E-selectivity Research Articles

Evaluation of Micro-Mechanism and High- and Low-Temperature Rheological Properties of Disintegrated High Volume Crumb Rubber Asphalt (DHVRA).

The aims of this paper are to prepare disintegrated high volume crumb rubber asphalt (DHVRA) with low viscosity, good workability and low-temperature performance by adding disintegrating agent (DA) in the preparation process, and to further analyze the disintegrating mechanism and evaluated high-temperature and low-temperature rheological properties. To obtain DHVRA with excellent comprehensive performance, the optimum DA dosage was determined. Based on long-term disintegrating tests and the Fluorescence Microscopy (FM) method, the correlations between key indexes and crumb rubber (CR) particle diameter was analyzed, and the evaluation indicator and disintegrating stage division standard were put forward. Furthermore, Fourier transform infrared spectroscopy (FT-IR) and Gel Permeation Chromatography (GPC) was used to reveal the reaction mechanism, and the contact angle test method was adopted to evaluate the surface free energy (SFE). In addition, the high-temperature and low-temperature rheological properties were measured, and the optimum CR content was proposed. Results indicated that the optimum DA dosage was 7.5‰, and the addition of DA promoted the melt decomposition of CR, reduced the viscosity and improved the storage stability. The 135 °C rotational viscosity (RV) of DHVRA from mixing for 3 h could be reduced to 1.475 Pa·s, and the softening point difference was even less than 2 °C. The linear correlation between 135 °C RV and the diameter of CR particle in rubber asphalt system was as high as 0.968, and the viscosity decay rate (VDR) was used as the standard to divide the disintegrating process into a fast disintegrating stage, stable disintegrating stage and slight disintegrating stage. Compared to common rubber asphalt (CRA), DHVRA has an absorption peak at 960 cm−1 caused by trans olefin = C-H, and higher molecular weight and polar component of surface energy. Compared with CRA, although the high-temperature performance of DHVRA decreases slightly, the low-temperature relaxation ability can be greatly improved.

Terminal alkyne semi-tritiation with Lindlar catalyst and its mechanism implications

It has been nearly 70 years since the discovery of Lindlar catalyst and its valuable ability to semi-hydrogenate alkynes to cis olefins. The Lindlar catalyst semi-hydrogenation mechanism appears to still be an ongoing discussion. Spectroscopy (NMR, MS) results are presented regarding the Lindlar catalyst semi-tritiation of terminal alkynes, providing evidence of a novel tritium for hydrogen exchange process taking place.

Photoreactivity of Oil and Alcoholic Diterpenic Varnishes: Key Role of the Polymerization Process

AbstractThe conservation of artworks is a key issue in cultural heritage. The alterations of the varnish could be linked mainly to the photopolymerization of the natural resin. The photodegradation of sandarac and rosin under simulated solar light was studied using a multianalytical approach using a second derivative FT‐IR spectroscopy and gas chromatography coupled to mass spectroscopy. In all resins, main terpene molecules underwent esterification and perestererifications via the formation of hydroperoxides preferentially formed on tertiary carbons. This polymerization could be associated with the advice of important art manuals and treatises recommending exposing varnishes to the sunlight while drying. In a second step, these esters and perester bonds underwent photoscissions by Norrish I and II reactions, leading to the production of free terpenes in the varnishes. In sandarac varnishes, the vinyl bonds showed a strong reactivity associated with the formation of trans and cis secondary alkenes. These photochemical reactions could explain an important part of the problem of conservation of the varnish layers experienced by museums all over the world.

Singlet and triplet cyclonona‐3,5,7‐trienylidenes and their α, ά‐halogenated derivatives at DFT

AbstractSinglet and triplet boat‐shaped cyclonona‐3,5,7‐trienylidene (1H‐S and 1H‐T) and their corresponding α, ά‐tetrahalo derivatives (1F‐S, 1F‐T, 1Cl‐S, 1Cl‐T, 1Br‐S, 1Br‐T, 1I‐S, and 1I‐T) are compared at M06‐2X/AUG‐cc‐pVDZ, B3LYP/AUG‐cc‐pVTZ, and B3LYP/6‐311++G** levels of theory. Apart from 1H‐S which turns out as a transition state for having a negative force constant, the other structures appear as minima on their potential energy surfaces. 1H, 1Cl, and 1Br carbenes have triplet ground states, while 1F and 1I which initially take on a singlet multiplicity and eventually transform to a more stable intramolecular [1 + 2] cycloaddition product, and trans alkene, respectively. Except for 1H‐T and 1F‐T carbenes, the other triplet states emerge as allene isomer after optimization. We conclude that halogen substitution increases stability based on absolute value of singlet‐triplet energy separation (|ΔES‐T|), in the following order 1Br < 1H ≤ 1Cl < 1I < 1F at B3LYP/AUG‐cc‐pVTZ, and 1Br < 1Cl < 1H < 1I < 1F at M06‐2X/AUG‐cc‐pVDZ. Nucleophilicity of the carbenic center is decreased due to higher electronegativity of substituted halogens. The scrutinized carbenes 1Cl‐S, 1Br‐S, and 1I‐S show higher nucleophilicity and higher electrophilicity than the synthesized common NHCs (2‐6). Hence, compared with 2‐6 carbenes, these halogenated carbenes exhibit ambiphilic character owing to the presence of a σ2‐orbital and vacant 2pπ‐orbital that makes them theoretically more susceptible for attacking an electrophile or a nucleophile.

Synthesis and Preliminary Biological Evaluation of Two Fluoroolefin Analogs of Largazole Inspired by the Structural Similarity of the Side Chain Unit in Psammaplin A.

Largazole, isolated from a marine Cyanobacterium of the genus Symploca, is a potent and selective Class I HDAC (histone deacetylation enzymes) inhibitor. This natural 16-membered macrocyclic depsipeptide features an interesting side chain unit, namely 3-hydroxy-7-mercaptohept-4-enoic acid, which occurs in many other natural sulfur-containing HDAC inhibitors. Notably, one similar fragment, where the amide moiety replaces the trans alkene moiety, appears in Psammaplin A, another marine natural product with potent HDAC inhibitory activities. Inspired by such a structural similarity, we hypothesized the fluoroolefin moiety would mimic both the alkene moiety in Largazole and the amide moiety in Psammaplin A, and thus designed and synthesized two novel fluoro olefin analogs of Largazole. The preliminary biological assays showed that the fluoro analogs possessed comparable Class I HDAC inhibitory effects, indicating that this kind of modification on the side chain of Largazole was tolerable.

Photoredox-Mediated Ring-Opening Metathesis Polymerization: Methods, Scope, and Scalability

Photoredox-mediated ring-opening metathesis polymerization (PhotoROMP) is a novel method for the preparation of ROMP polymers. As opposed to metal-mediated ROMP, which requires metal initiators, PhotoROMP utilizes organic photo-oxidants and vinyl ether initiators. In a recent study, a variety of pyrylium and thiopyrylium photo-oxidants were evaluated, demonstrating that milder photo-oxidants achieved higher conversions, and that thiopyrylium salts generally out-performed pyrylium salts. Further studies showed that polyolefins with polar functionality, such as protected alcohols and halides, can be prepared via PhotoROMP. This functionality could be leveraged for post-polymerization processing and functionalization. An evaluation of the stereochemistry of these polymers demonstrated that they are atactic, with roughly 75% trans olefin content. Ongoing work will probe the scalability of this reaction via flow chemistry. The polymer produced will be injection molded and the mechanical properties of PhotoROMP polymers and copolymers will be determined. Figure 1

Photoinduced Rearrangement of Dienones and Santonin Rerouted by Amines

AbstractThe photoinduced rearrangement pathways of simple 2,5‐dienones and the natural product santonin were found to be effectively rerouted by amines, giving rise to unprecedented products. Either cis olefins or cyclobutenes were obtained from 4,4‐disubstituted 2,5‐dienone upon irradiation (365 nm) in the presence of various amines depending on the solvent. Previously undescribed [4.4.0] and [5.3.0] fused‐ring‐containing products were obtained when santonin was irradiated (365 nm) in the presence of methylamine. The amines present in these reactions were incorporated into the products by means of amide‐group formation.

Photoinduced Rearrangement of Dienones and Santonin Rerouted by Amines.

The photoinduced rearrangement pathways of simple 2,5-dienones and the natural product santonin were found to be effectively rerouted by amines, giving rise to unprecedented products. Either cis olefins or cyclobutenes were obtained from 4,4-disubstituted 2,5-dienone upon irradiation (365 nm) in the presence of various amines depending on the solvent. Previously undescribed [4.4.0] and [5.3.0] fused-ring-containing products were obtained when santonin was irradiated (365 nm) in the presence of methylamine. The amines present in these reactions were incorporated into the products by means of amide-group formation.

Synthesis of the molecular hybrid inspired by Largazole and Psammaplin A

One important class of HDAC (histone deacetylation enzymes) inhibitors is the sulfur-containing marine natural products with structural diversity. Inspired by two structurally distinguishing examples, Largazole and Psammaplin A, which possess macrocyclic depsipeptide and simple linear amide scaffold respectively, we designed one novel molecular hybrid by replacing the alkene moiety in Largazole with a semirigid amide bond. This hybrid compound has been synthesized from l-malic acid in 10 steps with an overall yield of 7%. The preliminary biological assays suggest that the replacement of trans olefin moiety with amide bond will lead to an unbeneficial effect on the inhibition against HDACs.

Investigation of Tacticity and Living Characteristics of Photoredox-Mediated Metal-Free Ring-Opening Metathesis Polymerization.

This study investigated the microstructures of polymers produced via photoredox-mediated metal-free ring-opening metathesis polymerization. Polynorbornene, poly(exo-dihydrodicyclopentadiene), and poly(endo-dicyclopentadiene) were found to have cis olefin contents of 23%, 24%, and 28%, respectively. Additionally, the cis/trans ratio remained consistent during the course of norbornene polymerization. Polymer tacticity was evaluated by quantitative 13 C NMR spectroscopy, which revealed each polymer to be largely atactic. Specifically, the three polymers were estimated to be 33%, 58%, and 55% syndiotactic, respectively. In parallel, this study also explored the ability to produce diblock copolymers from norbornene and exo-dihydrodicyclopentadiene. Successful diblock copolymerization was achieved using either monomer order. In each case, however, the results suggested to us that chain-chain coupling (increased molecular weight) and irreversible termination (dead chains observed during attempted chain extension) occurred when reaction times were extended.

Fast-Initiating, Ruthenium-based Catalysts for Improved Activity in Highly E-Selective Cross Metathesis.

Ruthenium-based olefin metathesis catalysts bearing dithiolate ligands have been recently employed to generate olefins with high E-selectivity (>99% E) but have been limited by low to moderate yields. In this report, 1H NMR studies reveal that a major contributing factor to this low activity is the extremely low initiation rates of these catalysts with trans olefins. Introducing a 2-isopropoxy-3-phenylbenzylidene ligand in place of the conventional 2-isopropoxybenzylidene ligand resulted in catalysts that initiate rapidly under reaction conditions. As a result, reactions were completed in significantly less time and delivered higher yields than those in previous reports while maintaining high stereoselectivity (>99% E).

High Trans Kinetic Selectivity in Ruthenium-Based Olefin Cross-Metathesis through Stereoretention.

The first kinetically controlled, highly trans-selective (>98%) olefin cross-metathesis reaction is demonstrated using Ru-based catalysts. Reactions with either trans or cis olefins afford products with highly trans or cis stereochemistry, respectively. This E-selective olefin cross-metathesis is shown to occur between two trans olefins and between a trans olefin and a terminal olefin. Additionally, new stereoretentive catalysts have been synthesized for improved reactivity.

Selective Stereochemical Catalysis Controlled by Specific Atomic Arrangement of Ordered Alloys

AbstractUnprecedented surface stereochemistry governed by specific atomic arrangement of intermetallic compounds (ordered alloys) is shown. Hydrogen‐mediated cis–trans alkene isomerization is selectively catalyzed without overhydrogenation to alkanes by Rh‐based intermetallic compounds displaying an orthorhombic Pnma structure such as RhSb. Multiple characterization techniques combined with DFT calculations revealed that (2 1 1), (0 2 0), and (0 1 3) planes comprising one‐dimensionally aligned Rh rows separated by Sb atoms (1D‐planes) dominated RhSb surfaces. Systematic DFT calculations indicated that geometric constraints of these 1D‐planes and steric hindrance from one alkyl group of cis‐alkene limited hydrogen access to the alkenyl carbon to one direction, enabling one‐atom hydrogenation for isomerization, but inhibiting two‐atom hydrogenation for overhydrogenation.

Formation of Ruthenium Carbenes by gem‐Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans‐Hydrogenation

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para-hydrogen (p-H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter-ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over-reduction, which frequently interfere with regular alkyne trans-hydrogenation.

Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-Hydrogenation.

Insights into the mechanism of the unusual trans‐hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para‐hydrogen (p‐H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans‐reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter‐ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over‐reduction, which frequently interfere with regular alkyne trans‐hydrogenation.

ChemInform Abstract: Regio‐ and Steroselective Cross‐Metathesis of Silylacetylenes with Terminal Olefins and α,ω‐Dienes.

Abstract2‐Silyl‐1,3‐dienes (III) and 2‐silyl‐1,3,n‐trienes (V) are efficiently obtained with complete E‐selectivity.

ChemInform Abstract: Synthesis of Ketenaminals via Catalyst‐Free Hydroamination of Ynamides and Diphenylsulfonimide.

AbstractThe scalable transformation of aryl ynamides proceeds with excellent E selectivity whereas alkyl ynamides form mixtures of E/Z isomers.

Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst.

A new bis-facial dinuclear ruthenium complex, {[Ru(II)(bpy)]2(μ-bimp)(μ-Cl)}(2+), 2(2+), containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic, and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The in situ generated bis-aqua complex, {[Ru(II)(bpy)(H2O)]2(μ-bimp)}(3+), 3(3+), is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min(-1), are achieved using PhIO as oxidant. Moreover, 3(3+) presents an outstanding stereospecificity for both cis and trans olefins toward the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on density functional theory (DFT) calculations in order to better understand the putative cooperative effects within this dinuclear catalyst.

Isomerization of Olefins Triggered by Rhodium‐Catalyzed CH Bond Activation: Control of Endocyclic β‐Hydrogen Elimination

AbstractFive‐membered metallacycles are typically reluctant to undergo endocyclic β‐hydrogen elimination. The rhodium‐catalyzed isomerization of 4‐pentenals into 3‐pentenals occurs through this elementary step and cleavage of two CH bonds, as supported by deuterium‐labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β‐hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.

Isomerization of Olefins Triggered by Rhodium-Catalyzed C-H Bond Activation: Control of Endocyclic β-Hydrogen Elimination.

Five-membered metallacycles are typically reluctant to undergo endocyclic β-hydrogen elimination. The rhodium-catalyzed isomerization of 4-pentenals into 3-pentenals occurs through this elementary step and cleavage of two C–H bonds, as supported by deuterium-labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β-hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.