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CH Bond (ch + bond)
Terms modified by CH Bond Selected Abstracts5-Benzyl-3-methylimidazolidin-4-one-Derived Reactive Intermediates of Organocatalysis , A Comforting Resemblance of X-Ray, NMR, and DFT Solid-Phase, Liquid-Phase, and Gas-Phase StructuresHELVETICA CHIMICA ACTA, Issue 1 2009Abstract The X-ray crystal structures of three (E)-1-cinnamoylidene iminium PF6 salts of 5-benzyl-3-methylimidazolidin-4-ones (2,2-dimethyl-, cis -2-(tert -butyl)-, and cis -2-styryl-substituted; 2,4, resp.) are reported (Figs.,3,5). In the 2,2-dimethyl and in the cis -2-styryl derivative, 2 and 4, respectively, a CH bond of the cis -substituent in 2-position points to the center of the benzene ring of the benzyl group above the five-membered ring (Fig.,6,,a and b). NMR Measurements (Fig.,8) provide evidence that the same structure is present in solution, and that a fourth derivative of this type, 5 (Scheme), has (Z)- instead of (E)-configuration around the CN bond. In the cis -2-(tert -butyl) derivative 3, the benzyl group is located over the iminium , -system (Figs.,4 and 6,,c). Overlays with DFT-calculated crotonylidene analogs, A and B, show that the theoretical and experimental structures are almost superimposable (Fig.,9 and Table). The structures are discussed in view of their role as reactive intermediates in organocatalysis and in view of the help synthetic organic chemists may experience from theory. [source] Benzo[a]heptalenes from Heptaleno[1,2- c]furans.HELVETICA CHIMICA ACTA, Issue 4 2007Abstract It is shown in this ,Part 2' that heptaleno[1,2- c]furans 1 react thermally in a Diels,Alder -type [4+2] cycloaddition at the furan ring with vinylene carbonate (VC), phenylsulfonylallene (PSA), , -(acetyloxy)acrylonitrile (AAN), and (1Z)-1,2-bis(phenylsulfonyl)ethene (ZSE) to yield the corresponding 1,4-epoxybenzo[d]heptalenes (cf. Schemes,1, 5, 6, and 8). The thermal reaction of 1a and 1b with VC at 130° and 150°, respectively, leads mainly to the 2,3- endo -cyclocarbonates 2,3- endo - 2a and - 2b and in minor amounts to the 2,3- exo -cyclocarbonates 2,3- exo - 2a and - 2b. In some cases, the (P*)- and (M*)-configured epimers were isolated and characterized (Scheme,1). Base-catalyzed cleavage of 2,3- endo - 2 gave the corresponding 2,3-diols 3, which were further transformed via reductive cleavage of their dimesylates 4 into the benzo[a]heptalenes 5a and 5b, respectively (Scheme,2). In another reaction sequence, the 2,3-diols 3 were converted into their cyclic carbonothioates 6, which on treatment with (EtO)3P gave the deoxygenated 1,4-dihydro-1,4-epoxybenzo[d]heptalenes 7. These were rearranged by acid catalysis into the benzo[a]heptalen-4-ols 8a and 8b, respectively (Scheme,2). Cyclocarbonate 2,3- endo - 2b reacted with lithium diisopropylamide (LDA) at ,70° under regioselective ring opening to the 3-hydroxy-substituted benzo[d]heptalen-2-yl carbamate 2,3- endo - 9b (Scheme,3). The latter was O -methylated to 2,3- endo -(P*)- 10b. The further way, to get finally the benzo[a]heptalene 13b with MeO groups in 1,2,3-position, could not be realized due to the fact that we found no way to cleave the carbamate group of 2,3- endo -(P*)- 10b without touching its 1,4-epoxy bridge (Scheme,3). The reaction of 1a with PSA in toluene at 120° was successful, in a way that we found regioisomeric as well as epimeric cycloadducts (Scheme,5). Unfortunately, the attempts to rearrange the products under strong-base catalysis as it had been shown successfully with other furan,PSA adducts were unsuccessful (Scheme,4). The thermal cycloaddition reaction of 1a and 1b with AAN yielded again regioisomeric and epimeric adducts, which could easily be transformed into the corresponding 2- and 3-oxo products (Scheme,6). Only the latter ones could be rearranged with Ac2O/H2SO4 into the corresponding benzo[a]heptalene-3,4-diol diacetates 20a and 20b, respectively, or with trimethylsilyl trifluoromethanesulfonate (TfOSiMe3/Et3N), followed by treatment with NH4Cl/H2O, into the corresponding benzo[a]heptalen-3,4-diols 21a and 21b (Scheme,7). The thermal cycloaddition reaction of 1 with ZSE in toluene gave the cycloadducts 2,3- exo - 22a and - 22b as well as 2- exo,3- endo - 22c in high yields (Scheme,8). All three adducts eliminated, by treatment with base, benzenesulfinic acid and yielded the corresponding 3-(phenylsulfonyl)-1,4-epoxybenzo[d]heptalenes 25. The latter turned out to be excellent Michael acceptors for H2O2 in basic media (Scheme,9). The Michael adducts lost H2O on treatment with Ac2O in pyridine and gave the 3-(phenylsulfonyl)benzo[d]heptalen-2-ones 28a and 3- exo - 28b, respectively. Rearrangement of these compounds in the presence of Ac2O/AcONa lead to the formation of the corresponding 3-(phenylsulfonyl)benzo[a]heptalene-1,2-diol diacetates 30a and 30b, which on treatment with MeONa/MeI gave the corresponding MeO-substituted compounds 31a and 31b. The reductive elimination of the PhSO2 group led finally to the 1,2-dimethoxybenzo[a]heptalenes 32a and 32b. Deprotonation experiments of 32a with t -BuLi/N,N,N,,N,-tetramethylethane-1,2-diamine (tmeda) and quenching with D2O showed that the most acid CH bond is HC(3) (Scheme,9). Some of the new structures were established by X-ray crystal-diffraction analyses (cf. Figs.,1, 3, 4, and 5). Moreover, nine of the new benzo[a]heptalenes were resolved on an anal. Chiralcel OD-H column, and their CD spectra were measured (cf. Figs.,8 and 9). As a result, the 1,2-dimethoxybenzo[a]heptalenes 32a and 32b showed unexpectedly new Cotton -effect bands just below 300,nm, which were assigned to chiral exciton coupling between the heptalene and benzo part of the structurally highly twisted compounds. The PhSO2 -substituted benzo[a]heptalenes 30b and 31b showed, in addition, a further pair of Cotton -effect bands in the range of 275,245,nm, due to chiral exciton coupling of the benzo[a]heptalene chromophore and the phenylsulfonyl chromophore (cf. Fig.,10). [source] Interaction of FeO+ cation with benzene, aniline, and 3-methylaniline: DFT study of oxygen insertion mechanismINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2008Karolina Kwapien Abstract The reaction pathways and energetics for oxygen insertion into CH bond in benzene, aniline, and 3-methylaniline by FeO+ in the gas phase were investigated by means of the DFT methodology with the B3LYP exchange-correlation functional and 6-311G** basis set. The main aim of this work was to elucidate the influence of substituents in phenyl ring on stationary points along the energy profile on sextet and quartet surfaces for the reaction of FeO+ with substituted benzenes. The studies show that the amino and methyl groups change the energetics of oxygen insertion by lowering the energy profile along the reaction pathway. The substituents studied in this work facilitate the insertion of oxygen into the aromatic CH bond by stabilizing the intermediate sigma complex (,-complex), the amino group being by far more effective. On the other hand, both functional groups increase the activation energy of the rate-determining step in the gas phase, so that they have unfavorable influence on the kinetics. The comparison of the energy diagrams for the sextet and quartet spin states indicates the dominance of the low-spin reactivity in oxygen insertion into aromatic CH bond. Aniline and 3-methylaniline oxidation occurs via electrophilic addition while the conversion of benzene to phenol by FeO+ is mediated by a ,-complex with mixed radical and cationic character. Present results are also discussed in the context of oxyferryl group reactivity. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Stability of carbon-centered radicals: Effect of functional groups on the energetics of addition of molecular oxygenJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2009James S. Wright Abstract In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the CH bond. We use theoretical calculations to explore whether the carbon-centered radicals R, which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R, + O2 , ROO, becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for RH and ROO, bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between ROO, and RH BDEs for a wide variety of structures. They also show an equally strong correlation between the ROO, BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Theoretical study on the gas-phase reaction mechanism between nickel monoxide and methane for syngas productionJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2009Hua-Qing Yang Abstract The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH3 is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH3NiOH and HNiOCH3, locate on the deep well, while CH3NiOH is more energetically favorable than HNiOCH3 on both the triplet and singlet states. The main products shall be syngas once HNiOCH3 is created on the singlet state, whereas the main products shall be methyl radical if CH3NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH3 formation involving an oxidative addition of NiO molecule into the CH bond of methane, with an energy barrier of 120.3 kJ mol,1. The syngas formation would be more effective under higher temperature and photolysis reaction condition. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Tunnelling corrections in hydrogen abstractions by excited-state ketonesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2010Monica Barroso Abstract Hydrogen abstraction from 1-phenylethanol by triplet acetophenone occurs from both CH and OH bonds. The reaction path of the Interacting-State Model (ISM) is used with the Transition-State Theory (TST) and the semiclassical correction for tunnelling (ISM/scTST) to help rationalizing the experimental kinetic results and elucidate the mechanisms of these reactions. The weak exothermicity of the abstraction from the strong OH bond is compensated by electronic effects, hydrogen bonding and tunnelling, and is competitive with the more exothermic abstraction from the ,-CH bond of 1-phenylethanol. The alkoxy radical formed upon abstraction from OH reacts within the solvent cage and the primary product of this reaction is 1-phenylethenol. The corresponding kinetic isotope effect is ca. 3 and is entirely consistent with a tunnelling correction ca. 9 for H abstraction. We therefore demonstrate that the tunnelling correction is the major contributor to the kinetic isotope effect. Copyright © 2010 John Wiley & Sons, Ltd. [source] Stereoelectronic effects in radical processes,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8-9 2006Paolo Brandi Abstract A kinetic study of the H-abstraction reaction from cyclic and acyclic alkylarene substrates by the nitroxyl radical (dubbed BTNO) of 1-hydroxy-benzotriazole (HBT) has been carried out in MeCN solution at 25°C. BTNO was generated from one-electron oxidation of HBT by cerium(IV) ammonium nitrate. The H-abstraction reactivity measured with the cyclic alkylarenes is invariably higher than that with the acyclic counterparts. This is explained as the contribution of hyperconjugation between the aromatic ,-system and the scissile benzylic CH bond of the substrate, which weakens the CH bond in the transition state and promotes its cleavage. Stereoelectronic considerations enable to appreciate why the weakening effect is more pronounced in the cyclic system than in the acyclic counterpart, thereby justifying the higher reactivity of the former. Evidence for the intervention of stereoelectronic effects is embodied by the dissociation energies of the CH bonds, having always lower values for the cyclic substrates investigated. Copyright © 2006 John Wiley & Sons, Ltd. [source] Applications of non-steady-state kinetics in physical organic chemistry: guidelines for the resolution of the kinetics of complex reaction mechanismsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2001Vernon D. Parker Abstract The resolution of the kinetics of the reversible consecutive second-order reaction mechanism involving the formation of a kinetically significant intermediate, which does not reach steady state before late in the first half-life, followed by an irreversible product-forming reaction is discussed. It is shown that an apparent second-order rate constant kapp and an extent of reaction,time profile are the only experimental data necessary for the evaluation of kf and kb (the forward and reverse rate constants) as well as kp (the microscopic rate constant for the product forming reaction). When the product-forming step involves the cleavage of a CH bond, for which there is a deuterium kinetic isotope effect on kp, the resolution of the kinetics is enhanced. In this case, the experimental data include two apparent rate constants ( and ) and two extent of reaction,time profiles, one for normal reactants and the other for isotopically substituted reactants. Under these circumstances, a unique highly resolved experimental to theoretical data fit is found that results in the evaluation of all four microscopic rate constants: and . An alternative, when a kinetic isotope effect is not involved, is to fit the extent of reaction,time profiles for two or more concentrations of reactants concurrently. This procedure results in the resolution of the three microscopic rate constants for the reaction. Copyright © 2001 John Wiley & Sons, Ltd. [source] Studies on inclusion complexes of calix[4]arenes capped by diamide bridges with small organic molecules,MAGNETIC RESONANCE IN CHEMISTRY, Issue 8 2008Barbara Balázs Abstract The inclusion of small neutral organic guests (acetonitrile, toluene, pyrazine, butylamine, nitromethane) by cyclic calix[4]arene diamide receptors was studied by 1H NMR spectroscopy. The binding constants determined by 1H NMR titration, and the results obtained by T1 relaxation measurements and DOSY confirm the importance of the acidity of the CH bond of the guests and highlight the role of steric interactions including conformational properties of the receptors in the recognition process. Copyright © 2008 John Wiley & Sons, Ltd. [source] Divergent Insertion Reactions of Pt,Carbenes Generated from [3+2] Cyclization of Platinum-Bound PyrylliumsCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2009Chang, Ho Oh Prof. Good fusion: We describe the reactivity of Pt,carbene B, derived from enynals 1 via A, with Pt catalysts to afford fused cyclopropanes 2, which are obtained by insertion into the CH bond of the , position. Acid-catalyzed rearrangement of the cyclopropanes 2 smoothly occurred to form spiranes 3 (see scheme). [source] Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh MoietyCHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2008Alexander Abstract [RhIIIH{(tBu2PCH2SiMe2NSiMe2CH2PtBu{CMe2CH2})}], ([RhH(PNP*)]), reacts with O2 in the time taken to mix the reagents to form a 1:1 ,2 -O2 adduct, for which OO bond length is discussed with reference to the reducing power of [RhH(PNP*)]; DFT calculations faithfully replicate the observed O,O distance, and are used to understand the oxidation state of this coordinated O2. The reactivity of [Rh(O2)(PNP)] towards H2, CO, N2, and O2 is tested and compared to the associated DFT reaction energies. Three different reagents effect single oxygen atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at and above ,60,°C and by DFT calculations, is a ground-state triplet, is nonplanar, and reacts, above about +15,°C, with its own tBu CH bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe2CH2PtBu2)(SiMe2CH2PtBu{CMe2CH2})}]. [source] Transition-State Energy and Position along the Reaction Coordinate in an Extended Activation Strain Model,CHEMPHYSCHEM, Issue 8 2007G. Theodoor de Jong Dr. Abstract We investigate palladium-induced activation of the CH, CC, CF, and CCl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants' properties. To this end, we extend the activation strain model (in which the activation energy ,E, is decomposed into the activation strain ,E,strain of the reactants and the stabilizing TS interaction ,E,int between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate ,, that is, ,E(,)=,Estrain(,)+,Eint(,). This extension enables us to understand qualitatively, trends in the position of the TS along , and, therefore, the values of the activation strain ,E,strain=,Estrain(,TS) and TS interaction ,E,int=,Eint(,TS) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated CC versus CH activation originates from steric shielding of the CC bond in ethane by CH bonds. Thus, before a favorable stabilizing interaction with the CC bond can occur, the CH bonds must be bent away, which causes the metal,substrate interaction ,Eint(,) in CC activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the CH bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative SN2 pathway, and the effect of ring strain. [source] Car,Parrinello Molecular Dynamics Study of the Blue-Shifted F3CH,,,FCD3 System in Liquid N2CHEMPHYSCHEM, Issue 6 2006Pawel Rodziewicz Dr. Abstract Fluoroform, as confirmed by both experimental and theoretical studies, can participate in improper H-bond formation, which is characterized by a noticeable increase in the fundamental stretching frequency ,(CH) (so-called blue frequency shift), an irregular change of its integral intensity, and a CH bond contraction. A Car,Parrinello molecular dynamics simulation was performed for a complex formed by fluoroform (F3CH) and deuterated methyl fluoride (FCD3) in liquid nitrogen. Vibrational analysis based on the Fourier transform of the dipole moment autocorrelation function reproduces the blue shift of the fundamental stretching frequency ,(CH) and the decrease in the integral intensity. The dynamic contraction of the CH bond is also predicted. The stoichiometry of the solvated, blue-shifted complexes and their residence times are examined. [source] Reaction of para -Hydroxy-Substituted Diphenylmethanes with tert -Butoxy RadicalCHEMPHYSCHEM, Issue 8 2004Catarina F. Correia What is the outcome of this reaction? In acetonitrile solution, the methylenic CH bond is approximately 25 kJ,mol,1 weaker than the OH bond in the same molecule (see picture), as demonstrated by time-resolved photoacoustic calorimetry and quantum chemical methods. However, as shown by electron spin resonance spectroscopy, the tert -butoxy radical selectively abstracts the hydrogen atom from the OH group. [source] An analysis of the dynamic , polarization in the V state of etheneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2010Celestino Angeli Abstract The importance of the dynamic , polarization (absent in methods where the , skeleton is treated at a mean-field level) for the correct description of the V state of the ethene molecule has been recognized by many authors in the past. In this article, this physical effect is analyzed and it is seen as arising from the sum of two contributions: the polarization of the , CC bond and of the , CH bonds. In both cases it is described in a valence bond scheme and the types of excitations needed in a molecular orbital frame to introduce such effects are identified. The effect of the dynamic , polarization on the spatial extent of the V state (,x2,) is described. The analysis here reported has been used in a recent article (Angeli, J Comp Chem 2009, 30, 1319) for the accurate calculation of the V state and of its vertical excitation energy. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110:2436,2447, 2010 [source] Theoretical study of hydrogen-bonded complexes of benzene with hydrides of astrochemical interestINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2008M. Nait Achour Abstract Post Hartree,Fock and DFT calculations have been performed for studying the possibility for a benzene support to be linked to various hydrides through a quasi Bz···HA bond. Interaction energy of compounds, including CH bonds (CH4, CH3F, CH2O, CHN, CHNO), NH bonds (NH3, NH2F, NHC, NHCO, NH3O), and OH bonds (OH2, OHF, NCOH), were evaluated, taking basis set superposition error (BSSE) and zero point vibrational energy (ZPVE) corrections into account. Numerical convergence of results with respect to the ingredients included at different steps of theory (basis set, DFT functionals, correlation treatments, geometry optimization) was tested mainly on the example of the water adduct and, for comparison, the Bz···H3O+ system containing a cation instead of a neutral molecule. A rather large range of adsorption energies is obtained, from about 1 kcal/mol for methane to more than 6 kcal/mol for cyanic acid, according to the acidic character of the adsorbed species in each family of Bz···HA bonds. Some consequences for astrophysical problems involving PAHs in the interstellar medium are pointed out. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Metal-Free, Selective Alkane FunctionalizationsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9-10 2003Andrey Abstract The present overview of alkane functionalization reactions presents comparisons between radical and metal-initiated (sometimes metal-catalyzed) methodologies. While metal-catalyzed processes are excellent approaches to this problem, metal-free alternatives are equally if not, at least from an environmental and cost perspective, more useful. This conclusion is supported by the fact that many so-called metal-catalyzed reactions also work without the metal present, and the large variety of metals showing the same product distributions emphasizes that the metal often just aids in the generation of the active species, i.e., the metal itself is not participating in the crucial CH activation step. Highly selective alkane functionalization reactions such as those derived from nitroxyl and related radicals as well as through radical reactions conducted in phase-transfer catalyzed systems are available but generally underutilized. These systems, in contrast to typical metal-catalyzed approaches, are also applicable to highly strained alkanes and offer the highest 3°/2° CH selectivities reported to date in a radical reaction. The article closes with representative experimental protocols for the PTC bromination of cubane as an example of the applicability of this method to strained hydrocarbons and the direct iodination of cyclohexane as well as adamantane as typical alkanes bearing secondary and tertiary CH bonds. [source] Charge parameterization of the metal centers in cytochrome c oxidaseJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2008Mikael P. Johansson Abstract Reliable atomic point charges are of key importance for a correct description of the electrostatic interactions when performing classical, force field based simulations. Here, we present a systematic procedure for point charge derivation, based on quantum mechanical methodology suited for the systems at hand. A notable difference to previous procedures is to include an outer region around the actual system of interest. At the cost of increasing the system sizes, here up to 265 atoms, including the surroundings achieves near-neutrality for the systems as well as structural stability, important factors for reliable charge distributions. In addition, the common problem of converting between CH bonds and CC bonds at the border vanishes. We apply the procedure to the four redox-active metal centers of cytochrome c oxidase: CuA, haem a, haem a3, and CuB. Several relevant charge and ligand states are considered. Charges for two different force fields, CHARMM and AMBER, are presented. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Stereoelectronic effects in radical processes,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8-9 2006Paolo Brandi Abstract A kinetic study of the H-abstraction reaction from cyclic and acyclic alkylarene substrates by the nitroxyl radical (dubbed BTNO) of 1-hydroxy-benzotriazole (HBT) has been carried out in MeCN solution at 25°C. BTNO was generated from one-electron oxidation of HBT by cerium(IV) ammonium nitrate. The H-abstraction reactivity measured with the cyclic alkylarenes is invariably higher than that with the acyclic counterparts. This is explained as the contribution of hyperconjugation between the aromatic ,-system and the scissile benzylic CH bond of the substrate, which weakens the CH bond in the transition state and promotes its cleavage. Stereoelectronic considerations enable to appreciate why the weakening effect is more pronounced in the cyclic system than in the acyclic counterpart, thereby justifying the higher reactivity of the former. Evidence for the intervention of stereoelectronic effects is embodied by the dissociation energies of the CH bonds, having always lower values for the cyclic substrates investigated. Copyright © 2006 John Wiley & Sons, Ltd. [source] One-bond 13C,13C coupling constants in alkyl-substituted cyclopropenes: experimental and theoretical studies,MAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2002Krystyna Kamie, ska-Trela Abstract Measurements of one-bond carbon,carbon coupling constants, 1J(C, C), were performed for two series of compounds, alkyl-substituted cyclopropenes and cyclopropanes. The experimental data were complemented by a set of DFT-calculated J couplings for the parent cyclopropene (1), its methyl and silyl derivatives and, additionally, for 1-methylcyclobutene (3), 1-methylcyclopentene (4) and 1-methylcyclohexene (5) and good agreement was observed between the experimental and the calculated data; all the trends are perfectly maintained, including a dramatic decrease in the couplings across endocyclic single bonds in cyclopropene and its derivatives, and a significant decrease in the corresponding couplings in cyclobutene. Using the data obtained, the s characters of the carbon hybrid orbitals involved in the formation of the cyclopropene were calculated. The results clearly show that the ring closure and the related strain exerted upon the cyclopropene molecule only slightly disturb the electron structure of the double bond. The s character of the corresponding carbon orbital is 0.314 in cyclopropene vs the theoretical value of 0.333 in ethene. This is at variance with the endo- and exocyclic single bonds, where the s characters of the orbitals forming the endocyclic single bonds are much smaller than those of the bonds in the open-chain compounds, i.e. 0.229 (C-1 and/or C-2) and 0.166 (C-3). The s values calculated for the exocyclic CH bonds are 0.334 for C-3 and 0.456 for C-1 and/or C-2. Copyright © 2002 John Wiley & Sons, Ltd. [source] Significant Influence of Zn on Activation of the C-H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite,CHEMPHYSCHEM, Issue 17 2008Alexander G. Stepanov Prof. Abstract Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C1,n -C4 alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of CH bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n- butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C1,n -C4 alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites. [source] Transition-State Energy and Position along the Reaction Coordinate in an Extended Activation Strain Model,CHEMPHYSCHEM, Issue 8 2007G. Theodoor de Jong Dr. Abstract We investigate palladium-induced activation of the CH, CC, CF, and CCl bonds in methane, ethane, cyclopropane, fluoromethane, and chloromethane, using relativistic density functional theory (DFT) at ZORA-BLYP/TZ2P. Our purpose is to arrive at a qualitative understanding, based on accurate calculations, of the trends in activation barriers and transition state (TS) geometries (e.g. early or late along the reaction coordinate) in terms of the reactants' properties. To this end, we extend the activation strain model (in which the activation energy ,E, is decomposed into the activation strain ,E,strain of the reactants and the stabilizing TS interaction ,E,int between the reactants) from a single-point analysis of the TS to an analysis along the reaction coordinate ,, that is, ,E(,)=,Estrain(,)+,Eint(,). This extension enables us to understand qualitatively, trends in the position of the TS along , and, therefore, the values of the activation strain ,E,strain=,Estrain(,TS) and TS interaction ,E,int=,Eint(,TS) and trends therein. An interesting insight that emerges is that the much higher barrier of metal-mediated CC versus CH activation originates from steric shielding of the CC bond in ethane by CH bonds. Thus, before a favorable stabilizing interaction with the CC bond can occur, the CH bonds must be bent away, which causes the metal,substrate interaction ,Eint(,) in CC activation to lag behind. Such steric shielding is not present in the metal-mediated activation of the CH bond, which is always accessible from the hydrogen side. Other phenomena that are addressed are anion assistance, competition between direct oxidative insertion (OxIn) versus the alternative SN2 pathway, and the effect of ring strain. [source] | ||||||||||||||||||||||