scholarly journals Guanidine-catalyzed asymmetric Strecker reaction: modes of activation and origin of stereoselectivity

2016 ◽  
Vol 94 (12) ◽  
pp. 1099-1108 ◽  
Author(s):  
Hansong Xue ◽  
Choon-Hong Tan ◽  
Ming Wah Wong
Keyword(s):  
Density Functional ◽  
Catalytic Mechanism ◽  
Interaction Analysis ◽  
Transition States ◽  
Enantiomeric Excess ◽  
Energy Transition ◽  
Density Functional Theory Calculations ◽  
Noncovalent Interaction ◽  
Acid Activation ◽  
Strecker Reaction

Density functional theory calculations were employed to study the catalytic mechanism, modes of activation, and origin of enantioselectivity of guanidine-catalyzed asymmetric Strecker reaction of N-benzhydryl imine with hydrogen cyanide. Two types of bifunctional activation mode were identified, namely conventional bifunctional Brønsted acid activation and unconventional bifunctional Brønsted–Lewis acid activation. The lowest-energy transition states correspond to the conventional bifunctional mode of activation. The calculated enantiomeric excess, based on eight lowest-energy C–C bond forming transition states, is in good accord with observed enantioselectivity. NCI (noncovalent interaction) analysis of the key transition states reveals extensive noncovalent interactions, including aromatic interactions and hydrogen bonds, between the guanidinium catalyst and substrates. Multiple aryl–aryl interactions between the phenyl groups of guanidine catalyst and the phenyl rings of N-benzhydryl imine are the key stabilizations in the most stable (R)-inducing transition state. Differential attractive aryl–aryl stabilization is the major factor for stereoinduction.

Pentanidium-Catalyzed Asymmetric Phase-Transfer Conjugate Addition: Prediction of Stereoselectivity via DFT Calculations and Docking Sampling of Transition States, and Origin of Stereoselectivity

2016 ◽  
Vol 69 (9) ◽  
pp. 983 ◽  
Author(s):  
Choon Wee Kee ◽  
Ming Wah Wong
Keyword(s):  
Density Functional ◽  
Catalytic Mechanism ◽  
Phase Transfer ◽  
Interaction Analysis ◽  
Hybrid Approach ◽  
Transition States ◽  
Interaction Model ◽  
Conjugate Addition ◽  
Experimental Result ◽  
Covalent Interaction

Density functional theory (DFT) study, at the M06–2X/6–311+G(d,p)//M06–2X/6–31G(d,p) level, was carried out to examine the catalytic mechanism and origin of stereoselectivity of pentanidium-catalyzed asymmetric phase-transfer conjugate addition. We employed a hybrid approach by combining automated conformation generation through molecular docking followed by subsequent DFT calculation to locate various possible transition states for the enantioselective conjugate addition. The calculated enantioselectivity (enantiomeric excess), based on the key diastereomeric C–C bond-forming transition states, is in good accord with experimental result. Non-covalent interaction analysis of the key transition states reveals extensive non-covalent interactions, including aromatic interactions, hydrogen bonds, and non-classical C–H⋯O interactions between the pentanidium catalyst and substrates. The origin of stereoselectivity was analysed using a strain-interaction model.

scholarly journals Theoretical Determination of Size Effects in Zeolite-Catalyzed Alcohol Dehydration

2019 ◽  
Author(s):  
Larrissa Y. Kunz ◽  
Lintao Bu ◽  
Brandon C. Knott ◽  
Cong Liu ◽  
Mark R. Nimlos ◽  
...  
Keyword(s):  
Electrostatic Interactions ◽  
Density Functional ◽  
Activation Barrier ◽  
Knudsen Diffusion ◽  
Transition States ◽  
Density Functional Theory Calculations ◽  
Detailed Examination ◽  
Amyl Alcohol ◽  
Primary Objective ◽  
Alcohol Dehydration

In the upgrading of biomass pyrolysis vapors to hydrocarbons, dehydration accomplishes a primary objective of removing oxygen and acidic zeolites represent promising catalysts for dehydration reaction. Here, we utilize density functional theory calculations to estimate adsorption energetics and intrinsic kinetics of alcohol dehydration over H-ZSM-5, H-BEA, and H-AEL zeolites. ONIOM calculations of adsorption energies were observed to be inconsistent when benchmarked against QM/Hartree-Fock and periodic boundary condition calculations. However, reaction coordinate calculations of adsorbed species and transition states were consistent across all levels considered. Comparison of ethanol, iso-propanol (IPA), and tert-amyl alcohol (TAA) over these three zeolites allowed for a detailed examination of how confinement impacts reaction mechanisms and kinetics. TAA, seen to proceed via a carbocationic mechanism, was found to have the lowest activation barrier, followed by IPA and then ethanol, both of which dehydrate via a concerted mechanism. Barriers in H-BEA were consistently found to be lower than in H-ZSM-5 and H-AEL, attributed to late transition states and either elevated strain or inaccurately estimating long-range electrostatic interactions in H-AEL, respectively. Molecular dynamics simulations revealed that the diffusivity of these three alcohols in H-ZSM-5 are significantly overestimated by Knudsen diffusion, which will complicate experimental efforts to develop a kinetic model for catalytic fast pyrolysis.

scholarly journals Computational Design of SCS Nickel Pincer Complexes for the Asymmetric Transfer Hydrogenation of 1-Acetonaphthone

Catalysts ◽  
2019 ◽  
Vol 9 (1) ◽  
pp. 101 ◽  
Author(s):  
Bing Qiu ◽  
Wan Wang ◽  
Xinzheng Yang
Keyword(s):  
Density Functional ◽  
Enantiomeric Excess ◽  
Density Functional Theory Calculations ◽  
Computational Design ◽  
Transfer Hydrogenation ◽  
Pincer Complexes ◽  
Free Energy Barrier ◽  
Asymmetric Transfer Hydrogenation ◽  
Functional Theory ◽  
Prochiral Ketones

Inspired by the active site structures of lactate racemase and recently reported sulphur–carbon–sulphur (SCS) nickel pincer complexes, a series of scorpion-like SCS nickel pincer complexes with an imidazole tail and asymmetric claws was proposed and examined computationally as potential catalysts for the asymmetric transfer hydrogenation of 1-acetonaphthone. Density functional theory calculations reveal a proton-coupled hydride transfer mechanism for the dehydrogenation of (R)-(+)-1-phenyl-ethanol and the hydrogenation of 1-acetonaphthone to produce (R)-(+)-1-(2-naphthyl)ethanol and (S)-(−)-1-(2-naphthyl)ethanol. Among all proposed Ni complexes, 1Ph is the most active one with a rather low free energy barrier of 24 kcal/mol and high enantioselectivity of near 99% enantiomeric excess (ee) for the hydrogenation of prochiral ketones to chiral alcohols.

DFT interpretations for cycloadditions of an electron deficient C-aryl-N-phenyl nitrone

2014 ◽  
Vol 13 (01) ◽  
pp. 1450007 ◽  
Author(s):  
Nivedita Acharjee
Keyword(s):  
Density Functional ◽  
Transition States ◽  
Density Functional Theory Calculations ◽  
Basis Set ◽  
Dipolar Cycloaddition ◽  
Functional Theory ◽  
Concerted Mechanism ◽  
Reaction Channels ◽  
Calculated Activation Energy ◽  
Calculated Activation

1,3-dipolar cycloaddition reactions of an electron deficient C-aryl-N-phenyl nitrone to benzylidene derivatives (with different electrophilicities) have been analyzed by density functional theory calculations. The transition states corresponding to the endo and exo approaches along the feasible regioisomeric reaction channels have been located for each cycloaddition. The reactions follow a concerted mechanism with asynchronous transition states. The asynchronicity along the regiochemical reaction modes depends on the β-carbon electrophilicities of the olefins. The regio and stereochemistries predicted from the calculated activation energy barriers (with solvent and higher basis set corrections) of the located transition states are in conformity with the experimental results. The local electrophilicities, softness matching indices and the interaction energies were then calculated to analyze how well these reactivity parameters can interpret the regioselectivities of such reactions. The electronic populations at the reactive sites computed from electrostatic potential-driven atomic charges provided correct and consistent predictions for each theoretical model contrary to the natural orbital based charges.

scholarly journals A density functional theory study on the mechanism of the allylpalladium-catalyzed dehydrogenation of aldehydes and cyclic ketones

2021 ◽  
Vol 46 ◽  
pp. 146867832110206
Author(s):  
Anan Haj Ichia Arisha
Keyword(s):  
Density Functional Theory ◽  
Gas Phase ◽  
Density Functional ◽  
Transition States ◽  
Density Functional Theory Calculations ◽  
Carbonyl Complex ◽  
Cyclic Ketones ◽  
Complex Catalyst ◽  
Functional Theory ◽  
Hydride Elimination

The results of density functional theory calculations at the APFD/SDD level are detailed herein in order to study the main steps in the α,β-dehydrogenation of aldehydes and cyclic ketones in the presence of an allylpalladium complex catalyst. The mechanism is believed to proceed via an allylpalladium enolate complex (A) in equilibrium with the carbon-bonded complex (B), followed by β-hydride elimination to yield the allylpalladium hydride coordinated to the α,β-unsaturated carbonyl (complex C). The optimized structures and detailed energy profiles of these intermediates and their corresponding transition states are presented herein. The results indicate that the intermediates and their transition states are more stable in THF solution than in the gas phase. In detail, the energy barriers for the two steps are found to be 25.22 and 11.13 kcal/mol, respectively, in THF, and 29.93 and 9.77 kcal/mol, respectively, in the gas phase.

scholarly journals Theoretical Determination of Size Effects in Zeolite-Catalyzed Alcohol Dehydration

Catalysts ◽  
2019 ◽  
Vol 9 (9) ◽  
pp. 700 ◽  
Author(s):  
Larissa Y. Kunz ◽  
Lintao Bu ◽  
Brandon C. Knott ◽  
Cong Liu ◽  
Mark R. Nimlos ◽  
...  
Keyword(s):  
Electrostatic Interactions ◽  
Density Functional ◽  
Activation Barrier ◽  
Knudsen Diffusion ◽  
Transition States ◽  
Density Functional Theory Calculations ◽  
Detailed Examination ◽  
Primary Objective ◽  
Alcohol Dehydration ◽  
Molecular Mechanics Calculations

In the upgrading of biomass pyrolysis vapors to hydrocarbons, dehydration accomplishes a primary objective of removing oxygen, and acidic zeolites represent promising catalysts for the dehydration reaction. Here, we utilized density functional theory calculations to estimate adsorption energetics and intrinsic kinetics of alcohol dehydration over H-ZSM-5, H-BEA, and H-AEL zeolites. The ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) calculations of adsorption energies were observed to be inconsistent when benchmarked against QM (Quantum Mechanical)/Hartree–Fock and periodic boundary condition calculations. However, reaction coordinate calculations of adsorbed species and transition states were consistent across all levels considered. Comparison of ethanol, isopropanol (IPA), and tert-amyl alcohol (TAA) over these three zeolites allowed for a detailed examination of how confinement impacts on reaction mechanisms and kinetics. The TAA, seen to proceed via a carbocationic mechanism, was found to have the lowest activation barrier, followed by IPA and then ethanol, both of which dehydrate via a concerted mechanism. Barriers in H-BEA were consistently found to be lower than in H-ZSM-5 and H-AEL, attributed to late transition states and either elevated strain or inaccurately estimating long-range electrostatic interactions in H-AEL, respectively. Molecular dynamics simulations revealed that the diffusivity of these three alcohols in H-ZSM-5 were significantly overestimated by Knudsen diffusion, which will complicate experimental efforts to develop a kinetic model for catalytic fast pyrolysis.

scholarly journals Structures and reaction rates of the gaseous oxidation of SO<sub>2</sub> by an O<sub>3</sub><sup>−</sup>(H<sub>2</sub>O)<sub>0-5</sub> cluster – a density functional theory investigation

2012 ◽  
Vol 12 (8) ◽  
pp. 3639-3652 ◽  
Author(s):  
N. Bork ◽  
T. Kurtén ◽  
M. B. Enghoff ◽  
J. O. P. Pedersen ◽  
K. V. Mikkelsen ◽  
...  
Keyword(s):  
Density Functional Theory ◽  
Atmospheric Chemistry ◽  
Density Functional ◽  
Transition States ◽  
Density Functional Theory Calculations ◽  
Reaction Rates ◽  
Water Molecules ◽  
Functional Theory ◽  
High Entropy ◽  
Collision Complexes

Abstract. Based on density functional theory calculations we present a study of the gaseous oxidation of SO2 to SO3 by an anionic O3−(H2O)n cluster, n = 0–5. The configurations of the most relevant reactants, transition states, and products are discussed and compared to previous findings. Two different classes of transition states have been identified. One class is characterised by strong networks of hydrogen bonds, very similar to the reactant complexes. The other class is characterised by sparser structures of hydration water and is stabilised by high entropy. At temperatures relevant for atmospheric chemistry, the most energetically favourable class of transition states vary with the number of water molecules attached. A kinetic model is utilised, taking into account the most likely outcomes of the initial SO2 O3−(H2O)n collision complexes. This model shows that the reaction takes place at collision rates regardless of the number of water molecules involved. A lifetime analysis of the collision complexes supports this conclusion. Hereafter, the thermodynamics of water and O2 condensation and evaporation from the product SO3−O2(H2O)n cluster is considered and the final products are predicted to be O2SO3− and O2SO3−(H2O)1. The low degree of hydration is rationalised through a charge analysis of the relevant complexes. Finally, the thermodynamics of a few relevant reactions of the O2SO3− and O2SO3−(H2O)1 complexes are considered.

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