E-selectivity Research Articles

Silver-Catalyzed Olefination of Aryl Aldehydes Using Propiolates.

Olefination of aldehydes is one of the fundamental reactions in organic synthesis. The commonly used Wittig olefination reaction however uses stoichiometric quantities reagents under basic conditions resulting in stoichiometric amounts of byproducts. Known catalytic alternate to the Wittig reaction requires stoichiometric amounts of silane reducing agents and high temperature. Herein, we report a base-free olefination of aryl aldehydes using propiolates as a surrogate for the Witting reagent under silver catalysis. Trimethyl orthoformate, in the presence of a silver catalyst adds to the alkynoate to form the nucleophilic silver allenolate which reacts with the reactive oxocarbenium ion formed from aldehyde under the reaction conditions. Subsequently, decarbonylation occurs to form the olefin. Trans olefin is formed exclusively from simple aryl aldehydes and cinnamaldehydes. Such a silver allenolate is conceptually novel and has not been explored so far.

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates.

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

Stereodivergent sila-germylenation vs. sila-stannylenation of an internal alkyne.

We report on the insertion of electron deficient alkyne, dimethyl acetylene dicarboxylate (DMAD), into the E-Si bond of hypersilyl tetrylenes, PhC(NtBu)2ESi(SiMe3)3 (E = Ge and Sn), at room temperature. Uniquely, the germylene leads to cis alkenes, while the stannylene gives access to trans alkenes, and the insight into divergent stereoselectivity has been obtained by DFT studies.

Lewis acid catalyzed cis (liquid) to trans (solid) isomerization of Jojoba oil in supercritical CO2

Isomerization of the olefin from cis to trans configuration is often a desirable reaction pathway that leads to products with different properties and chemical reactivities. In this work, we report a novel process involving the use of boron trifluoride diethyl etherate, BF3·O(C2H5)2, in supercritical CO2 that converts a compound with an isolated cis to a trans olefin in quantitative yield. No polymerization was observed. The utility of this reaction has been demonstrated by the conversion of jojoba oil (a natural wax ester, comprising a monoester of long-chain fatty acids with cis olefins) to its trans isomers. Jojoba oil is liquid at room temperature with a melting point of about 10 °C; the reacted product is a solid with a melting point of 37.6 °C (around the normal human body temperature). The simplicity of the reaction and the use of supercritical CO2 are the attractive features of this reaction.

Carbonyl Oxide Stabilization from Trans Alkene and Terpene Ozonolysis.

The pressure dependence of carbonyl oxide (Criegee intermediate) stabilization can be measured via H2SO4 detection using chemical ionization mass spectrometry. By selectively scavenging OH radicals in a flow reactor containing an alkene, O3, and SO2, we measure an H2SO4 ratio related to the Criegee intermediate stabilization, and by performing experiments at multiple pressures, we constrain the pressure dependence of the stabilization. Here, we present results from a set of monoterpenes as well as isoprene, along with previously published results from tetramethylethylene and a sequence of symmetrical trans alkenes. We are able to reproduce the observations with a physically sensible set of parameters related to standard pressure falloff functions, providing both a consistent picture of the reaction dynamics and a method to describe the pressure stabilization following ozonolysis of all alkenes under a wide range of atmospheric conditions.

Sustainable production of eco-friendly rubberized asphalt binders through chemically crosslinking with polymer modifier

The incorporation of waste tire rubber into asphalt pavement can facilitate the development of sustainable transportation infrastructure. Recently, terminal blend (TB) rubberized asphalt incorporating polymers has gained popularity due to its improved performance and eco-friendliness. However, the crosslinking mechanism between terminal blend (TB) rubberized asphalt and supplemented polymer modifier is not well understood. The present paper aims to investigate the crosslinking mechanism between degraded rubber, styrene-butadiene-styrene (SBS) polymer and crosslinking agent, and to clarify the effect of crosslinking reaction on the properties and production of eco-friendly rubberized asphalt. In this paper, Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), dynamic shear rheological (DSR), multiple stress creep recovery (MSCR) and bending beam rheometer (BBR) tests were employed to investigate the chemical and physical properties of TB rubberized asphalt after incorporating various contents of SBS and crosslinking agent. FTIR test indicates that the conjugated olefins released from crumb rubber have the possibility to react with sulfur. However, sulfur alone fails to greatly improve the rheological performance of TB rubberized asphalt, for the level of crosslinking reaction between sulfur and TB rubberized asphalt is weak. Therefore, it is necessary to further add SBS polymer to enhance the crosslinking reaction. Further study showed that there are two types of crosslinking reactions when involving conjugated olefins. Type I is the reaction of sulfur with conjugated olefins provided by the degraded rubber, which helps the conjugated olefins to crosslink themselves, resulting in increase of the amount of trans olefins. Type II is that sulfur bridges the SBS polymer and degraded rubber through the chemical link between the opened trans olefins double bonds located in SBS and opened conjugated olefins from degraded rubber, which leads to reduction of the amount of trans olefins. Type II crosslinking reaction can effectively improve the high temperature performance of TB rubberized asphalt. The findings of this study not only provides valuable insights into the interaction mechanism of eco-friendly rubberized asphalt, but also promote the recycling of waste tire rubber.

Metal-Free Intermolecular Allylic C–H Amination of Alkenes Using Primary Carbamates

The allylic C–H amination of alkenes is a powerful method for the regioselective formation of new C–N bonds. Though many methods for introducing a wide range of nitrogen groups intermolecularly have been developed, few of these utilize carbamates, which are an important class of bioactive compounds and are commonly used as protecting groups for free amines. Here, we report a convenient metal-free protocol for intermolecular allylic C–H amination using unactivated primary carbamates as the nitrogen source. A sterically hindered NHC-selenium catalyst was found to be critical to obtaining high yields. A wide range of trisubstituted and 1,1-disubstituted alkenes were regioselectively aminated, including terpenoid natural products. A range of common carbamate-protecting groups such as Cbz, Teoc, and Alloc could be successfully incorporated into alkene substrates. Additionally, trifluoroacetamide was shown to be a viable nitrogen source. The observed regio- and stereoselectivities can be explained by a sequential ene reaction/[2,3]-sigmatropic rearrangement mechanism in which cis C–H bonds are preferentially activated, but subsequent rearrangement results in selective formation of trans alkene products.

Silk sericin conjugated magnesium oxide nanoparticles for its antioxidant, anti-aging, and anti-biofilm activities

The Silk sericin protein was conjugated with magnesium oxide (MgO) nanoparticles to form SS-MgO-NPs . UV, XRD, FTIR, SEM, DLS, and EDX were used to confirm the formation of SS-MgO-NPs. The absorption band of SS-MgO-NPs using UV–visible spectra was observed at 310 nm, with an average size of the nanoparticles was 65–88 nm analyzed from DLS. The presence of alcohol, CN, and CC, alkanes, alkenes, and cis alkenes, in silk sericin, is confirmed by FT-IR and may act as a stabilizing agent. Later SS-MgO-NPs were evaluated for antioxidant, antibacterial, anti-biofilm, ,anti-aging, and anticancer properties. The SS-MgO-NPs inhibited the formation of biofilm of Pseudomonas aeruginosa and Bacillus cereus. The blood compatibility of SS-MgO-NPs, delaying coagulation was observed using human, blood, and goat blood samples. The SS-MgO-NPs exhibited significant anticancer activity on MCF-7 (IC50 207.6 μg/mL) cancer cell lines. Correspondingly, SS-MgO-NPs demonstrated dose-dependent inhibition of the enzymes in the following order collagenase > elastase > tyrosinase > hyaluronidase, with IC50 values of 75.3, 85.3, 133.6, and 156.3 μgmL−1, respectively. This exhibits the compoundposses anti-aging properties. So, in in vitro settings, SS-MgO-NPs can be used as an antibacterial, anti-aging, and anticancer agent. Additionally, in vivo research is necessary to validate its therapeutic applications.

Molecular conformation evolutions of trans HNS under high pressure

The molecular conformation evolution of Hexanitrostilbene (HNS) under high pressure was systematically investigated using Raman and Fourier transform infrared (FTIR) spectroscopy. The vibration modes of HNS associated with C-H, nitro groups, CC and the ring have been analyzed and clarified in detail under ambient conditions. trans-HNS is symmetrically distributed about –CHCH–, and six nitro groups are symmetrically distributed under ambient conditions. Two molecular conformation changes of HNS were observed at 1.4 GPa and 5 GPa due to the variations of hydrogen-bond interaction between C-H (in the ring) and N-O and the distortion of trans olefin, respectively. The hydrogen-bond interaction between C-H (in the ring) and N-O strengthened at 1.4 GPa. It induced the degenerated symmetry of the nitro groups and the Raman changes of νas (NO2), ν(CC), ν(C-C) and ν(C-H). In addition, the nonplanarity property of HNS and the sensitivity of trans olefin to pressure promoted the deformation of trans olefin, as well as the hydrogen bond interaction between C-H (in trans olefin) and N-O at about 5 GPa. When further loading pressure on HNS, the variations in the hydrogen-bond interaction between C-H and N-O restricted the vibrations of C-H, NO2 and the ring. It blocked the nonradiative pathway and activated the strong fluorescent background in the Raman spectra as the pressure increased above 5.7 GPa. These current results reveal that there is no structural transformation and only conformational changes under high pressure for HNS.

Bench-Stable Dinuclear Mn(I) Catalysts in E-Selective Alkyne Semihydrogenation: A Mechanistic Investigation.

Dinuclear manganese hydride complexes of the form [Mn2 (CO)8 (μ-H)(μ-PR2 )] (R=Ph, 1; R=iPr, 2) were used in E-selective alkyne semi-hydrogenation (E-SASH) catalysis. Catalyst speciation studies revealed rich coordination chemistry and the complexes thus formed were isolated and in turn tested as catalysts; the results underscore the importance of dinuclearity in engendering the observed E-selectivity and provide insights into the nature of the active catalyst. The insertion product obtained from treating 2 with (cyclopropylethynyl)benzene contains a cis-alkenyl bridging ligand with the cyclopropyl ring being intact. Treatment of this complex with H2 affords exclusively trans-(2-cyclopropylvinyl)benzene. These results, in addition to other control experiments, indicate a non-radical mechanism for E-SASH, which is highly unusual for Mn-H catalysts. The catalytically active species are virtually inactive towards cis to trans alkene isomerization indicating that the E-selective process is intrinsic and dinuclear complexes play a critical role. A reaction mechanism is proposed accounting for the observed reactivity which is fully consistent with a kinetic analysis of the rate limiting step and is further supported by DFT computations.

Conformational bias in density functional theory ring strain energy calculations of cyclopentene derivatives: Towards predictive design of chemically recyclable elastomers

AbstractEquilibrium ring opening metathesis polymerization of low strained cycloolefins is opportunistic for the development of novel materials capable of chemical recycling to monomer (CRM). However, many of the potential materials for CRM contain complex side chains complicating predictions of their ring strain energies (RSE). The effects of different conformational considerations on RSE predictions using density functional theory (DFT) are explored. New homodesmotic equations are investigated to capture changes in olefin conformation upon polymerization. The employment ofcis‐2‐butene as a corrective factor with a 2,7‐nonadiene linear analog bearing onecisand onetransolefin (H2cis) resulted in RSEs similar to previously reported ΔHpvalues. Different consideration of possible conformers aside from their lowest energy counterparts leads to a range of predicted RSE values. Similarly, the application of a Boltzmann distribution resulted in negligible differences in RSE. Therefore, RSE predictions using the lowest energy structures with H2ciscalculated at B3LYP/6‐31+G* in toluene is a sufficient approach for predicting RSE of monomers with multiple conformers. This method can be used to screen a monomer's potential for CRM to reduce the time, cost, waste, and effort necessary to research new materials towards a more circular polymer economy.

Stereoselective Synthesis of Trisubstituted Alkenes by Nickel‐Catalyzed Benzylation and Alkene Isomerization

AbstractCatalytic strategies that provide stereoselective access to highly substituted alkenes from abundant monosubstituted substrates are exceedingly sought‐after but rare. Here, we show that a N‐heterocyclic carbene‐NiI catalytic species mediates efficient union of electronically polarized terminal olefins with benzyl chlorides, in the presence of trimethylsilyl triflate and trimethylamine additives, to generate trisubstituted boron‐ and arene‐containing trans alkenes in excellent regio‐ and stereoselectivities. Control experiments provide evidence for a mechanism involving branched‐selective Heck‐type benzylation that overrides substrate control, followed by trans‐selective 1,3‐hydrogen shift. The method represents a significant addition to the toolbox of reactions for the concise synthesis of unsaturated biologically active compounds.

Stereoselective Synthesis of Trisubstituted Alkenes by Nickel-Catalyzed Benzylation and Alkene Isomerization.

Catalytic strategies that provide stereoselective access to highly substituted alkenes from abundant monosubstituted substrates are exceedingly sought-after but rare. Here, we show that a N-heterocyclic carbene-NiI catalytic species mediates efficient union of electronically polarized terminal olefins with benzyl chlorides, in the presence of trimethylsilyl triflate and trimethylamine additives, to generate trisubstituted boron- and arene-containing trans alkenes in excellent regio- and stereoselectivities. Control experiments provide evidence for a mechanism involving branched-selective Heck-type benzylation that overrides substrate control, followed by trans-selective 1,3-hydrogen shift. The method represents a significant addition to the toolbox of reactions for the concise synthesis of unsaturated biologically active compounds.

Synthesis, characterization and catalytic activities of nonheme manganese(III) complexes: Preferential formation of cis olefin oxide owing to steric hindrance

Three mononuclear nonheme MnIII(salophen) complexes, 1a-1c, with tetradentate ligands containing two deprotonated phenolates ([(X2-tert-butyl-salophen)Mn(OAc)(H2O)] (tert-butyl-salophen = N,N′-bis(6-di-tert-butylsalicylidene)-1,2-phenylenediaminato, 1a for X = Cl, 1b for X = H, and 1c for X = CH3)) were synthesized and characterized using 1H NMR, 13C NMR, elemental analysis and ESI-Mass spectrometry. These Mn(III) complexes were used to efficiently catalyze the epoxidation reactions of diverse aliphatic, aromatic and terminal alkenes to form the corresponding epoxides with MCPBA (m-chloroperoxybenzoic acid) as an oxidant under mild conditions. Notably, catalysts 1a-1c preferably react with the cis-alkene because of the steric hindrance between the reactive intermediate MnIII-OOC(O)R and the trans-type substrate. A Hammett study and product analysis using PPAA (peroxyphenylacetic acid) as a mechanistic indicator suggested that the peracid reacted with the Mn(III) complex to generate the MnIII-OOC(O)R intermediate, which underwent both homolysis and heterolysis to form MnIV=O or MnV=O. The reactive MnV=O might participate in the alkene epoxidation with good stereospecificity, whereas the MnIV=O species might trigger radical-type oxidation to produce non-stereospecific by-products, such as ketones and aldehyde. On the other hand, MnIII-OOC(O)R (2) could oxidize the reactive cyclohexene to the epoxide, whereas it was unable to epoxidize the poorly reactive 1-octene.

Living ring‐opening metathesis polymerization of norbornenes bay‐functionalized perylene diimides

AbstractRing‐opening metathesis polymerization (ROMP)‐derived poly(oxanorbornene imide)s bearing bay‐linked mono‐alkoxy‐M1 and 1,7‐di‐alkoxy M2 functionalized perylene diimides (PDIs) were synthesized using Grubb's third (G3) and Hoveyda‐Grubbs second generation (HG2) ruthenium‐alkylidene metathesis initiators. The mono‐alkoxy‐derived PDI‐based non‐ladderphane polymer polyM1 displayed 67% to 77% of the trans olefin content in the polymer chain depending on the initiator used for the polymerization. When using the symmetrical 1,7‐di‐alkoxy‐derived PDI‐based polymer polyM2 having the ladderphane type‐structure, this displayed a significant amount of cis and trans olefin contents in the polymer chains, irrespective of the type of initiators used for the polymerization. ROMP of both monomers M1 and M2 proceeded in a well‐controlled manner with a linear dependence of molecular weight on the monomer/initiator ratio using G3 as initiator. Optical properties of the ladderphane‐based polyM2 and non‐ladderphane‐based polyM1 were characterized in both solution and the film state. X‐ray diffraction (XRD) analysis for all the polymers showed significant π‐stacking in the thin film state with ordered molecular packing and closer values of d‐spacing for both polyM1 and polyM2. Film morphology examined by AFM elucidated homogenous smooth polymer surface for both polymers in general, but with some irregularities observed for polyM1. In addition, CV analysis revealed both polymers could be good candidates as electron‐accepting materials, with excellent film‐forming ability, and thermal stability.

Olefin Functionalization/Isomerization Enables Stereoselective Alkene Synthesis.

Despite tremendous efforts aimed at devising methods for stereoselective alkene synthesis, critical challenges are yet to be addressed. Direct access to a diverse range of 1aryl(boryl)-1-methyl-functionalized tri- and tetrasubstituted trans alkenes, entities that are prevalent in many important molecules of interest, through a catalytic manifold from readily available α-olefin substrates remains elusive. Here, we demonstrate that catalytic amounts of a nonprecious N-heterocyclic carbene-Ni(I) complex in conjunction with a sterically bulky base promote site- and trans-selective union of monosubstituted olefins with a wide array of electrophilic reagents to deliver tri- and tetrasubstituted alkenes in up to 92% yield and >98% regio- and stereoselectivity. The protocol is amenable to the preparation of carbon- and heteroatom-substituted C=C bonds, providing distinct advantages over existing transformations. Utility is highlighted through concise stereoselective synthesis of biologically active compounds.

A Colorimetric Method for Quantifying Cis and Trans Alkenes Using an Indicator Displacement Assay.

A colorimetric indicator displacement assay (IDA) amenable to high-throughput experimentation was developed to determine the percentage of cis and trans alkenes. Using 96-well plates two steps are performed: a reaction plate for dihydroxylation of the alkenes followed by an IDA screening plate consisting of an indicator and a boronic acid. The dihydroxylation generates either erythro or threo vicinal diols from cis or trans alkenes, depending upon their syn- or anti-addition mechanisms. Threo diols preferentially associate with the boronic acid due to the creation of more stable boronate esters, thus displacing the indicator to a greater extent. The generality of the protocol was demonstrated using seven sets of cis and trans alkenes. Blind mixtures of cis and trans alkenes were made, resulting in an average error of ±2 % in the percentage of cis or trans alkenes, and implementing E2 and Wittig reactions gave errors of ±3 %. Furthermore, we developed variants of the IDA for which the color may be tuned to optimize the response for the human eye.

A Colorimetric Method for Quantifying Cis and Trans Alkenes Using an Indicator Displacement Assay

AbstractA colorimetric indicator displacement assay (IDA) amenable to high‐throughput experimentation was developed to determine the percentage of cis and trans alkenes. Using 96‐well plates two steps are performed: a reaction plate for dihydroxylation of the alkenes followed by an IDA screening plate consisting of an indicator and a boronic acid. The dihydroxylation generates either erythro or threo vicinal diols from cis or trans alkenes, depending upon their syn‐ or anti‐addition mechanisms. Threo diols preferentially associate with the boronic acid due to the creation of more stable boronate esters, thus displacing the indicator to a greater extent. The generality of the protocol was demonstrated using seven sets of cis and trans alkenes. Blind mixtures of cis and trans alkenes were made, resulting in an average error of ±2 % in the percentage of cis or trans alkenes, and implementing E2 and Wittig reactions gave errors of ±3 %. Furthermore, we developed variants of the IDA for which the color may be tuned to optimize the response for the human eye.

An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization

AbstractStereochemistry can have a profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal‐mediated ring‐opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal‐free ROMP. Ion‐pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98 %. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (−78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.

An Ion‐Pairing Approach to Stereoselective Metal‐Free Ring‐Opening Metathesis Polymerization

Stereochemistry can have a profound impact on polymer and materials properties. Unfortunately, straightforward methods for realizing high levels of stereocontrolled polymerizations are often challenging to achieve. In a departure from traditional metal-mediated ring-opening metathesis polymerization (ROMP), we discovered a remarkably simple method for controlling alkene stereochemistry in photoredox mediated metal-free ROMP. Ion-pairing, initiator sterics, and solvation effects each had profound impact on the stereochemistry of polynorbornene (PNB). Simple modifications to the reaction conditions produced PNB with trans alkene content of 25 to >98 %. High cis content was obtained from relatively larger counterions, toluene as solvent, low temperatures (-78 °C), and initiators with low Charton values. Conversely, smaller counterions, dichloromethane as solvent, and enol ethers with higher Charton values enabled production of PNB with high trans content. Data from a combined experimental and computational investigation are consistent with the stereocontrolling step of the radical cationic mechanism proceeding under thermodynamic control.