Seven-membered Ring Research Articles

Regiospecific Synthesis and Structural Studies of 3,5-Dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-ones and Comparison with 1,3-Dihydro-2H-benzo[b][1,4]diazepin-2-ones.

Nine 3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-ones (17–25), some of which contain fluoro-substituents, have been regiospecifically prepared by reaction of 2,3-diaminopyridines with ethyl aroylacetates. In two cases, open intermediates have been isolated and these are related to the reaction pathway. The X-ray crystal structure of 1-methyl-4-phenyl-3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-one (23) has been solved (formula, C15H13N3O; crystal system, monoclinic; space group, C2/c). This is an asymmetric unit constituted by a single nonplanar molecule and its conformational enantiomer due to the presence of the seven-membered diazepin-2-one moiety, which introduces a certain degree of torsion in the adjacent pyridine ring. The 1H, 13C, 15N, and 19F NMR spectra were obtained and the chemical shifts, together with those of the previously published 1,3-dihydro-2H-benzo[b][1,4]diazepin-2-ones (1–16), i.e., a total of 544 values, were successfully compared with the chemical shifts calculated at the gauge invariant atomic orbital (GIAO)/Becke, three-parameter, Lee–Yang–Parr (B3LYP)/6-311++G(d,p) level. The seven-membered ring inversion barrier in 5-benzyl-2-phenyl-3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-one (25) was determined and, in conjunction with the data from the literature, compared with the B3LYP/6-311++G(d,p) computed values. This allowed the determination of several structural effects. The rotation about the exocyclic N1–CR bond was also calculated and its dynamic properties were discussed.

Superelectrophilic aluminium(iii)–ion pairs promote a distinct reaction path for carbonyl–olefin ring-closing metathesis

Catalytic carbonyl–olefin metathesis reactions represent powerful synthetic strategies for alkene formation. Successful approaches for carbonyl–olefin ring-closing, ring-opening and cross metathesis have been developed in recent years, but current limitations hamper the generality of these transformations. Stronger, more efficient catalytic systems are needed to further broaden the scope of these transformations while they prevent undesired reaction pathways. Here we report the development of an aluminium-based heterobimetallic ion pair as a superior catalyst that promotes carbonyl–olefin ring-closing metathesis via a distinct reaction mechanism and allows access to six- and seven-membered rings, which suffer from low yields and poor conversion under previously reported conditions. Mechanistic investigations support a distinct reaction profile in which two productive reaction pathways competitively form metathesis products. These insights are expected to have important implications in the catalyst design and development for carbonyl–olefin metathesis and enable future advances to ultimately expand the synthetic utility of these transformations. Carbonyl–olefin metathesis reactions are a valuable tool in synthetic chemistry, but there are still some limitations in scope. Now, a catalyst system allows the activation of previously unreactive substrates for such a reaction by aluminium(iii)–ion pairs acting as Lewis acidic superelectrophiles.

Crystal structure of 6-azido-6-de-oxy-1,2-O-iso-propyl-idene-α-d-gluco-furan-ose.

Short syntheses to high Fsp 3 index natural-product analogues such as imino-sugars are of paramount importance in the investigation of their biological activities and reducing the use of protecting groups is an advantageous synthetic strategy. An iso-propyl-idene group was employed towards the synthesis of seven-membered ring imino-sugars and the title compound, C9H15N3O5, was crystallized as an inter-mediate, in which the THF ring is twisted and the dioxolane ring adopts an envelope conformation: the dihedral angle between the rings is 67.50 (13)°. In the crystal, the hydroxyl groups participate in O-H⋯(O,O) and O-H⋯N hydrogen-bonding inter-actions, which generate chains of mol-ecules propagating parallel to the a-axis direction. There is a notable non-classical C-H⋯O hydrogen bond, which cross-links the [100] chains into (001) sheets.

Synthetic Approaches to Non-Tropane, Bridged, Azapolycyclic Ring Systems Containing Seven-Membered Carbocycles

AbstractThis Short Review highlights various synthetic approaches to bridged azabicyclic ring systems containing seven-membered carbocyclic rings. Such ring systems are common to a number of biologically active natural products. The seven-membered ring in such systems is generally formed in one of four ways: 1) cyclization of an acyclic precursor; 2) ring expansion or rearrangement of a different ring size; 3) cycloaddition; and 4) use of a synthetic building block with the seven-membered ring already present. Representative examples of each approach from both total synthesis and methodological studies are discussed, with an emphasis on work publishedin the last twenty years.1 Introduction2 Cyclization Reactions3 Ring Expansions and Rearrangements4 Cycloadditions5 Strategies Involving Seven-Membered Ring Building Blocks6 Conclusion

Controlled Cyclopolymerization of 1,5-Hexadiynes to Give Narrow Band Gap Conjugated Polyacetylenes Containing Highly Strained Cyclobutenes.

For decades, cyclopolymerization of α,ω-diyne derivatives has been an effective method to synthesize various soluble polyacetylenes containing five- to seven-membered rings in the backbone. However, cyclopolymerization to form four-membered carbocycles was considered impossible due to their exceptionally high ring strain (∼30 kcal/mol). Herein, we demonstrate the successful cyclopolymerization of rationally designed 1,5-hexadiyne derivatives to afford various polyacetylenes containing highly strained cyclobutenes in each repeat unit. After screening, Ru catalysts containing bulky diisopropylphenyl groups promoted challenging four-membered ring cyclization efficiently from various monomers, enabling the synthesis of high molecular weight (up to 40 kDa) polyacetylenes in a controlled manner. Furthermore, living polymerization allowed for block copolymer synthesis by combining with ring-opening metathesis polymerization as well as block copolymerization of two different 1,5-hexadiyne monomers to give a fully conjugated polyacetylene. These new polymers unexpectedly showed much narrower band gaps than conventional substituted polyacetylenes by >0.2 eV. Interestingly, computational studies showed much smaller bond length alternation in the conjugated backbone containing cyclobutenes, resulting in highly delocalized π electrons along the polymer chain and lower band gaps.

Synthetically Important Ring-Opening Acylations of Alkoxybenzenes

AbstractCyclic ketones, anhydrides, lactams and lactones are a particular class of molecules that are often used in synthesis, wherein their electrophilic properties are leveraged to enable facile Friedel–Crafts ring openings through nucleophilic attack at the carbonyl sp2 centre. The use of electron-rich alkoxybenzenes as nucleophiles has also become important since the discovery of the Friedel–Crafts reaction. As a result, various isomeric alkoxybenzenes are used for preparing starting materials in target-oriented syntheses. This review covers the instances of different alkoxybenzenes that are used as nucleophiles in ring-opening acylations with carbonyl-containing cyclic electrophiles, for the construction of important building blocks for multistep transformations. This review summarizes the ring-opening functionalization of three- to seven-membered molecular rings with alkoxybenzenes in a Friedel–Crafts fashion. Sometimes the rings need subtle or considerable activation by the help of Lewis acid(s), followed by nucleophilic attack. This review is aimed to be a summary of the important acylations of electron-rich alkoxybenzenes by nucleophilic ring-opening of cyclic molecules. The works cited employ a wide range of conditions and differently substituted substrates for target-oriented syntheses.1 Introduction and Scope2 Arenes for Acylative Ring Opening2.1 Three-Membered Rings: Ring Opening of Oxirane-2,3-dione2.2 Four-Membered Rings2.2.1 Ring Opening of Cyclobutanones2.2.2 Ring Opening of β-Lactams2.2.3 Ring Opening of β-Lactone2.3 Five-Membered Rings2.3.1 Ring Opening of Phthalimides2.3.2 Ring Opening of γ-Lactones2.3.3 Ring Opening of Anhydrides2.4 Six-Membered Rings2.5 Seven-Membered Rings3 Conclusion

Fabrication of DABCO functionalized poly(ionic liquids): Vital role of ferric oxides in the formation of mesoporous structure and used as highly efficient and recyclable catalysts for multi-component reactions

The preparation of well-defined porous materials as carriers is critically fundamental for the design of heterogeneous catalysts. In this study, 1,4-Diazabicyclo[2.2.2]octane (DABCO) based hypercross-linked ionic copolymers with well-developed mesopores were fabricated, in which the alkene modified ferric oxides nanoparticles play a vital role for the formation of the mesoporous structure. The magnetic copolymers, fully characterized by N2 adsorption-desorption, scanning electron microscopy (SEM)/transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM), exhibited abundant active ionic centers, rich mesoporosity and moderate surface area. The as-prepared materials were found to be used as efficient catalysts for one-pot synthesis of indole derivatives and 3,4-fused indoles with seven-membered ring and good to excellent products yields were achieved within 10 h. The kinetic study revealed that the meso-porosity increased the catalytic efficiency in terms of reaction rate and yield, albeit with mass transfer resistance produced by mesoporous channels. Finally, the catalyst could be readily recovered using external magnetic force and recycled at least ten times without significant loss in catalytic activities, which makes it attractive for green transformation in chemical industry.

Natural tropolones form via radical route

Tropolones are a class of seven-membered aromatic rings that are naturally occurring, have unusual electronic properties, and are hard to synthesize. TropC, a fungi-based α-ketoglutarate-dependent ...

Reactivity of Coordinated 2-Pyridyl Oximes: Synthesis, Structure, Spectroscopic Characterization and Theoretical Studies of Dichlorodi{(2-Pyridyl)Furoxan}Zinc(II) Obtained from the Reaction between Zinc(II) Nitrate and Pyridine-2-Chloroxime

This work reports our first results in the area of the reactivity of coordinated chloroximes. The 1:2:2:2 Zn(NO3)2∙6H2O/Eu(NO3)3∙6H2O/ClpaoH/Et3N reaction mixture in MeOH, where ClpaoH is pyridine-2-chloroxime, resulted in complex [ZnCl2(L)] (1); L is the di(2-pyridyl)furoxan [3,4-di(2-pyridyl)-1,2,5-oxadiazole-2-oxide] ligand. The same complex can be isolated in the absence of the lanthanoid. The direct reaction of ZnCl2 and pre-synthesized L in MeOH also provides access to 1. In the tetrahedral complex, L behaves as a Npyridyl,N′pyridyl-bidentate ligand, forming an unusual seven-membered chelating ring. The Hirshfeld Surface analysis of the crystal structure reveals a multitude of intermolecular interactions, which generate an interesting 3D architecture. The complex has been characterized by FTIR and Raman spectroscopies. The structure of 1 is not retained in DMSO (dimethylsulfoxide) solution, as proven by NMR (1H, 13C, 15N) spectroscopy and its molar conductivity value. Upon excitation at 375 nm, solid 1 emits blue light with a maximum at 452 nm; the emission is of an intraligand character. The geometric and energetic profiles of possible pathways involved in the reaction of ClpaoH and Zn(NO3)2∙6H2O in MeOH in the presence of Et3N has been investigated by DFT (Density Functional Theory) computational methodologies at the PBE0/Def2-TZVP(Cr)∪6-31G(d,p)(E)/Polarizable Continuum Model (PCM) level of theory. This study reveals an unprecedented cross-coupling reaction between two coordinated 2-pyridyl nitrile oxide ligands.

A Modular Approach to Dibenzo-fused ϵ-Lactams: Palladium-Catalyzed Bridging-C-H Activation.

Tricyclic ring systems possessing a dibenzo structure joined to a seven-membered heterocyclic ring frequently show important biological activities. However, a modular approach to these molecules based on efficient intermolecular reaction of readily available chemicals is lacking. Herein, an unprecedented palladium-catalyzed formal [4+3] annulation for modular construction of these tricyclic systems is described. This reaction features easily accessible reactants (o-haloarylaldehydes and N-tosylhydrazones), broad substrate scope, and excellent functional group compatibility. The synthetic potential is demonstrated by the easy scale-up reactions, late-stage modification of complex molecules, and collective synthesis of bioactive molecules and approved drugs.

Influence of the flexible tetrapyridines on electrocatalytic water oxidation by cobalt complexes

Effective, robust, and low-cost catalysts are highly demanded for water oxidation reaction (WOR) which is the bottleneck of water splitting. Herein reported is water oxidation catalysis of two cobalt complexes [Co(tpbn)](ClO4)2 (1) and [Co(tpen)](ClO4)2.3CH3CN (2) of tetrapyridyl ligands N,N,N',N'-tetrakis(2-pyridyl-methyl)-1,4-butanediamine (tpbn) and N,N,N',N'-tetrakis(2-pyridyl-methyl)-1,2-ethanediamine (tpen). 1 was a stable molecular electrocatalyst for WOR in alkaline solutions with a rate constant (kcat) of 0.97 s−1. A turnover number (TON) was estimated to be 10. Complex 2 was a much weaker catalyst than 1. The difference in the catalytic activity of 1 and 2 could be due to the flexibility of the coordination sphere of the Co center. The Co-N bonds in 1 were longer than those in 2. In comparison to the five-membered ring (N-C2-N-Co) in 2, a seven-membered ring (N-C4-N-Co) in 1 was more flexible and easily adaptive for the incoming H2O or OH– groups to access the Co center, which was vital for the formation of O2. A nucleophilic attack catalytic mechanism was proposed for WOR. This work demonstrated that the flexibility of the organic ligands and the adaptability of the coordination configuration of the metal center had a great impact on the catalytic properties of metal complexes.

Local structure elucidation and reaction mechanism of light naphtha aromatization over Ga embedded H-ZSM-5 zeolite: Combined DFT and experimental study

Abstract Local structures and mechanisms for n-pentane aromatization on Ga embedded H-ZSM-5 zeolite (Ga/ZSM-5) were elucidated using Synchrotron-based X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations to understand the role of Ga/ZSM-5 zeolite in aromatics synthesis. XAS data suggests that Ga ligates with four oxygen or four hydrogen atoms. Catalytic tests results suggest that conversion by Ga/ZSM-5 catalyst cannot occur via C6–C8 non-aromatic intermediates, while the availability of Ga metal sites promotes the aromatization of C2 and C3 species. Therefore, conversion of n-pentane to benzene or toluene comprises four steps, i) cracking, ii) GaH2 activation, iii) cyclization, and iv) dehydrogenation. Our model predicts the key intermediate in n-pentane aromatization on Ga/ZSM-5 zeolite to be a five-membered Ga-C4 ring structure. The ring undergoes expansion to form a seven-membered Ga-C6 ring. Moreover, we discuss thermodynamics and kinetic results for the benzene and toluene formation pathways. Our results provide new finding for the role of Ga/ZSM-5 zeolites in n-pentane aromatization processes.

Tandem Oxidative Ring Expansion for Synthesis of Dibenzocyclooctaphenanthrenes.

A novel tandem single-electron oxidative ring expansion reaction has been developed for the construction of the saddle-shaped polycyclic arenes fused with cyclooctatetraene, that is, dibenzo[3,4:7,8]cycloocta[1,2-l]phenanthrenes (dbCOTPs). The combination of Cu(OTf)2 catalyst with DDQ triggered the selective oxidation of o-biphenyl-tethered methylenecirculenes fused with a seven-membered ring, giving rise to the formation of the corresponding eight-membered ring fused dbCOTPs. The present tandem ring expansion of a seven- to an eight-membered ring takes place via the selective single-electron oxidation of the benzylidene moiety, intramolecular spirocyclization, and 1,2-aryl migration sequence.

Density Functional Theory Study on the Mechanism of Iridium-Catalyzed Benzylamine ortho C-H Alkenylation with Ethyl Acrylate.

Iridium-catalyzed oxidative o-alkenylation of benzylamines with acrylates was enabled by the directing group pentafluorobenzoyl (PFB). Density functional theory calculations were performed to explore the detailed reaction mechanism. The calculated results reveal that N-deprotonation prior to C–H activation is favored over direct C–H activation. Moreover, C–H activation is reversible and not the rate-determining step, which has been supported by the experimental observation. The regio- and stereoselectivity of ethyl acrylate insertion are controlled by the steric effect and the carbon atom with a larger orbital coefficient of the π* antibonding orbital in the nucleophilic attack, respectively. The migratory insertion of ethyl acrylate is computationally found to be rate-determining for the whole catalytic cycle. Finally, the seven-membered ring intermediate IM11 undergoes a sequential N-protonation and β-H elimination with the assistance of AcOH, rather than β-H elimination and reductive elimination proposed experimentally, to afford the o-alkenylated product. IM11 is unable to directly cyclize through C–N reductive elimination because both sp3-hybridized N and C atoms are unfavorable for N–C reductive elimination. The origin of the directing group PFB preventing the product and intermediates undergoing aza-Michael addition has been explained.

A Macrocycle Based on a Heptagon-Containing Hexa-peri-hexabenzocoronene.

A cyclophane is reported incorporating two units of a heptagon-containing extended polycyclic aromatic hydrocarbon (PAH) analogue of the hexa-peri-hexabenzocoronene (HBC) moiety (hept-HBC). This cyclophane represents a new class of macrocyclic structures that incorporate for the first time seven-membered rings within extended PAH frameworks. The saddle curvature of the hept-HBC macrocycle units induced by the presence of the nonhexagonal ring along with the flexible alkyl linkers generate a cavity with shape complementarity and appropriate size to enable π interactions with fullerenes. Therefore, the cyclophane forms host-guest complexes with C60 and C70 with estimated binding constants of Ka =420±2 m-1 and Ka =(6.49±0.23)×103 m-1 , respectively. As a result, the macrocycle can selectively bind C70 in the presence of an excess of a mixture of C60 and C70 .

O-Annulation to Polycyclic Aromatic Hydrocarbons: A Tale of Optoelectronic Properties from Five- to Seven-Membered Rings.

We take advantage of the Pummerer oxidative annulation reaction to extend PAHs through the formation of an intramolecular C-O bond with a suitable phenol substituent. Depending on the peripheral topology of the PAH precursor (e.g., pyrene, boron-dipyrromethene, or perylene diimide) five-, six-, and seven-membered O-containing rings could be obtained. The effect of the O-annulations on the optoelectronic properties were studied by various methods with the pyrano-annulated pyrene and BODIPY derivatives depicting quantitative emission quantum yields.

Formation of Samin Diastereomers by Acid-Catalyzed Transformation of Sesamolin with Hydrogen Peroxide.

The conversion of sesame lignans is of interest because the derived products may have potential applications. Here, in investigating the transformation of sesamin and sesamolin, main endogenous sesame lignans in sesame seeds, in both acidic aqueous and anhydrous systems, 7R,7'S-samin was identified as one of the major products of sesamolin in both systems catalyzed with common inorganic acids, but sesaminol was not generated. In investigating the effect of different oxidizing agents on the acid-catalyzed conversion of sesame lignans, 7R,7'S-samin was still the major product of sesamolin, whereas sesamolin as well as 7R,7'S-samin stereoselectively rendered 7R,7'R-samin in the presence of hydrogen peroxide. Hydrogen peroxide may play a role in stabilizing the transitional oxonium ions, derived from acid hydrolysis of sesamolin or 7R,7'S-samin by forming a seven-membered ring intermediate through hydrogen bonding, to consequently produce 7R,7'R-samin as the final product.

Leveraging Hierarchical Self-Assembly Pathways for Realizing Colloidal Photonic Crystals.

Colloidal open crystals are attractive materials, especially for their photonic applications. Self-assembly appeals as a bottom-up route for structure fabrication, but self-assembly of colloidal open crystals has proven to be elusive for their mechanical instability due to being low-coordinated. For such a bottom-up route to yield a desired colloidal open crystal, the target structure is required to be thermodynamically favored for designer building blocks and also kinetically accessible via self-assembly pathways in preference to metastable structures. Additionally, the selection of a particular polymorph poses a challenge for certain much sought-after colloidal open crystals for their applications as photonic crystals. Here, we devise hierarchical self-assembly pathways, which, starting from designer triblock patchy particles, yield in a cascade of well-separated associations first tetrahedral clusters and then tetrastack crystals. The designed pathways avoid trapping into an amorphous phase. Our analysis reveals how such a two-stage self-assembly pathway via tetrahedral clusters promotes crystallization by suppressing five- and seven-membered rings that hinder the emergence of the ordered structure. We also find that slow annealing promotes a bias toward the cubic polymorph relative to the hexagonal counterpart. Finally, we calculate the photonic band structures, showing that the cubic polymorph exhibits a complete photonic band gap for the dielectric filling fraction directly realizable from the designer triblock patchy particles. Unexpectedly, we find that the hexagonal polymorph also supports a complete photonic band gap, albeit only for an increased filling fraction, which can be realized via postassembly processing.

Magnetization plateaus of spin- 12 system on a 5/7 skewed ladder

Magnetization plateaus are some of the most striking manifestations of frustration in low-dimensional spin systems. We present numerical studies of magnetization plateaus in the fascinating spin-1/2 skewed ladder system obtained by alternately fusing five- and seven-membered rings. This system exhibits three significant plateaus at $m = 1/4$, $1/2$ and $3/4$, consistent with the Oshikawa-Yamanaka-Affleck condition. Our numerical as well as perturbative analysis shows that the ground state can be approximated by three weakly coupled singlet dimers and two free spins, in the absence of a magnetic field. With increasing applied magnetic field, the dimers progressively become triplets with large energy gaps to excited states, giving rise to stable magnetization plateaus. Finite-temperature studies show that $m=1/4$ and $1/2$ plateaus are robust and survive thermal fluctuations while the $m=3/4$ plateau shrinks rapidly due to thermal noise. The cusps at the ends of a plateau follow the algebraic square-root dependence on $B$.

Molecular Dynamics Study of sp-Defect Migration in Odd Fullerene: Possible Role in Synthesis of Abundant Isomers of Fullerenes

To explain recent experiment showing the role of odd fullerenes in formation of abundant fullerene isomers a reactive molecular dynamics (MD) study has been performed. Three types of bond rearrangement reactions are found by MD simulations at 3000 K in odd fullerenes which contain an extra sp atom among all other sp$^2$ atoms. The first type is sto-chastic sp-defect migration analogous to exchange mechanism of adatom migration on a surface. The second type cor-responds to changes in the ring configuration of the sp$^2$-structure assisted by the sp atom which can lead to annealing of seven-membered rings or separation of five-membered rings. The third type is formation of short-living one-coordinated atoms or two additional sp atoms. Annihilation of a pair of sp defects has been also observed in the MD simulations. It is shown that the frequency of sp-defect migration at a lower temperature, as estimated from performed density functional theory calculations of the barriers of sp-defect migration events, is sufficient to deliver the sp atom to defects of sp$^2$ structure during the fullerene formation time. Based on these results, we propose to supplement the self-organization paradigm of fullerene formation by the following four-stage atomistic mechanism of formation of abundant isomers of fullerenes: 1) attachment of single carbon atoms, 2) sp-defect migration to sp$^2$-structure defects, 3) sp$^2$-defect annealing assisted by the sp atom and 4) subsequent annihilation of pairs of sp defects.