A density functional theory mechanistic study of thermal decomposition reactions of nitroethyl carboxylates: undermine of “pericyclic” insight

Quantum chemical study of thermal decomposition reactions of model nitroethyl carboxylates were studied using various density functional theory levels. It was found that conversion esters into nitroalkenes demean according to one‐step mechanism. However, it is not an expected “pericyclic” mechanism but a “one‐step two‐stage” process. Subsequently, nitrous acid extrusion was also analyzed.     

Theoretical study on the aromaticity of transition states in pericyclic reactions

The aromaticity of transition states in pericyclic reactions such as electrocyclic reactions, cycloaddition reactions, and sigmatropic shifts was studied by the IDA (index of deviation from aromaticity) on the basis of a CASSCF wave function. The aromaticity defined by the IDA classified the allowed and forbidden transition states of pericyclic reactions treated here. The order of the aromaticity levels corresponds to that of the energy barriers of some reactions. The difference between the aromaticity defined by the IDA and that by the magnetic properties as a NICS is also discussed.     

Transition structures and energy barriers of pericyclic reactions in the CASSCF approach

Five energy hypersurfaces of the most examined pericyclic reactions have been investigated by using ab initio SCF , CASSCF , and the semiempirical AM 1 methods. The systems are H₄, H₆, C₃H₆, C₄H₄, and C₃OH₄. Stationary points and their sets of harmonic vibrational frequencies have been calculated by means of analytical gradient techniques in the frameworks of the respective approaches. ZPE corrected energy barriers are based on single-point calculations including dynamical correlation corrections by CAS -CI (SD )+DC , CASCEPA , or MP 2.     

Half-projected Hartree–Fock natural orbitals for defining CAS–SCF active spaces

We have extended the range of systems to which the half-projected Hartree–Fock (HPHF ) method has been applied, including the triplet state of the wave function. In our implementation, DIIS overcomes the convergence difficulties reported in earlier studies. HPHF allows generation of a symmetry-broken wave function in regions of the potential energy surface where the RHF wave function is triplet-stable. The fractionally occupied natural orbitals (FONOS ) of the HPHF wave function are good starting vectors for CAS –SCF calculations. A CAS –SCF in the space defined by the HPHF FONOS should be used instead of the unrestricted natural orbital CAS –SCF method in regions of triplet stability and for small active space problems. We draw extensive comparisons between the results of both the UNO –CAS and HPNO –CAS methods and those of full CAS –SCF calculations. © 1993 John Wiley & Sons, Inc.     

Half-Projected and Projected Hartree-Fock Calculations for Singlet Ground States. i. four-Electron Atomic Systems

The half-projected Hartree-Fock function (HPHF ) for singlet states is defined as a linear combination of two Slater determinants, which contains only spin eigenfunctions with even quantum number. Using a self-consistent procedure based on the generalized Brillouin's theorem, the RHF , HPHF and PHF functions are deduced for the ground states of the Li^?, Be, B⁺, and C²⁺ systems, in a limited basis set. It is found that the HPHF function yields better energy values than the RHF function, very close to that of the PHF one. The HPHF scheme seems thus to be useful as a substitute for the PHF model, specially in the case of large electronic systems in which the latter method becomes unmanageable.     

Half-projected and projected Hartree-Fock calculations for singlet ground states. II. Lithium hydride

The half-projected Hartree–Fock function (HPHF ) for singlet states is defined as a linear combination of two Slater determinants which contains only spin eigenstates with even spin quantum numbers. The possible uses of such an approach for determining molecular properties are investigated computing the potential energy curve, binding energy, force constant, and dipole moment variation corresponding to the lithium hydride ground state. Full projected and restricted Hartree–Fock calculations (PHF and RHF ) are performed simultaneously for comparison purposes. It is found that the HPHF model yields very satisfactory results, very close to those of the PHF scheme. Both models predict properly the molecular behavior as a function of nuclear separation, whereas the RHF one fails. A discussion is given in terms of configuration equivalents. It is concluded that the HPHF scheme seems to be useful for determining molecular properties specially in the case of large systems in which the more sophisticated methods are unmanageable.     

An “All‐Green” Catalytic Cycle of Aqueous Photoionization

Hydrated electrons are highly aggressive species that can force chemical transformations of otherwise unreactive molecules such as the reductive detoxification of halogenated organic compounds. We present the first example of the sustainable production of hydrated electrons through a homogeneous catalytic cycle driven entirely by green light (532 nm, coinciding with the maximum of the terrestrial solar spectrum). The catalyst is a metal complex serving as a “container” for a radical anion. This active center is generated from a ligand through quenching by a sacrificial electron donor, is shielded by the complex such that it stores the energy of the photon for much longer than a free radical anion could, and is finally ionized by another photon to regenerate the ligand and recover the starting complex quantitatively. The sacrificial donor can be a bioavailable reagent such as ascorbic acid.     

Carbon–Nitrogen Bond‐Forming Reactions of Dialkyl Azodicarboxylate: A Promising Synthetic Strategy

Azodicarboxylates have found applications in electrophilic amination reactions and in pericyclic reactions. The nucleophilic trigger in Mitsunobu reactions, that is, the zwitterion formed from triphenylphosphine and dialkyl azodicarboxylate, has been utilized recently in various heterocyclic constructions. This Focus Review summarizes the potential utility of azodicarboxylates in various carbon–nitrogen bond‐forming reactions.     

Donor-Acceptor-Substituted Cyclic π-Electron Systems—Probes for Theories and Building Blocks for New Materials

Donor and acceptor substituents stabilize (4n)π-electron systems and destabilize those with (4n+2)π electrons. The same is true for the transition states of pericyclic reactions, which explains the appearance of dipolar intermediates in symmetry-allowed cycloadditions and sigmatropic rearrangements. Donor-acceptor-substituted semibullvalenes undergo rapid Cope rearrangement, as do tetraazabarbaralanes. In contrast, tetraazasemibullvalenes can not be isolated, since the isomeric tetrazocines always result. The usefulness of the donor-acceptor concept in preparative chemistry is demonstrated by numerous stable cyclic (4n)π-electron systems, like donor-acceptor-substituted cyclobutadienes, tetraaminobenzene, and p-benzoquinone dications, benzodiazepinyl anions, and donor-acceptor-substituted cyclopentadienyl cations and their heteroatom-containing analogues. The new compounds are of interest in the fields of organic metals and ferromagnets, nonlinear optics, and dyestuffs, among others.     

DFT study of pericyclic and pseudopericyclic thermal cheletropic decarbonylations. Evaluation of magnetic properties

A comprehensive B3LYP/6-31G** study of various thermal cheletropic decarbonylations was conducted. The complete pathway for each reaction was determined, and changes in magnetic susceptibility and its anisotropy were monitored with a view to estimating the aromatization associated to each process. This information, together with the energy and structural results, allowed us to clarify cases that are not clearly pseudopericyclic or pericyclic from previous work. Also, our results reveal that pericyclic reactions involve no disconnection in the cyclic array of overlapping orbitals. Therefore, a pseudopericyclic reaction involves at least one such disconnection. In any case, pseudopericyclic reactions involving two disconnections exhibit much clearer features, which facilitates their classification.     

Ethynylogization of a coarctate fragmentation

Coarctate reactions form a separate class of elementary closed-shell processes in addition to polar and pericyclic reactions. Hence, they also follow a different homology principle. Whereas vinylogous polar and pericyclic reactions differ in the length of the reacting system by a double bond, coarctate reactions can be homologized (ethynylogized) by extending a known system by a triple bond. The prediction, which is based on theoretical considerations, is confirmed experimentally by the fragmentation of cyclopropylethynyl nitrene to cyano acetylene and ethylene, a reaction that is "ethynyloguous" to the known fragmentation of cyclopropyl nitrene to ethylene and HCN.     

Analysis of the half-projected Hartree–Fock function: Density matrix,natural orbitals,and configuration interaction equivalence

The half-projected Hartree–Fock function for singlet states (HPHF ) is analysed in terms of natural electronic configurations. For this purpose the HPHF spinless density matrix and its natural orbitals are first deduced. It is found that the HPHF function does not contain any contribution from odd-times excited configurations. It is seen in addition, in the case of the singlet ground states, this function is approximately equivalent to two closed-shell configurations, although the nature of the excited one depends on the nuclear geometry. An example is given in the case of the LiH ground state. Finally, the application of this model for studying systems of more that two atoms is criticized.     

Dienamine‐Induced Divinylcyclopropane–Cycloheptadiene Rearrangements

Sigmatropic rearrangements constitute an important group of pericyclic reactions. In contrast to cycloaddition reactions, examples of catalytic variants of electrocyclic reactions and sigmatropic rearrangements are still scarce in the chemical literature. Herein, we report the first organocatalytic Cope rearrangement of in situ‐generated divinylcyclopropanes. The reactive motif was generated by condensation of 4‐(2‐vinylcyclopropyl)but‐2‐enal derivatives with a secondary amine catalyst to form a transient dienamine. The cycloheptadiene products could be obtained in high yield and excellent diastereoselectivity. Importantly, the reaction was demonstrated to be stereospecific, proceeding under mild conditions, while exhibiting broad functional group tolerance.     

Electronic energy inequalities for isoelectronic molecular systems

Within the Born-Oppenheimer approximation an inequality relation is derived between points of electronic energy hypersurfaces of pairs of isoelectronic molecules. The inequality is directly applicable to point pairs fulfilling a symmetry criterion for the nuclear frameworks and it may be extended to coordinate domains on both hypersurfaces. The result is applied to special examples of conformational problems, dissociation reactions and impurity —vacancy centres in solid clusters.     

Sterilization of foods with low-energy electrons (“soft-electrons”)

Electrons with an energy of 300 keV or lower were defined as “Soft-electrons”, which showed several advantages over conventional irradiation with gamma-rays or high-energy electrons in decontamination of grains and spices. Energies of electrons necessary to reduce microbial loads to levels lower than 10 CFU/g were 60 keV for brown rice, 75 keV for wheat, 100 keV for white pepper, coriander and basil, 130 keV for buckwheat, 160 keV for rough rice, and 210 keV for black pepper. Electrons with such energies did not significantly influence the quality.     

A simple model for predicting the effect of substituents on the rates of thermal pericyclic reactions

An operationally useful model is proposed for predicting the qualitative effect of substituents on the rates of thermal pericyclic reactions. The method provides a unified interpretation of several previously unrelated experimental observations. It also enables the prediction of substituent effects in pericyclic reactions for which experimental data are not yet available.     

Cascading pericyclic reactions: building complex carbon frameworks for natural product synthesis

Tandem reactions have emerged as powerful strategies to synthesize complex structures, in particular, processes involving pericyclic reactions. This article describes recent advancement by our group in the development of novel tandem pericyclic reactions aimed at constructing diterpene frameworks.     

从Hückel分子轨道理论导出激发态周环反应的奇偶定则

本文从Huckel分子轨道理论,导出激发态周环反应的奇偶定则。在推导中Huckel久期行列式的非零元均视为反应坐标的函数,对其数值不作特殊假定。本文的推导也有助于对周环反应的Mobius-Huckel结构理论获得进一步的了解。     

The half-projected Hartree-Fock method

A SCF method based on the solution of two eigenvalue problems, in the same manner as for the normal UHF procedure, is formulated for determining the half-projected Hartree-Fock (HPHF) function for singlet ground states of molecules, the HPHF function being defined as a linear combination of two Slater determinants containing only spin eigenfunctions with even quantum number. A computer program has been written and is described, and results are presented for two simple linear molecules. An important part of the correlation energy is obtained for these molecules.