Theoretical Studies on Cycloaddition Reactions between Keteniminium Cations and Olefins

The mechanisms of seven reactions between keteniminium cations and olefins have been theoretically explored at BHandHLYP/6-31G* level. It is found that these seven reactions always form a relatively stable hydrogen-bonded type of ion−molecule complex first except for reactions 1d+2a and 1e+2a, which have no hydrogen atom attached to nitrogen atom in keteniminium cations. Some reactions take place via a concerted but unsynchronous mechanism, and the others are stepwise processes. The substituent effects are also studied. The data reveal that the electron-pushing substituents on keteniminium cations disfavor the reaction, and the electron-attracting substituents on keteniminium cations favor the reactions. The substituent effects on ethene are contrary to the former case

An Explicit Interpretation of the Directing Group Effect for the Pd(OAc)₂‑Catalyzed Aromatic C–H Activations

A comprehensive DFT investigation has been performed for a series of the Pd­(OAc)₂-catalyzed C–H activations, updating and extending the understanding of directing group effect. In the beginning, the directed and undirected C–H activation mechanisms, based on 10 model reactions, have been discussed comparatively, which disclosed that directing group can exert a thermodynamic driving force, not necessarily a kinetic promotion, on the C–H activation process. Formation of the open palladation species via the undirected pathway is thermodynamically unspontaneous (Δ<i>G</i> = 4–9 kcal/mol), in sharp contrast to that of the cyclopalladation species via the directed pathway (Δ<i>G</i> < 0). Further calculations revealed that the free-energy barriers of proton-transfer are in fact not so high on the undirected pathway (17–24 kcal/mol), while mediation of some O-center groups in the directed pathway would increase the free-energy barriers of proton-transfer. For pyridine <i>N</i>-oxide systems, the undirected mechanism was estimated to be more plausible than the 4-member-directed one both thermodynamically and kinetically. In addition, the uncommon 7-membered cyclopalladation has been tentatively explored using two current examples, predicting that electron-rich directing groups can help to stabilize the 7-membered palladacycles formed

Catalytic C–H Activation/C–C Coupling Reaction: DFT Studies on the Mechanism, Solvent Effect, and Role of Additive

A series of density functional theory (DFT) experiments, employing the B3LYP+IDSCRF/BS1 and B3LYP+IDSCRF/DZVP methods, have been carried out for the Pd­(OAc)₂-catalyzed enamide–siloxane C–H activation/C–C coupling reactions. The results reveal that there are four processes, namely C–H activation, transmetalation (TM), reductive elimination (RE), and separation of product (SP) and recycling of catalyst (RC), each of which is consist of different steps. In order to fully understand the origin of regiospecific C–H activation/C–C coupling on the alicyclic ring experimentally observed, the conformational preference, kinetic aspects, and relative stabilities of the competitive products have been explored. In addition, the roles of additive silver salt AgF and solvent dioxane have also been addressed, providing valuable details upon which to rationally optimize experimental conditions

Theoretical Studies on Cycloaddition Reactions between the 2-Aza-1,3-butadiene Cation and Olefins

Density functional (B3LYP) calculations, using the 6-31G** basis set, have been employed to study the title reactions. For the model reaction (H2CC−NH+CH2 + H2CCH2), a complex has been formed with 6.2 kcal/mol of stabilization energy and the transition state is 4.0 kcal/mol above this complex, but 2.1 kcal/mol below the reactants. However, the substituent effects are quite remarkable. When ethene is substituted by electron-withdrawing group CN, the reaction could also yield six-membered-ring products, but the energy barriers are all more than 7 kcal/mol, which shows that CN group unfavors the reaction. The other substituents, such as CH3O and CH3 groups, have also been considered in the present work, and the results show that they are favorable for the formation of six-membered-ring adducts. The calculated results have been rationalized with frontier orbital interaction and topological analysis

DFT Studies on the Mechanisms of Carboamination/Diamination of Unactivated Alkenes Mediated by Pd(IV) Intermediates

Density functional theory (DFT) calculations have been employed to investigate the mechanism of carboamination and diamination of unactivated alkenes mediated by Pd(IV) intermediates. Both reactions share a common Pd(IV) intermediate, serving as the starting point for either the carboamination or the diamination pathway. The formation of this Pd(IV) intermediate encompasses a transition state that substantially impacts the turnover frequency (TOF) of catalytic cycles, with an apparent activation free-energy barrier of 26.1 kcal mol–1. Carboamination of unactivated alkenes proceeds through the coordination of a toluene molecule, C–H activation, inner reductive elimination, and the separation of the carboamination product from this intermediate, while diamination of unactivated alkenes involves the formation of the ion nucleophile, SN2 attack, and the separation of the diamination product. A comparison of the free-energy profiles for carboamination and diamination of unactivated alkenes can elucidate the origin of the chemoselectivity, and Bader’s atoms in molecules (AIM) wave function analyses have been performed to analyze the contributions of the outer C–N bonding in the diamination process

DFT Studies on the Dirhodium-Catalyzed [3 + 2] and [3 + 3] Cycloaddition Reactions of Enol Diazoacetates with Isoquinolinium Methylide: Mechanism, Selectivity, and Ligand Effect

The reaction mechanisms of dirhodium-catalyzed [3 + 2] and [3 + 3] cycloaddition between enol diazoacetate and isoquinolinium methylide have been studied in detail using density functional theory and a solution-phase translational entropy model. The reaction starts with the formation of a metallic carbene intermediate first, from which two competing reaction channels of [3 + 2] and [3 + 3] cycloaddition take place. For <b>CAT1</b>-catalyzed reactions, the calculated activation free energy barriers for [3 + 3] and [3 + 2] cycloaddition reactions are 14.3 and 16.0 kcal mol^–1, respectively, which is in good agreement with the ratio of products. Both the steric and electronic effects have been considered for <b>CAT2</b>- and <b>CAT3</b>-catalyzed reactions, with which the ratio of products has also been rationalized

DFT Studies on the Dirhodium-Catalyzed [3 + 2] and [3 + 3] Cycloaddition Reactions of Enol Diazoacetates with Isoquinolinium Methylide: Mechanism, Selectivity, and Ligand Effect

The reaction mechanisms of dirhodium-catalyzed [3 + 2] and [3 + 3] cycloaddition between enol diazoacetate and isoquinolinium methylide have been studied in detail using density functional theory and a solution-phase translational entropy model. The reaction starts with the formation of a metallic carbene intermediate first, from which two competing reaction channels of [3 + 2] and [3 + 3] cycloaddition take place. For <b>CAT1</b>-catalyzed reactions, the calculated activation free energy barriers for [3 + 3] and [3 + 2] cycloaddition reactions are 14.3 and 16.0 kcal mol^–1, respectively, which is in good agreement with the ratio of products. Both the steric and electronic effects have been considered for <b>CAT2</b>- and <b>CAT3</b>-catalyzed reactions, with which the ratio of products has also been rationalized

An ab Initio Study toward Understanding the Mechanistic Photochemistry of Acetamide

The potential energy surfaces for CH3CONH2 dissociation into CH3 + CONH2, CH3CO + NH2, CH3CN + H2O, and CH3NH2 + CO in the ground and lowest triplet states have been mapped with DFT, MP2, and CASSCF methods with the cc-pVDZ and cc-pVTZ basis sets, while the S1 potential energy surfaces for these reactions were determined by the CASSCF/cc-pVDZ optimizations followed by CASSCF/MRSDCI single-point calculations. The reaction pathways leading to different photoproducts are characterized on the basis of the computed potential energy surfaces and surface crossing points. A comparison of the reactivity among HCONH2, CH3CONH2, and CH3CONHCH3 has been made, which provides some new insights into the mechanism of the ultraviolet photodissociation of small amides

An ab Initio Study toward Understanding the Mechanistic Photochemistry of Acetamide

The potential energy surfaces for CH3CONH2 dissociation into CH3 + CONH2, CH3CO + NH2, CH3CN + H2O, and CH3NH2 + CO in the ground and lowest triplet states have been mapped with DFT, MP2, and CASSCF methods with the cc-pVDZ and cc-pVTZ basis sets, while the S1 potential energy surfaces for these reactions were determined by the CASSCF/cc-pVDZ optimizations followed by CASSCF/MRSDCI single-point calculations. The reaction pathways leading to different photoproducts are characterized on the basis of the computed potential energy surfaces and surface crossing points. A comparison of the reactivity among HCONH2, CH3CONH2, and CH3CONHCH3 has been made, which provides some new insights into the mechanism of the ultraviolet photodissociation of small amides

DFT Case Study on the Comparison of Ruthenium-Catalyzed C–H Allylation, C–H Alkenylation, and Hydroarylation

Density functional calculations at the B3LYP-D3+IDSCRF/TZP-DKH­(-dfg) level of theory have been performed to understand the mechanism of ruthenium-catalyzed C–H allylation reported in the literature in depth. The plausible pathway consisted of four sequential processes, including C–H activation, migratory insertion, amide extrusion, and recovery of the catalyst, in which C–H activation was identified as the rate-determining step. The amide extrusion step could be promoted kinetically by trifluoroacetic acid since its mediation lowered the free-energy barrier from 32.1 to 12.2 kcal/mol. Additional calculations have been performed to explore other common pathways between arenes and alkenes, such as C–H alkenylation and hydroarylation. A comparison of the amide extrusion and β-H elimination steps established the following reactivity sequence of the leaving groups: protonated amide group > β-H group > unprotonated amide group. The suppression of hydroarylation was attributed to the sluggishness of the Ru–C protonation step as compared to the amide extrusion step. This study can unveil factors favoring the C–H allylation reaction