Stereospecific [2+2] cycloaddition reactions of diphosphorus with alkenes

[2+2] Cycloaddition reactions of P₂ with alkenes were predicted to have concerted paths, that is, pseudoexcitation, distorted 2π_s+2π_s, and 2π_s+2π_a processes without any interventions of intermediates. The pseudoexcitation and/or distorted 2π_s+2π_s paths with retention of configuration of alkenes are kinetically preferred to the 2π_s+2π_a path with inversion of configuration. The reactions were predicted from the appreciable difference in the calculated enthalpies of activation to be stereospecific.     

Nonreactivity of rydberg states woodward-hoffmann forbidden [2s+2s] photochemical cycloadditions

A theoretical analysis is presented for the [2_s + 2_s] cyclodimerisation of molecules in which the lowest unoccupied MO is of Rydberg nature. The concerted mechanism is forbidden by the Woodward-Hoffmann rules for the thermal and the photochemical path. However, the frontier MO approach predicts an enhancement of these reactions. Experiments related to this analysis are discussed.     

Thermal addition of xathenethione to allenes: “A Concerted Two Step” (π2s+π2s+π2s)-cycloaddition

(2+2)- and (4+2)-cycloadducts are formed by thermal reaction of xanthenethione with allenes. Activation parameters for ths “concerted two step” (_π2_s+_π2_s+_π2_s) reaction are presented.     

Transformations thermiques des photooxydes meso des acenes—III: Dimeres de l'o-quinodimethane issu du photooxyde d'anthracene

Thermolysis of the anthracene photooxide 1a, in solution, can afford successively three dimers (I→II→III) the structures of which have been established by NMR and X-ray analysis for the first one. Their origin is the unstable o-quinodimethane diether 3a, coming from 1a by isomenzation, which at 80° dimerizes to 8a (dimer I) by an unusual [_π8_s+_π6_s] concerted cycloaddition. Above 110°, 8a isomerizes into 9a (dimer II) by a concerted [1,5] suprafacial sigmatropic migration. Finally, at higher temperature (~200°), both dimers are partly converted into the symetrical dimer 10a (dimer III), probably by a radical pathway. Thermolysis runned in presence of N-methylmaleimide show the non-reversibility of these reactions, giving two adducts, 15a and 16a, derived from 9a, the structures of which are deduced from NMR data. An explanation of the nature and high selectivity of the proceeding processes is given by considering orbital factors.     

Water exchange rates and mechanisms in tetrahedral [Be(H2O)4]2+ and [Li(H2O)4]+ complexes using DFT methods and cluster‐continuum models

The water exchange reactions in aquated Li⁺ and Be²⁺ ions were investigated with density functional theory calculations performed using the [Li(H₂O)₄]⁺·14H₂O and [Be(H₂O)₄]²⁺·8H₂O systems and a cluster‐continuum approach. A range of commonly used functionals predict water exchange rates several orders of magnitude lower than the experimental ones. This effect is attributed to the overstabilization of coordination number four by these functionals with respect to the five‐coordinated transition states responsible for the associative ( A ) or associative interchange ( I_a ) water exchange mechanisms. However, the M06 and M062X functionals provide results in good agreement with the experimental data: M062X/TZVP calculations yield a concerted I_a mechanism for the water exchange in [Be(H₂O)₄]²⁺·8H₂O that gives an average residence time of water molecules in the first coordination sphere of 260 μs. For [Li(H₂O)₄]⁺·14H₂O the water exchange reaction is predicted to follow an A mechanism with a residence time of inner‐sphere water molecules of 25 ps.     

Reaction HFCO + 2H2O — Calculated Mechanisms

Three possible reaction mechanisms of methanoyl fluoride with 2H₂O include a concerted and a stepwise hydrolysis of HFCO into HCOOH + HF, and a pure catalytic decomposition of HFCO into HF + CO. Among these, the two H₂O molecules acting as catalyst to decompose HFCO has the lowest calculated barrier, 25.1 kcal/mol with respect to the reactant‐adduct complex, whereas the barriers for the concerted and stepwise hydrolytic reactions in which one H₂O acts as a reactant and the other H₂O as catalyst are similar, 30.8 kcal/mol for concerted and 29.9 kcal/mol for stepwise. The formation of transoid HCOOH in the hydrolysis of HFCO is more favorable than cisoid HCOOH.     

Electron-transfer and chemical reactivity following collisions of Ar with C2H2

The reaction between Ar²⁺ and C₂H₂ has been studied, at centre-of-mass collision energies ranging from 3 to 7 eV, using a position-sensitive coincidence technique to detect the monocation pairs, which are formed. Sixteen different reaction channels generating pairs of monocations have been observed, these channels arise from double-electron-transfer, single-electron-transfer and chemical reactions forming ArC⁺. Examination of the scattering diagrams and energetic information extracted from the coincidence data indicate that double-electron-transfer is a direct process, which does not involve a collision complex, and the derived energetics point towards a concerted, not stepwise, mechanism for the two-electron-transfer. As is commonly observed, single-electron-transfer from C₂H₂ to Ar²⁺ takes place via a direct mechanism, again not involving complexation. Most of the C₂H₂⁺ products that are formed in the single-electron-transfer reactions possess significant (12–15 eV) internal energy and fragment rapidly within the electric field of the partner Ar⁺ ion. The chemical reactions appear to proceed via a direct mechanism involving the initial formation of ArCH⁺, which subsequently fragments to form ArC⁺.     

Nucleophilic substitution reactions of diethyl 4‐nitrophenyl phosphate triester: Kinetics and mechanism

The reactions of diethyl 4‐nitrophenyl phosphate ( 1 ) with a series of nucleophiles: phenoxides, secondary alicyclic (SA) amines, and pyridines are subjected to a kinetic study. Under excess of nucleophile, all the reactions obey pseudo‐first‐order kinetics and are first order in the nucleophile. The nucleophilic rate constants (k_N) obtained are pH independent for all the reactions studied. The Brønsted‐type plot (log k_N vs. pK_a nucleophile) obtained for the phenolysis is linear with slope β=0.21; no break was found at pK_a 7.5, consistent with a concerted mechanism. The Brønsted‐type plots for the SA aminolysis and pyridinolysis are linear with slopes β=0.39 and 0.43, respectively, also suggesting concerted processes. The concerted mechanisms for the latter reactions are proposed on the basis of the lack of break in the Brønsted‐type plots and the instability of the hypothetical pentacoordinate intermediates formed in these reactions. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 708–714, 2011     

A density functional theory study of the stereoselectivity of Cu(OTf)2-catalyzed [3+2] cycloaddition of trifluoromethylated N-acylhydrazones and isoprene: A concerted asynchronous mechanism

Here we employ density functional theory calculations to systematically investigate the underlying mechanism of Cu(OTf)₂-catalyzed [3+2] cycloaddition reactions in the synthesis of CF₃-substituted pyrazolidines. About eight possible initial configurations of the [3+2] reaction is considered, and all relevant reactants, transition states, and products are optimized. Based on these structures, internal reaction coordinate paths, and wavefunction analysis results, we conclude that the Cu(OTf)₂-catalyzed [3+2] cycloaddition follows a concerted asynchronous mechanism. The C N bond forms immediately after the formation of the C C bond. Among the eight reaction paths, the energy barrier for the [3+2] reaction that leads to the CF₃-substituted syn-pyrazolidine is the lowest, ∼5.4 kcal/mol, which might result in the diastereoselectivity that is observed in the experiment. This work not only gives the detailed mechanism of the Cu(OTf)₂-catalyzed [3+2] cycloaddition but can also be helpful for the future designation of Cu(OTf)₂-based cycloaddition processes.     

Validation of a thermal decomposition mechanism of formaldehyde by detection of CH2O and HCO behind shock waves

The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH₂O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH₂O + M → HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k_1a = 5.0 × 10¹⁵ exp(‐308 kJ mol^?1/RT) cm³ mol^?1 s^?1. The pressure dependence, the rate of the competing molecular channel (1b), CH₂O + M → H₂ + CO + M, and the branching fraction β = k_1a/(kA_1a + k_1b) was characterized by a two‐channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1–50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH₂O, CO, and H atom measurements at temperatures of 1200–3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self‐reaction of two CH₂O molecules. At high initial CH₂O mole fractions the reverse of reaction (6), CH₂OH + HCO ? CH₂O + CH₂O becomes noticeable. The rate of the forward reaction was roughly measured to be k₆ = 1.5 × 10¹³ cm³ mol^?1 s^?1. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 157–169 2004     

Reactions of the HO2 radical with OH,H, Fe2+ and Cu2+ at elevated temperatures

The temperature dependence of the rate constant for the reactions of HO₂ with OH, H, Fe²⁺ and Cu²⁺ has been determined using pulse radiolysis technique. The following rate constants, k (dm³ mol⁻¹ s⁻¹) at 20°C and activation energies, E_a (kJ mol⁻¹) have been found. The reaction with OH was studied in the temperature range 20–296°C (k=7.0×10⁹, E_a=7.4) and the reaction with H in the temperature range 5–149°C (k=8.5×10⁹, E_a=17.5). The reaction with Fe²⁺ was studied in the temperature range 16–118°C (k=7.9×10⁵, E_a=36.8) and the reaction with Cu²⁺ in the temperature range 17–211°C (k=1.1×10⁸, E_a=14.9).     

Preparation and crystallographic investigation of the spinel series: Mn1 + 2sCr2 − 3sSbsO4 (0.05 ⩽ , s ⩽ 0.30)

Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄, a new series of spinels, have been prepared and studied using X-ray powder data. For s going from 0.05 to 0.30, a gradually increases from 8.441(1) to 8.472(1) Å, and u slightly decreases, from 0.262 to 0.258. Interatomic distances are given. The Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄ (0.05 < s < 0.30) series may be conceived as the result of partial substitution of Cr^{3 +} by 2/3Mn^{2 +} + 1/3Sb^{5 +} in the normal spinel, MnCr₂O ₄.     

Zum Chemischen Transport von Cr2O3 mit Cl2 und mit HgCl2 ? Experimente und Modellrechungen

On the Chemical Transport of Cr₂O₃ with Cl₂ and with HgCl₂ — Experiments and Model Calculations The migration of Cr₂O₃ in a temperature gradient (1 000°C → 900°C) in the presence of low concentrations of chlorine and water (from the wall of silica ampoules) is a result from the endothermic reactions (1) Cr₂O_3,s + H₂O_g + 3 Cl_2,g = 2 CrO₂Cl_2,g + 2 HCl_g (2) Cr₂O_3,s + 1/2 O_2,g + 2 Cl_2,g = 2 CrO₂Cl_2,g With higher concentrations of chlorine, the transport reaction is (3) Cr₂O_3,s + 5/2 Cl_2,g = 3/2 CrO₂Cl_2,g + 1/2 CrCl_4,g The gas phase of the transport system Cr₂O₃/Cl₂ can be reduced step by step by adding small amounts of chromium, so that CrCl₃ and finally also CrCl₂ become more important. Further, at a lower ratio n°(Cl)/n°(Cr) three transport reactions have to be taken into consideration; with the participation of CrOCl_2,g (5). (4) Cr₂O_3,s + 9/2 CrCl_4,g = 3/2 CrO₂Cl_2,g + 5 CrCl_3,g (5) Cr₂O_3,s + 3 CrCl_4,g = 3 CrOCl_2,g + 2 CrCl_3,g (6) Cr₂O_3,s + H_2,g + 4 HCl_g = 2 CrCl_2,g + 3 H₂O_g The reactions (1), (2) and (6) become possible through the cooperation of two transport agents at a time. The migration of Cr₂O₃ with HgCl₂ can also be described with reactions (1) – (3). The decomposition of HgCl₂ Produces the small chlorine pressure for the transport reaction. The oxidation potential of the transport agent HgCl₂ is too low for the oxidation of Cr^III to Cr^VI.     

Revisiting the concept of the (a)synchronicity of diels‐alder reactions based on the dynamics of quasiclassical trajectories

A number of model Diels‐Alder (D‐A) cycloaddition reactions (H₂C?CH₂ + cyclopentadiene and H₂C?CHX + 1,3‐butadiene, with X = H, F, CH₃, OH, CN, NH₂, and NO) were studied by static (transition state ‐ TS and IRC) and dynamics (quasiclassical trajectories) approaches to establish the (a)synchronous character of the concerted mechanism. The use of static criteria, such as the asymmetry of the TS geometry, for classifying and quantifying the (a)synchronicity of the concerted D‐A reaction mechanism is shown to be severely limited and to provide contradictory results and conclusions when compared to the dynamics approach. The time elapsed between the events is shown to be a more reliable and unbiased criterion and all the studied D‐A reactions, except for the case of H₂C?CHNO, are classified as synchronous, despite the gradual and quite distinct degrees of (a)symmetry of the TS structures. © 2015 Wiley Periodicals, Inc.     

Evidence against the (π2s + π2a) mechanism for the cycloadditions of allenes

The chemoselectivities in the (_π4_s + _π2_s) cycloaddition reactions of tetraphenylcyclopentadienone (TPCD) with ethyl- and 1,1-dimethylallene have been determined and are compared with those observed for reaction with maleic anhydride.     

[2 + 2] Cycloadditions. A concerted pathway for acetylene plus ethylene

The [2_s + 2_a] cycloaddition of ethylene and acetylene has been studied. A transition structure of C₂ symmetry was located on the potential surface. Activation energies for the process are also reported.     

The kinetics of the B2H6/O (3P) system at room temperature

An experimental study of the kinetics of the B₂H₆/O (³P) system at room temperature, is presented. Modeling was based on a multiple-parameter fitting process to a complex kinetic mechanism. The aim of the study was to propose and evaluate a preferred set of elementary reactions which might be important for the oxidation of boron-hydrides. In this study, relative concentration-vs.-time profiles of the radicals OH and BO₂ were measured in a low pressure flow reactor, by the technique of laser-induced-fluorescence. A range of almost two orders of magnitude in the initial fuel to oxygen ratio was covered, while the residence times of the gases in the reactor were up to 1 s. A comprehensive kinetic mechanism was constructed from the available measured and estimated data in literature. After the fitting process and dropping all of the reactions with negligible contribution, a 46 elementary reactions mechanism was obtained. In this mechanism, 27 reactions are not measured and their rate coefficients were used as the adjustable (within reasonable limits deduced from kinetic and thermodynamic considerations) parameters of the fitting process. Good agreement was obtained between all of the measured and calculated profiles. From sensitivity analysis it was found that only a limited group of these reactions highly contributes to the calculated concentrations of OH and BO₂. With only the highly contributing reactions and all the oxygenhydrogen reactions which are measured and relatively well known, a 30 elementary reactions mechanism was obtained. In this mechanism only 13 reactions are not measured and the agreement between the measured and the calculated profiles is still reasonable. To demonstrate the possible usefulness of the proposed mechanism, the rate coefficient of the reaction B₂H₆ + OH → B₂H₅ + H₂O which was not measured before, was directly measured in our experimental set-up. The rate coefficient that was obtained in the direct measurement is (3.3 ± 1.1) × 10¹¹ cm³mol^?1s^?1, in excellent agreement with the predicted one by the multiple-parameter-fitting process, which was 2 × 10¹¹ cm³mol^?1s^?1. © 1995 John Wiley & Sons, Inc.     

Liquid phase kinetic study of the formation and decomposition of methylsulfonyl and cyclohexylsulfonyl radicals in the cyclohexane–MeSO2C1-SO2 System at 150°

The thermally and radiolytically induced chain decomposition of methanesulfonyl chloride (MeSO₂Cl) in liquid cyclohexane (RH) was studied at 150°C. The main products, chlorocyclohexane, sulfur dioxide, and methane, are formed in almost equal yields, and a relatively small amount of methyl chloride is also observed. The formation and addition of SO₂ strongly inhibit the chain decomposition reaction. By kinetic analysis it is shown that the formation of the main products can be explained only in terms of a mechanism that postulates the decomposition of MeSO₂, and that the alternative mechanism of methane and SO₂ formation via the methanesulfinic acid is inconsistent with the kinetic behavior of the system. For the reactions Me + MeSO₂Cl → MeCl + MeSO₂(2b), Me + RH → MeH + R (4), and Me + CCl₄ → MeCCl₄ → MeCl + CCl₃ (15b) the following rate constant ratios are determined; k_2b/k₄=2.17±0.20 and k_2b/k_15b=2.63±0.52. For the reactions R + MeSO₂Cl → RCl + MeSO₂(2a) and R + CCl₄ → RCl + CCl₃ (15a), k_2a/k_15a is equal to 1.55±0.05. In addition the equilibrium constant K₇ for the reaction R + SO₂ ? RSO₂ (7) is estimated as being equal to (9.4±3) × 10³ mole/l.     

A study of the self reaction of CH2ClO2 and CHCl2O2 radicals at 298 K

A low‐pressure discharge‐flow system equipped with laser‐induced fluorescence (LIF) detection of NO₂ and resonance‐fluorescence detection of OH has been employed to study the self reactions CH₂ClO₂ + CH₂ClO₂ → products (1) and CHCl₂O₂ + CHCl₂O₂ → products (2), at T = 298 K and P = 1–3 Torr. Possible secondary reactions involving alkoxy radicals are identified. We report the phenomenological rate constants (k_obs) k_1obs = (4.1 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ k_2obs = (8.6 ± 0.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and the rate constants derived from modelling the decay profiles for both peroxy radical systems, which takes into account the proposed secondary chemistry involving alkoxy radicals k₁ = (3.3 ± 0.7) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ k₂ = (7.0 ± 1.8) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ A possible mechanism for these self reactions is proposed and QRRK calculations are performed for reactions (1), (2) and the self‐reaction of CH₃O₂, CH₃O₂ + CH₃O₂ → products (3). These calculations, although only semiquantitative, go some way to explaining why both k₁ and k₂ are a factor of ten larger than k₃ and why, as suggested by the products of reaction (1) and (2), it seems that the favored reaction pathway is different from that followed by reaction (3). The atmospheric fate of the chlorinated peroxy species, and hence the impact of their precursors (CH₃Cl and CH₂Cl₂), in the troposphere are briefly discussed. HC(O)Cl is identified as a potentially important reservoir species produced from the photooxidation of these precursors. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 433–444, 1999     

Kinetics and mechanism of autocatalyzed reaction between Phenyl Hydrazine and Toluidine blue in aqueous solution

The kinetics and mechanism of reduction of aqueous toluidine blue (TB⁺) by phenyl hydrazine (Pz), which exhibits nonlinear behavior, is studied spectrophotometrically at 630 nm. Typical kinetic curves exhibited autocatalytic characteristics. The role of H⁺ as an autocatalyst is established. Rate constants for the uncatalyzed and acid catalyzed reactions are determined. The forward rate constants for the uncatalyzed and acid catalyzed reactions were 1.4 × 10⁻² M⁻¹ s⁻¹ and 60 M⁻¹ s⁻¹. Reaction products are toluidine white, phenol, and an azo dye. From the stoichiometric ratios, the major reaction is Pz + 2 TB⁺ + H₂O = PhOH + 2 TBH + 2 H⁺ + N₂. The rate expression and a detailed 12‐step reaction mechanism supported by simulations are proposed. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 83–88, 1999