The proton exchange reaction between indene and the indenyl anion

The proton exchange reaction between the indenyl carbanion and its parent compound indene has been studied by NMR as a function of temperature. The rate of this bimolecular reaction is very low and has been found to be strongly dependent on the polarity of the solvent. In solvents like dimethoxyethane (? = 7·2) and diglyme the reaction becomes manifest in the NMR spectrum only at elevated temperatures (T > 150°C). In hexamethylphosphortriamide (? = 30) the rate is much greater and line broadening may be observable at room temperature. The reaction in this solvent is characterised by a frequency factor f = 7 × 10⁷ 1 mol^?1 s^?1, an activation enthalpy ΔH ≠ = 9·5 kcal mol^?1 and an entropy of activation ΔS≠ = ?23 e.u. The low reaction rate and its solvent dependence are briefly discussed.     

Rhodium-catalyzed disulfide exchange reaction

A system of RhH(PPh3)4, trifluoromethanesulfonic acid, and (p-tol)3P catalyzes the disulfide exchange reaction. Treatment of two symmetrical dialkyl disulfides with the catalyst provides an equilibrium mixture of three disulfides within 15 min in refluxing acetone. The catalyst is active after reaching the equilibrium, and addition of a disulfide to the mixture changes the ratio of the products. The use of 4 mol equiv excess of one of the disulfides provides the unsymmetrical disulfide in a yield exceeding 80%. Disulfide-containing peptides also undergo an exchange reaction. The reactions of diaryl disulfides and dialkyl disulfides are even faster, and reach equilibrium within 5 min at room temperature in the presence of the rhodium complex and 1,2-bis(diphenylphosphino)ethane (dppe). This exchange reaction is considerably affected by the substituents on the disulfides. Treatment of diphenyl disulfide, di(p-tolyl) disulfide, and bis(sec-butyl) disulfide yields phenyl p-tolyl disulfide at room temperature with unchanged bis(sec-butyl) disulfide; random disproportionation occurs at reflux. The rhodium catalysis can be used for the exchange reaction of disulfides and diselenides giving selenosulfides as well as disulfides and ditellurides giving tellurinosulfides.     

New, unexpected, and dominant mechanisms in the hydrogen exchange reaction

A quasiclassical trajectory study of the state specific H+D(2)(upsilon = 0,j = 0) --> HD(upsilon' = 0,j' = 0) + D reaction at a collision energy of 1.85 eV (total energy of 2.04 eV) found that the scattering is governed by two unexpected and dominant new mechanisms, and not by direct recoil as is generally assumed. The new mechanisms involve strong interaction with the sloping potential around the conical intersection, an area of the potential energy surface not previously considered to have much effect upon reactive scattering. Initial investigations indicate that more than 50% of reactive scattering could be the result of these new mechanisms at this collision energy. Features in the corresponding quantum mechanical results can be attributed to these new (classical) reaction mechanisms.     

The second step of the halogen/metal exchange reaction

[reaction: see text] The conversion of the iodine ate-complex 5 to the Grignard reagent 6 follows a rate law first order in 5 and in magnesium ions. In addition to this normal pathway of the halogen-metal exchange process, the conversion of 5 to 6 is catalyzed by a number of diverse "electrophiles". This points to a SET-initiated radical chain process as a second pathway for the conversion of the iodine ate-complex 5 to the Grignard reagent 6.     

A new phosphine-sulfides-to-phosphine-oxides exchange reaction

An unusual phosphine sulfide to phosphine oxide exchange reaction was found during the study of the annulation of (3,5-dimethylphenyl)-(2-hydroxy-2-methylpropyl)methylphosphine sulfide (3) using the mild cyclodehydration reagent methanesulfonic acid/phosphorus pentoxide.     

磷酰氨基酸的酯交换反应研究

本文利用^3^1P NMR , 对比研究了磷酰化组氨酸与其它磷酰化氨基酸和一级醇的磷上酯交换的反应速度,实验表明磷酰化组氨酸的反应速度最快. 由此提出了以下机理:由于咪唑环的参与,磷可以形成六配位过渡态,从而加快了反应速度.     

Rhodium-catalyzed alkylthio exchange reaction of thioester and disulfide

The Wilkinson complex RhCl(PPh₃)₃ catalyzes equilibrating alkylthio exchange reaction of thioesters with disulfides. The treatment of a thioester and a dialkyl disulfide in refluxing diethyl ketone in the presence of RhCl(PPh₃)₃ (2.5 mol %) for 1.5 h gave an alkylthio exchanged thioester. The reaction of S-methyl ester was conducted shifting the equilibrium by removing volatile dimethyl disulfide.     

Ligand exchange and reaction mechanisms offluorinated compounds

The topic of ligand exchange and reaction mechanisms of fluorinated compounds is reviewed, with emphasis on the main group fluorides. Mechanisms are divided into a series of elementary steps of bond formation and bond dissociation, using the coordination model of reaction mechanisms as an organizing principle. Included in this review is an analysis of the stercochemical behavior of pentacoordinated molecules, as well as a discussion of the role of impurities, anionie, cationie, free radical and fluorine-bridged intermediates, and fluoride-induced reactions.     

Homogeneous isotope exchange reaction between hydrogen and deuterium

The isotope exchange reaction between hydrogen and deuterium was studied in incident and reflected shock waves over the temperature range 1200–1800 K by vacuum ultraviolet absorption spectroscopy. The observed exchange reaction ensued after long induction periods at rates that proved to account for the amounts of exchange previously seen in single-pulse shock-tube and reflected-wave mass-spectrometric investigations. From the absence of detectable reaction during the induction period, lower bounds of 70 and 45 kcal were placed on the barriers to molecular exchange in four- and six-center transition structures, respectively.     

State-to-state dynamics of the hydrogen exchange reaction

Nascent product quantum state distributions have been measured for reactive, H + D₂ → HD + D, and inelastic, H + D₂ → D₂? + H, collisions of H with D₂, at collision energies of 0.98 eV, 1.1 eV, and 1.3 eV. These distributions are extracted from coherent anti-Stokes Raman scattering (CARS) spectra of HD and D₂ (recorded under single-collision conditions) following laser photodissociation of HI to generate translationally hot H atoms in a D₂/HI gas mixture. Variation of the photolysis wavelength allows selection of the H atom translational energy. These rotational and vibrational state distributions are compared with those obtained from quasiclassical trajectory calculations on an ab initio potential energy surface. The agreement between the calculated and measured distributions is extremely good. TheHD and D₂ product quantum state distributions are well represented by simple linear surprisal functions, with large positive vibrational and rotational surprisal.     

The kinetics of cation exchange reaction of europium propylenediaminetetraacetate with ytterbium ion

The kinetics of cation exchange reaction of Eu(III) propylenediaminetetraacetate with Yb(III) has been studied in an aqueous solution by the polarographic procedure. The measurements were made at 25°C and at an ionic strength of 0.5 M KCl. The dissociation of the europium complex has been found to be catalyzed by hydrogen ions. The influences of the inductive effect and the steric hindrance of a C-methyl group on the ethylene of PDTA are discussed by comparing it to analogous EDTA and CyDTA system.     

On amide exchange reaction of polymide precursor

The reaction of poly(amic acid) (PAA) derived from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) with 1-aminopyrene (APy) in solution as a model of amide exchange reaction between PAAs was studied in the temperature range of 0–60°C using viscometry and light-scattering (LS) measurements. The decrease in the weight-average molecular weight (M?_w) of PAA in N,N-dimethylacetamide (DMAc) solution with time and the acceleration of M?_w drop due to the increase in storage temperature or the addition of APy into PAA solution were observed. Apparent activation energies (E_a) for scission of PAA chains were similar: about 13 kcal/mol in PAA/DMAc and PAA/APy/DMAc, respectively. When stored at 60°C for a week, the number of scissions per polymer chain in PAA/DMAc is about 2, but is about 5 in PAA/DMAc with a large amount of APy. The result indicates that the M?_w drop accelerated by the addition of APy is attributed to amide exchange reaction between PAA Chains and monofunctional APy. It was concluded from the dependence of M?_w drop on Apy concentration that the exchange reaction between different PAA molecules during storage of PAA/PAA solution may scarcely occur under the conditions (storage time and temperature) used for preparation of PAA/PAA blends.     

Synthesis of a muscarinic receptor antagonist via a diastereoselective Michael reaction, selective deoxyfluorination and aromatic metal-halogen exchange reaction

An efficient synthesis of a structurally unique, novel M(3) antagonist 1 is described. Compound 1 is conveniently disconnected retrosynthetically at the amide bond to reveal the acid portion 2 and the amine fragment 3. The synthesis of key intermediate 2 is highlighted by a ZnCl(2)-MAEP complex 19 catalyzed diastereoselective Michael reaction of dioxolane 7 with 2-cyclopenten-1-one (5) to establish the contiguous quaternary-tertiary chiral centers and a subsequent geminal difluorination of ketone 17 using Deoxofluor in the presence of catalytic BF(3).OEt(2). The synthesis of the amine moiety 3 is highlighted by the discovery of a novel n-Bu(3)MgLi magnesium-halogen exchange reaction for selective functionalization of 2,6-dibromopyridine. This new and practical metalation protocol obviated cryogenic conditions and upon quenching with DMF gave 6-bromo-2-formylpyridine (26) in excellent yield. Further transformations afforded the amine fragment 3 via reductive amination with 35, Pd-catalyzed aromatic amination, and deprotection. Finally, the highly convergent synthesis of 1 was accomplished by coupling of the two fragments. This synthesis has been used to prepare multi-kilogram quantities of the bulk drug.     

Mechanism of isotope exchange reaction between methane and deuterium

The rate of CH₄ disappearance in a shock-heated CH₄:D₂:Ar = 9.70:9.86:80.44 mixture was monitored by coincidence absorption of the 2948 cm^?1 He-Ne laser line over the shockfront temperature range of 1900–2300K. Comparison with CH₄ pyrolysis results by means of computer simulations suggested that atom and free radical chains are responsible for the homogeneous D/H exchange reaction on CH₄. Additional simulations for the experimental conditions of previous single-pulse shock tube experiments led to the recognition of a high sensitivity of the exchange rate to trace amounts of hydrocarbon impurity and to the dissociation rate of CH₄.