Double Bond Isomerization Research Articles

Alkene Transfer Hydrogenation with Alkaline-Earth Metal Catalysts.

The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN''2 catalysts [Ae=Ca, Sr, Ba; N''=N(SiMe3 )2 ] using 1,4-cyclohexadiene (1,4-CHD) as a H source. Reaction of 1,4-CHD with AeN''2 gave benzene, N''H, and the metal hydride species N''AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN''2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1-hexene) needed a higher temperature (120 °C) but proceeded without double-bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4-CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4-CHD followed by H transfer. Convenient access to larger quantities of BaN''2 , its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.

Improving oxidative stability of biodiesel by cis-trans isomerization of carbon-carbon double bonds in unsaturated fatty acid methyl esters

Biodiesel is a renewable fuel source comprising of fatty acid methyl esters (FAMEs). However, biodiesel (FAMEs) is susceptible to autoxidation. Therefore, structural modification of FAMEs through decreasing the number of double bonds (e.g., hydrogenation) or trans isomerization of original double bonds in the cis configuration, could improve the oxidative stability of FAMEs. In this work, the trans isomerization of carbon-carbon double bonds in unsaturated FAMEs obtained from edible oils has been successfully achieved using simple and safe p-toluenesulfinic acid as catalyst. And the effects of reaction time, temperature, catalyst loading, amount of water and antioxidants on the trans isomerization were investigated systematically. The results indicated that the amount of water (0–20 wt%) and antioxidants (<200 ppm) had no significant effect on the trans isomerization of unsaturated FAMEs. And the final product (trans FAMEs) was characterized by both GC and FT-IR. The conversion of trans double bonds of the isomerized lard FAMEs could reach 79.9% after heating at 100 °C for only 30 min without changing both the double bonds location and unsaturation degree. In addition, this convenient method was successfully applied in a variety of FAMEs prepared from different vegetable oils (e.g., safflower seed oil, camellia seed oil, high oleic sunflower oil and soybean oil) and approximately 80.0% selectivity of trans isomerization for all FAMEs were obtained. The present study showed a promising method to provide trans-FAMEs products for producing biodiesels with better oxidative stability (for high oleic FAMEs) and without adding of antioxidants.

Design, two-directional synthesis, DFT study of new pyrimido[5,4-d]pyrimidine-2,8-dione derivatives

A facile two-directional synthesis of new pyrimido[5,4-d]pyrimidine-2,8-dione was reported via an efficient reaction of premade bis-aldehydes and 1-(2-amino-1,2-dicyanovinyl)-3-phenylurea in the presence of triethylamine as the base and Cu (II) as catalyst. As there is controversy about the formation of two types of products, that is, purine or pyrimidine ring containing compounds in the reaction of diaminomaleonitrile with isocyanates and aldehydes, the computational model chemistry has been employed to obtain new insight about this reaction and determining the dominant pathway of the process. Using DFT model, two alternative pathways have been explored and geometrical isomerization of central double bond has been considered. Accordingly, the evaluated energy barriers affirm the formation of six-membered pyrimidine ring as the major product in the presence of CuCl2 as the catalyst and MeOH as solvent.

Photoswitchable catalysis based on the isomerisation of double bonds.

Photoswitchable catalysis is a young but rapidly evolving field that offers great potential for non-invasive dynamic control of both activity and selectivity in catalysis. Within this context, the E/Z photoisomerisation of double bonds in molecular switches and motors is one of the most popular tools to control the catalytic activity essentially due to its reversible nature, the large concomitant geometrical changes, and the high tunability of such photochromic entities. This Feature Article summarises the key developments accomplished over the past years through the incorporation of photoswitchable double bonds into the structure of catalytically competent molecules and shows some perspectives on the remaining challenges and possibilities arising from this, yet still somehow immature, exciting area of research.

Состав добавки для улучшения характеристик современного дизельного топлива

The aim of the study was to create an essential composition that improves the environmental and performance properties of contemporary diesel fuel obtained by deep oil refining including the stages of hydrotreating and hydrodesulphurisation. In order to improve the environmental properties of contemporary diesel fuels, such as cetane number and tribological characteristics, a mixture of biodiesel and low molecular weight ethers was proposed for application. Biodiesel was obtained by the transesterification reaction of triacylglycerols from non-edible (rancid) sunflower oil and methanol. By a quantum chemical calculation, the most likely attack direction of a nucleophilic reagent on a triacylglycerol molecule was determined as via a β-carbonyl carbon atom. During the chemical reaction, the absence of cis-trans isomerisation of double bonds was established in the radicals of unsaturated carboxylic acids presented in composition of both the triacylglycerols of vegetable oil and biodiesel molecules. Low molecular weight esters were obtained via the esterification reaction. The physico-chemical properties are determined for several ester mixtures consisting of biodiesel (30–90%) admixed by different saturated esters of lower molecular weight. As the content of saturated low molecular weight esters in mixtures increases, the physico-chemical characteristics of ester mixtures (for example, viscosity, density, cloud point and pour point) tend to improve. However, if the content of low-boiling saturated ethers increases to 60–70%, the boiling point becomes too low, leading to engine interruptions, e.g. the formation of air blocks. In the case of a decrease in the content of low-boiling saturated ethers to 10–30%, ester mixtures are characterised by a heavier fractional composition, as well as a density and viscosity exceeding the levels necessary for the uninterrupted operation of a diesel engine. As a result of the analysis of physico-chemical indicators, the composition of an ester mixture is proposed comprising 50 % vol. of biodiesel and 50 % vol.of low molecular weight saturated esters. The physico-chemical characteristics of such a mixture meet the requirements of the interstate standard for diesel fuels. Thus, the addition of the developed mixture of esters will lead to an increase in the cetane number of hydrotreated diesel fuels, subsequently reflecting in improvement of their lubricating and environmental characteristics.

Ring-closing-metathesis-based synthesis of annellated coumarins from 8-allylcoumarins.

8-Allylcoumarins are conveniently accessible through a microwave-promoted tandem Claisen rearrangement/Wittig olefination/cyclization sequence. They serve as a versatile platform for the annellation of five- to seven-membered rings using ring-closing olefin metathesis (RCM). Furano-, pyrano-, oxepino- and azepinocoumarins were synthesized from the same set of precursors using Ru-catalyzed double bond isomerizations and RCM in a defined order. One class of products, pyrano[2,3-f]chromene-2,8-diones, were inaccessible through direct RCM of an acrylate, but became available from the analogous allyl ether via an assisted tandem catalytic RCM/allylic oxidation sequence.

MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis.

Highly active olefin metathesis catalysts were prepared by thermal spreading MoO3 and/or MoO2(acac)2 on MWW zeolites (MCM-22, delaminated MCM-56) and on two-dimensional MFI (all in NH4+ form). The catalysts‘ activities were tested in the metathesis of neat 1-octene (as an example of a longer chain olefin) at 40 °C. Catalysts with 6 wt % or 5 wt % of Mo were used. The acidic character of the supports had an important effect on both the catalyst activity and selectivity. The catalyst activity increases in the order 6MoO3/HZSM-5(25) (Si/Al = 25) << 6MoO2(acac)2/MCM-22(70) < 6MoO3/2D-MFI(26) < 6MoO3/MCM-56(13) < 6MoO3/MCM-22(28) reflecting both the enhancing effect of the supports‘ acidity and accessibility of the catalytic species on the surface. On the other hand the supports‘ acidity decreases the selectivity to the main metathesis product C14 due to an acid-catalyzed double bond isomerization (followed by cross metathesis) and oligomerization. 6MoO3/2D-MFI(26) with a lower concentration of the acidic centres resulting in catalysts of moderate activity but with the highest selectivity.

Oxidative and reductive cyclization in stiff dithienylethenes.

The electrochemical behavior of stiff dithienylethenes, undergoing double bond isomerization in addition to ring-closure, has been investigated. Electrochromism was observed in almost all cases, with the major pathway being the oxidatively induced cyclization of the open isomers. The influence of the ring size (to lock the reactive antiparallel conformation) as well as substituents (to modulate the redox potential) on the electrocyclization was examined. In the series of derivatives with 6-membered rings, both the E- and the Z-isomer convert to the closed isomer, whereas for the 7-membered rings no cyclization from the E-isomer was observed. For both stiff and normal dithienylethenes bearing benzonitrile substituents an additional and rare reductive electrocyclization was observed. The mechanism underlying both observed electrocyclization pathways has been elucidated.

Base‐Promoted Cycloisomerization for the Synthesis of Indolizines and Related Heterocycles

AbstractThe base‐mediated 5‐exo‐dig type cyclization reaction of homopropargyl pyridines for the construction of indolizines has been reported for the first time. A variety of indolizine derivatives could be obtained in moderate to excellent isolated yields by treatment of homopropargyl pryidines with cesium carbonate in DMSO at 100 °C. This strategy could also be adapted to the synthesis of pyrrolo[1,2‐a]quinolines and pyrrolo[1,2‐a]pyrimidines from the corresponding homopropargylquinolines and homopropargylpyrimidines, respectively. The mechanism involves α C−H deprotonation, 5‐exo‐dig cyclization, and isomerization of the exocyclic double bond to provide the heteroaromatic system. The transformation reported herein does not require the aid of transition metal catalysts or halogens, and is thus environmentally friendly.

Harvesting the Benefits of Inherent Reactive Functionalities in Fully Biosourced Isomeric Benzoxazines

Isomerization of double bonds from an allylic to propenyl position is generally mediated by expensive metal catalysts, demanding an additional synthetic step, thereby reducing sustainability of the...

Experimental and theoretical investigation of heterogeneous catalyzed oligomerization of a mixed C4 stream over modified amorphous aluminosilicates

The oligomerization of a mixed butene stream in liquid phase over amorphous aluminosilicates was investigated. The reaction products were analyzed with gas chromatography with special attention to the octene isomers. By varying temperature, content of iso-butene, and space velocity, the origin of skeletal isomers of octene was identified. Some octene isomers have been considered in the literature as a consequence of methyl group migration or as a cracking product. The catalytic results, however, indicate a direct formation of these C8 side products during dimerization. The correlation of catalytic measurements and thermodynamic data, calculated by quantum-chemical methods on the explicitly correlated coupled cluster single, double, and perturbative triple excitations level (CCSD(F12∗)(T)), prove a competitive reaction between the formation of 4,4-dimethylhex-2-ene (4,4-DMH) and the main product trans-3,4-dimethylhex-2-ene (3,4-DMH). 3-Ethyl-2-methylpentene is also formed in competition with 3,4-dimethylhexene. The skeletal isomers 2,3-dimethylhexene and 2,4-dimethylhexene show a dependence on the proportion of 1-butene at the acid site, whereas a rapid double bond isomerization strongly influences the proportion of 1-butene.

Asymmetric Synthesis of α,β-Unsaturated δ-Lactones through Copper(I)-Catalyzed Direct Vinylogous Aldol Reaction

A simple methodology for the asymmetric synthesis of chiral α,β-unsaturated δ-lactones was achieved by copper(I)-catalyzed direct vinylogous aldol reaction (DVAR) of β,γ-unsaturated esters and various aldehydes, including aromatic aldehydes, heteroaromatic aldehydes, α,β-unsaturated aldehydes, and aliphatic aldehydes. For aromatic and heteroaromatic aldehydes, a one-pot reaction consisting of DVAR, isomerization of the unsaturated carbon-carbon double bond from ( E)-form to ( Z)-form, and subsequent intramolecular transesterification was required to get the lactones in moderate to high yields with high enantioselectivity. For α,β-unsaturated and aliphatic aldehydes, the DVAR proceeded directly to afford the lactones in moderate yields with high enantioselectivity. In the DVAR, various functional groups were well tolerated. Moreover, the methodology was nicely applicable to the aldehyde group distributed in natural products, derivatives of natural product, and derivatives of drug molecules (atomoxetine and naproxen). The mechanism studies revealed that α-addition was reversible and not favored, which accounted for the excellent regioselectivity in the DVAR. The copper(I)-dienolate species generated through deprotonation was proposed to form an equilibrium with an allylcopper(I) species, which reacted with aldehydes to afford the DVAR products through a catalytic asymmetric allylation of aldehydes. Finally, the robustness of the present reaction was demonstrated by a gram-scale reaction, and the utility of the present methodology was showcased by the formal asymmetric synthesis of ezetimibe and fostriecin.

Third DWF1 paralog in Solanaceae, sterol Δ24-isomerase, branches withanolide biosynthesis from the general phytosterol pathway

A large part of chemodiversity of plant triterpenes is due to the modification of their side chains. Reduction or isomerization of double bonds in the side chains is often an important step for the diversification of triterpenes, although the enzymes involved are not fully understood. Withanolides are a large group of structurally diverse C28 steroidal lactones derived from 24-methylenecholesterol. These compounds are found in the Indian medicinal plant Withania somnifera, also known as ashwagandha, and other members of the Solanaceae. The pathway for withanolide biosynthesis is unknown, preventing sustainable production via white biotechnology and downstream pharmaceutical usages. In the present study, based on genome and transcriptome data we have identified a key enzyme in the biosynthesis of withanolides: a DWF1 paralog encoding a sterol Δ24-isomerase (24ISO). 24ISO originated from DWF1 after two subsequent duplication events in Solanoideae plants. Withanolides and 24ISO appear only in the medicinal plants in the Solanoideae, not in crop plants such as potato and tomato, indicating negative selection during domestication. 24ISO is a unique isomerase enzyme evolved from a reductase and as such has maintained the FAD-binding oxidoreductase structure and requirement for NADPH. Using phylogenetic, metabolomic, and gene expression analysis in combination with heterologous expression and virus-induced gene silencing, we showed that 24ISO catalyzes the conversion of 24-methylenecholesterol to 24-methyldesmosterol. We propose that this catalytic step is the committing step in withanolide biosynthesis, opening up elucidation of the whole pathway and future larger-scale sustainable production of withanolides and related compounds with pharmacological properties.

Selective Production of Terminally Unsaturated Methyl Esters from Lactones Over Metal Oxide Catalysts

Metal oxide catalysts were studied for their selectivity for the production of a terminally unsaturated methyl ester, methyl 5-hexenoate (M5H), from a 6 carbon, 6-membered ring lactone, δ-hexalactone (DHL). A 15 wt% Cs/SiO2 catalyst had a selectivity of 55% to M5H. This selectivity was the highest of the metal oxide catalysts studied, which were Cs/SiO2, MgO, SrO, CeO2, ZrO2, Ta2O5, MgAl2O4, and a Mg–Zr mixed oxide. The Cs/SiO2 catalyst was utilized for the ring-opening of γ-valerolactone (GVL), a 5 carbon, 5-membered ring lactone. The catalyst was 88% selective to the terminally unsaturated methyl ester, methyl 4-pentenoate (M4P). Weight hourly space velocity studies determined that the unsaturated ester distributions remained constant and no C=C double bond isomerization occurred. Liquid phase transesterification reactions with DHL and methanol and nuclear magnetic resonance spectroscopy confirmed that DHL undergoes ring-opening transesterification to produce an ω-1 hydroxy methyl ester, methyl 5-hydroxyhexanoate (M5HH). Liquid phase transesterification reactions and thermochemistry calculations established that the equilibrium for GVL transesterification with methanol was favored towards the ring-closed lactone instead of the ring-opened hydroxy ester because of the decreased ring strain of GVL compared to DHL. The difference in terminally unsaturated methyl ester selectivity between GVL and DHL manifests from the difference in ring-strain energy. DHL passes through the M5HH intermediate as a result of greater ring strain, while the production of M4P from GVL most likely occurs through a direct, concerted mechanism.

Stereospecific Ring Opening and Cycloisomerization of Aziridines with Propargylamines: Synthesis of Functionalized Piperazines and Tetrahydropyrazines.

Stereospecific Cu-catalyzed nucleophilic ring opening of N-sulfonylaziridines with propargylamines and subsequent hydroamination afford piperazines, which leads to double-bond isomerization to furnish tetrahydropyrazines. Optically active aziridines can be cross-coupled with high enantiomeric purities (>98% ee).

Sequential Alkene Isomerization and Ring-Closing Metathesis in Production of Macrocyclic Musks from Biomass.

We report successful utilization of sequential alkene isomerization and ring-closing metathesis of dec-9-enoic acid based dienes in synthesis of macrocyclic lactones that possess a strong scent of musk. This catalytic sequence was essential to trim the chain length of starting dienes to yield macrocycles of the right size. Dec-9-enoic acid is conveniently obtainable from oleic esters by Ru-catalysed ethenolysis.

Design and synthesis of Pt/ZSM-22 catalysts for selective formation of iso-Dodecane with branched chain at more central positions from n-Dodecane hydroisomerization

The formation of centrally branched iso-dodecane was controlled by absorption modes of alkene intermediates on ZSM-22, which included pore mouth type and key lock type. In this paper, we have designed and synthesized two kinds of ZSM-22 catalysts for selective formation of iso-dodecane with branched chain at more central positions from n-dodecane hydroisomerization. Aluminosilicate ZSM-22 was modified via carbonization of template to block the micropores and borosilicate ZSM-22 was synthesized. The characterization results and performance evaluation validated that micropore blockage could limit the pore mouth type at low reaction temperatures which favored the formation of iso-dodecane with side chain near the end of the carbon chain. Owing to the weak acid strength, higher temperatures, which favored the key lock mode, were needed to run the n-dodecane hydroisomerization over Pt/borosilicate ZSM-22. Both the two strategies for catalyst design achieved key lock adsorption mode and improvement of the selectivity for centrally branched iso-dodecane. Most importantly, weak acid sites on borosilicate ZSM-22 could catalyze double-bond isomerization reactions, which enhanced centrally branched iso-dodecane distribution up to more than 47%. Our research provides a prospective strategy for selective formation of centrally branched iso-alkanes and Pt/borosilicate ZSM-22 is expected to have potential applications in n-alkane hydroisomerization.

The studies on the chemoenzymatic synthesis of 2-benzyl-3-butenoic acid

The studies on the chemoenzymatic synthesis of 2-benzyl-3-butenoic acid 3 were performed. The alkylation of dienolates derived from the crotonic and vinylacetic acids with different types of alkylating agents was examined. In all cases the inseparable mixture of isomeric acids 3 and 4 were obtained. Chemoselective enzymatic esterification of those mixtures provides ester 6 in 86% yield as a single product without double bond isomerization. None common method can be used for its hydrolysis since under basic conditions only isomeric acid 5 is formed. To overcome this limitation enzyme catalysed hydrolysis process was design. This protocol afforded isomerically pure acid 3 in 98% yield.

Carbon–carbon double bond isomerization in heterocyclic hydrazine derivatives

Double bond isomerization by in situ generated ruthenium hydride catalyst in 6- to 9-membered hydrazine heterocycles including an endocyclic N–N bond was investigated. Based on the results of the experiments, a conclusion was made that the generation of resonance-stabilized enehydrazides is generally favored, but in some cases, the reaction pathway is presumably also highly dependent of steric interactions. With the current method, two novel enehydrazide heterocycles and one enehydrazine heterocycle were produced and characterized.

Samholides, Swinholide-Related Metabolites from a Marine Cyanobacterium cf. Phormidium sp.

Cancer cell cytotoxicity was used to guide the isolation of nine new swinholide-related compounds, named samholides A–I (1–9), from an American Samoan marine cyanobacterium cf. Phormidium sp. Their structures were determined by extensive analysis of 1D and 2D NMR spectroscopic data. The new compounds share an unusual 20-demethyl 44-membered lactone ring composed of two monomers, and they demonstrate structural diversity arising from geometric isomerization of double bonds, sugar units with unique glyceryl moieties and varied methylation patterns. All of the new samholides were potently active against the H-460 human lung cancer cell line with IC50 values ranging from 170 to 910 nM. The isolation of these new swinholide-related compounds from a marine cyanobacterium reinvigorates questions concerning the evolution and biosynthetic origin of these natural products.