Base-free Conditions Research Articles

A Simple Work-Up-free, Solvent-free Approach to Novel Amino Acid Linked 1,4-Disubstituted 1,2,3-Triazoles as Potent Antituberculosis Agents.

An efficient, green strategy for synthesis of 1,4-disubstituted-1,2,3-triazole has been developed using 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) acetate ionic liquid (200 μL) under a solvent- and external base-free condition. This protocol is further applied for the synthesis of novel amino acid containing 1,2,3-triazole molecules, which were then evaluated for potential antitubercular and antibacterial activities. Cytotoxicity assay of the compounds was also performed. In silico analysis of the promising compounds selected through experimental analysis was thereafter performed for visualizing molecular interactions and predicting binding affinities between our synthesized molecules, which exhibited good activity in experimental studies and the DprE1 target protein of Mycobacterium tuberculosis. Durg-likeness studies also show potential of the synthesized molecules as drug candidates.

Base-free selective conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over a CoOx-CeO2 catalyst

Ce oxide-based catalysts with different molar ratio of Co/Ce (denoted as CoCe-x, x represents the Co to Ce ratio) were examined as the catalyst of 5-hydroxymethylfurfural (5-HMF) oxidation to 2,5-furandicarboxylic acid (FDCA). The Co to Ce ratio, catalyst amount and reaction temperature all have remarkable effects on yield and selectivity. Complete conversion of 5-HMF with 86.3 % selectivity of FDCA has been achieved over the CoCe-0.15 catalyst at 130 °C with 0.6 MPa O2 pressure (gauge) for 4 h. The good performance is attributed to the high specific surface area and the high oxygen vacancy concentration to promote the mobility and adsorption of oxygen species. In addition, the CoCe-0.15 catalyst is reused for five times without significant activity loss. Finally, the reaction mechanism over the CoCe-0.15 catalyst under base-free condition is discussed. 5-HMF is oxidized by the lattice oxygen via Mars-van Krevelen mechanism with the redox of Ce4+/Ce3+ and Co3+/Co2+, and the lattice oxygen is replenished by adsorbing and activating O2 molecules.

Selective oxidation of glycerol to dihydroxyacetone over N-doped porous carbon stabilized CuxO supported Au catalysts

In this work, N-doped porous carbon stabilized CuxO (Cu-NPCs) were synthesized by a one-pot method using copper nitrate as copper precursor with the aid of Pluronic F127 under acidic condition. The characterization results confirmed that CuxO species (CuO and Cu2O) existed in NPC in a highly dispersed state due to the strong interaction between CuxO and the doped N atoms. The obtained Cu-NPCs were used as supports to prepare Au catalysts for glycerol (GLY) oxidation to dihydroxyacetone (DHA) under base-free conditions. During the reaction, GLY preferentially adsorbed onto Au nanoparticles via the secondary OH groups due to the porous characteristics of the supports, the enhanced surface basicity from the N functionalities accelerates the alkoxide formation by abstracting proton from GLY, and the surface Cu+ sites instead of Au nanoparticles activate O2 molecules to produce active oxygen species, as a result, Au/Cu-NPC-15 %-H, which is the best catalyst in Au/Cu-NPCs, exhibited much higher catalytic activity and selectivity than those of Au/CuO. Moreover, due to the stabilization of CuxO species by NPC, relatively stable surface Cu+ content, and the strong interaction between Au nanoparticles and the supports, Au/Cu-NPC-15 %-H also possessed much better catalytic stability than that of Au/CuO during four consecutive catalytic runs.

Copper-catalyzed radical oxidative C(sp3)–H/C(sp3)–H cross-coupling between arylacetonitriles and benzylic compounds

Abstract For the first time, a novel copper-catalyzed direct C(sp3)–H/C(sp3)–H cross-coupling of arylacetonitriles with unactivated benzylic compounds was described, allowing various a-benzylated arylacetonitriles to be readily accessible under base-free conditions. Mechanistic investigations suggested that the reaction proceeds through radical process and the C(sp3)–H cleavage of arylacetonitriles probably is the rate-determining step.

Visible-light assisted of nano Ni/g-C3N4 with efficient photocatalytic activity and stability for selective aerobic C−H activation and epoxidation

A selective, economical, and ecological protocol has been described for the oxidation of methyl arenes and their analogs to the corresponding carbonyl compounds and epoxidation reactions of alkenes with molecular oxygen (O2) or air as a green oxygen source, under mild reaction conditions. The nano Ni/g-C3N4 exhibited high photocatalytic activity, stability, and selectivity in the C−H activation of methyl arenes, methylene arenes, and epoxidation of various alkenes under visible- light irradiation without the use of an oxidizing agent and under base free conditions.

Cationic NHC-Phosphine Iridium Complexes: Highly Active Catalysts for Base-Free Hydrogenation of Ketones.

Novel bidentate N‐heterocyclic carbene‐phosphine iridium complexes have been synthesized and evaluated in the hydrogenation of ketones. Reported catalytic systems require base additives and, if excluded, need elevated temperature or high pressure of hydrogen gas to achieve satisfactory reactivity. The developed catalysts showed extremely high reactivity and good enantioselectivity under base‐free and mild conditions. In the presence of 1 mol % catalyst under 1 bar hydrogen pressure at room temperature, hydrogenation was complete in 30 minutes giving up to 96 % ee. Again, this high reactivity was achieved in additive‐free conditions. Mechanistic experiments demonstrated that balloon pressure of hydrogen was sufficient to form the activate species by reducing and eliminating the 1,5‐cyclooctadiene ligand. The pre‐activated catalyst was able to hydrogenate acetophenone with 89 % conversion in 5 min.

Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of sulfoxonium ylides with dioxazolones

A highly novel and direct synthesis of benzoxazinones was developed via Cp*Co(III)-catalyzed C–H activation and [3 + 3] annulation between sulfoxonium ylides and dioxazolones. The reaction is conducted under base-free conditions and tolerates various functional groups. Starting from diverse readily available sulfoxonium ylides and dioxazolones, a variety of benzoxazinones could be synthesized in one step in 32%-75% yields.

Ion-paired Ligands for Palladium-catalyzed Allylic Alkylation under Base-free Conditions

The effectiveness of a catalytic amount of bromide ions for significantly accelerating the palladium-catalyzed allylation of carbonyl compounds with allylic carbonates is reported. In particular, t...

Electrochemically Enabled Sulfonylation of Alkynes with Sodium Sulfinates

An electrochemical sulfonylation of alkynes with sodium sulfinates was achieved for the first time at room temperature. Employing this electrolysis strategy, the reaction occurs efficiently under transition-metal-free, external oxidant-free, and base-free conditions and furnishes diverse alkynyl sulfones in satisfactory yield with broad functional group tolerance.

Titanium dioxide as an efficient heterogeneous catalyst for quick C–B bond cleavage of aryl/hetero arylboronic acid on water at room temperature

A simple and convenient protocol for the conversion of aryl/heteroarylboronic acids to corresponding phenols via oxidative hydroxylation has been developed, using titanium dioxide (TiO2) as heterogeneous catalyst and aqueous hydrogen peroxide as oxidant. The reusability of the said catalyst is assessed and it could be recycled up to 5th consecutive cycles without significant loss of catalytic activity. The reaction pathway is greener with ligand and base free reaction condition, short reaction time, reusable heterogeneous catalytic system and room temperature aqueous reaction medium.

Selective 1,2-Aryl-Aminoalkylation of Alkenes Enabled by Metallaphotoredox Catalysis.

A highly chemo- and regioselective intermolecular 1,2-aryl-aminoalkylation of alkenes by photoredox/nickel dual catalysis is described here. This three-component conjunctive cross-coupling is highlighted by its first application of primary alkyl radicals, which were not compatible in previous reports. The readily prepared α-silyl amines could be transferred to α-amino radicals by photo-induced single electron transfer step. The radical addition/cross-coupling cascade reaction proceeds under mild, base-free and redox-neutral conditions with good functional group tolerance, and importantly, provides an efficient and concise method for the synthesis of structurally valuable α-aryl substituted γ-amino acid derivatives motifs.

A Systems Approach to a One-Pot Electrochemical Wittig Olefination Avoiding the Use of Chemical Reductant or Sacrificial Electrode.

An unprecedented one‐pot fully electrochemically driven Wittig olefination reaction system without employing a chemical reductant or sacrificial electrode material to regenerate triphenylphosphine (TPP) from triphenylphosphine oxide (TPPO) and base‐free in situ formation of Wittig ylides, is reported. Starting from TPPO, the initial step of the phosphoryl P=O bond activation proceeds through alkylation with RX (R=Me, Et; X=OSO2CF3 (OTf)), affording the corresponding [Ph3POR]+X− salts which undergo efficient electroreduction to TPP in the presence of a substoichiometric amount of the Sc(OTf)3 Lewis acid on a Ag‐electrode. Subsequent alkylation of TPP affords Ph3PR+ which enables a facile and efficient electrochemical in situ formation of the corresponding Wittig ylide under base‐free condition and their direct use for the olefination of various carbonyl compounds. The mechanism and, in particular, the intriguing role of Sc3+ as mediator in the TPPO electroreduction been uncovered by density functional theory calculations.

Cationic palladium(II)–acetylacetonate complexes containing phosphine and aminophosphine ligands and their catalytic activities in telomerization of 1,3-butadiene with methanol

The Pd(II) complexes [Pd(acac)(L)n]BF4 were synthesized by the reaction of [Pd(acac)(MeCN)2]BF4 with L (n = 2: L = PCyPh2, PCy2Ph, P(NMe2)3, P(NEt2)3, tri-2-furylphosphine, tri-2-thienylphosphine; n = 1: L = 2 dicyclohexylphosphino-2′,6′-dimethoxybiphenyl, 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl). The structural features of the complexes were analyzed by NMR and FTIR spectroscopies. Complexes [Pd(acac)(PCyPh2)2]BF4 and [Pd(acac)(PCy2Ph)2]BF4 were characterized by X-ray diffractometry. The cationic palladium catalyst precursors were found to be active catalysts for telomerization of 1,3- butadiene with methanol under base-free conditions. Complexes [(acac)Pd(PCy2Ph)2]BF4 and [(acac)Pd(TOMPP)2]BF4 in terms of yield, selectivity, and stability are effective for the catalytic telomerization (TOMPP = tris(2-methoxyphenyl)phosphine). Using 0.001–0.0025 mol% palladium loading methoxy-2,7-octadienes were obtained with chemoselectivities of 76–94% and TON up to 60000 mol 1,3-butadiene per mol Pd.

Oxidative Esterification of 5-Hydroxymethylfurfural with an N-doped Carbon-supported CoCu Bimetallic Catalyst.

The direct fabrication of furan-2,5-dimethylcarboxylate (FDMC), a promising renewable monomer, from biomass-derived 5-hydroxymethylfurfural (HMF) is a cutting-edge process. In this contribution, an elaborately designed N-doped carbon-supported CoCu bimetallic catalyst (Cox Cuy -NC; x/y=9:1, 7:3, 4:6, which represents the designed molar ratio of Co and Cu in the catalyst), which could offer a desirable FDMC yield of 95 % under mild and base-free conditions (Co7 Cu3 -NC, 2 bar O2 , 80 °C, 4 h) is described for the oxidative esterification of HMF. Notably, an FDMC formation rate of 6.1 molFDMC molCo -1 h-1 was achieved over Co7 Cu3 -NC, which represents the highest catalytic efficiency so far among Co-based catalytic systems. It has been demonstrated that Cu-doping in Co7 Cu3 -NC catalyst brings about more active sites (Co-Nx species) with stronger molecular oxygen activation ability. The increase of surface N content of Co7 Cu3 -NC also improves basicity of the catalyst, which favors the hydrogen abstraction process during the HMF oxidative esterification reaction. These findings may pave an efficient and green way for the synthesis of sustainable bio-based polymer monomers.

Multicomponent click reactions catalysed by copper(I) oxide nanoparticles (Cu2ONPs) derived using Oryza sativa

A procedure for the expedient synthesis of well-characterized Cu(I) oxide nanoparticles (Cu2ONPs) from Cu(II) salts by employing Rice (Oryza sativa) as a cheap and ready source of reducing as well as stabilizing agent has been demonstrated. The judicious choice of rice as a catalyst has helped in the symbiotic combination of two events: viz., acidic hydrolysis of starch to form glucose and the subsequent formal reduction of Cu2+ by the in-situ generated monosaccharide reducing sugar (glucose) under the alkaline condition to produce Cu2O. Further, rice was also found to be effectively stabilizing the nanoparticles from agglomeration. Optical and microscopic techniques were suitably employed for the characterization of the nanoparticles of approximately 10 nm size. Furthermore, the specifically generated nanoparticles were found to be active catalysts in an aqueous medium for Azide-alkyne Huisgen cycloaddition (Click reaction) under base free condition via one-pot multi-component addition for the synthesis of mono-, bis- and tris-1,2,3-triazoles in good to excellent yields. Cu2ONPs synthesized using hydrolysed Rice, an active catalyst for synthesis of mono-, bis- and tris-1,4-disubstituted 1,2,3-triazoles via one pot multicomponent reactions in water.

2,5-Furandicarboxylic acid production via catalytic oxidation of 5-hydroxymethylfurfural: Catalysts, processes and reaction mechanism

Catalytic oxidation is a promising manner to produce 2,5-furandicarboxylic acid (FDCA) from 5-Hydroxymethylfurfural (HMF). This review summarizes the progress in the thermocatalysis, electrocatalysis, and photocatalysis for FDCA production, including catalyst screening, synthesis processes, and reaction mechanism. Biomass conversion to value-added chemicals has received tremendous attention for solving global warming issues and fossil fuel depletion. 5-Hydroxymethylfurfural (HMF) is a key bio-based platform molecule to produce many useful organic chemicals by oxidation, hydrogenation, polymerization, and ring-opening reactions. Among all derivatives, the oxidation product 2,5-furandicarboxylic acid (FDCA) is a promising alternative to petroleum-based terephthalic acid for the synthesis of biodegradable plastics. This review analytically discusses the recent progress in the thermocatalytic, electrocatalytic, and photocatalytic oxidation of HMF into FDCA, including catalyst screening, synthesis processes, and reaction mechanism. Rapid fundamental advances may be possible in non-precious metal and metal-free catalysts that are highly efficient under the base-free conditions, and external field-assisted processes like electrochemical or photoelectrochemical cells.

Pickering HIPEs derived hierarchical porous nitrogen-doped carbon supported bimetallic AuPd catalyst for base-free aerobic oxidation of HMF to FDCA in water

As an important renewable alternative to the production of petroleum-derived terephthalic acid, selective aerobic oxidation of biomass derivative 5-hydroxymethylfurfural (HMF) into high value-added 2,5-furandicarboxylic acid (FDCA) is attracting increasing attention. Herein, a novel hierarchical porous nitrogen-doped carbon (NC) supported bimetallic AuPd catalyst was synthesized by Pickering high internal phase emulsions (Pickering HIPEs) templating method, subsequent carbonization and the sol-curing technique. By varying Pickering HIPEs parameter, nitrogen-doped content and the loaded AuPd nanoparticles, AuxPdy/NC catalysts with adjustable porous structure and basicity were obtained. Benefiting from the synergistic effects of the high basicity of NC support and efficient bimetallic AuPd catalyst, tandem oxidation of HMF to FDCA reactions can be carried out under base-free condition in water, which makes the biomass transformation process environmental friendliness and economic. And the highest yield of FDCA at 96.7% was achieved under the optimal reaction conditions. Regeneration test revealed the obtained AuxPdy/NC catalyst was recovered and reused at least five times without significant loss of catalytic activity. The present work offers a versatile strategy for developing nitrogen-doped carbon supported highly efficient bimetallic catalysts with hierarchical porous structure and adjusting basicity for a wide range of base-free catalytic oxidation in water.

Borrowing hydrogen activity of NH2-MIL-125 for N-alkylation of amines with alcohols under solvent and base free condition

The NH2-MIL-125 showed excellent activity in solvent and base free N-alkylation of amines with alcohols via borrowing hydrogen mechanism. The Ti–O clusters of NH2-MIL-125 provide acidic-basic active sites for the catalysis of reaction. The NH2-MIL-125 was found to be stable and reusable in the reaction.

The relationship of morphology and catalytic performance of CeO2 catalysts for reducing nitrobenzene to azoxybenzene under the base-free condition

CeO2 morphology was proposed to be a crucial factor for reducing nitrobenzene to azoxybenzene under the base-free condition. Herein, the structure-activity relationship of CeO2 catalysts was explored to improve the azoxybenzene yield. A series of CeO2 catalysts were synthesized with seven morphologies to obtain different Ce3+ proportion and various surface areas. Notably, the catalytic performance of these samples for reducing nitrobenzene to azoxybenzene enhanced with the increasing Ce3+ proportion. With the highest surface Ce3+ proportion, the Rod-CeO2 catalyst exhibited 100% conversion of nitrobenzene and 89.8% azoxybenzene selectivity in 7 h at 150 °C under 1 MPa CO. Moreover, the preliminary mechanistic analysis indicated that the inhabitation of azoxybenzene to by-product azobenzene resulted in the high selectivity of azoxybenzene.

Cobalt/Bisoxazolinephosphine-Catalyzed Asymmetric Alkynylation of Isatins.

A new catalyst system based on Co(OAc)2/bisoxazolinephosphine has been developed to catalyze the direct addition of terminal alkynes to isatins under base-free conditions. Chiral propargyl alcohols with an oxindole skeleton could be prepared in up to 99% yield and 99% ee with the help of the chiral tridentate ligand. A variety of functionalized aliphatic or aromatic alkynes and isatins were utilized in this method, and gram-scale synthesis could be achieved with 1 mol % catalyst.