Conformational analysis and inversion process in some perhydrodipyrido[1,2‐b;1′2′‐e]‐1,4,2,5‐ dioxadiazines

The stereoselectivity in the cyclodimerization of several six‐membered cyclic nitrones has been investigated. The configurational/conformational analysis of the dimers (i.e. perhydrodipyrido[1,2‐b;1′2′‐e]‐1,4,2,5‐dioxadiazines) has been carried out by NMR spectroscopy. The NMR spectra of the dimers at lower temperatures indicated the presence of either a single or two invertomer(s). The nitrogen inversion barriers are determined using complete line–shape analysis. The invertomer ratios have been used to estimate the relative energies associated with the cis and trans ring fusion in these tricycles. A mechanistic rationale for the observed stereochemistry of the dimerization process has been presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Structures and conformations of heteroatom‐bridged calixarenes

The structures and conformational energies of twelve MeN‐, O‐, S‐, MeP‐, CO‐bridged homocalix[4]arenes and two kinds of O‐bridged alternate hybrid‐calix[4]arenes have been calculated at the B3LYP/6‐31G* level of theory. The 1,3‐alternate or twisted‐1,3‐alternate conformations are found to be the lowest energy structures in all cases except for MeP‐bridged calix[4]pyridine and calix[4]benzene, for which the twist‐pinched cone and partial cone are the most stable conformations, respectively. The conformational energy differences calculated between the lowest energy and the next conformation are on the order of 2.0–3.0 kcal/mol, but smaller for the S‐ and MeP‐bridged compounds. Analysis of the structures and relative energies shows that the phenyl hydrogens have electrostatic attractions with the lone pairs of the heteroatoms and steric repulsions with the methyl groups on bridgehead groups. Conversely, the lone electron pair in the pyridyl compounds engage in a repulsive electrostatic interaction. The interactions of 1,3‐aromatic rings play a minor but important role in the relative stability sequence. This detailed understanding of the factors governing the conformational space of hetero calixarences can be used to design conformationally biased analogs of these interesting compounds. Copyright © 2011 John Wiley & Sons, Ltd.     

Microwave‐Assisted One‐Pot Synthesis of Metal‐Free Nitrogen and Phosphorus Dual‐Doped Nanocarbon for Electrocatalysis and Cell Imaging

Metal‐free nitrogen and phosphorus dual‐doped, electrocatlytically active, functionalized nanocarbon (FNC) and photoluminescent carbon nanodots (PCNDs) are simultaneously synthesized using a facile one‐pot microwave‐assisted process. The successful incorporation of phosphorus and nitrogen to oxygen rich PCNDs and FNC are confirmed using surface morphological and spectral studies. The characterization studies of FNC further reveals the presence of edge‐plane‐like sites/defects and remarkable electrocatalytical activity. In addition to the electroctalytical activity, FNC shows attractive properties as a metal‐free oxygen reduction catalyst and is resistant to methanol crossover effects in alkaline media. The 5–10 nm PCNDs, which exhibit blue fluorescence under UV exposure, are successfully used for bioimaging applications.     

Hydrothermal Synthesis of Nickel Phosphate Nanorods for High‐Performance Flexible Asymmetric All‐Solid‐State Supercapacitors

Ni₂₀[(OH)₁₂(H₂O)₆][(HPO₄)₈(PO₄)₄]·12H₂O nanorods are successfully synthesized via a one‐pot hydrothermal reaction. A high‐performance flexible asymmetric all‐solid‐state supercapacitor based on the obtained Ni₂₀[(OH)₁₂(H₂O)₆][(HPO₄)₈(PO₄)₄]·12H₂O nanorods (positive electrode) and graphene nanosheets (negative electrode) is successfully assembled. It is the first report of this nanomaterial applied for all‐solid‐state supercapacitors. Interestingly, a maximum volumetric energy density of 0.446 mW h cm^?3 at a current density of 0.5 mA cm^?2 and a maximum power density of 44.1 mW cm^?3 at a current density of 6.0 mA cm^?2 are achieved by the as‐assembled device. What's more, the device also shows excellent mechanical flexibility and little capacitance change after over 5000 charge/discharge cycles at a current density of 0.5 mA cm^?2.     

A DFT‐based exploration augmented by X‐ray and NMR of the stereoselectivity in the 1,3‐dipolar cycloaddition of 1‐pyrroline‐1‐oxide to methyl cinnamate and benzylidene acetophenone

A B3LYP/6–31G* study was carried out for the reactions of 1‐pyrroline‐1‐oxide (N1) with methyl cinnamate (E1) and benzylidene acetophenone (E2) for getting a quantitative rationalization of the experimental findings. The product ratios were determined by NMR studies of the crude reaction mixtures. The conformation and stereochemistry of the isolated cycloadducts were finally confirmed by 2D NMR and X‐ray diffraction. The endo/exo‐selectivities were predicted through the computation of activation parameters on the basis of assumed concerted mechanism. The regioselectivity and reactivity were amply predicted by local and global electrophilicity indices and were found to be in good agreement with the experimental findings which were supportive of polar character and of the direction of charge transfer (CT) accompanying the cycloaddition. It was found that the cycloaddition involving methyl cinnamate was endo‐selective, while that with benzylidene acetophenone produced the exo‐isomer as the major adduct. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and Raman investigation of sol–gel‐derived relaxor ferroelectric thin films

Pb(Fe_2/3W_1/3)O₃ (PFW) thin films were deposited on platinized silicon substrate by a chemical solution deposition technique. Room‐temperature X‐ray diffraction (XRD) revealed a pure cubic crystal structure of the investigated material. The microstructure indicated good homogeneity and density of the thin films. A Raman spectroscopic study was carried out on PFW to study the polar nano‐regions in the temperature range 85–300 K. The Raman spectra showed a change in the peak intensity and a shift towards the lower wavenumber side with temperature. The Raman spectra also revealed the transition from the relaxor to the paraelectric state of PFW. There was no evidence of a soft mode in the low‐temperature region, in contrast to the normal ferroelectric behavior. The polar nano‐regions tend to grow and join at low temperatures (∼85 K), which become smaller with increase in temperature. The presence of strong Raman spectra in the cubic phase of the material is due to the presence of distributed Fm3m(Z = 2) symmetry nano‐ordered regions in the Pm3m(Z = 1) cubic phase. Copyright © 2007 John Wiley & Sons, Ltd.     

Experimental and theoretical push‐pull Chemo‐ and regioselectivity in 1,3‐Dipolar cycloaddition reactions: the case of benzotriazepin‐5‐one with mesitylnitrile oxide

A novel heterocyclic compound 3‐mesityl‐5‐methyl‐4,5,11,11a‐tetrahydro‐6H‐[1,2,4]oxadiazolo [5,4‐b][1,3,4]benzotriazépin‐6‐one 4 has been synthesised by a 1,3 dipolar cycloaddition (13DC) reaction of 1,3,4‐benzotriazepin‐5‐one 1 with mesitylnitrile oxide 3 . The reaction, beside its synthetic interest, has shown to be completely chemo‐ and regioselective. The structure of the compound was determined by X‐ray crystallography and analysed by spectral methods (NMR and mass spectrometry). The molecular mechanism for the reaction has been studied using quantum mechanical calculations at the B3LYP/6‐31G* theory level. Two mechanisms are possible for the formation of the cycloadduct 4 . The first one involves a 13DC reaction between 1 , as dipolarophile and 3 , as dipole. Analysis of the results indicates that it takes place along asynchronous concerted bond‐formation process with a very low polar character. The regioselectivity obtained from the calculations are in complete agreement with the unique formation of the cycloadduct 4 . The second mechanism is initiated by the nucleophilic attack of the N3 nitrogen of the tautomer form of 2 , to the C5 carbon of the nitrile oxide 3 to yield an amidoxime. However, the large energy involved in this addition prevents this mechanism. The large energy difference between the tautomers 1 and 2 , makes that only the C?N site of benzotriazepin‐5‐one 1 could act as a dipolarophile site. This fact makes the 13DC reaction to be chemoselective. The analysis of global electrophilicity of the reagents allows explaining the low polar character of these 13DC reactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis and electrochemical evaluation of 2‐substituted imidazolium salts

Herein, we report the synthesis, electrochemical, and computational evaluation of six 2‐substituted imidazolium bromides and six 2‐substituted imidazolium triflates. All final compounds were obtained in 2 or fewer synthetic steps from inexpensive starting materials and display a single, irreversible electrochemical reduction. The reduction potentials span a range greater than 1 V depending on the electron withdrawing power of the 2‐substituent. Imidazolium bromides such as Bn₂(H)ImBr reduce with E_1/2 = ?2.70 V vs Fc/Fc⁺, whereas the electron‐withdrawing Br‐containing analog Bn₂(Br)ImBr reduces at only ?1.58 V vs Fc/Fc⁺. The reduction potential of imidazolium bromides obeys a linear free energy relationship to σ_m Hammett constants, whereas imidazolium triflates correlate better with the σ_p Hammett constants. These results indicate that the stabilizing effect of the 2‐substituent is anion‐sensitive, changing from induction to resonance upon exchanging bromide for triflate. Predicted electron affinities from density functional theory–optimized structures of imidazolium cations and reduced species more closely match experimental data for the triflates, suggesting that a triflate anion does not electronically perturb the imidazolium core as much as a bromide. Taken together, these data highlight the dual modularity of imidazolium salts by changing both 2‐substituent and anion.     

Mechanism and regioselectivity of the 1,3‐dipolar cycloaddition of thiocarbonyl S‐imide with cyclopent‐3‐ene‐1,2‐dione and methoxyethene: a density functional theory approach

A theoretical study on the regioselectivity of 1,3‐dipolar cycloaddition reaction between an uncommon dipole (thiocarbonyl S‐imide) with cyclopent‐3‐ene‐1,2‐dione (DPh1) and methoxyethene (DPh2) has been carried out by means of several theoretical approaches, namely, activation energy, Houk's rule based on the frontier molecular orbital theory and density functional theory (DFT) reactivity indices. The calculations were performed at the DFT‐B3LYP/6‐31G(d) level of theory using GAUSSIAN 09. The present analysis shows that the 1,3‐dipolar cycloaddition of thiocarbonyl S‐imide with DPh1 and DPh2 has normal‐electron demand and inverse‐electron demand character, respectively. Moreover, the results obtained from energetic point view are in agreement with electronic approaches, and the Houk's rule is capable to predict true regioselectivity. Copyright © 2012 John Wiley & Sons, Ltd.     

Substrate‐Induced Synthesis of Nitrogen‐Doped Holey Graphene Nanocapsules for Advanced Metal‐Free Bifunctional Electrocatalysts

The development of efficient metal‐free electrocatalysts for oxygen electrocatalysis is of great significance for various energy conversion devices. Herein, novel nitrogen‐doped holey graphene nanocapsules (NHGNs) are reported prepared by self‐assembly of graphene oxide nanosheets on the surface of amino‐functionalized silica template and NH₃ activation with simultaneously enhanced nitrogen doping and etching of nanopores in graphene, followed by template etching. The silica template is demonstrated to show a substrate‐enhanced effect on nitrogen doping and etching of nanopores in graphene based on density functional theory calculations. Benefiting from the large surface area, unique pore distribution, and high surface functionality of nitrogen doping, the resulting NHGNs exhibit superior bifunctional electrocatalytic activity and durability for both oxygen reduction reaction and oxygen evolution reaction, which is similar to that of the commercial Pt/C and RuO₂ electrocatalysts, respectively. This work presents an advance in developing new nitrogen‐doped graphene species for highly efficient metal‐free electrocatalysis.     

Ring inversion process for a series of 3,5‐dialkyl‐1‐oxa‐3,5‐diazacyclohexanes: 1H DNMR study and semiempirical calculations

The ring inversion process for a series of 3,5‐dialkyl‐1‐oxa‐3,5‐diazacyclohexanes was studied using proton dynamic nuclear magnetic resonance (¹H DNMR) spectroscopy in conjunction with semiempirical calculations. At low temperature, the ring methylene protons decoalesced into two AB spin systems in a 2:1 ratio. Lineshape simulations of the DNMR spectra provided first‐order rate constants for magnetic exchange. The energy barrier for each inversion reaction was calculated from the respective rate constants. In general, as the size of the N‐alkyl group increased, the barrier to ring inversion decreased. A similar trend was seen in semiempirical calculations that modeled the ring inversion process. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and electrochemical study of nitrophenyl derivatives of β‐cyclodextrin

A series of nitrophenyl β‐cyclodextrin derivatives: mono[6‐deoxy‐6‐(4‐nitrobenzamido)]‐per‐ O‐methyl‐β‐cyclodextrin (R₁? Ph? NO₂), mono[6‐deoxy‐6‐(3‐nitrobenzamido)]‐per‐O‐methyl‐β‐cyclodextrin (R₂? Ph? NO₂) and heptakis[6‐deoxy‐6‐(4‐nitrobenzamido)‐2,3‐di‐O‐methyl]‐β‐cyclodextrin [R₃? (Ph? NO₂)₇] were synthesized. Purity and composition of the obtained substances were checked. Electroreduction of nitro groups of the new synthesized compounds was investigated on mercury electrode using cyclic voltammetry and chronocoulometry. The parameters of the reduction processes of ? NO₂ groups of the investigated compounds were found not to be comparable to the reduction of nitrobenzene under the same experimental conditions. Moreover, the electroreduction of nitro groups in these nitrophenyl derivatives was dependent on pH, the type of the studied compound, and slightly on the solvent composition. All the reactants were strongly adsorbed on mercury electrode. In the case of R₃? (Ph? NO₂)₇, its seven nitro groups were reduced practically at the same potential, and no radical anion formation was observed. Copyright © 2007 John Wiley & Sons, Ltd.     

A DFT study of the mechanism and selectivities of the [3 + 2] cycloaddition reaction between 3‐(benzylideneamino)oxindole and trans‐β‐nitrostyrene

The mechanism and regioselectivities and stereoselectivities of the [3 + 2] cycloaddition (32CA) reaction of 3‐(benzylideneamino) oxindole (AY) and trans‐β‐nitrostyrene have been studied using both B3LYP and ωB97XD density functional theory methods together with the standard 6‐31G(d) basis set. Four reactive pathways associated with the ortho and meta regioselective channels and endo and exo stereoselective approaches modes have been explored and characterized. While the B3LYP functional fails to predict the experimental regioselectivity, the ωB97XD one succeeds to predict the experimentally observed meta regioselectivity favoring the formation of meta/endo cycloadduct as the major isomer. Inclusion of solvent effects increases the regioselectivity and decreases the experimentally observed stereoselectivity. Analysis of the density functional theory global reactivity indices and the Parr functions of the reagents in its ground state allows explaining the reactivity and the meta regioselectivity of this zwitterionic‐type 32CA reaction, which account for the high polar character of this reaction. Non‐covalent interaction analysis of the most favorable meta/endo transition state structure reveals that the formation of a hydrogen‐bond between 1 nitro oxygen and the AY N–H hydrogen is responsible for the selectivity experimentally found in this polar zwitterionic‐type 32CA reaction.     

Synthesis and vibrational analysis of N‐(2′‐Furyl)‐Imidazole

The N‐(2′‐furyl)‐imidazole ( 1 ) has been prepared and characterized using infrared, Raman and multidimensional nuclear magnetic resonance spectroscopies. Theoretical calculations have been carried out by employing the Density Functional Theory (DFT) method, in order to optimize the geometry of their two conformers in the gas phase and to support the assignments of the vibrational bands of 1 to their normal modes. For a complete assignment of the compound, DFT calculations were combined with Scaled Quamtum Mecanic Force Field (SQMFF) methodology in order to fit the theoretical wavenumber values to the experimental one. Furthermore, Natural Bond Orbital (NBO) and topological properties by Atoms In Molecules (AIM) calculations were performed to analyze the nature and magnitude of the intramolecular interactions. The result reveals that two conformers are expected in liquid phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Understanding the molecular mechanism of the [3 + 2] cycloaddition reaction of benzonitrile oxide toward electron‐rich N‐vinylpyrrole: a DFT study

The [3 + 2] cycloaddition (32CA) reaction of benzonitrile oxide, BNO 2 , with an electron‐rich N‐vinylpyrrole derivative, NVP 3a , in the presence of dichloromethane, has been theoretically studied using density functional theory (DFT) methods at the B3LYP/6‐31G(d) level. This 32CA reaction presents a relatively high activation Gibbs free energy as a result of the low polar character of this zwitterionic‐type (zw‐type) reaction. Analyses of the calculated relative Gibbs free energies and transition state geometries indicate that the studied 32CA reaction, in excellent agreement with experimental outcomes, takes place in a complete regioselective manner as a consequence of the steric repulsions that appear at the most unfavorable transition state. An electron localization function (ELF) topological analysis of the bonding changes along this 32CA reaction supports a non‐concerted two‐stage one‐step molecular mechanism in which the formation of the O3‐ C5 single bond takes place at the end of the reaction after the complete formation of the C1‐C4 one. Copyright © 2016 John Wiley & Sons, Ltd.     

Understanding the stereo‐ and regioselectivities of the polar Diels–Alder reactions between 2‐acetyl‐1,4‐benzoquinone and methyl substituted 1,3‐butadienes: a DFT study

The polar Diels–Alder (DA) reactions of 2‐acetyl‐1,4‐benzoquinone (acBQ) with methyl substituted 1,3‐butadienes have been studied using DFT methods at the B3LYP/6‐31G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3‐dienes to the β conjugated position of the acBQ followed by ring‐closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions. Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics of base‐catalyzed cyclization of 2,6‐dinitrophenylsulfanyl ethanenitrile and 2,4,6‐trinitrophenylsulfanyl ethanenitrile

The kinetics of base catalyzed cyclization of 2,6‐dinitrophenylsulfanyl ethanenitrile and 2,4,6‐trinitrophenylsulfanyl ethanenitrile giving 2‐cyano‐7‐nitrobenzo[d]thiazole‐3‐oxide and 2‐cyano‐5,7‐dinitrobenzo[d]thiazole‐3‐oxide respectively was studied in methanolic methoxyacetate, acetate, trichlorophenoxide, N‐methylmorpholine, and N‐methylpiperidine buffers at 25 °C and I = 0.1 mol L^?1. It was found that reaction involves both general acid and general base catalyses whose manifestation depends on the pK_a of the acid‐buffer component and the ratio of both buffer components. In weakly basic buffers the rate‐limiting step is C? H bond breaking in the cyclic intermediate, while in strongly basic buffers the rate‐limiting step is the general acid‐catalyzed elimination of hydroxyl group from the intermediate. Copyright © 2008 John Wiley & Sons, Ltd.     

Dramatic solvent and concentration dependence in the reaction of (anthracen‐9‐yl)methanamines with suitable electron‐deficient acetylenes

Herein, we describe diverse reactivity of (anthracen‐9‐yl)methanamines with a few electron‐deficient acetylenes. Depending on the solvent and concentration, (anthracen‐9‐yl)methanamines reacted with acetylenes through one electron transfer, two electron transfer, or Diels–Alder pathways; and under certain conditions, we observed multiple reaction pathways. We have proposed plausible mechanisms to account for various reactions observed by us. Copyright © 2014 John Wiley & Sons, Ltd.     

Theoretical study on the mechanisms of Proline‐catalyzed Mannich reaction between acetaldehyde and N‐Boc imines

The mechanisms of the single and double Mannich reactions between acetaldehyde and N‐Boc imines are clarified by density functional theory calculations. For single addition of Mannich reaction, the energy difference between the transition states of different configurations correspond to an enantiomeric excess value of 90.58% (without solvent) and 98.46% (in acetonitrile) in favor of the (S)‐configuration product. For bis‐addition of Mannich reaction, the calculated enantiomeric excess value is 95.02% (without solvent) and 98.57% (in acetonitrile) in favor of the (S, S)‐configuration product. These calculated results are in good agreement with the experimental results. The calculations clearly demonstrate that the hydrogen‐bonding determine the stereochemistry of the reactions. Copyright © 2012 John Wiley & Sons, Ltd.     

Thermal reaction of electron deficient 4‐oxo‐4H‐pyrazole 1,2‐dioxide with cycloheptatriene: the first examples of the formation of an endo‐[4π + 6π]‐cycloadduct and an intramolecular 1,3‐dipolar reaction leading to a heterocage

The reaction of 3,5‐bis(methoxycarbonyl)‐4‐oxo‐4H‐pyrazole 1,2‐dioxide (1a) with 1,3,5‐cycloheptatriene (2b) gave a mixture of the novel endo‐[4 + 6]‐cycloadduct (4ab), anti‐exo‐[4 + 2]‐cycloadduct (5ab), and the heterocage (6ab) derived from the intramolecular 1,3‐dipolar cycloaddition reaction of the syn‐endo‐[4 + 2]‐cycloadduct. Analogous endo‐[4 + 6] selectivity in 1,3‐dipolar cycloadditions has not been reported previously. The X‐ray analysis indicates that 6ab has a very long N_sp3–N_sp3 bond distance of 1.617(4) Å. The cycloaddition behaviour is discussed on the basis of transition‐state structures optimized at the B3LYP/6‐31G(d) level of theory, from which predictions of the peri‐, regio‐, and stereoselectivities agreed well with the experimental results. Copyright © 2012 John Wiley & Sons, Ltd.