Acid-catalyzed Ring-opening Reaction Research Articles

Pyrrole Compounds from the Two-Step One-Pot Conversion of 2,5-Dimethylfuran for Elastomer Composites with Low Dissipation of Energy.

A one-pot, two-step process was developed for the preparation of pyrrole compounds from 2,5-dimethylfuran. The first step was the acid-catalyzed ring-opening reaction of 2,5-dimethylfuran (DF), leading to the formation of 2,5-hexanedione (HD). A stoichiometric amount of water and a sub-stoichiometric amount of sulfuric acid were used by heating at 50 °C for 24 h. Chemically pure HD was isolated, with a quantitative yield (up to 95%), as revealed by 1H-NMR, 13C-NMR, and GC-MS analyses. In the second step, HD was used as the starting material for the synthesis of pyrrole compounds via the Paal-Knorr reaction. Various primary amines were used in stoichiometric amounts. 1H-NMR, 13C-NMR, ESI-Mass, and GC-Mass analyses confirmed that pyrrole compounds were prepared with very good/excellent yields (80-95%), with water as the only co-product. A further purification step was not necessary. The process was characterized by a very high carbon efficiency, up to 80%, and an E-factor down to 0.128, whereas the typical E-factor for fine chemicals is between 5 and 50. Water, a co-product of the second step, can trigger the first step and therefore make the whole process circular. Thus, this synthetic pathway appears to be in line with the requirements of a sustainable chemical process. A pyrrole compound bearing an SH group (SHP) was used for the functionalization of a furnace carbon black (CB). The functionalized CB (CB/SHP) was utilized in place of silica, resulting in a 15% mass reduction of reinforcing filler, in an elastomeric composite based on poly(styrene-co-butadiene) from solution anionic polymerization and poly(1,4-cis-isoprene) from Hevea Brasiliensis. Compared to the silica-based composite, a reduction in the Payne effect of about 25% and an increase in the dynamic rigidity (E' at 70 °C) of about 25% were obtained with CB/SHP.

Catalytic one-pot conversion of biomass-derived furfural to ethyl levulinate over bifunctional Nb/Ni@OMC

As one of the important chemical feedstocks and biofuels, ethyl levulinate (EL) can be produced from biomass-derived furfural by consecutive hydrogenation, etherification and ring-opening reactions. However, this process normally requires two separate steps in different reaction systems, which is complex and energy consumption. Herein, Nb–Ni bifunctional ordered mesoporous carbons (Nb/Ni@OMCs) were synthesized by a two-step process where Ni@OMC was firstly synthesized by a solvent evaporation induced self-assembly (EISA) process using F127 as a template agent, polyphenol gallic acid as carbon precursor and Ni2+ as cross-linker, following with the supporting of Nb species in the Ni@OMC by an incipient wetness impregnation method. The as-synthesized Nb/Ni@OMC samples were applied for the one-pot catalytic transformation of furfural to EL, and exhibited good activity with the highest EL yield of 83% under an optimized condition. Two possible pathways for the one-pot production of EL were proposed according to two intermediates: (1) hydrogenation of furfural to form furfuryl alcohol (FFA) that is then etherified with ethanol to form 2-(ethoxymethyl)furan (2-EMF); and (2) acetalization and etherification of furfural with ethanol to produce 2-(diethoxymethyl)furan (2-DMF). Then both 2-DMF and 2-EMF are converted to EL through acid catalyzed ring opening reaction. The dominant reaction pathway varied with the properties of the catalyst and the applied reaction conditions.

Electrochemical Aziridination of Tetrasubstituted Alkenes with Ammonia

Ammonia (NH3) is an ideal nitrogen source in terms of availability, reactivity, safety, atom economy, environmental compatibility, and ease of isolation. However, its utility for amine synthesis is...

P‐Menthadienes as Biorenewable Feedstocks for a Monoterpene‐Based Biorefinery

AbstractA terpene‐based biorefinery is described that uses crude sulfate turpentine (CST) and gum turpentine (GT) to produce mixtures of p‐menthadienes (p‐MeDs) as biorenewable terpene feedstocks. An acid catalyzed ring opening reaction (6 m aq. H2SO4, 90 °C) is first used to convert the major bicyclic monoterpenes (α‐pinene, β‐pinene, and 3‐carene) in untreated CST (or GT with 5 mol% Me2S) into mixtures of monocyclic p‐MeDs. These unpurified sulfurous p‐MeD mixtures (α‐terpinene, γ‐terpinene, and isoterpinolene) are then used as feedstocks for oxidative aromatization (OA), ozonolysis, Diels–Alder, and hydrogenation reactions to produce p‐cymene, fragrances, anti‐oxidants, drugs, biopolymers, and biofuels. Mechanistic studies of the OA reaction used to convert the p‐MeDs into p‐cymene reveal that p‐cymene hydroperoxide acts as an initiator to produce polar radical intermediates that are stabilized by DMSO generated in situ through aerobic oxidation of Me2S. This enables CST and GT to be converted into biorenewable p‐cymene in 50–60% yields (two steps) using a process that only requires aqueous acid, oxygen, heat, and a final distillation step.

How Lewis Acids Catalyze Ring-Openings of Cyclohexene Oxide.

We have quantum chemically studied the Lewis acid-catalyzed epoxide ring-opening reaction of cyclohexene epoxide by MeZH (Z = O, S, and NH) using relativistic dispersion-corrected density functional theory. We found that the reaction barrier of the Lewis acid-catalyzed epoxide ring-opening reactions decreases upon ascending in group 1 along the series Cs+ > Rb+ > K+ > Na+ > Li+ > H+. Our activation strain and Kohn–Sham molecular orbital analyses reveal that the enhanced reactivity of the Lewis acid-catalyzed ring-opening reaction is caused by the reduced steric (Pauli) repulsion between the filled orbitals of the epoxide and the nucleophile, as the Lewis acid polarizes the filled orbitals of the epoxide more efficiently away from the incoming nucleophile. Furthermore, we established that the regioselectivity of these ring-opening reactions is, aside from the “classical” strain control, also dictated by a hitherto unknown mechanism, namely, the steric (Pauli) repulsion between the nucleophile and the substrate, which could be traced back to the asymmetric orbital density on the epoxide. In all, this work again demonstrates that the concept of Pauli-lowering catalysis is a general phenomenon.

Synthesis of Azocane- and Oxocane-Annulated Furans by a [2+2] Photocycloaddition–Ring-Opening Cascade

AbstractThe title compounds were synthesized from readily available quinolone and coumarin derivatives by a cascade reaction (12 examples, 90–98% yield). The cascade comprised a [2+2] photocycloaddition which occurred upon sensitized irradiation at λ = 420 nm (or direct UV irradiation at λ = 366 nm) and a subsequent acid-catalyzed ring-opening reaction. A variety of substituents are compatible with the conditions and a 3-alkyl group in the coumarin (or quinolone) is crucial to achieve a high chemoselectivity. Key to the success of the ring opening is the formation of a 4,5,5a-trihydrocyclobuta-2H-furan containing a strained bridgehead double bond which stems from the allenyl group tethered to the 4-position of the starting materials.

Mechanism of Pd/C-catalyzed hydrogenation of furfural under hydrothermal conditions

Catalytic hydrogenation of furfural under hydrothermal conditions is of great importance and has recently attracted huge attention as an approach to obtaining a wide range of valuable chemicals from a renewable source. However, the data concerning the reaction mechanism of aqueous-phase furfural hydrogenation are very scarce, and it is still not clear which reactions lead to various carbonyl compounds, such as cyclopentanone, 4-oxopentanal, 5-hydroxy-2-pentanone. Herein, we present the results of an experimental study of the mechanism of reactions occurring in the furfural hydrogenation under hydrothermal conditions over the 1% Pd/CNTs catalyst. The following approaches have been used to study the reaction mechanism: hydrogenation of key intermediates formed upon the furfural conversion, and isotopic labeling experiments using D2O as a tracer in hydrogenation of furfural, which provides direct evidence of the occurring reactions. It was found that the furfural hydrogenation process is realized through four pathways including various reductive and acid-catalyzed furan ring opening reactions. The methods and approaches used in the present work, as well as the results obtained, can be useful in studying the mechanism of aqueous-phase hydrogenation of a wide range of oxygen-containing substrates in the presence of catalysts of different compositions.

Rac- and meso-Cyclohexanoids: Their α-, β-glycosidases, antibacterial, antifungal activities, and molecular docking studies.

An efficient and versatile synthesis method has been postulated for hydroxymethylated rac- and meso-cyclohexanoid derivatives. The synthesis of these stereoisomers was achieved easily with traditional methods using hexahydroisobenzofuran 6, prepared from commercially available cis-hydrophthalic anhydride. The study, involving diastereoselective epoxidation and cis-hydroxylation, was conducted to obtain epoxy-, cis-, and trans-diol-furans 7, 8, and 9. After sulfamic acid-catalyzed ring-opening reaction of the epoxide and furan rings, rac- and meso-tetraacetates 14, 15, and 16 were afforded. Hydrolysis of acetate groups with ammonia in absolute methanol yielded the desired tetrols rac-17, meso-18, and meso-19. All structures, after purification by chromatographic methods and elucidation by spectral techniques, were screened against α- and β-glucosidases. Compounds 7, 8, 10, 17, 18, and 19 were also evaluated for their antibacterial and antifungal activity against some selected synthesized compounds with varying degrees of inhibitory effects on the growth of different pathogenic microorganisms by the well-diffusion method. In addition, Saccharomyces cerevisiae α-glucosidase molecular modeling studies were performed for all rac- and meso-compounds 7, 8, 10, 17, 18, and 19.

Thermal reaction of 1,8-diketone: A computational study

Thermal reaction of 1,8-diketone in presence of Lawesson's reagent (LR) or P4S10 under toluene reflux condition provided expected fused five-membered thiophene and dithiin compounds along with 2,4-diphenylthiophene (2,4-DPT), 2,5-diphenylthiophene (2,5-DPT) and 3,6-diphenylthieno[3,2-b][1,4]dithiine (3,6-DPTDT). DFT level computations conducted to shed light on this intriguing mechanism indicate in situ generation of phenylthiirene from phenylthiiran analogue, which leads to the formation of the 2,4-DPT and 2,5-DPT based on the literature. Phenylthiiran undergoes successive intramolecular reactions yielding a precursor 3-phenyl-1,2-dithiete, which renders 2,5- and 2-6-diphenyl-1,4-dithiin. These 1,4-dithiins result in 2,4-DPT and 2,5-DPT by overcoming relatively high activation barriers. On the other hand, acid catalyzed ring opening reactions of two phenylthiirenes followed by intramolecular reactions yield the 2,4-DPT and 2,5-DPT. Formation of the fused five-membered thiophene and 3,6-DPTDT takes place in successive reactions of 1,4-dithiin. These results decipher the intricate mechanism of thermal reaction of 1,8-diketone in presence of P4S10.

Prediction of Transformation Products of Monensin by Electrochemistry Compared to Microsomal Assay and Hydrolysis

The knowledge of transformation pathways and identification of transformation products (TPs) of veterinary drugs is important for animal health, food, and environmental matters. The active agent Monensin (MON) belongs to the ionophore antibiotics and is widely used as a veterinary drug against coccidiosis in broiler farming. However, no electrochemically (EC) generated TPs of MON have been described so far. In this study, the online coupling of EC and mass spectrometry (MS) was used for the generation of oxidative TPs. EC-conditions were optimized with respect to working electrode material, solvent, modifier, and potential polarity. Subsequent LC/HRMS (liquid chromatography/high resolution mass spectrometry) and MS/MS experiments were performed to identify the structures of derived TPs by a suspected target analysis. The obtained EC-results were compared to TPs observed in metabolism tests with microsomes and hydrolysis experiments of MON. Five previously undescribed TPs of MON were identified in our EC/MS based study and one TP, which was already known from literature and found by a microsomal assay, could be confirmed. Two and three further TPs were found as products in microsomal tests and following hydrolysis, respectively. We found decarboxylation, O-demethylation and acid-catalyzed ring-opening reactions to be the major mechanisms of MON transformation.

Lewis Acid Catalyzed Nucleophilic Ring Opening and 1,3-Bisfunctionalization of Donor-Acceptor Cyclopropanes with Hydroperoxides: Access to Highly Functionalized Peroxy/(α-Heteroatom Substituted)Peroxy Compounds.

The Lewis acid catalyzed ring opening reaction of Donor-Acceptor (D-A) cyclopropanes with alkyl hydroperoxides is reported to furnish various peroxycarbonyls and 1,3-haloperoxygenated compounds in good to excellent yields. This method adds another instance to scarcely reported noncyclilizing 1,3-bisfunctionalization of D-A cyclopropanes with two different functional groups and relies on the dual role of peroxide as nucleophile and oxidant through an orchestrated reaction sequence. The products obtained, including α-heterosubstituted peroxy compounds, are amenable to useful synthetic elaboration.

GaCl3-Catalyzed Ring-Opening Carbonyl-Olefin Metathesis.

The development of a Lewis acid-catalyzed ring-opening cross-metathesis reaction which enables selective access to acyclic, unsaturated ketones as the carbonyl-olefin metathesis products is described. While catalytic amounts of FeCl3 were previously identified as optimal to catalyze ring-closing metathesis reactions, the complementary ring-opening metathesis between cyclic alkenes and carbonyl functionalities relies on GaCl3 as the superior Lewis acid catalyst.

Acid-catalyzed ring-opening reactions of a cyclopropanated 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with alcohols.

The acid-catalyzed ring-opening reactions of a cyclopropanated 3-aza-2-oxabicylic alkene using alcohol nucleophiles were investigated. Although this acid-catalyzed ring-opening reaction did not cleave the cyclopropane unit as planned, this represent the first examples of ring-openings of cyclopropanated 3-aza-2-oxabicyclo[2.2.1]alkenes that lead to the cleavage of the C–O bond instead of the N–O bond. Different acid catalysts were tested and it was found that pyridinium toluenesulfonate in methanol gave the best yields in the ring-opening reactions. The scope of the reaction was successfully expanded to include primary, secondary, and tertiary alcohol nucleophiles. Through X-ray crystallography, the stereochemistry of the product was determined which confirmed an SN2-like mechanism to form the ring-opened product.

Determination of the acidic properties of carboxylated carbocatalysts in an acid-catalyzed ring-opening reaction using kinetic profiling

The acid-catalyzed ring-opening reaction of styrene oxide was used as a probe reaction for evaluating the acidic properties of carboxylated carbocatalysts. Significant discrepancies in the initial reaction rates were normalized using the total number of carboxyl groups, and demonstrated that the average catalytic activities of the carboxyl moieties on the carbocatalysts differed. Comparisons between the apparent activation energy E a and the pre-exponential factor A, derived from Arrhenius analysis, demonstrated that A varied more significantly, and therefore had a more significant effect on the reaction rates than E a. The variation in the calculated pKa values of the carboxyl groups was attributed to the electronic effects of the nitro groups. This hypothesis was supported by the temperature programmed desorption profiles of nitrogen monoxide ions.

Facile Conversion of Ni(II) Cyclopropylchlorins into Novel β-Substituted Porphyrins through Acid-Catalyzed Ring-Opening Reaction.

The conversion of cyclopropylchlorins into porphyrins represents a key step in the synthetic manipulation of macrocycles with tunable physical and chemical properties. Herein, we report a facile method for the synthesis of novel β-substituted porphyrins from cyclopropylchlorins. A series of Ni(II) cyclopropylchlorins was converted into the corresponding Ni(II) and free base porphyrins using TFA and H2SO4 under mild reaction conditions in good yields (75-86%). The new chlorins and porphyrins were characterized by various spectroscopic techniques and the single-crystal X-ray diffraction method. The reaction proceeds very fast (<5 min.) with complete conversion of chlorin into porphyrin with distinct color change. Facile conversion, shorter reaction time scale, and good yield (75-86%) without any side products are the significant features of this new protocol. These porphyrinoids exhibited red-shifted electronic spectral features with varying degrees nonplanar conformation, tunable redox properties, and porphyrin core basicity.

Lewis Acid-catalyzed Ring-opening Addition Reactions of Alcohols to Vinylcyclopropane

Abstract Lewis acids such as Sn(OTf)2, GaCl3, Sc(OTf)3, and Al(OTf)3 catalyzed ring-opening 1,5-addition reactions of aliphatic alcohols to dimethyl 2-vinylcyclopropane-1,1-dicarboxylate, resulting in moderate to high yields of methyl 4-alkoxy-2-methoxycarbonyl-5-hexenoates with a catalytic turnover number of up to 80. A deuterium-labeling experiment and comparison with the reactivity of dimethyl cyclopropane-1,1-dicarboxylate suggested that this reaction involves a 1,3-dipole intermediate stabilized by the donor and the acceptor substituents. The ring-opening addition of phenols demonstrated several reaction modes, including Friedel–Crafts 1,5- and 1,7-additions.

ChemInform Abstract: The First Total Synthesis of Synparvolide C.

The first stereoselective total synthesis of synparvolide C has been accomplished. An acid-catalyzed epoxide ring-opening reaction and ring-closing metathesis (RCM) were used for the cyclic ether and δ-lactone formation, respectively, to furnish the core structure of the target molecule. These reactions along with a succession of functional group manipulations led to the preparation of the natural product. The spectral and analytical data were used to establish the absolute configuration.

Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts

A ReO(x)-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C-O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends, NH(3) temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C-O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh-ReO(x)/C (1:0.5) was confirmed by NH(3) TPD, and the estimated acid site density and standard enthalpy of NH(3) adsorption were 40 μmol g(-1) and -100 kJ mol(-1), respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside α to carbon in the C-O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C-O hydrogenolysis of biomass-derived feedstocks.

Et2MeN×HI-Catalyzed Reaction of Arylboronic Acids with 2-Acyl-2,3-dihydro-4H-pyrans Leading to 2-Aryltetrahydrocyclopenta[1,3,2]dioxaboroles

The reaction of arylboronic acids with 2-acyl-2,3-dihydro-4H-pyrans proceeded smoothly in the presence of a catalytic amount of Et 2 MeN·HI to give acyl-substituted 2-aryltetrahydrocyclopenta[ 1,3,2]dioxaboroles in good yields. The reaction is likely to involve the acid-catalyzed ring-opening reaction of pyrans, an intramolecular aldol reaction, and condensation with boronic acids.

Kinetics and Products of the Acid-Catalyzed Ring-Opening of Atmospherically Relevant Butyl Epoxy Alcohols

Epoxydiols are produced in the gas phase from the photo-oxidation of isoprene in the absence of significant mixing ratios of nitrogen oxides (NO(x)). The reactive uptake of these compounds onto acidic aerosols has been shown to produce secondary organic aerosol (SOA). To better characterize the fate of isoprene epoxydiols in the aerosol phase, the kinetics and products of the acid-catalyzed ring-opening reactions of four hydroxy-substituted epoxides were studied by nuclear magnetic resonance (NMR) techniques. Polyols and sulfate esters are observed from the ring-opening of the epoxides in solutions of H(2)SO(4)/Na(2)SO(4). Likewise, polyols and nitrate esters are produced in solutions of HNO(3)/NaNO(3). In sulfuric acid, the rate of acid-catalyzed ring-opening is dependent on hydronium ion activity, sulfate ion, and bisulfate. The rates are much slower than the nonhydroxylated equivalent epoxides; however, the hydroxyl groups make them much more water-soluble. A model was constructed with the major channels for epoxydiol loss (i.e., aerosol-phase ring-opening, gas-phase oxidation, and deposition). In the atmosphere, SOA formation from epoxydiols will depend on a number of variables (e.g., pH and aerosol water content) with the yield of ring-opening products varying from less than 1% to greater than 50%.