The Mechanism of the Palladium Hydride β-Elimination Step in the Heck Reaction

The kinetics of competitive phenylation of alkenes with iodobenzene over palladium complexes (the Heck reaction) was studied. The effect of one alkene on the arylation rate of another alkene conflicts with the conventional mechanism of the Heck reaction in which the arylated alkene is formed through the unimolecular step of palladium hydride -elimination. Based on experimental data obtained, another mechanism is proposed in which a reaction product is formed through the transfer of palladium hydride to the initial alkene molecule.     

The proton affinities of styrene,trsns-β-methylstyrene,and indene

We report experimental and theoretical AM1 proton affinities of styrene,-methylstyrenes, and indene. The computed AM1 proton affinities for the species of interest were in good agreement with the experimental values.trans--Methylstyrene was found to have a proton affinity slightly lower than that of styrene. This is an unusual result since methyl substitution in most classes of compounds increases the proton affinity by 2–4 kcal mol^–1. The lower basicity oftrans--methylstyrene compared to styrene is due to the greater stabilizing effect of the methyl group in the neutral species compared to the cation.     

Effects of protonation on the hydrolysis of triphosphate in vacuum and the implications for catalysis by nucleotide hydrolyzing enzymes

Background

Nucleoside triphosphate (NTP) hydrolysis is a key reaction in biology. It involves breaking two very stable bonds (one P–O bond and one O–H bond of water), in either a concurrent or a sequential way. Here, we systematically examine how protonation of the triphosphate affects the mechanism of hydrolysis.

Results

The hydrolysis reaction of methyl triphosphate in vacuum is computed with protons in various numbers and position on the three phosphate groups. Protonation is seen to have a strong catalytic effect, with the reaction mechanism depending highly on the protonation pattern.

Conclusion

This dependence is apparently complicated, but is shown to obey a well-defined set of rules: Protonation of the α- and β-phosphate groups favors a sequential hydrolysis mechanism, whereas γ-protonation favors a concurrent mechanism, the two effects competing with each other in cases of simultaneous protonation. The rate-limiting step is always the breakup of the water molecule while it attacks the γ-phosphorus, and its barrier is lowered by γ-protonation. This step has significantly lower barriers in the sequential reactions, because the dissociated γ-metaphosphate intermediate (P_γO₃ ^?) is a much better target for water attack than the un-dissociated γ-phosphate (?P_γO₄ ^2?). The simple chemical logic behind these rules helps to better understand the catalytic strategy used by NTPase enzymes, as illustrated here for the catalytic pocket of myosin.A set of rules was determined that describes how protonating the phosphate groups affects the hydrolysis mechanism of methyl triphosphate: Protonation of the α- and/or β- phosphate groups promotes a sequential mechanism in which P-O bond breaking precedes the breakup of the attacking water, whereas protonation of the γ-phosphate promotes a concurrent mechanism and lowers the rate-limiting barrier of water breakup. The role played by individual protein residues in the catalytic pocket of triphosphate hydrolysing enzymes can be assigned accordingly.

Graphical abstract

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Electrochemical Methoxylation of Cinnamic Acid and Its Derivatives

Electrochemical methoxylation of cinnamic acid is accompanied by decarboxylation and yields 1,1,2-trimethoxy-2-phenylethane through formation of -truxillic acid in the initial stage. Under the same conditions methyl cinnamate and cinnamamide give rise to hydrodimerization and reduction products. The results show that introduction of electron-acceptor substituents into the -position of the aliphatic chain of styrene hinders methoxylation.     

Oxidative Addition Step in Reactions Involving Palladium Activation of Carbon-Halogen and Carbon-Oxygen Bonds

Different modifications of the Heck reaction involving the activation of carbon-halogen and carbon-oxygen bonds by palladium (styrene phenylation with iodobenzene or benzoic anhydride and iodobenzene carbonylation, reductive coupling, and reduction) are studied by in situ ³¹P NMR spectroscopy. The catalytic cycles of the reactions include oxidative addition to Pd(0) formed in situ. The product composition in this step depends strongly on the composition of the reaction mixture, which is related to PhX conversion in the main catalytic process and with the nature of the catalyst precursor. A new hypothesis as to the mechanism of the catalytic cycle in alkene arylation in the presence of phosphine ligands is suggested. This hypothesis is consistent with NMR monitoring data and with the value of the kinetic isotope effect.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 529–535.Original Russian Text Copyright © 2005 by Shmidt, Smirnov.     

α-phenylisocinchomeronic and 4-azafluorenone 3-carboxylic acids

3,6-Dimethyl-6-phenylpyridine, obtained on phenylation of 2,5-lutidine, has been used in the synthesis of -phenylisocinchomeronic acid, derivatives of it, and also for the preparation of 4-azafluorenone 3-carboxylic acid. It was established that 4-hydroxy-3,6-dimethyl-2-phenylpyridine was formed on phenylation of 2,5-lutidine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 645–648, May, 1986.     

Synthesis of Copolymers of Unsaturated β-Diketones with Styrene and Methyl Methacrylate

Copolymerization of styrene and methyl methacrylate with various unsaturated -diketones was studied. The influence exerted by the reaction temperature, structure of -diketone, and its content in the comonomer mixture on the kinetic parameters of copolymerization and molecular weights of the products was examined.     

Computer simulation for the cooligomerization system

In order to clarify the kinetic features of the styrene (A)–methyl methacrylate (B)–CCl₄(S) cooligomerization system, a computer simulation was carried out. The experimental data on the degree of polymerization and the deviation of the cooligomer composition from the statistical steady-state composition were comparatively well explained by calculations based on the kinetic equations derived from the assumed reaction scheme and the values of the velocity coefficients, although the values of the four velocity coefficients in the initiation step and the velocity coefficient of the termination by the coupling of two solvent radicals were estimated. The results of the calculation of the rate of each component reaction show that the following two reactions are the most important in the initiation and in the transfer and termination steps when the [S]/([A] + [B]) ratio is large: where, A, A*, and P are styrene, polystyryl radical, and the cooligomer, respectively. Moreover, it was concluded that the deviation of the cooligomer composition from the statistical steady-state composition was caused by these two reactions.     

Kinetic role of t-amines in the ligand inhibited osmium tetraoxide catalyzed trimethylamine-N-oxide dihydroxylation of alkenes

Kinetic studies on the t-amine inhibited catalytic OsO₄ dihydroxylation of cyclooctene, octene, styrene and 4-methyl-, 4-methoxy-, 4-chloro-, 4-(chloromethyl), 4-(trifluoromethyl) and 3-chloro-derivatives of styrene with Me₃NO in t-BuOH have been carried out at 50 °C. The reactions follow identical kinetics: first order in total osmium species, first order in Me₃NO and zero order in alkene. All t-amines have been found to retard the catalysis and the reaction order in t-amine changes from inverse first order to zero. The involvement of dioxomonoglycolataosmium(VI) esters and their monoamine adducts in the rate-determining oxidation step was established by the linear plots of 1/k ₂ versus. 1/[L] where k ₂ is the decrease in second order rate constant in the presence of [L] concentration of t-amine ligand. Beyond a definite concentration of t-amine, the rate reaches a minimum and remains constant. Activation parameters were evaluated for catalytic OsO₄ dihydroxylation of 4-(chloromethyl)styrene at different t-amine concentrations. The values of H and S are consistent with the proposed mechanism.     

Substituent effects on the rate of cyclization of N-phenyl-N′-(o-cyanophenyl)urea in solution

Conclusions The isomerizational cyclization of N-phenyl-N-(o-cyanophenyl)urea in solution in the presence of bases is a one-step reaction with an equilibrium step involving proton abstraction as the rate-limiting step.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1440–1443, June, 1985.     

ESR studies of the radiation-induced multiple graft polymerization

The radiation-induced multiple-graft polymerization was studied by an ESR method. When methyl methacrylate vapor was introduced onto preirradiated polyethylene already grafted with styrene, the second step of grafting of methyl methacrylate occurred mainly in the polyethylene portion. The kinetic treatment proved that the termination rate constant k_t of methyl methacrylate decreased with the amount of styrene grafted in advance. On the other hand, when styrene vapor was introduced onto polyethylene grafted with methyl methacrylate, only radicals of poly(methyl methacrylate) decreased. In this case, the second step of grafting of styrene occurred in the poly-(methyl methacrylate) portion which covered the whole surface of the polyethylene powder. When monomer vapors were alternately introduced onto preirradiated polyethylene powder, the second step of grafting occurred at the growing chain end of the first monomer.     

The kinetics of the addition of gem-denitroethane to methyl acrylate and acrylonitrile

Summary A study was made of the rate of addition of dinitroethane to methyl acrylate and acrylonitrile. The limiting reaction step is the attack of the-carbon atom of the unsaturated compound by the dinitroethane anion.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimisheskaya, No. 7, pp. 1283–1285, July, 1965See [1] for preceding communication.     

Analysis of ″stepwise″ heterocoagulation process of small cationic polymer particles onto large anionic polymer particles using dynamic light scattering

In order to clarify in detail the process of the stepwise heterocoagulation of small polymer particles (SP) onto large polymer particles (LP), which we proposed to prepare anomalous polymer particles, the particle-size distribution in each step was estimated using dynamic light scattering.SP andLP have surface charges opposite to each other in emulsion states.SP were produced by emulsion copolymerization of styrene and methacryloyloxyethyltrimethylammonium chloride, andLP by emulsion terpolymerization of styrene, butyl acrylate, and methacrylic acid. Both emulsions in which nonionic emulsifier had been added were blended without coagulation and then the heterocoagulation was carried out by adjusting of the pH and heating.Part CXVII of the series Studies on Suspension and Emulsion.     

The phenylation of 3βpicoline. Isolation and establishment of the structure of β-pyridyl-α-dehydropiperidine

It was established that the side product that is formed in substantial amounts in the phenylation of -picoline by phenyllithium is 3-methyl-2-phenyl-5-(3-methyl-2-phenyl-3,4-dehydropiperidyl-6)pyridine — a structural analog of anabasine. Its structure was demonstrated by spectral methods and by chemical conversions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 115–119, August, 1984.     

Oxidation of aromatic hydrocarbons with oxygen in a radiofrequency plasma

The reactions of three aromatic hydrocarbons, benzene, toluene, and styrene, with oxygen in a radiofrequency (r) plasma were investigated. Benzene was oxidized to yield phenol as a single volatile organic product. Similarly, toluene gave the ring oxidation products, cresols, as well as considerable amounts of methyl oxidation products, consisting mainly of benzaldehyde and benzyl alcohol. In contrast, the oxidation of styrene took place predominantly on the olefinic double bond to produce styrene oxide On basis of the products and effects of reaction variables, r power and flow rates of hydrocarbons and oxygen, on the reaction rate, the oxidation mechanism was discussed, particularly focusing on the intermediate species responsible for the formation of major products.     

Effect of the potential of an external electron donor on C<Subscript>2</Subscript>H<Subscript>2</Subscript> reduction catalyzed by the nitrogenase active center (FeMoco) isolated from the enzyme

The effect of potential value and chemical properties of an external electron donor on C₂H₂ reduction catalyzed by nitrogenase active center (cluster [(₆-N)Fe₇MoS₉·homocitrate] FeMoco isolated from the enzyme) has been investigated in the presence of proton donors of different acidity. The temperature—reaction rate dependences of these reactions have been studied. It has been shown that the rate-limiting steps of the reactions differ depending on the proton donor used. When thiophenol or water are used as proton donors, and electrochemical step — the electron transfer from cathode to adsorbed catalytic cluster — has been found to be a rate-limiting one. The effective activation energy of ethane formation as a product of four-electron C₂H₂ reduction is found to be 1.5 times lower than that of ethylene, namely, 13 kcal mol^–1. When stronger acid, pentafluorothiophenol, is used as a proton donor, the chemical step of intramolecular rearrangement of the catalyst—substrate complex taking place in solution becomes a rate-limiting one. The effective activation energies of both ethylene and ethane become equal to 32 kcal mol^–1.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1583–1591, August, 2004.     

Phosphine‐Catalyzed Remote β‐CH Functionalization of Amines Triggered by Trifluoromethylation of Alkenes: One‐Pot Synthesis of Bistrifluoromethylated Enamides and Oxazoles

An unprecedented phosphine‐catalyzed remote β‐C? H functionalization of amine derivatives triggered by trifluoromethylation of an alkene with Togni’s reagent was disclosed. This reaction proceeded through the highly selective and concomitant activation of an unactivated alkene and the β‐C? H bond of an amine derivative, providing bistrifluoromethylated enamides in excellent yields with good regio‐, chemo‐, and stereoselectivity. Furthermore, the newly developed one‐pot protocol provides a facile and step‐economical access to valuable trisubstituted 5‐(trifluoromethyl)oxazoles. Mechanistic studies showed that this reaction may initiate with a novel phosphine‐catalyzed radical trifluoromethylation of unactivated alkene via a phosphorus radical cation.     

Substituent effects in acid-catalyzed hydration of alkenes, measured under consistent reaction conditions

A set of convenient conditions for the title reaction is used to determine directly relative reactivities of alkenes with widely-varying steric requirements. Previous determinations did not use one set of conditions for all alkenes, but developed parameters in order to compare alkene reaction rates run in different solvents and in different concentrations; such an indirect reactivity comparison can introduce errors. Relative rates established under this common set of reaction conditions are compared to those previously calculated by using correction parameters. Comparison reveals that the approximate method overestimated effects of some groups attached to CC, such as methyl and chloro.