The gas phase aldose‐ketone isomerization mechanism: Direct interconversion of the model hydroxycarbonyls 2‐hydroxypropanal and hydroxyacetone

We report a novel mechanism for the interconversion of 2‐hydroxypropanal with its more‐stable ketone isomer hydroxyacetone. Reaction proceeds via concerted transfer of two H atoms, requires a barrier of only ～40 kcal mol^?1, bypasses the enediol intermediate, and is general for α‐hydroxy carbonyls. A similar isomerization mechanism is shown to persist for β, γ, and δ‐hydroxy carbonyls; these compounds are skeletal forms of the monosaccharides and this work, therefore, discloses the gas‐phase mechanism for aldose‐ketose isomerization. As an example, the isomerization of glyceraldehyde to dihydroxyacetone is shown to proceed via this mechanism with a barrier of 31 kcal mol^?1. Rate coefficients and thermochemical properties are reported for the isomerization of 2‐hydroxypropanal and hydroxyacetone for use in detailed kinetic models. Additionally, RRKM theory k (E ) values for this reaction suggest that it may transpire in the troposphere following solar excitation.     

Ab initio calculations on 1,2-hydrogen shifts in the benzene radical cation and on carbon scrambling via an isomerization to the fulvene structure

Multireference configuration interaction (MRCI//6-31G**) ab initio calculations show that the barrier for hydrogen scrambling in the benzene radical cation is about 50 kcal mol^?1. Once the internal energy is sufficient for a 1,2-hydrogen shift, the moving hydrogen can go to any position in the ring. The barrier for carbon scrambling via an isomerization to the fulvene structure is about 17 kcal mol^?1 higher than that for hydrogen scrambling. Both of these values are far below the dissociation limit.     

Dynamics of the isomerization of trans-stilbene in n-alcohols studied by ultraviolet picosecond absorption recovery

We have used a simple At⁺ synchronously pumped and cavity-dumped dye-laser system to generate UV picosecond pulses with energies sufficiently high for absorption recovery experiments. With these pulses, we have studied the dynamics of the isomerization of trans-stilbene in n-alcohols as a function of viscosity and temperature. It is concluded that the excited-state barrier of trans-stilbene in n-alcohols is less than 1 kcal mol^?1 and that the trans → cis isomerization yield is 0-54 ± 0.05.     

The [CH2 = CHOH/H2O]+˙ system: A theoretical study of distonic ions,hydrogen-bridged ions and ion–dipole complexes

Several isomeric forms of the vinyl alcohol/water radical cation have been investigated by high-level ab initio molecular orbital theory calculations, including electron correlation effects. Of the ions considered here, the anti form of the ? O ?H ?O? bridged complex is calculated to be the lowest in energy, having a stabilization energy of 100 kJ mol^?1 with respect to the dissociation products [CH₂CHOH]⁺˙ and H₂O. Although the isomeric ions may formally be represented as distonic ions, hydrogen-bridged ions and ion–dipole complexes, the only significant barrier separating the isomers appears to be the anti?syn isomerization barrier. However, in the ? O ?H ?O? bridged complex this barrier is found to be considerably lowered relative to the anti?syn isomerization barrier for the free vinyl alcohol radical cation.     

Solvent effects on the syn-anti isomerization rates of hydrazones of acetone

The coalescence temperature for the C-methyl peaks in the NMR spectra of hydrazones of acetone are above 200°, indicating an energy barrier for syn-anti isomerization greater than 28 kcal mole^?1. the presence of halogenated compounds decreases the energy barrier, due to the formation of a charge transfer complex between the hydrazone acting as a donor and the halogenated compound acting as the electron acceptor.     

Ab initio calculations on the isomerization of alkene radical cations

Ab initio calculations on the isomerization of butene and pentene radical cations indicate that, for all classical ion structures, the lowest barrier for a rearrangement to the most stable ion structure is below the dissociation limit. Isomerizations of linear butene radical cations to the isobutene structure take place via the CH₃CC₂H₅^·+ structure, whereas in the pentene case the connection between linear and branched ion structures proceeds via the 1,2-dimethylcyclopropane radical cation. From the results a qualitative model is derived which suggests that for larger alkene radical cations an isomerization to structures with four alkyl substituents on the double bond may be in close competition with dissociation.     

Permutational isomerization of caged polycyclic oxyphosphoranes

The reaction of 1-phospha-2,8,9-trioxaadamantane with hexafluoroacetone gives a crystal-line caged polycyclic pentaoxyphosphorane. The ambient temperature fourier transform ¹³C NMR spectrum of the phosphorane in CH₂Cl₂ solution, with proton noise-decoupling, shows one singlet for three equivalent methine carbons, and one singlet for three equivalent methylene carbons. Hence, the phosphorane undergoes a rapid permutational isomerization, which is confirmed by the observation of equivalency of the three methine protons and of the three groups of methylene protons in the ¹H NMR, and of equivalency of the four CF₃ groups in the ¹⁹F NMR. The variable temperature ¹⁹F NMR spectra in a vinyl chloride-CHFCl₂ solvent system disclose that the permutational isomerization of this phosphorane cannot be prevented even at ? 165°, although there is a progressive line-broadening indicative of a decrease in the exchange rate. The ³¹P NMR spectrum of the phosphorane has a signal at a higher magnetic field (?³¹P = +42·4 ppm) than H₃PO₄; the shift is nearly identical in CH₂Cl₂ and in the acidic (CF₃)₂CHOH. The remarkably low energy barrier (< ca 5 kcal/mole) for the permutational isomerization of a compound as constrained as the caged oxyphosphorane is attributed to: (1) A small deviation from the perfect trigonal bipyramidal configuration, which raises the ground state energy. (2) The existence of a barrier configuration which can accommodate the constrains of the caged molecule, and is therefore of relatively low energy. The barrier configuration is deduced from the turnstile rotation mechanism of permutational isomerization.     

Kinetic Study on the Isomerization of Perindopril by HPLC

The isomerization of perindopril has been investigated using dynamics chromatography and an unified equation introduced by Trapp that was based on stochastic and theoretical plate models to determine the energies. The isomerization rate constants and Gibbs activation energies of isomerization are directly calculated from chromatographic peak parameters, i.e., retention times of the inter-converting species, peak width at half height, and relative plateau height. From the rate constant \( k_{1}^{ue} (T) \), measured at variable temperatures, the kinetic eyring activation parameters ΔG ^#, ΔH ^# and ΔS ^# of isomerization of perindopril were obtained. By variation of the flow rate of the mobile phase, the expected independence of the isomerization barrier from the chromatographic time scale was demonstrated for the first time. The relationships between peak shape and chromatographic conditions, such as flow rate, temperature, pH, organic modifier, and β-cyclodextrin, such as an additive, were investigated. In addition, an NMR investigation on perindopril was described.

     

Unraveling the Mechanism of the IrIII-Catalyzed Regiospecific Synthesis of α-Chlorocarbonyl Compounds from Allylic Alcohols

We have used experimental studies and DFT calculations to investigate the Ir^III-catalyzed isomerization of allylic alcohols into carbonyl compounds, and the regiospecific isomerization–chlorination of allylic alcohols into α-chlorinated carbonyl compounds. The mechanism involves a hydride elimination followed by a migratory insertion step that may take place at Cβ but also at Cα with a small energy-barrier difference of 1.8 kcal mol⁻¹. After a protonation step, calculations show that the final tautomerization can take place both at the Ir center and outside the catalytic cycle. For the isomerization–chlorination reaction, calculations show that the chlorination step takes place outside the cycle with an energy barrier much lower than that for the tautomerization to yield the saturated ketone. All the energies in the proposed mechanism are plausible, and the cycle accounts for the experimental observations.     

Impact of Water Molecules on the Isomerization of CH3S（OH）CH2 to CH3S（O）CH3： A Computational Investigation

The isomerization of CH3S（OH）CH2 to CH3S（O）CH3 in the absence and presence of water has been investigated at the G3XMP2//B3LYP/6-311 ＋ G（2df, p） level. The naked isomerization, the reaction without water, gives the high barrier height （21.56 kcal.mol^-1）. Three models are constructed to describe the water influence on the isomerization, that is, water molecules are the catalyst and the microsolvation, and water molecules act as the catalyst and microsolvation simultaneously. Our results show that the isomerization barrier heights of CH3S（OH）CH2 to CH3S（O）CH3 are reduced by 12.32, 11.04, and 7.80 kcal.mol^-1, respectively, when one, two, and three water molecules are performed as catalyst, in contrast to the naked isomerization. Moreover, the rate constants of the isomerization are calculated using the transition state theory with the Wigner tunneling correction over the temperature range of 240-425 K. We find that the rate constant of a single water molecule as the catalyst is 1.58 times larger than the naked isomerization at 325 K, whereas it is slower by 6 orders of magnitude when water molecule serves as the microsolvation at 325 K, compared to naked reaction. So the water-catalyzed isomerization of CH3S（OH）CH2 to CH3S（O）CH3 is predicted to be the key role in lowering the activation energy. The isomerization involving water molecules acting as mierosolvation is unfavorable under atmospheric conditions.     

Conformational isomerization of formic acid by vibrational excitation at energies below the torsional barrier

Photoinduced conformational isomerization of formic acid has been studied in a low-temperature argon matrix. It is found that conformational isomerization occurs when the photon energy is below the energy barrier for this process. The quantum yield for the process near the top of the barrier is comparable with the quantum yield above the barrier and drops at lower energies. The isomerization takes place via a tunneling mechanism.     

Reversible Photothermal Isomerization of Carborane-Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene-Stabilized Carborane-Fused Borirane

A fully reversible photothermal isomerization between carborane-fused trigonal-planar azaborole (dark-purple) and tetrahedral borirane (pale-yellow) has been observed, leading to the isolation and structural characterization of the first example of carborane-fused borirane. DFT calculations indicate that the azaborole is thermodynamically more stable than the borirane by 11.2 kcal mol⁻¹, and the energy barrier for the thermal conversion from azaborole to borirane is 35.5 kcal mol⁻¹. The reactivity studies show that the B−C(cage) bond in borirane can be broken in the reaction with CuCl, HCl, or elemental sulfur.     

Mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines

A theoretical study of the mechanism of the rhodium-catalyzed asymmetric isomerization of allylamines to enamines by using density functional theory with the B3LYP functional leads us to discard the so far accepted nitrogen-triggered mechanism, in which the isomerization occurs on N-bonded intermediates and transition states, in favor of a variation of the classical allylic mechanism for olefin isomerization. The modified allylic mechanism consists of four main steps: 1) N-coordination of the allylamine to Rh(I); 2) intramolecular isomerization from kappa(1)-(N)-coordination to eta(2)-(C,C)-coordination of the allylamine; 3) oxidative addition of C(1)--H to form a distorted octahedral eta(3)-allyl complex of Rh(III); and 4) hydrogen transfer to C(3) (reductive C(3)--H elimination). The two hydrogen transfer steps (oxidative addition and reductive elimination) have the highest barriers of the overall process. The oxidative addition barrier, which includes solvent effects, is 28.4 kcal mol(-1). For the reductive elimination, the value in solvent is 28.6 kcal mol(-1), very similar to the oxidative addition barrier.     

Direct measurement of the energy thresholds to conformational isomerization. II. 3-indole-propionic acid and its water-containing complex

The methods of stimulated emission pumping-hole-filling spectroscopy (SEP-HFS) and population transfer spectroscopy (SEP-PTS) were used to place direct experimental bounds on the energetic barriers to conformational isomerization in 3-indole-propionic acid (IPA) and its water-containing complex. By contrast with tryptamine (Paper I), IPA has only two conformations with significant population in them. The structures of the two conformers are known from previous work [P. M. Felker, J. Phys. Chem. 96, 7844 (1992)]. The energy thresholds for A-->B and B-->A isomerizations are placed at 854 and 754 cm(-1), respectively. Lower bounds on the isomerization barrier in the two directions are determined from the last transition not observed in the SEP-PT spectra. These are placed at 800 and 644 cm(-1) for A-->B and B-->A, respectively. The combined results place bounds on the relative energies of the A and B minima, with E(B)-E(A)=46-210 cm(-1). Like the IPA monomer, the IPA-H2O complex forms two conformational isomers. Both these isomers incorporate the water molecule as a bridge between the carbonyl and OH groups of the carboxylic acid. Previous rotational coherence measurements (L. L. Connell, Ph.D. thesis, UCLA, 1991) have determined that these complexes retain the same IPA conformational structure as the monomers. SEP-PTS and SEP-HFS were carried out on the IPA-H2O complexes. It was demonstrated that it is possible to use SEP to drive conformational isomerization between the two conformational isomers of IPA-H2O. Bounds on the energy barriers to conformational isomerization are not effected greatly by the presence of the water molecule, with Ebarrier(A-->B)=771-830 cm(-1) and Ebarrier(B-->A)=583-750 cm(-1). This is a simple consequence of the fact that the barrier is an intramolecular barrier, and the water molecule is held fixed in the COOH pocket, where it interacts with the ring only peripherally during the isomerization process. Finally, changes in the SEP-PT spectral intensity in transitions near the top of the barrier to isomerization as a function of the position of SEP excitation relative to the pulsed valve exit provide some insight to the competition between vibrational relaxation and isomerization in a molecule the size of IPA.     

Isomerization of Functionalized Olefins by Using the Dinuclear Catalyst [PdI(μ-Br)(PtBu3)]2: A Mechanistic Study

In a combined experimental and computational study, the isomerization activity of the dinuclear palladium(I) complex [Pd^I(μ-Br)(P^tBu₃)]₂ towards allyl arenes, esters, amides, ethers, and alcohols has been investigated. The calculated energy profiles for catalyst activation for two alternative dinuclear and mononuclear catalytic cycles, and for catalyst deactivation are in good agreement with the experimental results. Comparison of experimentally observed E/Z ratios at incomplete conversion with calculated kinetic selectivities revealed that a substantial amount of product must form via the dinuclear pathway, in which the isomerization is promoted cooperatively by two palladium centers. The dissociation barrier towards mononuclear Pd species is relatively high, and once the catalyst enters the energetically more favorable mononuclear pathway, only a low barrier has to be overcome towards irreversible deactivation.     

Substituent Effect on the Stability and 14N NQR Parameters of Linkage Isomers of Nitriles in a Rhodium Half‐Sandwich Metallacycle: A Theoretical Study

In this work, using the MPW1PW91 method, the substituent effect on the stability and on the ¹⁴N NQR parameters of linkage isomers of nitriles in a rhodium half‐sandwich metallacycle is illustrated. After determination of the corresponding isomerization transition state (TS), the substituent effect on the barrier energy and on the activation thermodynamic parameters (ΔG^‡ and ΔH^‡) of isomerization is explored. The electric field gradient tensor, nuclear quadrupole coupling constant, asymmetry parameter, and nuclear quadrupole resonance frequencies of the studied isomers are calculated. Also, linear correlations between these parameters and Hammett constant of the substituent are explored.     

Thermal isomerization of acetylnitrene: a quantum-chemical study

The electronic structure and pathways of thermal isomerization of formylnitrene and acetylnitrene were studied by the B3LYP/6-311G(d,p) density functional method and ab initio G2(MP2,SVP) computational procedure using the geometries obtained from B3LYP calculations. According to G2 calculations, both nitrenes have singlet ground states while the energies of the corresponding triplet states are 2.8 and 5.7 kcal mol^–1 higher. For acetylnitrene, the activation barrier to the nitrene isocyanate isomerization was estimated at 28.9 kcal mol^–1 (G2). Calculations revealed no pathway for single-step isomerization of nitrene into cyanate in both systems. The formation of methyl cyanate from isocyanate is thermodynamically unfavorable (E = 26.5 kcal mol^–1) and requires a high activation barrier (89.4 kcal mol^–1) should be overcome. Based on the results obtained, the pathways of transformation of nitrene formed in thermal decomposition of acetyl azide (Curtius rearrangement) were analyzed.     

Theoretical calculation of rate constants for the thermal isomerization from 1,2-butadiene to 1,3-butadiene

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Reversible Photothermal Isomerization of Carborane‐Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene‐Stabilized Carborane‐Fused Borirane

A fully reversible photothermal isomerization between carborane‐fused trigonal‐planar azaborole (dark‐purple) and tetrahedral borirane (pale‐yellow) has been observed, leading to the isolation and structural characterization of the first example of carborane‐fused borirane. DFT calculations indicate that the azaborole is thermodynamically more stable than the borirane by 11.2 kcal mol⁻¹, and the energy barrier for the thermal conversion from azaborole to borirane is 35.5 kcal mol⁻¹. The reactivity studies show that the B−C(cage) bond in borirane can be broken in the reaction with CuCl, HCl, or elemental sulfur.