Theoretical analysis of concerted and stepwise mechanisms of Diels-Alder reactions of butadiene with silaethylene and disilene

The concerted and the stepwise mechanisms of the Diels-Alder reactions of butadiene with silaethylene and disilene were studied by ab initio MO methods. For the reaction of butadiene and silaethylene, an asymmetric concerted process that is almost stepwise and two stepwise processes were located. For the first step of the stepwise process, the C-Si bond formation is more favorable than the C-C bond formation. The activation energy barrier of the concerted transition state is only 0.89 kcal/mol lower than that of the first-step transition state of the C-Si bond formation for the stepwise process by the CASPT2 calculation level. For the reaction of butadiene and disilene, the activation energy barrier of the concerted-type transition state constrained with Cs symmetry is about 9 kcal/mol higher than that of the stepwise transition state by the CASSCF method. The energy barrier of the first step of the stepwise reaction disappears at the CASPT2/6-311++G(d,p) calculation level including the nondynamical correlation energy, although the reaction of the butadiene with disilene occurs through the stepwise-like process.     

Studies on the thickening reaction of polyester resins employed in sheet molding compounds

The mechanism of the thickening reaction between alkaline earth metal oxides and acid group-terminated polyester resins was studied. The thickening can occur either by the increase in molecular weight of the resin through neutral salt formation or by complex formation between the basic salt and carbonyl oxygens. Simultaneous analysis of reacted resins for moisture content by near infrared spectroscopy and metal content by atomic spectroscopy was carried out to differentiate between the two possibilities. It was established that the amount of basic salt present in the reaction mixtures was small and hence the thickening appears to be due to neutral salt formation. The neutral salt formation reaction, being similar to condensation polymerization, could be modelled to predict the course of the thickening process as a function of time. A model proposed hypothesizes the thickening process to be heterogeneous in nature and to occur in a sequence of three steps: diffusion of metal oxide into the resin, reaction of the dissolved metal oxide with the resin to form the basic salt, and the further reaction of the basic salt with the resin to form the neutral salt. The model predicts all the observed features of the thickening process qualitatively. Further, quantitative agreement was obtained between experimental data and the theoretical predictions of plateau viscosities and in several aspects of the kinetics of the thickening process.     

Multiple channel dynamics in the O(1D) reaction with alkanes

In this article, we briefly review the recent experimental studies of the multiple channel dynamics of the O((1)D) reaction with alkane molecules using the significantly improved universal crossed molecular beam technique. In these reactions, the dominant reaction mechanism is found to be an O atom insertion into the C-H bond, while a direct abstraction mechanism is also present in the OH formation channel. While the reaction mechanism is similar for all of these reactions, the product channels are quite different because of the significantly different energetics of these reaction channels. In the O((1)D) reaction with methane, OH formation is the dominant process while H atom formation is also a significant process. In the O((1)D) reaction with ethane, however, the CH(3) + CH(2)OH is the most important process, OH formation is still significant and H atom formation is of minor importance. A new type of O atom insertion mechanism (insertion into a C-C bond) is also inferred from the O((1)D) reaction with cyclopropane. Through these comprehensive studies, complete dynamical pictures of many multiple channel chemical reactions could be obtained. Such detailed studies could provide a unique bridge between dynamics and kinetics research.     

Control over the chemoselectivity of Pd-catalyzed cyclization reactions of (2-iodoanilino)carbonyl compounds

The factors that control the chemoselectivity of palladium-catalyzed cyclization reactions of (2-iodoanilino)carbonyl compounds have been explored by an extensive experimental computational (DFT) study. It was found that the selectivity of the process, that is, the formation of fused six- versus five-membered rings, can be controlled by the proper selection of the initial reactant, reaction conditions, and additives. Thus, esters or amides produce ketones by a nucleophilic addition process, whereas the addition of PhO(-) ions leads to the formation of indolines by an α-arylation reaction. In contrast, the corresponding ketone reactants yield a mixture of both reaction products, the ratio of which depends on the base used, in the presence of phenol. The outcome of the processes can be explained by the formation of a common four-membered palladacycle intermediate from which the competitive nucleophilic addition and α-arylation reactions occur. The remarkable effect of phenol in the process, which makes the α-arylation reaction easier, favored the formation of enol complexes, which are stabilized by an intramolecular hydrogen bond between the hydroxy group of the enol moiety and the oxygen atom of the phenoxy ligand. Moreover, the chemoselectivy of the process can be also controlled by the addition of bidendate ligands that lead to the almost exclusive formation of indoles at expenses of the corresponding alcohols.     

Highly diastereoselective formation of 3,7-dioxabicyclo[3.3.0]octan-2-ones in reaction of 2-arylcyclopropanedicarboxylates with aromatic aldehydes using 1,2-zwitterionic reactivity type

A new reaction of 2-arylcyclopropane-1,1-dicarboxylates (ACDC) with aromatic aldehydes that occurs via generation of a 1,2-zwitterionic intermediate in the presence of GaCl₃ has been developed. This process results in the 3,7-dioxabicyclo[3.3.0]octane frame by addition of two aldehyde molecules to an ACDC molecule. The reaction is a complex anionic-cationic cascade process involving the formation of two C–C bonds, two C–O bonds, and five stereocenters. The process occurs with high diastereoselectivity to give only one diastereomer. A probable reaction mechanism is suggested and confirmed by NMR monitoring data.     

劣质渣油性质对受热生焦趋势影响的研究

以四种劣质渣油为原料,研究组成性质对其热转化过程初期生焦特性的影响。结果表明,不同油样生焦诱导期受温度改变的影响程度可以用敏感度参数衡量,生焦诱导期越短的油样,敏感度参数越大,反应温度升高其生焦诱导期降低率越大。相同反应条件下,劣质渣油的生焦特性取决于其自身的基本性质,且各个性质对其生焦特性的影响程度不一。残炭、灰分、分子量、沥青质沉淀点和稳定性参数与其生焦诱导期相关性较强,其中,沥青质沉淀点和稳定性参数均反映的是油样胶体稳定性的突出影响。劣质渣油自身的胶体稳定性与其生焦密切相关,胶体稳定性越差的渣油,越容易生焦,油样的生焦过程是热反应过程中胶体体系逐渐被破坏的过程。     

A New Discovery of Calcium Phosphate Urinary Stones Formation Induced by Melamine: Nanocrystalline Assembly Mechanism

Melamine (Mel), a contaminant that has received much attention in recent years, has been adulterated into milk powder, causing a large number of infants suffering from kidney stone disease. To study the process and mechanism of calcium phosphate (CaP) stone formation induced by Mel, we simulated the formation process of CaP stones and studied the effect of Mel on crystallization in aqueous and synthetic urine systems, respectively. Ion selective electrode method was used to study the thermodynamic parameters and reaction rate of the crystallization process. It was firstly discovered that Mel could induce the formation of CaP crystals significantly under weak acidic urine conditions in which CaP cannot be stably present, so it may cause people to produce CaP stones. Thermodynamic parameter and reaction rate analysis indicated that Mel could increase the reaction tendency and accelerate the formation of CaP crystals, which was achieved by two process of electrostatic adsorption and release of calcium ions. This research is expected to provide scientific guidance for the prevention and treatment of Mel‐related stones.     

Classical trajectory study of the formation of XeH+ and XeCl+ in the Xe++HCl collision

The collision-induced reaction of Xe+ with HCl has been studied by use of classical dynamics procedures at collision energies 2-20 eV using empirical potential parameters. The principal reaction pathway on the potential energy surface is the formation of XeH+ with the maximum reaction cross section, 1.2 A2, occurring at E=9 eV. At lower energies, the cross section for the charge transfer process Xe++HCl-->Xe+HCl+ is comparable to that for XeH+ formation, but at higher energies, it is larger by a factor of 2. The cross section of the XeCl+ formation is an order of magnitude smaller than that of XeH+. For both XeH+ and XeCl+ formations, the reaction threshold is approximately 2 eV. The XeH+ formation takes place immediately following the turning point in a direct-mode mechanism, whereas an indirect-mode mechanism operates in the formation of XeCl+. Both XeH+ and XeCl+ formations come mainly from the perpendicular configuration, Xe+...HCl, at the turning point. Product vibrational excitation is found to be strong in both XeH+ and XeCl+.     

Photochemical electrocyclization of the indolinylphenylethenes involving a C-N bond formation

[reaction: see text] A novel oxidative photodehydrocyclization of indolinylphenylethenes to a polycyclic heteroaromatic cation with good yields was described. Starting from the trans derivative, the phototransformation is a multistep process. The process includes two photochemical reactions and a trans-cis isomerization reaction, followed by an 1-aza-1,3,5-hexatrienic electrocyclic reaction involving the formation of a C-N bond. The cyclized product gives the stable heteroaromatic cations from hydride elimination with oxygen from air or iodine.     

Mechanism of olefin cyclopropanation by diazomethane catalyzed by palladium dicarboxylates: a density functional study.

The reaction of diazomethane with ethylene in the presence of palladium diformate has been studied through density functional calculations. Several mechanistic paths leading to the formation of cyclopropane have been studied. The results obtained show that the reaction of palladium diformate with diazomethane is more favorable than the reaction with ethylene. The reaction with diazomethane may lead to two different isomeric complexes: a methylene-inserted complex and a palladium-carbene complex. Insertion of methylene is the most favorable process, but the resulting complex is not suitable for cyclopropanation. The reaction with two additional diazomethane molecules makes the formation of the bismethylene-inserted palladium-carbene complex favorable. Attack of ethylene on this palladium-carbene complex leads to the formation of cyclopropane.     

Novel synthesis of isoquinolines using isobenzofuran-nitrile Diels-Alder reactions

[reaction: see text] The synthesis of isoquinolines through coupling of 2-alkynylbenzaldehyde derivatives with beta-cyanocarbene complexes has been examined. The reaction involves formation of an isobenzofuran followed by intramolecular Diels-Alder reaction with the nitrile, a process with limited precedent. The unique success of this process in this system has been attributed to deoxygenation of the initial adduct to form the isoquinoline ring system.     

Formal Aminocyanation of α,β‐Unsaturated Cyclic Enones for the Efficient Synthesis of α‐Amino Ketones

Installation of amino functionality on organic molecules through direct C N bond formation is an important research objective. To achieve this goal, a 1,2‐aminocyanation reaction was developed. The reaction occurs through the formation of pyrazolines by means of a formal dipolar cycloaddition of cyclic α,β‐unsaturated ketones with lithium trimethylsilyldiazomethane followed by novel protonolytic N N bond cleavage under mild conditions. This two‐step process provides a diverse array of structurally complex free and mono‐alkylated α‐amino ketones in excellent yields.     

Gold-nanoparticle-catalyzed synthesis of propargylamines: the traditional A3-multicomponent reaction performed as a two-step flow process

The alkyne, aldehyde, amine A(3)-coupling reaction, a traditional multicomponent reaction (MCR), has been investigated as a two-step flow process. The implicated aminoalkylation reaction of phenylacetylene with appropriate aldimine intermediates was catalyzed by gold nanoparticles impregnated on alumina. The aldimine formation was catalyzed by Montmorillonite K10 beforehand. The performance of the process has been investigated with respect to different reaction regimes. Usually, the A(3)-multicomponent reaction is performed as a "one-pot" process. Diversity-oriented syntheses using MCRs often have the shortcoming that only low selectivity and low yields are achieved. We have used a flow-chemistry approach to perform the A(3)-MCR in a sequential manner. In this way, the reaction performance was significantly enhanced in terms of shortened reaction time, and the desired propargylamines were obtained in high yields.     

Control over the Chemoselectivity of Pd‐Catalyzed Cyclization Reactions of (2‐Iodoanilino)carbonyl Compounds

The factors that control the chemoselectivity of palladium‐catalyzed cyclization reactions of (2‐iodoanilino)carbonyl compounds have been explored by an extensive experimental computational (DFT) study. It was found that the selectivity of the process, that is, the formation of fused six‐ versus five‐membered rings, can be controlled by the proper selection of the initial reactant, reaction conditions, and additives. Thus, esters or amides produce ketones by a nucleophilic addition process, whereas the addition of PhO^? ions leads to the formation of indolines by an α‐arylation reaction. In contrast, the corresponding ketone reactants yield a mixture of both reaction products, the ratio of which depends on the base used, in the presence of phenol. The outcome of the processes can be explained by the formation of a common four‐membered palladacycle intermediate from which the competitive nucleophilic addition and α‐arylation reactions occur. The remarkable effect of phenol in the process, which makes the α‐arylation reaction easier, favored the formation of enol complexes, which are stabilized by an intramolecular hydrogen bond between the hydroxy group of the enol moiety and the oxygen atom of the phenoxy ligand. Moreover, the chemoselectivy of the process can be also controlled by the addition of bidendate ligands that lead to the almost exclusive formation of indoles at expenses of the corresponding alcohols.     

Immobilized Osmium Clusters in the Processes of Liquid-Phase Oxidation of Cycloxehene

The reaction of catalytic hydroxylation of olefins by organic hydroperoxides in the presence of osmium carbonyl clusters supported on polymer matrices is studied. The process occurs with the predominant formation of unsaturated alcohols. The oxidative coupling of olefins with the formation of nonconjugated dienes and in which the double bonds remain intact occurs in parallel. In the course of the reaction, changes in the chemical structures of the initial osmium clusters are not observed.     

Reaction mechanism between carbonyl oxide and hydroxyl radical: a theoretical study

The reaction mechanism of carbonyl oxide with hydroxyl radical was investigated by using CASSCF, B3LYP, QCISD, CASPT2, and CCSD(T) theoretical approaches with the 6-311+G(d,p), 6-311+G(2df, 2p), and aug-cc-pVTZ basis sets. This reaction involves the formation of H2CO + HO2 radical in a process that is computed to be exothermic by 57 kcal/mol. However, the reaction mechanism is very complex and begins with the formation of a pre-reactive hydrogen-bonded complex and follows by the addition of HO radical to the carbon atom of H2COO, forming the intermediate peroxy-radical H2C(OO)OH before producing formaldehyde and hydroperoxy radical. Our calculations predict that both the pre-reactive hydrogen-bonded complex and the transition state of the addition process lie energetically below the enthalpy of the separate reactants (DeltaH(298K) = -6.1 and -2.5 kcal/mol, respectively) and the formation of the H2C(OO)OH adduct is exothermic by about 74 kcal/mol. Beyond this addition process, further reaction mechanisms have also been investigated, which involve the abstraction of a hydrogen of carbonyl oxide by HO radical, but the computed activation barriers suggest that they will not contribute to the gas-phase reaction of H2COO + HO.     

Simulation of methane oxidation on Pt

The authors present a generic model of CH4 oxidation on Pt with the emphasis on the role of surface-oxide formation. The latter process is treated in terms of the theory of first-order phase transitions. The corresponding Monte Carlo simulations indicate that the surface-oxide formation may result in stepwise features in the reaction kinetics. Specifically, with increasing CH4 pressure and/or decreasing O2 pressure, the model predicts a sharp transition from a low-reactive state with the surface completely covered by oxide to a high-reactive state with the surface covered by chemisorbed oxygen. In the former case, the reaction is first order in CH4 and zero order in O2. In the latter case, both reaction orders are positive. All these findings help in interpreting available experiments.     

Acrylate formation via metal-assisted C-C coupling between CO2 and C2H4: reaction mechanism as revealed from density functional calculations

The reaction path for the formation of a binuclear hydrido-acrylate complex in a CO(2)-C(2)H(4) coupling process is explored in detail by locating the key intermediates and transition states on model potential energy surfaces derived from density functional calculations on realistic models. The formation of the new C-C bond is shown to take place via oxidative coupling of coordinated CO(2) and C(2)H(4) ligands resulting in a metalla-lactone intermediate, which can rearrange to an agostic species allowing for a beta-hydrogen shift process. The overall reaction is predicted to be clearly exothermic with all intermediates lying below the reactants in energy, and the highest barrier steps correspond to C-C coupling and beta-hydrogen transfer. The phosphine ligands are found to play an important role in various phases of the reaction as their dissociation controls the coordination of CO(2), the formation of the agostic intermediate, and the dimerization process; furthermore, their presence facilitates the oxidative coupling by supplying electrons to the metal center. Our results provide a theoretical support for the reaction mechanism proposed from experimental observations. The effect of the solvent medium on the relative energy of reaction intermediates and transition states is examined and found important in order to predict reliable energetics.     

C2-amidoglycosylation. Scope and mechanism of nitrogen transfer

A one-pot C2-amidoglycosylation reaction for the synthesis of 2-N-acyl-2-deoxy-beta-pyranosides from glycals is described. Glycal donors activated by the reagent combination of thianthrene-5-oxide (11) and Tf2O, followed by treatment with an amide nucleophile and a glycosyl acceptor, lead to the formation of various C2-amidoglycoconjugates. Both the C2-nitrogen transfer and the glycosidic bond formation proceed stereoselectively, allowing for the introduction of both natural and nonnatural amide functionalities at C2 with concomitant anomeric bond formation in a one-pot procedure. Tracking of the reaction by low-temperature NMR spectroscopy employing 15N- and 18O-isotope labels suggests a mechanism involving the formation of the C2-sulfonium glycosyl imidate 39 as well as oxazoline 37 as key intermediates in this novel oxidative glycosylation process.     

Rapid and quantitative disulfide bond formation for a polypeptide chain using a cyclic selenoxide reagent in an aqueous medium

To elucidate the reaction mechanism of the disulfide (SS) bond formation reaction of a polypeptide molecule with a water‐soluble selenoxide reagent, trans‐3,4‐dihydroxyselenolane oxide (DHS^ox), short‐term oxidation experiments were carried out for the reduced state (R) of a recombinant hirudin CX‐397 variant at pH 7.0 and 25 °C. In the reaction, R was oxidized sequentially to one‐SS, two‐SS, and three‐SS intermediate ensembles within 1 min. The kinetic analysis revealed that the three second‐order rate constants for the SS formation are proportional to the number of thiol groups existing in the reactant SS intermediates, indicating the stochastic nature of the SS formation. Ab initio calculation at the HF/6‐31++G(d,p) level in water by using the polarizable continuum model suggested that the SS formation reaction is highly exothermic and proceeds via a reactive thioselenurane intermediate with a distorted linear O‐Se‐S linkage. The results clearly demonstrated that the rate‐determining step of the SS formation reaction is the first bimolecular process between a thiol substrate and DHS^ox rather than the subsequent process to release a SS product.