Understanding the differences between iron and palladium in cross-coupling reactions

Rational design of iron catalysts for C-X bond activation

We have quantum chemically studied the iron-mediated CX bond activation (X = H, Cl, CH₃ ) by d⁸ -FeL₄ complexes using relativistic density functional theory at ZORA-OPBE/TZ2P. We find that by either modulating the electronic effects of a generic iron-catalyst by a set of ligands, that is, CO, BF, PH₃ , BN(CH₃ )₂ , or by manipulating structural effects through the introduction of bidentate ligands, that is, PH₂ (CH₂ )_n PH₂ with n = 6-1, one can significantly decrease the reaction barrier for the CX bond activation. The combination of both tuning handles causes a decrease of the CH activation barrier from 10.4 to 4.6 kcal mol^-1 . Our activation strain and Kohn-Sham molecular orbital analyses reveal that the electronic tuning works via optimizing the catalyst-substrate interaction by introducing a strong second backdonation interaction (i.e., "ligand-assisted" interaction), while the mechanism for structural tuning is mainly caused by the reduction of the required activation strain because of the pre-distortion of the catalyst. In all, we present design principles for iron-based catalysts that mimic the favorable behavior of their well-known palladium analogs in the bond-activation step of cross-coupling reactions.

Computational insights into the inverse electron-demand Diels-Alder reaction of norbornenes with 1,2,4,5-tetrazines: norbornene substituents' effects on the reaction rate

The study of the reaction rates and mechanism of click chemistry reactions still remains an interesting challenge in organic chemistry. In this regard, the inverse electron demand Diels-Alder (IEDDA) reaction represents a promising metal-free alternative with enhanced reaction rates compared to other reactions of the click chemistry toolbox. Among the different types of dienophiles used in the IEDDA reactions, norbornenes have been widely used given their high stability and fast reaction rates. The inverse electron-demand Diels Alder reaction of 3,6-dipyridin-2-yl-1,2,4,5-tetrazine with a series of norbornene derivatives was studied with quantum mechanical calculations at the M06-2X/6-311+G(d,p) level of theory. The theoretical predictions were confirmed with the experimental data and analyzed with the use of the distortion/interaction model. The obtained results will help in obtaining a better understanding of the factors that affect the relative cycloaddition rates of norbornenes with tetrazines, which are crucial for selectively tuning their efficacy.

Mechanistic Aspects of the Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling Reaction

The story of C-C bond formation includes several reactions, and surely Suzuki-Miyaura is among the most outstanding ones. Herein, a brief historical overview of insights regarding the reaction mechanism is provided. In particular, the formation of the catalytically active species is probably the main concern, thus the preactivation is in competition with, or even assumes the role of the rate determining step (rds) of the overall reaction. Computational chemistry is key in identifying the rds and thus leading to milder conditions on an experimental level by means of predictive catalysis.

Chemical reactivity from an activation strain perspective

Chemical reactions are ubiquitous in the universe, they are at the core of life, and they are essential for industrial processes. The drive for a deep understanding of how something occurs, in this case, the mechanism of a chemical reaction and the factors controlling its reactivity, is intrinsically valuable and an innate quality of humans. The level of insight and degree of understanding afforded by computational chemistry cannot be understated. The activation strain model is one of the most powerful tools in our arsenal to obtain unparalleled insight into reactivity. The relative energy of interacting reactants is evaluated along a reaction energy profile and related to the rigidity of the reactants' molecular structure and the strength of the stabilizing interactions between the deformed reactants: ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). Owing to the connectedness between the activation strain model and Kohn-Sham molecular orbital theory, one is able to obtain a causal relationship between both the sterics and electronics of the reactants and their mutual reactivity. Only when this is accomplished one can eclipse the phenomenological explanations that are commonplace in the literature and textbooks and begin to rationally tune and optimize chemical transformations. We showcase how the activation strain model is the ideal tool to elucidate fundamental organic reactions, the activation of small molecules by metallylenes, and the cycloaddition reactivity of cyclic diene- and dipolarophiles.

Rationalizing the AlI -Promoted Oxidative Addition of C-C Versus C-H Bonds in Arenes

The factors controlling the oxidative addition of C-C and C-H bonds in arenes mediated by Al^I have been computationally explored by means of Density Functional Theory calculations. To this end, we compared the processes involving benzene, naphthalene and anthracene which are promoted by a recently prepared anionic Al^I -carbenoid. It is found that this species exhibits a strong tendency to oxidatively activate C-H bonds over C-C bonds, with the notable exception of benzene, where the C-C bond activation is feasible but only under kinetic control reaction conditions. State-of-the-art computational methods based on the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis have been used to rationalize the competition between both bond activation reactions as well as to quantitatively analyze in detail the ultimate factors controlling these transformations.

How Alkali Cations Catalyze Aromatic Diels-Alder Reactions

We have quantum chemically studied alkali cation-catalyzed aromatic Diels-Alder reactions between benzene and acetylene forming barrelene using relativistic, dispersion-corrected density functional theory. The alkali cation-catalyzed aromatic Diels-Alder reactions are accelerated by up to 5 orders of magnitude relative to the uncatalyzed reaction and the reaction barrier increases along the series Li+ < Na+ < K+ < Rb+ < Cs+ < none. Our detailed activation strain and molecular-orbital bonding analyses reveal that the alkali cations lower the aromatic Diels-Alder reaction barrier by reducing the Pauli repulsion between the closed-shell filled orbitals of the dienophile and the aromatic diene. We argue that such Pauli mechanism behind Lewis-acid catalysis is a more general phenomenon. Also, our results may be of direct importance for a more complete understanding of the network of competing mechanisms towards the formation of polycyclic aromatic hydrocarbons (PAHs) in an astrochemical context.

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.

Dual Activation of Aromatic Diels-Alder Reactions

The unusually fast Diels-Alder reactions of [5]cyclophanes were analyzed by DFT at the BLYP-D3(BJ)/TZ2P level of theory. The computations were guided by an integrated activation-strain and Kohn-Sham molecular orbital analysis. It is revealed why both [5]metacyclophane and [5]paracyclophane exhibit a significant rate enhancement compared to their planar benzene analogue. The activation strain analyses revealed that the enhanced reactivity originates from 1) predistortion of the aromatic core resulting in a reduced activation strain of the aromatic diene, and/or 2) enhanced interaction with the dienophile through a distortion-controlled lowering of the HOMO-LUMO gap within the diene. Both of these physical mechanisms and thus the rate of Diels-Alder cycloaddition can be tuned through different modes of geometrical distortion (meta versus para bridging) and by heteroatom substitution in the aromatic ring. Judicious choice of the bridge and heteroatom in the aromatic core enables effective tuning of the aromatic Diels-Alder reactivity to achieve activation barriers as low as 2 kcal mol-1 , which is an impressive 35 kcal mol-1 lower than that of benzene.