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Nucleophilic aromatic substitution reactions play a crucial role in the synthesis of organic compounds and are considered to be one of the most important synthetic transformations. Traditionally, these reactions have been described as stepwise processes in textbooks. However, with further research, the existence of concerted nucleophilic aromatic substitution reactions has been widely confirmed. This paper provides a brief overview of the stepwise pathway of nucleophilic aromatic substitution reactions and focuses on several examples of concerted reactions, highlighting the importance of reaction design in mechanistic studies. Integrating cutting-edge research findings into teaching can enhance students’ understanding of nucleophilic aromatic substitution reactions. [ABSTRACT FROM AUTHOR]

The stepwise and concerted Ionic Diels-Alder reaction between phenyl (pyridin-2-ylmethylene) oxonium and styrene derivatives are explored using theoretical method. The results support using computational method via persistent intermediates. The DFT method was essential to reproduce a reasonable potential energy surface for these challenging systems.

[Display omitted] • The incomplete α-cage on [1 0 0]-D4R-e surface can adsorb more than four ethylene molecules. • The Si–OH and the Si–O–Al bridge on [1 0 0]-D4R-e surface are the active sites of ethylene oligomerization. • The reaction network of ethylene oligomerization is divided into stepwise polymerization path, concerted polymerization path and concerted butene path. • The energy barrier of the rate-determining step of ethylene oligomerization on K-LTA zeolite is significantly higher than that on other zeolites. • Trace n-butene will be formed when the temperature reaches 473 K, and oligomerization will occur at 513 K, which may lead to carbon deposition. The regeneration gas used in the K-LTA zeolite dryer of the MTO plant is planning to be changed from high purity nitrogen to the top gas of the demethanizer tower with high ethylene content, which is helpful to energy saving and emission reduction. However, there is a risk of ethylene oligomerization and carbon deposition when the regeneration gas passes through the K-LTA zeolite. The reaction mechanism of ethylene oligomerization on the K-LTA zeolite surface must be clarified. DFT calculations showed that ethylene could be physically adsorbed on Si–OH, Al–OH, the incomplete α- and β-cage on [1 0 0]-D4R-e surface. Butoxide forms through ethylene dimerization by stepwise and concerted mechanisms first. Then the reaction of ethylene and butoxide to n-hexyloxide leads to the continued growth of the carbon chain. The oligomerization reaction barriers of rate-determining steps are between 1.65∼1.75 eV, which is more difficult to occur than other zeolites. The ethylene oligomerization reaction can be divided into stepwise polymerization path, concerted polymerization path and concerted butene path. kMC simulation found that the dominant reaction paths are different under different temperatures. When the regeneration temperature reaches 473 K, trace n-butene is generated. At 513 K, ethylene oligommerization occurs and carbon deposition is formed. The results put forward higher requirements for strictly controlling the temperature of the dryer to ensure the excellent performance of K-LTA zeolite. [ABSTRACT FROM AUTHOR]

The mechanism of the Wolff rearrangement of some deprotonated diazocarbonyl compounds has been studied using density functional calculations, for both the gas and aqueous phases. The anionic rearrangement of the deprotonated analogues proceeds in a similar manner to the neutral counterparts. In the gas phase, the syn and anti conformers form the deprotonated ketene product concertedly. In certain analogues, the reaction of the anti conformer occurs non-concertedly, involving a deprotonated carbene-like intermediate. The migratory aptitude is similar to that in the neutral rearrangement, except that methoxy and amino substituents exchange positions. [ABSTRACT FROM AUTHOR]

For several decades, the concerted or stepwise character of the mechanism of the 1,3-dipolar cycloaddition reaction has been one of the most debated issues in the field of organic chemistry. The significance of this problem is due to the fact that in a catalyst-free 1,3-dipolar cycloaddition, when the mechanism switches from concerted to stepwise, the stereospecificity is lost and thus unwanted stereoisomers may emerge. The first proposals about the mechanism of the 1,3-dipolar reaction were due to Huisgen (concerted model) and subsequently by Firestone (two-step diradical channel) in the 1960s. After a decade of debate, most researchers accepted the concerted model for the reaction, but during these years, researchers reported some examples of the stepwise mechanism for catalyst-free 1,3-dipolar cycloadditions. This review attempts to find a number of factors that could influence the reaction channels and switch the mechanism from concerted to stepwise, or vice versa. [ABSTRACT FROM AUTHOR]

The Wolff rearrangement is a 1,2-rearrangement reaction of α-diazocarbonyl compounds, leading to ketene formation and the extrusion of nitrogen. The reaction may undergo a concerted mechanism or involve a carbene as an intermediate. There is also the possibility of its rearrangement to oxirene. In this work, we have performed density functional calculations to elucidate the mechanism of the thermal Wolff reaction for both the gas and aqueous phases. The concerted mechanism appears to be slightly favored over the stepwise mechanism for alkyl and aryl substitution of the aldehydic hydrogen. Various diazocarbonyl systems have been investigated in order to explore the influence of the migrating group on the activation barrier and course of reaction. The stepwise rearrangement is preferred in case of methoxy and amino substitution. The intermediate in the stepwise rearrangement is found to have an interesting structure, neither completely a carbene nor an oxirene. [ABSTRACT FROM AUTHOR]

It is a well-known fact that by changing the 1,3-dipolar cycloaddition (1,3- DC) reaction mechanism from concerted to stepwise, the stereospecificity is lost; since in synthesizing the required heterocyclic molecules that reaction is a requisite, it is important to study the concertedness of that reaction. Several papers on this subject have already stated that the existence of electron withdrawing groups ( EWG) or electron donor groups ( EDG) on dipole or dipolarophile leads to a high-energy differentiation between the dipole HOMO and dipolarophile LUMO (or vice versa) as well as the emergence of an intermediate in the reaction pathway. This paper seeks answering the question of when an EWG on dipole and an EDG on dipolarophile could be a factor in making the reaction mechanism stepwise, and does repositioning of functional groups in replacing dipole and dipolarophile switches the reaction mechanism from stepwise into concerted or vice versa? [ABSTRACT FROM AUTHOR]

The double-proton transfer mechanism in the newly synthesized molecule 1,8-dihydroxydibenzo[a,c]phenazine containing two hydrogen bonds is investigated by means of the time-dependent density functional theory and resolution-of-identity second-order approximate coupled-cluster methods. Both reaction paths of the concerted and the stepwise mechanisms are examined, and the results show that the double-proton transfer is likely to occur in the excited state following the stepwise mechanism in the gas phase at the lowest excitation energy. Based on the calculated potential energy scheme, the single-proton transfer and the double-proton transfer are likely to coexist, with the former is expected to occupy in a significant number. The concerted reaction path of C2 symmetry is found to be less preferred compared with the stepwise reaction path, and also could be dynamically open due to the small activation energy. [ABSTRACT FROM AUTHOR]

Highlights: [•] The alcoholysis and thiolysis of four triesters have been studied using density functional theory. [•] The reactions proceed through a concerned mechanism or an associative mechanism. [•] The thiolysis are less favorable than the corresponding alcoholysis reactions. [•] The activation enthalpies in solution are higher than those in the gas phase. [Copyright &y& Elsevier]

The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd-electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC-TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t-butyldimethylsilyl oxime ethers with oxygen in a β-position, the McLafferty rearrangement was accompanied by loss of the t-butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β-position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C-C bond breaking was the rate-determining step. However, for both the oximes and t-butyldimethylsilyl oxime ethers with oxygen at the β-position, the hydrogen transfer step was rate limiting, whereas with a CH2 group at the β-position, the C-C bond breaking was again rate determining. n-Propoxy-acetaldehyde, bearing an oxygen atom at the β-position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the ( E)- and ( Z)-isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism of the aminolysis of dimethyl phenylphosphinate with ammonia was investigated by the density functional theory at the B3LYP level with basis set 6-31G(d,p) in the gas phase. Single point energy evaluations were also performed for more precise energy predictions at the B3LYP and the BB1K levels with the 6-311++G(d,p) basis set. Solvent effects on the uncatalyzed process were assessed by the polarized continuum model with B3LYP/6-311++G(d,p) and BB1K/6-311++G(d,p) based on the gas-phase optimized geometries. Transition state structures and energies were determined for concerted and neutral stepwise pathways. The theoretical results show that the stepwise pathway is more favorable than the concerted process. The general base catalysis of the process was also examined. The catalytic role of a second ammonia molecule is executed by facilitating the hydrogen transfer processes and by decreasing all energy barriers. The results show that the most favorable pathway of the reaction is through the general-base-catalyzed neutral stepwise mechanism. [Copyright &y& Elsevier]

Molecular complexes are constructed to simulate proton transfer channels of the influenza A virus and of the active site of carbonic anhydrase. These complexes consist of proton donor and acceptor groups connected by a chain of water molecules. Quantum chemical calculations on the methylimidazole(H+)&bond;H2O&bond;CH3COO- model of the M2 virus channel indicate free translational motion of the water molecule between donor and acceptor, as well as concerted transfer of both H-bond protons. The proton transfer barrier does not depend on the position of the bridged water molecule and varies linearly with the difference of electrostatic potentials between the donor and acceptor. When the water chain is elongated, and with various donor and acceptor models, periodicity appears in the H-bond lengths and the progression of proton transfer in each link. This “wave” is shown to propagate along the chain, as it is driven by the displacement of a single proton. One can thereby estimate the velocity of the proton wave and proton conduction time. Computations are performed to examine the influence of immersing the system within a polarizable medium. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [ABSTRACT FROM AUTHOR]