Formation Of Five-membered Ring Research Articles

Enzyme-Like Catalysis of Vinyl Copolymer Carrying Boron Directly Connected to Backbone: Catalytic Esterification through Cooperation of Boron with Neighboring Carboxylic Anhydride.

Alternating-rich copolymer of vinylboronic acid pinacol ester (VBpin) and maleic anhydride (MAH) was found to catalyze direct dehydrative esterification of carboxylic acid and alcohol. The key to the catalytic function is the activation of the MAH unit by the neighboring Lewis acidic boron directly connected to the backbone through the formation of five-membered ring. The effects of the side-chain cooperation were clarified through comparisons with the polymers having similar structures and a conventional titanium catalyst as well as the analyses of reactions with carboxylic acid or alcohol. The catalytic activity was enhanced as the molecular weight was higher, which is owing to the structural feature that boron is directly attached to the backbone. The cooperative catalysis is of interest because of its conceptual similarity with enzyme.

Density functional theory study of molecular pillared graphene for High-Performance Sodium-Ion batteries

This study investigates the potential of molecular pillared graphene (MPG) as anode materials for Na-ion batteries using density functional theory (DFT) calculations. Two distinct MPG structures were designed by varying the organic pillar molecule’s chemical composition. The investigated MPGs comprise two parallel graphene layers linked by naphthalene or pyrene via the formation of five-membered boroxine rings. Cohesive energy analysis and ab-initio molecular dynamics simulations confirm the structural stability of these materials. The results demonstrate high Na mobility within the MPG structures due to the low energy barrier for diffusion. Furthermore, the layered structure facilitates Na-ion insertion, leading to exceptional theoretical capacitances calculated to be 832 and 858 mAh/g. The combination of low diffusion barriers and high theoretical capacities suggests MPG as a promising candidate for Na-ion battery anodes. Additionally, Na adsorption induces metallic behavior in the MPG structures, a crucial prerequisite for efficient ion diffusion in anode materials.

Formation of five-membered ring structures via reactions of o-benzyne

The formation of five-membered ring structures is important for the generation of curved and bowl-shaped polycyclic aromatic hydrocarbons (PAHs) under combustion conditions. Here, we report the identification of the indenyl (C9H7) radical – the simplest aromatic hydrocarbon radical carrying an adjacent five- and six-membered ring, as the major product of the o-benzyne (o-C6H4) reaction with propargyl (C3H3). Because real flames exhibit a complex chemistry, elucidation of a specific reaction is very challenging. Instead, we studied the o-C6H4 + C3H3 reaction in a resistively heated microtubular SiC reactor at controlled conditions of 1150 K and a pressure near 10–20 Torr. To this end, the reactants o-benzyne and propargyl were pyrolytically generated from benzoyl chloride and propargyl bromide. We identified the reactants and the indenyl radical isomer-selectively utilizing photoion mass-selected threshold photoelectron spectroscopy (ms-TPES). The experimentally observed predominant formation of indenyl radicals finally confirms the theoretical predictions of Matsugi and Miyoshi [Phys. Chem. Chem. Phys. 14 (2012), 9722–9728] and highlights a versatile route for the formation of five-membered rings and curved PAHs via reactions of o-benzyne that favor the formation of multiring species over aliphatically substituted aromatic species.

High-purity graphene and carbon nanohorns prepared by base-acid treated waste tires carbon via direct current arc plasma

As the accumulation of waste tires continues to rise year by year, effectively managing and recycling these discarded materials has become an urgent global challenge. Among various potential solutions, pyrolysis stands out due to its superior environmental compatibility and remarkable efficiency in transforming waste tires into valuable products. Thus, it is considered the most potential method for disposing these tires. In this work, waste tire powder is pyrolyzed at 560 °C to yield pyrolysis carbon black, and meanwhile, the purification effects of base-acid solutions on pyrolysis carbon black are discussed. High-purity few-layer graphene flakes and carbon nanohorns are synthesized by a direct current arc plasma with H2 and N2 as buffer gases and high-purity pyrolysis carbon black as raw material. Under an H2 atmosphere, hydrogen effectively terminates the suspended carbon bonds, preventing the formation of closed structures and facilitating the expansion of graphene sheets. During the preparation of carbon nanohorns, the nitrogen atoms rapidly bond with carbon atoms, forming essential C–N bonds. This nitrogen doping promotes the formation of carbon-based five-membered and seven-membered rings and makes the graphite lamellar change in the direction of towards negative curvature. Consequently, such change facilitates the formation of conical structures, ultimately yielding the coveted carbon nanohorns. This work not only provides an economical raw material for efficient large-scale synthesis of few-layer graphene and carbon nanohorns but also broadens the intrinsic worth of pyrolysis carbon black, which is beneficial to improving the recycling value of waste tires.

Practical synthesis of 1,3-benzoazaphosphole analogues.

Despite the promising pharmacological activity and material properties of five-membered heterocyclic compounds containing phosphorus and nitrogen, synthetic examples of them have been rather limited due to the instability of phosphorus toward air and water. In this study, 1,3-benzoazaphosphol analogues were selected as target molecules, and various synthetic routes were examined to establish a fundamental technology for the introduction of phosphorus groups into aromatic rings and formation of five-membered rings containing phosphorus and nitrogen by cyclization. As a result, we found that 2-aminophenyl(phenyl)phosphine is an extremely promising synthetic intermediate with high stability and easy handling. Furthermore, 2-methyl-3-phenyl-2,3-dihydro-1H-benzo[d][1,3]azaphosphole and 3-phenyl-2,3-dihydro-1H-benzo[d][1,3]azaphosphole-2-thione as synthetically useful 1,3-benzoazaphosphol analogues were successfully synthesized by using 2-aminophenyl(phenyl)phosphine as a key intermediate.

Study of the reaction mechanism based on the formation of the first carbocyclic ring from propargyl and diacetylene

The formation of the first carbon ring is a crucial rate-controlling step in developing polycyclic aromatic hydrocarbons (PAHs). It is vital to investigate the mechanism of the creation of the first carbon ring to inhibit the formation of PAHs. To explore the growth process of the first carbocyclic ring, this work used the average localized ionization energy (ALIE) and electrostatic potential (ESP) to predict the reaction sites. Moreover, the reaction paths and chemical kinetic parameters for the generation of the first carbocyclic ring from propargyl (C3H3) + diacetylene (C4H2) are calculated based on the density functional theory (DFT) method and transition state theory (TST). The results showed that the addition reaction of C3H3 +C4H2 can form five-, six- and seven-membered ring molecules, in which the five-membered ring formation is fastest and the six-membered ring formation is slowest. During the formation of the first carbon ring, the activation energy required for the H transfer and cyclization reactions is large, and the reaction rate is slow, which determines the formation rate of the first carbon ring. The rate of H-transfer reaction on each carbon ring depends on the number of C atoms of the carbon ring, with the five-membered ring being the fastest and the six-membered ring the slowest. This paper improves the reaction kinetics and thermodynamic data of the first carbon ring formation during the combustion of hydrocarbon fuels, which offers a powerful theoretical basis for predicting the generation of PAHs.

Reactivity of Oxygen-Bridged Geminal Al/P and Si/P Frustrated Lewis Pairs towards Heterocumulenes.

The two oxygen-bridged geminal frustrated Lewis pairs (FLP) tBu2 P-O-AlBis2 (Bis=CH(SiMe3 )2 ; 1) and tBu2 P-O-Si(C2 F5 )3 (2) were reacted with the heterocumulenes PhNCO, PhOCN, PhNCS, CS2 and PhNSO as well as SO2 . With isocyanate and cyanate, both 1 and 2, form addition products under formation of five-membered rings. With CS2 , isothiocyanate and sulfinylaniline, only 1 forms stable adducts, whereas 2 shows reactivity towards sulfinylaniline, but the product decomposed after a few minutes. The reaction of 1 with SO2 led to partial cleavage of the P-O-Al and Al-C units, as confirmed by X-ray diffraction studies of a complex aggregate. The reaction of 2 with SO2 affords the 1,2-addition product. All adducts were characterized by means of multinuclear NMR spectroscopy, X-ray crystallography and CHN analyses.

The synthesis and the mechanism of a five-membered ring formation between an isothiocyanate and an amide leading to the yield of Enzalutamide anticancer API; a joint experimental and theoretical study

This paper has been prepared to investigate the question "why the five-membered ring formation to give Enzalutamide, needs at least a portion of dimethyl sulfoxide (DMSO) as the solvent and a high level of temperature about 95 °C?". By understanding the role of DMSO as solvent in the mechanism of such ring closing, this question might be answered. In addition, with the aid of the potential energy surface (PES) (dE) calculations, and the density functional theory (DFT) investigations, the PES curves of the reaction were designed to find the probable energy valleys and barriers of the process.The results of the experimental as well as theoretical studies showed that by choosing a suitable catalyst in DMSO: Isopropyl acetate (IPOAC) as the solvent mixture, we could handle the synthesis of Enzalutamide anti-cancer active pharmaceutical ingredient (API) in a milder condition with a lower temperature (about 70 °C), and also a shorter time (about 18 h instead of 24 h). Also, the developed HPLC-UV method showed that after crystallization, the API powder would have a total purity profile up to 99%.

Palladium-Catalyzed Hydroboration/Cyclization of 1,n-Dienes

While the hydroboration of alkenes is well established, the corresponding cyclization reaction of dienes remains challenging. Here, we report a new method for hydroboration/cyclization applicable to various 1,n-dienes and hydroboranes. The method features the direct synthesis of borylalkyl cyclopentanes from common 1,6-dienes, which is highlighted by syntheses of elaborated pyrrolidine cores from easily accessible diallylamines. Notably, 1,n-dienes (n > 6) also undergo five-membered ring formation, offering "remote" hydroboration/cyclization that would be otherwise difficult to achieve.

Synthesis, structural analysis, DFT study, antioxidant activity of metal complexes of N-substituted thiourea

In this work, N-substituted thiourea and its metal complexes were synthesized and characterized by Nuclear magnetic resonance (NMR), Infrared (IR) spectroscopy and X-ray phase analysis. DFT/B3LYP level of theory with 6–31++G(d,p) basis set have been used to determine the geometry, energy and electronic parameters, characterized the stable structures of these compounds. Frontier molecular orbitals energies, energy gap, chemical reactivity descriptors, atomic charges, dipole moment and polarizability have been calculated, the sites of electrophilic and nucleophilic regions where the molecular interactions likely to happen are identified for reagent and complex. The electron density transfer and hyper conjugative interactions are elucidated. The data obtained allowed us to predict the reactivity CuCl2 substituent. It was revealed that the nature of the mentioned substituent favors the formation of a stable five-membered ring via coordinate bonding N6-Cu25 and N17-Cu25. In addition, the thiocarbonyl and methyl groups form a pseudo six-membered ring via the C10H13…S5 hydrogen bond in this complex. It was found that the experimentally and theoretically obtained parameters of the proposed models of thiourea and its complex with CuCl2 have match well. The antioxidant properties of the synthesized compounds were studied in model reactions. Since the study of the mechanism of the antioxidant action of oxidation inhibitors under real conditions of oxidation of lubricating oils is very difficult, the antioxidant effect of the compounds synthesized by us was studied in the oxidation reaction of a model hydrocarbon-cumene, the oxidation mechanism of which was studied in detail. The study of cumene autoxidation in the presence of these compounds showed that they effectively inhibit the autoxidation process.

Tunable band gap of diamond twin boundaries by strain engineering

Diamond thin films are promising wide band gap semiconductor materials for applications in electronic and microwave devices. Revealing the mechanism of how twin boundaries impact the band gap of diamond is of critical importance since they are the most common defects in polycrystalline diamond films. Here, nanocrystalline diamond films are synthesized by microwave plasma-enhanced chemical vapor deposition. The atomic and electronic structures of diamond lamellar and fivefold twins, and their evolution behaviors with the increase of axial tensile strain, are investigated by combining aberration-corrected transmission electron microscopy with first-principles calculations. There is no intrinsic stress concentration at the lamellar twin boundary, and its band gap equals to that of the bulk diamond ( i.e. , 5.3 eV). An intrinsic in-plane compressive stress field is formed and the band gap is increased evidently ( i.e. , 0.6 eV) in the center of fivefold twins. When applying an axial tensile strain up to 15%, the band gaps of the bulk diamond, lamellar and fivefold twins reduce significantly to 2.2 eV, 2.1 eV and 2.4 eV, respectively, which are mainly due to the decrease of p z orbital energy caused by the increase of axial bond length during the tensile process. Under the same axial tensile strain, the band gap of the fivefold twin is always larger than those of the lamellar twin and the bulk diamond due to the formation of in-plane five-membered carbon rings in the center. The findings of the tunable band gap by strain engineering will benefit for the innovation and design of advanced diamond functional devices. • Atomic and electronic structures of diamond lamellar and fivefold twins under tensile strains have been investigated. • The band gap of fivefold twins is evidently larger than that of the bulk diamond. • Under axial tensile strains, the band gap of bulk diamond, lamellar and fivefold twins reduces significantly. • The reduction of band gap is due to the decrease of p z orbital energy caused by the increase of axial bond length.

Stereoselective synthesis and anticancer profile of C-alkyl pyrrolidine-diols with a sphingoid base-like backbone

The chiron approach to pyrrolidine-containing isophytosphingosine base-like compounds 21.HCl-23.HCl was developed from the known d-glucofuranose template. The key step of the synthesis was a highly diastereoselective aza-Claisen rearrangement, which created the cornerstone of the target molecules carrying three consecutive stereocentres. Subsequent cross metathesis reaction allowed the installation of a long-chain unit. The five-membered ring formation was accomplished via an intramolecular SN2 process. A single crystal X-ray analysis of the key intermediates confirmed the (2R,3R,4R)- and (2S,3R,4R)-configuration seen in the final derivatives. A comparison of the cytotoxic activity of the novel sphingomimetics (21.HCl-23.HCl) with recently described analogues (compounds 61, 62 and their antipodes) revealed that their antiproliferative profile strongly depends on the stereochemistry of the pyrrolidine substituents as well as the length of the side chain unit. Moreover, with regard to their structure and recent literature findings, compound 22.HCl may possess β-GCase inhibitory activity.

Influence of alkali metal Na on coal-based soot production

This work aims to investigate the effects of Na on the characteristics of coal-derived soot. Soot, which has long been regarded as an unwelcome source of air pollution, has been classified as a pollutant. However, with the proposed clean and efficient utilization of coal, soot has been found to be effectively inhibited by adding metal catalysts and changing the reaction environment, and it can even realize the transformation of pollutants to functional materials. To understand the effect on physiochemical properties, pyrene was used as a coal tar substitute, and the structural transformation of soot was thoroughly investigated by DFT calculation method in combination with elemental analysis, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The presence of Na changed the π electron distribution of benzene ring, promoted the dehydrogenation condensation and oxidation exfoliation of the surface microchip layer, and accelerated the condensation of PAHs and the formation of five-membered ring in the microchip layer. Na participates in the oxidation and growth process of soot, which can regulate the micromorphology of soot. With the participation of Na, the diameter of soot particles decreases, the microcrystal length and curvature become larger, and the internal holes increase.

Carbon charge population and oxygen molecular transport regulated by program-doping for highly efficient 4e-ORR

Oxygen reduction reaction (ORR) is a key reaction in both hydrogen peroxide (H2O2) generation and fuel cells system. Carbon is a promising catalyst to overcome the sluggish kinetics of ORR, while its intrinsic activity is contentious. It has been verified that both heteroatoms doping and intrinsic defects upgrade the carbon performance. Nevertheless, the charge population is the fundamental origin of carbon activity. In this work, N and S are used to co-regulate the carbon charge population. Results demonstrate that the N in the six-member ring and thiophene-S are electron-withdrawing configurations, leading to more positive charge distribution. Nevertheless, pyrrole-N is an electron-donor and destroys the charge delocalization when it co-existed with a six-member N ring or thiophene-S. Consequently, S is preferentially released to inhabit the five-membered ring formation site, inducing the precise formation of six-membered N ring. Importantly, the transfer enhancement of O2 molecules by physical absorption and reaction consumption is confirmed by in-situ concentration detection. The synthesized material possesses more positive half-wave potential (E1/2 ∼ 0.851 VRHE), high selectivity and stability than that of Pt/C (E1/2∼0.823 VRHE).

The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings.

The regions of the C13H11 potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C2H2 reactions have been explored by ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9H-fluorene product. The 1-/2-methylnaphthyl + C2H2 reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C13H11 PES, these straightforward pathways are dominant in the high-temperature regime (above ∼1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta(a)naphthalene, for 1-methylnaphthyl + C2H2, and 1H-cyclopenta(b)naphthalene and 3H-cyclopenta(a)naphthalene, for 2-methylnaphthyl + C2H2. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of ∼10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and σ-aryl + C2H2. The 1-methylnaphthyl + C2H2 and 2-methylnaphthyl + C2H2 reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.

Synthesis of phosphiranes via organoiron-catalyzed phosphinidene transfer to electron-deficient olefins.

Herein is reported the structural characterization and scalable preparation of the elusive iron-phosphido complex FpP( t Bu)(F) (2-F, Fp = (Fe(η5-C5H5)(CO)2)) and its precursor FpP( t Bu)(Cl) (2-Cl) in 51% and 71% yields, respectively. These phosphide complexes are proposed to be relevant to an organoiron catalytic cycle for phosphinidene transfer to electron-deficient alkenes. Examination of their properties led to the discovery of a more efficient catalytic system involving the simple, commercially available organoiron catalyst Fp2. This improved catalysis also enabled the preparation of new phosphiranes with high yields ( t BuPCH2CHR; R = CO2Me, 41%; R = CN, 83%; R = 4-biphenyl, 73%; R = SO2Ph, 71%; R = POPh2, 70%; R = 4-pyridyl, 82%; R = 2-pyridyl, 67%; R = PPh3 +, 64%) and good diastereoselectivity, demonstrating the feasibility of the phosphinidene group-transfer strategy in synthetic chemistry. Experimental and theoretical studies suggest that the original catalysis involves 2-X as the nucleophile, while for the new Fp2-catalyzed reaction they implicate a diiron-phosphido complex Fp2(P t Bu), 4, as the nucleophile which attacks the electron-deficient olefin in the key first P-C bond-forming step. In both systems, the initial nucleophilic attack may be accompanied by favorable five-membered ring formation involving a carbonyl ligand, a (reversible) pathway competitive with formation of the three-membered ring found in the phosphirane product. A novel radical mechanism is suggested for the new Fp2-catalyzed system.

Nickel(II)-Complex of Ceftibuten Dihydrate: Synthesis, Characterization and Thermal Study

Ceftibuten dihydrate is a semisynthetic, orally administered, third generation cephalosporin antibiotic which is effective against most of the pathogens causing infections in the respiratory tract. Complexation of ceftibuten dehydrate (Ligand, L) was performed with hydrated Ni(II) salt (Metal, M) in the ratio of 2:1 (L:M) in aqueous medium at 90 oC. The metal complex was then characterized by spectral techniques and thermal analyses. The FT-IR spectral data of metal complex suggested the monodentate bonding of metal ion to carboxylate group. Spectral evidence also supported the formation of five-membered ring via coordination of metal ion to β-lactam nitrogen and carboxylate group of parent drug. Thermal behavior of ligand and complex were studied. Thus, thermoanalytical (DSC and TGA) results also supported the formation of new metal complex, indicating the successful interaction of metal ion to ligand. Dhaka Univ. J. Pharm. Sci. 20(2): 219-225, 2021 (December)

The role of oxygenated species in the growth of graphene, fullerenes and carbonaceous particles

The growth of carbonaceous materials was studied using a Kinetic Monte Carlo model that captures the growth and oxidation of six-member and partially-embedded five-member rings. A novel algorithm was used to resolve the migration of partially-embedded five-member rings. Circumcoronene molecules were grown at 1500 K and 1 atm in the presence of varying mole fractions of atomic and molecular oxygen and constant mole fractions of hydrogen and acetylene. Four regions of carbon growth associated with different carbonaceous products were identified. Graphene was formed in the presence of high mole fractions of atomic oxygen 10−4<XO≤10−2. Fullerenes were formed in the presence of low mole fractions of atomic oxygen and high mole fractions of molecular oxygen (XO ≤ 10−4 and 10−2 < XO2 ≤ 10−1). Low mole fractions of both atomic and molecular oxygen (XO ≤ 10−4 and XO2 ≤ 10−2) resulted in structures that became curved as time progressed. Small structures were found at the highest mole fractions of atomic oxygen (XO > 10−2). The production and consumption of partially-embedded five-member rings appear to explain the formation of the observed structures. The oxidation of partially-embedded five-member rings produces armchair sites that grow to form flat graphenic structures. Formation and subsequent embedding of partially-embedded five-member rings result in curved structures that resemble fullerenes.

Unveiling the catalytic effects of Brønsted acidic ionic liquid on quantitative α-glucose conversion to 5-HMF: Experimental and computational studies

Effective biomass conversion to 5-HMF(5-hydroxymethylfurfural) is still a challenge and needs to be improved because of the 5-HMF importance as building blocks for valuable monomers and fuel precursors. We suggest for the first time utilization of low-cost metal-free Brønsted acidic Ionic liquid (IL) N,N-Diethyl-1,4-phenylenediamine hydrogen sulfate, [DPhDA]HSO4 as a catalyst for the α-glucose dehydration to 5-HMF. Quantitative α-glucose conversion is achieved via optimizing the reaction condition: 91.4% 5-HMF yield with 30 mol% [DPhDA]HSO4 in the presence of DMSO as a solvent at 160 °C in 30 min (TOF 6.1 h−1). 3-fold increase in reaction time leads to higher (94%) 5-HMF yield at 160 °C with lower TOF 1.6 h−1. The IL surprising catalytic performance is scrutinized with its amphiprotic nature (availability of basic and acidic spots in its structure (Fig. 1)). Further computational mechanistic studies revealed the function of the catalytic sites in the α-glucose dehydration. We calculated five-membered ring formation and water extrusion in one concerted transition state (TS2A, ΔG‡ = 38.8 kcal/mol) following the α-glucose ring-opening (TS1, ΔG‡ = 20.4 kcal/mol). Further four steps (sp3-s, C–H bond cleavages (TS3, TS6) and dehydroxylation (TS4A, TS5)) are quite straightforward to reach the product (5-HMF).

Reaction pathways for the formation of five-membered rings onto polyaromatic hydrocarbon framework

Experimental and theoretical evidences accumulated over the years have highlighted the role of polycyclic aromatic hydrocarbons as molecular precursors to soot particles. However, many of their physical and chemical characteristics are still under debate, as well as the mechanisms that drive their transition from gaseous species to solid carbonaceous particles. The formation of five-membered rings can be described with three types of reactive sites present on hydrocarbons: (1) a “free-edge” reactive site and a C3 gas phase species (C2 + C3), (2) a “zig-zag” site and a C2 gas phase species (C3 + C2), and (3) an “armchair” site and a C1 gas phase species (C4 + C1). In this work, we focus our attention on the last two categories and use ab initio G3-type electronic structure calculations to explore systematically possible reaction pathways leading to the formation of five-membered rings. Specifically, our study reports on reaction pathways leading to the formation of acenaphthene and acenaphthylene starting from the zig-zag type site on naphthalene and to the formation of five-membered ring in the form of fluorene and 4H-cyclopenta[def]phenanthrene starting from the “armchair” site on biphenyl or phenanthrene. The relative importance of the new reaction pathways has been investigated in a 0-D Closed Homogeneous Batch Reactor, where the conditions are taken from experimental data. Furthermore, the new reaction pathways, together with temperature dependent rate constants, provide a more complete description of the formation of embedded five-membered rings, such as methylene-bridged PAHs identified in catechol pyrolysis, that can play an important role not only in the gas-phase chemistry of aromatics but also as pathways to five-membered rings on armchair sites and cross-linking for soot models.