Transition state

About: Transition state is a research topic. Over the lifetime, 4978 publications have been published within this topic receiving 117965 citations. The topic is also known as: transition state of elementary reaction.

Development of a Cysteine-Catalyzed Enantioselective Rauhut−Currier Reaction

Abstract: Herein, we report a full account of the development of an enantioselective Rauhut−Currier process that affords products in high yields and enantioselectivities using a cysteine-based catalyst. While traditional Morita−Baylis−Hillman catalysts were found to be essentially ineffective, various derivatives of protected cysteine were found to exhibit extraordinary reactivity and enantiotopic control. Extensive screening of reaction conditions led us to discover the enhanced effects of water as an additive and the chelating power of potassium in achieving higher enantiomer ratios. Mechanistic experiments also provided insight on the potential mechanism of the reaction in addition to possible transition states that provide the absolute stereochemistry formed in the observed products. Also included is a brief survey of the reaction scope involving different ring sizes as well as functionalized substrates.

Relationship of Leffler (Brønsted) α values and protein folding Φ values to position of transition-state structures on reaction coordinates

Abstract: The positions of transition states along reaction coordinates (r‡) for simple chemical reactions are often estimated from Leffler α values, the slope of plots of ΔG‡ (activation energy) versus ΔG0 (equilibrium free energy) for a series of structural variants. Protein folding is more complex than simple chemical reactions and has a multitude of reaction coordinates. Φ-Value analysis measures degree of structure formation at individual residues in folding transition states from the ratio ΔΔG‡/ΔΔG0 for mutations. α values are now being used to analyze protein folding by lumping series of Φ values into single plots. But, there are discrepancies in the values of α for folding with more classical measures of the extent of structure formation, which I rationalize here. I show for chemical reactions with just a single reaction coordinate that α = r‡ only for limiting cases, such as for reactants and products being in parabolic energy wells of identical curvature. Otherwise, α can differ radically from r‡, with α being determined just by the angles of intersection of reactant and product energy surfaces. Φ is an index of the progress of a local, energy-based reaction coordinate at the global transition state: Φ 0.5 means >50%. Protein Leffler plots can force different local indexes to a single fit and give skewed underestimates of the extent of global structure formation in transition states that differ from other measures of structure formation.

Thermal decomposition of ethanol. II. A computational study of the kinetics and mechanism for the H+C2H5OH reaction

Abstract: The kinetics and mechanism for the H+C2H5OH reaction, a key chain-propagation step in the high temperature decomposition and combustion of ethanol, have been investigated with the modified GAUSSIAN -2 (G2M) method using the structures of the reactants, transition states and products optimized at the B3LYP/6-311+G(d,p) level of theory. Four transition states have been identified for the production of H2+CH3CHOH (TS1), H2+CH2CH2OH (TS2), H2+C2H5O (TS3), and H2O+C2H5 (TS4) with the corresponding barriers, 7.18, 13.30, 14.95, and 27.10 kcal/mol. The predicted rate constants and branching ratios for the three H-abstraction reactions have been calculated over the temperature range 300–3000 K using the conventional and variational transition state theory with quantum-mechanical tunneling corrections. The predicted total rate constant, kt=3.15×103T3.12 exp(−1508/T) cm3 mol−1 s−1, agrees reasonably with existing experimental data; in particular, the result at 423 K was found to agree quantitatively with an availab...

Reactivity of N-Phenyl-1-Aza-2-Cyano-1,3-Butadienes in the Diels−Alder Reaction

Abstract: It is found that N-phenyl-2-cyano-1-azadiene 4, prepared via a two-step, one-pot, sequence from acrylanilide, undergoes efficient [4 + 2] cycloaddition with a complete range of electron rich, electron poor, and neutral dienophiles under remarkably mild thermal conditions (90-120 degrees C for 20-48 h). Regiospecific formation of the alpha-cycloadduct wherein the dienophile substituent is alpha to nitrogen is observed for vinyl ethers and styrene, whereas the Diels-Alder reactions with methyl acrylate and methyl vinyl ketone (MVK) produce alpha/beta mixtures in which the alpha-cycloadduct is the major regioisomer (approximately 4-5:1). An essentially identical reaction pattern was observed in the Diels-Alder reaction of N-(p-methoxyphenyl)-2-cyano-1-azadiene 18 and the 4-methyl-substituted azadiene 27. For compound 19 derived from cycloaddition of 18 with ethyl vinyl ether, facile conversion to the dihydropyridine 21 through loss of EtOH on brief acid treatment was also noted. The 2,4-cis-disubstitution pattern confirmed by X-ray diffraction for the major cycloadduct 29 isolated from the reaction of 27 with styrene provides evidence for the endo mode of cycloaddition in the Diels-Alder reaction of N-phenyl(aryl)-2-cyano-1-azadienes. Calculation of the frontier orbital energies and coefficients, as well as the transition state geometries for the [4 + 2] cycloaddition of N-phenyl-2-cyano-1-azadiene 4 with methyl vinyl ether, styrene, and MVK were carried out at the RHF AM1 level (MOPAC, Version 5.0). The FMO treatment indicates that the reaction of 4 with methyl vinyl ether occurs under LUMO(diene) control, whereas in contrast, the corresponding cycloaddition with MVK occurs preferably under HOMO(diene) control. A high degree of asynchronicity is observed in the calculated transition states for reaction of 4 with the three representative dienophiles. In all cases the transition states leading to the alpha-cycloadducts are lower in energy than those giving the beta-products. However, at the AM1 level the exo cycloaddition mode is found to be the preferred, this result contrasting with experimental results for azadiene 27.