1,5] Sigmatropic hydrogen shifts in cyclic 1,3-dienes

Density functional calculations have been carried out for [1,5] hydrogen shifts in 1,3-cycloalkadienes (cyclohexadiene, cycloheptadiene, and cyclooctadiene). The complexity of the potential surfaces of these reactions was found to increase with ring size. For 1,3-cyclohexadiene a single transition structure for the [1,5] hydrogen shift was located, which connects the two enantiomeric conformers. For 1,3-cycloheptadiene two enantiomeric transition structures for the [1,5] hydrogen shift were located, which interconnect three conformers of the diene, a pair of enantiomeric conformers and a third achiral conformer. Finally for 1,3-cyclooctadiene two diastereomeric transition structures were found in addition to six conformers (three pairs of enantiomeric conformers) of the diene. Calculated activation energies for the [1,5] hydrogen shifts were found to be in qualitative agreement with experiment. Variation in these energies are attributed to strain energies present in either the diene or the transition structure.     

[1,5] Sigmatropic shifts in a 1,4-diphospholylbenzene

The central phenylene ring of 1,4-bis(3,4-dimethylphosphol-1-yl)benzene undergoes [1,5] shifts around both phospholyl rings above 150°C to give 1,4-bis(3,4-dimethyl-5H-phosphol-2-yl)benzene, which can be trapped by tolane, [CpFe(CO)₂]₂, or Mn₂ (CO)₁₀ to yield the corresponding bis-1-phosphanor-bornadiene, bis-phosphaferrocene, or bis-phosphacymantrene respectively.     

Sigmatropic shifts and cycloadditions on neutral, cationic, and anionic pentadienyl+butadiene potential energy surfaces

Quantum chemical calculations were used to characterize [4,5] sigmatropic shifts in systems containing butadiene and pentadienyl substructures. Cationic, anionic, and radical systems with various tethers connecting these substructures were examined. In many cases, concerted [4,5] sigmatropic shifts with asynchronous bond making and breaking events were observed. The fundamental connections between [4,5] sigmatropic shifts and (5+4) cycloadditions are highlighted.     

Sigmatropic rearrangements in 5-allyloxytetrazoles

Mechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3']-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3']- and [3,3']-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3']-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products.     

Quantitative measurement of endo preference in homodienyl hydrogen shifts

From the kinetics of the gas-phase pyrolysis of a series of $\text{cis}$-1,1-dimethyl-2-alkenylcyclopropanes, the preference for the endo transition state in the homodienyl hydrogen shift (retro-ene reaction) is estimated to be at least 12 kcal/mol.     

Vicinal deuterium perturbations on hydrogen NMR chemical shifts in cyclohexanes

The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at -80 degrees C or lower, all four possible perturbations of H by D as H-C-C-H is changed to D-C-C-H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Delta delta perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal J(H-H) coupling constants as a function of dihedral angles.     

Sigmatropic Rearrangements in Phosphorylated 2-Azaallylic Systems

Abstract

Allylic and heteroallylic compounds are classical objects of organic chemistry and can serve as models in the investigation of various theoretical problems. The anions and 1,3-dipoles generated from azaallylic derivatives are widely used in the synthesis of cyclic and acyclic nitrogen-containing compounds.     

Sigmatropic rearrangements in the allyl substituted oxazolinone system

Thermolysis and photolysis of several allyl substituted oxazolin-5-ones results in a sigmatropic rearrangement in accord with orbital symmetry predictions.     

Frequency shifts in the hydrogen spectrum due to interaction with the electromagnetic field

Frequency shifts of hydrogen atom spectral lines are analyzed, both from the “exact” numerical solutions of the coupled equations and the perturbation theory. A numerical method for solving the set of the time-dependent equations is described that includes both the bound and the continuumstates of the hydrogen atom which is stable on a long time scale.     

Performance of density functional theory methods to describe intramolecular hydrogen shifts

The performance of three exchange and correlation density functionals, LDA, BLYP and B3LYP, with four basis sets is tested in three intramolecular hydrogen shift reactions. The best reaction and activation energies come from the hybrid functional B3LYP with triple-ζ basis sets, when they are compared with high-level post-Hartree-Fock results from the literature. For a fixed molecular geometry, the electrophilic Fukui function is computed from a finite difference approximation. Fukui function shows a small dependence with both the exchange and correlation functional and the basis set. Evolution of the Fukui function along the reaction path describes important changes in the basic sites of the corresponding molecules. These results are in agreement with the chemical behavior of those species.     

Stereochemistry of [2,3]Sigmatropic Rearrangements

Stereoselective synthesis is attracting more and more attention. Successful stereoselective syntheses require the availability of reactions of known stereochemistry which give maximum yields of the desired products. This is particularly so with reactions proceeding via cyclic transition states. Of these, the stereochemistry of [3,3]sigmatropic processes has long been known. This article now summarizes the factors determining the stereoselectivity of [2,3]sigmatropic rearrangememts.     

Sigmatropic mechanism of the chlorotropic isomerization of phosphazopentachloroethane

The sigmatropic and dissociative-recombination mechanisms of the chlorotropic isomerization of benzodioxachlorophosphazopentachloroethane and its 1:2 solvate with chloroform have been studied by the MNDO-PM3 method in the supermolecular approximation. It is observed that the thermodynamically more stable chlorotropic isomer corresponds to the phosphorane structure with axial-equatorial disposition of two chlorine atoms and the two oxygen atoms in the dioxaphosphole ring, in complete agreement with literature data on teh³⁵Cl NQR spectrum. It is shown that the most likely pathway for phosphorus-carbon chlorotropic rearrangement for both phosphazopentachloroethane and its solvate with two molecules of chloroform is the sigmatropic route. Institute of Bioorganic and Oil Chemistry, National Academy of Sciences of Ukraine, 1 Murmanskaya ul., Kiev 02094, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 3, pp. 177–181, May–June, 2000.     

Communication: Frequency shifts of an intramolecular hydrogen bond as a measure of intermolecular hydrogen bond strengths

Infrared-ultraviolet double resonance spectroscopy has been applied to study the infrared spectra of the supersonically cooled gas phase complexes of formic acid, acetic acid, propionic acid, formamide, and water with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. In these complexes each binding partner to 9HFCA can function as both proton donor and acceptor. Relative to its frequency in free 9HFCA, the 9-hydroxy (9OH) stretch is blue shifted in complexes with formic, acetic, and propionic acids, but is red shifted in the complexes with formamide and water. Density functional calculations on complexes of 9HFCA to a variety of H bonding partners with differing proton donor and acceptor abilities reveal that the quantitative frequency shift of the 9OH can be attributed to the balance struck between two competing intermolecular H bonds. More extensive calculations on complexes of glycolic acid show excellent consistency with the experimental frequency shifts.