1,5-hydrogen shift in 1,3-cycloheptadienes

Conclusions It was shown for methyl-1,3-cycloheptadienes that the system of double bonds in the cyeloheptadiene ring migrates at temperatures above 120°, apparently as a consequence of a 1,5-hydrogen shift, to form an equilibrium mixture of isomers with respect to the position of the intracyclic double bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1641–1642, July, 1972.     

Theoretical study of the 1,3-hydrogen shift of triazene in water

The 1,3-hydrogen shift of triazene in aqueous solution was studied with a combination of QM/MM methods. First, the different species involved were characterized and the activation free-energies calculated with ASEP/MD, a method that makes use of the mean field approximation. Then the reaction dynamics was simulated with a QM/MM/MD method. A very strong influence of the solvent was observed, both specific, with the participation of a water molecule, and from the rest of the solvent. The effect of solvation on the geometry and electron distribution of triazene is important: N-N bond lengths tend to be more similar and the molecule acquires a planar structure. For the transition state structure, a substantial degree of ionic nature was found. Dynamic solvent effects were also analyzed.     

Tunneling in the 1,5-hydrogen shift reactions of 1,3-cyclopentadiene and 5-methyl-1,3-cyclopentadiene

MPW1K/6-31+G(d,p) calculations which include the effects of small curvature tunneling find that, around room temperature, thermally activated tunneling dominates the 1,5-hydrogen shift reactions of 1,3-cyclopentadiene (2a) and 5-methyl-1,3-cyclopentadiene (2c). The calculated temperature dependence of the H/D kinetic isotope effect (KIE) for the latter rearrangement agrees well with experimental measurements that were published nearly 40 years ago. It is argued that the experimental KIEs provide prima facie evidence for tunneling in this reaction. The calculations also predict that it should be possible, at least in principle, to confirm this conclusion by observing curvature in the Arrhenius plot for the rearrangement of 2c.     

Localized MO analysis of the 1,2-hydrogen shift mechanism

For the 1.2-hydrogen shift, a localized SCF MO analysis shows that two different electronic rearrangement mechanisms are possible. In the first case, the migrating hydrogen behaves as a hydride along the entire reaction path; in the other case. the migrating hydrogen first behaves as a hydride, but in the second half of the reaction behaves as a naked proton. The difference between the two mechanisms appears to be related to the spatial orientation of the lone-pair orbitals.     

Photochemical ring opening of 7-benzoyl- and 7-methoxycarbonyldibenzonorcaradienes. competing 1,2-hydrogen shift and cyclization reactions of 1,3-diradicals

[see reaction]. The UV irradiation of dibenzonorcaradienes bearing an acyl or alkoxycarbonyl substituent in the 7-position results in formation of substituted phenanthrenes, as well as cis-trans isomerization of the starting material. This reaction apparently proceeds via intermediate formation of a short-lived (tau = 1-20 ns) 1,3-diradical, which is produced by photochemical cleavage of one cyclopropane bond, while no evidence of alpha-carbonylcarbene formation was found.     

Extension of the bebo method for the calculation of activation energies of intraradical 1,3-, 1,4- and 1,5-hydrogen shift reactions

The bond-energy—bond-order (BEBO) method has been extended for the calculation of activation energies of the radical isomerization reactions occurring via 1,3-, 1,4- and 1,5-hydrogen atom shifts. The energy of the cyclic activated complexes comprises four contributions, i.e. the energy change in formation of the transition state due to the occurrence of fractional and strained bonds, the triplet repulsion, the deformation energy and the non-bonding interaction. The method has been applied to a set of 11 reactions. The agreement between the calculated and the experimental activation energies is satisfactory.     

Multipath variational transition state theory: rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical

We propose a new formulation of variational transition state theory called multipath variational transition state theory (MP-VTST). We employ this new formulation to calculate the forward and reverse thermal rate constant of the 1,4-hydrogen shift isomerization of the 2-cyclohexylethyl radical in the gas phase. First, we find and optimize all the local-minimum-energy structures of the reaction, product, and transition state. Then, for the lowest-energy transition state structures, we calculate the reaction path by using multiconfiguration Shepard interpolation (MSCI) method to represent the potential energy surface, and, from this representation, we also calculate the ground-state vibrationally adiabatic potential energy curve, the reaction-path curvature vector, and the generalized free energy of activation profile. With this information, the path-averaged generalized transmission coefficients <γ> are evaluated. Then, thermal rate constant containing the multiple-structure anharmonicity and torsional anharmonicity effects is calculated using multistructural transition state theory (MS-TST). The final MP-VTST thermal rate constant is obtained by multiplying k(MS-T)(MS-TST) by <γ>. In these calculations, the M06 density functional is utilized to compute the energy, gradient, and Hessian at the Shepard points, and the M06-2X density functional is used to obtain the structures (conformers) of the reactant, product, and the saddle point for computing the multistructural anharmonicity factors.     

Effect of phenyl substitution on the lifetime and product distribution of cyclobutylidene: preference change in the rearrangements via 1,2-carbon shift and 1,2-hydrogen shift

Steady state and laser flash photolytic experiments with precursors 6 and 11 revealed that diphenyl substitution affects the lifetime and reaction mode of cyclobutylidene. 2,2-Diphenylcyclobutylidene 3 (τ <0.1 ns) produces methylenecyclopropane 1 via 1,2-carbon in significant preference to the positional isomer 2 or cyclobutene 4. On the other hand, 3,3-diphenylcyclobutylidene 5 (τ = ca. 4 ns) gives 1,2-hydrogen shift product 4 more favorably than 1,2-carbon shift product 2 together with formal carbene dimer 14. MRMP2//MP2 calculations afford useful results to understand the interrelationship among substitution, structure, and reactivity.     

An ab initio study of substituent effects in [1,3]-hydrogen shifts

Reports in the literature place the TS for the [1,3]-H shift in propene comparable to or higher in energy than loss of the allylic H. However, [1,3]-H shifts have been repeatedly observed experimentally in enolates. We used GAUSSIAN 98 to examine the origin of this apparent contradiction. We found the first TS for an antarafacial [1,3]-H shift that is clearly lower in energy than simple dissociation of the migrating H. This occurs in the [1,3]-H shift in the acetone enolate. Symmetrical substituents (H, O(-), ethynyl) have TSs with C(2) symmetry, implying that they, and probably most [1,3]-H shift TSs, are antarafacial. Conjugating substituents at C2 lower the energy of [1,3]-H shifts and raise the energy of dissociation by loss of a hydrogen atom from C3, increasing the likelihood of the former type of reaction. Strongly electron-donating and electron-withdrawing substituents are more effective than neutral substituents in lowering the energy requirement of [1,3] shifts. Our best calculations predict that a [1,3]-H shift is lower in energy than dissociation by loss of the H by 27.8 kJ/mol in 2-methyl-1-butene-3-yne, by 36.8 kJ/mol in isoprene, by 55.9 kJ/mol in 2-aminopropene, by 114.5 kJ/mol in the acetone enolate, and by 120.8 kJ/mol in the 1-methylacryloyl cation. Thus, there is a chance of experimental observation of [1,3] shifts in conjugated alkenes and related species.     

Preparation of 1,3-diols by metalation-hydroboration of alkynes: A 1,2-hydrogen migration from boron to carbon

Metalation of a series of 1- and 2-alkynes with subsequent treatment with diborane and oxidation gave 1,3-diols exclusively. Deuteroboration of the lithiated acetylenes established the occurrence of a displacement of one of the B atoms by deuterium in gem-diboronated compounds. Hydroboration of several acetylenes and acetylides was also studied.     

Very low-pressure pyrolysis (VLPP) of penta-1,3-dienes. Kinetics of the unimolecular 1,4-hydrogen elimination from cis-penta-1,3-diene

The thermal unimolecular reactions of cis- and trans-penta-1,3-diene (c-PTD and t-PTD) have been studied over the temperature range of 1002–1235 K using the technique of very low-pressure pyrolysis (VLPP). c-PTD decomposes via 1,4-hydrogen elimination analogous to that previously reported for cis-but-2-ene. RRKM calculations incorporating a six-center transition state show that the experimental rate constants are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol, and the A factor was assumed to be the same as that for cis-but-2-ene. The activation energy is in excellent agreement with that obtained for cis-but-2-ene. t-PTD also undergoes decomposition by H₂ elimination presumably via the prior rapid isomerization to c-PTD the results are in exact agreement with those for c-PTD.     

Ruthenium-catalyzed cyclization of 2-alkyl-1-ethynylbenzenes via a 1,5-hydrogen shift of ruthenium-vinylidene intermediates

Catalytic cyclization of 2-alkyl-1-ethynylbenzene derivatives was implemented by TpRuPPh3(CH3CN)2PF6 (10 mol %) in hot toluene (105 degrees C, 36-100 h) to form 1-substituted-1H-indene and 1-indanone products; such cyclizations proceeded more efficiently for substrates bearing electron-rich benzenes. We propose that the cyclization mechanism involves a 1,5-hydrogen shift of initial metal-vinylidene intermediate.     

DFT study of the mechanisms of in water Au(I)-catalyzed tandem [3,3]-rearrangement/Nazarov reaction/[1,2]-hydrogen shift of enynyl acetates: a proton-transport catalysis strategy in the water-catalyzed [1,2]-hydrogen shift

A computational study with the Becke3LYP density functional was carried out to elucidate the mechanisms of Au(I)-catalyzed reactions of enynyl acetates involving tandem [3,3]-rearrangement, Nazarov reaction, and [1,2]-hydrogen shift. Calculations indicate that the [3,3]-rearrangement is a two-step process with activation free energies below 10 kcal/mol for both steps. The following Nazarov-type 4pi electrocyclic ring-closure reaction of a Au-containing dienyl cation is also easy with an activation free energy of 3.2 kcal/mol in CH2Cl2. The final step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a computed activation free energy of 20.2 kcal/mol) when dry CH2Cl2 is used as the solvent. When this tandem reaction was conducted in wet CH2Cl2, the [1,2]-hydride shift step in dry solution turned to a very efficient water-catalyzed [1,2]-hydrogen shift mechanism with an activation free energy of 16.4 kcal/mol. Because of this, the tandem reaction of enynyl acetates was found to be faster in wet CH2Cl2 as compared to the reaction in dry CH2Cl2. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule is involved in catalysis or a proton-relay stabilizer when a water cluster is involved in catalysis. Water is found to act as a proton shuttle in the proton-transport catalysis strategy. Theoretical discovery of the role of the acetoxy group in the water-catalyzed [1,2]-hydrogen shift process suggests that a transition metal-catalyzed reaction involving a similar hydrogen shift step can be accelerated in water or on water with only a marginal effect, unless a proton-accepting group such as an acetoxy group, which can form a hydrogen bond network with water, is present around this reaction's active site.     

A study of 1,5-hydrogen shift and cyclization reactions of an alkali isomerized methyl linolenoate

Heating a mixture formed by alkali isomerization of methyl linolenoate (1) produces a complex mixture with the bicyclic hexahydroindenoic esters 4β-(7-methoxycarbonylheptyl)-5α-methyl-2,3,3aα,4,5,7aαhexahydroindene (CL5) and 4β-ethyl-5α-(6-methoxycarbonylhexyl)-2,3,3aα,4,5,7aα-hexahydroindene (CL6) as main components. Similar isomerization reactions of three synthetic model compounds, methyl 9Z,13E,15Z-octadecatrienoate (2), 9Z,14E,16E-octadecatrienoate (4) and 9Z,11E,15Z-octadecatrienoate (5) corroborated the results obtained with alkali isomerized methyl linolenoate.     

Reactions induced by the γ-hydrogen shift in the molecular ion of 1-nitropropane

It is shown that the γ- or 1,5-hydrogen shift in the molecular ion of 1-nitropropane leads to three primary fragmentation reactions. They are the loss of a hydroxyl radical, a molecule of ethylene and a molecule of nitric oxide. The structures and chemistry of the resulting ions have been investigated by a series of experiments including deuterium labelling, spontaneous and collisionally induced dissociations and accurate mass measurements.     

1,1-二甲基-2-(9-菲基)-环丙烷的直接同面1,3氢迁移

二甲基被氘代的标题环丙烷光解生成2-甲基d3-4-(9-菲基)-1-丁烯-d3,它的结构系根据光谱和化学降解推断.基于分子轨道跟踪法和激发分子基础轨道对间的键序变化讨论.标题化合物最合理的开环模式是同面的1,3氢迁移.     

Direct suprafacial 1,3-hydrogen migration of 1,1-dimethyl-2-(9-phenanthryl)cyclopropane

Irradiation of the title compound with two D-labeled methyl groups gave 2-methyl-d₃-4-(9-phenanthryl)-1-butene-d₃, which was eluciated on the bases of the spectral evidences and chemical degradation. Based on the discussions of MO following and bond order change between the designed basis orbital pairs on excitation, the most reasonable ring-opening mode of the title compound appeared to be the direct suprafacial 1,3-hydrogen migration.