopropane has been comprehensively investigated.Irradiation of vinylcyclopropane at relatively low pressures with the partially focused output of a CO2 laser leads to a mixture of the C5 products cyclopentene, cyclopentadiene, 1,4-pentadiene, and cis- and trans-1,3-pentadiene.The composition of the product mixture as well as the total product yields are a sensitive function of experimental parameters.The effects of bath gas pressure, laser power, laser intensity, laser frequency, and number of pulses have been systematically examined.A simple physical picture of the multiphoton activation and resulting decomposition is developed.RRKM theory is employed to calculate energy-dependent unimolecular reaction rates.The model is quite successful in rationalizing these data, providing good evidence for the qualitative validity of a rate equation description of infrared multiphoton dissociation.
Crystal Structure and Photochemistry of Four α-Cycloalkyl-p-chloroacetophenones
Evans, Stephen V.,Trotter, James
, p. 63 - 72 (1988)
The photochemical fragmentation or cyclization reactions of α-cycloalkylacetophenones have been studied by crystal-structure analyses of four chloro derivatives with varying cycloalkyl-ring sizes, and by correlation of crystal- and molecular-structure par
Characterization and reactivity of γ-Al2O3 supported Pd-Ni bimetallic nanocatalysts for selective hydrogenation of cyclopentadiene
Feng, Yi-Si,Hao, Jian,Liu, Wei-Wei,Yao, Yun-Jin,Cheng, Yue,Xu, Hua-Jian
, p. 709 - 713 (2015)
Abstract Several γ-Al2O3 supported Pd-Ni bimetallic nanocatalysts (Pd-Ni (x:y)/Al2O3; where x and y represent the mass ratio of Pd and Ni, respectively) were prepared by the impregnation method and used for selective hydrogenation of cyclopentadiene to cyclopentene. The Pd-Ni/Al2O3 samples were confirmed to generate Pd-Ni bimetallic nanoparticles by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The catalytic activity was assessed in view of the effects of different mass ratios of Pd and Ni, temperature, pressure, etc. Among all the samples, the Pd-Ni (1:1)/Al2O3 (PN-1:1) catalyst showed extremely high catalytic ability. The conversion of cyclopentadiene and selectivity for cyclopentene can be simultaneously more than 90%.
