Activation Strain Model Of Reactivity Research Articles

Activation of σ‐Bonds by Group 13/Ylide‐Based Ambiphiles: Understanding and Design

AbstractThe factors controlling the activation of σ‐bonds promoted by hidden Frustrated Lewis Pairs have been computationally explored using quantum chemical tools. To this end, the influence of both the nature of the group 13 element acting as Lewis acid as well as the cooperative action of the Lewis antagonists on the bond activation was quantitatively analyzed by means of the activation strain model of reactivity in combination with the energy decomposition analysis method. It is found that while the activation of the polar EX−H bonds (E15=group 15 element; E16=group 16 element) is feasible, the analogous processes involving non‐polar E14−H bonds (CH4, SiH4 or H2) proceed with much higher barriers. Nevertheless, these processes, and in particular the dihydrogen activation, can be realizable (i. e. proceeding with a feasible barrier) through rational design.

Origin of the Felkin-Anh(-Eisenstein) model: a quantitative rationalization of a seminal concept.

Quantum chemical calculations were carried out to quantitatively understand the origin of the Felkin-Anh(-Eisenstein) model, widely used to rationalize the π-facial stereoselectivity in the nucleophilic addition reaction to carbonyl groups directly attached to a stereogenic center. To this end, the possible approaches of cyanide to both (S)-2-phenylpropanal and (S)-3-phenylbutan-2-one have been explored in detail. With the help of the activation strain model of reactivity and the energy decomposition analysis method, it is found that the preference for the Felkin-Anh addition is mainly dictated by steric factors which manifest in a less destabilizing strain-energy rather than, as traditionally considered, in a lower Pauli repulsion. In addition, other factors such as the more favorable electrostatic interactions also contribute to the preferred approach of the nucleophile. Our work, therefore, provides a different, more complete rationalization, based on quantitative analyses, of the origin of this seminal and highly useful concept in organic chemistry.

Understanding the Reactivity of Diazaborinines towards the Activation of σ-Bonds.

The poorly understood factors governing the small molecule activation reactions mediated by diazaborinines have been computationally explored in detail using quantum chemical tools. To this end, the activation of E-H σ-bonds (E = H, C, Si, N, P, O, S) has been investigated. These reactions, which proceed in a concerted manner, are exergonic and, in general, associated with relatively low activation barriers. In addition, the barrier becomes lower for the E-H bonds involving the heavier element in the same group (ΔG≠ : C>Si; N>P; O>S). This reactivity trend together with the mode of action of the diazaborinine system are quantitatively analyzed by means of the activation strain model of reactivity in combination with the energy decomposition analysis method.

Understanding the enhanced reactivity of strained intramolecular Frustrated Lewis Pairs

AbstractThe poorly understood factors controlling the enhanced reactivity of strained intramolecular frustrated Lewis pairs (FLPs) having a rigid biphenylene tether have been quantitatively explored in detail by means of computational methods. With the help of the activation strain model of reactivity and the energy decomposition analysis methods, the challenging allene activation reaction has been selected and compared to the analogous process mediated by a related intramolecular FLP having a more flexible tether, which is significantly less reactive. In addition, the influence of the nature of the Lewis acid atom on the reactivity of the strained FLP has been considered as well showing that the reactivity steadily decreases when going down in group 13.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans.

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

Theoretical study of water-assisted reaction between methane and nitrosonium cation

The water-assisted reaction between CH4 and NO+ has been studied employing post Hartree–Fock and density functional theory methods. Two reaction pathways were considered: (i) a frontside NO+ attack on the C–H bond of methane with retention of configuration and (ii) a backside attack with inversion of configuration. The first pathway leads to a thermodynamically more favorable hydrate of N-protonated nitrosomethane, and the second one leads to its O-protonated isomer. The catalytic effect of a single water molecule is expressed in decreasing the activation energy for the elementary step by about a factor of two for the frontside mechanism and by a factor of four to five for the backside one when compared with the calculated literature data on water-free methane nitrosation. The combination of the activation strain model of reactivity and the energy decomposition analysis reveals that the activation barriers are largely determined by the relative stability of the termolecular reactant complexes. The crucial factor that stabilizes these complexes comes from the electrostatic attraction. The catalytic effect of the key water molecule is decreased with introduction of additional one or two explicit water molecules, which form a coordinate O→N bonding with NO+. On the contrary, an additional water molecule hydrogen bonded to the key catalytic water molecule enhances the catalytic effect.

Understanding the reactivity of frustrated Lewis pairs with the help of the activation strain model-energy decomposition analysis method.

This Feature article presents recent representative applications of the combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis methods to understand the reactivity of Frustrated Lewis Pairs (FLPs). This approach has been helpful to not only gain a deeper quantitative insight into the factors controlling the cooperative action between the Lewis acid/base partners but also to rationally design highly active systems for different bond activation reactions. Issues such as the influence of the nature of the FLP antagonists or the substituents directly attached to them on the reactivity are covered herein, which are crucial for the future development of this fascinating family of compounds.

Influence of the CH/B replacement on the Reactivity of Boranthrene and Related Compounds.

The influence of the replacement of CH groups by boron atoms on the reactivity of planar polycyclic aromatic hydrocarbons has been explored by means of computational tools. To this end, [4 + 2]-cycloaddition reactions involving anthracene and neutral boranthrene with different dienophiles such as ethylene, acetylene, and CO2 have been compared. In addition, the influence of additional fused aromatic rings (pentacene or borapentacene) on the reactivity of these species has been also explored. It was found that the B-doped systems are systematically much more reactive than their all-carbon counterparts from both kinetic and thermodynamic points of view. The observed trends in reactivity are quantitatively analyzed in detail using state-of-the-art methods, namely, the activation strain model of reactivity and the energy decomposition analysis method. Our calculations reveal the importance of molecular orbital interactions as the key factor responsible for the enhanced reactivity of the B-doped systems.

Factors Controlling the Aluminum(I)-meta-Selective C-H Activation in Arenes.

The so far poorly understood factors controlling the complete meta‐selectivity observed in the C−H activation reactions of alkylarenes promoted by aluminyl anions have been explored in detail by means of Density Functional Theory calculations. To this end, a combination of state‐of‐the‐art computational methods, namely the activation strain model of reactivity and energy decomposition analysis, has been applied to quantitatively unveil the origin of the selectivity of the transformation as well as the influence of the associated potassium cation. It is found that the selectivity takes place during the initial nucleophilic addition step where the key LP(Al)→π*(C=C) molecular orbital interaction is more stabilizing for the meta‐pathway, which results in a stronger interaction between the reactants along the entire transformation.

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 AlI 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 AlI -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.

The origin of the high reactivity of triazolinediones (TADs) in Diels-Alder reactions from a theoretical perspective

We investigate on the origin of the high reactivity of triazolinediones compared to maleimides in Diels-Alder reactions by using a combination of Molecular Orbital Theory and the Activation Strain Model of reactivity. Calculations at M06-2X/6–311++G(d,p)//M06-2X/6-31+G(d) level show that the energy barrier of the cycloaddition between anthracene and triazolinedione is much lower than that for maleimides. The analysis of frontier molecular orbitals (FMO) reveals that for the TAD system there is a much efficient charge transfer as consequence of a more delocalized HOMO over the dienophile fragment at the transition state structure. The Activation Strain Model revealed that the higher reactivity of TAD in the cycloaddition is related to the lower distortion of both fragments to attain the geometry of the transition state.

Unraveling the Selectivity Patterns in Phosphine-Catalyzed Annulations of Azomethine Imines and Allenoates.

The mechanism and selectivity of phosphine-catalyzed [3 + 2] and [3 + 3] annulations of azomethine imines and allenoates have been computationally studied. Exploration of the potential energy surface reveals that the cyclization step is a key step controlling the selectivity of the process. This contrasts with previous studies on related transformations where the initial nucleophilic addition involving the activated allenoate was found to exclusively control the regioselectivity of the transformation. Among the possible reaction pathways, the energetically low-lying reaction channel involves an intramolecular Michael addition leading to the experimentally observed [3 + 2] product. The factors controlling the observed regioselectivity have been quantitatively rationalized by means of state-of-the-art computational methods, namely, the activation strain model of reactivity in combination with the energy decomposition analysis.

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds.

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

Understanding the Reactivity of Neutral Geminal Group 14 Element/Phosphorus Frustrated Lewis Pairs.

The influence of the nature of the group 14 elements (E = Si, Ge, Sn) on the reactivity of (F5C2)3E-CH2-P(tBu)2 geminal frustrated Lewis pairs (FLPs) has been computationally explored by means of density functional theory calculations. To this end, the experimentally described activation reactions of CO2 and phenyl isocyanate have been investigated and compared to the analogous processes involving the corresponding B/P geminal FLP. It is found that the reactivity of these species is kinetically enhanced when going down the group 14 (Si < Ge < Sn). This trend of reactivity is quantitatively analyzed in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis method, which identify the interaction energy between the deformed reactants as the main factor controlling the reactivity of these group 14 containing geminal FLPs.

Impact of C=C/B−N Replacement on the Diels–Alder Reactivity of Curved Polycyclic Aromatic Hydrocarbons

The influence of the replacement of C=C bonds by isoelectronic B-N moieties on the reactivity of π-curved polycyclic aromatic hydrocarbons has been computationally explored by means of density functional theory calculations. To this end, we selected the Diels-Alder cycloaddition reactions of the parent corannulene and its BN-doped counterparts with either cyclopentadiene or maleic anhydride. In addition, the analogous reactions involving larger buckybowls, such as BN-hemifullerene, BN-circumtrindene, and BN-fullerene, have been also considered. It has been found that whereas corannulene behaves as a dienophile, its BN counterpart better acts as a diene. In contrast, the larger BN-curved systems cannot be used as dienes in Diels-Alder reactions, but undergo facile (i.e., low barrier) cycloaddition reactions with cyclopentadiene. The observed trends in reactivity, which cannot be directly explained by using typical frontier molecular orbital arguments, are quantitatively described in detail by means of state-of-the-art computational methods, namely the activation strain model of reactivity combined with the energy decomposition analysis method. The results of our calculations highlight the crucial role of the curvature of the system on the reactivity and its influence on the strength of the orbital interactions between the deformed reactants during their transformations.

Factors Controlling the Reactivity of Strained-Alkyne Embedded Cycloparaphenylenes.

The factors controlling the reactivity of the strained-alkyne embedded cycloparaphenylenes have been computationally explored by means of Density Functional Theory calculations. To this end, the Diels-Alder cycloaddition reaction involving cyclopentadiene and these macrocyclic systems has been selected in order to understand the influence of the strained nature of the alkyne in their structures as well as the size of the system on their reactivity. It is found that the cycloaddition reactions involving those macrocycles having more strained alkynes not only are more exothermic and exhibit lower activation barriers but also are associated with earlier transition states. The combination of the Activation Strain Model of reactivity and the Energy Decomposition Analysis method suggests that the enhanced reactivity of bent alkynes, as compared to linear C≡C triple bonds, finds its origin not only in the lower deformation energy required to adopt the corresponding transition state structure but also in the stronger interaction energy between the deformed reactants.

Understanding exo-selective Diels-Alder reactions involving Fischer-type carbene complexes.

The factors controlling the selectivity of the Diels-Alder cycloaddition reactions involving Fischer-type carbene complexes and cyclopentadiene have been explored computationally by means of density functional theory calculations. To this end, the influence of the substituents directly attached to the carbene ligand on the endo : exo ratio has been compared to the available experimental data and quantitatively analysed in detail by means of the combination of the activation strain model of reactivity and energy decomposition analysis methods. The insight gained in this computational study may be important for the rational design of exo-selective Diels-Alder reactions.

Influence of the Lewis Acid/Base Pairs on the Reactivity of Geminal E-CH2 -E' Frustrated Lewis Pairs.

The influence of the nature of the acid/base pairs on the reactivity of geminal frustrated Lewis pairs (FLPs) (Me2 E-CH2 -E'Ph2 ) has been computationally explored within the density functional theory framework. To this end, the dihydrogen-activation reaction, one of the most representative processes in the chemistry of FLPs, has been selected. It is found that the activation barrier of this transformation as well as the geometry of the corresponding transition states strongly depend on the nature of the E/E' atoms (E=Group 15 element, E'=Group 13 element) in the sense that lower barriers are associated with earlier transition states. Our calculations identify the geminal N/Al FLP as the most active system for the activation of dihydrogen. Moreover, the barrier height can be further reduced by replacing the phenyl group attached to the acidic atom by C6 F5 or 3,5-(CF3 )2 C6 H3 (Fxyl) groups. The physical factors controlling the computed reactivity trends are quantitatively described in detail by means of the activation strain model of reactivity combined with the energy decomposition analysis method.

Hydrogenation of Multiple Bonds by Geminal Aminoborane-Based Frustrated Lewis Pairs.

The hydrogenation reaction of multiple bonds that is mediated by geminal aminoborane-based frustrated Lewis pairs (FLPs) has been explored by means of density functional theory calculations. It was found that the release of the activated dihydrogen occurred in a concerted, yet highly asynchronous, manner. The physical factors that control the transformation were quantitatively described in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. This approach suggested a cooperative double hydrogen-transfer mechanism, which involves the initial migration of the protic (N)H followed by the nucleophilic attack of the (B)H hydride to the carbon atom of the multiple bond. The influence of both the substituents directly attached to the boron atom of the initial FLP and the nature of the multiple bond on the transformation was also investigated.

Rationalizing the Regioselectivity of the Diels-Alder Biscycloaddition of Fullerenes.

The physical factors governing the regioselectivity of the double functionalization of fullerenes have been explored by means of density functional theory calculations. To this end, the second Diels-Alder cycloaddition reactions involving 1,3-butadiene and the parent C60 fullerene as well as the ion-encapsulated system Li+@C60 have been selected. In agreement with previous experimental findings on related processes, it is found that the cycloaddition reaction, involving either C60 or Li+@C60, occurs selectively at specific [6,6]-bonds. The combination of the activation strain model of reactivity and the energy decomposition analysis methods has been applied to gain a quantitative understanding into the markedly different reactivity of the available [6,6]-bonds leading to the observed regioselectivity in the transformation.