Showing papers on "Transition state published in 1979"

.alpha.-Deuterium isotope effects and transition-state structure in an intramolecular model system for methyl-transfer enzymes

Abstract: The ..cap alpha..-deuterium isotope effects for the general base catalyzed ring-closure reaction of 2-(2-cis-hydroxycyclopentyl)ethylmethyl-4-nitrophenylsulfonium tetrafluoroborate (1) are k/sub 2H/K/sub 2D/ = 1.7 +- 0.02 (water catalysis), 1.104 +- 0.014 (carbonate buffer catalysis), and 0.998 +- 0.001 (hydroxide ion catalysis) in aqueous solution (..mu.. = 1.0) at 40.00 +- 0.05/sup 0/C. These are all considerably more normal (i.e., larger in the direction k/sub H/ > k/sub D/) than the effect for the catechol O-methyltransferase reaction (k/sub 3H//k/sub 3D/ = 0.83 +- 0.05) for which the reactions of 1 are a model. This indicates the model-reaction transition states to be looser (i.e., to possess a longer oxygen-to-sulfur distance) than the enzymic transition state and suggests that the enzyme may achieve all or part of its catalysis of methyl transfer from processes that are associated with compression of the transition state.

Isotope effects in nucleophilic substitution reactions. III. The effect of changing the leaving group on transition state structure in SN2 reactions

Abstract: The nucleophilic substitution reactions of a series of 4-substituted phenylbenzyldimethyl-ammonium ions with thiophenoxide ions at 0 °C in N,N-dimethylformamide have been used to demonstrate how a change in the leaving group alters the structure of the SN2 transition state. Heavy atom (nitrogen) kinetic isotope effects, secondary α-deuterium kinetic isotope effects and Hammett ρ values provide qualitative descriptions of both the nucleophile–α-carbon and α-carbon–leaving group bonds in the transition states of these reactions. The results indicate that changing to a better leaving group causes the bond between the α-carbon and the nucleophile to be much more fully formed while the bond to the leaving group is essentially unchanged. The results are discussed in the light of current theories of substituent effects on SN2 reactions and a possible explanation for the surprising results (i) that the greatest effect is in the bond more remote from the point of structural change and (ii) that more nucleophilic a...

Ab initio approach to organic reaction rates. Kinetic isotope effects in the reaction H + C/sub 2/H/sub 4/. -->. C/sub 2/H/sub 5/

Abstract: An ab Initio Approach to Organic Reaction Rates. Kinetic Isotope Effects in the Reaction H + C/sub 2/H/sub 4/..-->..C/sub 2/H/sub 5/. The results of a transition state theoretical (TST) study, reinforced by the relevant ab initio molecular orbital computation, of the addition reaction H+C/sub 2/H/sub 4/..-->..C/sub 2/H/sub 5/ are reported. The TST procedure has proved to be valuable as a quantitative tool for kinetic considerations. Kinetic isotope effects of various deuterium isotope combinations were considered. The vibrational frequencies of the deuterated reactants as well as the transition states were calculated in exactly the same manner as for the nondeuterated case. The resulting rate constants are included. The ab initio molecular orbital calculation seems to be capable of supplying sufficient information on the characteristics of the transition state for elementary reactions of moderately complex organic compounds. The importance of the entropy or preexponential factor in the rate processe should be emphasized in this connection. (3 tables)

Molecular Orbital Studies of Enzyme Catalysed Reactions. Rearrangement of Chorismate to Prephenate

Abstract: Molecular orbital calculations are used to describe the reaction surface for the non-enzymic Claisen rearrangement of chorismate to prephenate, which may proceed through either a boat-like or a chair-like transition state. Detailed molecular geometries are obtained for the neutral and dianionic forms of chorismate, prephenate, and the alternative transition states. The transition states are asymmetric structures in which the breaking C-O bond (c. 1.45 A) is significantly shorter than the making C-C bond (c. 1.95 A). The alternative reaction pathways have almost identical enthalpies of activation (chair, 277.4 kJ/mol ; boat, 282.8 kJ/mol; dianionic forms) which result partly from a loss of internal bond strength and partly from repulsive interactions between the polar carboxyl groups. Protonation stabilizes the transition states (chair, 247.3 kJ/mol; boat, 248.5 kJ/mol ; diacid forms) by delocalization of charge in the carboxyl groups, and a similar mechanism is proposed for the greatly reduced enthalpy of activation in aqueous solution (86.6 kJ/mol). The enthalpy difference between the alternative reaction pathways is insufficient to define a preferred transition state structure, and either pathway may be favoured for the non-enzymic reaction in aqueous solution. For the enzyme-catalysed reaction the chair pathway is used, and the calculated transition state structures and enthalpy barriers provide information relevant to the catalytic mechanism. They indicate that an active site comprising only two essential binding groups is sufficient to account for catalysis; the orientation of these groups within the active site should allow simultaneous bond formation, accompanied by charge delocalization, to both carboxyl groups of the transition state, but not to those of substrate or product. The calculated structure for the chair transition state, taken in conjunction with those for chorismate and prephenate, thus provides a template for the active sites of chorismate mutases.

Observation of a Condon reflection products state distribution in the collinear H + Cl2 reaction

Abstract: A classical and semiclassical investigation of conditions required for the observation of a Condon reflection pattern in the products state distribution for the collinear H + Cl2 reaction is reported. Both vibrational and translation excitation of the reactants are considered. The use of semiclassical arguments in this context is justified by a high level of agreement with exact quantum mechanical results, although a significant threshold anomaly remains to be investigated. Competition with the inelastic channel above a certain threshold, which decreases with increasing reactant vibrational quantum number, is found to invalidate any simple Condon reflection prediction. The nature and importance of this competition is shown to be simply characterisable in terms of the properties of certain trapped trajectories, or nascent transition states, in the products valley of the potential surface.

Application of force field calculations to organic chemistry. Part 7. Steric interpretation of thermolysis, homoketonization, ring enlargement, and acid-catalysed rearrangement of strained cage molecules

Abstract: Empirical force field calculations are applied to elucidate various types of reaction mechanisms for strained cage molecules. Rates of novel thermal decarbonylation in homologous series of homocubanone derivatives (1; R = phenyl) increase with the calculated strain of the C(2)–C(3) bond, which in turn is strongly influenced by the length of alkylene bridge X, two bonds away from the C(2)–C(3) bond. The remarkable sensitivity of the rates on the calculated C(2)–C(3) bond strain is interpreted in terms of a reactant-like transition state. In contrast, thermolyses, homoketonizations, cationic ring enlargements, and anionic rearrangements of birdcage and homocubyl systems are shown to proceed through product-like transition states based on highly selective formation of the thermodynamically most stable product. The most favourable pathway is presented for a novel acid-catalysed multi-step rearrangement of a bisethanocubanedione derivative (29) to a bisnordiamantanedione derivative (30) based on enthalpy calculations of the intermediate carbonium ions. The failure to produce the ‘stabilomer’ upon acid treatment of (29) under ordinary conditions is attributed to the absence of a mechanistically acceptable path for further skeletal isomerization.

Model Calculations of Isotope Effects. IV. Transition State Structures and the Question of Tunnelling in Proton Transfer Reactions

Abstract: Model calculations of primary hydrogen isotope effects in proton transfer reactions are reported. The geometries and force fields of transition state models have been systematically varied with respect to both reactant-like and product-like character and to tight against loose character. The models include both hypothetical cut-off molecules and 2-nitropropane. Values of kH/kD greater than 17 are calculated for loose, symmetrical transition states in which the sum of the bond orders pertaining to the transferring proton is set at 0.6, and higher than normal values of (ED-EH) and ADIAH are also associated with such transition states. It is suggested that transition state looseness is a consequence of repulsive donor-acceptor steric interactions, and that several sets of experimental results which have hitherto been rationalized by the invocation of proton tunnelling may equally well be explained by postulating loose transition states.

The Interconversion of Two Elemadienolides Through Two Consecutive Cope Rearrangements

Abstract: Callitrin (1) at 220o partly isomerizes to epicallitrin (9). This result, together with those of the Cope rearrangements of some other sesquiterpenoids, is explained in terms of the accessibility of suitable transition states and the relative stabilities of the possible products.

Estimating the positions of transition states from experimental data

Abstract: A procedure is suggested to estimate the positions of a transition state (TS) along the reaction coordinate in reaction series involving several radical reactions. It is based on the dependence between experimental activation energies U± and heat effects Q. Taking into account the shift of a TS position, the following relation between these quantities is obtained: U± −; U = A (Q - Q0)2 + B(Q - Q0). The subscript 0 indicates the standard reference reaction. For three series, namely, the hydrogen abstraction from hydrocarbon substrates by radicals CH3˙, CF˙3, and Br·, the bond lengths characterizing the TS reaction center are evaluated. The TS positions appear to vary significantly in the reaction series, which accounts for significant changes in the experimentally observed activation volumes. The derivation of the Hammond rule and the range of its validity for the series with polar and steric substituent effects are discussed.

Reaction of 1,1,3,3-tetramethylthiourea with methyl iodide: kinetic and thermodynamic aspects

Abstract: In the reaction of 1,1,3,3-tetramethylthiourea with methyl iodide limiting forms of the rate law arise in solvents in which the concentration of free ions and ion-pairs is small in comparison with higher aggregates. The solvent effects on the rate of the forward reaction were linearly correlated with those on the Menschutkin reaction. The effects were quite close to those on the pyridine–methyl iodide reaction. The value (Δ2V‡/Δ2°), which is expected to be an index of the position of the transition state along the reaction co-ordinate, is ca. 0.27 in acetonitrile and propylene carbonate. The position of the transition state, nT, calculated from the enthalpy term, is ca. 0.30 for the present reaction and for the NN-dimethylaniline–methyl iodide reaction. These indices of the transition state position are qualitatively in agreement with each other. The results of CNDO/2 calculations, performed for the assumed transition state structures, are in accord with the results of the experimental observations on the solvent effects on reaction rates.

Solvent effect on the reaction of alkenes with phosphorus pentachloride

Abstract: 1. The solvent effect on the conditions and course of reactions of PCl5 with alkenes was studied. The temperature at the onset of these reactions decreased with increasing polarity of the medium and donor properties of the solvents. 2. The reaction pathways for PCl5 with alkenes are determined not only by the alkene structure, as had been assumed previously, but also by the temperature conditions for the condensation and decomposition of the adducts and also by the solvent properties. Depending on these factors, the final reaction products may be chloroalkenes, dichloroalkanes, phosphorylated alkenes, or 2-chloroalkanes. 3. Proposals were offered for the nature of the transition states in the reactions of PCl5 with alkenes.

Theoretical Study of Some Simple Organic Reactions

Abstract: The mechanisms of four gas phase organic reactions are studied through ab-initio LCAO-MO-SCF calculations. These reactions are: (i) cis-trans isomerization of cyclopropane; (ii) decomposition of cyclobutane into two ethylene molecules; (iii) the Diels-Alder reaction; (iv) decomposition of 1-pyrazolines into cyclopropane and nitrogen. The structure and the electronic properties of the various transition states are discussed. In particular, it is shown that a diradical can be either a transition state, a secondary intermediate, or a “transient point” on a hillside.

Stereochemistry of the transition state in Diels-Alder reactions

Abstract: A comparison of the compactness of hypothetical models of isomer transition states (TS) of competitive reactions makes it possible to draw conclusions regarding the ratio between their volumes, which can be verified experimentally by investigating the influence of the pressure on the isomeric composition of the products [i]. Thus, the possibility of choosing among alternative models of transition states has been opened up. The application of this method to Diels--Alder reactions has led to the conclusion that in the reactions of transpiperylene with methyl acrylate, as well "as of l-alkoxybutadienes with carbonyl-containing dineophiles, the transition state is more similar with respect to its spatial structure to the prereaction complex (PRC) than to the adduct (ADD), which has a half-chair conformation [i, 2]. A similar analysis of the results of the present work (Table i) made it possible for the first time to follow the motion of a transition state along the reaction coordinate due to the variation of R in the reaction of ~-l-alkylgutadiene (R~u) with methyl acrylate. As previously, the experimental values of (Vo~ -V~t) and (V~c -V~G) have the same signs as