Transition-State Energy and Position along the Reaction Coordinate in an Extended Activation Strain Model

Mechanistic Insights into the Palladium-Catalyzed Aziridination of Aliphatic Amines by C-H Activation

Detailed kinetic studies and computational investigations have been performed to elucidate the mechanism of a palladium-catalyzed C-H activation aziridination. A theoretical rate law has been derived that matches with experimental observations and has led to an improvement in the reaction conditions. Acetic acid was found to be beneficial in controlling the formation of an off-cycle intermediate, allowing a decrease in catalyst loading and improved yields. Density functional theory (DFT) studies were performed to examine the selectivities observed in the reaction. Evidence for electronic-controlled regioselectivity for the cyclopalladation step was obtained by a distortion-interaction analysis, whereas the aziridination product was justified through dissociation of acetic acid from the palladium(IV) intermediate preceding the product-forming reductive elimination step. The understanding of this reaction mechanism under the synthesis conditions should provide valuable assistance in the comprehension and design of palladium-catalyzed reactions on similar systems.

Selective C-H and C-C Bond Activation: Electronic Regimes as a Tool for Designing d(10) MLn Catalysts

We wish to understand how a transition-metal catalyst can be rationally designed so as to selectively activate one particular bond in a substrate, herein, C-H and C-C bonds in ethane. To this end, we quantum chemically analyzed the activity and selectivity of a large series of model catalysts towards ethane and, for comparison, methane, by using the activation strain model and quantitative molecular orbital theory. The model catalysts comprise d(10) MLn complexes with coordination numbers n=0, 1, and 2; metal centers M=Co(-), Rh(-), Ir(-), Ni, Pd, Pt, Cu(+), Ag(+), and Au(+); and ligands L=NH3, PH3, and CO. Our analyses reveal that rather subtle electronic differences between bonds can be exploited to induce a lower barrier for activating one or the other, depending, among other factors, on the catalysts electronic regime (i.e., s-regime versus d-regime catalysts). Interestingly, the concepts and design principles emerging from this work can also be applied to the more challenging problem of differentiating between activation of the C-H bonds in ethane versus those in methane.

The activation strain model and molecular orbital theory

The activation strain model is a powerful tool for understanding reactivity, or inertness, of molecular species. This is done by relating the relative energy of a molecular complex along the reaction energy profile to the structural rigidity of the reactants and the strength of their mutual interactions: ΔE(ζ) = ΔE_strain(ζ) + ΔE_int(ζ). We provide a detailed discussion of the model, and elaborate on its strong connection with molecular orbital theory. Using these approaches, a causal relationship is revealed between the properties of the reactants and their reactivity, e.g., reaction barriers and plausible reaction mechanisms. This methodology may reveal intriguing parallels between completely different types of chemical transformations. Thus, the activation strain model constitutes a unifying framework that furthers the development of cross-disciplinary concepts throughout various fields of chemistry. We illustrate the activation strain model in action with selected examples from literature. These examples demonstrate how the methodology is applied to different research questions, how results are interpreted, and how insights into one chemical phenomenon can lead to an improved understanding of another, seemingly completely different chemical process. WIREs Comput Mol Sci 2015, 5:324-343. doi: 10.1002/wcms.1221.

Combined activation strain model and energy decomposition analysis methods: a new way to understand pericyclic reactions

The recently introduced activation strain model (ASM) has allowed us to gain more insight into the intimacies of different fundamental processes in chemistry. In combination with the energy decomposition analysis (EDA) method, we have nowadays a very useful tool to quantitatively understand the physical factors that govern the activation barriers of reactions within organic and organometallic chemistry. In this Perspective article, we present selected illustrative examples of the application of this method to pericyclic reactions (Diels-Alder and double group transfer reactions) to show that this methodology nicely complements other more traditional, widely used theoretical methods.

Tuning the reactivity of Fe(V)(O) toward C-H bonds at room temperature: effect of water

The presence of an Fe(V)(O) species has been postulated as the active intermediate for the oxidation of both C-H and C═C bonds in the Rieske dioxygenase family of enzymes. Understanding the reactivity of these high valent iron-oxo intermediates, especially in an aqueous medium, would provide a better understanding of these enzymatic reaction mechanisms. The formation of an Fe(V)(O) complex at room temperature in an aqueous CH3CN mixture that contains up to 90% water using NaOCl as the oxidant is reported here. The stability of Fe(V)(O) decreases with increasing water concentration. We show that the reactivity of Fe(V)(O) toward the oxidation of C-H bonds, such as those in toluene, can be tuned by varying the amount of water in the H2O/CH3CN mixture. Rate acceleration of up to 60 times is observed for the oxidation of toluene upon increasing the water concentration. The role of water in accelerating the rate of the reaction has been studied using kinetic measurements, isotope labeling experiments, and density functional theory (DFT) calculations. A kinetic isotope effect of ∼13 was observed for the oxidation of toluene and d8-toluene showing that C-H abstraction was involved in the rate-determining step. Activation parameters determined for toluene oxidation in H2O/CH3CN mixtures on the basis of Eyring plots for the rate constants show a gain in enthalpy with a concomitant loss in entropy. This points to the formation of a more-ordered transition state involving water molecules. To further understand the role of water, we performed a careful DFT study, concentrating mostly on the rate-determining hydrogen abstraction step. The DFT-optimized structure of the starting Fe(V)(O) and the transition state indicates that the rate enhancement is due to the transition state's favored stabilization over the reactant due to enhanced hydrogen bonding with water.

Computational study on the mechanism and enantioselectivity of Rh2(S-PTAD)4 catalyzed asymmetric [4+3] cycloaddition between vinylcarbenoids and dienes

In this paper, the mechanism of asymmetric [4+3] cycloaddition between a vinylcarbenoid and a diene to form cycloheptadiene has been studied using a two-layer ONIOM methodology consisting of density functional theory and semiempirical PM6.

Distortion–interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes

The reactivity of uncatalyzed Alder-ene type reactions of hetero-substituted propylene is interpreted by distortion–interaction analysis of both the transition states and the complete reaction pathways.

Mechanistic insights into the Pd-catalyzed intermolecular asymmetric allylic dearomatization of multisubstituted pyrroles: understanding the remarkable regio- and enantioselectivity

In this article we report a comprehensive density functional theory study on the Pd-catalyzed intermolecular asymmetric allylic dearomatization reactions of multisubstituted pyrroles. The calculated results are in line with the previous experimental observations (J. Am. Chem. Soc. 2014, 136, 6590), and the remarkable regio- and enantioselectivity are well explained. Of all the potential nucleophilic sites around the multisubstituted pyrrole ring, the reaction always occurs at the position where the HOMO of the molecule distributes most significantly. In contrast to the common view on the enantioselectivity of the Pd-catalyzed asymmetric allylic substitution reactions, we find that the steric interaction between the nucleophile and the chiral ligand does not have the dominating effect on the enantioselectivity of the reaction. Instead, the interaction between the allyl moiety and the incoming nucleophile plays an important role in the enantioselectivity-determining process.

Origin of Reactivity Trends of Noble Gas Endohedral Fullerenes Ng2@C60 (Ng = He to Xe)

We have computationally studied the factors governing the enhanced Diels-Alder reactivity of noble gas endohedral fullerenes Ng2@C60 when Ng = Ar, Kr, and Xe as compared to Ng = none, He, and Ne. To this end, we have employed the activation strain model of reactivity in combination with the energy decomposition analysis (EDA) method in DFT calculations on the Diels-Alder cycloaddition reaction between Ng2@C60 and 1,3-butadiene. Our results indicate that when heavier noble gas dimers are introduced inside the C60 cage, dramatic effects on both the geometrical and electronic structure of the fullerenic cage occur leading to a remarkable enhanced interaction between the deformed reactants along the entire reaction coordinate.

The activation strain model and molecular orbital theory: understanding and designing chemical reactions

In this Tutorial Review, we make the point that a true understanding of trends in reactivity (as opposed to measuring or simply computing them) requires a causal reactivity model. To this end, we present and discuss the Activation Strain Model (ASM). The ASM establishes the desired causal relationship between reaction barriers, on one hand, and the properties of reactants and characteristics of reaction mechanisms, on the other hand. In the ASM, the potential energy surface ΔE(ζ) along the reaction coordinate ζ is decomposed into the strain ΔEstrain(ζ) of the reactants that become increasingly deformed as the reaction proceeds, plus the interaction ΔEint(ζ) between these deformed reactants, i.e., ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). The ASM can be used in conjunction with any quantum chemical program. An analysis of the method and its application to problems in organic and organometallic chemistry illustrate the power of the ASM as a unifying concept and a tool for rational design of reactants and catalysts.

Distortion/Interaction analysis reveals the origins of selectivities in iridium-catalyzed C-H borylation of substituted arenes and 5-membered heterocycles

The iridium-catalyzed borylation of mono- and disubstituted arenes and heteroarenes has been studied with density functional theory. The distortion/interaction model was employed to understand the origins of selectivities in these reactions. Computations revealed that the transition states for C-H oxidative addition are very late, resembling the aryl iridium hydride intermediate with a fully formed Ir-C bond. Consequently, the regioselectivity is mainly controlled by differences in the interaction energies between the iridium catalyst and arene carbon.

Ene-ene-yne reactions: activation strain analysis and the role of aromaticity

The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, A=B-C=D-E≡F, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level. These reactions proceed through formally aromatic transition states to form a bent-allene intermediate with relatively high activation barriers. Activation-strain analyses show that the major factor controlling this Hopf cyclization is the geometrical strain energy associated with the rotation of the terminal [A] group. This rotation is necessary for achieving a favorable HOMO-LUMO overlap with the yne-moiety [F] associated with the formation of the new A-F single bond. In addition, the relationship between the aromaticity of the corresponding cyclic transition states (all six-membered rings) and the computed activation barriers were analyzed. The calculations also indicate that the aromatization of the bent-allene structures takes place through two consecutive 1,2-hydrogen shifts, the second one exhibiting negligible energy barriers.

Indenyl effect due to metal slippage? Computational exploration of rhodium-catalyzed acetylene [2+2+2] cyclotrimerization

The mechanism of CpRh (Cp=cyclopentadienyl) and IndRh (Ind=indenyl)-catalyzed acetylene [2+2+2] cyclotrimerization has been revisited aiming at finding an explanation for the better performance of the latter catalyst found experimentally. The hypothesis that an ancillary ligand of the precatalyst remains bonded to the metal center throughout the whole catalytic cycle, based on the experimental evidence that the nature of this ligand can exert some control in cocyclotrimerization of different alkynes, is considered. Strong hapticity variations occur in both the CpRh- and IndRh-catalyzed processes. As the Ind ligand undergoes a more facile slippage than Cp, the energy profile is far smoother in the IndRh-catalyzed cyclotrimerization. This difference in the energetics of the process translates into an enhanced activity of the IndRh catalyst, in nice agreement with experiment.

Origin of the "endo rule" in Diels-Alder reactions

Detailed density functional theory calculations definitively rationalize the preference for the endo cycloadduct (also known as endo rule) in text-book thermal Diels-Alder reactions involving maleic anhydride and cyclopentadiene or butadiene. This selectivity is mainly caused by an unfavorable steric arrangement in the transition-state region of the exo pathway which translates into a more destabilizing activation strain. In contrast with the widely accepted, orbital-interaction-based explanation for the endo rule, it is found that neither the orbital interactions nor the total interaction between the deformed reactants contributes to the endo selectivity.

In silico design of heteroaromatic half-sandwich RhI catalysts for acetylene [2+2+2] cyclotrimerization: evidence of a reverse indenyl effect

A mechanistic density functional theory study of acetylene [2+2+2] cyclotrimerization to benzene catalyzed by Rh(I) half metallocenes is presented. The catalyst fragment contains a heteroaromatic ligand, that is, the 1,2-azaborolyl (Ab) or the 3a,7a-azaborindenyl (Abi) anions, which are isostructural and isoelectronic to the hydrocarbon cyclopentadienyl (Cp) and indenyl (Ind) anions, respectively, but differ from the last ones on having two adjacent carbon atoms replaced with a boron and a nitrogen atom. The better performance of either the classic hydrocarbon or the heteroaromatic catalysts is found to depend on the different mechanistic paths that can be envisioned for the process. The present analyses uncover and explain general structure-reactivity relationships that may serve as rational design principles. In particular, we provide evidence of a reverse indenyl effect.

Diastereoselective hydrogen-transfer reactions: an experimental and DFT study

Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.

Enabling nucleophilic substitution reactions of activated alkyl fluorides through hydrogen bonding

It was discovered that the presence of water as a cosolvent enables the reaction of activated alkyl fluorides for bimolecular nucleophilic substitution reactions. DFT calculations show that activation proceeds through stabilization of the transition structure by a stronger F···H2O interaction and diminishing C-F bond elongation, and not simple transition state electrostatic stabilization. Overall, the findings put forward a distinct strategy for C-F bond activation through H-bonding.

Acid-catalyzed nucleophilic additions to carbonyl groups: is the accepted mechanism the rule or an exception?

The transesterification reaction, and in particular the methanolysis of ethyl acetate with sulfuric acid as catalyst, is used as a model reaction to study the acid-catalyzed nucleophilic addition to a carbonyl group. Continuum solvation methods (SMD and IEF-PCM) and the MPWB1K functional are used. The reaction mechanism is studied in methanol and in acetonitrile as solvents. Our results indicate that the acid-catalyzed addition mechanism is stepwise, and the transition state (TS) is a contact ion-pair. The counteranion of the acid catalyst remains in the reaction site playing an important role in the TS of this reaction. Changes in the reaction kinetics and the ionic/nonionic nature of the TS with the ionizing ability of the solvent and the strength of the acid catalyst are explored. Additional calculations at the CBS-Q3 level of theory reinforce the conclusions of this paper. The results obtained allow the generalization of important ideas regarding the mechanism of the nucleophilic addition to carbonyl groups.

Mechanisms for the reactions of group 10 transition metal complexes with metal-group 14 element bonds, Bbt(Br)E═M(PCy3)2 (E = C, Si, Ge, Sn, Pb; M = Pd and Pt)

The electronic structures of the Bbt(Br)E═M(PCy(3))(2) (E = C, Si, Ge, Sn, Pb and M = Pt, Pd) complexes and their potential energy surfaces for the formation and water addition reactions were studied using density functional theory (B3LYP/LANL2DZ). The theoretical evidence suggests that the bonding character of the E═M double bond between the six valence-electron Bbt(Br)E: species and the 14 valence-electron (PCy(3))(2)M complexes has a predominantly high s-character. That is, on the basis of the NBO, this theoretical study indicates that the σ-donation from the E element to the M atom prevails. Also, theoretical computations suggest that the relative reactivity decreases in the order: Bbt(Br)C═M(PCy(3))(2) > Bbt(Br)Si═M(PCy(3))(2) > Bbt(Br)Ge═M(PCy(3))(2) > Bbt(Br)Sn═M(PCy(3))(2) > Bbt(Br)Pb═M(PCy(3))(2), irrespective of whether M = Pt or M = Pd is chosen. Namely, the greater the atomic weight of the group 14 atom (E), the larger is the atomic radius of E and the more stable is its Bbt(Br)E═M(PCy(3))(2) doubly bonded species toward chemical reactions. The computational results show good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.

DFT study of the stereoselective formation of cis-W(CO)4(η2-C2H4)2 in the gas-phase consecutive coordination of C2H4 onto doubly coordinatively unsaturated W(CO)4

B3LYP-based density functional theory (DFT) calculations are reported that provide insight into the stereoselective formation of cis-W(CO)(4)(η(2)-C(2)H(4))(2) observed in the pulsed 266 nm laser photolysis of tungsten hexacarbonyl (W(CO)(6)) in the presence of C(2)H(4) in the gas phase at room temperature (J. Phys. Chem. 1995, 99, 4558). W(CO)(4)(η(2)-C(2)H(4)) formed through the coordination of C(2)H(4) onto coordinatively unsaturated W(CO)(4) was found to have a pseudo-C(2v) symmetry (distorted trigonal bipyramid with an angle of ca. 90° between the two equatorial COs) with a bond dissociation enthalpy (BDE) of W-C(2)H(4) of 125 kJ mol(-1). In the subsequent coordination of C(2)H(4) onto the W(CO)(4)(η(2)-C(2)H(4)), having one vacant coordinatively unsaturated site, no barrier was found in the reaction path to cis-complex formation, while there was a barrier of about 89 kJ mol(-1) to the trans-complex. The calculations show that the stereoselective formation of cis-W(CO)(4)(η(2)-C(2)H(4))(2) is due to kinetic rather than thermodynamic control. The trans-W(CO)(4)(η(2)-C(2)H(4))(2) was calculated to be more stable than cis-W(CO)(4)(η(2)-C(2)H(4))(2) by about 10 kJ mol(-1). The BDE of W-C(2)H(4) in cis-W(CO)(4)(η(2)-C(2)H(4))(2) was estimated to be 61 kJ mol(-1).

Regiochemistry of the condensation of 2-aroyl-cyclohexanones and 2-cyanoacetamide: 13C-labeling studies and semiempirical MO calculations

Hydroxy-aryl-5,6,7,8-tetrahydroisoquinoline-4-carbonitriles represent interesting chemical scaffolds, but synthetic access to these compounds is limited. The reaction of 2-aroyl-cyclohexanones with 2-cyanoacetamide and base in ethanol has been reported to lead to the formation of the tetrahydroisoquinoline isomer. We show that depending on the electronic nature of the para-substituent on the aryl ring, formation of the regioisomeric tetrahydroquinoline isomer can significantly compete. The electron-donating or -withdrawing properties of the para-substituent of the aryl ring determines the ratio of product isomers. A series of 2-aroyl-cyclohexanones, with para-substituents ranging from electron-donating to electron-withdrawing, were reacted with [2-(13)C]-cyanoacetamide. The product ratio and absolute regiochemistry were directly determined by quantitative (13)C, HMBC, and NOESY NMR spectroscopy on the reaction mixtures. A clear relationship between the regioisomeric product ratio and the Hammett sigma values of the substituents is demonstrated. This is explained by the separate in situ yields, which reveal that the pathway leading to the tetrahydroquinoline regioisomer is significantly more sensitive toward the electronic nature of the para-substituent than the pathway leading to the tetrahydroisoquinoline. Semiempirical AM1 molecular orbital calculations on the starting electrophile 2-aroyl-cyclohexanone support a correlation between the energy of the LUMOs and the regioisomeric product ratio. Our results facilitate synthetic access to a range of these interesting synthetic intermediates.

Type-I dyotropic reactions: understanding trends in barriers

To understand the factors that control the activation barrier of type-I 1,2-dyotropic reactions (X-EH(2)-CH(2)-X*→X*-EH(2)-CH(2)-X, with E=C and Si, X=H, CH(3), SiH(3), F to I) and trends therein as a function of the migrating groups X, we have explored ten archetypal model reactions of this class using relativistic density functional theory (DFT) at ZORA-OLYP/TZ2P. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity, which had to be extended from bimolecular to unimolecular reactions. Thus, the above type-I dyotropic reactions can be conceived as a relative rotation of the CH(2)CH(2) and [X···X] fragments in X-CH(2)-CH(2)-X. The picture that emerges from these analyses is that reduced C-X bonding in the transition state is the origin of the reaction barrier. Also the trends in reactivity on variation of X can be understood in terms of how sensitive the C-X interaction is towards adopting the transition-state geometry. A valence bond analysis complements the analyses and confirms the picture emerging from the activation strain model.

Hydrogen-abstraction reactivity patterns from A to Y: the valence bond way

"Give us insight, not numbers" was Coulson's admonition to theoretical chemists. This Review shows that the valence bond (VB)-model provides insights and some good numbers for one of the fundamental reactions in nature, the hydrogen-atom transfer (HAT). The VB model is applied to over 50 reactions from the simplest H + H(2) process, to P450 hydroxylations and H-transfers among closed-shell molecules; for each system the barriers are estimated from raw data. The model creates a bridge to the Marcus equation and shows that H-atom abstraction by a closed-shell molecule requires a higher barrier owing to the additional promotion energy needed to prepare the abstractor for H-abstraction. Under certain conditions, a closed-shell abstractor can bypass this penalty through a proton-coupled electron transfer (PCET) mechanism. The VB model links the HAT and PCET mechanisms conceptually and shows the consequences that this linking has for H-abstraction reactivity.

Halogen Bonding versus Hydrogen Bonding: A Molecular Orbital Perspective

We have carried out extensive computational analyses of the structure and bonding mechanism in trihalides DX⋅⋅⋅A(-) and the analogous hydrogen-bonded complexes DH⋅⋅⋅A(-) (D, X, A=F, Cl, Br, I) using relativistic density functional theory (DFT) at zeroth-order regular approximation ZORA-BP86/TZ2P. One purpose was to obtain a set of consistent data from which reliable trends in structure and stability can be inferred over a large range of systems. The main objective was to achieve a detailed understanding of the nature of halogen bonds, how they resemble, and also how they differ from, the better understood hydrogen bonds. Thus, we present an accurate physical model of the halogen bond based on quantitative Kohn-Sham molecular orbital (MO) theory, energy decomposition analyses (EDA) and Voronoi deformation density (VDD) analyses of the charge distribution. It appears that the halogen bond in DX⋅⋅⋅A(-) arises not only from classical electrostatic attraction but also receives substantial stabilization from HOMO-LUMO interactions between the lone pair of A(-) and the σ* orbital of D-X.