Neutral Reaction Conditions Research Articles

An efficient synthesis of some new bipyrazole derivatives by three-component reaction between 3-methyl-1H-pyrazol-4(5H)-one, aroylphenylhydrazones and arylglyoxals

A simple one-pot and catalyst-free method is reported for synthesis of some new bipyrazole derivatives by three-component reaction between arylglyoxals, 3-methyl-1H-pyrazol-4(5H)-one and aroylphenylhydrazone derivatives. The reactions were carried out in ethanol as a green solvent in neutral reaction conditions and products were simply isolated by filtering of the precipitated solids in high yields. All products were characterized by their 1H and 13C NMR and IR spectral and elemental analysis data.

Synthesis of Functionalized Cycloheptadienones Starting from Phenols and Using a Rhodium/Boron Asymmetric Catalytic System.

Skeletal editing offers a unique route to assemble complex architectures from simple feedstocks that are otherwise difficult to obtain. However, the asymmetric version of skeletal editing has not been widely studied. Herein, we present a modular rhodium/boron asymmetric catalytic system that enables ring-expansion of phenols with cyclopropenes to synthesize highly functionalized cycloheptadienones in excellent chemo- and regioselectivities. This unique protocol features with low-catalyst loading, atom and step-economies, and mild neutral reaction conditions. Isotope-labelling experiments and DFT calculations have been conducted to reveal that boron reagent plays a vital role in the whole catalytic cycle.

Synthesis of Functionalized Cycloheptadienones Starting from Phenols and Using a Rhodium/Boron Asymmetric Catalytic System

Skeletal editing offers a unique route to assemble complex architectures from simple feedstocks that are otherwise difficult to obtain. However, the asymmetric version of skeletal editing has not been widely studied. Herein, we present a modular rhodium/boron asymmetric catalytic system that enables ring‐expansion of phenols with cyclopropenes to synthesize highly functionalized cycloheptadienones in excellent chemo‐ and regioselectivities. This unique protocol features with low‐catalyst loading, atom and step‐economies, and mild neutral reaction conditions. Isotope‐labelling experiments and DFT calculations have been conducted to reveal that boron reagent plays a vital role in the whole catalytic cycle.

Highly Reactive Glycosylation with 1-O-(Methylthio)thiocarbonyl Glycosyl Donors under Acidic to Neutral Reaction Conditions.

In this study, we have successfully developed a glycosylation method using 1-O-(methylthio)thiocarbonyl-glycoses as donors. Such xanthate donors are easily accessible and shelf-stable. The glycosylation reaction could be promoted by cations (acidic to neutral conditions) under mild conditions, exhibiting a reactivity intermediate between that with glycosyl trichloroacetimidate as the donor and that with thioglycoside as the donor. This methodology tolerates both "armed" and "disarmed" glycosyl donors, as well as various sugar acceptors, and affords the corresponding glycosides in good to excellent yields. Based on the relative higher reactivity of such xanthate donors than thioglycoside donors under the same glycosylation conditions, a trisaccharide was further synthesized in a one-pot glycosylation strategy.

Catalyst-free, one-pot synthesis of N,N-di(α-amino aryl ketones) from aryl glyoxals and aryl amines in presence of guanidine as a source of nitrogen

Three-component reaction between aryl glyoxals, aryl amines, and guanidine afforded N,N-di (α-amino aryl ketone) derivatives. This method include two C–N bonds formation in a one-pot and single synthetic step in neutral and simple reaction conditions. All reactions were conducted in EtOH as solvent at reflux conditions without using any catalyst, and the stable products were obtained in high yields. The structures of all products were proved by 1H and 13C NMR, IR spectral and elemental analysis data.

Low Palladium‐Loading HY Zeolite Catalyzed Hydrolysis of Hydrosilanes to Silanols and Alkoxysilanes

AbstractA low palladium‐loading heterogeneous catalyst has been developed for the hydrolysis of hydrosilanes to silanols and alkoxysilanes under mild and neutral reaction conditions with broad substrate scope. After five recycling runs, this low palladium‐loading catalyst keeps good activity for this transformation.

Durable high-entropy non-noble metal anodes for neutral seawater electrolysis

Enhancing hydrogen production is key to achieving a carbon–neutral world. Among the various technologies available, using renewable energy for water electrolysis stands out as a leading method for creating green hydrogen. However, standard water electrolysis requires substantial quantities of ultra-clean fresh water for optimal energy efficiency and operations. This poses a problem as fresh water only represents a minuscule 0.01 % of our planet’s water reserves. Seawater electrolysis is a potential solution to resolve this limitation and attain sustainable hydrogen production. Nevertheless, electrolysis of seawater in its neutral state demands stringent reaction conditions, making it imperative to thoughtfully design the electrodes, particularly the anodes. This study presents the development of highly durable, noble-metal-free, high-entropy alloy anodes suitable for neutral seawater under highly fluctuating power operation. The anode demonstrates impressive stability (up to 6,000 cycles), lasting over 100 h at 100 mA cm−2 in a 0.5 M aqueous NaCl electrolyte. Its Faradaic efficiency for O2 production is 42 %. Density functional theory calculations reveal that the oxidized surface (passivation layer) of the anode prevents Cl− adsorption, helping to protect the catalytically active sites and prevent catalyst degradation. This finding suggests that noble-metal-free anodes can serve as practical alternatives to those made from noble metals for neutral seawater electrolysis. Moreover, the utilization of seawater without acid or alkali treatment paves the way for safer and more sustainable hydrogen production. This is a significant stride toward realizing our goal of carbon neutrality.

Vanadium-catalyzed Hydration of 2-Cyanopyrazine to Pyrazinamide with Unique Substrate Specificity.

Pyrazinamide is an important medicine used for the treatment of tuberculosis(TB). The preparation of pyrazinamide via catalytic hydration of 2-cyanopyrazine is of great economic interest with high atomic economy. Heterogeneous non-precious transition metal-catalyzed hydration of nitriles under neutral reaction conditions would be rather attractive. Herein vanadium-nitrogen-carbon materials were fabricated and employed for selective hydration of nitriles using water as both the solvent and reactant. 2-Cyanopyrazine could be smoothly converted into to pyrazinamide with unique substrate specificity. Additives with different N and O atoms could significantly affect hydration of 2-cyanopyrazine due to competitive adsorption/coordination in the reaction system. This work provides a new approach for non-precious metal catalyzed hydration of nitriles.

Lewis Acidity of Hydrosilanes: Synthesis of β‐Alkoxy Alcohols via the Triphenylsilane‐Catalyzed Ring‐Opening Reaction of Epoxides

AbstractTriphenylsilane (Ph3SiH) works as a Lewis‐acidic catalyst for the ring‐opening reaction of di‐ and tri‐substituted epoxides with alcohols, yielding β‐alkoxy alcohols under nearly neutral reaction conditions. This operationally simple protocol allows the facile synthesis of β‐alkoxy alcohols in good‐to‐excellent yield.

Access to polysubstituted pyrimidine derivatives via the aza-Diels–Alder reaction of 1,2,3-triazine derivatives and amidines

The synthesis of highly substituted pyrimidine derivatives by the aza-Diels–Alder reaction of 1,2,3-triazines with amidine derivatives is described. This reaction provided direct access to a wide range of 6-aryl-2-substituted pyrimidine-4-carboxylates in good yields with good functional group compatibility. A broad substrate scope, and the operationally simple, metal-free, and neutral reaction conditions are the prominent features of this method.

Metal free synthesis of 2,3-dideoxy-α, β-unsaturated carbohydrate enals (Perlin aldehydes)

A metal free synthesis of enantiopure 2,3-dideoxy-α, β-unsaturated carbohydrate enals (Perlin aldehydes), in CH3CN-0.02 N H2SO4 in water (1:1, v/v) with 0.5 equivalent additives (4-hydroxy-6-methyl-2-pyrone or 4-amino coumarin), has been reported. This efficient protocol works well for the acetylated glycals (glucal, galactal and arabinal) and afforded Perlin aldehydes and hemiacetals in acceptable to good yields. Whereas, benzylated glycals furnished respective Perlin aldehydes, hemiacetals and the 2-deoxy derivatives, under similar reaction conditions. The products yields were significantly reduced when the additives were removed from the reaction mixture, indicating that they constitute an essential component of this approach. Further the use of 0.02 N H2SO4 in water: acetonitrile (1:1, v/v) solvent system is essential for the formation of Perlin aldehydes. The similar reactions under neutral reaction conditions (CH3CN:H2O, 1:1, v/v) with additives, afforded the hemiacetals as major product. This methodology is a metal free approach to Perlin aldehyde synthesis and therefore having additional benefit of its use in preparation of bioactive drug molecules, where metal toxicity is the major concern.

Site‐Selective Decarboxylative Direct Formylation of Nitrogen Heterocycles (Azaindoles, Indoles, Pyrroles) and Anilines Utilizing Glyoxylic Acid

AbstractA new method for formylation of nitrogen heterocycles overcoming the limitations of classical approaches using a cheap feedstock formylating agent especially under mild and neutral conditions is the subject of this current investigation. This work describes direct formylation of nitrogen heterocycles (azaindoles, indoles, pyrroles) and anilines using glyoxylic acid as the formyl source and K2S2O8 as the exclusive reagent. The key features include a general method for both C‐ and N‐formylation under neutral and mild reaction conditions. The application of the method is further demonstrated in the tandem synthesis of nitrogen heterocycles, pharmaceuticals, and a natural product. Unlike aroylation with arylglyoxylic acids, a previously unknown pathway for the direct formylation with glyoxylic acid in the presence of K2S2O8 is now unveiled.magnified image

Iridium-Catalyzed Branch-Selective Hydroalkylation of Simple Alkenes with Malonic Amides and Malonic Esters

We report the iridium-catalyzed branch-selective hydroalkylation of simple alkenes such as aliphatic alkenes and aromatic alkenes with malonic amides and malonic esters under neutral reaction conditions. A variety of aliphatic alkenes and aromatic alkenes bearing bromine, chlorine, ester, 2-thienylcarboxylate, silyl, and phthalimide groups were all found to be suitable for this hydroalkylation. The combination of this method with Krapcho dealkoxycarbonylation realized a one-pot synthesis of β-substituted amide and ester from β-amide ester and malonic ester. The hydroalkylated products derived from malonic amides are suitable for further transformation. The finely tuned reaction conditions realized the selective transformation of hydroalkylated products to 1,3-diamines or monoamides with the same reagent. Deuterium labeling experiments and measurement of the kinetic isotope effect indicated that the catalytic cycle involves a reversible step and cleavage of the C-H bond is not a rate-determining step. Density functional theory calculations provided insight into the reaction mechanism, where the carboiridation step is followed by C-H reductive elimination.

Phosphine-catalyzed ring-opening reaction of cyclopropenones with dicarbonyl compounds.

We developed a phosphine-catalyzed ring-opening reaction of cyclopropenones with dicarbonyl compounds as C-nucleophiles, leading to 1,3,3'-tricarbonyl compounds. During this neutral procedure, C-acylation is more dominant than O-acylation. This transition-metal free procedure features mild and neutral reaction conditions with good atom economy. As such, it represents a facile pathway to access 1,3,3'-tricarbonyl derivatives.

Metal-carbonate interface promoted activity of Ag/MgCO3 catalyst for aqueous-phase formaldehyde reforming into hydrogen

Aqueous-phase reforming of biomass-derived formaldehyde is one of efficient and sustainable routes to generate molecular hydrogen as clean energy resource. In this work, Ag/MgCO3 catalyst is prepared with constructed stable carbonate-modified metal-support interfaces. Under mild and neutral reaction conditions, it exhibits a near an order of magnitude higher low-temperature activity in formaldehyde reforming reaction for producing hydrogen in comparison with Ag/MgO. The catalytic and spectral observations reveal that the Ag/MgCO3-catalyzed reaction follows an O2-involved HCHO/H2O reforming reaction pathway through O2−, OOH and H radicals as highly active intermediates. Ag/MgCO3 catalyst shows high rates in isotopic H2-D2 exchange and HCHO/D2O reforming reactions and displays an apparent activation energy (Ea) as low as 7.5 kJ mol−1 within 10–50 °C, indicating facile activation of HCHO CH and H2O OH bonds. Furthermore, Ag/MgCO3 catalyst adsorbs HCHO molecule in a favorable configuration and strength, as evidenced by the HCHO desorption profile. Comprehensive positive factors benefit to the superior hydrogen production activity of carbonate-modified Ag/MgCO3 than Ag/MgO.

In situ polymerization of N-methylol acrylamide (NMA) for bamboo anti-mold modification

Bamboo is an eco-friendly and sustainable material but with limited application due to its susceptibility to mold infection. N-methylol acrylamide (NMA) is a cross-linking monomer with bifunctional groups, here NMA was used to modified bamboo to enhance its anti-mold property through in situ polymerization and cross-linking reaction. When the monomer concentration was 6% and above, no mold germinated at the NMA treated bamboo surface. SEM/EDX, FTIR was used to explore the anti-mold mechanism of NMA modified bamboo. The primary reason for the improvement in the anti-mildew property was the coating effect of PNMA on starch as well as the antimicrobial properties of PNMA. Thermal stability of bamboo before and after modification was almost unchanged. Results also indicated that the mechanical properties of modified bamboo are improved by around 50%, which was mainly attributed to more compact structure of cell wall obtained in the neutral reaction condition. The NMA in situ polymerization was proved to be a potential modification for bamboo to broaden its application.

Transesterification of Non‐Activated Esters Promoted by Small Molecules Mimicking the Active Site of Hydrolases

AbstractThe synthesis of small molecules able to mimic the active site of hydrolytic enzymes has been largely pursued in recent decades. The high reaction rates and specificity shown by natural hydrolases present an attractive target, and yet the preparation of suitable small‐molecule mimics remains challenging, requiring activated substrates to achieve productive outcomes. Here we present small synthetic artificial enzymes which mimic the catalytic site and the oxyanion hole of chymotrypsin and N‐terminal hydrolases and are able to perform, for the first time, the transesterification of a non‐activated ester such as ethyl acetate with methanol under mild and neutral reaction conditions.

Visible‐light‐promoted Radical Cyclization/Arylation Cascade for the Construction of α,α‐Difluoro‐γ‐Lactam‐Fused Quinoxalin‐2(1H)‐Ones

Comprehensive SummaryA visible‐light‐induced tandem radical intramolecular cyclization/arylation of quinoxalin‐2(1H)‐ones with bromodifluoroacetamides is described. This protocol allows efficient access to a variety of valuable α,α‐difluoro‐γ‐lactam‐fused quinoxalin‐2(1H)‐ones in moderate to good yields under metal‐free, mild and redox neutral reaction conditions. This strategy is tolerant of various functional groups and a broad range of substrates. The mechanism experiments suggested an involvement of 5‐exo‐trig cyclization and a radical process in this transformation.

Highly efficient C(CO)-C(alkyl) bond cleavage in ketones to access esters over ultrathin N-doped carbon nanosheets.

Selective oxidative cleavage of the C(CO)–C bond in ketones to access esters is a highly attractive strategy for upgrading ketones. However, it remains a great challenge to realize this important transformation over heterogeneous metal-free catalysts. Herein, we designed a series of porous and ultrathin N-doped carbon nanosheets (denoted as CN-X, where X represents the pyrolysis temperature) as heterogeneous metal-free catalysts. It was observed that the fabricated CN-800 could efficiently catalyze the oxidative cleavage of the C(CO)–C bond in various ketones to generate the corresponding methyl esters at 130 °C without using any additional base. Detailed investigations revealed that the higher content and electron density of the graphitic-N species contributed to the excellent performance of CN-800. Besides, the high surface area, affording active sites that are more easily accessed, could also enhance the catalytic activity. Notably, the catalysts have great potential for practical applications because of some obvious advantages, such as low cost, neutral reaction conditions, heterogeneous nature, high efficiency, and broad ketone scope. To the best of our knowledge, this is the first work on efficient synthesis of methyl esters via oxidative esterification of ketones over heterogeneous metal-free catalysts.

Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones.

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH4OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.