Monosubstituted Epoxides Research Articles

Attenuating Nucleophilicity of Titanocene Hydrides Beyond Steric Effects en Route to Fatty Alcohols.

Here, we introduce a new class of titanocene catalysts for epoxide hydrosilylation that frustrates their hydridicity and thereby emphasizes their electron transfer reactivity. This unique attenuation of hydridicity is accomplished by introducing Lewis acidic silicon centers to the cyclopentadienyl ligands for an intramolecular coordination of the titanium-bound hydride. The superiority of our rationally designed catalysts over classic titanocenes with alkyl-substituted cyclopentadienyl ligands is demonstrated in the dramatically improved regioselectivity of the hydrosilylation of monosubstituted epoxides to primary alcohols.

Polymorphism and Its Influence on Catalytic Activities of Lanthanide-Glutamate-Oxalate Coordination Polymers.

To study the relationship between polymorphism and catalytic activities of lanthanide coordination polymers in the cycloaddition reactions of CO2 with epoxides, the monoclinic and triclinic polymorphs of [LnIII(NH3-Glu)(ox)]·2H2O, where LnIII = LaIII (I), PrIII (II), NdIII (III), SmIII (IV), EuIII (V), GdIII (VI), TbIII (VII), and DyIII (VIII), NH3-Glu- = NH3+ containing glutamate, and ox2- = oxalate, were synthesized and characterized. Factors determining polymorphic preference, the discrepancy between the two polymorphic framework structures, potential acidic and basic sites, thermal and chemical stabilities, active surface areas, void volumes, CO2 sorption/desorption isotherms, and temperature-programmed desorption of NH3 and CO2 are comparatively presented. Based on the cycloaddition of CO2 with epichlorohydrin in the presence of tetrabutylammonium bromide under solvent-free conditions and ambient pressure, catalytic activities of the two polymorphs were evaluated, and the relationship between polymorphism and catalytic performances has been established. Better performances of the monoclinic catalysts have been revealed and rationalized. In addition, the scope of monosubstituted epoxides was experimented and the outstanding performance of the monoclinic catalyst in the cycloaddition reaction of CO2 with allyl glycidyl ether under ambient pressure has been disclosed.

Mechanistic Insights into Urea‐, Thiourea‐, and Isothiourea‐Based Bifunctional Tetraarylphosphonium Salt Catalysis for Conversion of Carbon Dioxide to Cyclic Carbonates

AbstractA novel bifunctional isothiourea derived from a tetraarylphosphonium salt catalyzes the coupling reaction of monosubstituted epoxides with 1 atm of carbon dioxide to afford five‐membered cyclic carbonates in high yields. Thiourea and urea groups are also effective hydrogen bond donors for the carbon dioxide fixation. Comprehensive mechanistic studies on this catalysis using theoretical calculations disclosed key hydrogen bond interactions between catalysts and intermediates, which relied upon orientation of the binding modes.

Enantioconvergent hydrolysis of racemic 1,2-epoxypentane and 1,2-epoxyhexane by an engineered Escherichia coli strain overexpressing a novel Streptomyces fradiae epoxide hydrolase

In order to excavate microbial epoxide hydrolases (EHs) with desired catalytic properties, a novel EH, SfEH1, was identified based on the genome annotation of Streptomyces fradiae and sequence alignment analysis with local protein library. The SfEH1-encoding gene, sfeh1, was then cloned and over-expressed in soluble form in Escherichia coli/BL21(DE3). The optimal temperature and pH of recombinant SfEH1 (reSfEH1) and reSfEH1-expressing E. coli (E. coli/sfeh1) were both determined as 30 ℃ and 7.0, also indicating that the influences of temperature and pH on reSfEH1′s activities were more obvious than those of E. coli/sfeh1 whole cells. Subsequently, using E. coli/sfeh1 as catalyst, its catalytic properties towards thirteen common mono-substituted epoxides were tested, in which E. coli/sfeh1 had the highest activity of 28.5 U/g dry cells for rac-1,2-epoxyoctane (rac-6a), and (R)-1,2-pentanediol ((R)-3b) (or (R)-1,2-hexanediol ((R)-4b)) with up to 92.5% (or 94.1%) eep was obtained at almost 100% conversion ratio. Regioselectivity coefficients (αS and βR) displayed in the enantioconvergent hydrolysis of rac-3a (or rac-4a) were calculated to be 98.7% and 93.8% (or 95.2% and 98.9%). Finally, the reason of the high and complementary regioselectivity was confirmed by both kinetic parameter analysis and molecular docking simulations.

Chemical recycling of CO2-based polycarbonates to sulfur-containing polymers

This work provides a “polymer A to polymer B” strategy for the upcycling of CO2-based poly(ether)carbonates to sulfur-containing polymers.

Tuning the Properties of Ester-Based Degradable Polymers by Inserting Epoxides into Poly(ϵ-caprolactone).

A series of ester-ether copolymers were obtained via the reaction between α,ω-dihydroxyl poly(ϵ-caprolactone) (PCL) and ethylene oxide (EO) or monosubstituted epoxides catalyzed by strong phosphazene bases. The two types of monomeric units were distributed in highly random manners due to the concurrence of epoxide ring-opening and fast transesterification reactions. The substituent of epoxide showed an interesting bidirectional effect on the enzymatic degradability of the copolymer. Compared with PCL, copolymers derived from EO exhibited enhanced hydrophilicity and decreased crystallinity which then resulted in higher degradability. For the copolymers derived from propylene oxide and 1,2-butylene oxide, the hydrophobic alkyl pendant groups also allowed lower crystallinity of the copolymers thus higher degradation rates. However, further enlarging the pendant groups by using styrene oxide or 2-ethylhexyl glycidyl ether caused a decrease in the degradation rate, which might be ascribed to the higher bulkiness hindering the contact of ester groups with lipase.

Structure-Guided Regulation in the Enantioselectivity of an Epoxide Hydrolase to Produce Enantiomeric Monosubstituted Epoxides and Vicinal Diols via Kinetic Resolution.

Structure-guided microtuning of an Aspergillus usamii epoxide hydrolase was executed. One mutant, A214C/A250I, displayed a 12.6-fold enhanced enantiomeric ratio (E = 202) toward rac-styrene oxide, achieving its nearly perfect kinetic resolution at 0.8 M in pure water or 1.6 M in n-hexanol/water. Several other beneficial mutants also displayed significantly improved E values, offering promising biocatalysts to access 19 structurally diverse chiral monosubstituted epoxides (97.1 - ≥ 99% ees) and vicinal diols (56.2-98.0% eep) with high yields.

Deoxygenation of Epoxides with Hexamethyldigermane Catalyzed by Au Nanoparticles on TiO2

AbstractMono‐ and disubstituted epoxides undergo quantitative deoxygenation into the corresponding alkenes by hexamethyldigermane (Me3GeGeMe3) in the presence of 1 mol% of supported Au nanoparticles on TiO2, with digermane being transformed into the corresponding digermoxane. Tri‐ and tetrasubstituted epoxides react either poorly or remain intact, allowing thus the selective intermolecular deoxygenation of a monosubstituted epoxide in the presence of a highly substituted one. Stereoisotopic studies and Hammett‐type kinetics indicate a dipolar transition state of the deoxygenation process, whose degree of polarity profile depends on the substituents. For alkyl‐substituted epoxides the deoxygenation is almost stereospecific (retention of configuration), while for the aryl‐substituted ones, in which the dipolar character is more profound, substantial loss of stereospecificity is observed.

Primary Alcohols via Nickel Pentacarboxycyclopentadienyl Diamide Catalyzed Hydrosilylation of Terminal Epoxides.

The efficient and regioselective hydrosilylation of epoxides co-catalyzed by a pentacarboxycyclopentadienyl (PCCP) diamide nickel complex and Lewis acid is reported. This method allows for the reductive opening of terminal, monosubstituted epoxides to form unbranched, primary alcohols. A range of substrates including both terminal and nonterminal epoxides are shown to work, and a mechanistic rationale is provided. This work represents the first use of a PCCP derivative as a ligand for transition-metal catalysis.

CO2 fixation by cycloaddition of mono/disubstituted epoxides using acyl amide decorated Co(II) MOF as a synergistic heterogeneous catalyst

Dual ligand 3D MOF {[Co(BDC)(L)]·2H2O.xG}n (CoMOF-2; G = guest) was synthesized via simple room temperature stirring method. Bulk Phase purity of CoMOF-2 was assessed by various physicochemical methods including X‐ray diffraction (XRD). CO2 adsorption isotherms indicate that activated CoMOF-2 is efficient in CO2 uptake, which has been utilized for the CO2-Epoxide cycloaddition. The catalytic ability of CoMOF-2 as a binary catalyst revealed excellent results for variety of monosubstituted epoxide under solvent‐free conditions (1 bar/40 °C/12 h). Interestingly CoMOF-2/KI also showed great potential as a heterogeneous catalyst for disubstituted epoxide (10 bar/120 °C/24 h) with high yields/selectivity. The catalytic efficiency of the present investigation for scantly explored disubstituted epoxide is better/on par with the earlier reports and the recyclability of the catalyst is an added advantage. Probable mechanism for the catalytic reaction is deduced and verified the representative energy profile for cycloaddition of CO2-Cyclohexane oxide (CHO) by DFT calculation.

MALDI-ToF MS Study of Macrocyclic Polyethers Generated by Electrophilic Zwitterionic Ring Expansion Polymerization of Monosubstituted Epoxides with B(C6F5)3

The electrophilic zwitterionic ring expansion polymerization of monosubstituted epoxides initiated by tris(pentafluorophenyl)borane [B(C6F5)3] is investigated to reveal the effect of substituents o...

Synthesis of Homo- and Heteronuclear Rare-Earth Metal Complexes Stabilized by Ethanolamine-Bridged Bis(phenolato) Ligands and Their Application in Catalyzing Reactions of CO2 and Epoxides.

A series of homonuclear rare-earth (RE) metal complexes (1Y, 2Yb, 3Nd, and 4La) and heteronuclear RE-Zn complexes (1Y-Zn, 3Nd-Zn, and 5Sm-Zn) stabilized by ethanolamine-bridged bis(phenolato) ligands was prepared and structurally characterized. Heteronuclear complexes are assembled through bridging acetate ligands, and their formation and characterization add to the diversity of 3d-4f complexes. Their activities in mediating reactions of CO2 and epoxides were evaluated and compared. Heteronuclear RE-Zn complexes found application in the copolymerization of cyclohexene oxide and CO2, giving rise to acetate-group-capped copolymers. Homonuclear complexes showed good activity in catalyzing the cycloaddition of variously monosubstituted epoxides and CO2 (1 bar), generating cyclic carbonates in 65-96% yield. For sterically hindered disubstituted epoxides, good yields of 60-91% were achieved in the presence of 10 bar CO2.

Kinetic Resolution of Epoxides with CO2 Catalyzed by a Chiral-at-Iridium Complex.

Chiral-at-metal bis-cyclometalated iridium(III) complexes are introduced as a new class of chiral catalysts for the reaction of epoxides with CO2 to form cyclic carbonates under conditions of kinetic resolution. Reactions are typically performed at room temperature in the presence of 1 mol % of iridium catalyst and 1.5 mol % of tetraethylammonium bromide as the nucleophilic cocatalyst to provide selectivity factors of up to 16.6. A variety of monosubstituted epoxides, including styrene epoxide, epoxides with aliphatic side chains, glycidyl ethers, and a glycidyl ester, are found to be suitable substrates. No polymerization side reaction is observed for any of the investigated substrates.

Sequence-Selective Terpolymerization from Monomer Mixtures Using a Simple Organocatalyst.

One-step synthesis of block copolymer from mixed monomers is of great interest and challenge. Using a simple non-nucleophilic organobase as the catalyst, we have achieved sequence-selective terpolymerization from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA). Alcohol-initiated alternating copolymerization of PA and epoxide occurs first and exclusively because PA is substantially more active than LA for reacting with base-activated hydroxyl. When PA is fully consumed, LA polymerizes from the termini of the first block while excess epoxide stays intact because of the mild basicity of the catalyst. The two polymerizations thus occur tandemly, both in chemoselective manners, so that an aromatic-aliphatic block copolyester is generated in this one-step synthesis. The effectiveness and versatility of this approach is demonstrated by the use of ethylene oxide and several monosubstituted epoxides as well as mono-, di-, or tetrahydroxy initiators.

CO2 cycloaddition to epoxides promoted by bis-thioether-phenolate Fe(II) and Fe(III) complexes

The synthesis and characterization of three new bis-thioether-phenolate monoanionic tridentate [SOS]-type ligands, and the synthesis of the corresponding Fe(II) and Fe(III) complexes are reported. Their application, in combination with tetrabutylammonium bromide (TBAB), as catalysts for the cycloaddition of CO2 to a number of monosubstituted epoxides and cyclohexene oxide was investigated. For example, epichlorohydrin is converted in the corresponding cyclic carbonate, with a turnover frequency of 155 h−1 (0.1 mol% of iron, 0.5 mol% of TBAB, 10 bar of CO2). With the aim to obtain further information about the catalyst structure-activity relationship and the reaction mechanism, the reaction between the catalyst precursors and TBAB was investigated, suggesting the formation of an anionic metallate species in the case of the Fe(III) complexes that is supposed to be true catalyst. A mechanistic scenario explaining the different behaviour of the Fe(II) and Fe(III) complexes is proposed.

Metal-free, regioselective and stereoregular alternating copolymerization of monosubstituted epoxides and tricyclic anhydrides

A metal-free, highly regioselective and stereoregular ring-opening alternating copolymerization of monosubstituted epoxides with tricyclic anhydrides was achieved to yield well-defined polyesters.

Conversion of Carbon Dioxide into Oxazolidinones Mediated by Quaternary Ammonium Salts and DBU

AbstractA straightforward method to construct oxazolidinones through a three‐component reaction involving CO2, epoxides, and amines promoted by a combination of Bu4NI and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene was developed. A wide range of aromatic and aliphatic amines and monosubstituted epoxides were converted into 3,5‐disubstituted‐2‐oxazolidines in up to 95 % yield. This metal‐free and easily available catalytic system was applicable to a broad range of substrates, including conventionally challenging ones such as aliphatic epoxides, at atmospheric pressure. Preliminary mechanistic studies suggested a reaction pathway involving β‐amino alcohols.

Chiral Macrocyclic Organocatalysts for Kinetic Resolution of Disubstituted Epoxides with Carbon Dioxide.

Among chiral macrocycles 1 synthesized, 1m with the 3,5-bis(trifluoromethyl)phenylethynyl group was the best organocatalyst for the enantioselective synthesis of cyclic carbonates from disubstituted or monosubstituted epoxides and CO2. The X-ray crystal structure of 1m revealed a well-defined chiral cavity with multiple hydrogen-bonding sites that is suitable for the enantioselective activation of epoxides. A catalytic cycle proposed was supported by DFT calculations.

Recyclable Single-Component Rare-Earth Metal Catalysts for Cycloaddition of CO2 and Epoxides at Atmospheric Pressure.

Ionic rare-earth metal complexes 1-4 bearing an imidazolium cation were synthesized, which, as single-component catalysts, showed good activity in catalyzing cyclic carbonate synthesis from epoxides and CO2. In the presence of 0.2 mol % catalyst, monosubstituted epoxides bearing different functional groups were converted into cyclic carbonates in 60-97% yields under atmospheric pressure. In addition, bulky/internal epoxides with low reactivity yielded cyclic carbonates in 40-95% yields. More importantly, the readily available samarium complex 2 was reused for six successive cycles without any significant loss in its catalytic activity. This is the first recyclable rare-earth metal-based catalyst in cyclic carbonate synthesis.

Poly(monothiocarbonate)s from the Alternating and Regioselective Copolymerization of Carbonyl Sulfide with Epoxides.

Carbonyl sulfide (COS) is an air pollutant that causes acid rain, ozonosphere damage, and carbon dioxide (CO2) generation. It is a heterocumulene and structural analogue of CO2. Relevant to organic synthesis, it is a source of C═O or C═S groups and thus an ideal one-carbon (C1) building block for synthesizing sulfur-containing polymers through the similar route of CO2 copolymerization. In contrast, traditional synthesis of sulfur-containing polymers often involves the condensation of thiols with phosgene and ring-opening polymerization of cyclic thiocarbonates that are generally derived from thiols and phosgene; thus, COS/epoxide copolymerization is a "greener" route to supplement or supplant current processes for the production of sulfur-containing polymers. This Accounts highlights our efforts on the discovery of the selective formation of poly(monothiocarbonate)s from COS with epoxides via heterogeneous zinc-cobalt double metal cyanide complex (Zn-Co(III) DMCC) and homogeneous (salen)CrX complexes. The catalytic activity and selectivity of Zn-Co(III) DMCC for COS/epoxide copolymerization are similar to those for CO2/epoxide copolymerization. (salen)CrX complexes accompanied by onium salts exhibited high activity and selectivity for COS/epoxide copolymerization under mild conditions, affording copolymers with >99% monothiocarbonate units and high tail-to-head content up to 99%. By way of contrast, these catalysts often show moderate or low activity for CO2/epoxide copolymerization. Of note, a specialty of COS/epoxide copolymerization is the occurrence of an oxygen-sulfur exchange reaction (O/S ER), which may produce carbonate and dithiocarbonate units. O/S ER, which are induced by the metal-OH bond regenerated by chain transfer reactions, can be kinetically inhibited by changing the reaction conditions. We provide a thorough mechanistic understanding of the electronic/steric effect of the catalysts on the regioselectivity of COS copolymerization. The regioselectivity of the copolymerization originates from the solely nucleophilic attack of the sulfur anion to methylene of the epoxide, and thus, the chiral configuration of the monosubstituted epoxides is retained. COS-based copolymers are highly transparent sulfur-containing polymers with excellent optical properties, such as high refractive index and Abbe number. Thanks to their good solubility and many available epoxides, COS/epoxide copolymers can potentially be a new applicable optical material. Very recently, crystalline COS-based polymers with or without chiral carbons have been synthesized, which may further expand the scope of application of these new materials.