Electrocyclic Research Articles

Synthesis of 2-arylchromeno[2,3,4,5-lmna]phenanthridines through a sequential multicomponent assembly, oxygenation and 6πe electrocyclization reactions

Synthesis of 2-arylchromeno[2,3,4,5-lmna]phenanthridines through a sequential multicomponent assembly, oxygenation and 6πe electrocyclization reactions

Electrocyclization for the Synthesis of Mono‐ and Disulfonyl‐substituted Pyrazoles From Sulfonyl Hydrazines and 1,3‐Diketones

AbstractThe tunable electrocyclic reactions for the synthesis of a series of mono‐ and disulfonyl‐substituted pyrazoles from sulfonyl hydrazines and 1,3‐diketones were investigated by utilizing inexpensive electrolytes under electrochemical conditions in an undivided cell. When nBu4NBF4 was used as an electrolyte, it afforded monosulfonyl‐substituted pyrazoles. By contrast, when NH4I was used as an electrolyte, it gave disulfonyl‐substituted pyrazoles. The mechanistic investigation shows that they all underwent a free radical process but formed N‐centered sulfonyl hydrazine radical and S‐centered sulfonyl radical in the synthesis of mono‐ and disulfonyl‐substituted pyrazoles, respectively.

Aza-Diarylethenes Undergoing Both Photochemically and Thermally Reversible Electrocyclic Reactions.

Exploring novel molecular photoswitches plays a crucial role in the field of photo-functional materials chemistry. In this study, we synthesized aza-diarylethenes with benzothiophene-S,S-dioxide as a part of the hexatriene structure and investigated their photochromic properties. Unlike previously reported aza-diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo-photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X-ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed-ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza-diarylethenes exhibited not only photochemical ring-closing and ring-opening reactions but also thermal ring-closing and ring-opening reactions, driven by a thermal equilibrium between the open- and closed-ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy-level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed-ring isomers adopt the same molecular structure, which was well explained based on the reaction mechanism of photochemical and thermal ring-closing reactions. These findings not only advance the field of aza-diarylethenes but also inspire future research in the development of new photoswitches.

Photochemical Synthesis of 7,12-Dioxa[8]helicene and Density Functional Theory Studies: Unravelling the One-Way Valve System Involving Steric Crowding and Aromatic Stability.

C2 symmetric chiral 7,12-dioxa[8]helicenes were synthesized through a series of photochemical E-Z isomerization, electrocyclic reaction, and oxidation steps in a stepwise sequential manner at both the ends of 2,9-di((E)-styryl)naphtho[2,1-b:7,8-b']difuran. The chemical transformations complemented with density functional theory (DFT) studies delineate some fundamental concepts, exhibiting counter current effects, namely, destabilization caused by increasing steric crowding and stabilization caused by aromatic units on the overall transformation. The calculated energy profile diagram unravels the formation of photoinduced intermediate species with increasing free energies for the E-Z isomerization and the electrocyclic reactions; the reverse processes for the said steps are prevented by a specific barrier-less oxidation step forming aromatic rings, presenting a one-way valve situation. The steric crowding-related increase in free energy and its counterbalancing by aromaticity have been illustrated for the helicene system using DFT studies. HOMA analysis shows that each individual ring in 7,12-dioxa[8]helicene exhibits a strong aromatic character, supporting the Fries model empirically. Interestingly, despite the nonplanar and sterically crowded geometry, 7,12-dioxa[8]helicene displayed a large HOMO-LUMO gap, typical of aromatic compounds.

Study of the Torquoselectivity of a Set of Unusual Ring‐Opening Electrocyclic Reactions: Determination of the Electronic Bonding Structure Through the Methodologies of Natural Bond Orbital Analysis and Quantum Theory of Atoms in Molecules, and Analysis of the Electronic Reaction Mechanism Through Bond Reactivity Descriptors

ABSTRACTIn this work, we have studied the torquoselectivity of a set of unusual ring‐opening electrocyclic reactions that have not been successfully rationalized using models based on orbital symmetry nor have they been explained by steric hindrance. Firstly, the corresponding transition states have been obtained, and it has been verified that the intrinsic reaction coordinates associated with these transition states are consistent with the reactants and products of the reactions studied. This has allowed us to theoretically calculate the reaction barriers (with their corresponding thermal corrections) and compare them with the corresponding experimental values. In a second step, we have analyzed the electronic bonding structure using the methodologies of Natural Bond Orbital Analysis and Quantum Theory of Atoms in Molecules, searching for interactions that may significantly stabilize the transition states. Finally, we have analyzed the reactivity using a recent model that has allowed us to calculate the corresponding bond reactivity descriptors.

Expanding the Chemistry of Pentafluorophenyl-N-Confused Porphyrin: Diketonate Substitution and Derivatizations at the External 3-C Position of the Inverted Pyrrole Ring.

In this study, we synthesized two new 3-C-substituted pentafluorophenyl-N-confused porphyrins (PFNCPs), one with acetylacetonate (PFNCP-acac, 2a) and the other with ylidene-2-propanone (PFNCP-ac, 3a), through a one-pot reaction in the absence of a catalyst. Under mild acidic and heating conditions, the acac-substituted compound underwent acyl cleavage degradation, yielding ac-substituted product 3a. Subsequent chelation of the acac-substituted PFNCP with BF2 resulted in a boron diketonate derivative, PFNCP-acacBF2 (4). Additionally, an electrocyclic reaction of the ac-substituted PFNCP 3a, without a catalyst, produced a tricyclic fused [6,6,5]-TF-PFNCP (5). This tricyclic product could also be obtained directly from PFNCP-acac 2a under heating conditions. The absorption spectra revealed that acac- and ac-substituted macrocycles exhibit either a single or split Soret band, respectively, in the 400-550 nm range, along with multiple Q bands spanning the 580-690 nm region. While BF2 derivatization caused a slight red shift in the absorption spectra, the [6,6,5]-tricyclic fused NCP demonstrated a significant red shift. All newly synthesized compounds were characterized by using single-crystal X-ray structures, 1H NMR spectroscopy, and mass spectrometry. Density functional theory (DFT) studies were conducted to elucidate the photophysical properties of these macrocycles.

Crystal Structure Landscape of Diarylethene-Based Crystalline Solids: A Comprehensive CSD Analysis.

Diarylethenes (DAEs) are an exciting class of stimulus-responsive organic molecules that exhibit electrocyclization reactions upon exposure to light, heat, or other stimuli. The rational design of DAE-based crystalline materials is, however, complicated by the presence of DAE atropisomers, only one of which is photoactive. Data mining of the CSD produced 1349 unique molecular DAE structures that were subsequently analyzed according to selected chemical and geometric attributes. Additional analyses were performed on 1078 dithienylethene (DTE) structures-the largest subgroup within the ensemble. The crystal structure landscape, based upon D-D parameterization and analysis, revealed a vast array of molecular geometries, many of which may not correspond to energetic minima. The analyses link various chemical and geometric parameters to isomers observed in the lattice and their reactivity; however, potential biases intrinsic to this ensemble of structures complicate the determination of causal relationships. We believe that this retrospective comprehensive analysis of DAE structures represents an important step for understanding more broadly the crystal landscape of this class of materials.

Effect of Sustainable and Confined Media on the Photoinduced [6π]‐Electrocyclization Reaction of Diphenyl and N‐Methyldiphenylamines.

Effect of Sustainable and Confined Media on the Photoinduced [6π]‐Electrocyclization Reaction of Diphenyl and N‐Methyldiphenylamines.

Strategies to Enhance the Stability of Non‐Fullerene Acceptor‐Based Organic Solar Cells

AbstractThrough comprehensive density functional theory calculations, the photodegradation mechanisms, including cis–trans isomerization, electrocyclization, and sigmatropic rearrangement reactions are investigated in indacenodithieno [3,2‐b] thiophene (IT)‐based non‐fullerene acceptors (NFAs). Various functional group substitutions on the core (fluorine, ethyl, and cyano groups) and end groups (fluorine) of NFA are introduced to elucidate the influence of chemical modifications on photodegradation pathways. The findings reveal that the core substitution can effectively suppress electrocyclization and 1,5‐sigmatropic shift reactions, which are major contributors to photodegradation. Furthermore, the electronic, excited‐state, and charge transport properties of pristine and degraded products are studied to gain insights into the impact of degradation on photovoltaic parameters. The results suggest that photodegradation leads to the formation of shallow energy trap states, hindering charge transport and increasing charge recombination, ultimately affecting the power conversion efficiency of organic solar cells (OSCs). The study not only provides a comprehensive understanding of photodegradation mechanisms but also offers valuable molecular design strategies to enhance the stability of NFAs for future large‐scale applications of OSCs. By establishing a clear connection between the chemical structure and photostability of NFA, this research represents a pivotal contribution to the field of organic electronics and sustainable energy technologies.

One-Pot Expeditious Synthesis of Pyrazoloindolones via Base-Promoted Electrocyclization, C–N Coupling and Intramolecular Oxidative Cyclization

AbstractPyrazoloindolones are synthesized from N-tosylhydrazones in a one-pot multistep process which include base-promoted (i) electrocyclization reaction of N-tosylhydrazones derived from α,β-unsaturated aldehydes, (ii) aromatic nucleophilic substitution, and (iii) a domino cyclization–oxidation process under aerobic conditions.

Skeletal Rearrangement of Oxazole to Azepine and Pyrrole through Dynamic 8π Electrocyclizations.

We present a novel approach for the skeletal rearrangement of an oxazole into an azepine and pyrrole through a dynamic electrocyclization process, showing an innovative, unconventional reaction sequence. This method enables precise control of regioselectivity in competitive 6π and 8π electrocyclization reactions, rendering the final products rich in functional groups that can be further developed for the synthesis of nitrogen-containing scaffolds. This is an unprecedented example of the selective synthesis of seven- and five-member heterocycles via dynamic electrocyclization ring opening or closure.

Polycyclic Heteroaromatic π‐Linkers Provide Dithienylethene Switches with Favorable Thermal and Photochemical Properties for Solar‐Energy Storage

AbstractDithienylethene photoswitches with an aromatic π‐linker as the bridge between the two thiophene units are attractive starting materials for developing molecular solar thermal energy (MOST) storage systems, partly because the aromaticity of their ring‐open forms is a favorable feature with regard to the energy‐storage densities of their ring‐closed forms produced by photoinduced electrocyclization (photocyclization) reactions. At the same time, this typically leads to small barriers for their thermal cycloreversion reactions, which are not desirable in this context. Here, we use computational methods to show that this problem can be circumvented with polycyclic heteroaromatic π‐linkers. Specifically, through the tuning of the aromatic character of the individual rings of such a π‐linker (like indole or isoindole), it is shown to be possible to strike a delicate balance between the seemingly contrasting requirements of simultaneously achieving both a high energy‐storage density and a large cycloreversion barrier. Furthermore, this design is also found to provide for a quick and efficient photocyclization reaction, owing to the onset of excited‐state antiaromaticity in the π‐linker upon light absorption of the ring‐open form. Altogether, dithienylethenes with polycyclic heteroaromatic π‐linkers appear to have both thermal and photochemical properties suitable for further development into future MOST systems.

Urea-Promoted Neat Synthesis of Fused Dihydroisoquinolines and Disubstituted Pyridines: A Mechanistic Observation with Molecular-Sensing Studies.

An efficient and short synthesis of fused dihydroisoquinolines, diaryl substituted pyridine derivatives in good to high yields has been established by using an environmentally safe, solvent-metal-oxidant-free tandem approach. In this article, we discuss how the electrocyclic reaction is more pronounced in the solid phase in the presence of urea, whereas the typical aza-Michael addition is more prominent in presence of arylamine in the solution phase for 3-(2-formylcycloalkenyl)acrylic ester derivative substrates. The wide range of substrates and urea-promoted neat synthesis made our approach more significant in medical and also analytical science. Moreover, an isoquinoline alkaloid decumbenine B analogue has been synthesized by using our newly developed neat methodology. We have also investigated the photophysical properties of the synthesized fused dihydroisoquinoline derivatives. One of the synthesized molecules was used as a sensor for the selective detection of toxic picric acid. Therefore, the effective neat synthesis and molecular sensing applications of these compounds made our approach more exciting in the field of heterocyclic chemistry.

A 21st Century View of Allowed and Forbidden Electrocyclic Reactions.

In 1965, Woodward and Hoffmann proposed a theory to predict the stereochemistry of electrocyclic reactions, which, after expansion and generalization, became known as the Woodward-Hoffmann Rules. Subsequently, Longuet-Higgins and Abrahamson used correlation diagrams to propose that the stereoselectivity of electrocyclizations could be explained by the correlation of reactant and product orbitals with the same symmetry. Immediately thereafter, Hoffmann and Woodward applied correlation diagrams to explain the mechanism of cycloadditions. We describe these discoveries and their evolution. We now report an investigation of various electrocyclic reactions using DFT and CASSCF. We track the frontier molecular orbitals along the intrinsic reaction coordinate and modeled trajectories and examine the correlation between HOMO and LUMO for thermally forbidden systems. We also investigate the electrocyclizations of several highly polarized systems for which the Houk group had predicted that donor-acceptor substitution can induce zwitterionic character, thereby providing low-energy pathways for formally forbidden reactions. We conclude with perspectives on the field of pericyclic reactions, including a refinement as the meaning of Woodward and Hoffmann's "Violations. There are none!" Lastly, we comment on the burgeoning influence of computations on all fields of chemistry.

Highly Enantioselective 6π Photoelectrocyclizations Engineered by Hydrogen Bonding.

Photochemical electrocyclization reactions are valued for both their ability to produce structurally complex molecules and their central role in elucidating fundamental mechanistic principles of photochemistry. We present herein a highly enantioselective 6π photoelectrocyclization catalyzed by a chiral Ir(III) photosensitizer. This transformation was successfully realized by engineering a strong hydrogen-bonding interaction between a pyrazole moiety on the catalyst and a basic imidazolyl ketone on the substrate. To shed light on the origin of stereoinduction, we conducted a comprehensive investigation combining experimental and computational mechanistic studies. Results from density functional theory calculations underscore the crucial role played by the prochirality and the torquoselectivity in the electrocyclization process as well as the steric demand in the subsequent [1,4]-H shift step. Our findings not only offer valuable guidance for developing chiral photocatalysts but also serve as a significant reference for achieving high levels of enantioselectivity in the 6π photoelectrocyclization reaction.

Density functional theory study of the styrylbenzoquinoline dyad and the related dibenzoquinolylcyclobutane formed in the [2 + 2] photocycloaddition reaction

AbstractThe structure and electronic properties of the biphotochromic dyad with two styrylbenzo[f]quinoline photochromes, as well as the corresponding cyclobutane with two benzo[f]quinoline (BQ) substituents, are studied by DFT at the M06‐2X/6‐31G* level, the cyclobutane being a product of the [2 + 2] photocycloaddition (PCA) reaction of the dyad. According to calculations, the dyad forms π‐stacked folded conformers, which, when excited, can form excimers that are precursors of cyclobutane. TD DFT calculations and natural transition orbital (NTO) analysis indicated that the lowest singlet excited S1 state in the dyad is localized on the SBQ photochrome, including the ethylene group that undergoes PCA. Thus, the conditions for concerted electrocyclic reactions are satisfied, and the direct PCA follows the Woodward–Hoffmann rules. In contrast, in cyclobutane, the S1 state is localized on the BQ substituent rather than on the cyclobutane core. Therefore, the reverse ring‐opening (retro‐PCA) reaction cannot follow the Woodward‐Hoffmann rules and inevitably involves a step of excitation energy transfer from BQ to cyclobutane, which means the predissociation mechanism.

Reversible Boron‐Insertion into Aromatic C−C Bonds

AbstractFormation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron‐insertion into aromatic C−C bonds in the photo‐promoted skeletal rearrangement reaction of triarylboranes bearing an ortho‐phosphino substituent (ambiphilic phosphine‐boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X‐ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron‐doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z‐isomerization of an alkene moiety proceed via a highly strained trans‐borepin intermediate.

Reversible Boron-Insertion into Aromatic C-C Bonds.

Formation of borabicyclo[3.2.0]heptadiene derivatives was achieved via boron-insertion into aromatic C-C bonds in the photo-promoted skeletal rearrangement reaction of triarylboranes bearing an ortho-phosphino substituent (ambiphilic phosphine-boranes). The borabicyclo[3.2.0]heptadiene derivatives were fully characterized by NMR and X-ray analyses. The dearomatized products were demonstrated to undergo the reverse reaction in the dark at room temperature, realizing photochemical and thermal interconversion between triarylboranes and boron-doped bicyclic systems. Experimental and theoretical studies revealed that sequential two electrocyclic reactions involving E/Z-isomerization of an alkene moiety proceed via a highly strained trans-borepin intermediate.

Synthesis towards Phainanoid F: Photo-induced 6π-Electrocyclization for Constructing Contiguous All-Carbon Quaternary Centers.

In this paper, we report an efficient strategy for synthesizing the DEFGH rings of phainanoid F. The key to the construction of the 13,30-cyclodammarane skeleton of the molecule was a photo-induced 6π-electrocyclization and a homoallylic elimination. Notably, this is a rare example of using electrocyclization reaction to simultaneously construct two vicinal quaternary carbons in total synthesis. The strategy outlined here forms the basis of our total synthesis of Phainanoid F, and it could also serve as a generally applicable approach for synthesizing other natural products containing the 13,30-cyclodammarane skeleton.

Diarylethene Photoswitches Undergoing 6π Azaelectrocyclic Reaction: Disrotatory Thermal Cycloreversion of the Closed-Ring Isomer.

Gaining insight into the dynamics of electrocyclic reactions is very important from both fundamental and application perspectives. In this study, we developed novel diarylethene photoswitches that undergo 6π azaelectrocyclic reaction. We found that they exhibit fast thermally reversible type (T-type) photochromism, in contrast to the fact that common diarylethenes exhibit photochemically reversible type (P-type) photochromism. The quantum chemical calculations revealed that the fast T-type photochromism originates from the unprecedented disrotatory thermal cycloreversion of the closed-ring isomer. Our results provide useful information not only for the dynamics of the 6π azaelectrocyclic reaction but also for the further development of diarylethene photoswitches utilizing the 6π azaelectrocyclic reaction.