A priori rate constants for kinetic modeling

The advent of computer-aided methods for constructing detailed kinetic models of multicomponent reacting systems provides fresh motivation for the development of efficient and accurate methods for estimating rate constants. There is now the real likelihood that a priori rate estimates, formerly of primarily academic interest, could directly impact major public policy and business decisions. This opportunity brings many challenges. The process of building a computer model for a real-world system can require hundreds of thousands of rate estimates, making most existing rate calculation techniques impractical. Also, the demands for tight error bars on model predictions used to make major decisions often imply levels of accuracy unachievable with existing rate calculation techniques. Past and recent progress towards developing fast and accurate rate estimates is selectively reviewed, and our methodology is outlined. New rate estimates for several types of reactions involving O and HO₂ are presented. Several technical issues requiring further work by the theoretical chemistry community are highlighted. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00214-002-0368-4. Received: 6 February 2002 / Accepted: 2 June 2002 / Published online: 2 October 2002 Acknowledgements. This work was partially supported by the National Computational Science Alliance under Grant CTS010006 N and utilized the Origin 2000 High-performance Computing and UniTree Mass Storage systems. We are grateful for financial support from the EPA Center for Airborne Organics, the NSF CAREER program, Alstom Power, Dow Chemical, and the Office of Basic Energy Science, U.S. Department of Energy through grant DE-FG02-98ER14914. The authors acknowledge the contribution of Hans-Heinrich Carstensen in the initial stages of this work. Correspondence to: W.H. Green Jr. e-mail: whgreen@mit.edu Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00214-002-0368-4     

Laser flash photolysis kinetic studies of enol ether radical cations. Rate constants for heterolysis of alpha-methoxy-beta-phosphatoxyalkyl radicals and for cyclizations of enol ether radical cations

Two series of enol ether radical cations were studied by laser flash photolysis methods. The radical cations were produced by heterolyses of the phosphate groups from the corresponding alpha-methoxy-beta-diethylphosphatoxy or beta-diphenylphosphatoxy radicals that were produced by 355 nm photolysis of N-hydroxypryidine-2-thione (PTOC) ester radical precursors. Syntheses of the radical precursors are described. Cyclizations of enol ether radical cations 1 gave distonic radical cations containing the diphenylalkyl radical, whereas cyclizations of enol ether radical cations 2 gave distonic radical cation products containing a diphenylcyclopropylcarbinyl radical moiety that rapidly ring-opened to a diphenylalkyl radical product. For 5-exo cyclizations, the heterolysis reactions were rate limiting, whereas for 6-exo and 7-exo cyclizations, the heterolyses were fast and the cyclizations were rate limiting. Rate constants were measured in acetonitrile and in acetonitrile solutions containing 2,2,2-trifluoroethanol, and several Arrhenius functions were determined. The heterolysis reactions showed a strong solvent polarity effect, whereas the cyclization reactions that gave distonic radical cation products did not. Recombination reactions or deprotonations of the radical cation within the first-formed ion pair compete with diffusive escape of the ions, and the yields of distonic radical cation products were a function of solvent polarity and increased in more polar solvent mixtures. The 5-exo cyclizations were fast enough to compete efficiently with other reactions within the ion pair (k approximately 2 x 10(9) s(-1) at 20 degrees C). The 6-exo cyclization reactions of the enol ether radical cations are 100 times faster (radical cations 1) and 10 000 times faster (radical cations 2) than cyclizations of the corresponding radicals (k approximately 4 x 10(7) s(-1) at 20 degrees C). Second-order rate constants were determined for reactions of one enol ether radical cation with water and with methanol; the rate constants at ambient temperature are 1.1 x 10(6) and 1.4 x 10(6) M(-1) s(-1), respectively.     

一个制备手性烯醇硅醚的新方法

本文合成了8个手性烯醇硅醚, 其中5个为新化合物, 其结构为IR, 1HNMR和MS所证实。     

A highly erythro selective aldol reaction between chromium-complexed benzaldehyde derivatives and cyclic silyl enol ethers

Reaction of chromium-complexed benzaldehydes having a trimethylsilyl group at the position with cyclic silyl enol ethers gave, after decomplexation, the products in a highly selective manner.     

A general computer approach for calculating rate constants from near-equilibrium kinetic studies

A general computer approach for estimating rate constants from relaxation times is described. The programme CORNEK is essentially a least-squares refinement programme applied to non-linear systems. It uses directly the differential forms of the first derivatives of mass-balance and rate equations, thus avoiding the time-consuming derivations of near-equilibrium rate equations. The programme has been tested for binary systems such as Cu-histamine, Cu-serine, and the ternary system Cu-histamine-serine.     

A silver-catalyzed spirocyclization of alkynyl silyl enol ethers

Ring out the old: The cycloisomerization of alkynyl silyl enol ethers proceeds at ambient temperature under the mild conditions of silver catalysis. Mono- or bicyclic spiro compounds can be obtained by 5-exo-dig reactions. Trapping the vinyl silver species with an iodide source, such as N-iodosuccinimide (NIS), afforded the alkenyl iodide derivatives.     

A general synthesis of substituted indoles from cyclic enol ethers and enol lactones

[reaction: see text] X = CH2, C[double bond]O, R2 = H, alkyl. A general method was developed for the one-pot synthesis of highly functionalized indoles from simple, commercially available aryl hydrazines and cyclic enol ethers. Enol lactones were also used as substrates, affording substituted indole acetic acid or indole propionic acid derivatives. This procedure affords 2,3-disubstituted indoles as single regioisomers from the appropriately substituted enol ether or enol lactone. This method was highlighted in the efficient synthesis of the antimigraine drug sumitriptan and the antiinflammatory drug indomethacin.     

Aromatic bisvinyl ethers: A new class of highly reactive thermosetting monomers

Aromatic bisvinyl ethers, prepared by the condensation of bisphenols with 2-chloroethyl vinyl ether in the presence of sodium hydroxide, are a new class of thermosetting monomers. These new materials can be polymerized cationically by using diaryliodonium salts as photo and thermal initiators to produce crosslinked resins whose physical and thermal characteristics resemble those of epoxy resins.     

Haloacetylated enol ethers. 2. Synthesis of 5-trifluoromethylpyrazoles

1-Methyl-5-(trifluoromethyl)-1H-pyrazoles 2, 3 and 4,5-dihydro-1-phenyl-5-(trifluoromethyl)-1H-pyrazol-5-ol 4 were prepared by reaction of 4-alkoxy-1,1,1-trifluoro-3-alken-2-ones 1 and hydrazine, methylhydrazine, and phenylhydrazine, respectively, in good yields. Compound 1 proved to be a versatile building block for the regiospecific construction of pyrazole rings having an 5-trifluoromethyl substituent.     

Phosphonyl radical addition to enol ethers. The stereoselective synthesis of cyclic ethers

Addition of diethyl phosphite or diethyl thiophosphite to enol ethers, in the presence of a radical initiator, results in the regioselective synthesis of organophosphonate or phosphonothioate derivatives, respectively, under mild conditions. This method can be applied to the stereoselective formation of substituted tetrahydrofurans and tetrahydropyrans, on cyclisation of vinyl ethers bearing unsaturated side chains.     

Application of carbene reactive intermediates in organic synthesis: III. Reaction of difluorocarbene with trimethylsilyl enol ethers

The addition of difluorocarbene generated from phenyl(trifluoromethyl)mercury and anhydrous sodium iodide, with trimethylsilyl enol ethers derived from cyclohexanone, cycloheptanone, 3-pentanone and acetophenone was described. The corresponding 2, 2-difluoro-1-trimethylsilyloxy-cyclopropanes were obtained in good yields. The thermal stability of these cyclopropyl derivatives was affected by the molecular strain and the nature of their substituents. Thus, 1-trimethylsilyloxy-cyclopentene reacted with difluorocarbene to give only 2-fluoro-2-cyclohexen-1-one instead of the expected 1-trimethylsilyloxy-6, 6-difluorobicyclo [3.1.0]hexane, and a mechanism for its formation was proposed.     

A semiclassical reactive flux method for the calculation of condensed phase activated rate constants

A semi classical reactive flux algorithm for calculating thermally activated rate constants is presented which is based on a semi-classical transition state theory due to Chapman, Garrett and Miller [J. Chem. Phys. 63 (1975) 2710]. This reactive flux technique, when combined with the semiclassical TST, enables one to describe dynamical recrossings of the transition state on the same footing as tunneling effects. Most importantly, the method is readily applied to nonlinear multidimensional systems over a wide range of temperatures. It will be shown that the method works very well for a variety of existing models.     

Silyl enol ethers as monomer. I. Radical copolymerizability of α-trimethylsilyloxystyrene

α-Trimethylsilyloxystyrene (TMSST), the silyl enol ether of acetophenone, was not homopolymerized either by a radical or a cationic initiator. Radical copolymerization of TMSST with styrene (ST) and acrylonitrile (AN) in bulk and the terpolymerization of TMSST, ST, and maleic anhydride (MA) in dioxane were studied at 60°C and the polymerization parameters of TMSST were estimated. The rate of copolymerization decreased with increased amounts of TMSST for both systems. Monomer reactivity ratios were found as follows: r₁ = 1.48 and r₂ = 0 for the ST (M₁)–TMSST (M₂) system and r₁ = 0.050 and r₂ = 0 for the AN (M₁)–TMSST (M₂) system. The terpolymerization of ST (M₁), TMSST (M₂), and MA (M₃) gave a terpolymer containing ca. 50 mol % of MA units with a varying ratio of TMSST to ST units and the ratio of rate constants of propagation, k₃₂/k₃₁, was found to be 0.39. Q and e values of TMSST were determined using the values shown above to be 0.88 and ?1.13, respectively. Attempted desilylation by an acid catalyst for the copolymer of TMSST with ST afforded polystyrene partially substituted with hydroxyl groups at the α-position.     

A highly efficient synthesis of 1,2,3,4-tetrahydroquinolines by molecular iodine-catalyzed domino reaction of anilines with cyclic enol ethers

A highly efficient method for the synthesis of 1,2,3,4-tetrahydroquinoline derivatives via a molecular iodine catalyzed domino reaction of anilines with cyclic enol ethers, such as 2,3-dihydrofuran and 3,4-dihydro-2H-pyran, is described. The reaction may proceed through an aza-Diels-Alder process between an in situ generated 2-azadiene and another equivalent of cyclic enol.     

Substituent effects on the rate constants for the photo-Claisen rearrangement of allyl aryl ethers

The photochemistry of 11 substituted allyl 4-X- and 3-X-aryl ethers 3 (ArOCH2-CH=CH2) has been examined in both methanol and cyclohexane as solvents. The ethers react by the photo-Claisen rearrangement to give allyl substituted phenols as the major primary photoproducts, as expected from the well-established radical pair mechanism. The excited singlet state properties (absorption spectra, fluorescence spectra, fluorescence quantum yields, and singlet lifetimes) were compared with a parallel set of unreactive 4-X- and 3-X-anisoles 4. The excited-state properties of three substituted 4-X-aryl 4-(1-butenyl) ethers 14 (ArOCH2CH2-CH=CH2) were also examined. The model compounds 4 and the reactive allyl ethers 3 have essentially identical rate constants for the excited-state processes with the exception of, the rate constant for homolytic cleavage from S(1) of the allyl ethers to give the radical pair. The difference between the fluorescence quantum yields and/or singlet lifetimes for 3 and 4 were used to obtain values of for all of the allyl ethers. These values exhibit a large substituent effect, spanning almost 2 orders of magnitude with electron-donating groups (CH3O, CH3) accelerating the reaction and electron-withdrawing ones (CN, CF3) slowing it down. The parallel range of rate constants observed in both methanol and cyclohexane indicates that ion pairs are not important intermediates in these rearrangements. Quantum yields of reaction (Phi(r)) for several of the more reactive ethers demonstrate that neither these values nor rate constants of reaction derived from them are reliable measures of the actual excited-state process. In fact, the values are significantly lower than the ones, indicating that the radical pairs undergo recombination to generate starting material. Finally, the rate constants were found to parallel a trend for the change in bond dissociation energy (deltaBDE) for the O-C (allyl) bond of the allyl ethers, indicating that other possible substituent effects are of minor importance.     

Preferred conformations of enol ethers in solution: Detection of an equilibrium by kinetic nuclear overhauser effects

n.O.e. difference spectra reveal that the preferred solution conformation of methyl enol ethers has the methyl group $\text{syn}$-periplanar to the double bond; n.O.e kinetics in a methoxy—heptatriene demonstrate the presence of both possible periplanar conformations, the energy difference being $\text{ca}$ 1 kcal/mole.     

A novel aminomethylation of silyl enol ethers with aminomethyl ethers catalyzed by iodotrimethylsilane or trimethylsilyl trifluoromethanesulfonate

The iodotrimethylsilane-catalyzed reaction of silyl enol ethers with aminomethyl ethers in acetonitrile gives aminomethylation products of the corresponding ketones readily. The reaction can slso be catalyzed by trimethylsilyl trifluoromethanesulfonate in dichloromethane.     

Determination of rate constants for quenching singlet oxygen by chemiluminescence technique

Overall rate constants have been found for quenching¹O₂ generated under thermal decomposition of hydrotrioxides ROOOH, by several unsaturated species at 0°C. CH₃CH(OH)OOOHm (CH₃)₂C(OH)OOOH, CH₃C(OC₂H₅)₂OOOH and CH₃C(O) CH(OOOH) (CH₂)₂CH₃ were used as ROOOH; unsaturated compounds were 1,3-diphenylisobenzofurane, 2-methylfurane, 2,3,4,5-tetraphenylfurane, furfuroldiacetate, tetramethylethylene, adamantylideneadamantane, exo,-exo-2,3-dioxymethyl-7-adamantylidenenorbornane, exo,-exo-2,3-(2¹-oxatrimethylene)-7-adamantylidenenorbornane, 1,3-cyclohexadiene, (E,E)-8, 10-dodecadienol, cyclooctatetraene and isoprene.

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¹O₂, ROOOH O°C. ROOOH CH₃CH(OH)OOOH, (CH₃)₂C(OH)OOOH, CH₃C(OC₂H₅)₂OOOH, CH₃C(O)CH(OOOH) (CH₂)₂CH₃; -1,3-, 2-, 2,3,4,5-, , , . , -2,3-(2¹-)-7-, 1,3-, (,)-8,10-, , .

     

Vibrational energy exchanges in nitrogen: application of new rate constants for kinetic modeling

The state-to-state collisional data on vibration-vibration and vibration-translation/rotation energy exchanged in N2(v)-N2(v') collisions recently obtained from accurate ab initio semiclassical calculations have been used to analyze the data measured in nitrogen under two different plasma conditions. In particular, the vibrational distribution function and the time-evolution of the gas temperature measured under post-discharge and glow discharge conditions, respectively, have been calculated and compared with the experimental observations. The theoretical analysis and the related results, generally in very good agreement with the experimental data, provide insight into the various energy-exchange mechanisms that lie behind the macroscopic behaviors of the nitrogen plasmas. In particular, the role played by the vibrationally excited nitrogen molecules in the gas kinetics is pointed out, as well as the importance of nitrogen atom production in the long time scales of the glow discharge.