Sigmatropic hydrogen shifts in aryltetraphenylcyclopentadienes

Dynamic NMR has revealed intramolecular migrations of hydrogen atom over the periphery of the five-membered ring in 5-(p-tolyl)-1,2,3,4-tetraphenylcyclopentadiene in a deuteronitrobenzene solution with energy barrier ΔG ₁₈₀ ^≠ = 24.8 kcal/mol. Quantum-chemical DFT calculations B3LYP/6-311++G** have shown that such migrations in 1,2,3,4,5-pentaphenylcyclopentadiene in the gas phase occur in a chiral conformation of propeller type by the mechanism of 1,5-sigmatropic hydrogen shifts with retention of configuration through asymmetric transition state with energy barrier ΔE _ZPE ^≠ = 25.9 kcal/mol. Enantiomers P and M can readily interconvert into each other (ΔE _ZPE ^≠ = 3.9 kcal/mol) owing to synchronous flip rotations of the phenyl groups.     

Quantum-chemical simulation of structure and conformational flexibility of 5,7-di(<Emphasis Type="Italic">tert</Emphasis>-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone

Density functional theory [DFT B3LYP/6-311++G(d,p)] simulation has revealed stable tautomers and conformers of polydentate ligand system based on 5,7-di(tert-butyl)-2-(8-hydroxyquinolin-2-yl)-1,3-tropolone with different structures of the coordination nodes, capable of formation of metal chelates. It has been shown that the tautomeric NH- and OH- forms with exo and endo location of the hydroxy group in the quinoline fragments (close in energy, ΔE_ZPE = 0.2–2.4 kcal/mol) are stabilized by intramolecular hydrogen bonds. Energy barriers of the interconversion of these forms via rotation about the C–OH bond of the phenolic fragment are of ΔE_ZPE^≠ = 2.1–4.2 kcal/mol, whereas the barrier of rotation about the bond between the quinoline and tropolone fragments is higher (ΔE_ZPE^≠ = 18.2 and 19.6 kcal/mol).     

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLIII. Solvent effect on activation parameters of dehydrochlorination of 3-chloro-3-methylbut-1-ene. Correlation analysis of solvation effects

The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in 17 aprotic and 13 protic solvents, ν = k[C₅H₉Cl], was studied by the verdazyl method. In aprotic solvents, the electrophilicity, ionizing power, and cohesion of solvents decrease ΔG ^≠ by increasing ΔS ^≠. The nucleophilicity and polarizability increase both ΔH ^≠ and ΔS ^≠ to equal extent and therefore do not affect ΔG ^≠. In protic solvents, the solvent nucleophilicity increases ΔH ^≠ to a greater extent than ΔS ^≠, and the overall effect of the nucleophilic solvation is small and negative.     

Theoretical and experimental studies on the structure and isomerization of isocyano and cyano cyclopolyenes

Quantum-chemical calculations in terms of the density functional theory showed that cyclopolyenyl isocyanides RNC are considerably less stable than the corresponding cyanides and that they are capable of undergoing RNC → RCN isomerization according to both 1,2-shift mechanism (cyclopropenyl and cyclopentadienyl isocyanides; ΔE ^≠ = 35.0 and 37.5 kcal/mol, respectively) and previously unknown 2,5-sigmatropic shift mechanism (cycloheptatrienyl isocyanide, ΔE ^≠ = 26.4 kcal/mol). Migration of cyano group in the cyclopentadiene and cycloheptatriene systems follows the 1,5-sigmatropic shift pattern. The activation barrier to 1,5-shift of cyano group around the cycloheptatriene ring was estimated by dynamic NMR in deuterated nitrobenzene (ΔG _190°C ^≠ = 26.5 kcal/mol).     

Quantum chemical modeling of superbase-catalyzed reactions of acetophenone and methyl mesityl ketone with acetylene

The mechanisms of furan formation and the competitive reaction of 1,5-diketone formation from 1-mesitylbut-2-en-1-one (1a) and 1-phenylbut-2-en-1-one (1b) were studied using the B3LYP/6-311++G**//B3LYP/6-31+G* quantum chemical approach taking into account solvation effects in terms of the IEFPCM model. The addition to 1a of the ethynide ion to form β-acetylene enolate ion (2a) occurs with the activation energy ΔG^≠ = 13.3 kcal mol^–1, while the further intramolecular O-vinylation with the closure of 2a to the dihydrofuran cycle of the dihydrofuran carbanion is the rate-determining step in the reaction pathway of substituted furan formation needing the activation energy G^≠ = 19.5 kcal mol^–1. The alternative addition of the mesitylethenolate ion to 1a to form the 1,5-diketone carbanion is associated with a higher (by 3.4 kcal mol^–1) free activation energy than that of ethynide ion addition. In contrast, in the case of 1b, the addition at the C=C bond of the phenylethenolate ion rather than of the ethynide ion to form the 1,5-diketone carbanion is more preferable, since its activation free energy is 1.5 kcal mol^–1 lower. The quantum chemical calculation results are consistent with the experimental data.     

Quantum chemical study of the structures and dynamic behavior of tricarbonyl complexes of Group 6 metals (Cr,Mo, W) with polyaromatic hydrocarbons using the density functional theory

The quantum chemical study of the mechanism was performed for tricarbonyl η⁶-complexes of coronene I-M and kekulene II-M (M = Cr, Mo, W) by the density functional method. The activation barriers of η⁶,η⁶-interring haptotropic rearrangements (IHR), being the migration of the metaltricarbonyl group M(CO)₃ from one six-membered aromatic ring to another, were determined. The processes of η⁶,η⁶-IHR in the metal tricarbonyl complexes with relatively high polycyclic aromatic hydrocarbons (PAH) I and II occur with close energy barriers (ΔG^≠ ≈ 20—25 kcal mol^–1), which are lower than the barriers (ΔG^≠ ~ 30 kcal mol^–1) of similar transformations measured or calculated earlier for the chromium tricarbonyl complexes of naphthalene and its derivatives and other PAH. For the molybdenum tricarbonyl complexes the activation barriers of η⁶,η⁶-IHR decrease additionally by ~ 5 kcal mol^–1 compared to those for the chromium tricarbonyl complexes, whereas for the tungsten tricarbonyl complexes they increase again and become approximately equal to the activation barriers of similar chromium tricarbonyl complexes. All stationary states on the potential energy surface determining the mechanism of η⁶,η⁶-IHR are characterized by a decrease in hapticity compared to the initial and final complexes.     

Structure and dynamics of eight-membered <Emphasis Type="Italic">P,N</Emphasis>-heterocycles in solution

Conformational analysis of eight-membered P,N-heterocycles (P₂^NR₂^R') in the solution has been performed by means of NMR spectroscopy and quantum chemical calculations. The equilibrium of the С₂-symmetrical (crown, major) and С_s-symmetrical (chair-boat, minor) forms of the compounds has been revealed, the transition barriers being of about 12 kcal/mol. The presence of an aromatic substituent at the nitrogen atom significantly shifts the equilibrium towards the dominating form.     

Compensation effect in reactions between <Emphasis Type="Italic">trans</Emphasis>-4,4'-dinitrostilbene oxide and arylsulfonic acids

The effect structure and temperature have on the rate and free activation energy of reactions between trans-4,4'-dinitrostilbene oxide and Y-substituted arylsulfonic acids YC₆H₄SO₃H in a mixture of dioxane with 1,2-dichloroethane (7: 3 vol/vol) at 265, 281, and 298 K is studied. It is found that as a result of the nonadditivity of the joint effect of substituents Y and temperature on the rate of the process of oxirane ring opening, the cross reaction series exhibits isoparametric properties in the aspect of enthalpy–entropy compensation. This allows the experimental determination of an isoparametric point with respect to the constant of substituent Y (σ_Y^IP= 0.52), in which activation entropy ΔS^≠ = 0 and free activation energy ΔG^≠ do not depend on temperature (ΔG^≠ = ΔH^≠), and to conduct the transition through this point with inversion of the order of the effect temperature has on the value of ΔG^≠ as a result of reversing the sign of ΔS^≠: in the series Y (σ_Y) = 4-OCH₃ (–0.27), 4-CH₃ (–0.17), H (0), 4-Cl (0.23), and 3-NO₂ (0.71), the values of ΔS^≠ (J/(mol K)) are–140,–119,–85,–42, and 44, respectively. The possibility of using isoparametric points as quantitative mechanistic criteria is demonstrated.     

Mechanisms of circumambulatory rearrangements of 5-halogeno-1,2,3,4,5-pentamethoxycarbonylcyclopentadienes

Possible paths of halogen atom migration in 5-halogeno-1,2,3,4,5-pentamethoxycarbonylcyclopentadienes were studied using the density functional theory. The calculations revealed preferential 1,5(in comparison with 1,3-) sigmatropic shifts of halogen atoms along the perimeter of the five-membered ring with the energy barriers ΔE _ZPE ^≠ = 42.9, 26.9, 19.8, and 15.4 kcal mol^–1 for the fluoro-, chloro-, bromo-, and iodosubstituted derivatives, respectively. The calculated charges of halogen atoms in the structures of transition states for 1,5-shifts change from negative for the fluorine atom to positive for the iodine atom (–0.356 (F), 0.019 (Cl), 0.052 (Br), 0.184 e (I)). The migration capacity increases in the order F < Cl < Br < I with an increase in the atomic radius of halogen.     

Synthesis of Fluorine-Containing Derivatives of 5-Arylisoxazoles and 4,5-Dichlorothiazole

Alkylation of fluorophenols and phenolic aldehydes with 5-phenyl(p-tolyl)-3-chloromethylisoxazole under the Williamson reaction conditions afforded the corresponding ethers. Condensation of the resulting phenolic aldehyde derivatives and 5-phenyl(p-tolyl)isoxazole-3-carbaldehydes with p-fluoroaniline resulted in fluorine-containing azomethines. Acylation of fluorinated amines with 5-(p-tolyl)isoxazole- and 4,5- dichloroisothiazole-3-carbonyl chlorides gave rise to fluorinated amides bearing isoxazole and isothiazole fragments, representatives of which showed a synergistic effect in compositions with insecticides.     

Synthesis and biologic activity of aryloxyaminopropanoles based on substituted <Emphasis Type="Italic">p</Emphasis>-tolylalkylamines

Reaction of 2,2-dimethyl-2-(p-tolyl)ethyl-, 1-p-tolylcyclopenthylmethyl- and 4-(p-tolyl)tetrahydro-2Н-pyran-4-ylmethylamines with phenoxymethyloxiranes substituted in the aromatic ring afforded new aryloxypropanolamines, some of which show moderate adrenoblocking activity.     

Synthesis,Structures, and Antibacterial Activities of Two Methanol Coordinated Zinc Complexes with Tetradentate Bis-Schiff Bases

Two methanol coordinated zinc(II) complexes, [Zn(L¹)(MeOH)] (I) and [Zn(L²)(MeOH)] (II), where L¹ and L² are the dianionic form of N,N'-bis(5-fluorosalicylidene)ethane-1,2-diamine and N,N'-bis(5-fluorosalicylidene)propane-1,2-diamine, respectively, have been synthesized and characterized by physical chemical methods and single crystal X-ray diffraction (CIF files CCDC nos. 950907 (I) and 950908 (II)). Crystallographic data for I: triclinic, P \(\bar 1\), a = 8.8765(8), b = 9.6577(9), c = 10.5117(9) Å, α = 114.590(2)°, β = 91.648(3)°, γ = 97.114(3)°, V = 809.87(13) ų, Z = 2, R ₁ = 0.0307, wR ₂ = 0.0698. Crystallographic data for II: orthorhombic, Pca2₁, a = 22.946(2), b = 7.6942(7), c = 9.6234(8) Å, V = 1699.0(2) ų, Z = 4, R ₁ = 0.0320, wR ₂ = 0.0676. X-ray crystal structural study indicated that the coordination environment around each zinc(II) atom in the complexes is a five-coordinated distorted pyramid in which the apical position is occupied by a methanol oxygen atom, and the basal plane is defined by the nitrogen and oxygen donor atoms of the Schiff base ligand. The antibacterial activities of the complexes were assayed.     

(Iodomethyl)fluorosilanes: Synthesis and Reactions

The methods of synthesis of bifunctional (iodomethyl)fluorosilanes of general formula ICH₂SiMe_nF_3–n (n = 0, 2) have been elaborated; the structure was proved by ¹H, ¹³C, ²⁹Si NMR spectroscopy. The reaction of (iodomethyl)dimethylfluorosilane with O-trimethylsilyl derivative of N,N'-dimethylhydrazide of trifluoroacetic acid gives rise to the formation of 2,2,4,4-tetramethyl-6-(trifluoromethyl)-3,4-dihydro-2Н-1,4,5,2-oxadiasilin-4-ium iodide with tetracoordinate silicone atom.     

Molecular structure,quantum chemical investigation,and thermal behavior of (DNAZ-CO)<Subscript>2</Subscript>

Bis-(3,3-dinitroazetidinyl)-oxamide ((DNAZ-CO)₂) is an acyl derivative of 3,3-dinitroazetidine (DNAZ). It is prepared and its crystal structure is determined. The crystal is orthorhombic, Fdd2 space group, a = 13.136(14) Å, b = 19.48(3) Å, c = 10.326(14) Å, V = 2642 (6) ų, Z = 8. A density functional theory (DFT) method of the Amsterdam Density Functional (ADF) package is used to calculate the geometry, frequencies, and properties. The optimized geometry, frontier orbital energy, and main atomic orbital percentage are obtained. The thermal behavior is studied under a non-isothermal condition by DSC and TG/DTG methods. The apparent activation energy (E _a) and pre-exponential factor (A) of the exothermic decomposition reaction of (DNAZ-CO)₂ are 164.10 kJmol^?1 and 10^13.38 s^?1 respectively. The critical temperature of thermal explosion is 272.20°C. The values of ΔS ^≠, ΔH ^≠, and ΔG ^≠ of this reaction are 6.44 Jmol^?1·K^?1, 163.76 kJmol^?1 and 160.34 kJmol^?1 respectively.     

A study of the polymerization of ethylene on Ti(III) monocyclopentadienyl compounds by the density functional theory method

Density functional theory was used to study model ethylene reactions with CpTi^IIIEt⁺A^? (A^? = CH₃B(C₆F₅) ₃ ^? , or B(C₆F₅) ₄ ^? ; A^? can be absent) compounds. The polymerization of ethylene on an isolated CpTiEt⁺ cation is hindered because of equilibrium between the CpTi(C₂H₄)Et⁺ primary complex and the primary product of CpTiBu⁺ insertion. At the same time, the polymerization of ethylene on CpTiEt⁺A^? ion pairs (A^? = CH₃B(C₆F₅) ₃ ^? or B(C₆F₅) ₄ ^? ) is thermodynamically allowed (ΔE from ?26.2 to ?25.6 kcal/mol and ΔG ₂₉₈ from ?10.9 to ?10.4 kcal/mol) and is not related to overcoming substantial energy barriers (ΔE ^# = 8.2?12.3 kcal/mol and ΔG ₂₉₈ ^≠ ) = 7.8?13.3 kcal/mol). The degree of polymerization can be low because of the effective occurrence of polymer chain termination by hydrogen transfer from the polymer chain to the monomer.     

<Emphasis Type="Italic">Ab initio</Emphasis> quantum-chemical study of vinylation of pyrrole and 2-phenylazopyrrole with acetylene in a KOH/DMSO system

Interaction between pyrrole and its 2-vinyl, 2-azo, and 2-phenylazo derivatives with acetylene in the gas phase and DMSO was studied using the MP2/6-311++G**//MP2/6-31G* ab initio approach and including the solvation effects within the framework of the continuum model. Possible reasons are considered for the hindered character of direct vinylation of azopyrroles with acetylene in superbasic media. The introduction of the azo group in the 2 position of the pyrrole ring leads to the increased stability of the pyrrole anion and increased acidity from pK _a = 22.1 for pyrrole and pK _a = 20.5 for vinylpyrrole to pK _a = 16.6 and 16.4 for 2-azopyrrole and 2-phenylazopyrrole, respectively. The binding energy between the pyrrole anion and the acetylene molecule decreases concurrently. The heat of formation of the pyrrole anion adducts with acetylene changes from ΔH = 4.8 kcal/mol for pyrrole to ΔH = 22.4 kcal/mol for 2-phenylazopyrrole. For all anion adducts under study, preferable isomers are Z isomers formed by the interaction of pyrrole anions with the cis-distorted acetylene molecule, but the formation of the E isomers corresponds to a lower activation barrier, which explains known Z stereoselectivity of the nucleophilic addition to monosubstituted acetylenes. When an azo group is introduced, the reaction becomes more endothermal, and the energy barriers to the formation of both Z and E isomers increase. Among other reasons for lowering of the activity of 2-arylazopyrroles during vinylation we consider possible reaction of acetylene addition at the most remote nitrogen atom of the azo group and participation of the anion center in cation chelation (K⁺ in the calculation).     

Synthesis,crystal structure,and kinetics of thermal decomposition of the Zn(II) complex with vanillin

A complex [Zn(C₈H₇O₃)₂(H₂O)₂] (C₈H₈O₃ is vanillin) has been synthesized and characterized by IR, elemental analysis, and X-ray diffraction single-crystal analysis. The crystals are monoclinic, space group C2/c, a = 22.236(8) Å, b = 10.594(2) Å, c = 7.8190(16) Å, α = 89.90(3)°, β = 106.87(4)°, γ = 89.99(3)°, V = 1762.6(8) ų, Z = 4, F(000) = 832, S = 1.079, ρ _c = 1.521g cm^?3, R = 0.0221, R _w = 0.0604, μ = 1.433 mm^?1. The Zn²⁺ ion is six-coordinated with a distorted octahedron geometry. The complex forms a three-dimensional network through intermolecular hydrogen bonds. The thermal decomposition kinetics of the complex for the second stage was studied under non-isothermal conditions by the TG and DTG methods. The kinetic equation can be expressed as dα/dt = Ae^?E/RT 2(1 ? α)[1 ? ln(1 ? α)]^1/2. The kinetic parameters (E, A), activation entropy ΔS ^≠, and activation free-energy ΔG ^≠ were also gained.     

μ<Subscript>2</Subscript>-Oxobis[(aroxo)tris(<Emphasis Type="Italic">para</Emphasis>-tolyl)antimony]: Synthesis and Structure

µ₂-Oxobis[(2,4,6-tribromophenoxo)tris(para-tolyl)antimony] (I), µ₂-oxobis[(2,3,4,5,6-pentachlorophenoxo) tris(para-tolyl)antimony] (II), and µ₂-oxobis(2,4-dinitrophenoxo)tris(para-tolyl)antimony] (III) have been synthesized with high yields by the reaction of tris(para-tolyl)antimony with 2,4,6-tribromo-, 2,3,4,5,6-pentachloro-, and 2,4-dinitrophenol, respectively, in ether in the presence of tert-butylhydroperoxide. The Sb atoms in complexes I, II, and III have a distorted trigonal bipyramidal coordination with the aroxyl ligands and the bridging oxygen atom in axial positions. The central Sb–O–Sb moiety in molecules of complexes I–III has an angular structure.     

Quality control of Amazonian cocoa (<Emphasis Type="Italic">Theobroma cacao</Emphasis> L.) by-products and microencapsulated extract by thermal analysis

The mineralogical characterization and pyrolysis kinetics of raw oil shale from Moroccan Rif region and the corresponding bitumen-free material were investigated using various analytical techniques. The structural analysis results showed the siliceous character of mineral matrix and the presence of complex organic components in both oil shales studied. Non-isothermal pyrolysis kinetic measurements indicated that bitumen-free oil shale exhibits a single behavior pyrolysis in the oil-producing stage as compared to raw oil shale. The activation energies estimated by using isoconversional methods reveal that the pyrolysis reaction occurred by one-step kinetic process. The kinetic parameters, determined from a nonlinear fitting method using various kinetic models g(α) and iterative Kissinger–Akahira–Sunose energy calculations, reveal that the pyrolysis mechanism is well described by the nth order kinetics (Fn), with n = 1.071, for bitumen-free oil shale, and n = 1.550, for kerogen of raw oil shale. The mechanism of the whole pyrolysis process of raw oil shale seems not to be affected by the elimination of bitumen, but only some kinetic changes have been recorded in the reaction order mechanism. The process pyrolysis is represented by independent reactions and consequently considered as parallel processes. Besides, the thermodynamic functions of activated complexes (?S ^≠ , ?H ^≠ and ?G ^≠) were also calculated and the pyrolysis is found as non-spontaneous process in agreement with the thermal analysis data.     

Low-dimensional compounds containing cyano groups. XV. Preparation,crystal structure and spectral properties of three copper(II) nitrosodicyanomethanide complexes

We describe the preparation and crystal structures of the ionic complexes [Cu(bipy)₂{ONC(CN)₂}]CF₃SO₃ (1b), [Cu(phen)₂{ONC(CN)₂}]PF₆ (2p) and [Cu(bipy)₂{ONC(CN)₂}]PF₆ (2b). In the complex cations [Cu(L)₂{ONC(CN)₂}]⁺ (L is 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen)) the two molecules of bipy or phen coordinate to the copper atom through two nitrogen atoms along with the oxygen atom of the nitrosodicyanomethanide anion, ONC(CN) ₂ ^? , to form a {CuN₄O} chromophore with a distorted square pyramidal coordination sphere in (1b) and (2b) and a distorted trigonal bipyramidal geometry in (2p). The basal plane in (1b) and (2b) is formed by an oxygen atom coordinated at the Cu1–O1 distance of 1.990(2) and 2.002(2) Å, respectively, and three nitrogen atoms coordinated to the copper atom at similar distances with the average of 2.01(2) and 2.00(3) Å, respectively. The axial position is occupied by the fourth N atom at the longer distance of 2.222(2) and 2.185(2) Å, respectively. The trifluoromethanesulfonate anion (triflate), CF₃SO ₃ ^? , in (1b) might be considered as very weakly coordinated in the opposite axial position (Cu1–O2 = 2.719(2) Å). The equatorial plane in (2p) is formed by an oxygen atom coordinated at the Cu1–O1 distance of 1.975(3) Å, and two nitrogen atoms from different phen molecules coordinated to the copper atom at the same distance within 2 σ with the average distance of 2.124(2) Å. The axial positions are occupied by remaining two nitrogen atoms coordinated at shorter distance (average Cu–N = 1.99(3) Å). The hexafluorophosphate anions, PF ₆ ^? , in (2p) and (2b) remain uncoordinated. Besides the ionic forces, the structures of (2p) and (2b) may be stabilized by very weak C–H···F whereas the structure of (1b) by very weak C–H···F, C–H···O and C–H···N hydrogen bonds. The structural–spectral correlations are also discussed.