D. V. Steglenko

The Stepwise Diels–Alder Reaction of 4-Nitrobenzodifuroxan with Danishefsky’s Diene

The Diels-Alder reaction of 4-nitrobenzodifuroxan (NBDF) with 1-methoxy-3-trimethylsilyloxy-1,3-butadiene has been investigated experimentally and theoretically. Treatment of NBDF with excess diene in chloroform at room temperature was found to afford a single product that contained a carbonyl functionality. Based on an X-ray structure and NMR spectroscopic data, the product appeared to be a result of the hydrolysis of the OSiMe(3) moiety of the thermodynamically more stable endo [2+4] cycloadduct, characterized by a cis arrangement of the MeO and NO(2) functionalities. In situ NMR investigations of the interaction were carried out at room temperature in CDCl(3) and at -40 °C in deuterated acetonitrile. Calculations at the B3LYP/6-31G* level in the gas phase and in acetonitrile were carried out under the assumption that the most stable cis conformation of the diene is also the most reactive in the interaction. The analysis revealed the NBDF/cis diene interaction involves the formation of a zwitterionic intermediate. Importantly, this intermediate is formed in two preferred conformations, which correspond to the endo and exo modes of approach of the reagents. Cyclization of these two identified conformations afforded the experimentally characterized endo and exo [2+4] cycloadducts. According to the calculations, the interconversion of the two conformers can either take place through a return to the pre-reaction complexes or it can occur by rotation through an intermediate conformation of lesser stability. In view of the stepwise character of the interaction, the possibility that the intermediate zwitterion is the result of the interaction between NBDF and the trans diene could not be excluded. Calculations carried out with the most stable and more populated s-trans conformer confirmed this idea and supported the role of the zwitterion in the overall interaction.

Usefulness of the σ-Aromaticity and σ-Antiaromaticity Concepts for Clusters and Solid-State Compounds.

In this Review we present examples of clusters, molecules, and solid-state compounds, for which the use of σ-aromaticity and σ-antiaromaticity concepts is essential for understanding of chemical bonding. We show that the bonding patterns in these σ-aromatic and σ-antiaromatic compounds are similar to those of the corresponding π-aromatic and π-antiaromatic chemical systems, respectively. Undoubtedly, σ-aromaticity helps us understand why the high symmetry isomers are the most stable among myriads of other potential structures. We also show that besides systems exhibiting either σ- or π-aromatic features, there are species, which can possess multiple aromaticity/antiaromaticity, or conflicting aromaticity patterns. We believe that the σ-aromaticity and σ-antiaromaticity concepts will be helpful in rationalizing chemical bonding, structure, stability, and molecular properties of chemical species in both organic and inorganic chemistry. We hope that they will also be useful for other areas of science such as material science, catalysis, nanotechnology, and biochemistry.

Computational modeling of LD LISC and LIESST rearrangements of a Fe(II) complex with phenanthroline modified by photochromic chromene

Quantum-chemical study (B3LYP*/6-311++G(d,p)) has shown that the Fe(II) complex containing the phenanthroline ligand with annelated chromene and dihydrobis(pyrazol-1-yl)borate anions is able to exhibit thermally induced spin crossover. The calculations predict that the magnetic properties of this compound can be controlled by irradiation (LIESST and LD LISC effects).

Experimental and quantum chemical study of the reactions of 2-methyloxirane with 5-alkyl-6-(2,6-dihalobenzyl)-2-thioxo1,2-dihydropyrimidine-4(3H)-one derivatives

The reactions of 5-alkyl-6-(2,6-dihalobenzyl)-2-thioxo-1,2-dihydropyrimidin-4(3H)-one with 2-methyloxirane in an alkaline medium afforded uracil and 2-[(2-hydroxyprop-1yl)sulfanyl]pyrimidin-4(3H)-one derivatives. The mechanism proposed for these transformations was studied using the DFT/B3LYP/311+G** quantum chemical method, and the influence of the Nathan—Baker effect on the chemical nature and composition of the reaction products was shown.

Quantum-chemical and NMR study of nitrofuroxanoquinoline cycloaddition

Using DFT/B3LYP and ab initio RHF quantum-chemical calculations in the triple-zeta basis set 6-31++G** the endo and exo cycloaddition mechanism for the interaction of ethyl vinyl ether, trimethylsilyloxybutadiene, or cyclopentadiene with 5-nitro-7,8-furoxanoquinolines was studied in details. Considering that both in solutions and crystals nitrofuroxanoquinoline exists as an inseparable mixture of two N-oxide tautomers, all cycloaddition processes were studied for both of them. The studied mechanisms practically do not depend on the location of the exocyclic oxygen atom in nitrofuroxanoquinoline molecule. At the first step of all reactions the conjugated nitroarene fragments C=C–N=O react with nucleophilic reagents following the mechanism of endo-[4+2] cycloaddition with inverse electronic demand. Further (in the cases of trimethylsilyloxybutadiene and cyclopentadiene) endo-[4+2] cycloadducts recyclize spontaneously according to the mechanism of [3,3] sigmatropic rearrangement into more thermodynamically stable, experimentally detected endo-[2+4] cycloadducts. Both endo-[4+2] and endo-[2+4] cycloadducts obtained from cyclopentadiene and nitrofuroxanoquinoline have been experimentally isolated and characterized. For this case, the kinetic and activation parameters of [4+2] → [2+4] transformation have been studied by 1H NMR method, which have shown an excellent agreement with quantum-chemical results. In all cases the exo processes are one-step reactions, less favorable kinetically than their endo competitors.

Intramolecular π-complexes based on nitroaryl derivatives of furotroponimine: structure and stereodynamics

Neutral aromatic electrophiles, such as trinitrobenzene and dinitrobenzofuroxane, covalently bound to the furotroponimine moiety form intramolecular through-space charge-transfer π-complexes both in solution and in the crystalline state. The kinetic and activation parameters of stereodynamic processes were determined by dynamic NMR spectroscopy. These parameters provide quantitative estimates of the kinetic stability of the p-complexes. The electronic and geometric characteristics of the π-complexes were calculated by the B3LYP/6-31G** method.

Self-association of α-tocopherol: a computer simulation

Self-association of α-tocopherol molecules in the gas phase was simulated using quantum chemical calculations in the B3LYP/6-31G(d,p)/6-31G(d) approximation. The influence of electric field on the energy characteristics of oligomerization of α-tocopherol was also considered.

Quantum chemical study of the self-assembly of tetrathiacalix[4]arenes and their oxygen analogs functionalized by hydrazide groups

The self-assembly of tetrathiacalix[4]arene and tetraoxacalix[4]arene molecules functionalized by hydrazide groups was studied by DFT calculations at B3LYP/6-31G(d,p) level. The calculations were performed for the cone and 1,3-alternate conformations. The associates of calix[4]arenes in the cone conformation are stabilized by multiple hydrogen bonds with inclusion of all hydrazide groups in the formation of the globule, which suppresses the formation of extended structures.