Reduction Of Esters Research Articles

Efficient elimination of 3-monochloropropane-1,2-diol fatty acid ester from palm oil via using activated carbon grafted with Tween80.

In the current study, we attempt to remove 3-monochloropropane-1,2-diol fatty acid ester (3-MCPD ester) from palm oil with developed composite adsorbent (Tween80 modified activated carbon [AC]), and scanning electron microscopy-energy dispersive spectrometer, Fourier transform-infrared spectroscopy, X-ray diffraction, nitrogen content adsorption-desorption and thermogravimetric analysis were used to characterize the modifications. We further examined the adsorption capability of the composite adsorbent for 3-MCPD ester and found that the highest removal efficiency was 87.36% (5.3% of adsorbent dose at 104°C for 29min). This is approximately three times higher than that of pristine AC, implying that the composite can be employed as a novel adsorbent for 3-MCPD ester reduction. Along with the adsorption mechanism of Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin models were also tested. It has been suggested that Freundlich model could best describe the adsorption process. Adsorption was found to be well-fitted by pseudo-second-order kinetics according to the kinetic study. An endothermic and spontaneous adsorption mechanism was demonstrated by the thermodynamic studies.

Direct Hydrogenation of Sterically Hindered, Unactivated Alkenes Catalyzed by Phosphino(silyl)‐Nickel Complexes

AbstractReadily accessible (PSi)Ni(II)‐benzyl complexes supported by bidentate phosphino(silyl) ligation were found to be effective pre‐catalysts for the direct hydrogenation of a variety of highly sterically hindered, unfunctionalized alkenes under relatively mild conditions (2.5‐5 mol% Ni, 10 atm H2, 50 °C) and without the need for additional additives or activator species. A range of substrates, including di‐, tri‐, and tetra‐substituted alkenes were evaluated in this regard and afforded good to moderate yields. Substrates featuring α,β‐unsaturated carbonyl functionality were also readily hydrogenated with no evidence for reduction of carbonyl or ester C−O functionalities. Deuteration experiments highlight the occurrence of chain walking which occurs in the background of the reported catalytic chemistry.

Ammonia-Borane Dependent Transfer Hydrogenation of Carboxylic Esters to Primary Alcohols.

Here, we present a user-friendly protocol that uses bench-stable NH3·BH3 (AB) as the hydrogen transfer agent for the reduction of both aromatic and aliphatic esters without an external catalyst and base, delivering a structurally diverse array of primary alcohols (80-98% yields). The broad functional-group tolerance (halogen, boronic ester, -NO2, -OH, etc.) under environmentally acceptable conditions implies high practical utility. Further, a tandem catalytic conversion of a plastic waste polyethylene terephthalate (PET) bottle to 1,4-benzenedimethanol including fatty esters into respective fatty alcohols was shown.

Blue‐Light Enhanced Iron Catalysed Hydrosilylation of Carboxamides and Carboxylic Esters at Ambient Conditions

AbstractWe report herein a blue‐light‐promoted iron‐catalyzed reduction of carboxamides and carboxylic esters. Conducting the reaction under hydrosilylation conditions and blue light (2×24 W, 450–460 nm) in the presence of an iron(0) based pre‐catalyst, Fe(CO)4(IMes), amines and alcohols were obtained, respectively, notably at ambient conditions, whereas using white light, higher reaction temperatures up to 100 °C were required. Preliminary mechanism studies highlighted the crucial role of blue light not only to generate the catalytic active species, but also in some steps of the catalytic cycle.

Tandem hydroformylation-amine condensation and reduction of canola fatty acid methyl esters for production of diesel renewable antioxidant additives

A canola fatty acid methyl esters (FAME) mixture was synthesized through transesterification and modified by rhodium homogeneous tandem hydroformylation-amine condensation, with primary amines, yielding imine products. The obtained imines were reduced by maceration with sodium borohydride and alumina, yielding secondary amine products. The synthesized fatty imines and amines were evaluated as additives for commercial diesel and biodiesel on concentrations of 0.5 and 1.0 %, while tested for oxidative stability fallowing the EN14112 standard method. The tested amino products exhibited higher oxidative stability, which justifies its application as antioxidant additives, while the diesel blends did not show expressive physicochemical modifications. The modified amino-FAME antioxidant performance, justifies a renewable biorefinery application, extend the biodiesel shelf-life prior to diesel blending, complies with UN’s Sustainable Development Goals (SDGs) and if prepared from residues, enhance the circular economy appeal.

Multielectron Reduction of Esters by a Diazabenzacenaphthenium Photoredox Catalyst.

A novel diazabenzacenaphthenium photocatalyst, N-BAP, with high photoredox abilities and visible-light absorption was designed and prepared in one step. Under visible-light irradiation, N-BAP promoted the four-electron reduction of esters in the presence of ammonium oxalate as a "traceless reductant" to generate carbinol anion intermediates that underwent protonation with water to give the corresponding alcohols. The resulting carbinol anions also exhibited nucleophilic reactivity under the photocatalytic conditions to undergo a 1,2-addition to a second carbonyl compound, affording unsymmetric 1,2-diols.

Counterintuitive chemoselectivity in the reduction of carbonyl compounds.

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

Aquatic plants as whole-cell biocatalysts for preparing optically active β-hydroxy esters

We investigated the asymmetric reduction of β-keto esters using aquatic plants as whole-cell biocatalysts under darkness and illumination by fluorescent light. The (S)- β-hydroxy esters were obtained with high enantiomeric excess (ee) values. Regardless of the light conditions, the amount of (R)- β-hydroxy ester produced did not change significantly as the substrate concentration increased, whereas the yield of (S)- β-hydroxy ester increased substantially. Moreover, increasing the quantity of plants resulted in a significant increase in the production of (S)- β-hydroxy ester and a slight increase in the production of (R)- β-hydroxy ester, reducing the ee value. The reaction proceeded more efficiently for substrates bearing bulkier ester moieties, of which the tert-butyl ester gave the best results. Ultimately, we concluded that aquatic plants can be used as whole-cell biocatalysts for preparing optically active β-hydroxy esters. This result represents a new use of aquatic plants and contributes to the achievement of carbon neutrality.

Ester reduction on bifunctional metal-acid catalysts: Effect of metal to acid ratio

Ethers are highly desirable commodities in food, cosmetics, rubber, and coatings due to their low chemical reactivity, stability, and miscibility with other organic solvents. The direct reduction of esters to ethers by reactions with H2 provides a pathway to create nonsymmetrical ethers from biomass-derived esters. Propyl acetate (PA) and H2 react over platinum nanoparticles supported on Brønsted acidic faujasite zeolite (Pt-FAU) by two primary pathways: hydrogenation to form ethyl propyl ether and water and hydrogenolysis to form acetaldehyde and propanol. Here, we evaluate how rates and selectivities for these pathways depend on the ratios of the metal to Brønsted acid sites and the densities of Brønsted acid sites within Pt-FAU. X-ray absorption spectroscopy and infrared spectroscopy of adsorbed CO show that Pt species exist in a metallic state in the presence of H2 at elevated temperatures. The vibrational frequencies of chemisorbed CO on surface Pt atoms (Pts) do not vary significantly with differences in Pt contents or Brønsted acid site (H+) density. Temperature programmed reactions of amines protonated upon Brønsted acid sites demonstrate that the fractions of Brønsted acid sites accessible to organic reactants decrease with increasing Pt content, which implies intimate contact between Pt nanoparticles and acid sites. Comparisons between electron microscopy and rate measurements indicate that the proximity between Pt and H+ matters even when Pt nanoparticles appear to be within pores. Even though rates for reactions between PA and H2 over Pt-FAU decrease significantly during the initial use of the catalysts, analysis of initial rates provides mechanistic insight into elementary steps that determine rates. Initial hydrogenation rates increase by 13-fold, and hydrogenolysis rates increase by 33-fold when the atomic ratio of Pts to H+ increases from 0.08 to 0.6. Hydrogenation and hydrogenolysis rates increase with Pts density but do not correlate with H+ densities, which indicates kinetically relevant steps occur on Pt sites. These elementary steps likely involve the formation of hydrogenated intermediates (i.e., hemiacetal) that diffuse and react rapidly upon Brønsted acid sites, which generates intraparticle hemiacetal gradients. These correlations confirm that ester reduction rates and selectivities depend sensitively on the metal to acid ratio and proximity between these functions in a manner reminiscent of bifunctional catalysts used for hydroisomerization and hydrocracking catalysis. Consequently, increasing the number of stable Pt sites close to H+ sites should increase rates per mass catalyst by increasing the concentration of hemiacetals at Brønsted acid sites.

Total syntheses of Tetrodotoxin and 9-epiTetrodotoxin

Tetrodotoxin and congeners are specific voltage-gated sodium channel blockers that exhibit remarkable anesthetic and analgesic effects. Here, we present a scalable asymmetric syntheses of Tetrodotoxin and 9-epiTetrodotoxin from the abundant chemical feedstock furfuryl alcohol. The optically pure cyclohexane skeleton is assembled via a stereoselective Diels-Alder reaction. The dense heteroatom substituents are established sequentially by a series of functional group interconversions on highly oxygenated cyclohexane frameworks, including a chemoselective cyclic anhydride opening, and a decarboxylative hydroxylation. An innovative SmI2-mediated concurrent fragmentation, an oxo-bridge ring opening and ester reduction followed by an Upjohn dihydroxylation deliver the highly oxidized skeleton. Ruthenium-catalyzed oxidative alkyne cleavage and formation of the hemiaminal and orthoester under acidic conditions enable the rapid assembly of Tetrodotoxin, anhydro-Tetrodotoxin, 9-epiTetrodotoxin, and 9-epi lactone-Tetrodotoxin.

Nickel-Catalyzed Reductive Arylation of Redox Active Esters for the Synthesis of α-Aryl Nitriles: Investigation of a Chlorosilane Additive.

A nickel-catalyzed reductive cross-coupling of redox active N-hydroxyphthalimide (NHP) esters and iodoarenes for the synthesis of α-aryl nitriles is described. The NHP ester substrate is derived from cyanoacetic acid, which allows for a modular synthesis of substituted α-aryl nitriles, an important scaffold in the pharmaceutical sciences. The reaction exhibits a broad scope, and many functional groups are compatible under the reaction conditions, including complex highly functionalized medicinal agents. Mechanistic studies reveal that reduction and decarboxylation of the NHP ester to the reactive radical intermediate are accomplished by a combination of a chlorosilane additive and Zn dust. We demonstrate that stoichiometric chlorosilane is essential for product formation and that chlorosilane plays a role beyond activation of the metal reductant.

Electrochemical reduction of benzoic acid esters using water as a H/D source

Benzoic esters are reduced with water as a hydrogen/deuteron source in DC/AC electrolysis in an undivided cell without a sacrificial electrode.

Formation of Arylsulfinates Using Electroreductive Cleavage of Cone-Calix[4]Arene Nosylates – EPR and Theoretical Study

Nosylates are used as protecting groups. In a non-redox hydrolytic process, the leaving group is the corresponding sulfonate anion. Upon electrochemical reduction of nosylates (here six cone-calix[4]arene-bis-nosylates; i.e. 4-nitrophenylsulfonate aryl esters) using mercury electrodes (DC-polarography and cyclic voltammetry - CV) combined with in-situ EPR-spectroelectrochemistry in aprotic DMF, the mechanism is different. All studied calix[4]arene-bis-nosylates are reduced in a first reversible step to bis-nitroradical anion) which was proved by EPR. The stability of the multiple radical ions proves the absence of electron communication among the redox centers on the calixarene macrocycle which was observed previously in other systems1-3.In the second reduction step next 2×2 electrons are transferred and both sulfonate ester groups are cleaved to two 4-nitro-benzenesulfinate ions and a calixarene bis-phenolate (95%) under splitting of the S-O bond.4 The mechanism of electroreductive generation of arylsulfinate anions is a significant finding from electrosynthetic point of view. Synthesis of activated arylsulfinates is generally difficult for their low stability. Nevertheless, they can be easily prepared by electrochemical reduction of calixarene nosyl esters. Acknowledgement This work was supported by the grant GAČR 21-23261S (Czech Science Foundation) with the institutional support RVO: 61388955. The authors are grateful to colleagues ing. Markéta Řezanková, prof. Jan Budka and prof. Pavel Lhoták (University of Chemistry and Technology, Prague) for synthesis and granting the substances.

Effect of high hydrostatic pressure on aroma volatile compounds and aroma precursors of Hami melon juice.

High hydrostatic pressure (HHP) treatment is an effective technique for processing heat-sensitive fruits and causes changes in volatile compounds and their precursors while maintaining quality. We investigated the changes and correlations of volatile compounds, related enzyme activities and precursor amino acids, and fatty acids in Hami melon juice under 350-500 MPa pressure. The application of HHP treatment resulted in a considerable reduction of esters and a substantial increase in aldehydes and alcohols in C6 and C9. Activities of lipoxygenase (LOX), alcohol acyltransferase (AAT), and phospholipase A2 (PLA2) were lower than those of the untreated group, alcohol dehydrogenase (ADH) activity was reversed. When compared to fresh cantaloupe juice, there was an increase in both the types and contents of amino acids with lower total fatty acid contents than the control group. Positive correlations were observed among six ester-related substances and eight alcohol-related substances. Additionally, the correlations between volatile compounds and fatty acids were more substantial compared to those between volatile compounds and amino acids. HHP treatment increases Hami melon flavor precursors and is an effective way to maintain the aroma volatile compounds and flavor of Hami melon juice.

Exhaustive Reduction of Esters, Carboxylic Acids and Carbamates to Methyl Groups Catalyzed by Boronic Acids

AbstractCarboxy to methyl reduction is an important transformation in organic synthesis, yet existing methodologies often require multi‐step procedures or use hazardous metal hydrides. Herein, a metal‐free catalytic system is reported for the one‐step reduction of esters, carboxylic acids, and carbamates to a methyl group, in the presence of catalytic amounts of boronic acids. By using ammonia borane as a hydrogen donor, a wide range of products bearing different functional groups can be obtained in high yields under relatively mild conditions. Mechanistic studies and control experiments elucidate the complexity of the mechanism and provide an explanation for the observed selectivity.

Catalyst- and Stoichiometry-Dependent Deoxygenative Reduction of Esters to Ethers or Alcohols with Borane-Ammonia.

A facile and selective room temperature deoxygenation of both aromatic and aliphatic carboxylic esters to ethers has been achieved by regulating the stoichiometry of the reductant, BH3-NH3, and the catalyst, TiCl4. This first, practical borane-mediated process is compatible with various potentially sensitive functional groups and is applicable to the deoxygenative ether formation from typically challenging aromatic acid esters. Substituting BF3-Et2O as the catalyst alters the reaction pathway, reducing the esters to alcohols. Mechanistic insights are provided by NMR spectroscopy, deuterium labeling, and kinetic isotope studies.

Enantioselective synthesis of structurally diverse aryl γ-/δ- lactones using an engineered carbonyl reductase

Optically pure aryl γ-/δ-lactones are valuable building blocks in the organic synthesis of natural products and pharmaceutical compounds. The asymmetric reduction of aryl keto acids or esters using carbonyl reductase provides a green and atom-economic synthetic route to these molecules. However, this method is still challenging due to the substantial steric hindrance of substrates, which has limited its practical application. Herein, we report the engineering of a carbonyl reductase SmCRV4, which showed excellent enantioselectivity (99% (S)) for methyl 3-benzoyl propionate (1a), to obtain the mutant SmCRW6 by directed evolution. The specific activity and catalytic efficiency of SmCRW6 towards 1a were 4.9-fold and 3.2-fold higher than those of the parent SmCRV4, respectively. The mutant SmCRW6 showed a broad substrate scope (26 examples) with improvement in specific activity and enantioselectivity for most substrates. Finally, 7 chiral aryl lactones and 10 aliphatic lactones were prepared using lyophilized enzyme powder. In scaled-up synthesis, three aryl γ-/δ-lactones were prepared with > 97% conversions, 76 − 88% yields and ee up to > 99%, and the highest space-time yield (STY) of 28.2 g L−1 h−1 reported so far for (S)-γ-phenyl-γ-butyrolactone was obtained through biocatalysis.

Fine-tuning NHC ligands for ruthenium-catalyzed hydrogenation of carboxylic esters to alcohols

An efficient and simple catalytic system has been developed for hydrogenation of carboxylic esters. Combination of commercially available Ru-salts and simple imidazolium salts bearing N-cyclohexyl, N-cycloheptyl and N-cyclooctyl substituents enable reduction of esters to alcohols at 40 °C and 40 bar of hydrogen pressure. The reaction protocol offers a practical access to alcohols using an easy operational setup.

Mechanistic Investigation of Ni-Catalyzed Reductive Cross-Coupling of Alkenyl and Benzyl Electrophiles.

Mechanistic investigations of the Ni-catalyzed asymmetric reductive alkenylation of N-hydroxyphthalimide (NHP) esters and benzylic chlorides are reported. Investigations of the redox properties of the Ni-bis(oxazoline) catalyst, the reaction kinetics, and mode of electrophile activation show divergent mechanisms for these two related transformations. Notably, the mechanism of C(sp3) activation changes from a Ni-mediated process when benzyl chlorides and Mn0 are used to a reductant-mediated process that is gated by a Lewis acid when NHP esters and tetrakis(dimethylamino)ethylene is used. Kinetic experiments show that changing the identity of the Lewis acid can be used to tune the rate of NHP ester reduction. Spectroscopic studies support a NiII-alkenyl oxidative addition complex as the catalyst resting state. DFT calculations suggest an enantiodetermining radical capture step and elucidate the origin of enantioinduction for this Ni-BOX catalyst.

Magnesium-mediated hydroboration under ambient condition: A reduction of esters, aldehydes, and ketones

Alkoxymagnesium halide is demonstrated as an effective catalyst for the carbonyl hydroboration of esters, aldehydes, and ketones. This study reports the smooth hydroboration of a variety of esters, aldehydes, and ketones with ethoxymagnesium chloride (EtOMgCl) and pinacolborane under mild reaction conditions (low catalyst loading 0.3 mol% for aldehydes and ketones, 5 mol% for esters, room temperature). Among the tested alkoxymagnesium chlorides, EtOMgCl was the optimum catalyst in terms of yield and selectivity for esters. Moreover, the designed protocol afforded the chemoselective hydroboration of esters in good yields with high selectivity.