Showing papers on "Epoxide published in 2018"
TL;DR: In this paper, the authors investigated whether the presence of acid functional groups accelerate epoxy-amine curing reaction and found that epoxy/Fe3O4 nanocomposite was considered as a case, where discussions are based on chemistry, mechanism behind curing reactions and fracture behavior influenced by acid assistance to crosslinking.
73 citations
TL;DR: Alcohol-initiated ring-opening alternating copolymerization (ROAP) of phthalic anhydride (PA) and a variety of mono-, di-, and trisubstituted epoxides has been performed with a weak phosphazene base (t-BuP1) as the catalyst as mentioned in this paper.
Abstract: Alcohol-initiated ring-opening alternating copolymerization (ROAP) of phthalic anhydride (PA) and a variety of mono-, di-, and trisubstituted epoxides has been performed with a weak phosphazene base (t-BuP1) as the catalyst. Each product exhibits a perfectly alternating sequence distribution, controlled molar mass (Mn up to 124 kg mol–1), and low dispersity (ĐM < 1.15, mostly). Full conversion of PA can be reached in 0.5–24 h depending on the substituent of the epoxide, the targeted degree of polymerization, and the amount of t-BuP1 used (0.2–5 mol % of PA) when the reactions are conducted under solvent-free conditions at 100 °C with a small excess of the epoxide (0.5 equiv of PA). The glass transition temperature of the polyester ranges from −14 to 135 °C. The living nature of the ROAP allows one-pot construction of well-defined block-alternating copolymers through sequential addition of two epoxides. Statistical-alternating copolymers have also been synthesized by copolymerization of PA and two mixed ep...
63 citations
TL;DR: In this article, the authors synthesized a poly(terphenyl)-anchored covalent aromatic polymer (CAP-DAP) from p-terphenol and 1,3,5-benzene tricarbonyl chloride, followed by subsequent functionalization with 1, 3-diamino-2-propanol for CO2 capture and metal-free catalysis in CO2-epoxide cycloaddition reactions.
Abstract: Hydroxylamine-anchored covalent aromatic polymer (CAP-DAP) was synthesized from p-terphenyl and 1,3,5-benzene tricarbonyl chloride, followed by subsequent functionalization with 1,3-diamino-2-propanol for CO2 capture and metal-free catalysis in CO2–epoxide cycloaddition reactions. The novel CAP-DAP material was characterized using various analytical techniques. It showed very good CO2 adsorption capacity of 153 mg/g along with a high (CO2/N2) selectivity of 86 at 273 K/1 bar, in contrast to bare CAP, which exhibited moderate CO2 adsorption of 136 mg/g with a CO2/N2 selectivity of 47. CAP-DAP also displayed high catalytic activity for CO2–epoxide cycloaddition reactions under mild and solvent-free conditions. The synergistic effect between metal-free CAP-DAP and tetrabutylammonium bromide (n-Bu4NBr) enabled a high epoxide conversion of 98% coupled with an excellent product selectivity of 99% at 60 °C, 1 bar CO2, and a reaction time of 12 h. Faster reaction kinetics with reaction times <6 h was possible at ...
60 citations
TL;DR: Mechanistic studies revealed that the hydroxyl group promoted the activation of the epoxide facilitating theEpoxide ring-opening by nucleophilic attack of the iodide.
60 citations
TL;DR: The developed method for epoxidation reaction facilitated by low-temperature plasma enabled the structural analysis of unsaturated fatty acids (FAs) and was applied in a shotgun lipidomics approach to characterize phospholipids in a bovine liver extract.
Abstract: The presence of carbon–carbon double bonds (C═Cs) in unsaturated phospholipids is closely related to lipid conformations and physiochemical activities. Previously, we have demonstrated that epoxidation reaction facilitated by low-temperature plasma (LTP) enabled the structural analysis of unsaturated fatty acids (FAs). Epoxidation of the C═C leads to the production of an epoxide, which can be easily cleaved via collision-induced dissociation (CID) to produce diagnostic ions indicative of the C═C bond locations in FAs. In this work, we further developed this method for analysis of phospholipids. Tandem mass spectrometry analysis with epoxidation reaction was performed in both positive and negative ion mode to analyze phosphatidylcholines (PCs), phosphatidic acids (PAs), phosphatidylethanolamines (PEs), phosphatidylglycerols (PGs), and phosphatidylinositols (PIs). The developed method was applied in a shotgun lipidomics approach to characterize phospholipids in a bovine liver extract.
60 citations
TL;DR: In this article, a simple non-nucleophilic organobase as the catalyst was used to achieve sequence-selective terpolymerization from a mixture of phthalic anhydride (PA), an epoxide, and rac-lactide (LA).
Abstract: 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.
56 citations
TL;DR: In this paper, a combination of ultrahigh vacuum and in situ experimental methods along with theory was used to show that the only species that has been shown to produce ethylene oxide, the so-called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4.
Abstract: Silver’s unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of ultrahigh vacuum and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so-called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4. This adsorbate is part of a 2D Ag/SO4 phase, where the nonstoichiometric surface variant, with a formally S(V+) species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well-studied heterogeneously catalyzed reaction.
55 citations
TL;DR: Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed and rational optimization strategies were identified.
Abstract: Novel scorpionate-type organocatalysts capable of effectively coupling carbon dioxide and epoxides under mild conditions to afford cyclic propylene carbonates were developed. On the basis of a combined experimental and computational study, a precise mechanistic proposal was developed and rational optimization strategies were identified. The epoxide ring-opening, which requires an iodide as a nucleophile, was enhanced by utilizing an immonium functionality that can form an ion pair with iodide, making the ring-opening process intramolecular. The CO2 activation and cyclic carbonate formation were catalyzed by the concerted action of two hydrogen bonds originating from two phenolic groups placed at the claw positions of the scorpionate scaffold. Electronic tuning of the hydrogen bond donors allowed to identify a new catalyst that can deliver >90% yield for a variety of epoxide substrates within 7 h at room temperature under a CO2 pressure of only 10 bar, and is highly recyclable.
47 citations
TL;DR: In this paper, a series of metal complexes containing a metal ion (Zn, Cu, Pb, Ni, Co) and two quaternary phosphonium salt units anchored on the ligands were designed and synthesized as efficient single-component bifunctional catalyst for the solvent-free coupling reaction of CO2 and epoxides without any co-catalysts.
Abstract: A series of metal complexes containing a metal ion (Zn, Cu, Pb, Ni, Co) and two quaternary phosphonium salt units anchored on the ligands were designed and synthesized as efficient single-component bifunctional catalyst for the solvent-free coupling reaction of CO2 and epoxides without any co-catalysts. The effects of various reaction variables on the catalytic activity were investigated systematically and the optimized reaction conditions were screened as (130 °C, 1 MPa and 5 h). The order of the catalytic activity for these single-component bifunctional metal complexes was found to be Zn-PPBCl (96%) > Co-PPBCl (90%) ≈ Ni-PPBCl (90%) > Pb-PPBCl (86%) > Cu-PPBCl (15%). Notably, a high turnover frequency (TOF) value (1230 h−1) for bifunctional catalyst Zn-PPBCl was achieved via adjusting reaction variables. Moreover, this series of catalysts can also catalyze the coupling reaction at atmospheric pressure, and most of them showed high conversion of epoxide (propylene carbonate (PC) yield > 90%) within 7 h. The catalysts are also applicable to a variety of epoxides and good catalytic performances were achieved for producing the corresponding cyclic carbonates in most cases. Furthermore, the catalyst can be easily recycled and reused for at least five times without dramatic activity loss after simple filtration and drying. Finally, kinetic studies were carried out preliminarily for four metal (Zn, Co, Ni, Pb) catalysts and the formation activation energies (Ea) of cyclic carbonate were obtained. The apparent activation energy Ea catalyzed by Zn-PPBCl is only 37.6 kJ/mol, while the Ea (Pb-PPBCl) is 70.0 kJ/mol, Ea (Ni-PPBCl) is 65.6 kJ/mol, and Ea (Co-PPBCl) is 43.1 kJ/mol. The sequence of Ea agrees well with the catalytic activity. Based on the kinetic studies and experimental results, an inferred mechanism was deduced.
37 citations
TL;DR: In this article, meso-octamethylcalix[4]pyrrole 1a, which was synthesized easily in one step by the acid-catalyzed condensation of pyrrole with acetone, was found to be a very active and robust organocatalyst.
35 citations
TL;DR: In this article, an enantioselective heterogeneous catalyst, GO-[Mn(TPyP)tart], was prepared by covalent attachment of Mn(III) complex of H2TPYP via the propyl linkage to graphene oxide (GO) nanosheet and using chiral tartrate counter ion.
TL;DR: The catalyst had an excellent reusability and maintained a continuous high selectivity and only a small amount of leaching was observed from the spent catalyst, as a result of the synergetic effect of the dual-functional sites.
Abstract: A pyridinium-based ionic-liquid-decorated 1 D metal-organic framework (MOF; IL-[In2 (dpa)3 (1,10-phen)2 ]; IL=ionic liquid; dpa=diphenic acid; 1,10-phen=1,10-phenanthroline) was developed as a bifunctional heterogeneous catalyst system for CO2 -oxirane coupling reactions. An aqueous-microwave route was employed to perform the hydrothermal reaction for the synthesis of the [In2 (dpa)3 (1,10-phen)2 ] MOF, and the IL-[In2 (dpa)3 (1,10-phen)2 ] catalyst was synthesized by covalent postfunctionalization. As a result of the synergetic effect of the dual-functional sites, which include Lewis acid sites (coordinatively unsaturated In sites) and the I- ion in the IL functional sites, IL-[In2 (dpa)3 (1,10-phen)2 ] displayed a high catalytic activity for CO2 -epoxide cycloaddition reactions under mild and solvent-free conditions. Microwave pulses were employed for the first time in MOF-catalyzed CO2 -epoxide cycloaddition reactions to result in a high turnover frequency of 2000-3100 h-1 . The catalyst had an excellent reusability and maintained a continuous high selectivity. Furthermore, only a small amount of leaching was observed from the spent catalyst. A plausible reaction mechanism based on the synergistic effect of the dual-functional sites that catalyze the CO2 -epoxide cycloaddition reaction effectively is proposed.
TL;DR: In this article, a series of cross-linked polymer nanoparticles grafted zinc-containing imidazolium ionic liquids (PNPs-Im2ZnBr2X2, X = Cl, Br, and I) were designed and facilely synthesized.
Abstract: A series of cross-linked polymer nanoparticles grafted zinc-containing imidazolium ionic liquids (PNPs-Im2ZnBr2X2, X = Cl, Br, and I) were designed and facilely synthesized. Evaluated in chemical fixation of CO2 with epoxides into cyclic carbonates in the absence of any solvent and co-catalyst, the PNPs-Im2ZnBr2X2 showed high and stable activity, superior to other mostly reported ionic liquid catalysts. The effects of reaction temperature, CO2 pressure, reaction time and the loading of catalyst on the synthesis of propylene carbonate (PC) from CO2 and propylene oxide were investigated. Under the optimized reaction conditions, PNPs-Im2ZnBr2I2-1 showed the highest catalytic activity, and PC was yielded to 95.5% with 99.5% of selectivity and 1910 h–1 of TOF. In the cycloaddition reaction of CO2 and epoxides, the catalysts shows good chemical stability, and can be separated conveniently from the products after reaction and reused for five runs without significant loss of activity. It is suggested that the superior catalytic performance of the as-fabricated catalysts may be attributed to the synergistic effects of zinc atom, imidazolium cation, and halide anion on the activation of epoxide and CO2, respectively. Due to the excellent stability, activity, and recyclability, the PNPs-Im2ZnBr2X2 catalysts have broad prospects for the practical conversion of CO2 in chemical industry.
TL;DR: Density functional theory calculations of the intrinsic reaction pathways showed that the reactions of the ketones 1 with the chloromethyl anions 2 yield two rotational isomers of the intermediate halohydrin anions 4, only one of which can cyclize while the other undergoes retroaddition because the barrier for rotation is higher than that for reversal to the reactants 1 and 2.
Abstract: The kinetics of epoxide formation by Darzens condensation of aliphatic ketones 1 with arylsulfonyl-substituted chloromethyl anions 2 (ArSO2CHCl–) have been determined photometrically in DMSO solution at 20 °C. The reactions proceed via nucleophilic attack of the carbanions at the carbonyl group to give intermediate halohydrin anions 4, which subsequently cyclize with formation of the oxiranes 3. Protonation of the reaction mixture obtained in THF solution at low temperature allowed the intermediates to be trapped and the corresponding halohydrins 4-H to be isolated. Crossover experiments, i.e., deprotonation of the halohydrins 4-H in the presence of a trapping reagent for the regenerated arylsulfonyl-substituted chloromethyl anions 2, provided the relative rates of backward (k–CC) and ring closure (krc) reactions of the intermediates. Combination of the kinetic data (k2exptl) with the splitting ratio (k–CC/krc) gave the second-order rate constants kCC for the attack of the carbanions 2 at the ketones 1. T...
TL;DR: A surprising pathway is revealed in which a thiourea catalyst serves as a nucleophile in the cleavage of the epoxide ring in cycloadditions of epoxides with CO2 to synthesize cyclic five-membered ring organic carbonates.
Abstract: Cycloadditions of epoxides with CO2 to synthesize cyclic five-membered ring organic carbonates are of broad interest from a synthetic, environmental, and green chemistry perspective, and the development of effective catalysts for these transformations is an ongoing challenge. A series of eight charge-containing thiourea salts that catalyze these reactions under mild conditions (i.e., 60 °C and atmospheric CO2 pressure) are reported. Substrate scope and mechanistic studies were also carried out, isotope effects were measured, and a reactive intermediate was isolated revealing a surprising pathway in which a thiourea catalyst serves as a nucleophile in the cleavage of the epoxide ring.
TL;DR: In this paper, the authors developed a limonene epoxidation process using environment-friendly H2O2, with high H 2O2 conversion (∼95%) and selectivity to the epoxide (100%).
Abstract: The development of a limonene epoxidation process using environment-friendly H2O2, with high H2O2 conversion (∼95%) and selectivity to the epoxide (100%), is reported in this paper. Parametric studies of temperature, oxidant, solvent, acid concentration and sodium sulphate amounts were performed with the focus on establishing a rapid and highly selective process. Approximately 95% conversion of H2O2 at 100% selectivity to limonene-1,2-epoxide was achieved in 15 minutes with a single-step addition of oxidant. The operating conditions included a 323 K temperature in a solvent-free environment, with a limonene/H2O2/catalyst molar ratio of 4 : 1 : 0.005, using a tungsten-based polyoxometalates. To prevent the hydrolysis of the epoxide, the reaction mixture was saturated with sodium sulphate. An acid concentration of lower than 0.04 M was used and found to have significant effect on the selectivity. Kinetic studies were performed to allow modelling of the reaction scheme. The activation energy was determined to be ∼36 kJ mol−1.
TL;DR: In this article, three Zn complexes based on the N4-tris(2-aminoethyl)-amine (tren) chelating ligand and presenting a C3 symmetrical axis have been synthesized and successfully applied in the coupling of CO2 with terminal and internal epoxides.
Abstract: Three Zn complexes based on the N4-tris(2-aminoethyl)-amine (tren) chelating ligand and presenting a C3 symmetrical axis have been synthesized and successfully applied in the coupling of CO2 with terminal and internal epoxides. These complexes proved to be efficient catalysts when associated with tetrabutylammonium iodide, even at low catalyst loading (0.005 mol%) or at room temperature, allowing for the production of cyclic carbonates in good to high yields. Variation of the substitution pattern on the tren ligand was shown to greatly impact the catalyst performance with the highest TON (up to 11200) being achieved with the less sterically hindered methyl substituted Zn(II)-azatrane complex, 2a. These binary Zn-azatrane/NBu4I catalytic systems could be applied to a wide range of epoxide substrates including the more challenging internal epoxides. Moreover, although soluble in the reaction medium, Zn-azatrane catalysts could be easily recovered and reused up to three times without substantial loss of activity, proving their robustness under the reaction conditions.
TL;DR: The chloro-substituted complex displayed remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO2, reaching turnover frequencies (TOF) up to 760 h-1 in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO2 pressure.
Abstract: Three phenoxyimine Fe(III)Cl complexes bearing electronically diverse -Cl, -H or -tBu substituents in the ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear structures with pentacoordinate iron centres and trigonal bipyramidal geometries. All three complexes demonstrated excellent catalytic activities towards CO2/epoxide coupling to selectively form cyclic carbonates, with catalyst activity correlating with the electron withdrawing nature of the ortho-substituent (Cl > H > tBu) and thus the Lewis acidity of the metal centre. The chloro-substituted complex displayed remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO2, reaching turnover frequencies (TOF) up to 760 h−1 in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO2 pressure. Importantly, the catalyst is also very robust, functioning with high substrate loading, under air or in the presence of water. The substrate scope was successfully extended to other terminal epoxides including epichlorohydrin (TOF = 900 h−1) and to the more challenging internal epoxide, cyclohexene oxide (TOF = 80 h−1). These are amongst the highest TOF values reported for an iron CO2/epoxide coupling catalyst to date.
TL;DR: Analysis of epidermal lipids indicates that linoleate is converted to a trihydroxy derivative by hydrolysis of an epoxy-hydroxy precursor, which implicate the activities of EPHX2 and E PHX3 in production of the linolesate triols detected as end products of the 12R-LOX pathway in the epidermis and implicate their functioning in formation of the mammalian water permeability barrier.
29 Mar 2018
TL;DR: Several Fe(II) complexes were prepared as catalysts for the synthesis of oxazolidinones from terminal epoxides, aryl amines, and carbon dioxide.
Abstract: Several Fe(II) complexes were prepared as catalysts for the synthesis of oxazolidinones from terminal epoxides, aryl amines, and carbon dioxide. DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) was identified as a highly active co-catalyst for the formation of oxazolidinones. Mechanistic studies reveal that Fe(II) complexes act as reservoirs of the two active species including Lewis acid and carbene. The epoxides were activated via a Fe-O σ-bond between the Fe atom of the Lewis acid and the oxygen atom of the epoxide. Simultaneously, a nucleophilic carbene activates CO2 to form a carbene−CO2 adduct. In addition, interactions between the Lewis acid and epoxides accelerate the reaction of epoxides with aryl amines. DBU plays a crucial role in the reaction between the β-amino alcohol and cyclic carbonate. Under mild reaction conditions, various epoxides and aryl amines were converted to the corresponding oxazolidinones with good yields in the presence of CO2.
TL;DR: In this article, the effect of ligand structure on the catalytic activity of salophen chromium(III) complexes in the ring-opening copolymerization of phthalic anhydride with a series of epoxides was studied.
TL;DR: In this article, a tetra-nitrogenated Zn-Schiff-base complexes have been synthetized and studied by X-ray diffraction, and two plausible pathways of CO2/epoxide coupling mechanism were proposed without halide intervention, where both reactants are activated by the same Zn metal center.
TL;DR: The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions.
Abstract: Mn(III)-iodosylarene porphyrin adducts, [Mn(III)(ArIO)(Porp)]+, were synthesized by reacting electron-deficient Mn(III) porphyrin complexes with iodosylarene (ArIO) at -60 °C and characterized using various spectroscopic methods. The [Mn(III)(ArIO)(Porp)]+ species were then investigated in the epoxidation of olefins under stoichiometric conditions. In the epoxidation of olefins by the Mn(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. For example, cyclohexene oxide was formed exclusively with trace amounts of allylic oxidation products; cis- and trans-stilbenes were oxidized to the corresponding cis- and trans-stilbene oxides, respectively. In the catalytic epoxidation of cyclohexene by an electron-deficient Mn(III) porphyrin complex and sPhIO at low temperature (e.g., -60 °C), the Mn(III)-iodosylarene porphyrin species was evidenced as the active oxidant that effects the olefin epoxidation to give epoxide as the product. However, at high temperature (e.g., 0 °C) or in the case of using an electron-rich manganese(III) porphyrin catalyst, allylic oxidation products, along with cyclohexene oxide, were yielded, indicating that the active oxidant(s) was not the Mn(III)-iodosylarene adduct but probably high-valent Mn-oxo species in the catalytic reactions. We also report the conversion of the Mn(III)-iodosylarene porphyrins to high-valent Mn-oxo porphyrins under various conditions, such as at high temperature, with electron-rich porphyrin ligand, and in the presence of base (OH-). The present study reports the first example of spectroscopically well-characterized Mn(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions. Other aspects, such as one oxidant versus multiple oxidants debate, also were discussed.
TL;DR: Rapid evolution of hydrogen gas was demonstrated, indicating that Cp2Ti(H)Cl is indeed a thermally unstable molecule, which undergoes intermolecular reductive elimination of hydrogen under the reaction conditions.
TL;DR: It is reported that an alkyloxy(diaryl)borane (C6F5)2BOR is readily formed as a catalytically competent species for the outer-sphere hydrosilylation of epoxides and cyclic ethers.
TL;DR: The graphene oxide (GO) assisted allylic alkylation of thiophenes with alcohols is presented and the cooperative action of the Brønsted acidity, epoxide moieties, and π-surface of the 2D-promoter is highlighted as crucial in the reaction course of the present Friedel-Crafts-type protocol.
TL;DR: In this article, a QM/MM computational study of the cycloaddition between carbon dioxide (CO2) and styrene oxide in two different metal-organic frameworks (MOFs), Co-MOF-74 and Mg-MOFs-74, was performed to obtain atomic level insights into the catalytic mechanism.
Abstract: We report a QM/MM computational study of the cycloaddition between carbon dioxide (CO2) and styrene oxide in two different metal–organic frameworks (MOFs), Co-MOF-74 and Mg-MOF-74, to obtain atomic-level insights into the catalytic mechanism. The results suggest that both reactions begin by forming Lewis acid–base pairs between the epoxide and an open metal site and between CO2 and a phenolate moiety in the linker. Consequently, higher electrophilic and nucleophilic reactivities are conferred on the epoxide and CO2 (CO2(A)), respectively, thereby facilitating the initial ring-opening of the epoxide moiety. The ring-opening process is followed by the adsorption of a second CO2 molecule (CO2(B)), which is necessary for the subsequent ring closure to occur. In the case of Co-MOF-74, the binding of CO2(B) to the alkoxide oxygen increases the flexibility of the substrate moiety, enabling the cyclization pathway in which CO2(A) is incorporated into the final product. In contrast, the carbonate intermediate in t...
TL;DR: In this article, three Mn(III) complexes and one Mn(II) complex were studied for their efficiency as catalysts for epoxidation of olefins with H2O2 at room temperature and 0 °C in the presence of ammonium acetate-acetic acid or triethylamine-perchloric acid system as co-catalyst/buffer.
TL;DR: Based on computational and kinetic studies, the mechanism of these reactions was found to proceed via activation of the epoxide by the Lewis acidic borane moiety followed by nucleophilic attack of the phosphine of a second FLP molecule.
Abstract: Treatment of the preorganized frustrated Lewis pairs (FLPs) tBu2 PCH2 BPh2 (1) and o-Ph2 P(C6 H4 )BCat (Cat=catechol) (4) with 2-methyloxirane, 2-phenyloxirane and 2-(trifluoromethyl)oxirane resulted in epoxide ring-opening to yield the six- and seven-membered heterocycles 2 a-c and 5 a-c, respectively. These zwitterionic products were characterized spectroscopically, and compounds 2 a, 2 b, 5 a and 5 c were structurally characterized by single-crystal X-ray structure analyses. Based on computational and kinetic studies, the mechanism of these reactions was found to proceed via activation of the epoxide by the Lewis acidic borane moiety followed by nucleophilic attack of the phosphine of a second FLP molecule. The resulting chain-like intermediates afford the final cyclic products by ring-closure and concurrent release of the second equivalent of FLP that behaves as catalyst in this reaction.