Scope of the asymmetric intramolecular stetter reaction catalyzed by chiral nucleophilic triazolinylidene carbenes

A highly enantioselective intramolecular Stetter reaction of aromatic and aliphatic aldehydes tethered to different Michael acceptors has been developed. Two triazolium scaffolds have been identified that catalyze the intramolecular Stetter reaction with good reactivity and enantioselectivity. The substrate scope has been examined and found to be broad; both electron-rich and -poor aromatic aldehydes undergo cyclization in high yield and enantioselectivity. The tether can include oxygen, sulfur, nitrogen, and carbon linkers with no detrimental effects. In addition, the incorporation of various tethered Michael acceptors includes amides, esters, thioesters, ketones, aldehydes, and nitriles. The catalyst loading may be reduced to 3 mol % without significantly affecting the reactivity or selectivity of the reaction.     

DFT study on mechanism of the classical Biginelli reaction

The condensation of benzaldehyde, urea, and ethyl acetoacetate according to the procedure described by Biginelli was investigated at the B3LYP/6-31G（d）, B3LYP/6-31 ＋G（d,p）, and B3LYP/6-311＋G（3df,2p）//B3LYP/6-31＋G（d,p） levels to explore the reaction mechanism. According to the mechanism proposed by Kappe, structures of five intermediates were optimized and four transition states were found. The calculation results proved that the mechanism proposed by Kappe is right.     

Mechanistic investigation of the enantioselective intramolecular stetter reaction: proton transfer is the first irreversible step

A study on the mechanism of the asymmetric intramolecular Stetter reaction is reported. This investigation includes the determination of the rate law, kinetic isotope effects, and competition experiments. The reaction was found to be first order in aldehyde and azolium catalyst or free carbene. A primary kinetic isotope effect was found for the proton of the aldehyde. Taken together with a series of competition experiments, these results suggest that proton transfer from the tetrahedral intermediate formed upon nucleophilic attack of the carbene onto the aldehyde is the first irreversible step.     

A study of the reaction mechanism of the catalytic intramolecular conversion of heterocycles

In order to study the reaction mechanism of the catalytic intramolecular conversion of heterocycles [1–3] and to broaden the field of their application, we have studied the dehydration over Al₂O₃ of 1-amino-3-(-tetrahydrofuryl) propane (I), 1-methylamino-1-(-tetrahydrofuryl) propane (II), and 2-(3-hydroxy-1-propyl)-1-methylpyrrolidine (III), leading in all three cases to pyrrolizidine (IV).     

DFT study on reaction mechanism of di-tert-butylphenol to di-tert-butylhydroxybenzoic acid

Structural Chemistry - Experimental studies on the Kolbe–Schmitt reaction and its side reactions have made great progresses; however, the relative theoretical studies fall behind. In order to...     

Multiple electronic state mechanism for carboryne reaction with benzene: A DFT study

The multiple electronic state mechanisms of the reaction of carboryne with benzene were investigated by M11 calculations. Mechanisms leading to [4 + 2] cycloaddition product P_{4 + 2} , [2 + 2] cycloaddition product P _{2 + 2} , C? C insert product P _C‐Cins and C? H insert product P _C‐Hins were considered. The barrier/stability to structural characteristics correlations revealed that, 1) [2 + 2] addition is a two‐step mechanism which exhibits three electronic state reactivity, and both the addition steps are controlled by the barriers on open‐shell singlet (OSS) potential energy surface (PES); 2) [4 + 2] product P _{4 + 2} is a kinetic product on the experimental condition, and other products should be obtained under more harsher condition. The theoretical results well explain the experimental facts.     

Catalytic reaction mechanism of homogentisate dioxygenase: a hybrid DFT study

Human homogentisate dioxygenase is an Fe(II)-dependent enzyme responsible for aromatic ring cleavage. The mechanism of its catalytic reaction has been investigated with the hybrid density functional method B3LYP. A relatively big model of the active site was first used to determine the substrate binding mode. It was found that binding of the substrate dianion with a vacant position trans to Glu341 is most favorable. The model was then truncated to include only the most relevant parts of the active-site residues involved in iron coordination and substrate binding. Thus, methylimidazole was used to model His292, His335, His365, and His371, while propionate modeled Glu341. The computational results suggest that the catalytic reaction of homogentisate dioxygenases involves three major chemical steps: formation of the peroxo intermediate, homolytic cleavage of the O-O bond leading to an arene oxide radical, and finally, cleavage of the six-membered ring. Calculated barriers for alternative reaction paths are markedly higher than for the proposed mechanism, and thus the computational results successfully explain the product specificity of the enzyme. Interestingly, the results indicate that the type of ring scission, intra or extra with respect to the substituents coordinating to iron, is controlled by the barrier heights for the decay of the arene oxide radical intermediate.     

DFT study into the reaction mechanism of CO methanation over pure MoS2

Mo‐based catalysts are commonly used in the direct methanation of CO; however, no integrated mechanism has been proposed due to limits in characterizing the nano‐sized active structures of MoS₂. Thus, we report our investigation into the mechanism of CO methanation over pure MoS₂ through density functional theory simulations, considering that only MoS₂ edge sites exhibit catalytic activity. Simulations revealed the presence of (010) and (110) surfaces on the MoS₂ edges. Both surfaces are reconstructed by the redistribution of surface sulfur atoms upon exposure to H₂/H₂S, and after sulfur coverage redistribution, S vacancies are generated for CO hydrogenation. The reaction mechanisms on both surfaces are discussed, with the S‐edge being better suited to CO methanation than Mo‐edge on the (010) surface. The rate‐controlling step differs between surfaces, and corresponds to the initial activation reaction, which was achieved more easily on the (110) surface.     

Dienophile twisting and substituent effects influence reaction rates of intramolecular Diels-Alder cycloadditions: a DFT study

Intramolecular cycloadditions of 5-vinyl-1,3-cyclohexadienes were studied with B3LYP/6-31G(d) density functional calculations. The one-atom tether dictates that the Z substituent becomes exo and the E substituent becomes endo in the TS. The geometry of the cycloaddition TS is typical of a pericyclic transformation except unusual twisting of the dienophile places the endo substituent in a relatively steric-free position and the exo substituent in a highly crowded position. The experimental rate differences between isomeric pairs of vinylcyclohexadienes can be explained by comparing reactant destabilization when a bulky group occupies the Z position of the starting alkene and transition state stabilization when a bulky group is endo in the cycloaddition TS.     

Insights into the reaction mechanism between propadienylidene and ethylene: a DFT study

The reaction mechanism between propadienylidene and ethylene has been systematically investigated employing the B3LYP/6-311++G** and MP2/cc-pVTZ levels of theory to better understand the reactivity of propadienylidene with unsaturated hydrocarbons. Geometry optimization, vibrational analysis, and energy property for the involved stationary points on the potential energy surface have been calculated. Two important initial reaction complexes characterized by three- and four-membered ring structures have been located firstly. After that, three different products possessing three-, four-, and five-membered ring characters have been obtained through three reaction pathways. In the first reaction pathway, a three-membered ring alkyne compound has been obtained. As for the second reaction pathway, it is a diffusion-controlled reaction, resulting in the formation of the four-membered ring conjugated diene compound. A five-membered conjugated diene compound has been obtained in the third reaction pathway, which is the most stable product in the available products thermodynamically. On the other hand, the second reaction pathway is the most favorable reaction to proceed kinetically.     

Reaction mechanism and stereoselectivity of ruthenium--porphyrin-catalyzed intramolecular amidation of sulfamate ester: a DFT computational study

The reaction mechanism of the ruthenium--porhyrin complex [Ru(por)(CO)]-catalyzed intramolecular C-H bond amidation was examined using density functional theory (DFT) calculations. The metal-nitrene reactive intermediate, Ru(por)(CO)-NSO3R1 (R1 = 1-methylclohexl-methyl) was found to be highly favorable to generate in terms of the free energy profile from the reaction of the starting materials. Ru(por)(CO)-NSO3R1 may exist in both singlet and triplet states since they are close in energy. In each state, six C-H bond amidation reaction pathways were characterized structurally and energetically. The predicted most probable diastereomeric product out of the four possible diasteromeric products examined in the calculations for the amidation reactions agree well with previously reported experimental results.     

A DFT study of the ambiphilic nature of arylpalladium species in intramolecular cyclization reactions

The remarkable structure-dependent reactivity observed in the cyclization of (2-haloanilino)-ketones with Pd-catalysts has been studied computationally within the density functional theory framework. The experimental reaction products ratio may be explained through the formation of a common palladaaminocyclobutane intermediate which can undergo a nucleophilic addition reaction and/or an enolate α-arilation process. The evolution of this metallacycle to the final products depends on two factors, the length of the tether joining the amino and the carbonyl groups, and the electronic nature of the substituent directly attached to the nitrogen atom. Thus, shorter chains (2 CH(2)) facilitate the nucleophic addition reaction by approximating the reactive aryl and Pd-coordinated carbonyl groups whereas longer chains (3 CH(2)) favor the enolate α-arylation proccess. For electron-withdrawing groups attached to the aniline nitrogen atom, the nucleophilic addition pathway becomes slightly disfavored, mainly due to the electron-withdrawing effect of the CO(2)Me group which avoids the delocation of the LP in the π-system, thus decreasing the nucleophilicity of the reactive arylic carbon atom. In contrast, the enolate α-arylation reaction is facilitated by the CO(2)Me group. This is translated into a small computed barrier energy difference of these competitive reaction pathways which should lead to a mixture of reaction products as experimentally found.     

DFT study of the mechanism of Cu(I)-catalyzed and uncatalyzed intramolecular cyclopropanation of iodonium ylides

The mechanism of the Cu(I)-catalyzed and uncatalyzed intramolecular cyclopropanation of ketoesteric and diesteric iodonium ylides has been thoroughly explored by means of electronic structure calculation methods (DFT). All crucial reaction steps encapsulated in the entire catalyzed and uncatalyzed reaction pathways were scrutinized, while the elementary steps, the intermediates and transition states were identified through monitoring the geometric and energetic reaction profiles. It was found that CuCl efficiently catalyze the cyclopropanation of iodonium ylides only for their diesteric derivatives and their diazo analogues via stabilization of the respective carbene upon complexation with the metal center. For the ketoesteric iodonium ylides the CuCl catalyst does not affect the kinetics of the intramolecular cyclopropanation reactions which could proceed easily without the catalyst, in line with available experimental observations.     

Relativistic DFT study on the reaction mechanism of second-row transition metal Ru with CO2

To estimate the importance of relativistic effects on the reaction mechanisms between Ru and CO2, the potential energy surfaces have been performed in the triplet and quintet electronic states using quasi-relativistic (Pauli), zero-order regularly approximated (ZORA), and nonrelativistic (NR) density functional theory (DFT) at the PW91/TZP level. The results demonstrate that there are two rival reaction mechanisms: one is an addition mechanism and the other is an insertion mechanism in the triplet state. The only mechanism in the quintet state is the insertion mechanism. The most favored reaction mechanism in Ru + CO2 is that the Ru atom in its ground state first attacks the CO bond of CO2, forming q-Ru(CO)O (5A') with the insertion mechanism, and then undergoes an intersystem crossing to t-Ru(CO)O (3A'). Then it crosses t-TS3 to produce t-ORuCO molecule. The relativistic effects are important for reactivity of the second-row transition metal to CO2. In the key step of t-Ru(CO)O via t-TS3 to t-ORuCO, relativistic effects reduce the barrier energy by 10.3 kcal/mol, which is nearly half the nonrelativistic barrier energy.     

Excited state intramolecular proton transfer in 5-hydroxy flavone: A DFT study

Potential energy surfaces (PES) for the ground and excited state intramolecular proton transfer (ESIPT) processes in 5-hydroxy-flavone (5HF) were studied using DFT-B3LYP/6-31G(d) and TD-DFT/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer (GSIPT) in 5HF. Excited states PES calculations support the existence of ESIPT process in 5HF. ESIPT in 5HF has been explained in terms of HOMO, LUMO electron density of the enol and keto tautomer of 5HF. PES scan by phenyl group rotation suggests that the twisted form, i.e., phenyl group rotated by 18.7° out of benzo-γ-pyrone ring plane is the most stable conformer of 5HF.     

The reaction mechanism of the gas‐phase thermal decomposition kinetics of neopentyl halides: A DFT study

The kinetics and mechanisms of the gas‐phase elimination reactions of neopentyl chloride and neopentyl bromide have been studied by means of electronic structure calculations using density functional methods: B3LYP/6‐31G(d,p), B3LYP/ 6‐31++G(d,p), MPW1PW91/6‐31G(d,p), MPW1PW91/6‐31++G(d,p), PBEPBE/6‐31G(d,p), PBEPBE /6‐31++G(d,p). The reaction channels that account in products formation have a common first step involving a Wagner‐Meerwein rearrangement. The migration of the halide from the terminal carbon to the more substituted carbon is followed by beta‐elimination of HCl or HBr to give two olefins: the Sayzeff and Hoffmann products. Theoretical calculations demonstrated that these eliminations proceed through concerted asynchronous process. The transition state (TS) located for the rate‐determining step shows the halide detached and bridging between the terminal carbon and the quaternary carbon, while the methyl group is also migrating in a concerted fashion. The TS is described as an intimate ion‐pair with a large negative charge at the halide atom. The concerted migration of methyl group provides stabilization of the TS by delocalizing the electron density between the terminal carbon and the quaternary carbon. The B3LYP/6‐31++G(d,p) allows to obtain reasonable energies and enthalpies of activation. The nature of these reactions is examined in terms of geometrical parameters, electron distribution, and bond order analysis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

PPh3催化Mannich反应机理的密度泛函研究

通过密度泛函理论研究了PPh3催化苯胺、苯甲醛和乙酰乙酸乙酯三组分Mannich反应的机理.计算结果表明该机理主要分3个步骤进行:PPh3催化乙酰乙酸乙酯发生酮式-烯醇式互变异构得到烯醇;烯醇辅助苯胺和苯甲醛缩合并脱水生成亚胺;亚胺和烯醇通过加成反应生成β-氨基羰基化合物.通过详细的机理研究,发现烯醇从亚胺的背面进攻其亲电C原子的过渡态的相对能量更低,容易得到反式的产物,对实验观察到的非对映选择性进行了合理的解释.