Aromatic Transition States Research Articles

Pressure-Induced Hydrogen Transfer in 2-Butyne via a Double CH···π Aromatic Transition State.

Hydrogen transfer (H-transfer) is an important elementary reaction in chemistry and bioscience. It is often facilitated by the hydrogen bonds between the H-donor and acceptor. Here, at room temperature and high pressure, we found that solid 2-butyne experienced a concerted two-in-two-out intermolecular CH···π H-transfer, which initiated the subsequent polymerization. Such double H-transfer goes through an aromatic Hückel six-membered ring intermediate state via intermolecular CH···π interactions enhanced by external pressure. Our work shows that H-transfer can occur via the CH···π route in appropriate conformations under high pressure, which gives important insights into the H-transfer in solid-state hydrocarbons.

C-C and C-H bond cleavage reactions in acenaphthylene aromatic molecule, an ab-initio density functional theory study

The ab-initio DFT method (B3LYP) is applied to the study of the C-C and C-H bond cleavage reactions in acenaphthylene molecule. It is found that the C-C bond cleavage proceeds via a singlet aromatic transition state, compelled through a disrotatoric ring opening reaction. A sigmatropic H atom shift follows the transition state in some of these reactions, where the formation of a methylene -CH2,acetylenyl-, allenyl- or butadienyl moiety in the final product is possible. The calculated activation and reaction energies for the C-C ring opening are 164-236 and 52-193 kcal/mol, respectively. The calculated cleavage reaction energies for the C-H bonds are 117-122 kcal/mol and the activation energies are 147-164 kcal/mol.

Mechanism-Guided Scaffold Diversification: Perturbing and Trapping the Intermediates of Maltol-Type Cascade Claisen Rearrangement.

A mechanism-guided study to harness different kinds of active species in the cascade Claisen rearrangement for scaffold diversification has been developed. These robust and practical processes furnished a series of architectures with a large chemical space, varying from planar to three-dimensional. In addition, several interesting reactions were observed, such as [3 + 3] dimerization, quinone-based vinylogous Nazarov-type cyclization, and a rare 12e [σ2a + π2s + π2a + π2s + (π2a + π2s)] Mobius aromatic transition state mediated rearrangement.

C-C and C-H bond cleavage reactions in the chrysene and perylene aromatic molecules: An ab-initio density functional theory study

The ab-initio DFT (B3LYP) method is applied for the study of C-C and C-H bond cleavage reactions in chrysene and perylene aromatic molecules. It is found that, the C-C bond cleavage proceeds via a singlet aromatic transition state, compelled through a disrotatoric ring opening reaction. A suprafacial H atom shift follows the transition state in some of these reactions, where the formation of a methylene -CH 2 ,acetylenyl-, allenyl- or butadienyl- moiety in the final product is possible. Activation energies are calculated for the ring opening and show the following values; for chrysene, 136.97-197.69 kcal/mol and for perylene, 160.87-187.33 kcal/mol. The reaction energies range from 95.57-162.42 kcal/mol for chrysene and 98.12 - 168.28 kcal/mol for perylene. The calculated cleavage reaction energies for all C-H bonds in both molecules are almost similar, 116-117 kcal/mol. Their activation energies however are different, for chrysene they range from 148.57-154.97 kcal/mol and for perylene 148.30-162.73 kcal/mol .

Computational insights into the S3 transfer reaction: A special case of double group transfer reaction featuring bicyclically delocalized aromatic transition state geometries.

An unusual pericyclic process that involves the intermolecular transfer of thiozone (S3 ) is computationally described. The process can be considered as a special case of double group transfer reaction whereby the two migrating groups are connected to the same substituent, taking place in a concerted manner via transition states featuring two five-membered C2 S3 rings fused together. Analysis of the aromaticity at the TS geometries by computing NICS values at the (3,+1) RCPS as well as ACID calculations confirms the aromatic character of each C2 S3 ring, thus resulting in bicyclically delocalized aromatic structures. The free energy barriers for the transfer of S3 are relatively similar (40-50 kcal mol-1 ) to those computed for typical double H group transfer reactions. The similarities and differences between these processes have been further analysed by applying ASM-EDA and NBO approaches to the model reactions between ethene and ethane, and ethene and 1,2,3-trithiolane. © 2017 Wiley Periodicals, Inc.

Reaction paths and transition states of the C-C and C-H bond cleavage in the aromatic anthracene and phenanthrene molecules

The reaction paths of the C-C and C-H bond cleavage in the anthracene and phenanthrene aromatic molecules are studied by applying the ab-initio DFT method. It is found that the C-C bond cleavage proceeds via a singlet aromatic transition state, compelled through a disrotatoric ring opening reaction. A suprafacial H atom shift follows the transition state, leading to the formation of a methylene -CH 2 and an acetylenic or allenic moiety. The calculated activation energies for anthracene range from 158.81-208.90 kcal/mol and the reaction energies from 96.106-156.976 kcal/mol. For phenanthrene, the energy values are 157.39-202.34 kcal/mol and 62.639-182.423 kcal/mol, respectively. For the C-H cleavage reactions, the calculated reaction energies for all C-H bonds in both molecules are almost similar, 116-117 kcal/mol. The activation energy values for anthracene and phenanthreneare 149.75-161.27 and 161.24-163.00 kcal/mol, respectively.

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.

C-C Bond Cleavage in Aromatic Molecules : Benzene , Toluene and Naphthalene

The calculated energy reaction path, for the C-C bond cleavage in benzene, applying the Density Functional Theory method, yielded the values for the activation energy, Ea= 221.561kcal/mol and reaction energy, Er= 146.938kcal/mol respectively. The corresponding values for the different bonds in toluene are; C1-C2, Ea= 200.640kcal/mol, Er =140.800 kcal/mol; C2-C3, Ea = 214.790kcal/mol, Er= 154.160kcal/mol; C3-C4, Ea= 209.560kcal/mol, Er= 174.110kcal/mol; C1-C7, Ea= 148.050kcal/mol, Er = 104.230kcal/mol. For naphthalene the calculated energy values were; C1-C2, Ea= 226.640kcal/mol, Er = 157.094kcal/mol; C2-C3, Ea= 255.677kcal/mol, Er= 205.147kcal/mol; C9-C10, Ea= 299.851kcal/mol, Er= 270.575kcal/mol. The geometries of all transition states were coplanar with (4n+2) electrons with the exception of toluene C2-C3 bond reaction where the C2 atom moves towards C6 to form a cyclopropene structure. The C2-C3 bond cleavage in naphthalene proceeds towards the splitting of two acetylene molecules from the mother molecule, beyond the transition state. The geometries of the aromatic transition states showed typical bond alternation distances i.e. classical C=C and C-C bonds.

σ, π aromaticity and anti-aromaticity as retrieved by the linear response kernel

The chemical importance of the linear response kernel from conceptual Density Functional Theory (DFT) is investigated for some σ and π aromatic and anti-aromatic systems. The effect of the ring size is studied by looking at some well known aromatic and anti-aromatic molecules of different sizes, showing that the linear response is capable of correctly classifying and quantifying the aromaticity for five- to eight-membered aromatic and anti-aromatic molecules. The splitting of the linear response in σ and π contributions is introduced and its significance is illustrated using some σ-aromatic molecules. The linear response also correctly predicts the aromatic transition states of the Diels-Alder reaction and the acetylene trimerisation and shows the expected behavior along the reaction coordinate, proving that the method is accurate not only at the minimum of the potential energy surface, but also in non-equilibrium states. Finally, the reason for the close correlation between the linear response and the Para Delocalisation Index (PDI), found in previous and the present study, is proven mathematically. These results show the linear response to be a reliable DFT-index to probe the σ and π aromaticity or anti-aromaticity of a broad range of molecules.

Aromatic Transition States in Nonpericyclic Reactions: Anionic 5-Endo Cyclizations Are Aborted Sigmatropic Shifts

The transition states (TSs) of 5-endo-dig and 5-endo-trig anionic ring closures are the first unambiguous examples of nonpericyclic reactions with TSs stabilized by aromaticity. Their five-center, six-electron in-plane aromaticity is revealed by the diatropic dissected nucleus-independent chemical shifts, -24.1 and -13.7 ppm, respectively, resulting from the delocalization of the lone pair at the nucleophilic center, a σ CC bond, and an in-plane alkyne (or alkene) π bond. Other seemingly analogous exo and endo cyclization TSs do not have these features. A symmetry-enhanced combination of through-space and through-bond interactions explains the anomalous geometric, energetic, and electronic features of the 5-endo ring closure transition state. Anionic 5-endo cyclizations can be considered to be "aborted" [2,3]-sigmatropic shifts. The connection between anionic cyclizations and sigmatropic shifts offers new possibilities for the design and electronic control of anionic isomerizations.

ChemInform Abstract: The Cope Rearrangement — The First Born of a Great Family

AbstractReview: 124 refs.

The Cope rearrangement—the first born of a great family

AbstractThis review takes the experimental and theoretical discussions on the Cope rearrangement as the basis to emphasize its tight connection to many closely related pericyclic reactions. The Cope reaction, which is rather simple in a formal sense, has led to a plethora of mechanistic discussions to determine the nature of biradical‐like intermediates or transition structures. After intense experimental and theoretical investigations starting in the late 1960s, the Cope rearrangement turned out to be a multifaceted reaction with a ‘chameleonic’ nature, ranging from homoaromatic transition structures to biradical intermediates in substituted cases, and six‐electron transition states in the parent system. Additionally, the Cope rearrangement served as an excellent example to probe theoretical methods for the computations of a variety of pericyclic reactions. However, the relationship of the Cope rearrangement with analogous reactions has been hampered by the notion that pericyclic reactions typically are assumed to proceed in a concerted fashion. Only in the past 10 years, the Cope reaction, with its 1,5‐hexadiene unit, emerged as the structural and mechanistic prototype of a large family, including sigmatropic, cycloaddition, and other electrocyclic reactions as well as thermal rearrangements of polyunsaturated systems, enediynes, eneyne‐allenes, and many others. The recurrent and interconnecting motif in this family of reactions is the questions regarding cyclic aromatic transition states and the possible involvement of biradical intermediates. © 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 172‐190 DOI: 10.1002/wcms.17This article is categorized under: Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics

Double Group Transfer Reactions: Role of Activation Strain and Aromaticity in Reaction Barriers

Double group transfer (DGT) reactions, such as the bimolecular automerization of ethane plus ethene, are known to have high reaction barriers despite the fact that their cyclic transition states have a pronounced in-plane aromatic character, as indicated by NMR spectroscopic parameters. To arrive at a way of understanding this somewhat paradoxical and incompletely understood phenomenon of high-energy aromatic transition states, we have explored six archetypal DGT reactions using density functional theory (DFT) at the OLYP/TZ2P level. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity. In this model, the shape of the reaction profile DeltaE(zeta) and the height of the overall reaction barrier DeltaE( not equal)=DeltaE(zeta=zeta(TS)) is interpreted in terms of the strain energy DeltaE(strain)(zeta) associated with deforming the reactants along the reaction coordinate zeta plus the interaction energy DeltaE(int)(zeta) between these deformed reactants: DeltaE(zeta)=DeltaE(strain)(zeta)+DeltaE(int)(zeta). We also use an alternative fragmentation and a valence bond model for analyzing the character of the transition states.

Electron reorganization in allowed and forbidden pericyclic reactions: multicenter bond indices as a measure of aromaticity and/or anti‐aromaticity in transition states of pericyclic electrocyclizations

AbstractThe multicenter bond indices (MCI), recently proposed as quantitative measures of cyclic delocalization in aromatic systems, have been applied to characterize the differences in the nature of the electron reorganization in a series of allowed and forbidden electrocyclic reactions of linear neutral polyenes of general formula CnHn+2 and related charged systems of formula CnHn+2(+) and CnHn+2(−) for n ranging from 4 to 7. The proposed methodology, which is based on the monitoring of the variation of the extent of cyclic delocalization along the concerted reaction paths, is shown to be completely consistent with the empirical Evans/Dewar classification anticipating aromatic transition states for allowed and anti‐aromatic transition states for forbidden electrocyclic reactions. Although the study reports the results of the analysis of electron reorganization just for the above‐mentioned particular class of electrocyclic reactions, the proposed approach is completely general and its conclusions remain valid for any pericyclic reaction. Copyright © 2009 John Wiley & Sons, Ltd.

A Preferred Disrotatory 4n Electron Möbius Aromatic Transition State for a Thermal Electrocyclic Reaction

Not forbidden: Thermal 4n electron electrocyclic reactions of Hückel topology structures proceed via "allowed" conrotatory pathways. However, for a Möbius topology, the Woodward-Hoffmann rules may be reversed and a "forbidden" disrotatory pathway can be preferred as shown theoretically for dodecahexaene 1 that transforms via a Heilbronner-Möbius aromatic transition structure 2 into a cyclic polyene 3.

Initial Hardness Response and Hardness Profiles in the Study of Woodward-Hoffmann Rules for Electrocyclizations.

The fundamental principles of pericyclic reactions are governed by the Woodward-Hoffmann rules, which state that these reactions can only take place if the symmetries of the reactants' molecular orbitals and the products' molecular orbitals are the same. As such, these rules rely on the nodal structure of either the wave function or the frontier molecular orbitals, so it is unclear how these rules can be recovered in the density functional reactivity theory (or "conceptual DFT"), where the basic quantity is the strictly positive electron density. A third, nonsymmetry based approach to predict the outcome of pericyclic reactions is due to Zimmerman which uses the concept of the aromatic transition states: allowed reactions possess aromatic transition states, while forbidden reactions possess antiaromatic transition states. Based on our recent work on cycloadditions, we investigate the initial response of the chemical hardness, a central DFT based reactivity index, along the reaction profiles of a series of electrocyclizations. For a number of cases, we also compute complete initial reaction coordinate (IRC) paths and hardness profiles. We find that the hardness response is always higher for the allowed modes than for the forbidden modes. This suggests that the initial hardness response along the IRC is the key for casting the Woodward-Hoffmann rules into conceptual DFT.

Aromaticity changes along the reaction coordinate connecting the cyclobutadiene dimer to cubane and the benzene dimer to hexaprismane

Aromaticity and reactivity are two deeply connected concepts. Most of the thermally allowed cycloadditions take place through aromatic transition states, while transition states of thermally forbidden reactions are usually less aromatic, if at all. In this work, we perform a numerical experiment to discuss the change of aromaticity that occurs along the reaction paths that connect two antiaromatic units of cyclobutadiene to form cubane and two aromatic rings of benzene to yield hexaprismane. It is found that the aromaticity profile along the reaction coordinate of the [4+4] cycloaddition of two antiaromatic cyclobutadiene molecules goes through an aromatic highest energy point and finishes to an antiaromatic cubane species. Up to our knowledge, this represents the first example of a theoretically and thermally forbidden reaction path that goes through an intermediate aromatic region. In contrast, the aromaticity profile in the [6+6] cycloaddition of two aromatic benzene rings show a slow steady decrease of aromaticity from reactants to the highest energy point and from this to the final hexaprismane molecule a plunge of aromaticity is observed. In both systems, the main change of aromaticity occurs abruptly near the highest energy point, when the distance between the centers of the two rings is about 2.2 A.

Characterization of Pericyclic Reactions Using Multicenter Electron Delocalization Analysis

This work investigates the applicability of multicenter delocalization analysis to the characterization of pericyclic reactions. The results indicate that multicenter delocalization indices are a powerful tool for studying concerted processes, allowing the characterization of aromatic transition states with a significant increase in the electron delocalization. Moreover, an advantage over magnetic-based indices is that multicenter delocalization indices are not influenced by local electron currents but by the electron delocalization along the multiple (n) centers, and provide, in a quantitative sense, more reliable results. A thorough comparison with magnetic-based indices is carried out for the trimerization reaction of acetylene. Tracking the values of multicenter delocalization indices along the reaction path allows investigation of the nature of concerted mechanisms. Six-center electron delocalization displays a maximum at the transition state of the Diels-Alder reaction, whereas a similar maximum of four-center electron delocalization is slightly displaced to butadiene for the ring opening of cyclobutene. The profile of multicenter electron delocalization indices along the reaction path of [2+2] cycloaddition of ketene to ethene shows the presence of the two independent mechanisms that agree with the two HOMO/LUMO orbital interactions previously proposed to dominate this reaction.

The unusual electronic mechanism of the [1,5] hydrogen shift in (Z)-1,3-pentadiene predicted by modern valence bond theory

The combination of modern valence bond theory in its spin-coupled (SC) form [SC(6)/6-31G*] and an intrinsic reaction coordinate calculation (MP2/6-31G* IRC) is used in order to obtain a model for the electronic mechanism of the gas-phase [1s,5s] hydrogen shift in (Z)-1,3-pentadiene. It is shown that this reaction follows an unusual heterolytic mechanism consistent with the C(s) symmetry of the transition state and involving the simultaneous movements of three well-defined orbital pairs. One of these is responsible for the sigma bond which initially attaches the migrating hydrogen to carbon 5 and, later on, to carbon 1. The second one realises the pi bond between carbons 3 and 4 which during the course of the reaction moves over carbons 2 and 3, while the third pair, initially involved in the pi bond between carbons 1 and 2 has no other choice but to embark on a long-range journey across the ring, ending up over carbons 4 and 5. While at first sight, it might appear that an electronic mechanism of this type would preclude the existence of an aromatic transition state, we have been able to show that the electronic structure of the transition state for this sigmatropic hydrogen shift has much in common with a hitherto apparently unknown alternative modern valence bond description of benzene involving 'antipairs' and so it can be considered to be aromatic.

Dynamic Processes in [16]Annulene: Möbius Bond-Shifting Routes to Configuration Change

Density functional and ab initio methods have been used to study the mechanisms for key dynamic processes of the experimentally known S4-symmetric [16]annulene (1a). Using BH&HLYP/6-311+G** and B3LYP/6-311+G**, we located two viable stepwise pathways with computed energy barriers (Ea = 8-10 kcal/mol) for conformational automerization of 1a, in agreement with experimental data. The transition states connecting these conformational minima have Möbius topology and serve as starting points for non-degenerate pi-bond shifting (configuration change) via Möbius aromatic transition states. The key transition state, TS1-2, that connects the two isomers of [16]annulene (CTCTCTCT, 1 --> CTCTTCTT, 2) has an energy, relative to the S4 isomer, that ranged from 6.9 kcal/mol (B3LYP/6-311+G**) to 16.7 kcal/mol (BH&HLYP/6-311+G**), bracketing the experimental barrier. At our best level of theory, CCSD(T)/cc-pVDZ(est), this barrier is 13.7 kcal/mol. Several other Möbius bond-shifting transition states, as well as Möbius topology conformational minima, were found with BH&HLYP energies within 22 kcal/mol of 1a, indicating that many possibilities exist for facile thermal configuration change in [16]annulene. This bond-shifting mechanism and the corresponding low barriers contrast sharply with those observed for cis/trans isomerization in acyclic polyenes, which occurs via singlet diradical transition states. All Möbius bond-shifting transition states located in [16]- and [12]annulene were found to have RHF --> UHF instabilities with the BH&HLYP method but not with B3LYP. This result appears to be an artifact of the BH&HLYP method. These findings support the idea that facile thermal configuration change in [4n]annulenes can be accounted for by mechanisms involving twist-coupled bond shifting.