Cyclic Conjugation Research Articles

Antiaromaticity of Cycloheptatrienyl Anions: Structure, Acidity, and Magnetic Properties.

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird-aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to -10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of antiaromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π-system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet-state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems.

Cyclic Azobenzene-BODIPY Hybrids.

Cyclic azobenzene-BODIPY hybrids were synthesized via cyclization by 1) acid-catalysed condensation of azobenzene-bridged dipyrroles with 3,5-di-tert-butylbenzaldehyde, 2) oxidation with DDQ, and 3) metalation with BF3 ⋅ Et2 O. The structures of many cyclic hybrids have been confirmed by single crystal X-ray analysis. The absorption spectra of the hybrids reveal the effective cyclic conjugation. The ultrafast measurements reveal that the photoexcited decays of these cyclic hybrids depend upon the ring size and connectivity.

Magnetic Antiaromaticity─Paratropicity─Does Not Necessarily Imply Instability.

Magnetically induced ring currents are a conventional tool for the characterization of aromaticity. Dia- and paratropic currents are thought to be associated with stabilization (aromaticity) and destabilization (antiaromaticity), respectively. In the present work, I have questioned the validity of the paratropic currents as a measure of antiaromaticity among monocyclic hydrocarbons. I have shown that while reduced/oxidized radical ions of hydrocarbons sustain strong paratropic currents, they often gain extra stabilization via cyclic conjugation compared to their acyclic counterparts.

Conjugated Nanohoop Polymers based on Antiaromatic Dibenzopentalenes for Charge Storage in Organic Batteries

AbstractWith their bent π‐systems, cyclic conjugation and inherent cavities, conjugated nanohoops are attractive for organic electronics applications. For ease of processing and morphological stability, an incorporation into polymers is desirable, but to date was hampered with few exceptions by synthetic difficulties. We herein present a unique strategy for the synthesis of conjugated nanohoop polymers using a dibenzo[a,e]pentalene (DBP) as central connector. We demonstrate this versatility by synthesizing three electronically diverse copolymers with dithienyldiketo(pyrrolopyrrol), fluorene and carbazole comonomers, and report the first donor‐acceptor nanohoop polymer. Optoelectronic investigations reveal the prevalence of cyclic or linear conjugation, depending on the comonomer unit, and ambipolar electrochemical properties through the antiaromatic character of the DBP units. As the first report on using conjugated nanohoops for charge storage as positive electrode materials, we show a significant improvement in battery performance in a nanohoop‐containing polymer compared to an equivalent nanohoop‐free reference polymer. We believe this study will pave the way for the synthesis of a diverse range of nanohoop polymers and further stimulate their exploration for charge storage in batteries.

Conjugated Nanohoop Polymers based on Antiaromatic Dibenzopentalenes for Charge Storage in Organic Batteries.

With their bent π-systems, cyclic conjugation and inherent cavities, conjugated nanohoops are attractive for organic electronics applications. For ease of processing and morphological stability, an incorporation into polymers is desirable, but to date was hampered with few exceptions by synthetic difficulties. We herein present a unique strategy for the synthesis of conjugated nanohoop polymers using a dibenzo[a,e]pentalene (DBP) as central connector. We demonstrate this versatility by synthesizing three electronically diverse copolymers with dithienyldiketo(pyrrolopyrrol), fluorene and carbazole comonomers, and report the first donor-acceptor nanohoop polymer. Optoelectronic investigations reveal the prevalence of cyclic or linear conjugation, depending on the comonomer unit, and ambipolar electrochemical properties through the antiaromatic character of the DBP units. As the first report on using conjugated nanohoops for charge storage as positive electrode materials, we show a significant improvement in battery performance in a nanohoop-containing polymer compared to an equivalent nanohoop-free reference polymer. We believe this study will pave the way for the synthesis of a diverse range of nanohoop polymers and further stimulate their exploration for charge storage in batteries.

Partition of topological indices of benzenoid hydrocarbons into ring contributions

AbstractThis work presents a simple method for partitioning the bond‐additive and atoms‐pair‐additive distance‐based topological indices of plane graphs into the sum of contributions of inner faces. The proposed method is applied to decompose several topological indices (Wiener, hyper‐Wiener, Tratch‐Stankevich‐Zefirov, Balaban, and Szeged indices) into the ring contributions for a series of benzenoid systems. It was found that the employed ring partitioning scheme is providing an accurate assessment of the dominant cyclic conjugation modes in the studied benzenoid hydrocarbons. Thus, the proposed method can be used as the alternative for the quantum‐chemistry‐based aromaticity indices which are significantly more computationally demanding.

Aromaticity enhancement of cyclopentadiene in piperidine derivatives: a DFT study on combination of the anomeric effect and Schleyer hyperconjugative aromaticity

The cyclopentadiene (Cp) hyperconjugative aromaticity on anomeric carbon of piperidines was studied using DFT method. The aromaticity indices showed that Cp has considerably enhanced cyclic conjugation in comparison with unsubstituted Cp.

Assessing an Elusive 3,4‐Dimethyl‐Chloroborole

AbstractThe attempted syntheses of 3,4‐dimethylborole derivatives further bearing bulky SiMe2t‐Bu groups in 2,5‐position are reported. Isolation of base‐stabilized adducts to the chloroborole were achieved. The respective free borole was found to be unstable and revealed 1,3‐hydrogen atom shift isomerization to a 1‐bora‐1,3‐butadiene with an s‐trans orientation and lacking cyclic conjugation of the π‐system. Computational probing indicates elevated C−H acidity of methyl groups attached to the butadiene backbone in boroles, exceeding the C−H acidity of respective boron‐bound methyl groups. Derivatization attempts of the new chloroborole are reported.

The generalized Zhang–Zhang polynomial of benzenoid systems – theory and applications

The generalized Zhang–Zhang (GZZ) polynomial was introduced recently aiming to increase the sensitivity of the well-known Zhang–Zhang polynomial onto π-electron cyclic conjugation of 10-membered rings. Here, the recursive formulas for the calculation of the GZZ of the benzenoid systems are derived. Then, an algorithm for calculating the GZZ of benzenoid chains is given. Lastly, testing the chemical applicability of GZZ is performed.

N]Cyclodibenzopentalenes as Antiaromatic Curved Nanocarbons with High Strain and Strong Fullerene Binding.

Conjugated nanohoops provide a platform to study structure-property relationships; they are attractive hosts for supramolecular chemistry as well as promising candidates as new organic materials. We herein present [n]cyclodibenzopentalenes ([n]CDBPs) as antiaromatic analogues of [n]cycloparaphenylenes. Platinum-mediated macrocyclization of dibenzopentalene boronic esters provided the trimer and tetramer with strain energies of up to 80 kcal mol-1. In the solid state, the cylindrical [4]CDBP molecules align to form columnar structures. The larger hoop [4]CDBP binds both fullerenes C60 and C70 with temperature-dependent exchange behavior, providing higher activation energies for the exchange compared to [10]CPP. The antiaromatic character of the [n]CDBPs paired with the cyclic conjugation leads to high HOMO energies and lowered LUMO energies with band gaps below 2 eV. This work presents a new class of the antiaromatic and nonalternant curved nanocarbons with intriguing supramolecular and ambipolar optoelectronic properties.

Heteroatom effects on aromaticity of five-membered rings in acenaphthylene analogs.

The pattern of cyclic conjugation was thoroughly studied in the series of N- and P-acenaphthylene derivatives using several different aromaticity indices: the energy effect (ef), multicenter delocalization index (MCI), harmonic oscillator model of aromaticity (HOMA) index, and nucleus independent chemical shifts (NICS). The Kekulé-structure-based reasoning predicts that there would be no cyclic conjugation in the "empty" five-membered heteroatom-containing rings in the studied molecules. It was found that according to the ef, MCI, and HOMA values, the extent of cyclic conjugation in the pentagonal rings is strongly influenced by the number and mutual arrangement of the hexagonal rings. In addition, it was revealed that in some of the examined molecules, the intensity of cyclic conjugation in the "empty" pentagons is even stronger than that of some hexagonal rings within the same molecule. The obtained results refute what one would expect based on "chemical intuition," which is usually strongly rooted to the Kekulé structures.Graphical abstract.

Bond Resonance Energy Verification of σ-Aromaticity in Cycloalkanes.

The Klein-Larson bonding model is best suited for describing σ-conjugation and σ-aromaticity of cycloalkanes. Methylene groups (-CH2-) in cycloalkanes are isoconjugates with ethylene groups (-CH═CH-) in annulenes, both contributing two electrons to cyclic conjugation and aromaticity. As in the case of π-aromatic stabilization, σ-aromatic stabilization must be associated with ring-current diamagnetism. Both nucleus-independent chemical shift (NICS) and bond resonance energy (BRE) values support this interpretation. Bond resonance energy has been used to determine local ring aromaticity in nonalternant hydrocarbons, alternant hydrocarbons, heterocyclic conjugated molecules, fullerenes, pure boron clusters, and porphyrins. This contribution represents the first application of bond resonance energy toward the evaluation of σ-aromaticity/σ-antiaromaticity of polycyclic alkanes.

Cycloparaphenylene (CPP) series are molecular models for graphene armchair edges: trends in their aromaticity and cyclic conjugation are evaluated

Cycloparaphenylene ([r]CPP) and cyclacene ([r]CA) series are models for short carbon nanotubes. It is shown that armchair edges in model cycloparaphenylenes possess greater aromaticity and cyclic conjugation than do zigzag edges in model cyclacenes. According to Aihara’s bond resonance energy (BRE) and Bosanac and Gutman energy effect (ef) measurements, cycloparaphenylenes are twice as aromatic as cyclacenes. The general solution of all eigenvalues of all members of the cycloparaphenylene series is given. The origin of the recurrence of some eigenvalues are determined.

Synthesis and Properties of Conjugated Nanohoops Incorporating Dibenzo[a,e]pentalenes: [2]DBP[12]CPPs.

Conjugated nanohoops allow studying the effect of cyclic conjugation and bending on the incorporated π-systems. To date, no such system containing antiaromatic units has been reported. We herein present [12]cycloparaphenylenes incorporating two dibenzo[a,e]pentalene units: [2]DBP[12]CPP nanohoops. Dibenzo[a,e]pentalene is a nonalternant hydrocarbon with antiaromatic character. The syntheses and optoelectronic properties of two different [2]DBP[12]CPP nanohoops with electronically modifying substituents are reported, accompanied by TDDFT calculations.

Ni(II) 10-Phosphacorrole: A Porphyrin Analogue Containing Phosphorus at the Meso Position.

Ni(II) 10-Phosphacorrole, a porphyrinoid containing phosphorus at the meso position, was synthesized from a bis(α,α'-dibromodipyrrin) Ni(II) complex and a phosphine anion via the palladium-catalyzed formation of a C-C and two C-P bonds. The optoelectronic properties of Ni(II) 10-phosphacorrole can be modulated effectively by oxidation or coordination of a metal to the phosphorus center. While Ni(II) 10-phosphacorrole exhibits a distinctly aromatic character due to the cyclic conjugation of 18 π-electrons, its oxide exhibited weak antiaromaticity, which was confirmed experimentally and theoretically.

Triplet state homoaromaticity: concept, computational validation and experimental relevance.

Cyclic conjugation that occurs through-space and leads to aromatic properties is called homoaromaticity. Here we formulate the homoaromaticity concept for the triplet excited state (T1) based on Baird's 4n rule and validate it through extensive quantum-chemical calculations on a range of different species (neutral, cationic and anionic). By comparison to well-known ground state homoaromatic molecules we reveal that five of the investigated compounds show strong T1 homoaromaticity, four show weak homoaromaticity and two are non-aromatic. Two of the compounds have previously been identified as excited state intermediates in photochemical reactions and our calculations indicate that they are also homoaromatic in the first singlet excited state. Homoaromaticity should therefore have broad implications in photochemistry. We further demonstrate this by computational design of a photomechanical "lever" that is powered by relief of homoantiaromatic destabilization in the first singlet excited state.

Aromaticity in Pericyclic Transition State Structures? A Critical Rationalisation Based on the Topological Analysis of Electron Density

AbstractThe nature of the electron delocalisation pattern within a cyclic structure, i. e. the aromatic character, is examined for six‐membered pseudocyclic transition state structures (TSs) involved in five representative examples of so‐called pericyclic reactions. Results of the electron localisation function (ELF) and the quantum theory of atoms in molecules (QTAIM) analyses of the electron density evidence that in four of the cases, at least one pair of atoms are not bound at the TS configuration, thus precluding a possible cyclic conjugation. These findings make it possible to rule out the aromatic character of these TSs. High values of the synchronicity Sy index at the TSs contrast with the bonding changes evidenced by ELF topological analysis. The magnitude of the nucleus independent chemical shift (NICS) computed for the five TSs becomes much more negative than that of the reference system, benzene, with no obvious relation to the strong evidence of the pattern of delocalisation revealed by the topological analysis of ELF and QTAIM for each TS. The topology of the anisotropy of current induced density (ACID) at each TS reveals the actual existence of strong non‐symmetrical patterns of electron delocalisation of the five TSs.

Catalytic Control in Cyclizations: From Computational Mechanistic Understanding to Selectivity Prediction.

This Account describes the use of quantum-chemical calculations to elucidate mechanisms and develop catalysts to accomplish highly selective cyclization reactions. Chemistry is awash with cyclic molecules, and the creation of rings is central to organic synthesis. Cyclization reactions, the formation of rings by the reaction of two ends of a linear precursor, have been instrumental in the development of predictive models for chemical reactivity, from Baldwin's classification and rules for ring closure to the Woodward and Hoffmann rules based on the conservation of orbital symmetry and beyond. Ring formation provides a productive and fertile testing ground for the exploration of catalytic mechanisms and chemo-, regio-, diastereo-, and enantioselectivity using computational and experimental approaches. This Account is organized around case studies from our laboratory and illustrates the ways in which computations provide a deeper understanding of the mechanisms of catalysis in 5-endo cyclizations and how computational predictions can lead to the development of new catalysts for enhanced stereoselectivities in asymmetric cycloisomerizations. We have explored the extent to which several cation-directed 5-endo ring-closing reactions may be considered as electrocyclic and demonstrated that reaction pathways and magnetic parameters of transition structures computed using quantum chemistry are inconsistent with this notion, instead favoring a polar mechanism. A rare example of selectivity in favor of 5-endo-trig ring closure is shown to result from subtle substrate effects that bias the reactant conformation out-of-plane, limiting the involvement of cyclic conjugation. The mode of action of a chiral ammonium counterion was deduced via conformational sampling of the transition state assembly and involves coordination to the substrate via a series of nonclassical hydrogen bonds. We describe how computational mechanistic understanding has led directly to the discovery of new catalyst structures for enantioselective cycloisomerizations. Calculations have revealed that stepwise C-C bond formation and proton transfer dictate the exclusive endo diastereoselectivity of the intramolecular Michael addition to form 2-azabicyclo[3.3.1]nonane skeletons catalyzed by primary amines. These insights have led to development of a highly enantioselective catalyst with higher atom economy than previous generations. This Account also explores transition-metal-catalyzed cycloisomerizations, where our theoretical investigations have uncovered an unexpected reaction pathway in the [5 + 2] cycloisomerization of ynamides. This has led to the design of new phosphoramidite ligands to enable double-stereodifferentiating cycloisomerizations in both matched and mismatched catalyst-substrate settings. Computational understanding of the factors responsible for the regio-, enantio-, and diasterocontrol is shown to generate tangible predictions leading to an acceleration of catalyst development for selective cyclizations.

The Raman fingerprint of cyclic conjugation: the case of the stabilization of cations and dications in cycloparaphenylenes.

Radical cations and dications of [n]CPP from n = 5 to n = 12 have been studied by Raman spectroscopy and density functional theory. Small [n]CPP dications owe their stability to the closed-shell electronic structure imposed by cyclic conjugation surpassing the destabilizing effect of ring strain and of the electron deficiency of the charged states. Large [n]CPP dications mitigate cyclic strain by forming diradicals. The Raman spectra reflect the balance among cyclic conjugation, cyclic strain and biradicaloid stabilization by finding an unexpected turning point in the frequency behavior of the G vibrational modes as a function of size. These Raman data represent the vibrational fingerprint of this rare form of conjugation which is central to stabilize both cations and dications of [n]CPP macrocycles.

ChemInform Abstract: Novel Aromatic and Antiaromatic Systems

AbstractReview: 48 refs.