Diastereoselectivity on Intramolecular Alder-ene Reaction of 1,6-Dienes

A detailed computational study of the intramolecular Alder-ene reaction of different 1,6-dienes at M06-2X(PCM)/TZ2P level of theory has been performed. We want to understand the influence of enophile-geminal substitution pattern in the cis : trans selectivity of the cyclization process. Our analysis of the reaction coordinate by means of activation strain model of chemical reactivity (ASM-distortion interaction model) reveals that the cis-selectivity observed for unactivated reagents is related with high stabilizing orbital interaction and lower strain energy, consequence of an early transition structure. On the other hand, the presence of activating groups increases the asynchronicity of the transition structures and reduces the activation barrier due to more stabilizing orbital and electrostatic interactions, favoring trans-selectivity.

Mechanism and the Origins of Periselectivity in Cycloaddition Reactions of Benzyne with Dienes

Density functional theory calculations have been used to explore the reaction mechanism of (4 + 2) and (2 + 2) cycloadditions of benzyne with classical dienes. The results indicate the following: (1) (4 + 2) products arise via concerted pathways, (2) (2 + 2) products arise via stepwise pathways with diradical intermediates, and (3) these diradical intermediates are formed via isomerization of carbene intermediates. The origins of periselectivity in these reactions are analyzed using distortion/interaction analysis for the key steps, and they indicate that the tiny distortion in the very early [4 + 2] transition structure, coupled with an entropic favorability, controls selective (4 + 2) cycloaddition.

Theoretical Investigate of the α-Substitution Effect on γ-C(sp3)−H Arylation of Amines: Structure-Reactivity Relationship (SRR) Studies

Structure-reactivity relationship (SRR) studies to understand the α-substitution effect toward Pd-catalyzed γ-C(sp3)−H arylation enabled by catalytic transient directing groups have carefully been performed by DFT calculations. Selected transient directing groups...

Understanding the intermolecular Diels-Alder cycloaddition promotion: Activation strain model/energy decomposition analysis model and conceptual density functional theory viewpoints

The present work reports the computational study of the major Diels-Alder reaction between 2-bromocycloalkenone and a variety of mono- and di-substituted dienes. Through density functional theory (DFT) calculations and subsequent activation strain model/energy decomposition analysis/conceptual DFT (C-DFT) analyses, the key factors governing the activation barriers heights, and thus reactivity, are characterized. In contrast with a previous study, steric effects do not appear to control reactivity. Conversely, in all presented cases, a subtle interplay between deformation and interaction energies is evidenced at transition states. In the end, neither term alone is enough to explain or predict reactivity. Yet a simple C-DFT descriptor allows to predict with a reasonable efficiency the activation barriers: the excitation energy needed to observe a charge transfer from the diene to the dienophile. Theoretical elements are provided to support the use of this descriptor.

A novel partitioning scheme for the application of the distortion/interaction - activation strain model to intramolecular reactions

The distortion/interaction or activation strain model, developed by Houk and Bickelhaupt, relates chemical reactivity to the reagents deformations and reciprocal electronic influences. However, in its original formulation, it struggles to elucidate the mechanistic insights of intramolecular reactions, those unimolecular processes in which two parts of a molecule, the reaction centers, linked by a connector, are brought together to yield a different chemical species. Here we present a modification of the distortion/interaction procedure for its application on intramolecular reactions. This new procedure allows the calculation of the influence exerted by the connector over the reaction pathway in an indirect way, from the distortions of the two reaction centers and their interaction energy. This procedure does not include additional, undesired interactions and offers the possibility of calculating very large connectors in a computationally inexpensive way. We applied this methodology in the normal electron-demand Diels–Alder reaction of 1,3,8-nonatriene derivatives, with different functionalizations and connector lengths. In-depth analysis of the IRC showed that the reaction pathway can be subdivided in three main regions, what we called the oncoming, conversion and relaxation phases, each of them characterized by different evolutions of the distortion and interaction energies, and with clear geometry changes. We suggest that this new formulation can provide additional information for intramolecular reactions, especially to those processes for which the connector is said to play a crucial role in the observed reaction rates.

Comparative DFT Study on Dehydrogenative C(sp)-H Elementation (E = Si, Ge, and Sn) of Terminal Alkynes Catalyzed by a Cationic Ruthenium(II) Thiolate Complex

Described herein is a comparative theoretical study of dehydrogenative C(sp)-H functionalizations of a terminal alkyne with group-14-based hydrides (HEEt₃; E = Si, Ge, Sn) catalyzed by an Ohki-Tatsumi complex-a cationic Ru(II) complex with a tethered thiolate ligand ([Ru-S] = [(DmpS)Ru(PiPr₃)][BAr₄^F]; Dmp = 2,6-(dimesityl)₂C₆H₃; Ar^F = 3,5-(CF₃)₂C₆H₃). The calculations indicate that the energy barriers for heterolytic cleavage of the H-EEt₃ bonds at the Ru-S sites of the Ohki-Tatsumi complex highly vary depending on the group 14 elements from 3.8 kcal/mol (E = Sn) to 10.5 kcal/mol (E = Ge) and 18.5 kcal/mol (E = Si), where Ru and S elements cooperatively serve as the Lewis acid and base, respectively. Likewise, the transfer of the group 14 cation (Et₃E⁺) to the C-C triple bond to generate the β-element-stabilized vinyl cations-the rate-determining step (RDS) of the overall reaction-is predicted to be susceptible to the element's identity [E_a = 36.8 for Sn < 42.9 and Ge < 50.7 for Si (kcal/mol)]. The key transition states involved in the RDS are compared in terms of energy and structure within each system of the group 14 hydrides. The distortion/interaction-activation strain (DIAS) model analysis of the transition states responsible for dehydrogenative stannylation and hydrostannation of a terminal alkyne sheds light on the origin of the experimentally observed kinetic preference toward dehydrogenative C-H stannylation over hydrostannation.

A computational investigation of the selectivity and mechanism of the Lewis acid catalyzed oxa-Diels-Alder cycloaddition of substituted diene with benzaldehyde

The selectivity and the mechanism of the uncatalyzed and AlCl₃ catalyzed hetero-Diels-Alder reaction (HDR) between ([E]-4-methylpenta-2,4-dienyloxy)(tert-butyl)dimethylsilane 1 and benzaldehyde 2 have been studied using density functional theory at the MPWB1K/6-31G(d) level of theory. The uncatalyzed HDR between diene 1 and alkene 2 is characterized by a polar character and proceeds via an asynchronous one-step mechanism for the meta paths and synchronous for the ortho ones. In the presence of AlCl₃ catalyst, the mechanism changes to be stepwise, while the first step is the rate-determining step. The activation energies widely decrease, and the polar character increases dramatically. A large analysis of the mechanism is performed using the activation strain model/energy decomposition analysis (ASM/EDA) model, the natural bond orbital (NBO) and state specific dual descriptors (SSDDs). The obtained results indicate that the combined interaction energy associated with the distortion of the reactants in these HDR are at the origin of the observed kinetics. NBO analyses were applied to estimate the Lewis-acid catalyst donor-acceptor interaction with the molecular system. The SSDD analysis shed light into the orientation effects on the reaction kinetics by providing important information about charge transfer interactions during the chemical reaction. It indicates that the more favorable HDR pathway have the lowest excitation energies, facilitating the interaction between diene 1 and benzaldehyde 2 moieties. Non-covalent interaction (NCI) and QTAIM analyses of the meta-endo structure indicate that the presence of several weak NCIs formed at this approach is at the origin of the meta-endo selectivity.

Understanding chemical reactivity using the activation strain model

Understanding chemical reactivity through the use of state-of-the-art computational techniques enables chemists to both predict reactivity and rationally design novel reactions. This protocol aims to provide chemists with the tools to implement a powerful and robust method for analyzing and understanding any chemical reaction using PyFrag 2019. The approach is based on the so-called activation strain model (ASM) of reactivity, which relates the relative energy of a molecular system to the sum of the energies required to distort the reactants into the geometries required to react plus the strength of their mutual interactions. Other available methods analyze only a stationary point on the potential energy surface, but our methodology analyzes the change in energy along a reaction coordinate. The use of this methodology has been proven to be critical to the understanding of reactions, spanning the realms of the inorganic and organic, as well as the supramolecular and biochemical, fields. This protocol provides step-by-step instructions-starting from the optimization of the stationary points and extending through calculation of the potential energy surface and analysis of the trend-decisive energy terms-that can serve as a guide for carrying out the analysis of any given reaction of interest within hours to days, depending on the size of the molecular system.

PyFrag 2019-Automating the exploration and analysis of reaction mechanisms

We present a substantial update to the PyFrag 2008 program, which was originally designed to perform a fragment-based activation strain analysis along a provided potential energy surface. The original PyFrag 2008 workflow facilitated the characterization of reaction mechanisms in terms of the intrinsic properties, such as strain and interaction, of the reactants. The new PyFrag 2019 program has automated and reduced the time-consuming and laborious task of setting up, running, analyzing, and visualizing computational data from reaction mechanism studies to a single job. PyFrag 2019 resolves three main challenges associated with the automated computational exploration of reaction mechanisms: it (1) computes the reaction path by carrying out multiple parallel calculations using initial coordinates provided by the user; (2) monitors the entire workflow process; and (3) tabulates and visualizes the final data in a clear way. The activation strain and canonical energy decomposition results that are generated relate the characteristics of the reaction profile in terms of intrinsic properties (strain, interaction, orbital overlaps, orbital energies, populations) of the reactant species. © 2019 The Authors. Journal of Computational Chemistry published by Wiley Periodicals, Inc.

Factors Controlling the Diels-Alder Reactivity of Hetero-1,3-Butadienes

We have quantum chemically explored the Diels-Alder reactivities of a systematic series of hetero-1,3-butadienes with ethylene by using density functional theory at the BP86/TZ2P level. Activation strain analyses provided physical insight into the factors controlling the relative cycloaddition reactivity of aza- and oxa-1,3-butadienes. We find that dienes with a terminal heteroatom, such as 2-propen-1-imine (NCCC) or acrolein (OCCC), are less reactive than the archetypal 1,3-butadiene (CCCC), primarily owing to weaker orbital interactions between the more electronegative heteroatoms with ethylene. Thus, the addition of a second heteroatom at the other terminal position (NCCN and OCCO) further reduces the reactivity. However, the introduction of a nitrogen atom in the backbone (CNCC) leads to enhanced reactivity, owing to less Pauli repulsion resulting from polarization of the diene HOMO in CNCC towards the nitrogen atom and away from the terminal carbon atom. The Diels-Alder reactions of ethenyl-diazene (NNCC) and 1,3-diaza-butadiene (NCNC), which contain heteroatoms at both the terminal and backbone positions, are much more reactive due to less activation strain compared to CCCC.

Theoretical studies on the mechanism of Pd2+-catalyzed regioselective C-H acylation of azoxybenzenes with α-oxocarboxylic acids

The reaction mechanism of the Pd2+-catalyzed regioselective C(sp2)-H acylation of azoxybenzenes with α-oxocarboxylic acids has been studied by density functional theory (DFT) calculations. This reaction mechanism involves five major steps: C-H activation, deprotonation, decarboxylation, reductive elimination and oxidation. Our calculation results indicate that the N-coordinated pathway is better than the O-coordinated pathway, which can be interpreted by distortion-interaction analysis of the C-H bond activation transition states. Furthermore, we also suggest that the C-H bond acylation of aryl 1 is more favorable than that of aryl 2, which can be attributed to the fact that five-membered ring transition states are more favorable than four-membered ring transition states and the ON-group has positive charge.

Reactivity of arynes toward functionalized alkenes: intermolecular Alder-ene vs. addition reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described.

Long distance unconjugated agostic-assisted 1,5-H shift in a Ru-mediated Alder-ene type reaction: mechanism and stereoselectivity

A long distance unconjugated novel 1,5-H shift mechanism for a Ru-catalyzed Alder-ene type alkene–alkyne coupling reaction was examined by DFT.

Fluorine substituent effect on the stereochemistry of catalyzed and non-catalyzed Diels–Alder reactions. The case of R-butenone with cyclopentadiene: a computational assessment of the mechanism

A semiquantitative prediction of stereoselectivity due to substitutions of dienophile is obtained.