Tricyclo[2.2.0.0]hexane: a new hypothetical molecule which should have only one inverted carbon atom

HF and MP2 calculations with the 6-31G** and 6-311G** basis sets and those at MP2/cc-pVTZ level were carried out for the hypothetical tricyclo[2.2.0.0^1,3]hexane. The results indicate that the molecule under study should have one carbon atom with highly unusual inverted configuration. The analysis of the vibrational frequencies of this molecule as well as the analysis of its plausible decomposition routes performed at the DFT level indicate that this unique molecule could be a plausible synthetic target.     

A quest for triplet silylenes XHSi3 at ab initio and DFT levels (X = H, F, Cl and Br)

Four ground state triplet silylenes are found among 30 possible silylenic XHSi₃ structures (X = H, F, Cl and Br), at seven ab initio and DFT levels including: B3LYP/6-311++G^∗∗, HF/6-311++G^∗∗, MP3/6-311G^∗, MP2/6-311+G^∗∗, MP4(SDTQ)/6-311++G^∗∗, QCISD(T)/6-311++G^∗∗ and CCSD(T)/6-311++G^∗∗. The latter six methods indicate that the triplet states of 3-flouro-1,2,3-trisilapropadienylidene, 1-chloro-1,2,3-trisilapropargylene and 3-chloro-1,2,3-trisilapropargylene are energy minima. These triplets appear more stable than their corresponding singlet states which cannot even exist for showing negative force constants. Also, triplet state of 1-flouro-1,2,3-trisilapropargylene is possibly accessible for being an energy minimum, since its corresponding singlet state is not a real isomer. Some discrepancies are observed between energetic and/or structural results of DFT vs. ab initio data.     

Comparative study of calix[4]arene derivatives: implications for ligand design

The first comparative theoretical study of three parent calix[4]arene analogues (calix[4]arene, thiacalix[4]arene, and homooxacalix[4]arene) has been performed using molecular dynamic simulations and density functional theory (MPWB1K/6-311G^∗∗//B3LYP/6-311G^∗∗) methods. The theoretical observations herein including optimized geometry, polarity, and atomic charge data provide that homooxacalix[4]arene would offer more efficient platform for metal ion recognition compared to thiacalix[4]arene or calix[4]arene.     

Syntheses and conformational analyses of mono- and trans-1,4-dialkoxy substituted cyclohexanes—the steric substituent/skeleton interactions

Mono- and trans-1,4-dialkoxy substituted cyclohexanes (alkyl=Me, Et, i-Pr, t-Bu) were prepared using the solvomercuration-demercuration (SM-DM) procedure. The axial?axial and axial,axial?equatorial, equatorial conformational equilibria of the products were studied by low temperature ¹H and ¹³C NMR spectroscopy in CD₂Cl₂. The structures and relative energies of the participating conformers were calculated at both the B3LYP (6-311G^∗//6-311+G^∗) and MP2 (6-311+G^∗//6-311G^∗) levels of theory. In the case of DFT, good correlations of ΔG^o_calcd versus ΔG^o_exptl were obtained. Both the structures and the energy differences of the conformers have been discussed with respect to established models of conformational analysis, viz. steric and hyperconjugative interactions. In addition, ¹J_H,C coupling constants were considered with respect to the hyperconjugation present.     

Dihydrodiazepine aus valenzisomeren diazatricyclo[3.2.0.0.2,4]Heptanen1)

Cycloaddition of diazoalkanes to diazabicyclo[2.2.0]hexenes and subsequent extrusion of nitrogen afford diazatricyclo[3.2.0.0^2,4]heptanes that are easily valence isomerized to novel dihydrodiazepines.     

Silver(I) induced rearrangement of the benzotricyclo[3.2.0.02,7]heptene ring system

The silver ion promoted isomerization of several substituted benzotricyclo- [3.2.0.0^2,7]heptenes to benzonorbornadienes has been found to readily occur.     

The quinones of bicyclo[3.1.0]hexatriene: A computational study of their chemistry and thermochemistry

Quinones of bicyclo[3.1.0]hexa-1,3,5-triene were examined computationally. The six compounds considered were the five possible classical and one non-classical quinone: bicyclo[3.1.0]hexa-1(6),4-diene-2,3-dione (and its monocyclic isomer with a long trans-annular bond), bicyclo[3.1.0]hexa-1(5),3-diene-2,6-dione, bicyclo[3.1.0]hexa-1,4-diene-3,6-dione (and its monocyclic isomer with a long trans-annular bond), and bicyclo[3.1.0]hexa-1(5),4-diene-2,4-dione-3,6-diyl, a non-classical (non-Kekulé) zwitterion. The two long trans-annular bond structures are akin to that found for m-benzyne. Geometries were calculated (BLYP/6-31G¹, CASSCF(2,2)/6-31G¹, MP2/6-31G¹) and electronic structural inferences were made from the geometries. Also calculated were relative energies and heats of formation (CBS-QB3), singlet and triplet energies (BLYP/6-31G¹), and ionization energies and electron affinities (HF/6-311+G⁷//BLYP/6-31G¹). The NICS(1) calculations were performed as a probe of the aromaticity of the diverse quinones.     

A DFT study on pyridine-derived N-heterocyclic carbenes

To appreciate the chemistry of N-heterocyclic carbenes (NHCs), eight carbenic tautomers of pyridine (azacyclohexadienylidenes) are studied at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G^∗ and B3LYP/6-311++G^∗∗//B3LYP/6-31+G^∗ levels of theory. Various thermodynamic parameters are calculated for these minima, along with a kinetic focus on carbene-pyridine tautomerization. Appropriate isodesmic reactions show stabilization energies of 2-azacyclohexa-3,5-dienylidene (1) and 4-azacyclohexa-2,5-dienylidene (6) as 119.4 and 104.1 kcal/mol, rather close to that of the synthesized 1,3-dimethylimidazol-2-ylidene (129.2 kcal/mol). Three different mechanisms are suggested for the tautomerizations including: [1,2]-H shift, [1,4]-H shift, and three sequential [1,2]-H shifts. The calculated energy barrier for [1,2]-H shift of 1 is 44.6 kcal/mol, while the first [1,2]-H shift for the proposed sequential mechanism of 6 requires 65.1 kcal/mol. Three preliminary minimum templates are introduced, which may possess the potential of synthetic consideration: 2,6-di(X)-3,5-dichloro-4-azacyclohexa-2,5-dienylidene for X=Mes, t-Bu, and Ad.     

Nitrochlorination of methyl tricyclo[4.1.0.02,7]heptane-1-carboxylate and phenyl tricyclo[4.1.0.02,7]hept-1-yl sulfone

Treatment of methyl tricyclo[4.1.0.0^2,7]heptane-1-carboxylate and phenyl tricyclo[4.1.0.0^2,7]hept-1-yl sulfone with a ～1:8 mixture of N₂O₄ and NOCl in diethyl ether at ?5 to 0°C gave products of formal anti-addition of NO₂Cl at the central C¹-C⁷ bond. In the reaction with phenyl tricyclo[4.1.0.0^2,7]hept-1-yl sulfone nitryl chloride acts as an effective chlorinating agent; as a result, a mixture of diastereoisomeric syn- and anti-6,7-dichlorobicyclo[3.1.1]hept-6-yl phenyl sulfones at a ratio of 7.5:1 is formed.     

Theoretical calculation of heat of formation for a number of moderate sized fluorinated compounds

Hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs) are considered to be potential replacements for chlorofluorocarbons (CFCs). The values of heat of formation have been calculated by using both the bond additivity corrected MP2 method (BAC-MP2/6-31G^∗∗) and atom additivity corrected MP2 method (AAC-MP2/6-31G^∗∗) for moderate sized HFCs and HFEs for which the literature values are not available. The reliability of the calculated values by these methods has been examined by comparing them with the values obtained with G2 and G2MP2 methods as well as with the literature values for a set of 42 simpler molecules.     

Gomadalactones A, B, and C: novel 3-oxabicyclo[3.3.0]octane compounds in the contact sex pheromone of the white-spotted longicorn beetle, Anoplophora malasiaca

The ether extract of the females of white-spotted longicorn beetle Anoplophora malasiaca showed activity as contact sex pheromone to males. The extract was fractionated, and a pheromonal activity was revealed only when three fractions; n-hexane, n-hexane/EtOAc 9:1, and EtOAc were blended. The relative structures of gomadalactone A, B, and C, three active components isolated from the EtOAc fraction, were determined by spectroscopic studies to be (1S^∗,4R^∗,5S^∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-7-en-2,6-dione, (1R^∗,4R^∗,5R^∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-7-en-2,6-dione, and (1S^∗,4R^∗,5S^∗,8S^∗)-5-hydroxy-4-[(E)-7-hydroxy-4-methylhept-3-enyl]-4,8-dimethyl-3-oxabicyclo[3.3.0]octan-2,6-dione, respectively.     

Hydrazones from hydroxy naphthaldehydes and N-aminoheterocycles: structure and stereodynamics

Schiff bases derived from 2-hydroxy-1-naphthaldehyde, or 1-hydroxy-2-naphthaldehyde, and different saturated N-aminoheterocycles have been prepared. Their structures have been elucidated in both solution and the solid state, including unequivocal X-ray diffraction analyses. Experimental data evidence the presence of imine (or hydrazone) structures as the most stable tautomers, while all attempts to switch to enamine (or enhydrazine) structures based on electronic and steric considerations were unsuccessful. A complete conformational analysis assisted by DFT calculations at B3LYP/6-31G^∗ and M06-2X/6-311++G^∗∗ levels has been performed on each series of representative structures.     

The effect of spiroadamantane substitution on the conformational preferences of N-Me pyrrolidine and N-Me piperidine: a description based on dynamic NMR spectroscopy and ab initio correlated calculations

Dynamic NMR spectroscopy and ab initio correlated calculations revealed that the attachment of a spiroadamantane entity at the C-2 position of N-methylpyrrolidine or N-methylpiperidine induces a severe steric crowding around nitrogen, which changes the conformational space of the heterocycle resulting in: (a) the complete destabilization of the N-Me(eq) conformer in spiranic structures; in contrast the N-Me(eq) conformer corresponds to the global minimum in N-methylpyrrolidine or N-methylpiperidine. The spiroadamantane structure raises the energy of the equatorial conformer because of the severe van der Waals repulsion between the N-Me(eq) group and adamantane C-H bonds. (b) The interconversion between the only populated enantiomeric N-Me(ax) conformers ax→[eq]→ax′; the interconversion eq→ax between N-Me(eq) and N-Me(ax) conformers, which are both populated, is observed in N-methylpyrrolidine or N-methylpiperidine. (c) The raising of ring and nitrogen inversion barriers ax→ts by ∼4-6 kcal mol⁻¹. The dynamic NMR study provides evidence that the most important process required for the enantiomerization between the axial N-Me conformers in spiropiperidine 4 and spiropyrrolidine 5 are different, i.e., a nitrogen inversion in 5 (9.10 kcal mol⁻¹) and a ring inversion in 4 (15.2 kcal mol⁻¹). While an enantiomerization interconverts N-Me axial conformers in spiropiperidine 5 and spiropyrrolidine 4, substitution of the pyrrolidine ring of 5 with a C-Me group effects a diastereomerization between two N-Me axial conformers and reduces effectively the nitrogen inversion barrier according to the protonation experiments and the calculations. In general, all the calculations levels used, i.e., the MM3, B3LYP/6-31+G^∗∗ and MP2/6-311++G^∗∗//B3LYP/6-31+G^∗∗, predict correctly the different stability of the local minima; however only MP2/6-311++G^∗∗//B3LYP/6-31+G^∗∗ was found to be reliable for the calculation of the nitrogen inversion barriers.     

On the conformational characteristics of the O,O,O-trimethyl selenophosphate molecule

Spectroscopic and theoretical studies were carried out for O,O,O-trimethyl selenophosphate molecule. DFT structural and vibrational calculations were performed at 6-311++G^∗∗ level. Ar/matrix-FTIR spectra were recorded. A coexistence of different conformers with C₃ and C₁ symmetries was detected at different temperatures. Spectral evidence of a lower energy C_s conformer was found. These conclusions are consistent with the results from DFT calculations. A tentative assignment of the features observed in the Ar/matrix-FTIR spectra is proposed.     

Regio- and π-facial selective Lewis acid interceded Diels-Alder reactions of α-dienyl-β-lactams: an indepth analysis

The regio-, diastereo-, and π-facial selective Lewis acid mediated Diels-Alder reactions of cis/trans-3-butadienyl-2-azetidinones with unsymmetrical dienophiles viz. methyl acrylate, dimethyl fumarate, and acrolein leading to the synthesis of diastereomerically pure and biologically potent 1,3,4-trisubstituted-2-azetidinones are reported. Theoretical calculations at HF/6-31G^∗∗ and 6-31G^∗∗/DFT levels have been performed to support the observed π-facial selectivity. The formation of diastereomerically pure ‘endo’ adducts is supported by the X-ray diffraction studies.     

A computational approach towards predicting π-facial selectivity in sterically unbiased olefins: an evaluation of the relative importance of electrostatic and orbital effects

Computational studies are presented to show that electrostatic interactions significantly impact the stereochemical outcome in electrophilic addition to a number of sterically unbiased alkenes. Transition states have been located for the reaction of different electrophiles with all the sterically unbiased alkenes studied here and the calculations effectively include interactions involving the σ and σ^∗ orbitals of the newly formed bond. Electrostatic interaction between the substrates and electrophiles was modelled by removing the electrophiles from the transition state geometry and placing the calculated charge at a distance from a selected atom as observed in TS structures. Electrostatic interactions between the electrophiles and the substrate seem to effectively determine the face selectivities in the systems studied and our model calculations indicate that it may not be important to invoke Cieplak type orbital interactions to rationalize the observed face selectivities. The face selectivities predicted for these alkenes and electrophiles with DFT B3LYP/6-31G^∗ and ab initio MP2/6-31G^∗ levels are generally in good agreement.     

Molecular structure of van der Waals complex of o- and m-methylaniline with CF3Cl, CF3H, CF4 and CH4 studied by laser induced fluorescence spectroscopy and ab initio calculations

The excitation LIF spectra of van der Waals complexes of o- and p-methylaniline and CF₃Cl, CF₃H, CH₄ and CF₄ in a supersonic free jet are reported. The spectra show a resolved structure characteristic due to the internal rotational transitions of the methyl group. The equilibrium geometries in the ground state of the complexes have been calculated at MP2/6-31+G^∗ level of calculation and the intermolecular interaction have been discussed.     

Synthesis, single crystal X-ray structure and optical properties of 3,4-dimethyl-dithieno[2,3-b:3′,2′-d]thiophene-7,7-dioxide

The synthesis of the title compound (4), starting from commercial 3-bromo-4-methylthiophene, is described. The single crystal packing mode of 4, as well as the absorption and photoluminescence properties in solution and in the solid state are reported and analyzed in relation to those of the isomeric 3,5-dimethyl-dithieno[3,2-b:2′,3′-d]thiophene-4,4-dioxide (7). The different reactivities of 4 and 7 towards bromination are analyzed in the light of the results of ab initio B3LYP/6-31G^∗ calculations on both compounds.     

Electrocatalytic cascade multicomponent assembling: stereoselective one-pot synthesis of the substituted 3-azabicyclo[3.1.0]hexane-1-carboxylate system from aldehyde, malononitrile, malonate and methanol

Electrolysis of aromatic aldehydes, malononitrile and malonate in methanol in an undivided cell in the presence of sodium bromide-sodium methoxide as double mediatory system results in the stereoselective formation of methyl (1R^∗,5R^∗,6R^∗) 6-substituted 5-cyano-4,4-dialkoxy-2-oxo-3-azabicyclo[3.1.0]hexane-1-carboxylates in 50-70% yields.     

Reaction products of methyl tricyclo[4.1.0.02,7]heptane-1-carboxylate and tricyclo[4.1.0.02,7]hept-1-yl phenyl sulfone with dinitrogen tetraoxide

The reaction of methyl tricyclo[4.1.0.0^2,7]hepatne-1-carboxylate with dinitrogen tetraoxide in diethyl ether at ?10 to 0°C, followed by treatment of the reaction mixture with methanol, gave approximately equal amounts of methyl exo,syn-6,7-dinitro-and exo-6-hydroxy-syn-7-nitrobicyclo[3.1.1]heptane-endo-6-carboxylates. Tricyclo[4.1.0.0^2,7]hept-1-yl phenyl sulfone reacted with dinitrogen tetraoxide under analogous conditions to produce a mixture of diastereoisomeric exo,syn-and endo,syn-6,7-dinitro-6-phenylsulfonylbicyclo-[3.1.1]heptanes and 6,6-dimethoxy-endo-7-nitrobicyclo[3.1.1]heptane at a ratio of 4.5:2:1. Probable factors responsible for the different stereoselectivities in the addition of N₂O₄ at the central C¹-C⁷ bond of the initial tricycloheptane compounds were discussed. The structural parameters of the dinitro ester and related dinitro sulfone were compared on the basis of the X-ray diffraction data.