Theoretical Insight into PtCl₂-Catalyzed Isomerization of Cyclopropenes to Allenes

To understand the mechanism of allene formation through the rearrangement of cyclopropenes catalyzed by PtCl₂, we have performed a detailed density functional theory calculation study on a representative substrate, 1-(trimethylsilyl)-2-(phenylethyl)­cyclopropene. Three reaction pathways proposed in the original study have been examined; however the calculated results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we present a novel mechanism of PtCl₂, which is fixed appropriately on the cyclopropene/allene to form the linear Cl–Pt–Cl disposition, a vital configuration for catalyzing the rearrangement of cyclopropene. The newly proposed mechanism involves an S_N2-type C–C bond activation of the cyclopropene by PtCl₂ fixed on a cyclopropene molecule via the d−π interaction between the metal center and the substrate to form the product precursor PtCl₂-allene with the metal center coordinated to the external CC bond in the allene framework. Once formed, the PtCl₂-allene immediately serves as a new active center to catalyze the rearrangement reaction rather than directly dissociating into the allene product and the PtCl₂ catalyst due to its high stability. During the catalytic cycle, an allene-PtCl₂-allene sandwich compound is identified as the most stable structure on the potential energy surface, and its direct dissociation results in the formation of the product allene and the regeneration of the catalytically active center PtCl₂-allene with an energy demand of 24.4 kcal/mol. This process is found to be the rate-determining step of the catalytic cycle. In addition, to understand the experimental finding that the H-substituted cyclopropenes do not provide any allenes, we have also performed calculations on the H-substituted cyclopropene system and found that the highest barrier to be overcome during the catalytic cycle amounts to 35.2 kcal/mol. This high energy barrier can be attributed to the fact that the C–H bond activation is more difficult than the C–Si bond activation. The theoretical results not only rationalize well the experimental observations but provide new insight into the mechanism of the important rearrangement reaction

DFT Insight into a Strain-Release Mechanism in Bicyclo[1.1.0]butanes via Concerted Activation of Central and Lateral C–C Bonds with Rh(III) Catalysis

Transition-metal-catalyzed, strain-release-driven transformations of “spring-loaded” bicyclo[1.1.0]butanes (BCBs) are considered potent tools in synthetic organic chemistry. Previously proposed strain-release mechanisms involve either the insertion of the central C–C bond of BCBs into a metal–carbon bond, followed by β-C elimination, or the oxidative addition of the central or lateral C–C bond on the transition metal center, followed by reductive elimination. This study, employing DFT calculations on a Rh(III)-catalyzed model system in a three-component protocol involving oxime ether, BCB ester, and ethyl glyoxylate for constructing diastereoselective quaternary carbon centers, introduces an unusual strain-release mechanism for BCBs. In this mechanism, the catalytic reaction is initiated by the simultaneous cleavage of two C–C bonds (the central and lateral C–C bonds), resulting in the formation of a Rh-carbene intermediate. The new mechanism exhibits a barrier of 21.0 kcal/mol, making it energetically more favorable by 11.1 kcal/mol compared to the previously suggested most favorable pathway. This unusual reaction mode rationalizes experimental observation of the construction of quaternary carbon centers, including the excellent E-selectivity and diastereoselectivity. The newly proposed strain-release mechanism holds promise in advancing our understanding of transition-metal-catalyzed C–C bond activation mechanisms and facilitating the synthesis of transition metal carbene complexes

New Insight into the Formation Mechanism of Imidazolium-Based Ionic Liquids from <i>N</i>‑Alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Autopromoted Process

To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from <i>N</i>-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of <i>N</i>-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]­Cl) IL. The reaction is shown to proceed via an S_N2 transition state with a free energy barrier of 34.4 kcal/mol in the gas phase and 27.6 kcal/mol in toluene solvent. The reaction can be remarkably promoted by the presence of ionic products and water molecules. The calculated barriers in toluene are 22.0, 21.7, and 19.9 kcal/mol in the presence of 1–3 ionic pairs of [Emim]­Cl and 23.5, 21.3, and 19.4 kcal/mol in the presence of 1–3 water molecules, respectively. These ionic pairs and water molecules do not participate directly in the reaction but provide a polar environment that favors stabilizing the transition state with large charge separation. Hence, we propose that the synthesis of imidazolium-based ILs from <i>N</i>-alkyl imidazoles and halogenated hydrocarbons is an autopromoted process and a polar microenvironment induced reaction, and the existence of water molecules (a highly polar solvent) in the reaction may be mainly responsible for the initiation of reaction

Theoretical Insight into the Conversion Mechanism of Glucose to Fructose Catalyzed by CrCl₂ in Imidazolium Chlorine Ionic Liquids

To better understand the efficient transformation of glucose to fructose catalyzed by chromium chlorides in imidazolium-based ionic liquids (ILs), density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl₂ in 1,3-dimethylimidazolium chlorine (MMImCl) ionic liquid (IL). The reaction is shown to involve three fundamental processes: ring opening, 1,2-H migration, and ring closure. The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol. Throughout all elementary steps, both CrCl₂ and MMImCl are found to play substantial roles. The Cr center, as a Lewis acid, coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate, and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property, while the Cl^– anion is identified as a Bronsted/Lewis base and also a H-bond acceptor. Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of O2–H hydroxyl group, which is in nature different from the stepwise mechanism proposed in the early literature. The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs

How Do the Thiolate Ligand and Its Relative Position Control the Oxygen Activation in the Cysteine Dioxygenase Model?

In the iron­(II)-thiolate models of cysteine dioxygenase, the thiolate ligand is a key factor in the oxygen activation. In this contribution, four model compounds have been theoretically investigated. This comparative study reveals that the thiolate ligand itself and its relative position are both important for the activation of O₂. Before the O₂ binding, the thiolate ligand must transfer charge to Fe­(II), and the effective nuclear charges of Fe­(II) is decreased, which results in a lower redox potential of compounds. In other words, the thiolate ligand provides a prerequisite for the O₂ activation. Furthermore, the relative position of the thiolate ligand is discovered to determine the reaction path of O₂ activation. The amount of charge transfer is crucial for these reactions; the more charge it transfers, the lower the related redox potentials. This work really helps think deeper into the O₂ activation process of mononuclear nonheme iron enzymes

Theoretical Elucidation of the Mechanism and Kinetic Experimental Phenomena on the Esterification of α‑Tocopherol with Succinic Anhydride: Catalysis of a Histidine Derivative vs an Imidazolium-Based Ionic Liquid

DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of α-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazolium-based ionic liquid (IL). The two catalytic reactions involve an intrinsically consistent molecular mechanism: a rate-determining, concerted nucleophilic substitution followed by a facile proton-transfer process. The histidine derivative or the IL anion is shown to play a decisive role, acting as a Brönsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated free energy barriers of two reactions (15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction) together with their respective characteristic features, the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL, rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time

Theoretical Insight into the Mechanism of CO Inserting into the N–H Bond of the Iron(II) Amido Complex (dmpe)₂Fe(H)(NH₂): An Unusual Self-Promoted Reaction

Density functional theory (DFT) calculations have been carried out to study the detailed mechanism of CO inserting into the N–H bond rather than the common Fe–N bond of the iron­(II) amido complex (dmpe)₂Fe­(H)­(NH₂) (dmpe = 1,2-bis­(dimethylphosphino)­ethane). Three mechanisms proposed in previous literature have been computationally examined, and all of them are found to involve high barriers and thus cannot explain the observed N–H insertion product. Alternatively, on the basis of the calculated results, a novel reactant-assisted (self-promoted) mechanism is presented, which provides the most efficient access to the insertion reaction via the assistance of a second reactant molecule. In detail, the reaction starts from direct attack of CO at the amide nitrogen atom of (dmpe)₂Fe­(H)­(NH₂), followed by a second reactant-assisted H abstraction/donation processes to afford the trans product of CO inserting into the N–H bond of the amido complex. The present theoretical results provide a new insight into the mechanism of the unusual insertion reaction and rationalize the experimental findings well

Theoretical Elucidation of Glucose Dehydration to 5‑Hydroxymethylfurfural Catalyzed by a SO₃H‑Functionalized Ionic Liquid

While the catalytic conversion of glucose to 5-hydroxymethyl furfural (HMF) catalyzed by SO₃H-functioned ionic liquids (ILs) has been achieved successfully, the relevant molecular mechanism is still not understood well. Choosing 1-butyl-3-methylimidazolium chloride [C₄SO₃HmimCl] as a representative of SO₃H-functioned IL, this work presents a density functional theory (DFT) study on the catalytic mechanism for conversion of glucose into HMF. It is found that the conversion may proceed via two potential pathways and that throughout most of elementary steps, the cation of the IL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bonding, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol, respectively, which are compatible with the observed catalytic performance of the IL under mild conditions (100 °C). The present results provide help for rationalizing the effective conversion of glucose to HMF catalyzed by SO₃H-functionalized ILs and for designing IL catalysts used in biomass conversion chemistry

DFT Study on the Mechanism of Formic Acid Decomposition by a Well-Defined Bifunctional Cyclometalated Iridium(III) Catalyst: Self-Assisted Concerted Dehydrogenation via Long-Range Intermolecular Hydrogen Migration

DFT calculations have been performed to gain insight into the mechanism of formic acid (HCOOH) decomposition into H₂ and CO₂, catalyzed by a well-defined bifunctional cyclometalated iridium­(III) complex (a Ir–H hydride) based on a 2-aryl imidazoline ligand with a remote NH functionality. It is shown that the reaction features the direct protonation of the Ir–H hydride by HCOOH with the hydrogen shuttling between the NH group and the carbonyl group of HCOOH. Importantly, the simultaneous presence of two HCOOH molecules is proposed to be important for the dehydrogenation, where one works as a hydrogen source and the other acts as a hydrogen shuttle to assist the long-range intermolecular hydrogen migration. The dehydrogenation mechanism is referred to as HCOOH self-assisted concerted hydrogen migration. With such a mechanism, the energetic span, i.e. the apparent activation energy of the catalytic cycle, is calculated to be 17.3 kcal/mol, which is consistent with the observed rapid dehydrogenation of HCOOH under mild conditions (40 °C). On one hand, the effectiveness of the self-assisted catalytic system is attributed to the d–pπ conjugation between the Ir center and the proximal nitrogen, which increases the electron density at the Ir center and hence promotes the Ir–H bond cleavage. On the other hand, the effectiveness is also closely related to the hydrogen-shared three-center–four-electron (3c-4e) bond between formate and formic acid, which stabilizes the transition states and hence reduces the free energy barriers of the reaction. In addition, calculated results also emphasize the importance of the concerted catalysis of the bifunctional catalyst: when the γ-NH functional group does not participate in the reaction or is replaced by an O atom, the reaction becomes remarkably less favorable. The present work rationalizes the experimental findings and provides important insights into understanding the catalysis of the bifunctional cyclometalated iridium­(III) complexes

DFT Study on the Formation Mechanism of Normal and Abnormal N‑Heterocyclic Carbene–Carbon Dioxide Adducts from the Reaction of an Imidazolium-Based Ionic Liquid with CO₂

To illustrate the formation mechanism of normal and abnormal N-heterocyclic carbene–carbon dioxide adducts (NHC–CO₂ and aNHC–CO₂), we implement density functional theory calculations on the reactions of two imidazolium-based ionic liquids ([C₂C₁Im]­[OAc] and [C₂C₁Im]­[CH₃SO₃]) with CO₂. The reaction of [C₂C₁Im]­[OAc] with CO₂ is mimicked using the gas phase model, implicit solvent model, and combined explicit–implicit solvent model. In the gas phase, the calculated barriers at 125 °C and 10 MPa are 12.1 kcal/mol for the formation of NHC–CO₂ and 22.5 kcal/mol for the formation of aNHC–CO₂, and the difference is significant (10.4 kcal/mol). However, the difference becomes less important (1.5 kcal/mol) as the solvation effect is considered more realistically using the combined explicit–implicit solvent model, rationalizing the experimental observation of aNHC–CO₂ adduct in the [C₂C₁Im]­[OAc]–CO₂ system. The anion of the ionic liquid is shown to play a substantial role, which can adjust the reactivity of imidazolium cation toward CO₂: upon replacement of the basic [OAc]⁻ anion with a less basic [CH₃SO₃]⁻ anion, the reaction becomes very difficult, as indicated by high free energy barriers involved (41.4 kcal/mol for the formation of NHC–CO₂ and 39.2 kcal/mol for the formation of aNHC–CO₂). This is in agreement with the fact that neither NHC–CO₂ or aNHC–CO₂ is formed in the [C₂C₁Im]­[CH₃SO₃]–CO₂ system, emphasizing the important dependence of the reactivity on the basicity of the anion of imidazolium-based ionic liquids for the formation of NHC– and aNHC–CO₂ adducts