Electrocyclic Ring-opening Reaction Research Articles

Ab initio molecular orbital and density functional studies on the ring-opening reaction of 2H-thiete

Electrocyclic ring opening (ERO) reaction of 2H-thiete has been carried out computationally in the gas phase and the ring opening barrier has been computed. When comparing the ERO reaction of 2H-thiete with the parent hydrocarbon (cyclobutene), the ring opening of cyclobutene is found to exhibit pericyclic behavior while 2H-thiete shows pseudopericyclic nature. Computation of the nucleus-independent chemical shift (NICS) of 2H-thiete adds evidence for pseudopericyclic behavior. Locking of the lone pair of electrons by hydrogen bonding was carried out to understand the extent of involvement of the lone pair of electrons during the course of the reaction. CR-CCSD(T)/6-311+G∗∗ computation was performed to assess the energies of the reactant, transition state and the product more accurately.

Analyzing torquoselectivity in electrocyclic ring opening reactions of trans-3,4-dimethylcyclobutene and 3-formylcyclobutene through electronic structure principles

The validity of maximum hardness, minimum electrophilicity and minimum polarizability principles is assessed to explain the phenomenon of torquoselectivity (inward and outward preference) in the conrotatory ring opening reactions of trans-3,4-dimethylcyclobutene into Z,Z- and E,E-butadienes and 3-formylcyclobutene into E- and Z-2,4-pentadienals. The hardness, average polarizability and electrophilicity profiles are computed along the intrinsic reaction coordinate and divided into three relevant stages. The transition states involved in the unfavorable inward conrotation of trans-3,4-dimethylcyclobutene and in the unfavorable outward conrotation of 3-formylcyclobutene are found to be higher in energy, softer, more electrophilic and more polarizable than the transition states corresponding to the torquoselective outward and inward conrotations, respectively. These observations are in conformity with the maximum hardness, minimum electrophilicity and minimum polarizability principles. The sharp changes in the local reactivity descriptors are also observed around the transition states in their respective profiles.

Ab initio molecular orbital and density functional studies on the ring-opening reaction of oxetene.

Electrocyclic ring opening (ERO) reaction of 2H-Oxete (oxetene) has been carried out computationally in the gas phase and ring opening barrier has been computed. When comparing the ERO reaction of oxetene with the parent hydrocarbon (cyclobutene), the ring opening of cyclobutene is found to exhibit pericyclic behavior while oxetene shows mild pseudopericyclic nature. Computation of the nucleus-independent chemical shift (NICS) of oxetene adds evidence for pseudopericyclic behavior of oxetene. By locking of lone pair of electrons by hydrogen bonding, it is seen that the pseudopericyclic nature of the ring opening of oxetene is converted into a pericyclic one. CASSCF(5,6)/6-311+G** computation was carried out to understand the extent of involvement of lone pair of electrons during the course of the reaction. CR-CCSD(T)/6-311+G** computation was performed to assess the energies of the reactant, transition state and the product more accurately.

ChemInform Abstract: Effect of the Trifluoromethyl Group on Torquoselectivity in the 4π Ring‐Opening Reaction of Oxetenes: Stereoselective Synthesis of Tetrasubstituted Olefins.

AbstractThe highly stereoselective synthesis of tetrasubstituted olefins such as (II)/(III), (V), (VII)/(VIII), and (X)/(XI) bearing a CF3‐group via the thermal 4π electrocyclic ring opening reaction of various oxetenes is described.

Substituent effects on rates and torquoselectivities of electrocyclic ring-openings of N-substituted 2-azetines.

Transition structures for the conrotatory electrocyclic ring-opening reactions of N-substituted 2-azetines were computed with the density functional M06-2X/6-31+G(d,p). A wide range of substituents from π acceptors (e.g., CHO, CN) to π donors (NMe2, OMe) was explored. Acceptor substituents delocalize the nitrogen lone pair and stabilize the reactant state of 2-azetines, while donors destabilize the 2-azetine reactant state. The conrotatory ring-opening is torquoselective, and the transition state for the outward rotation of the N-substituent and inward rotation of the nitrogen lone pair is preferred. This transition structure is stabilized by an interaction between the nitrogen lone pair and the vacant π* orbital. The activation free energies are linearly related to the reaction free energies and the Taft σR(0) parameter.

Peculiar structure of the potential energy surfaces of typical electrocyclic reactions in the areas of the symmetry-forbidden reaction paths

The mechanisms of the electrocyclic ring opening reactions of cyсlobutene to trans-l,3-butadiene and 1,3-cyclohexadiene to 1,3,5-hexatriene were computationally studied with the use of B3LYP/6-311++G(d,p), MP2/6-311G++(3df,2p), CCSD/6-311++G(df,p), CCSD/6-311G(df,p) and CCSD(T)/6-311++G(df,p)//CCSD methods. Special attention was paid to the study of the potential energy surfaces (PESs) of both systems in the domains pertaining to the disrotatory movement of the vicinal methylene groups. No symmetry-forbidden disrotatory reaction channel was found within the PES of the cyclobutene–1,3-butadiene system. Instead of that, disrotatory rotation of two methylene groups of cyсlobutene results in elimination of a molecule of dihydrogen giving rise to the formation of a low-stable π-complex H2·cyсlobutadiene. Similar type π-complex H2·benzene was also located on the 1,3-cyclohexadiene–1,3,5-hexatriene PES along the disrotatory reaction coordinate. The topologies of the PESs of both systems analyzed on the basis of the patterns of the gradient lines are characterized by an existence on both of them of valley-ridge inflection (VRI) points corresponding to the bifurcation of the disrotatory reaction paths leading to the formation of the π-complexes formed upon elimination of a dihydrogen molecule from cyclobutene and 1,3-cyclohexadiene.

Effect of the trifluoromethyl group on torquoselectivity in the 4π ring-opening reaction of oxetenes: stereoselective synthesis of tetrasubstituted olefins

A highly stereoselective synthesis of tetrasubstituted olefins bearing a trifluoromethyl group via the thermal 4π electrocyclic ring-opening reaction of oxetenes, simply prepared by the Pd-catalyzed [2+2] cycloaddition reaction of various alkynes with trifluoropyruvate, is achieved. In this reaction process, the trifluoromethyl group prefers inward rotational torquoselectivity because of the orbital interactions between the breaking C–O σ orbital on the oxetene moiety and the C–F σ* orbital in the transition state.

Nonlinear optical signatures of ultraviolet light-induced ring opening in α-terpinene

Photoinduced electrocyclic ring opening reactions in conjugated cylcoalkenes are among the most elementary processes in organic chemistry. One prototypical ring opening reaction transforms cyclohexadiene into hexatriene. It is known that a sequence of sub-100 fs internal conversion transitions precedes bond breaking in cyclohexadiene and some of its derivatives. However, these excited state dynamics have never been directly monitored in solution because of insufficient time resolution. Here we aim to uncover the extraordinary photophysics behind related ultrafast internal conversion processes in a derivative of cyclohexadiene, α-terpinene (α-TP), solvated in cyclohexane. Transient absorption anisotropy experiments conducted with 20 fs laser pulses at 267 nm expose non-exponential depopulation kinetics for the ππ* electronic state of α-TP. Our data show that population transfer rapidly accelerates within the first 100 fs after photoexcitation. In addition, recurrences in two-dimensional photon echo (2DPE) line shapes reveal strong vibronic coupling in a normal mode near 523 cm−1, which involves torsions of the C=C bonds and hydrogen out-of-plane (HOOP) wagging on a vinyl group. With the support of several experiments, we hypothesize that the excited state wavepacket inα-TP undergoes several recurrences in the C=C stretching coordinate before displacement along the C=C torsion/vinyl HOOP coordinate finally sets it free from the Franck–Condon region of the potential energy surface. The unconfined wavepacket departs the ππ* electronic state by way of a conical intersection with a lower energy excited state. The present observations are made possible by recent improvements to both the time resolution and detection sensitivity of our experimental setup. This work demonstrates that it is now possible to acquire 2DPE signals in the deep ultraviolet, which are comparable with high-quality measurements in the visible spectral region. These technical developments open the door to studies of many beautiful models for elementary chemical dynamics.

Predominance of the Participation of the Geminal over Vicinal Bonds: Torquoselectivity of Retro-Nazarov Reactions

Torquoselectivities of electrocyclic ring-opening reactions have been proposed to be controlled by the participation of electron-donating σ-bonds (σ-EDB) geminal to the breaking σ-bond and/or vicinal electron-accepting σ-bonds (σ-EAB) or lone pair(s) of the substituent(s). We designed reactions in which the effects of the vicinal bonds should be significantly diminished by the cationic nature of the reactants. The calculated enthalpies of activation of the retro-Nazarov reactions of some 3-substituted cyclopentenyl cations show inward rotation of the geminal σ-EDB, which is consistent with the theory of geminal bond participation.

ChemInform Abstract: The Ultrafast Pathway of Photon‐Induced Electrocyclic Ring‐Opening Reactions: the Case of 1,3‐Cyclohexadiene

AbstractReview: 74 refs.

A [4+4] annulation strategy for the synthesis of eight-membered carbocycles based on intramolecular cycloadditions of conjugated enynes

A [4+4] annulation strategy for the synthesis of eight-membered carbocycles is reported that proceeds via a cascade involving two pericyclic processes. In the first step, the [4+2] cycloaddition of a conjugated enyne with an electron-deficient cyclobutene generates a strained six-membered cyclic allene that isomerizes to the corresponding 1,3-cyclohexadiene. In the second step, this bicyclo[4.2.0]octa-2,4-diene intermediate undergoes thermal or acid-promoted 6-electron electrocyclic ring opening to furnish a 2,4,6-cyclooctatrienone. The latter transformation represents the first example of the promotion of 6-electron electrocyclic ring opening reactions by acid.

Catalyst-Controlled Torquoselectivity Switch in the 4π Ring-Opening Reaction of 2-Amino-2-azetines Giving β-Substituted α,β-Unsaturated Amidines

The torquoselectivity of the 4π electrocyclic ring-opening reaction of 2-azetines can be controlled by the Brønsted acidity of the catalyst and the polarity of the solvent. DFT calculations provided insight into the mechanism of this remarkable switch. Anti and syn stereoisomers of α,β-unsaturated amidines were selectively synthesized from ynamides and aldimines in the presence of Tf(2)NH and CSA, respectively.

The Ultrafast Pathway of Photon-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

Application of activation hardness in perturbed pericyclic reactions: a case study involving electrocyclic ring opening reactions of heterocyclobutenes

AbstractCyclobutene undergoes electrocyclic ring opening through conrotatory mode to give 1,3‐butadiene and similarly its skeletally substituted analogues viz. phosphetene, thietene, oxetene and azetene isomerize to give their respective heterodienes. Quantum mechanical investigations on these perturbed pericyclic isomerizations at B3LYP level with 6‐31G(d) and 6‐311 + G(d,p) basis sets reveal that hetero ‐ atoms significantly lower the barrier and alter the reaction free energies. Interestingly frontier orbital correlation diagram shows that σ and σ* levels are perturbed significantly while π and π* levels undergo little changes during the reaction; the lone pairs on hetero – atoms are slightly stabilized in the TS. This results in σ becoming HOMO in the TSs of phosphetene and thietene isomerizations and σ* becoming LUMO in all the cases. This necessitates defining hardness values based on closely interacting frontier orbitals for reactants and TSs and use them to compute activation hardness values to interpret relative reactivity. Copyright © 2010 John Wiley & Sons, Ltd.

ChemInform Abstract: Stereoselective Synthesis of Tetraalkyl Cyclobutene-1,2,3,4-tetracarboxylates. Synthesis of Tetraalkyl (Z,Z)-Buta-1,3-diene-1,2,3,4-tetracarboxylates.

Tetraalkyl cyclobutene-1,2,3,4-tetracarboxylates, prepared by intramolecular Wittig reaction between a vinylphosphonium salt and diethyl 2-oxobutanedioate, undergo electrocyclic ring-opening reactions, in boiling toluene, to produce highly electron-deficient 1,3-dienes.

ChemInform Abstract: Stereoselective Synthesis of Highly Functionalized Cyclobutenes. A Facile Route to Electron-Deficient 1,3-Dienes.

Abstract Acetoacetanilide undergoes a smooth reaction with triphenylphosphine and dialkyl acetylenedicarboxylates to produce dialkyl 2-(acetoacetanilide-2-yl)-3-(triphenylphosphoranilidene)-butandioates, which undergo intramolecular Wittig reaction to produce 2-methyl-3-( N -phenylcarbonyl)-1,4-dialkoxycarbonylcyclobutenes. These cyclobutene derivatives undergo electrocyclic ring-opening reactions in boiling toluene to produce highly electron-deficient 1,3-dienes.

Fluorine effect on pericyclic and pseudopericyclic processes: Evidences and ab initio theory

Electrocyclic ring opening (ERO) reactions of 2-pyrone, 2-pyranol and pyran and their fluoro compounds (1–6) have been studied at MP2/6-31G(d) level with special emphasis on the influence of fluorine on these pericyclic/pseudopericyclic processes. Calculations clearly predict that substitution of fluorine at C6 favour the reaction both kinetically and thermodynamically. Magnetic susceptibility anisotropy (Δχ aniso), NICS(0), NBO and bond critical property (BCP) analyses clearly illustrate the following; 2-pyrone (1) and 6-fluoro-2-pyrone (2) reactions are pseudopericyclic; 6-fluoro-2-pyranol (reaction 4) corresponds to a borderline case; 2-pyranol (3) and pyran (5) and 6-fluoro pyran (6) reactions are clearly pericyclic in character. Correspondingly pseudeopericyclic reactions show up orbital disconnections and fluorine delays the occurrence of orbital disconnections on the reaction trajectory.

Force-induced activation of covalent bonds in mechanoresponsive polymeric materials

Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups-so-called mechanophores-that the directional nature of mechanical forces can selectively break and re-form covalent bonds. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.

Electrocyclic ring-opening reactions may cause failure of enolate alkylation of 1,4-oxazin-2-one based chiral glycine equivalents

Attempted monoalkylation of 6-methyl-5,6-diphenyl-1,4-oxazin-2-one that had been designed as a chiral glycine equivalent gave even under optimized conditions only minute amounts of the desired monoalkylation product whereas an acyclic ketone was obtained repeatedly as the major product. This result indicates that the enolate of the chiral glycine equivalent is prone to an electrocyclic ring-opening reaction. This hypothesis could be further supported by additional experiments. Based on these results, it is reasonable to assume that electrocyclic ring-opening reactions are also the cause for the failure of alkylation reactions of structurally related glycine equivalents reported in the literature.

Cycloaddition/Ring Opening Reaction Sequences of N-Alkenyl Aziridines: Influence of the Aziridine Nitrogen on Stereoselectivity

The cycloaddition of (Z)-7-(prop-1-enyl)-7-azabicyclo[4.1.0]heptane with dimethyl acetylene dicarboxylate (DMAD) was reported previously to proceed with complete stereoselectivity. Quantum chemical calculations (B3LYP) were used to evaluate the mechanism of the cyclization process, and it was discovered that a stepwise pathway is preferred. The subsequent electrocyclic ring opening reaction of the cyclobutene was also studied, and it was found that ring opening to the "methyl-in" dienamine is preferred to the "methyl-out" product by some 4-5 kcal/mol.