Eight-membered Ring Research Articles

Experimental and Theoretical Study of Two 3D Difunctional Electrocatalytic Hybrid Vanadate-Containing Metal-Organic Motifs.

Two novel 3D inorganic-organic hybrids based on [V6O18]6-/[V3O9]3- clusters, [Cu18(bbpy)18(V6O18)6]·3H2O (1) and [Cu4Ag4(pty)4(V3O9)4]·H2O (2) (bbpy = 3,5-bis(1-benzimidazole) pyridine, pty = 4'-(4″-pyridyl)-2,2':6',2″-terpyridine), were isolated in the same POV/Cu/N-heterocycle ligand reaction systems. Hybrids 1 and 2 possess novel three-dimensional bimetallic frameworks derived from [V6O18]6-/[V3O9]3- clusters and Cu-organic complexes. In 1, bbpy ligands are grafted by Cu2+ to a grid ribbon 2D sheet, which are connected with benzene-like [V6O18]6- to yield a 3D framework. In 2, helical {O-V-O-V-}n chains are bridged by Cu(II) ions into a 2D layer including eight-membered rings {V6Cu2} and six-membered rings {V4Cu2}, and the adjoining sheets are joined by Ag-N coordination bonds to form a framework structure. Moreover, hybrid 1 has superior electrocatalytic properties for nitrite reduction and oxidation of ascorbic acid with electrocatalytic efficiencies of 397.2 and 96%, respectively. Hybrid 2 displays coordinated electrocatalytic performance toward oxidation reaction through V and Cu centers. Meanwhile, the corresponding theoretical studies were conducted to evaluate electrocatalytic active sites and charge distribution.

Enantiocontrolled Cyclization to Form Chiral 7- and 8-Membered Rings Unified by the Same Catalyst Operating with Different Mechanisms.

Chiral medium-sized rings, albeit displaying attractive properties for drug development, suffer from numerous synthetic challenges due to difficult cyclization steps that must take place to form these unusually strained, atropisomeric rings from sterically crowded precursors. In fact, catalytic enantioselective cyclization methods for the formation of chiral seven-membered rings are unknown, and the corresponding eight-membered variants are also sparse. In this work, we present a substrate preorganization-based, enantioselective, organocatalytic strategy to construct seven- and eight-membered rings featuring chirality that is intrinsic to the ring in the absence of singular stereogenic atoms or single bond axes of chirality. The reactions proceed under mild conditions and with high levels of stereocontrol. Notably, the same bifunctional iminophosphorane chiral catalyst orchestrates the cyclization of substrates of two different ring sizes, under two different mechanistic paradigms. We envision that the mechanistic and ring size versatility of this method could guide further applications of asymmetric catalysis to other challenging cyclization reactions.

Stabilizing Contorted Doubly-Reduced Tetraphenylene with Heavy Alkali Metal Complexation: Crystallographic and Theoretical Evidence.

The two-fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene, 1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C-C bond converts the central eight-membered ring into a twisted core with two fused five-membered rings. This C-C bond of 1.589(3)-1.606(6) Å falls into a single σ-bond range and generates two perpendicular π-surfaces with dihedral angles of 110.3(9)°-117.4(1)° in the 1TR 2- dianions. As a result, the highly contorted 1TR 2- ligand exhibits a "butterfly" shape and could provide different coordination sites for metal-ion binding. The K-induced reduction of 1 in THF affords a polymeric product with low solubility, namely [{K+(THF)}2(1TR 2-)] (K2-1TR 2-). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb+(18-crown-6)]2[1TR 2-] (Rb2-1TR 2-) and [Cs+(18-crown-6)]2[1TR 2-] (Cs2-1TR 2-). Both internal and external coordination are observed in K2-1TR 2-, while the bulky 18-crown-6 ligand only allows external metal binding in Rb2-1TR 2- and Cs2-1TR 2-. The reversibility of the two-fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the 1TR 2-TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected.

Pyridine-oriented transannular C-H functionalization of arenes.

We present a pyridine-oriented transannular Friedel-Crafts alkylation reaction. This reaction, despite being thermodynamically unfavorable, successfully yields a series of pyridine-containing fused seven- or eight-membered rings. The resulting products not only have a similar skeleton to some marketed pharmaceuticals, but also can be further transferred into synthetically promising pleiadienes.

Modulation of the Magnetic Anisotropy via the Ligand Field in Sandwiched Erbium Complexes.

Among lanthanide-based single-molecule magnets (SMMs), erbium(III) is a Kramers ion, apart from dysprosium(III), which provides magnetic bistability in the presence of a suitable coordination environment. However, Er-based SMMs exhibit significantly less magnetic anisotropy than Dy because their prolate electronic density necessitates equatorially correlated ligands to minimize the charge contact with the Er atom. Here, in this work, we have computationally investigated the heteroleptic organometallic complexes with an Er(III) atom sandwiched between two distinct cyclic rings (five- and eight-membered) with the aim of tuning the magnetic anisotropy via exploiting the ligand field. The ligand field is manipulated by substituting one of the C atoms from the five-membered ring with heteroatoms (groups 14 and 15), while the other (eight-membered) ring remains intact. The electronic and magnetic properties have been investigated using first-principles-based ab initio approaches. The distortion in the planarity of the five-membered ring generated by the larger heteroatom affects the bonding with magnetic Er and consequently the electronic structure. This is observed to modify the ligand field and the magnetic axis, thereby improving the magnetic relaxation barrier.

Enzymatic Peroxidation in the Chemoenzymatic Synthesis of 13-Oxoverruculogen

Verruculogens are fumitremorgin alkaloids that contain an eight-membered endoperoxide ring. Due to their unusual structure and bioactivity, there has been much interest in these natural products since their discovery over forty years ago. Similarly, interest in their biosynthesis resulted in the discovery of verruculogen synthase (FtmOx1) that catalyzes endoperoxide formation in these natural products. Herein, we describe our work in this area through the chemoenzymatic synthesis of 13-oxoverruculogen by endoperoxidation of a substrate analog using FtmOx1.

Catalytic Asymmetric Synthesis of Inherently Chiral Eight-Membered O-Heterocycles through Cross-[4+4] Cycloaddition of Quinone Methides.

Inherently chiral eight-membered rings embedded in tetraphenylene derivatives and hetero-analogues exhibit unique properties and allow diverse applications. A conceptually viable and straightforward approach to these frameworks is [4+4] cycloaddition, which still remains elusive. Herein, we describe the stereoselective cross-[4+4] cycloaddition of quinone methides (QMs), leading to the formation of oxa-analogues of tetraphenylene with exceptional chemo-, diastereo-, and enantioselectivity. The structures of these novel rigid eight-membered O-heterocycles were explored by single-crystal X-ray diffraction, and their stereochemical stability was elaborated through both density functional theory (DFT) calculations and thermal racemization experiments. The developed methodology exhibited broad substrate scope and the resulting cross-[4+4] cycloadducts could be readily transformed into valuable chiral building blocks. Our findings expand the library of inherently chiral medium-sized rings and also contribute to the advancement of asymmetric cross-[4+4] cycloadditions of quinone methides.

Selenium Disulfide from Sustainable Resources: An Example of "Redneck" Chemistry with a Pinch of Salt.

Selenium disulfide (often referred to as SeS2) encompasses a family of mixed selenium-sulfide eight-membered rings, traditionally used as an anti-dandruff agent in shampoos. SeS2 can be produced by reacting hydrogen sulfide (H2S) with selenite (SeO32-) under acidic conditions. This chemistry is also possible with natural spring waters that are rich in H2S, thus providing an avenue for the more sustainable, green production of high-quality SeS2 particles from an abundant natural source. The orange material obtained this way consists of small globules with a diameter in the range of 1.1 to 1.2 µm composed of various SexS8-x chalcogen rings. It shows the usual composition and characteristics of a Se-S interchalcogen compound in EDX and Raman spectroscopy. Since the mineral water from Bad Nenndorf is also rich in salts, the leftover brine has been evaporated to yield a selenium-enriched salt mixture similar to table salt. As the water from Bad Nenndorf-in comparison to other bodies of water around the world-is still rather modest in terms of its H2S content, especially when compared with volcanic waters, this approach may be refined further to become economically and ecologically viable, especially as a regional business model for small and medium-sized enterprises.

Ring Size Effects on the Structures of Sandwich Compounds with a Stoichiometry of C12H12M (M = Ti-Ni).

Ring size effects on geometries and electronic structures were investigated for the (C n H n )M(C m H m ) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti-Ni) systems using density functional theory. The lowest-energy C12H12M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η5-C5H5)Ti(η7-C7H7), doublet (η5-C5H5)V(η7-C7H7), and singlet (η6-C6H6)2Cr, respectively. The likewise experimentally known singlet (η6-C6H6)2Ti, doublet (η6-C6H6)2V, and singlet (η5-C5H5)Cr(η7-C7H7) are the second-lowest-energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C12H12M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C5H5)M(C7H7) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C7H7 rings with one or two uncomplexed C=C bonds. The (C4H4)M(C8H8) (M = Ti-Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C5H5)M(C7H7) and (C6H6)2M isomers.

The Tetrafluoroethylene-Mediated Ring-Closing Metathesis: Enabling Unfavored Reactions.

While ring-closing metathesis (RCM) is a powerful method for constructing medium and large cyclic alkenes, its application to the synthesis of heterocycles faces considerable limitations. For instance, RCM of divinyloxyalkanes does not proceed under the conventional conditions of RCM. The challenge lies in the formation of stable Fischer-type carbene intermediates with heteroatom(s) bound to the carbene carbon, impeding subsequent metathesis. In the present research, we develop a novel reaction system termed "tetrafluoroethylene (TFE)-mediated RCM", which enables the RCM of divinyloxyalkanes. In this system, a divinyloxyalkane and a TFE molecule are converted to the corresponding ring-closing product (cyclic 1,2-dioxyethene) and two molecules of 1,1-difluoroethylene. This method facilitates the construction of six- to eight-membered rings with various functional groups. The addition of TFE has two key effects: thermodynamically, it renders the entire reaction exergonic, while kinetically, it ensures that all ruthenium-carbene intermediates in the catalytic cycle are Fischer-type.

Wavy Graphene Nanoribbons Containing Periodic Eight-Membered Rings for Light-Emitting Electrochemical Cells.

Precision graphene nanoribbons (GNRs) offer distinctive physicochemical properties that are highly dependent on their geometric topologies, thereby holding great potential for applications in carbon-based optoelectronics and spintronics. While the edge structure and width control has been a popular strategy for engineering the optoelectronic properties of GNRs, non-hexagonal-ring-containing GNRs remain underexplored due to synthetic challenges, despite offering an equally high potential for tailored properties. Herein, we report the synthesis of a wavy GNR (wGNR) by embedding periodic eight-membered rings into its carbon skeleton, which is achieved by the A2B2-type Diels-Alder polymerization between dibenzocyclooctadiyne (6) and dicyclopenta[e,l]pyrene-5,11-dione derivative (8), followed by a selective Scholl reaction of the obtained ladder-type polymer (LTP) precursor. The obtained wGNR, with a length of up to 30 nm, has been thoroughly characterized by solid-state NMR, FT-IR, Raman, and UV/Vis spectroscopy with the support of DFT calculations. The non-planar geometry of wGNR efficiently prevents the inter-ribbon π-π aggregation, leading to photoluminescence in solution. Consequently, the wGNR can function as an emissive layer for organic light-emitting electrochemical cells (OLECs), offering a proof-of-concept exploration in implementing luminescent GNRs into optoelectronic devices. The fast-responding OLECs employing wGNR will pave the way for advancements in OLEC technology and other optoelectronic devices.

Rundqvistite-(Ce), Na3(Sr3Ce)(Zn2Si8O24), a new mineral from the Darai-Pioz alkaline massif, Tien-Shan Mountains, Tajikistan: mineral description and crystal structure

AbstractRundqvistite-(Ce), ideally Na3(Sr3Ce)(Zn2Si8O24), is a new mineral from the Darai-Pioz alkaline massif, Tien-Shan Mountains, Tajikistan. The mineral occurs as elongated grains up to 0.1 mm long and up to 0.03 mm thick embedded in quartz–pectolite aggregate in a silexite-like peralkaline pegmatite. Associated minerals are quartz, fluorite, pectolite, baratovite, aegirine, leucosphenite, neptunite, reedmergnerite, orlovite, sokolovaite, mendeleevite-(Ce), odigitriaite, pekovite, zeravshanite, kirchhoffite and garmite. The mineral is colourless with a vitreous lustre and a white streak, brittle, Dmeas. is 3.70(2) and Dcalc. is 3.709 g/cm3. Rundqvistite-(Ce) is monoclinic, space group P21/c, a = 5.1934(16), b = 7.8934(16), c = 26.011(5) Å, β = 90.02(3)° and V = 1066.3(4) Å3. The chemical composition of rundqvistite-(Ce) is SiO2 40.17, La2O3 2.64, Ce2O3 7.55, Pr2O3 0.80, Nd2O3 2.43, Sm2O3 0.33, Eu2O3 0.09, Gd2O3 0.24, Tb2O3 0.18, Dy2O3 0.21, PbO 1.03, SrO 19.83, FeO 0.37, ZnO 13.08, CaO 2.55, Na2O 8.04, total 99.54 wt.%. The empirical formula calculated on 24 O apfu (atoms per formula unit) is Na3.10Sr2.29Ca0.54Pb0.06(Ce0.55La0.19Nd0.17Pr0.06Sm0.02Gd0.02Eu0.01Tb0.01Dy0.01)Σ1.04Zn1.92Fe0.06 Si8.00O24Z = 2. The structural formula based on refined site-occupancies is (Na2.94Sr0.06)Σ3.00 [(Sr2.23Ca0.54Pb0.06Na0.13)Σ2.96Ln3+1.04]Σ4.00[(Zn1.92Fe2+0.06)Σ1.98Si8O24], where Ln3+1.04 = (Ce0.55La0.19Nd0.17Pr0.06Sm0.02Gd0.02Eu0.01Tb0.01Dy0.01)Σ1.04. The crystal structure of rundqvistite-(Ce) was refined to R1 = 2.76% on the basis of 3184 unique reflections [F > 4σ|F|]. In rundqvistite-(Ce), the main structural unit is a (Zn2Si8O24)12– sheet parallel to (100). In the sheet, the Si and Zn tetrahedra form four-, five- and eight-membered rings. The interstitial cations at the Na and M(1–3) sites sum to [Na3(Sr3Ce)]12– apfu. The Na and M(1–3) polyhedra share common edges to form a layer. Rundqvistite-(Ce) is a structural analogue of vladykinite, ideally Na3Sr4(Fe2+Fe3+)Si8O24. Rundqvistite-(Ce) and vladykinite are related by the following substitution: [8]Ce3+ + [4](Zn2+)2 ↔ [8]Sr2+ + [4](Fe2+Fe3+). The mineral is named after Dmitry Vasilievich Rundqvist (1930–2022), a prominent Russian geologist and an expert on the geology of ore deposits, metallogeny and mineralogy of Precambrian rocks.

The preferential occupying effect regulated atomically dispersed Cu site over Cu-SSZ-13 for enhancing NH3-SCR performance

The development of highly active catalysts for selective catalytic reduction of NOx with ammonia (NH3-SCR) at broad temperature range are desirable but challenging. Herein, a preferential occupying effect of Cd regulates atomically dispersed Cu site is proposed for NH3-SCR. Benefiting from the modulation effect of Cd, the temperature of NO conversion above 90 % for Cu-Cd-SSZ-13 is broadened to 160–550 ℃, and hydrothermal stability is significantly improved. The Cd ions prefer to occupy eight-membered rings (8 MRs) favor the generation of Z2Cu2+ at 6 MRs, and the electronegativity of Z2Cu2+ increases due to the introduction of Cd, thus boosting NH3-SCR process by reducing formation energy of key intermediate (NH4NO2) at active Z2Cu2+ site. Noticeably, the preferential occupying of Cd enhances the intrinsic stability of Z2Cu2+ by inhibiting its transformation into inactive CuOx. This work sheds new insight for regulating atomically dispersed Cu site to enhance NH3-SCR performance.

Visible light-induced pericyclic cascade reaction for the synthesis of eight-membered rings

A visible light-induced pericyclic cascade reaction for the synthesis of eight-membered rings is described. The reaction proceeds via [2 + 2]-photocycloaddition, base-promoted elimination, and retro-4π-electrocyclization. It exhibits broad substrate scope and good functional group tolerance.

The Total Synthesis of Hyperfirin via a Cyclooctadiene Strategy.

Polycyclic polyprenylated acylphloroglucinols (PPAPs) combine compelling structural complexity with effective biological activity. The total synthesis of Hyperfirin is reported as one linear sequence. Key to this novel modular strategy is to access the bicyclo[3.3.1]nonane-2,4,9-trione framework via transannular acylation of a decorated eight-membered ring, followed by late stage bridgehead substitution. The described route adds flexibility to PPAP construction and broadens the scope of eight-membered ring chemistry.

Synthesis of Bifluorenylidene- and Cycloocta[1,2,3-jk:6,5,4-j′k′]difluorene-Extended Tetrathiafulvalenes: Novel Multi-Redox Systems

AbstractHerein, we present the synthesis of a novel 9,9′-bifluorenylidene-extended tetrathiafulvalene (TTF) as an E/Z isomeric mixture that was found to readily undergo an oxidative dimerization to furnish a TTF vinylogue with the two dithiafulvene units connected to an eight-membered ring fused to a bifluorenylidene core. This compound exhibited multiredox behavior and was found to take six charge states under cyclic voltammetry conditions: –2, –1, 0, +1, +2, +3. The dication corresponds to a cycloocta[1,2,3-jk:6,5,4-j′k′]difluorene with two appended 1,3-dithiolium rings.

Synthesis of Oxa-bridged Medium-Sized Carbocyclic Rings

Abstract: Natural products with oxa-bridged medium-sized carbocyclic ring structure units play an important role in nature. Therefore, developing novel and efficient methods is essential for synthesizing complex natural products containing oxygen-bridged rings. In this article, progress toward the synthesis of oxygen-bridged seven and eight-membered rings is reviewed in terms of the strategies employed.

Development of a 2D Network of Zinc Organophosphate with an 8-Membered Ring Core Having Corrosion Resistance Properties.

A 2D framework, i.e., zinc organophosphate [Zn(tmbiph)(solv)]2 (tmbiphH4 = tetramethyl-[1,1'-biphenyl]-4,4'-diyl bis(dihydrogen phosphate); solv = 2H2O) with an eight-membered ring core has been developed. The hierarchical and rational design of the zinc organophosphate has been achieved by carefully designing an organic bisphosphate ligand with two phosphate monoester groups connected through extended aromatic rings with methyl groups at ortho positions. The development of the 2D network does not require any ancillary ligand due to participating two phosphate groups in the coordination. The molecular structure of the zinc phosphate material was determined using single-crystal X-ray diffraction technique. The corresponding Co and Cu organophosphate materials have also been obtained using the same synthetic method; however, these compounds could not be confirmed structurally due to nondiffractive crystal quality. The structure of zinc organophosphate obtained through single-crystal X-ray diffraction has also been supported by theoretical calculations using density function theory. Furthermore, the zinc organophosphate material has been employed as an corrosion inhibitor for mild steel. The effect of the zinc phosphate framework on mild steel in 1 M HCl was investigated using polarization and electrochemical impedance spectroscopy. This study suggests that such phosphate inhibitors can be employed in the form of a thin protective layer on the electrode surface.

Unveiling the enhanced reactivity of NO ozonation on NH4-SAPO-34 zeolite: Ab initio molecular dynamics combined with experimental characteristics

An environmental-friendly NH4-SAPO-34 catalyst, easily obtained via ion exchange, was prepared and tested for its catalytic ozonation activity in NO oxidation under O3-deficient conditions. Characterization results revealed the successful loading of NH4+ on the Brønsted acidity of SAPO-34 after NH4NO3 ion exchange. Surprisingly, the NH4-SAPO-34 zeolite exhibited superior performance in the catalytic ozonation of NO. Ab initio molecular dynamics simulations demonstrated that the facile binding of NO and O3, along with rapid dissociation of the intermediate (NO2 dimer), contributed to the enhanced catalytic performance of NH4-SAPO-34 compared to H-SAPO-34. Additionally, the impact of various ammonium salts on the catalytic activity was investigated, stating clearly the NH4+ coordination in the eight-membered ring windows as the active site. Thus, this study deepens our understanding of the NO ozonation dynamics catalyzed by NH4-SAPO-34 and provides a promising method for synthesizing catalysts without expensive transition metals.

Blue-Emissive Antioxidant Carbon Dots Enhance Drought Resistance of Pea (Pisum sativum L.).

Prolonged drought conditions are a critical challenge for agricultural advancement, threatening food security and environmental equilibrium. To overcome these issues, enhancing plant resilience to drought is essential for plant growth and sustainable agriculture. In this study, blue-emitting antioxidant carbon dots (B-CDs), synthesized from citric acid and ascorbic acid, emerged as a promising solution to enhance the drought resistance of peas (Pisum sativum L.). B-CDs can efficiently scavenge reactive oxygen species (ROS), which are harmful in excess to plants under stress conditions. Through detailed experimental analyses and density functional theory (DFT) studies, it is found that these B-CDs possess structures featuring eight-membered aromatic rings with abundant oxygen-containing functional groups, providing active sites for reactions with ROS. The practical benefits of the B-CDs are evident in tests with pea plants exposed to drought conditions. These plants show a remarkable reduction in ROS accumulation, an increase in photosynthetic efficiency due to improved electron transfer rates, and significant growth enhancement. Compared to untreated controls under drought stress, the application of B-CDs results in an impressive increase in the fresh and dry weights of both the shoots and roots of pea seedlings by 39.5 and 43.2% for fresh weights and 121.0 and 73.7% for dry weights, respectively. This suggests that B-CDs can significantly mitigate the negative effects of drought on plants. Thus, leveraging B-CDs opens a novel avenue for enhancing plant resilience to abiotic stressors through nanotechnology, thereby offering a sustainable pathway to counter the challenges of drought in agriculture.