Four-membered Transition State Research Articles

Experimental and theoretical study of the reaction of POCl3− with O2

The oxidation of the trichlorooxyphosphorus anion (POCl(3) (-)), which takes place in combustion flames, has been examined experimentally at a variety of temperatures and theoretically via ab initio and density functional methods. The reaction was examined in a turbulent ion flow tube and kinetics was measured between 300 and 626 K, estimating an overall reaction barrier of 1.23 kcal/mol. Calculations at the density functional, Moller-Plesset second order perturbation, and coupled cluster levels of theory with basis sets up to augmented triple-zeta quality point to a multistep reaction mechanism involving an initial [OP(Cl)(3)(OO)](-) intermediate, an adduct between triplet O(2) with POCl(3) (-), subsequent formation of a four-membered nonplanar P-O-O-Cl ring transition state, with concomitant breaking of the P-Cl and O-O bonds to provide a transient intermediate [OP(Cl)(2)OO...Cl](-), which, in turn, converts to the product complex (POCl(2) (-))(ClO) upon formation of the Cl-O bond without barrier. The calculated energy of the four-membered transition state is considered to be in good agreement with the small overall barrier found by experiment. The final step is responsible for the large exothermicity of the reaction.

Evidence for a Rapid Degenerate Hetero‐Cope‐Type Rearrangement in [Cp*W(S)2S‐CH2‐CHCH2

Treatment of the salt [PPh(4)]+[Cp*W(S)3]- (6) with allyl bromide gave the neutral complex [Cp*W(S)2S-CH2-CH=CH2] (7). The product 7 was characterized by an X-ray crystal structure analysis. Complex 7 features dynamic NMR spectra that indicate a rapid allyl automerization process. From the analysis of the temperature-dependent NMR spectra a Gibbs activation energy of DeltaG(not equal) (278 K) approximately 13.7+/-0.1 kcal mol(-1) was obtained [DeltaH(not equal) approximately 10.4+/-0.1 kcal mol(-1); DeltaS(not equal) approximately -11.4 cal mol(-1) K(-1)]. The DFT calculation identified an energetically unfavorable four-membered transition state of the "forbidden" reaction and a favorable six-membered transition state of the "Cope-type" allyl rearrangement process at this transition-metal complex core.

Transition State Energy Decomposition Study of Acetate-Assisted and Internal Electrophilic Substitution C−H Bond Activation by (acac-O,O)2Ir(X) Complexes (X = CH3COO, OH)

Chelate-assisted and internal electrophilic substitution type transition states were studied using a DFT-based energy decomposition method. Interaction energies for benzene and methane C−H bond activation by (acac-O,O)_2Ir(X) complexes (X = CH_3COO and OH) were evaluated using the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). A ratio of ~1.5:1 for forward to reverse charge-transfer between (acac-O,O)_2Ir(X) and benzene or methane transition state fragments confirms “ambiphilic” bonding, the result of an interplay between the electrophilic iridium center and the internal base component. This analysis also revealed that polarization effects account for a significant amount of transition state stabilization. The energy penalty to deform reactants into their transition state geometry, distortion energy, was also used to understand the large activation energy difference between six-membered and four-membered acetate-assisted transition states and help explain why these complexes do not activate the methane C−H bond.

Stereoelectronic Effects on Type I 1,2‐Dyotropic Rearrangements in Vicinal Dibromides

The effects of different substituents on type I-dyotropic rearrangements of open-chain and cyclic 1,2-dibromo hydrocarbons have been studied by means of DFT calculations. The activation energy (E(a)) of this transformation decreases with the pi-donor ability of the substituent attached to the reacting ethylenic system. This is due to donation of electronic density by conjugation or hyperconjugation. This donation leads to longer C--C and C--Br bond lengths in the corresponding four-membered transition states (TSs). Linear relationships between the E(a) and either the sigma(p) Hammett substituent constants and the C--C bond length of the TSs were also found. In all cases, the processes have a high value of synchronicity, which is mainly independent on the substituents. A model based on the second-order perturbational analysis for one ethylene unit with two apical bromine radicals accounts for all the computed results.

Electrophilic C−H Activation at {Cp*Ir}: Ancillary-Ligand Control of the Mechanism of C−H Activation

Density functional calculations on the low-temperature cyclometalation of dimethylbenzylamine with [IrCl2Cp*]2/NaOAc have characterized a novel electrophilic activation pathway for C-H bond activation. C-H activation occurs from [Ir(DMBA-H)(kappa2-OAc)Cp*]+, and OAc plays a central role in determining the barrier for reaction. Dissociation of the proximal OAc arm sets up a facile intramolecular deprotonation via a geometrically convenient six-membered transition state. Dissociation of the distal OAc arm, however, leads to a higher energy four-membered (sigma-bond metathesis) transition state, while oxidative addition is even higher in energy. For this Ir3+ system, these three mechanisms appear to lie within a continuum in which the participation of the metal center and an H-accepting ancillary ligand are inversely related. The ability of the ancillary ligand to act as a proton acceptor is the key factor in determining which mechanism pertains.

Ab Initio and NBO Studies of the Decomposition ([2+2] and [2+4]Elimination) Mechanisms of Alkylisothiocyanates (alkyl = ethyl-, iso-propyl- and tert-butyl)

Ab initio Molecular Orbital (MO), Density Functional Theory (DFT), and Natural Bond Orbital (NBO) population analysis were used to investigate the mechanism of the decomposition of ethylisothiocyanate (1), iso-propylisothiocyanate (2), and tert-buthylisothicyanate (3). HF/6-31G**//HF/6-31G** and B3LYP/6-31G**//HF/6-31G** results indicate that for the decomposition of compounds 1– 3, the six-membered transition state structures via concerted [2+4]elimination mechanisms have lower energies than the four-membered transition state geometries via [2+2]eliminations. The results at B3LYP/6-31G**//HF/6-31G** and HF/6-31G**//HF/6-31G** levels of theory revealed that the barrier height for the decomposition of compound 1, through the six-membered transition state structure via the concerted [2+4] elimination mechanism, is 46.31 kcal mol −1 and 59.51 kcal mol −1 , while the barrier height for the four-membered transition state structure via [2+2] elimination is 71.06 kcal mol −1 and 72.09 kcal mol −1 , respectively. [2+4]elimination mechanism by B3LYP/6-31G**//HF/6-31G** method (46.31 kcal mol−1) is in a good agreement with the reported experimental results (45.35 kcal mol−1). The obtained barrier heights via the [2+4]elimination mechanism at the B3LYP/6-31G**//HF/6-31G** and HF/6-31G**//HF/6-31G** levels of theory show also that the decomposition energy barriers via the [2+4]elimination mechanism for compound 1–3 are in the following order: 1 > 2 > 3. The obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the C sp3 atom (which is attached to the N atom). NBO analysis shows that the occupancies of σ Csp3−N bonds decrease in the following order: 3 < 2 < 1 and the occupancies of σ * Csp3−N bonds increase in the opposite order. This fact illustrates a comparatively easier unsymmetrical dissociation of the σ Csp3−N bond in compound 3 compared to compound 2, and in compound 2 compared to compound 1.

Quantum investigation on the mechanism of isomerization of 1-butylene catalyzed by Rh-complex

The mechanism of 1-butylene isomerization is studied by means of the density functional theory (DFT). As shown, the isomerization of 1-butylene goes mainly through coordination of 1-butylenen to the catalyst, insertion of the C1–C2 bond into the Rh-H bond, elimination of β-hydrogen, and dissociation of isomerized products. The C1–C2 bond insertion and the β-hydrogen elimination are accomplished through four-membered transition states consisted of H, Rh, and two C atoms. The rate-determining step for the catalytic cycle is the elimination of β-hydrogen. Trans-2-butylene is the preferred product and the selectivity for trans-2-butylene is kinetically and thermodynamically controlled.

Energy Profiles for Ketene Cyclizations. Interconversion of 1,3-Oxazin-6-ones, Mesoionic 1,3-Oxazinium Olates and Acylketenes, Imidoylketenes, Oxoketenimines, and Cyclization Products

The energy surface connecting oxazinium olates 9, several possible conformers of ketenes 10 and 11, and the final cyclization products 12, 13 and 14, as well as the isomeric 1,3-oxazine-6-ones 15, ring opening of the latter to N-acylimidoylketenes 16, and subsequent rearrangement of 16 to oxoketenimines 17, azetinones 18, and the cyclization products 19 and 20 are evaluated computationally at the B3LYP/6-31G and B3LYP/6-311+G//B3LYP/6-31G levels. The cyclizations of ketenes to oxazinium olates 9 and oxazines 15 have the characteristics of pseudopericyclic reactions. Plots of the energy vs internal reaction coordinate for the cyclization of transoid acylketenes such as 10 to 9 (via TS1) and 16 to 15 (via TS7) feature two inflection points and indicate that the part of the energy surface above the lower inflection points describe internal rotation of the acyl function in the ketene moiety, and the part below this point describes the cyclization of the cisoid ketene to the planar mesoionic oxazinium olate 9 or oxazinone 15. The 1,3-shifts of the OR group that interconvert ketenes 16 and ketenimines 17 via four-membered cyclic transition states TS8 behave similarly, the first portion (from the ketenimine side) of the activation barrier being due largely to internal rotation of substituents, and the top part being due to the 1,3-shift proper.

Ring Cleavage of Aziridines by Difluoroamine: Mechanistic Insights from ab Initio and DFT Study

cis-2,3-Dimethylaziridine reacts with difluoroamine to give the corresponding alkene and nitrogen with retention of configuration. We have carried out a DFT study of this reaction to clarify the reaction mechanism by considering a multistep reaction pathway with possible intermediacy of several three- and four-membered cyclic intermediates and transition states (TSs). The energetics of this reaction shows that the reaction takes place in four steps including a three-membered azamine intermediate. Both the energetics and the stereochemical outcome of this reaction rule out the formation of a four-membered diazetine intermediate during the reaction. Although the first N-N bond formation step is rate determining, the final step, asynchronous concerted cleavage of the azamine intermediate, explains the stereochemistry of this reaction. The asynchronous nature of the final step makes the reaction Woodward-Hoffmann allowed, as reported by Yamabe and Minato (J. Phys. Chem. A 2001, 105, 7281). Computations at HF and MP2 levels confirm the same trends in energetics. Single point energy computations at B3LYP, MP2, and QCISD levels with the 6-311++G(d,p) and cc-pVTZ basis sets show that the larger basis sets predict higher free energies of activation and less negative free energies of reaction. Intrinsic reaction coordinate (IRC) analyses reveal the asynchronous nature of the first and the last steps of the reaction. The deamination of trans-2,3-dimethylaziridine was shown to follow a course of reaction similar to that of the cis isomer.

Theoretical study on the ring-opening hydrolysis reaction of cAMP

The ring-opening hydrolysis reaction mechanism of cyclic 3′, 5′-adenosine monophosphate (cAMP) have been theoretically investigated at the B3LYP/6-31G** level. It is found that each ring-opening hydrolysis reaction of cAMP takes place via a four-membered transition state. For the reactions, H 2O has two attacking sites (α and β). At each site, H 2O has two attacking objects. One is the P atom, the other is the bond P–O, and they are competitive each other. The potential energy surface of reaction 4 is the lowest. The computations strongly support the experimental result that the nonenzymatic hydrolysis of cAMP yields 3′-AMP and in-line nonenzymatic hydrolysis of cAMP to 3′-AMP is the most advantageous hydrolysis reaction path. Our present calculations have rationalized and verified all the possible reaction channels.

The thermal degradation of poly(vinyl acetate) measured by thermal analysis–Fourier transform infrared spectroscopy

The elimination process which occurs during the thermal degradation of poly(vinyl acetate) has been studied using thermal analysis–Fourier transform infrared spectroscopy. It was found that elimination of acetate groups initially began slowly, but increased as degradation proceeded due to an additional process. The increase in rate was found to depend on the concentration of unsaturated groups in the polymer chain. The activation energy for the initial step was found to be 190kJmol−1, while that for the additional process was 130kJmol−1. The additional process of elimination was considered to be due to a four-membered transition state, activated by double bonds adjacent to the acetate unit.

Kinetics and mechanism of the pudovik reaction in the azomethine series: I. Addition of dimethyl hydrogen phosphite to N-isopropylbenzalimines

For a series of phenyl-substituted N-isopropylbenzalimines, the effect of substituent on their capability to add dimethyl hydrogen phosphite was studied qualitatively in the condensed phase (DTA) and quantitatively (with determination of the kinetic and activation parameters) in 2-propanol solutions with spectrophotometric monitoring of the reaction. A reaction mechanism was proposed, involving formation of a highly organized four-membered transition state.

Studies on aluminium mediated asymmetric Friedel–Crafts hydroxyalkylation reactions of pyridinecarbaldehydes

The reaction of pyridinecarbaldehydes with aromatic C–H bonds mediated by chiral and achiral aluminium complexes has been investigated both experimentally and theoretically. It was found that pyridine-2-carbaldehyde activated by (R)-BINOL–AlCl reacts with e.g.N,N-dimethylaniline in a Friedel–Crafts hydroxyalkylation reaction to give the resulting alcohol in moderate yield and enantiomeric excess, whereas the pyridine-3- and -4-carbaldehydes give a diarylated product. The mechanism of the reaction has been investigated using theoretical calculations. The calculations show that the uncatalyzed reaction proceeds via a concerted, one-step mechanism, having a four-membered transition state with an energy barrier of 81 kcal mol−1. However, for the Lewis acid-mediated reaction using AlCl3 as the Lewis acid, the reaction proceeds through a zwitterionic intermediate in which the transition-state energy is significantly reduced. For the reaction of pyridine-2-carbaldehyde, the aluminium Lewis acid can coordinate in both a mono- and bidentate fashion to the substrate. Furthermore, two Friedel–Crafts intermediates have been found depending on the orientation of the incoming aromatic compound. Several transition states have been calculated for the formation of the Friedel–Crafts intermediates starting from the two different coordination modes of pyridine-2-carbaldehyde to AlCl3. The lowest energy for the reaction has been calculated to be 5.6 kcal mol−1 for pyridine-2-carbaldehyde activated by AlCl3 in a monodentate fashion. The mechanistic aspects of these observations will be discussed.

Topological analysis of the electron localization function applied to the study of the [1,3] sigmatropic shift of fluorine in 3-fluorpropene

The [1,3] sigmatropic shift of the fluorine atom in the 3-fluorpropene system has been studied from the topological analysis of the electron localization function (ELF). All calculations have been carried out at the B3PW91/6-311g** level of theory. The allowed suprafacial pathway through a concerted four-membered Cs transition state (TS) has been characterized in terms of an ion pair structure which shows the halogen atom bearing −0.6e. The ionic nature of the TS is supported by the absence of any polisynaptic basin connecting the fluorine and the allyl fragments in this stationary point. Finally, the evolution of the bonding characteristics along the intrinsic reaction coordinate reaction path has been analyzed in detail in terms of the basin properties of the ELF function.

A spin‐coupled investigation of the electrophilic addition of hydrochloric acid to ethylene

The gas-phase reaction of hydrochloric acid and ethylene to give chloroethane is investigated in the framework of spin-coupled (SC) theory, with emphasis on the widely accepted bimolecular mechanism. The transition-state structure and intrinsic reaction coordinate path are studied in detail. Our results indicate that the driving force in the reaction is the electrophilic attack of the hydrogen atom in HCl on one of the ethylenic carbons. There is little evidence supporting the occurrence of a four-membered transition state; rather, the spin-coupled results suggest that the breaking of the HCl bond occurs before the CCl bond formation. These findings are supported by the results from a topological analysis of the total electron density. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 231–239, 1999

A spin-coupled investigation of the electrophilic addition of hydrochloric acid to ethylene

The gas-phase reaction of hydrochloric acid and ethylene to give chloroethane is investigated in the framework of spin-coupled (SC) theory, with emphasis on the widely accepted bimolecular mechanism. The transition-state structure and intrinsic reaction coordinate path are studied in detail. Our results indicate that the driving force in the reaction is the electrophilic attack of the hydrogen atom in HCl on one of the ethylenic carbons. There is little evidence supporting the occurrence of a four-membered transition state; rather, the spin-coupled results suggest that the breaking of the HCl bond occurs before the CCl bond formation. These findings are supported by the results from a topological analysis of the total electron density. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 74: 231–239, 1999

Syntheses, structures and reactions of some new benzyl-substituted cyclopentadienyl titanium complexes

Twenty eight new benzyl-substituted cyclopentadienyl titanium complexes have been synthesized. Among them, five typical molecular structures have been studied by X-ray analysis. It was found that in the preparation of ortho-MeO-containing benzyl-substituted titanium complexes, when both two substituents on benzylic carbon atom are Et or more large group, titanoxacycle complexes are always formed. The cyclization was promoted by the presence of halides. The activity order of halides for the cyclization is: LiI-NaI > LiBr-Br 2 > NaBr-KI > KBr > I 2. High temperature is favourable to the cyclization. At 80°C, the normal ortho-MeO benzyl-substituted cyclopentadienyl titanium complex 26 may be spontaneously converted into metallocyclic complex 19. The position of the MeO group on the benzene ring is the key factor to the cyclization. In the case of meta- and para-MeO benzyl-substituted titanium complexes 23 and 25, there was no possibility to form a titanoxacycle complex, such as 19, because the MeO group is too far from titanium. A probable mechanism involving a four-membered transition state was proposed.

Synthesis and Reactivity of d0Bis(imido) Silyl and Germyl Complexes of Molybdenum and Tungsten

The syntheses and reactivities of d0 bis(imido) molybdenum/tungsten silyl chloride complexes (2,6-iPr2C6H3N)2M[Si(SiMe3)3]Cl (1, M = Mo; 2, M = W) and the corresponding germyl complexes (2,6-iPr2C6H3N)2M[Ge(SiMe3)3]Cl (3, M = Mo; 4, M = W) are described. The complex (2,6-iPr2C6H3N)2Mo[Si(SiMe3)3]Cl (1), prepared by the reaction of (2,6-iPr2C6H3N)2MoCl2(dme) with (THF)3LiSi(SiMe3)3, has been structurally characterized. In general, these complexes are rather stable and do not react with CO, H2, or CH3CN. Complex 1 reacts with 2,6-Me2C6H3NC to provide the insertion product (2,6-iPr2C6H3N)2Mo[η2-C(N-2,6-Me2C6H3)Si(SiMe3)3](Cl) (5) and with AgOTf to give the silyl triflate complex (2,6-iPr2C6H3N)2Mo[Si(SiMe3)3]OSO2CF3 (6) in high yield. Complexes 1−6 react with neopentylmagnesium chloride to produce the silyl neopentyl complexes (2,6-iPr2C6H3N)2M[E(SiMe3)3](CH2CMe3) (7, M = Mo, E = Si; 8, M = W, E = Si; 9, M = Mo, E = Ge; 10, M = W, E = Ge). Complex 7, which was characterized by X-ray crystallography, contains an agostic interaction involving the α hydrogen of the neopentyl ligand (d(Mo−H) 2.55(4)Å). The neopentyl complexes 7−10 readily react with hydrogen (1 atm) to generate free neopentane and HSiMe3, probably via hydrogenation of the Mo−C bond to generate a highly unstable silyl hydride intermediate. The mechanism of HSiMe3 formation is unknown but may involve decomposition of the silyl hydride species via a four-membered transition state to generate a highly reactive silylene species. The corresponding tungsten analog 8 undergoes a similar reaction, but at a much slower rate. Attempts to trap the possible silylene intermediates (2,6-iPr2C6H3N)2MSi(SiMe3)2 (M = Mo, W) were unsuccessful.

Synthesis and characterization of intra- and intermolecular hydrogen bonding isomers of PH (apical) phosphoranes bearing a hydroxyl group and their thermal cyclization

The reaction of spirophosphorane 1 with 3 equivalents of RLi (R = Me, n-Bu, t-Bu) in Et 2O at −78 °C followed by treatment with aq. HCl gave monocyclic PH phosphoranes 2–4 bearing an apical hydrogen atom in good yields. Particularly in the case of 4, an isomer 4a with intramolccular hydrogen bonding and an isomer 4b with intermolecular hydrogen bonding with a solvent molecule could be separately isolated. Monocyclic phosphoranes 2–4 cyclized intramolecularly with elimination of H 2 to form spirophosphoranes 5–7 upon heating. Kinetic examination of cyclizations implied that they proceed via four-membered ring transition states involving hexacoordinate phosphorus.

Speculations on a possible mechanism of a tungsten catalyzed 1,2,3,3-tetraphenylprop-2-en-1-one polymerization

A mechanism is suggested for the reaction of 1,2,3,3-tetra-phenylprop-2-en-1-one (1) under the influence of some tungsten compounds containing WO or WC bonds. The processes involved in the polymerization of 1 are described as redistribution of atoms (C, O and W), involving four-membered metallacyclic transition states. The latter can be visualized by the schemes of olefin metathesis, carbonyl-olefin exchange reaction, reductive coupling of carbonyl compounds etc. These conclusions are confirmed by the fact that the processes under investigation can be catalyzed not only by tungsten oxocompounds but also by the stable Casey carbene.