Cyclooctene Research Articles

Changing the Linking Functionality in a Triblock Co-Polymer for Electrolyzer or Fuel Cell Membrane or Ionomer for Chemical Stability Improvements and Potentially Other Consequences.

We have developed a versatile scalable triblock polymer system for anion exchange membrane (AEM) based electrochemical energy conversion devices. This system is uniquely suited to solving both the membrane and ionomer challenges for the commercialization of AEM water electrolysis or fuel cells. Using our advanced fundamental understanding of AEMs based on our work which began in 2010, we can design systems from first principles to match the needs of the device. A triblock ABA polymer is readily fabricated that has both chemical and mechanical stability. The synthesis of these systems is robust, high yield and amenable to high volume production. It utilizes a novel dual chain transfer agent to mediate the chain-transfer ring-opening metathesis polymerization (ROMP) of cyclooctene (COE) or the polymerization of isoprene (IP) to give the hydrophobic B block of the polymer. This is followed by the reversible addition-fragmentation chain transfer radial polymerization of chloromethylstyrene (CMS). The polyCMS (pCMS) blocks are then quaternized with an amine to give the cationic hydrophilic A block. Before quaternization the B block is hydrogenated to give either semi-crystalline polyethylene (pE) or amorphous polymethylbutylene (pMB) from pCOE and pIP respectively. The obvious achilles heal in this system from a chemical durability standpoint is the linking ether functionality between the hydrophobic and hydrophilic blocks. The material we have synthesized to date have exceptional chemical and mechanical stability. It performs very well in ex-situ testing with KOH solutions and in device testing we have demonstrated >500 h at 50°C in water electrolysis in 5 cm2 cells in 1M carbonate electrolyte at 0.5 A cm-2. We attribute the chemical stability to the fact that we post quaternize the membrane to give methylpipiridinium cations and we assume that this reaction does not fully penetrate down the hydrophilic chain. This leaves a hydrophobic un-quaternized region close to the hydrophobic block that does not allow access to the ether linkage by the hydrated hydroxide anion. Recently a new approach to the telehelic mid block pCOE has been reported that has no ether linkages and allows the hydrophobic outer blocks to be linked by a methylene linker. This gives use an opportunity to directly contrast the chemical stability of two identical triblock co-polymers one with an ether linkage and one with a methylene linkage between blocks. In this presentation we will show the results from ex-situ chemical stability test one at low temperatures and higher hydroxide concentration and one at higher temperature and lower hydroxide concentration. There are many examples in polymer electrolyte chemistry where a simple change leads to unintended consequences. The geometry of the ether linkage versus the methylene linkage will change the way that the polymer chains will organize and entangle in ways that are not currently predictable. To probe the mechanical, chemical and morphological difference between these two polymers we will fully characterize and contrast both polymers physical chemical properties including water uptake, ionic conductivity, DSC, TGA, tensile strength and morphology through SAXS, WAXS and HRTEM. Data for the two polymer membranes will also be contrasted and compared in small laboratory 5 cm2 single cells for both electrolysis in dilute carbonate and H2/O2 fuel cells using conventional catalysts.

Cross-Linked Polyolefins through Tandem ROMP/Hydrogenation.

Cross-linked polyolefins have important advantages over their thermoplastic analogues, particularly improved impact strength and abrasion resistance, as well as increased chemical and thermal stability; however, most strategies for their production involve postpolymerization cross-linking of polyolefin chains. Here, a tandem ring-opening metathesis polymerization (ROMP)/hydrogenation approach is presented. Cyclooctene (COE)-co-dicyclopentadiene (DCPD) networks are first synthesized using ROMP, after which the dispersed Ru metathesis catalyst is activated for hydrogenation through the addition of hydrogen gas. The reaction temperature for hydrogenation must be sufficiently high to allow mobility within the system, as dictated by thermal transitions (i.e., glass and melting transitions) of the polymeric matrix. COE-rich materials exhibit branched-polyethylene-like crystallinity (25% crystallinity) and melting points (Tm = 107 °C), as well as excellent ductility (>750% extension), while majority DCPD materials are glassy (Tg = 84 °C) and much stiffer (E = 710 MPa); all materials exhibit high tensile toughness. Importantly, hydrogenation of olefins in these cross-linked materials leads to notable improvements in oxidative stability, as saturated networks do not experience the same substantial degradation of mechanical performance as their unsaturated counterparts upon prolonged exposure to air.

Ring Opening Metathesis Polymerization of Cyclooctadiene and Cyclooctene with Dihydrofuran: Influence of Ru Fischer Carbene.

Vinyl ethers are commonly used to deactivate Grubbs catalysts and terminate ring opening metathesis polymerization (ROMP) by forming Fischer carbene species with attenuated metathesis reactivity. However, we recently demonstrated that a cyclic enol ether, 2,3-dihydrofuran (DHF), can in fact be homopolymerized or copolymerized with norbornene derivatives. 1,5-Cyclooctadiene (COD) and cyclooctene (COE) consist of an important class of ROMP monomers, and we describe here a study of their copolymerization with DHF. Addition of DHF greatly suppressed the ROMP activity of COD and COE and resulted in significant alkene isomerization of COD. Chloranil was found to be an effective additive to prevent undesired isomerization and promote copolymerization. As a result, high molecular weight COD/COE and DHF copolymers were synthesized. Hydrolysis of the enol ether main chain linkages yields polyalkenamers with alcohol and aldehyde end groups. This study encourages further exploration of the in situ formed Ru Fischer carbene species in ROMP to access degradable polymers.

Original Synthesis of Ethylene‐Acrylic Acid (ester) Copolymers Derivatives from 5‐substituted Cyclooctenes with A Controllable Way Via Ring‐Opening Metathesis Polymerization

AbstractHigh‐value grades of ethylene‐acrylic acid (EAA) and ethylene‐acrylic ester copolymers have been producing produced from ethylene, acrylic acid (AA), and the alcohol derivative via radical reaction in autoclave reactor at a temperature of 150‒230 °C and pressure of 150–300 MPa. In this paper, EAA and its ester derivatives are originally created from the 5‐substituted cyclooctenes (COE) with side chains of carboxylic acid or ester derivatives by ring‐opening metathesis polymerization (ROMP) at normal temperature and pressure. Especially, the controllable insertion of side chain is effectively achieved by altering the molar ratio of COE and 5‐substituted COE. Furthermore, the polar carboxyl groups (ester groups) have a detrimental effect on the order assigns of the linear backbone and thus hindered crystallization, indicating relatively lower crystallizing and melting temperature with an increase in the content of side chain. As the amounts of polar groups raised, the stiffness of chain segments enhanced, thereby leading to an elevation in the copolymer's energy storage modulus (E') and glass transition temperature (Tg). Moreover, the Flynn‐Wall‐Ozawa (FWO) technique is employed to characterize and fit the non‐isothermal crosslinking kinetics of the ethylene‐acrylic ester derivatives, and the crosslinking activation energy is determined to lie between 97 and 140 kJ mol−1.

Nanocomposites of polyethylene with Fe3O4 nanoparticles via surface-initiated ROMP: Thermomechanical, shape memory and photothermal properties

The nanocomposites of polyethylene with Fe3O4 nanoparticles (NPs) were synthesized via surface-initiated ring-opening metathesis polymerization (SI-ROMP). First, the surfaces of pristine Fe3O4 NPs were functionalized with (5-nicyclo[2.2.1]hept-2-enyl)ethyl]triethoxylsilane. The as-obtained Fe3O4 NPs functionalized with norbornene were used to mediate the ROMP of cyclooctene (COE). By controlling the mass ratios of COE to the surface-functionalized Fe3O4 NPs, the polycyclooctene-grafted Fe3O4 NPs (denoted PCOE-g-Fe3O4 NPs) were obtained with variable contents of Fe3O4 NPs. Finally, these PCOE-g-Fe3O4 NPs were hydrogenated into the corresponding polyethylene-grafted Fe3O4 NPs (PE-g-Fe3O4 NPs). The nanocomposites of PE with Fe3O4 NPs were successfully synthesized as evidenced by the morphologies that the Fe3O4 NPs were well dispersed in PE matrix at the scale of nanometer. The nanocomposites displayed the mechanical properties superior to plain PE. More importantly, the nanocomposites were capable of displaying superparamagnetic, thermally-induced shape memory and photothermal properties. Furthermore, the photothermal behavior can be utilized to trigger the shape memory properties in a non-contact fashion.

Low-Temperature Dehydrogenation of Cyclooctane by Using Pincer Iridium Complexes Bearing an N,N′-Diarylated PNCNP Ligand

PNCNP pincer iridium complexes possessing arylnitrene linkers (−N(Ar)−) catalyze transfer dehydrogenation of cyclooctane (COA) under low-temperature conditions, giving 200 turnovers at 125 °C using tert-butylethylene (TBE) and 60 turnovers at 50 °C using cyclohexene (CHE) as a sacrificial hydrogen acceptor. Despite prior reports dealing with the synthetic difficulty of pincer iridium complexes bearing di(isopropyl)phosphoramine sidearms, our design using chemically inert (3,5-disubstituted-aryl)nitrene linkers allowed for the successful complexation with iridium. The structural characterization and computational study revealed that steric congestion provided by arylnitrene linkers destabilizes the resting internal alkene adducts, thereby reducing the overall activation barrier of the rate-limiting C–H addition. The steric congestion also hinders undesirable disproportionation of CHE, allowing for its use as an effective hydrogen acceptor. With their straightforward synthesis coupled with fine tunability through the 3,5-substituents on the aryl groups, the N,N′-diarylated PNCNP ligands presented in this work will be a valuable addition to the highly successful PCP pincer framework.

Alternating Ring-Opening Metathesis Polymerization Promoted by Ruthenium Catalysts Bearing Unsymmetrical NHC Ligands

In this paper, Grubbs- and Hoveyda–Grubbs-type olefin metathesis catalysts featuring N-cyclopentyl/N’-mesityl backbone-substituted N-heterocyclic carbene (NHC) ligands were synthesized. Their propensity to promote the alternating ring-opening metathesis copolymerization (ROMP) of norbornene (NBE) with cyclooctene (COE) or cyclopentene (CPE) was evaluated and compared to that shown by analogous N-cyclohexyl complexes. High degrees of chemoselectivity were achieved in both copolymerizations. The presence of the N-cyclopentyl substituent allowed for the achievement of up to 98% and 97% of alternating diads for NBE-COE and NBE-CPE copolymers, respectively, at low comonomer ratios. Density functional theory (DFT) studies showed that both the sterical and electronic effects of NHC ligands influence catalyst selectivity.

Iridium(I)- and Rhodium(I)-Olefin Complexes Containing an α-Diimine Supporting Ligand.

Iridium(I) complexes of the type IrX(olefin)(α-diimine) (α-diimine = 1,4-bis(2,6-xylyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; X = Cl, I, Me, O2CCF3; olefin = ethylene, cyclooctene (COE)) were synthesized from the readily available precursor [IrCl(COE)2]2. These complexes display unusual 1H NMR spectra and have large UV-vis extinction coefficients. NOESY and HSQC NMR experiments were used to provide rigorous NMR spectral assignments, and IrCl(C2H4)(α-diimine), 1, and IrCl(COE)(α-diimine), 4, were structurally characterized by X-ray crystallography. The related rhodium complex [RhCl(α-diimine)]2, 6, was also synthesized and characterized by NMR and X-ray crystallography. 6 was observed to be in equilibrium with RhCl(C2H4)(α-diimine), 7, under an ethylene atmosphere.

Periodically Functionalized Linear Polyethylene with Tertiary Amino Groups via Regioselective Ring-Opening Metathesis Polymerization

Polymeric materials modified with tertiary amino groups exhibit attractive properties for use in aqueous environments. Ring-opening metathesis polymerization (ROMP) is one of the versatile tools to synthesize functionalized polymers; however, the ROMP of monocyclic cycloolefins with tertiary amino groups is challenging because of the polymerization inhibition by the nitrogen atom coordination to the catalyst center. In this study, the ROMP of tertiary amino-functionalized monocyclic cycloolefins was first accomplished in high yield by utilizing the regioselective ROMP of 3-substituted cycloheptene (CHE) and cyclooctene (COE). This study suggests that the regioselective ROMP, which is achieved by the steric repulsion between the N-heterocyclic carbene ligand of the Grubbs second-generation catalyst and the functional group, might contribute to inhibit the coordination of donor functional groups to the Ru catalytic center, resulting in high monomer conversions. Head-to-tail regioregular polymers with tertiary amino side chains on every seventh and eighth backbone carbons, poly{3-[2-(N,N-dialkylamino)ethoxycarbonyl]-1-cycloolefin}s [P(3DR-CHE)s and P(3DR-COE)s, R = methyl and ethyl], were successfully synthesized. Hydrogenation that followed afforded the regioregular model linear poly[ethylene-co-2-(N,N-dialkylamino)ethyl acrylate] copolymers (alkyl = methyl and ethyl). The obtained novel amino-functionalized polymers exhibited unique behaviors in hydration, crystallization, and coating stability under physiological aqueous environment.

The hydrogenation of cycloalkenes over direct-synthetized well-defined zero-valent Pt nanoparticles incorporated TUD-1 mesoporous material

Well-defined Pt nanoparticles were integrated into the mesoporous material TUD-1 by applying one-pot sol–gel methodology and denoted as Pt-TUD-1. The characterization data of Pt-TUD-1 confirmed the presence of highly dispersed nanoparticles of zero-valent platinum (Pt0) of 5–10 nm average size incorporated into the three-dimensional silica framework. Pt-TUD-1 was used to catalyze the liquid-phase hydrogenation of four cycloalkenes; cyclopentene (CPE), cyclohexene (CXE), cyclooctene (COE), and cyclooctadiene (COD) without the use of any solvents in room temperature. The obtained results showed that cyclopentene is the fastest to be hydrogenated where 57.7% was converted within one hour’s reaction. The activity of Pt-TUD-1 benchmarked not only the commercial Pt nanopowder but also the Pt supported SiO2 and SBA-15. Moreover, Pt-TUD-1 showed high stability and it was recycled successfully for four consecutive runs without activity loss.

Polyolefin/Polyether Alternated Copolymers: Silyl-Modified Polymers as Promising Monocomponent Precursors to Adhesives

α,ω-Bis(trialkoxysilyl) telechelic polyether/polyolefin copolymers were synthesized and evaluated as monocomponent adhesive precursors. The Ru-catalyzed tandem ring-opening insertion-metathesis polymerization (ROIMP) and cross-metathesis (CM) reactions in the presence of a bis(trialkoxysilyl)alkene as chain-transfer agent (CTA) were first explored toward the synthesis of such silyl-modified polymers (SMPs). The one-pot, two-step ROIMP/CM of cyclooctene (COE) and poly(propyleneglycol) diacrylate (PPG(*)) with {(EtO)3Si(CH2)3NHC(O)OCH2CH═}2 (CTAEt) catalyzed by Grubb’s second generation catalyst (Method II) successfully enables the preparation of α,ω-[(EtO)3Si]2-PCOE/PPG(*) alternated copolymers, isolated in 10–100 g. The polyether/polyolefin copolymers were thoroughly characterized by SEC, 1D and 2D NMR, and FTIR spectroscopies and mass spectrometry analyses. The good thermal stability and rheofluidifying profile of the PCOE/PPG* copolymers are maintained following their catalyzed moisture-curing, as revealed by TGA, DSC, and rheology. The corresponding siloxane cross-linked copolymers then demonstrate strain-at-break, elongation-at-break, and wood-adhesion properties greater than those of the benchmarked SMP reference from Bostik, namely, Polyvest E100 (strain-at-break on wood up to 4.0 vs 2.6 MPa, respectively). These characteristics can be tuned according to the PCOE/PPG* ratio and also revealed significantly better than the ones of our previously reported alike siloxane cross-linked PCOE/PPG* copolymers similarly prepared, yet from a one-pot, one-step ROIMP/CM approach (Method I). Such polyolefin/polyether monocomponent SMPs are thus promising adhesive precursors.

Telechelic Carboxyl‐Terminated Polynorbornenes and Copolym‐ers via Chain‐Transfer Ring‐Opening Metathesis Polymerization

AbstractCarboxyl‐terminated polynorbornenes (CTPNB) have been prepared via ring‐opening metathesis polymerization (ROMP) catalyzed by Grubbs II catalyst (G2) in the presence of maleic acid (MA) as a chain transfer agent (CTA). The influences of reaction conditions on the molecular weight of products, including molar ratios of C=C/G2, C=C/CTA and amount of solvent were investigated. The results indicated that the molecular weight of CTPNBs could be controlled by varying the molar ratio of C=C/G2 or C=C/CTA. In addition, norbornene (NB) was copolymerized with cyclooctene (COE) in order to obtain carboxyl‐terminated copolymers via chain‐transfer ROMP. The unsaturated carboxyl‐terminated (co)polymers were further hydrogenated by using p‐toluenesulfonyl hydrazide/tri‐n‐propylamine reagents. The structures of polymers were characterized by FT‐IR, NMR and MALDI‐TOF mass spectroscopy, and their thermal properties were determined by DSC and TGA.

Thermoregulated Phase-Transfer Pd Nanocatalyst for Selective Hydrogenation of 1,5-Cyclooctadiene at Atmospheric Hydrogen Pressure

Thermoregulated phase-transfer Pd nanocatalyst was firstly employed in the selective hydrogenation of 1,5-cyclooctadiene (1,5-COD) at atmospheric hydrogen pressure. Under the optimal conditions, > 99% conversion and 95% selectivity to cyclooctene (COE) were obtained. The Pd nanocatalyst can be easily separated from products and directly reused. The turnover frequency (TOF) of 1411 h−1 is the highest value ever reported among Pd nanocatalysts at atmospheric hydrogen pressure.

Synthesis and reactivity at the Ir-MeTpm platform: from κ1-N coordination to κ3-N-based organometallic chemistry.

Reaction of [Ir(μ-Cl)(COE)2]2 (COE = cis-cyclooctene) with tris(3,5-dimethylpyrazol-1-yl)methane (MeTpm) affords [IrCl(κ1-N-MeTpm)(COD)] (1) (COD = 1,5-cyclooctadiene). The formation of 1 implies the transfer dehydrogenation of a COE ligand to give COD and COA (cyclooctane). A mechanistic proposal based on DFT calculations that explains this iridium promoted process has been disclosed. Additionally, reactivity studies have allowed the preparation and characterization, including determination of the molecular structures of a number of iridium complexes with the MeTpm ligand in κ1, κ2 or κ3-N coordination modes. Moreover, the first example of an Ir-cyclooctyl complex featuring hydride and carbonyl ligands, whose solid state structure has been determined by X-ray diffraction methods, is reported.

In-situ activation of Pd-TUD-1 during the selective reduction of 1,5-cyclooctadiene

Abstract Here, different loading of palladium (Pd), ranged from 0.5 to 5 wt%, was incorporated into the three dimensional siliceous mesoporous material; TUD-1. The synthesis procedure was carried out without the use of solvents, surfactants, and/or copolymers. Only triethanolamine and water with the precursors of Si and Pd were applied in one-pot synthesis procedure to prepare Pd-TUD-1 materials. The prepared materials were characterized by means of elemental analysis, XRD, Raman spectroscopy, N2 sorption measurements, SEM and TEM. Characterization data confirmed the formation of PdO particles inside the pores of TUD-1 matrix, moreover bulky PdO particles were also detected as an extra-framework separated phase in the high loading samples. The prepared samples were divided into two batches, the first was activated by hydrogenation at 300 °C for 3 h, and the second batch did not activated. The catalytic activity of the two batches was examined in the selective hydrogenation of 1,5-cyclooctadiene (COD) at 80 °C under solvent-free condition to produce cyclooctene (COE). The obtained results showed that the samples of the two batches were able to catalyze the reaction with a very comparable activity. The non-hydrogenated sample with a 0.5% Pd gave 98.5% COD conversion with a COE selectivity of 92.8%, while the same sample which pre-hydrogenated gave 96.8% of COD conversion and 93.5% selectivity towards COE. The characterization of the used catalysts confirmed that PdO was in-situ reduced to Pd nanoparticles during the reaction.

α,ω-Epoxide, Oxetane, and Dithiocarbonate Telechelic Copolyolefins: Access by Ring-Opening Metathesis/Cross-Metathesis Polymerization (ROMP/CM) of Cycloolefins in the Presence of Functional Symmetric Chain-Transfer Agents.

Epoxide- and oxetane-α,ω-telechelic (co)polyolefins have been successfully synthesized by the tandem ring-opening metathesis polymerization (ROMP)/cross-metathesis (CM) of cyclic olefins using Grubbs’ second-generation catalyst (G2) in the presence of a bifunctional symmetric alkene epoxide- or oxetane-functionalized chain-transfer agent (CTA). From cyclooctene (COE), trans,trans,cis-1,5,9-cyclododecatriene (CDT), norbornene (NB), and methyl 5-norbornene-2-carboxylate (NBCOOMe), with bis(oxiran-2-ylmethyl) maleate (CTA 1), bis(oxetane-2-ylmethyl) maleate (CTA 2), or bis(oxetane-2-ylmethyl) (E)-hex-3-enedioate (CTA 3), well-defined α,ω-di(epoxide or oxetane) telechelic PCOEs, P(COE-co-NB or -NBCOOMe)s, and P(NB-co-CDT)s were isolated under mild operating conditions (40 or 60 °C, 24 h). The oxetane CTA 3 and the epoxide CTA 1 were revealed to be significantly more efficient in the CM step than CTA 2, which apparently inhibits the reaction. Quantitative dithiocarbonatation (CS2/LiBr, 40 °C, THF) of an α,ω-di(epoxide) telechelic P(NB-co-CDT) afforded a convenient approach to the analogous α,ω-bis(dithiocarbonate) telechelic P(NB-co-CDT). The nature of the end-capping function of the epoxide/oxetane/dithiocarbonate telechelic P(NB-co-CDT)s did not impact their thermal signature, as measured by DSC. These copolymers also displayed a low viscosity liquid-like behavior and a shear thinning rheological behavior.

Azlactone Telechelic Polyolefins as Precursors to Polyamides: A Combination of Metathesis Polymerization and Polyaddition Reactions

Expanding on our strategy to design telechelic polyolefins through the catalyzed ring-opening metathesis polymerization/cross metathesis/ring-closing metathesis polymerization of cyclic olefins using bifunctional symmetric alkene chain transfer agents (CTAs), we have now explored the metathesis of cyclooctene (COE), 1,5,9-cyclododecatriene (CDT), and norbornene (NB), using azlactone (AZL)-functional CTAs, toward the synthesis of original AZL telechelic polyolefins. The monofunctional 2-vinyl-4,4-dimethylazlactone predominantly gives, as anticipated, (isomerized) monofunctional and/or difunctional PCOEs. On the other hand, the new symmetric AZL-based CTAs 1–3, namely, (E)-2,2′-(ethene-1,2-diyl)bis(4,4-dimethyloxazol-5(4H)-one) (1), (E)-2,2′-(but-2-ene-1,4-diyl)bis(4,4-dimethyloxazol-5(4H)-one) (2), and (E)-2,2′-(but-2-ene-1,4-diyl)bis(3-oxa-1-azaspiro[4.5]dec-1-en-4-one) (3), selectively give α,ω-di(AZL) telechelic PCOEs, along with minor amounts of cyclic PCOE. The synthesis of related AZL telechelic CDT ...

Imidazole end-functionalized polycyclooctenes from chain-transfer ring-opening metathesis polymerization and aminolysis reactions

Imidazole end-functionalized polycyclooctene derivatives were synthesized using a two-step procedure; combining ring-opening metathesis polymerization (ROMP)/cross-metathesis (CM) and aminolysis reaction. ROMP/CM of cyclooctene (COE) in the presence of methyl-10-undecenoate (MU) as a chain transfer agent in the presence of Grubbs first generation catalyst (G1) at 40 °C afforded mono ester end-functionalized (MF) as the major and bis ester end-functionalized (DF) polycyclooctene as the minor product. No isomerized mono ester end-functionalized (IMF) polycyclooctene was formed during G1 catalyzed ROMP/CM reactions. The post-polymerization modification of MF in the presence of 1-(3-aminopropyl) imidazole and different catalysts (Sn(Oct)2, Ti(OiPr)4 and triazabicyclodecene (TBD) in THF at 70 °C afforded imidazole end-functionalized polyolefins in excellent yields. All polymers were characterized by means of MALDI ToF-MS, 1H and 13C NMR spectrometry and Size Exclusion Chromatography (SEC) analyses. The solvent selectivity and catalyst screening experiments were carried out for both ROMP/CM and aminolysis reactions to determine the optimum reaction conditions.

Transfer Hydrogenation of Alkenes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex: Scope and Mechanism.

The first general catalytic approach to effecting transfer hydrogenation (TH) of unactivated alkenes using ethanol as the hydrogen source is described. A new NCP-type pincer iridium complex (BQ-NCOP)IrHCl containing a rigid benzoquinoline backbone has been developed for efficient, mild TH of unactivated C-C multiple bonds with ethanol, forming ethyl acetate as the sole byproduct. A wide variety of alkenes, including multisubstituted alkyl alkenes, aryl alkenes, and heteroatom-substituted alkenes, as well as O- or N-containing heteroarenes and internal alkynes, are suitable substrates. Importantly, the (BQ-NCOP)Ir/EtOH system exhibits high chemoselectivity for alkene hydrogenation in the presence of reactive functional groups, such as ketones and carboxylic acids. Furthermore, the reaction with C2D5OD provides a convenient route to deuterium-labeled compounds. Detailed kinetic and mechanistic studies have revealed that monosubstituted alkenes (e.g., 1-octene, styrene) and multisubstituted alkenes (e.g., cyclooctene (COE)) exhibit fundamental mechanistic difference. The OH group of ethanol displays a normal kinetic isotope effect (KIE) in the reaction of styrene, but a substantial inverse KIE in the case of COE. The catalysis of styrene or 1-octene with relatively strong binding affinity to the Ir(I) center has (BQ-NCOP)IrI(alkene) adduct as an off-cycle catalyst resting state, and the rate law shows a positive order in EtOH, inverse first-order in styrene, and first-order in the catalyst. In contrast, the catalysis of COE has an off-cycle catalyst resting state of (BQ-NCOP)IrIII(H)[O(Et)···HO(Et)···HOEt] that features a six-membered iridacycle consisting of two hydrogen-bonds between one EtO ligand and two EtOH molecules, one of which is coordinated to the Ir(III) center. The rate law shows a negative order in EtOH, zeroth-order in COE, and first-order in the catalyst. The observed inverse KIE corresponds to an inverse equilibrium isotope effect for the pre-equilibrium formation of (BQ-NCOP)IrIII(H)(OEt) from the catalyst resting state via ethanol dissociation. Regardless of the substrate, ethanol dehydrogenation is the slow segment of the catalytic cycle, while alkene hydrogenation occurs readily following the rate-determining step, that is, β-hydride elimination of (BQ-NCOP)Ir(H)(OEt) to form (BQ-NCOP)Ir(H)2 and acetaldehyde. The latter is effectively converted to innocent ethyl acetate under the catalytic conditions, thus avoiding the catalyst poisoning via iridium-mediated decarbonylation of acetaldehyde.

Nitrogen‐Doped Carbon Xerogels Supporting Palladium Nanoparticles for Selective Hydrogenation Reactions: The Role of Pyridine Nitrogen Species

AbstractNitrogen‐doped carbon xerogels (NDCX) functionalized with various nitrogen functionalities are synthesized and used as supports for Pd nanoparticles (NPs) for selective hydrogenation reactions. The nitrogen doping not only induces the formation of surface basic sites that can connect with the Pd NPs by donating electrons to the unoccupied d orbital of Pd atoms and improve the Pd dispersion, but it also leads to C–N–C hybrid rings that can interact with substrates containing electron‐rich carbon rings, increasing the reaction activity. So far, the Pd/NDCX shows >95 % 1,5‐cyclooctadiene (COD) conversion and cyclooctene (COE) selectivity within 90 min for selective hydrogenation of COD to COE, and the activity can be recovered after washing the catalyst. Moreover, the catalyst can be applied to various substrates containing C=C and/or −NO2 groups with considerable conversion and selectivity. Experimental results and DFT theoretical calculations suggest that the pyridinic nitrogen plays an important role in the reaction, as it is not only the adsorption site of Pd NPs for H2 activation, but it also serves as the active site for COD activation. On these bases, a mechanism for the reaction is proposed.