Zur Kinetik der Bildung von Arylaminomethylenverbindungen aus Triethoxymethan,Arylaminen und CH2-aciden Verbindungen in einer Dreikomponentenkondensation

Condensation of triethoxymethane and aniline with dimedone gives 2-anilinomethylene dimedone as the main product. An¹H-NMR-spectroscopic investigation of the kinetics in chloroform-d ₁ and methanol-d ₄ shows different rate determining steps in these solvents. There are two predominant rate determining steps in a complicated multistep reaction sequence.The initial one involves condensation of aniline with triethoxymethane to form diphenyl formamidine via ethyl phenyl formimidate. The second step involves reaction of the intermediate diphenyl formamidine with dimedone. The rates are strongly dependent upon the nature of the solvent and the concentration of catalytic acid. In methanol the reaction of dimedone with the intermediate diphenyl formamidine is rate determining. For preparative purposes the isolation of the intermediate diphenyl formamidine and the subsequent use of less polar solvents offer an advantage, because the second step is found to be accelerated.

6. Mitt.:Wolfbeis, O. S., Mh. Chem.112, 369 (1981).     

Effects of heteroatoms on 1,2-rearrangements of 3-membered ring carbenes

1,2-rearrangements of carbenes: CCH₂X(X = CH₂, NH and O) are studied by using ab initio gradient method. Heteroatoms N and O stabilize the carbene and decrease its reactivity, mainly by changing frontier molecular orbitals, but retain the way of the reaction. The reaction starts from the attack of the migrating hydrogen on the carbene p AO and ends with the entrance of the hydrogen into the carbene σ orbital. Reactivities are in the order of X = CH8>NH>0. The reaction is exothermic or endothermic according to whether the product is a 4n+2 or 4n π electron molecule.     

Synthesis and reactivity of formamidinato rhodium(I) complexes

The syntheses of [Rh(diol)(formamidine)]₂ complexes (diol  cycloocta-1,5-diene (1); diol  norbornadiene (2); formamidine  N,N′-di-p-tolylformamidine) are reported. These complexes are dimeric and contain the bridging formamidino ligand. They react with CO, dppe and PPh₃ with displacement of the diene ligand to yield the known [Rh(CO)₂(formamidine)]₂, [Rh(dppe)₂]⁺ and [Rh(PPh₃)₂(formamidine)], respectively; the last complex, in which the formamidine acts as a chelating ligand, was isolated only as the O₂ adduct. With HCl or HBF₄ aqueous 1 and 2 do not form hydrides but instead the formamidino cation [p-tolyl-NHCHNHtolyl-p]⁺ and the complexes [Rh(diol)X]₂ (X  Cl, F); a possible scheme for the reaction with HCl is proposed. The [Rh(C₈H₁₂)(formamidine)]₂ complex reacts with heterocumulenes as CS₂, SO₂, PhNCS and PhNCO with diene displacement; the only product isolated was [Rh(CS₂)₂(formamidine], to which a polymeric structure is assigned.     

Relaxation Effect of Basis Orbitals on the Properties of Atomic and Molecular Hydrogen Systems

The effect of orbital relaxation on the properties of atomic and molecular hydrogen systems H, H ₂ ⁺ , H₂, H ₂ ⁻ , and H ₃ ⁺ calculated using the minimal basis set, the split valence shell basis set including the polarization function, and an extended basis set of grouped natural orbitals is considered. Inclusion of orbital relaxation in calculations results in a decreased total energy and more accurate energies of electron affinity. The strongest effect is produced on the calculated characteristics of the anions. The calculated activation energy of the radical reaction of hydrogen elimination. H₂ + H = H + H₂ depends strongly on the degree to which electron correlation is taken into account. Due to inclusion of orbital relaxation, the activation energy also approximates the experimental value, although to a lesser extent. The semiempirical PM3 method fails to adequately describe the transition state of this reaction, but this disadvantage is eliminated by using the exponent of the relaxed orbital of hydrogen.Original Russian Text Copyright © 2004 by A. I. Ermakov, A. E. Merkulov, A. A. Svechnikova, and V. V. Belousov__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 6, pp. 979–985, November–December, 2004.     

A heuristic potential function for intramolecular and intrasupermolecular chemical reactions

A heuristic fitting procedure to obtain an analytical potential function for describing a reactive potential energy surface in the neighborhood of the intrinsic reaction coordinate (IRC ) has been developed. For discussion, the pairwise potential function form, ∑a_nr^?n, is assumed in order to fit ab initio quantum mechanical calculations of intramolecular (or intrasupermolecular) interaction energies and its use is found advantageous because all the calculation can be carried out by the linear least squares method. Normal modes perpendicular to IRC are utilized to prepare an initial data base for the potential fitting in the neighborhood of IRC . Some trial molecular dynamics (MD ) simulations are performed in order to check the fitted potential function and, unless they lead to reasonable energies within the tolerance assumed, their results are utilized to construct an improved data base (the dynamic sampling). The present systematic optimization procedure has been applied to the proton transfer reaction of the formamidine–water (FW ) system. The normal mode analysis in both the transition state (TS ) and the stable state (SS ) regions suggests that the present fitted potential function can reproduce satisfactorily the Born–Oppenheimer (BO ) adiabatic surface obtained by ab initio molecular orbital (MO ) calculations. We conclude that our procedure works well for the chemical reaction molecular dynamics (CRMD ) simulation.     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces

CNDO /2 calculations have been performed on the clusters X₄H₉ and X₄Y₉ modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp³ hybrid orbital. It is shown that in the CNDO /2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp² orbital of the silicon atom. Using the CNDO /2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp³ orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.     

Ab initio Mechanism Study on the Reaction of Chlorine Atom with Formic Acid

The potential energy surface(PES) for the reaction of Cl atom with HCOOH is predicted using ab initio molecular orbital calculation methods at UQCIDS(T,full)6-311 G(3df,2p)//UMP2(full)/6-311 G(d,P) level of theory with zero-point vibrational energy (ZPVE) correction.The calculated results show that the reaction mechanism of Cl atom with formic acid is a C-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom with a 3.73kJ/mol reaction barrier height,leading to the formation of cis-HOCO radical which will reacts with Cl atom or other molecules in such a reaction system.Because the reaction barrier height of O-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom which leads to the formation of HCO2 radical is 67.95kJ/mol,it is a secondary reaction channel in experiment,This is in good agreement with the prediction based on the previous experiments.     

Electron spin resonance study on UV-irradiated single crystal of (NH3iPr)6[Mo8O28(CHO)2] · 2H2O

EPR study on UV-irradiated single crystal of (NH₃iPr)₆ [Mo₈ O₂₈(CHO)₂]·2H₂O was carried out at room temperature. Three sets of EPR lines were observed. Spectra I and II are attributed respectively to paramagnetic Mo (V) centre (A) and (B). Line III may be assigned to an unstable-paramagnetic centre. Using the least-squares-fitting method to calculate the Hamiltonian parameters, we got the accurate principal values of g, hyperfine A_MO and ligand hyperfine A_H tensors, as well as-the direction cosines of their principal axes with respect to experimental coordinate system (xyz). The electron spin density was calculated from A value. The results showed that the unpaired electron occupies 4d orbital of molybdenum which appreciably contributes to the molecular orbital and the mechanism of direct spin coupling between the unpaired spin orbital of molybdenum and hydrogen orbital.     

Oxydationsprodukte von Thioharnstoff

Oxidation Products of Thiourea The oxidation of thiourea with hydrogen peroxide forms in dependence of test conditions thiourea S-dioxide (H₂N)₂C? SO₂ or in the presence of HCl dithiobisformamidinium dichloride [(H₂N)₂C? S? S? C(NH ₂)₂]Cl₂. Their electron absorption, infrared, nuclear magnetic resonance, and mass spectra are communicated. Both compounds as well as unstable bis(formamidine)disulfane do not react with carbon disulfide to produce the corresponding dithiocarbamate neither in the presence of help-bases. Also no reaction happens with alkyl esters of chlorodithioformic acid.     

Theoretical study of the hydrogen chemisorption on a ZnO surface

Reactions of a hydrogen molecule with a ZnO surface are studied by an ab initio method. For simulating the ZnO (10 1 0) surface, one ZnO molecule both with and without a Madelung potential is used. Since the electrostatic potential due to the ionic layer decreases exponentially, the effect of the layers deeper than the second one can be neglected. The Madelung potential is, therefore, expressed by the 32 point charges of ±0.5 situated on the first and second layers. Several low-lying states of ZnO and the ZnO + H₂ system have been calculated by the symmetry-adapted cluster (SAC ) and SAC –CI methods. It is found that the ¹Σ⁺ state of ZnO is the ground state and catalytic active and the other states are inactive. ZnO (¹Σ⁺) reacts with H₂ and dissociatively adsorbs it with making Zn? H and O? H bonds. This occurs both with and without the Madelung potential. Without the Madelung potential, the heat of reaction is 81.3 kcal/mol and the reaction barrier is 14.0 kcal/mol. With the Madelung potential, the heat of reaction decreases to 73.5 kcal/mol and the barrier decreases to 11.5 kcal/mol. The mechanism of this reaction is the electron donation from the 2pπ orbital of O to the antibonding σ_u MO of H₂ and the back-donation from the bonding σ_g MO of H₂ to the LUMO of ZnO. In the intermediate stage of the reaction, the dipole of ZnO works to increase the overlap of the active MOS to make the reaction easier. Throughout the reaction, the in-plane 2pπ orbital of O and the HOMO of ZnO are inactive and work to keep the ZnO bond stable during the catalytic process.     

Synthesis and Structure of MnCl2(HDpyF) (HDpyF = N,N′‐di(2‐pyridyl)formamidine): New Crystallographic Disorder and Bonding Mode of the HDpyF Ligand

The reaction of N,N′‐di(2‐pyridyl)formamidine (HDpyF) with MnCl₂‐4H₂O afforded the complex MnCl₂(HDpyF), which was characterized by X‐ray crystallography. The HDpyF ligand chelates to the Mn(II) center through the first and the third nitrogen atoms to form a six‐membered ring, leaving the second and the fourth nitrogen atoms uncoordinated. The HDpyF ligand is crystallographically disordered such that two different molecules can be solved. The neutral HDpyF ligand adopts the new s‐cis‐syn‐s‐trans conformation.     

Zn(II) and Cu(II) unsymmetrical formamidine complexes as effective initiators for ring‐opening polymerization of cyclic esters

A series of Zn(II) and Cu(II) complexes were synthesized using unsymmetrical N,N′‐ diarylformamidine ligands, i.e. N‐(2‐methoxyphenyl)‐N′‐2,6‐dichorophenyl)‐formamidine ( L1 ), N‐(2‐methoxyphenyl)‐N′‐phenyl)‐formamidine ( L2 ), N‐(2‐methoxyphenyl)‐N′‐(2,6‐dimethylphenyl)‐formamidine ( L3 ) and N‐(2‐methoxyphenyl)‐N′‐(2,6‐diisopropylphenyl)‐formamidine ( L4 ). The complexes, [Zn₂( L1 )₂(OAc)₄] ( 1) , [Zn₂( L2 )₂(OAc)₄] ( 2 ), [Zn₂( L3 )₂(OAc)₄] ( 3 ), [Zn₂( L4 )₂(OAc)₄] ( 4 ), [Cu₂( L1 )₂(OAc)₄] ( 5 ), [Cu₂( L2 )₂(OAc)₄] ( 6 ), [Cu₂( L3 )₂(OAc)₄] ( 7 ) and [Cu₂( L4 )₂(OAc)₄] ( 8 ), were prepared via a mechanochemical method with excellent yields between 95 ‐ 98% by reacting the metal acetates and corresponding ligands. Structural studies showed that both complexes are dimeric with a paddlewheel core structure in which the separation between the two metal centres are 2.9898 (8) and 2.6653 (7) Å in complexes 3 and 7 , respectively. Complexes 1 – 8 were used in ring‐opening polymerization of ε‐caprolactone (ε‐CL) and rac‐lactide (rac‐LA). Zn(II) complexes were more active than Cu(II) complexes, with complex 1 bearing electron withdrawing chloro groups being the most active (k_app = 0.0803 h^‐1). Low molecular weight poly‐(ε‐CL) and poly‐(rac‐LA) ranging from 1720 to 6042 g mol^‐1, with broad molecular weight distribution (PDIs, 1.78 – 1.87) were obtained. Complex 2 gave reaction orders of 0.56 and 1.52 with respect to ε‐CL and rac‐LA, respectively.     

CpRu(PPh3)2SSiiPr3与SCNR (R＝Ph, 1-Naphthyl)反应的结构、成键与机理的理论研究

应用密度泛函理论(DFT), 通过CpRu(PH₃)₂SSiⁱPr₃ (Cp＝环戊二烯负离子; ⁱPr＝异丙基)与SCNH模型化反应, 探讨了CpRu(PPh₃)₂SSiⁱPr₃ 与SCNR (R＝苯基, 萘基)的反应机理, 分析了反应所涉及的各相关化合物的结构与成键特征. 反应中首先失去一个膦配体, 生成一个中间体. 该中间体中, 硫原子采取sp²杂化, 硫原子剩余的一个p轨道与金属中心上的d轨道具有相同的对称性, 因而该p轨道上的孤电子对可与金属中心上的d轨道形成π键, 导致Cp环中心, Ru, S1, P和Si原子在同一平面内, 而不是S1, P和Si原子偏离该平面. 计算结果预测, S＝C双键中的p键打开, 生成含金属中心的四元环螯合物一步为反应的决速步骤. 空间位阻的减小、p共轭体系的生成以及螯合环的存在, 是导致该反应热力学有利的重要原因.     

A study of hydrogen bond strengths of neutral water clusters (H2O) n using modified MNDO

Semi-empirical molecular orbital methods proposed up to now seriously fail to describe hydrogen bonded systems associated with (H₂O) _n . A new scheme of parametrization using a semi-empirical method is proposed. We tested hydrogen bonding associated with the water clusters (H₂O) _n . The results are found to be close to ab initio Hartree-Fock quality, indicating a good promise for studying hydrogen-bonding systems other than O-H...O moiety.     

Kinetic study on hydrolysis and oxidation of formamidine disulfide in acidic solutions

Hydrolysis and oxidation of formamidine disulfide in acidic medium were investigated using high-performance liquid chromatography(HPLC) and mass spectrometry(MS) at 25 °C.By controlling the slow reaction rate and choosing appropriate mobile phase,HPLC provides the unique advantages over other methods(UV-Vis,chemical separation) in species tracking and kinetic study.In addition to thiourea and formamidine sulfinic acid,two unreported products were also detected in the hydrolysis reaction.Mass spectrometry measurement indicates these two products to be formamidine sulfenic acid and thiocyanogen with mass weights of 92.28 and 116.36,respectively.In the oxidation of formamidine disulfide by hydrogen peroxide,besides thiourea,formamidine sulfenic acid,formamidine sulfinic acid,thiocyanogen and urea,formamidine sulfonic acid and sulfate could be detected.The oxidation reaction was found to be first order in both formamidine disulfide and hydrogen peroxide.The rate constants of hydrolysis and oxidation reactions were determined in the pH range of 1.5-3.0.It was found both rate constants are increased with the increasing of pH.Experimental curves of different species can be effectively simulated via a mechanism scheme for formamidine disulfide oxidation,including hydrolysis equilibrium of formamidine disulfide and irreversible hydrolysis of formamidine sulfenic acid.     

氢分子与超氧阴离子自由基反应的微观机理

为了认识氢气生物学效应的分子机制,采用量子化学的M06-2X/6-311+G(d,p)和CCSD(t)/aug-cc-pVTZ方法模拟了人体条件(310K、液相)下氢分子与超氧阴离子自由基的反应机理。研究表明,反应的吉布斯自由能变化值为117.2kJ·mol~(-1),活化自由能垒为156.2kJ·mol~(-1),从热力学及动力学角度该反应都不容易进行。然后从电子结构和轨道作用层面对反应的微观机制进行了探讨,发现从反应物变为过渡态过程中,复合物轨道的组成和轨道能级发生显著变化(尤其是第8号轨道能级升高最多,达到2.73eV),O~-_2片段向H_2片段的电子转移数增加了0.1760个,并且转移的电子主要集居于第8号轨道,这削弱了H_2片段两个H原子间的化学键,也是反应活化能的主要来源。     

Magnesium Catalysis for the Hydroboration of Carbodiimides

A β‐diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}₂}MgnBu] (Dipp=2,6‐iPr₂C₆H₃), was shown to be an effective pre‐catalyst for the first reported catalytic hydroboration of alkyl‐ and aryl‐substituted carbodiimides with pinacol borane (HBpin). The catalytic reactions proceed under mild conditions to afford the corresponding N‐borylated formamidine compounds in good yields. The reactions were observed to proceed through the intermediacy of magnesium amidinate and formamidinatoborate intermediates and an example of one of these latter species has been structurally characterised by an X‐ray diffraction analysis. Crucially, no formation of the N‐boryl formamidine products was observed in the absence of additional equivalents of the carbodiimide and HBpin substrates. This observation, supported by the evolution of a sigmoidal kinetic profile for the hydroboration of dicyclohexylcarbodiimide, has been rationalised as the consequence of an allosteric effect of the pinacol borane and carbodiimide on the magnesium formamidinatoborate intermediates.     

The multieffects of DMF and DBU on the [5 + 1] benzannulation of nitroethane and α‐alkenoyl ketene‐(S,S)‐acetals: Hydrogen bonding and electrostatic interactions

A density functional theory (DFT) study was performed to elucidate the mechanism for the [5 + 1] benzannulation of nitroethane and α‐alkenoyl ketene‐(S,S)‐acetals. The calculation results are consistent with experimental findings, showing that the reaction proceeds via deprotonation of nitroethane, nucleophilic addition, intramolecular cyclization, elimination of HNO₂, and the keto‐enol tautomerization sequence. It was disclosed that N,N‐dimethylformamide (DMF) and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) act as not only solvent and nonnucleophilic base, respectively, but also catalysts in the reaction by stabilizing the transition states (TSs) and intermediates via intermolecular hydrogen bonds and electrostatic interactions. Besides, the effective orbital interaction of the reaction site in TS also contributes to the intramolecular cyclization step. The new mechanistic insights obtained by DFT calculations highlight that the hydrogen bonds and electrostatic interactions are key factors for the [5 + 1] benzannulation of nitroethane and α‐alkenoyl ketene‐(S,S)‐acetals. © 2015 Wiley Periodicals, Inc.