Norbornene Ring Research Articles

Gold-Catalyzed Cyclization of Alkynyl Heteroarenes with a Norbornene Bridge

AbstractIn this study, gold(III)-catalyzed cyclization reactions of alkynyl norbornene derivatives were developed to provide benzofused (hetero)arenes with substitutions at the benzocyclic ring. The combination of Au(OAc)3 and AgNTf2 allowed for the annulation of (hetero)arenes by intramolecular hydro(hetero)arylation followed by retro-Diels–Alder reactions to afford fully conjugated benzofused (hetero)arenes in yields higher than those obtained by our previously developed Pd(OAc)2-catalyzed protocol. Furthermore, when Au(OAc)3 was replaced with AuCl3, (hetero)arenes with the norbornene bridge installed at the more electron-rich and sterically demanding position underwent a rearrangement to afford the corresponding regioisomeric products in a divergent manner. The results indicate that the rigid norbornene ring creates a steric effect, in addition to placing two reactive sites in close proximity and serving as an acetylene synthon. This illustrates the importance of steric effects in addition to the electronic effects of the catalyst and the substrate on the rearrangement.

MOF-templated fabrication of Ni@C/g-C3N4 catalyst with high-dense accessible active sites achieving dicyclopentadiene hydrogenation at ambient conditions and comprehensive mechanism insight

The catalytic hydrogenation of dicyclopentadiene (DCPD) is one of the pivotal processes in the production of green high-energy-density liquid hydrocarbon fuels, jet propellent-10 (JP-10). However, the high dependency of the current catalysts on high temperature and pressure, whether the non-noble (>120 ℃, >15 bar) or even the noble catalyst (80 ℃, >7.6 bar), leads to enormous energy consumption and technical challenges in environment and sustainable development. Herein, we prepared a carbon layer wrapped Ni nanoparticles/g-C3N4 catalyst (Ni@C/g-C3N4) derived from the in-situ-formed Ni-MOF/g-C3N4 composite. Benefiting from the 53.2 wt% small-sized Ni nanoparticles and nanosheet structure, resulting in high-density accessible active sites (5.38 × 1020 sites g−1) and the mass transfer enhancement, Ni@C/g-C3N4 achieved DCPD hydrogenation under ambient conditions. > 99.9 % DCPD conversion and 98.1 % endo-tetrahydrodicyclopentadiene yield was obtained at 30 ℃, 1 bar, significantly outperforming the benchmark 5 % Pd/C. To the best of our knowledge, it’s the mildest condition for DCPD hydrogenation. Further, comprehensive mechanistic insights were gained from combining means of kinetic investigations, in situ characterization, and theoretical calculations. The discrimination result of 27 kinetic models indicated that DCPD hydrogenation followed the Langmuir-Hinshelwood-Haugen-Watson formulism regarding the reaction between the chemisorbed dihydrodicyclopentadiene (DHDCPD) and dissociatively adsorbed hydrogen as the rate-determining step. In-situ DRIFTS confirm that the norbornene ring is preferentially adsorbed and hydrogenated. The DFT calculations further substantiate the hydrogenation pathway. The present work provides guidance for the development of highly efficient non-noble metal catalysts and opens up an environmental-friendly and economically viable avenue for DCPD hydrogenation.

Pd‐Catalyzed Cyclization of Alkynyl Norbornene Derivatives for the Synthesis of Benzofused Heteroarenes

AbstractModular approaches, which allow a systematic variation of heteroaromatic cores and substituents, are crucial for the development of heteroaromatic drug candidates and organic functional materials. A new strategy involving the cyclization of heteroarenes tethered with alkynes through a norbornene bridge was developed. The precursors were readily prepared by a three‐component coupling process of heteroaryl halides, norbornadiene, and terminal alkynes. The Pd catalytic system derived from Pd(OAc)2and 2‐(pyrazol‐1‐yl)pyridine transformed a variety of five‐membered heteroarenes to the corresponding benzofused products, including (di)benzothiophene, indazole, carbazole, indole, and benzofuran, with aryl and alkyl substituents at the C4(C7) position. During the cyclization process, the norbornene ring underwent a retro‐Diels‐Alder reaction, serving as an acetylene synthon. This approach was used to synthesize naphthalene derivatives from electron‐rich arenes, demonstrating its versatility in the annulation of (hetero)aromatic rings.magnified image

Norbornene-Functionalized Plant Oils for Biobased Thermoset Films and Binders of Silicon-Graphite Composite Electrodes.

We herein report the functionalization of plant oil with norbornene (NB) and subsequent polymerization to prepare biobased thermoset films and biobased binders for silicon/mesocarbon microbead (MCMB) composite electrodes for use in lithium-ion batteries. A series of NB-functionalized plant oils were prepared as biobased thermoset films via ring-opening metathesis polymerization (ROMP) in the presence of a second-generation Grubbs catalyst with tunable thermomechanical properties. Increasing the catalyst loading and cross-linking agent increased cross-link density, storage modulus (E′), and glass transition temperature (Tg), while the numbers of unreacted or oligomeric components in the films were reduced. High number of NB rings per triglyceride in the plant oil encouraged monomer incorporation to form a polymer network, therefore accounting for the high Tg and E′ values. Furthermore, the NB-functionalized plant oil and 2,5-norbornadiene (NBD) were copolymerized as bioderived binders for silicone/MCMB composite electrodes of lithium-ion batteries via ROMP during electrode preparation. Cell performance investigation showed that the silicone/MCMB composite electrode bearing the NBD-cross-linked NB-functionalized plant oil binder exhibited a higher C-rate and cycle-life performance than that using a conventional poly(vinylidene fluoride) (PVDF) binder. Finally, the electrode based on the bioderived binder exhibited a high specific charge capacity of 620 mA h g–1 at 0.5 C.

Ring-opening carbonyl-olefin metathesis of norbornenes.

A computational and experimental study of the hydrazine-catalyzed ring-opening carbonyl–olefin metathesis of norbornenes is described. Detailed theoretical investigation of the energetic landscape for the full reaction pathway with six different hydrazines revealed several crucial aspects for the design of next-generation hydrazine catalysts. This study indicated that a [2.2.2]-bicyclic hydrazine should offer substantially increased reactivity versus the previously reported [2.2.1]-hydrazine due to a lowered activation barrier for the rate-determining cycloreversion step, a prediction which was verified experimentally. Optimized conditions for both cycloaddition and cycloreversion steps were identified, and a brief substrate scope study for each was conducted. A complication for catalysis was found to be the slow hydrolysis of the ring-opened hydrazonium intermediates, which were shown to suffer from a competitive and irreversible cycloaddition with a second equivalent of norbornene. This problem was overcome by the strategic incorporation of a bridgehead methyl group on the norbornene ring, leading to the first demonstrated catalytic carbonyl–olefin metathesis of norbornene rings.

Radical Copolymerization of N-Substituted 2-Azanorborn-5-enes with N-Vinylpyrrolidone

The radical copolymerization of N-methyl-2-azanorborn-5-ene, N-benzyl-2-azanorborn-5-ene, N-allyl-2-azanorborn-5-ene, and N-(2-azanorborn-5-en-2-yl)methyl acetate with N-vinylpyrrolidone is carried out in bulk in the presence of AIBN as an initiator. It is shown that the copolymerization yields statistical copolymers enriched in N-vinylpyrrolidone units. It is demonstrated that the rate of copolymerization decreases with increasing fraction of azanorbornene in the initial monomer mixture. Using 13С NMR spectroscopy, it is found that the copolymerization of azanorbornenes with N-vinylpyrrolidone occurs via the double bond of the norbornene ring. The allyl group of N-allyl-2-azanorborn-5-ene is uninvolved in copolymerization at both the equimolar comonomer ratio and the double excess of N-vinylpyrrolidone.

Hydrogenation of dicyclopentadiene into endo-tetrahydrodicyclopentadie over supported different metal catalysts

Abstract The hydrogenation of dicyclopentadiene (DCPD) to tetrahydrodicyclopentadiene (THDCPD) has been investigated over 5 wt% Ni, 1 wt% Pt and 1 wt%Ru supported on SBA-15. The catalysts were characterized to different phydico-chemial techniques founder standing their structural and textural properties. Interestingly, Ni/SBA-15 and Ru/SBA-15 provided only one fully hydrogenated producttetrahydrodicyclopentadiene (THDCPD), but Pt/SBA-15 produced both the two dihydro-products such as (3,4-and 8,9-dihydrodicyclopentadiene; DHDCPD) and the complete hydrogenated THDCPD. The yield of the DHCPDs passed through maxima at different run times, suggesting the reaction to be a consecutive one. The 8,9-DHCPD was usually obtained in larger amounts than the 3,4-isomer due to the greater reactivity of the 8,9-double bond in the norbornene ring. The apparent activation energy values for the hydrogenation of DCPD to THDCPD over Pt/SBA-15, Ni/SBA-15 and Ru/SBA-15 were found to be value of 23.8 kJ/mol 24.6 kJ/mol and 35 kJ/mol respectively catalysts. Interestingly, the dihydro intermediates were found to be higher conversion formed upon addition of quinoline in small amounts (0.16–0.78 mmol) to the reaction mixture in runs with Ni/SBA-15. The addition of quinoline had no effect on runs with Ru/SBA-15. Reactions with Raney Ni catalyst were also performed, and this catalyst also yielded the two dihydrogenated intermediates (3,4 – DHDCPD and 8,9 - DHDCPD)are found.

Domino Palladium-Catalyzed Double Norbornene Insertion/Annulation Reaction: Expeditious Synthesis of Overcrowded Tetrasubstituted Olefins.

A Pd-catalyzed domino process involving a double norbornene insertion/annulation reaction was developed for the expeditious synthesis of overcrowded olefins. In this one-pot reaction, four new C-C bond formations were achieved by three consecutive Heck carbopalladations and C-H activation across the C≡C triple bond of 2-alkynyl bromobenzenes with two norbornene rings via a reactive vinylic-Pd(II) species. This protocol provides a step-economical synthetic system to access the structurally identical molecular machine motifs of overcrowded olefins with high yields.

Ion Conducting ROMP Monomers Based on (Oxa)norbornenes with Pendant Imidazolium Salts Connected via Oligo(oxyethylene) Units and with Oligo(ethyleneoxy) Terminal Moieties

A matrix of 22 two-armed norbornene-based imidazolium TFSI monomers (8) was synthesized to determine the optimal structure in terms of single ion conductivity. For the chain tethering the imidazolium ring to the norbornene ring three or four oxyethylene units are optimal. A terminal group of two ethyleneoxy units was optimal. NMR studies indicated that both the tether oxyethylene units and the terminal ethyleneoxy units interact with the imidazolium cation via hydrogen bonding. 8r (X = 4, Y = 2) exhibited a conductivity of 9.57 × 10–5 S/cm at 25 °C and a Tg of −46 °C. Low Tg values do not correlate with higher conductivity as a result of the H-bonding interactions. Stability toward autopolymerization and reasonable conductivities provide an acceptable platform for ion conducting ROMP polymers. Four one-armed norbornene-based imidazolium TFSI monomers (15) were prepared with tetra(ethyleneoxy) linkers/spacers and variable terminal groups. All of these exhibited low Tgs (<−55 °C) and room temperature conduc...

Facile, Efficient Copolymerization of Ethylene with Norbornene-Containing Dienes Promoted by Single Site Non-Metallocene Oxovanadium(V) Catalytic System.

Non-metallocene oxovanadium(V) complexes bearing either [ONNO]-type amine pyridine bis(phenolate) ligands or [ONN]-type amine pyridine phenolate ligands were employed as efficient catalysts to copolymerize ethylene with several unsymmetrical norbornene-containing dienes, such as 5-vinyl-2-norbornene (VNB), 5-ethylidene-2-norbornene (ENB) or dicyclopentadiene (DCPD), producing copolymers with high comonomer incorporations (VNB: 33.0 mol %; ENB: 30.4 mol %; DCPD: 31.6 mol %, respectively) and high molecular weight (VNB: 86.4 kDa; ENB: 256 kDa; DCPD: 86.4 kDa, respectively). The enchainment of the dienes was proven to be exclusive of vinyl-addition via the C=C double bond of the norbornene ring while the other double bond was retained near the backbone without crosslinking. During the copolymerization of ethylene with ENB, a positive ‘comonomer effect’ was observed. The catalytic activities of the catalysts as well as the molecular weights and comonomer incorporations of the resultant copolymers could be tuned within a wide range by varying the structures of the catalysts and copolymerization conditions. The [ONN]-type oxovanadium(V) complexes showed higher catalytic activities than those of [ONNO]-type oxovanadium(V) complexes, irrespective of the structure of the dienes. In addition, the dominant chain transfer pathway of the non-metallocene oxovanadium(V) catalytic system promoted copolymerization was proven to be transfer to aluminum compounds.

Polyaddition of Azide-Containing Norbornene-Based Monomer through Strain-Promoted 1,3-Dipolar Cycloaddition Reaction.

The azide-alkyne "click" reaction has been well known in the past decade, however, another kind of 1,3-dipolar cycloaddition, the azide-alkene reaction is not fully explored in polymer science to date. This contribution reports, for the first time, the discovery of a polyaddition of norbornene based monomer (NC11N3 ) containing both strained double bond and azide moieties. The reaction product is characterized by Fourier transform infrared spectroscopy (FTIR), NMR, gel permeation chromatography (GPC), and mass spectrometry (MS), which confirmed the mechanism that is through cycloaddition of azide to strained double bond on norbornene ring to form triazoline linkage. The reaction can proceed at room temperature as indicated by the increase of molecular weight and viscosity during storage. Monomer, dimer, trimer, tetramer, etc., and species with loss of N2 due to lability of triazoline moiety are identified in the mixture of reaction product. As a unique feature, elimination of N2 in the five-membered ring of triazoline affords a chance to form highly reactive materials, such as with aziridine, which can be a very powerful tool in chemical functionalizations, and find promising applications in reactive polymer resin industries.

Design and synthesis of oxa-bowls via Diels–Alder reaction and ring-rearrangement metathesis as key steps

Ring-rearrangement metathesis of norbornene derivatives prepared by Diels–Alder reaction has been used to design various oxa-bowls. Enyne ring-rearrangement metathesis of norbornene ring systems with propargyl moiety produced diene containing oxa-bowls, suitable for further expansion by Diels–Alder strategy.

Broadband Dielectric Relaxation Spectroscopy of Functionalized Biobased Castor Oil Copolymer Thermosets

Novel biorenewable copolymer thermosets are successfully synthesized by the ring‐opening metathesis polymerization (ROMP) of norbornenyl‐functionalized castor oil (NCO) and norbornenyl‐functionalized castor oil alcohol (NCA) with controlled amounts of 0.8 and 1.8 norbornene rings per fatty acid chain, respectively. The NCO and NCA monomers are mixed in different concentrations and simultaneously polymerized via ROMP using Grubbs catalyst. Broadband dielectric relaxation spectroscopy (BDRS) is used to investigate the molecular dynamics of the fully cured copolymer thermosets over a wide range of frequencies (5 × 10−2 to 0.5 × 107 Hz) at different constant temperatures (−70 to 125 °C). Four phenomena, namely α‐, β‐, and γ‐relaxation processes and ionic conductivity are observed for all the measured samples. image

Remote Exo/Endo Selectivity in Selective Monohydrolysis of Dialkyl Bicyclo[2.2.1]heptane-2,3-dicarboxylate Derivatives

High exo-facial selectivity was observed in the selective monohydrolysis of a series of near-symmetric diesters that possess an exo-ester group and an endo-ester group attached on a norbornane or norbornene skeleton. The selectivities were found to be clear-cut, although the reaction center in these reactions is one covalent bond distant from the norbornane or norbornene ring, where the difference of the environment between the exo face and endo face is therefore expected to be negligible. The effect of the co-solvent we studied earlier for the selective monohydrolysis reaction was also confirmed and contributed to improvement of the yields of the half-esters.

Castor oil-based thermosets with varied crosslink densities prepared by ring-opening metathesis polymerization (ROMP)

Two castor oil-based monomers, (1) norbornenyl-functionalized castor oil (NCO), which has ∼0.8 norbornene rings per fatty acid chain and (2) norbornenyl-functionalized castor oil alcohol (NCA), which has ∼1.8 norbornene rings per fatty acid chain, have been prepared. Ring-opening metathesis polymerization (ROMP) of different ratios of NCO/NCA using the 2nd generation Grubbs catalyst results in rubbery to rigid biorenewable-based plastics with crosslink densities ranging from 318 to 6028mol/m3. Increased crosslink densities result in shorter gelation times, better incorporation of the monomers into the polymer network, and much less soluble materials in the bulk materials. The increased crosslink densities obtained by adding NCA enhance the thermal properties, including the glass transition temperature (Tg) and room temperature storage modulus, which increase from −17.1°C to 65.4°C and from 2.4MPa to 831.9MPa, respectively. The TGA results, where T10 increased from 285°C to 385°C, illustrate that improved thermal stabilities can be obtained for thermosets with higher crosslink densities. Young’s modulus (11–407MPa), tensile strength (1.6–18MPa) and toughness (0.14–1.6MPa) are also improved dramatically with higher crosslink densities.

Facile, efficient functionalization of polyethylene via regioselective copolymerization of ethylene with cyclic dienes

AbstractThe copolymerizations of ethylene with cyclic dienes [dicyclopentadiene (DCPD) and 2,5‐norbornadiene (NBD)] using bis(β‐enaminoketonato)titanium complexes [PhN = C(R2)CHC(R1)O]2TiCl2 (1a: R1 = CF3, R2 = CH3; 1b: R1 = t‐Bu, R2 = CF3; 1c: R1 = Ph, R2 = CF3) have been investigated. In the presence of modified methylaluminoxane, these complexes exhibited high catalytic activities in the copolymerization of ethylene with DCPD or NBD, affording high molecular weight copolymers with unimodal molecular weight distributions. 1H and 13C‐NMR spectra reveal ethylene/DCPD copolymerizations by catalysts 1a–c proceeds through the enchainment of norbornene ring. Catalysts 1a and 1c showed a tendency to afford alternating copolymers. More noticeably, catalysts 1b and 1c bearing bulky substituents on the ligands promote ethylene/NBD copolymerization without crosslinking, affording the copolymer containing intracyclic double bonds. The NBD incorporation as high as 27.2 mol % has been achieved by catalyst 1c. Moreover, the microstructures of the copolymers were further confirmed by the measurement of reactivity ratios and dyad monomer sequences as well as mean sequence lengths. The intracyclic double bonds of ethylene/DCPD or ethylene/NBD copolymers can be fully converted into polar groups such as epoxy, amine, silane, and hydroxyl groups under mild conditions. Convenient synthesis of hydroxylated polyethylene can be provided for the first time through the ring opening reaction of epoxide. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1764–1772, 2010

Lewis Acid Catalyzed Three-Component Hetero-Diels−Alder (Povarov) Reaction of N-Arylimines with Strained Norbornene-Derived Dienophiles

Generally, the hetero-Diels-Alder reaction (Povarov reaction) of N-arylimine dienes are limited to reaction with activated, electron-rich alkenes. However, introduction of ring strain in the dienophile, as with moderately strained bicyclo[2.2.1]heptenes (norbornene), enables three-component Povarov reaction with in situ formed N-arylimines under Lewis acid catalyzed conditions (BF(3).OEt(2)). The reactions proceed efficiently with a diverse set of commercially available anilines and benzaldehydes, as well as a variety of substituted norbornenes. The corresponding tetrahydroquinolines are formed with high complexity in a multicomponent fashion and are obtained in good yield and high diastereoselectivity. In addition, more reactive ethyl glyoxylate derived imines were utilized to achieve faster, room temperature reactions with norbornene. In all cases, attack of the N-arylimine dienes occurred exclusively from the exo-face of the norbornene ring, but the relative stereochemistry of the substituent alpha to the tetrahydroquinoline nitrogen, as well as the regioselectivity of reaction, was shown to depend upon subtle substituent effects on the aniline precursors. In most cases, a preference for the formation of exo-exo diastereomeric adducts was observed, but for reactions of ortho- or meta-substituted anilines, the formation of exo-endo adducts was also observed. These observations may be rationalized by two competing mechanistic models, involving either a concerted asynchronous [4 + 2]-like mechanism or a stepwise mechanism.

Hydrogenation of Dicyclopentadiene over amorphous nickel alloy catalyst SRNA-4

Hydrogenation of dicyclopentadiene (DCPD) to tetrahydrodicyclopentadiene (THDCPD) over nickel alloy catalyst SRNA-4 has been studied. GC–MS analysis showed that the reaction is a consecutive process with two hydrogenated intermediates. DFT simulation confirmed the double bond in norbornene ring is easier to saturate than that in cyclopentene ring, so the major intermediate is 8,9-dihydrodicyclopentadiene (8,9-DHDCPD). In comparison with Raney Ni, SRNA-4 shows a significantly higher activity and makes the reaction proceed at lower temperature. The reaction conditions including temperature and hydrogen pressure were optimized. The optimal hydrogen pressure is 1.5 MPa. To avoid the decomposition of DCPD, a two-stage operation was designed: 373 K for 1 h and then 403 K for another 4 h. Under these conditions, the yield of THDCPD reaches 98.5%. The apparent kinetics was also calculated using the concentration–time data obtained in the experiments. The activation energy for DCPD to 8,9-DHDCPD and 8,9-DHDCPD to THDCPD reaction is 22.8 kJ/mol and 40.9 kJ/mol, respectively.

Photo‐Fries Rearrangement in Polymeric Media: An Investigation on Fully Aromatic Esters Containing the Naphthyl Chromophore

AbstractThe present paper deals with the photo‐Fries rearrangement of fully aromatic esters in polymeric media. Low molecular weight aryl esters bearing the 1‐naphthyl chromophore were investigated with respect to their photochemical reactivity and the yield of hydroxyketones as the photo‐Fries rearrangement product. In particular, the phenyl, 4‐methylphenyl and 1‐naphthyl ester of 1‐naphthoic acid, as well as benzoic acid, 1‐naphthyl ester were dissolved in a polysiloxane matrix and exposed to 313 nm UV light. The progress of the photoreaction was assessed by quantitative FT‐IR spectroscopy. Both 1‐naphthoic acid, phenyl ester and benzoic acid, 1‐naphthyl ester underwent the photoreaction upon 313 nm irradiation. This indicates that in these fully aromatic esters, the initial CO bond scission also occurs upon selective photoexcitation of the acyl unit in the molecule. It was found that phenyl and 4‐methylphenyl esters of 1‐naphthoic acid gave the highest yields of hydroxyketone when irradiated in polymer matrix. Based upon these results, a photoreactive monomer was designed which contains naphthoic acid, phenyl ester units attached to a norbornene ring. This monomer was then polymerized by ring opening metathesis polymerization (ROMP). The polymer showed a high yield in the photo‐Fries reaction upon 313 nm illumination. Moreover, a large increase in the refractive index (Δn up to +0.043 at λ = 370 nm) was observed which makes this polymer interesting for optical applications (e.g., waveguiding and diffractive optics).magnified image

Isothermal cure characterization of dicyclopentadiene

Abstract Conversion (α) and the glass transition temperature (T g) were investigated during the isothermal cure of endo-dicyclopentadiene (DCPD) with a Grubbs catalyst for different temperatures using differential scanning calorimetry. Conversion vs. In (time) data at an arbitrary reference temperature were superposed by horizontal shift and the shift factors were used to calculate an Arrhenius activation energy. Glass transition temperature vs. conversion data fell on a single curve independent of cure temperature, implying that reaction of the norbornene and cyclopentene ring of DCPD proceeds in a sequential fashion. Implications of the isothermal reaction kinetics for self-healing composites are discussed.