Facile Ring Opening Research Articles

Aluminadichalcogeniranes and Aluminatrichalcogenetanes: Heavier Analogs of Dioxiranes and Trioxetanes.

Dioxiranes and their heavier chalcogen analogs have long been recognized as pivotal reagents and intermediates in synthetic chemistry, while trioxetanes have largely remained theoretical constructs. In this work, we present the synthesis of neutral, isoelectronic aluminum/chalcogen analogs of dioxiranes and trioxetanes, specifically aluminadiselenirane, aluminaditellurirane, aluminatriselenetane, aluminatritelluretane, and a mixed Se/Te analog of aluminatrichalcogenetane. These compounds, featuring strained AlCh2 and AlCh3 ring (Ch = Se, Te), exhibit significant polarization between the aluminum and chalcogen components. Reactivity studies have revealed that both aluminadiselenirane and aluminaditellurirane show pronounced ambiphilic behavior when interacting with proximal substrates, undergoing facile ring-opening (cyclo)additions. Additionally, the coordination chemistry of aluminatriselenetane with a Lewis acid has been documented. These investigations not only broaden the repertoire of unique main group species but also provide access to heterocyclic structures that present synthetic challenges through conventional methods.

Electrosynthesis of methyl 2-ureidobenzoates via a C2–C3 bond cleavage of isatins

A facile and efficient ring opening of isatin has been achieved by electrochemical anodic oxidation directly at room temperature. The results show that a constant current electrolysis of isatins in an undivided cell affords both the ester group and the urea group simultaneously. This reaction opens a new avenue for the synthesis of methyl 2-ureidobenzoates, enriches the research on the reactions of isatins involving the cleavage of the C2–C3 bond and shows a good range of substrates for both isatin and simple organic amine in an acidic buffer solutions of HOAc and Cu(OAc)2. In addition, this electrochemical C2–C3 bond cleavage reaction could be scaled up.

Reactivity of 1.1.1-Propellane with (silox)3M (M = Ti, V, Cr): Structures of (silox)3V═(cC4H4)═CH2 and [(silox)3Cr–(1.1.1-C5H6)−]2

Application of the two-orbital, two-electron, four-state paradigm to 1.1.1-propellane (111P), which contains an unusual “inverted” central bond, suggests that an electrophilic metal center can activate it toward ring opening. Low-valent, early-metal complexes ((silox)3M (M = Ti (1), V (2) (or M = V(THF) (2-THF), Cr (3))) were employed to test this assessment with mixed results. Facile ring opening of 111P occurs for 1, producing known degradation products, and 2, which generates the alkylidene (silox)V═(cC4H4)═CH2 (2═C5H6), a species capable of catalyzing the ROMP (ring-opening metathesis polymerization) of norbornene. For 3 and 111P, a radical-like dimerization to [(silox)3Cr-(1.1.1-C5H6)−]2 (or (silox)3Cr(C5H6)2Cr(silox)3 (32P2)) effectively competes with ring opening. In conjunction with the experiments, high-level calculations provide a rationale for the trichotomy in reactivity.

Utilization of CO2-Available Organocatalysts for Reactions with Industrially Important Epoxides

Recent knowledge in chemistry has enabled the material utilization of greenhouse gas (CO2) for the production of organic carbonates using mild reaction conditions. Organic carbonates, especially cyclic carbonates, are applicable as green solvents, electrolytes in batteries, feedstock for fine chemicals and monomers for polycarbonate production. This review summarizes new developments in the ring opening of epoxides with subsequent CO2-based formation of cyclic carbonates. The review highlights recent and major developments for sustainable CO2 conversion from 2000 to the end of 2021 abstracted by Web of Science. The syntheses of epoxides, especially from bio-based raw materials, will be summarized, such as the types of raw material (vegetable oils or their esters) and the reaction conditions. The aim of this review is also to summarize and to compare the types of homogeneous non-metallic catalysts. The three reaction mechanisms for cyclic carbonate formation are presented, namely activation of the epoxide ring, CO2 activation and dual activation. Usually most effective catalysts described in the literature consist of powerful sources of nucleophile such as onium salt, of hydrogen bond donors and of tertiary amines used to combine epoxide activation for facile epoxide ring opening and CO2 activation for the subsequent smooth addition reaction and ring closure. The most active catalytic systems are capable of activating even internal epoxides such as epoxidized unsaturated fatty acid derivatives for the cycloaddition of CO2 under relatively mild conditions. In case of terminal epoxides such as epichlorohydrin, the effective utilization of diluted sources of CO2 such as flue gas is possible using the most active organocatalysts even at ambient pressure.

Ruthenium-Hydride Assisted Remarkable Diversity Towards Non-Spectator Feature of Benzodifuroxan.

The present article demonstrates that ruthenium-hydride [RuII (H)(Cl)(CO)(PPh3 )3 ] mediated diverse functionalization modes of benzodifuroxan (BDF) encompassing two furoxan rings. Hydride transfer from the metal precursor facilitated multiple cascade reactions involving unsymmetrical cleavage of the furoxan rings of BDF, leading to the one-pot formation of a series of ruthenium (II) coordinated functionalized ligands exhibiting bidentate κ2 -N,O, κ2 -N,N' and bis-bidentate μ-bis(κ2 -N,O) modes. Further, a moderately stable intermediate species was also encountered in the reaction sequence in which the transformed deoxygenated ligand coordinated to the metal ion via the rarely manifested furazan ring (κ2 -N,N'' mode). The products were authenticated by their single-crystal X-ray structures and other spectroscopic/analytical techniques. Redox non-innocence of the functionalized ligands in the complexes was illustrated by spectroelectrochemistry (cyclic voltammmetry, UV-Vis. and EPR) in conjunction with DFT/TD-DFT calculations. Mechanistic outline for the facile ring opening processes of BDF including interconversions of complexes (e. g. reductive ring opening) were also addressed.

Experimental and Computational Studies of Microwave-Assisted, Facile Ring Opening of Epoxide with Less Reactive Aromatic Amines in Nitromethane.

Nucleophilic ring opening reactions of epoxides with aromatic amines are in the forefront of the synthetic organic chemistry research to build new bioactive scaffolds. Here, convenient, green, and highly efficient regioselective ring opening reactions of sterically hindered (2R,3S)-3-(N-Boc-amino)-1-oxirane-4-phenylbutane with various poorly reactive aromatic amines are accomplished under microwave irradiation in nitromethane. All the reactions effectively implemented for various aromatic amines involve the reuse of nitromethane that supports its dual role as a solvent and catalyst. The corresponding new β-alcohol analogs of hydroxyethylamine (HEA) are isolated in 41–98% yields. The reactions proceed under mild conditions for a broad range of less reactive and sterically hindered aromatic amines. Proton NMR experiments suggest that the nucleophilicity of amines is influenced by nitromethane, which is substantiated by the extensive computational studies. Overall, this methodology elucidates the first-time use of nitromethane as a solvent for the ring opening reactions under microwave conditions involving an equimolar ratio of epoxide and aromatic amine without any catalyst, facile ring opening of complex epoxide by less reactive aromatic amines, low reaction time, less energy consumption, recycling of the solvent, and simple workup procedures.

Selective Catalytic Synthesis of 1,2- and 8,9-Cyclic Limonene Carbonates as Versatile Building Blocks for Novel Hydroxyurethanes.

The selective catalytic synthesis of limonene‐derived monofunctional cyclic carbonates and their subsequent functionalisation via thiol–ene addition and amine ring‐opening is reported. A phosphotungstate polyoxometalate catalyst used for limonene epoxidation in the 1,2‐position is shown to also be active in cyclic carbonate synthesis, allowing a two‐step, one‐pot synthesis without intermittent epoxide isolation. When used in conjunction with a classical halide catalyst, the polyoxometalate increased the rate of carbonation in a synergistic double‐activation of both substrates. The cis isomer is shown to be responsible for incomplete conversion and by‐product formation in commercial mixtures of 1,2‐limomene oxide. Carbonation of 8,9‐limonene epoxide furnished the 8,9‐limonene carbonate for the first time. Both cyclic carbonates underwent thiol–ene addition reactions to yield linked di‐monocarbonates, which can be used in linear non‐isocyanate polyurethanes synthesis, as shown by their facile ring‐opening with N‐hexylamine. Thus, the selective catalytic route to monofunctional limonene carbonates gives straightforward access to monomers for novel bio‐based polymers.

Cure kinetics of epoxy/graphene oxide (GO) nanocomposites: Effect of starch functionalization of GO nanosheets

Graphene oxide (GO) nanoflakes are reactive towards both epoxy resin and amine curing agent, but their inevitable agglomeration worsens resin/hardener/nanoflake interfacial interactions. In this work, starch was used to functionalize GO nanoflakes. Isoconversional cure kinetics of low concentration epoxy/GO and epoxy/starch-functionalized GO (GO-St) nanocomposites was then studied based on nonisothermal differential scanning calorimetry (DSC) analyses to evaluate the effect of starch-aided epoxy/amine/GO curing reactions. Qualitative analysis of epoxy network formation governed by GO and GO-St was done in terms of Cure Index, where Good and Excellent cure were interestingly observed as a result of starch functionalization. Quantitative cure analyses based on integral and differential isoconversional methods were also allowed for understanding conversion-dependent cure kinetics in terms of activation energy (Eα) alteration. The activation energy increased for neat epoxy system at later stage of curing reaction. However, an almost constant trend was seen for the activation energy of GO incorporated epoxy nanocomposites. Observation of a decline in Eα values for epoxy/GO-St system with respect to epoxy/GO was due to the autocatalytic curing reaction, which was attributed to facile epoxide ring opening by OH groups of starch. A good confidence was observed between calculated and observed calorimetric data.

Ring-Opening Functionalization of Simple gem-Difluorocyclopropanes by Single-Electron Oxidants.

It was reported for the first time that single-electron oxidants such as CAN or K2S2O8 affected facile ring opening of simple gem-difluorocyclopropanes to afford 1,3-dibromo-2,2-difluoropropanes in good yields by the action of KBr, and the appropriate choice of conditions allowed to incorporate not only second halogen atoms but also hydroxy or acetamido groups at the C1 position in the difluoropropane structures in a regiospecific fashion after initiation of the reaction by the introduction of the first bromine atom at the C3 position.

Microwave-assisted synthesis of 3-aminoarylquinolines from 2-nitrobenzaldehyde and indole via SnCl2-mediated reduction and facile indole ring opening

A simple and efficient one-pot two-step synthesis of substituted 3-aminoarylquinolines has been achieved from 2-nitrobenzaldehyde and indoles under microwave irradiation. Firstly 2-nitrobenzaldehydes is reduced to 2-aminobenzaldehyde in situ by commonly used chemo selective reductant SnCl2 followed by condensation of indole. The acidic nature of the resultant reaction mixture due to SnCl2 helps in the condensation and facile ring opening of indole leading to the formation of 3-aminoarylquinoline derivatives in good to moderate yields.

Commercial Polymer Microsphere Grafted TBD-Based Ionic Liquids as Efficient and Low-Cost Catalyst for the Cycloaddition of CO2 with Epoxides

Development of efficient, cheap and recyclable catalysts for the synthesis of cyclic carbonates from CO2 and epoxides is still a very attractive topic. Herein, the polymer grafted TBD-based ionic liquids (ILs) were fabricated from commercially available polystyrene (PS) and ingredients of the ILs, and used as heterogeneous catalysts for the conversion of CO2 into cyclic carbonates in the absence of solvent and co-catalyst. To improve the catalytic performance, various substitutes (such as –COOH, –OH and –NH2) were functionalized on the TBD cation. Among the as-obtained catalysts, carboxyl-containing catalyst (PS–[CETBD]Br) showed superior activity than others, which may be attributed to the stronger polarization capability of –COOH on the C–O bond of epoxide through the formed hydrogen bonding. Additionally, the combined synergistic effect of the nucleophilic attack by the halide anions also account for the facile ring-opening of epoxide. It is noted that CO2 can be activated by the formation of carbamate between CO2 and alkaline nitrogen of the TBD cations, thus facilitating the formation of cyclic carbonates. Moreover, the catalyst shows good chemical stability and catalytic reusability, which is very important for the practical conversion of CO2 in chemical industry.

Facile catalytic ring opening polymerization of lactic acid: Comparing the performance of Fe and Zn metal species

As a bio‐compatible/degradable polymer, poly(lactic acid) (PLA) has been widely noticed in tissue engineering and medical applications. Proper catalytic ring‐opening polymerization of lactic acid is pertinent to development of active metal species capable of producing high‐molecular‐weight polymers. In this work, ligand catalyst (2,4‐di‐tertio‐butyl‐6‐{[(2‐di‐methyl‐amino ethyl) methyl amino] methyl} phenol) was synthesized and then attached to Fe‐based and Zn‐based metal substitutes, and performance of metal species toward ring opening polymerization of lactic acid has been investigated. Regulation of reaction condition at 180°C for 36 h with Zn‐based catalyst allowed for late‐stage production of a high‐molecular‐weight PLA, while Fe‐based facilitated polymerization at early stages leading to PLAs with comparatively twofold higher molecular weight at lower time, monomer‐to‐initiator ratio, and temperature (at 170°C). A ratio of monomer to initiator of 5000 with Fe catalyst resulted in the highest molecular weight. Thus, polymerization has been facilitated by the use of Fe‐based catalyst. For this class of catalyst with Fe substitute, the product was characterized by means of 1H‐NMR, Fourier transform infrared spectroscopy, and differential scanning calorimetry analyses. J. VINYL ADDIT. TECHNOL., 25:215–224, 2019. © 2018 Society of Plastics Engineers

A Facile Epoxide Aminolysis Promoted by (t-BuO)2Mg and Its Application to the Synthesis of Efinaconazole

A novel and efficient method for the aminolysis of trizole epoxides is described. This method consists of a facile ring opening of the epoxides mediated by t-BuOMgCl generated in situ from amine hydrochlorides and (t-BuO)2Mg. The desired β-amino alcohols were obtained in good yields without employing other heavy metals or precious catalysts. The optimized conditions were successfully applied to the synthesis of a number of potential triazole antifungal compounds as well as efinaconazole on up to a 700 g scale.

On the role of oxocarbenium ions formed in Brønsted acidic condition on γ-Al2O3 surface in the ring-opening of γ-valerolactone

The ring-opening and decarboxylation reaction of γ-valerolactone (GVL), yielding a mixture of butene isomers, constitutes an important transformative step for the upgradation of biomass-derived precursors into high value fuels and chemicals. In this study, density functional theory (DFT) simulations were utilized to gain insights into the ring-opening of GVL in presence of Brønsted acid promoted γ-Al2O3 catalyst. DFT simulations suggested towards the formation of stable oxocarbenium ion of GVL on the γ- Al2O3 surface via a proton transfer to the carbonyl oxygen in presence of the Brønsted acid. The surface oxygen atoms of γ-Al2O3 were observed to show high affinity for H-abstraction from the oxocarbenium ion to yield the carbenium ion intermediate, which subsequently gave the product butene via a combination of hydride transfer and decarboxylation steps. Higher reactivity of the Brønsted acid promoted γ-Al2O3 surface, was attributed to the direct ring-opening (Ea = 13 kJ/mol) of the surface adsorbed oxocarbenium ion of GVL, leading to the formation of the γ-carbenium ion, which was unlikely to form with the adsorbed GVL structure. In addition, H-abstraction by the surface oxygen from the C5 of adsorbed oxocarbenium ion (Ea = 13 kJ/mol) was calculated to be of lower activation barrier as compared to the H-abstraction from C5 of the adsorbed GVL (Ea = 40 kJ/mol), which may also lead to facile ring-opening in presence of a Brønsted acid on γ-Al2O3.

Facile ring opening reaction of oxazolone enables efficient amidation for aminoisobutyric acid.

4,4-Dimethyloxazolones derived from N-protected aminoisobutyric acid (AIB) are particularly known as poor electrophiles due to the steric hindrance around the carbonyl and not employed as useful intermediates for amidation whereas numerous examples have been reported to support the utility of other oxazolones in amidation. AIB is an important and strategical synthon in medicinal chemistry but the peptide bond formation of the N-protected urethane derivatives of AIB is known to be often unproductive due to the rapid formation of the stable 4,4-dimethyloxazolone via an intramolecular cyclization. We discovered that the 4,4-dimethyloxazolone of an AIB urethane is in fact an excellent electrophile that enables efficient amidation even with weakly reactive nucleophiles. The 4,4-dimethyloxazolone can be stored in a pure form and used as a reagent offering an efficient and convenient synthetic tool for generating AIB-peptide analogs.

Direct Aziridination of Nitroalkenes Affording N-Alkyl-C-nitroaziridines and the Subsequent Lewis Acid Mediated Isomerization to β-Nitroenamines

A mild and highly diastereoselective one-pot synthesis of trans-N-alkyl-C-nitroaziridines was achieved by treatment of nitroalkenes with aliphatic amines and N-chlorosuccinimide. Treatment of the obtained aziridines with a Lewis acid resulted in a facile ring opening reaction, accompanied by rearrangement and isomerization into functionalized (Z)-β-nitroenamines.

A Peerless Aproach: Organophotoredox/Cu(I) Catalyzed, Regioselective, Visible Light Facilitated, Click Synthesis of 1,2,3-Triazoles via Azide–Alkyne [3 + 2] Cycloaddition

A visible light initiated, mild, one pot, multicomponent copper catalyzed azide alkyne [3 + 2] cycloaddition in the presence of organo photoredox catalyst Eosin Y using EtOH:H2O as reaction medium for the synthesis of substituted 1,2,3-triazoles is reported. A facile regioselective ring opening of epoxides followed by 1, 3 dipolar cycloaddition with alkynes under CFL (Compact fluorescent light) irradiation as a source of visible light resulting in the formation of C–N bonds, is the characteristic feature of the present strategy. Scope of the present protocol is further extended by using benzyl chloride in place of epoxide.

Hydrodeoxygenation of Furylmethane Oxygenates to Jet and Diesel Range Fuels: Probing the Reaction Network with Supported Palladium Catalyst and Hafnium Triflate Promoter

Catalytic hydrodeoxygenation of furylmethane oxygenates to high carbon branched chain jet and diesel fuel range alkanes under mild reaction conditions is a promising strategy for energy-efficient production of fuels with minimal C–C cracking to undesired products. Here, we report that a strong Lewis acidic promoter can overcome the energy barrier for furylmethane hydrodeoxygenation at lower temperature. Furan rings of furylmethanes are first hydrogenated to fully saturated cyclic ethers by a hydrogenation catalyst, which then undergo facile ring opening and deoxygenation by the promoter. A cyclic intermediate between ethereal O and the Lewis acidic metal center, assisted by the triflate ligand of the promoter, is formed in the ring-opening step. Probing the reaction pathway with symmetric single furan ring surrogate molecules suggests that the promoter is necessary for the ring opening. Deoxygenation of ring-opened oxygenates takes place more quickly for single furan ring surrogates than for the multiple ...

Mechanistic insights into the bleaching of melanin by alkaline hydrogen peroxide

This work aims to determine the roles of reactive oxygen species HO∙ and HO2- in the bleaching of melanins by alkaline hydrogen peroxide. Experiments using melanosomes isolated from human hair indicated that the HO∙ radical generated in the outside solution does not contribute significantly to bleaching. However, studies using soluble Sepia melanin demonstrated that both HO2- and HO∙ will individually bleach melanin. Additionally, when both oxidants are present, bleaching is increased dramatically in both rate and extent. Careful experimental design enabled the separation of the roles and effects of these key reactive species, HO∙ and HO2-. Rationalisation of the results presented, and review of previous literature, allowed the postulation of a simplified general scheme whereby the strong oxidant HO∙ is able to pre-oxidise melanin units to o-quinones enabling more facile ring opening by the more nucleophilic HO2-. In this manner the efficiency of the roles of both species is maximised.

Alane-Centered Ring Expansion of N-Heterocyclic Carbenes

The β-diketiminato aluminum dihydrides, [HC{(Me)CNAr}2AlH2] [4: Ar = 2,6-di-isopropylphenyl (Dipp), 5: 2,4,6-trimethylphenyl (Mes)] react directly with N-aryl-substituted N-heterocyclic carbenes (NHCs) by C–N bond activation to afford aluminum amido-alkyl derivatives of the form [HC{(Me)CNAr}2AlCH2(N(Ar′)CH)2]. The more sterically congested alane (4), bearing N-Dipp substitution, does not react with either 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), even under forcing conditions. In contrast, in situ generation of 1,3-bis(phenyl)imidazol-2-ylidene through deprotonation of the corresponding imidazolium tetrafluoroborate by KN(SiMe3)2 in the presence of compound 4 provides facile ring opening of the NHC at room temperature to yield [HC{(Me)CNDipp}2AlCH2(N(Ph)CH)2]. Although compound 5 also does not react with IPr, relaxation of the steric demands of the supporting β-diketiminate ligand to N-mesityl substitution enables analogous ring op...