Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway

To study the effect of adjacent hydroxyl to the active sites, several acid catalysts, i.e. substituted benzoic acids with adjacent carboxyl are employed in the fructose dehydration to 5-hydroxymethylfurfural(HMF).Experimental results reveal that Br?nsted acid sites with adjacent carboxyl present higher catalytic ability than isolated ones. Computational results suggest that the adjacent sites lead to co-interaction on fructose, corresponding more stable transition state and faster HMF formation rate. Based on the enhancement from the adjacent sites, a novel ordered mesoporous carbon(OMC) full of carboxyls in surface is prepared and turns out to be an effective solid catalyst for HMF production from fructose derived from biomass.

Conjugate addition‐enantioselective protonation to forge tertiary stereocentresαto azaarenes via cooperative hydrogen atom transfer and chiral hydrogen‐bonding catalysis

Cooperative hydrogen atom transfer and chiral hydrogen‐bonding catalysis as a new platform for the asymmetric synthesis of azaarene derivatives is reported.By using a tetrabutylammonium decatungstate as the photocatalyst and a chiral phosphoric acid as the hydrogen‐bonding catalyst,transformations of a variety of commercially available hydrocarbons and silanes with diverseα‐branched 2‐vinylazaarenes could efficiently experience a tandem radical conjugate addition and enantioselective protonation process,providing a convenient and fully atom economical approach to access a range of valuable enantioenrichedα‐tertiary azaarenes in high yields with good to excellent enantioselectivities(up to 93%ee).Through the direct use of tert‐butyl methylcarbamate as the feedstock,this method enables a highly practical and concise synthesis of the enantiomerically pure medicinal molecule pheniramine(Avil).

Acid-base cooperativity of heterogeneous catalyst containing acidic framework and sterically hindered base for aldol condensation

A bifunctional heterogeneous catalyst containing two mutually incompatible acidic and basic sites, which exhibits cooperative catalytic behavior in the aldol condensation of acetone and various aldehydes, was synthesized by postgrafting of 1,5,7- triazabicyclo[4.4.0] dec-5-ene （TBD, a sterically hindered organic base） onto AI-MCM-41 molecular sieve. 2009 Xiao Bing Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Enantioselective intermolecular[2+2]photocycloadditions of vinylazaarenes with triplet-state electron-deficient olefins

The development of catalytic asymmetric radical reactions is an attractive but formidable task.The high reactivity of radicals enables the use of readily accessible feedstocks and mild reaction conditions,but it leads to substantial difficulty for chiral catalysts to provide sufficient enantiocontrol.Moreover,a racemic background process is often inevitable,further deteriorating enantioselectivity.In this regard,an effective protocol has been established for enantioselective intermolecular[2+2]photocycloadditions to overcome the challenges,which is capitalising on the ground-state preassociations of chiral catalysts with photoactivated substrates.Here,we report the viability of substrate-differentiating synergistic catalysis for this important reaction.In this new platform,energy transfer occurs between DPZ as a photosensitizer and enones or(E)-2-substituted vinylazaarenes for producing triplet-state species,and chiral phosphoric acid interacts with ground-state 2-vinylazaarenes via hydrogen bonding for subsequent enantiofacial cycloaddition.Although all active species are dispersed in the reaction system,valuable enantioenriched mono-and di-azaarene-functionalized cyclobutanes are obtained efficiently and selectively.In addition to constructing all-carbon quaternary stereocentres,flexible modulation of azaaryl groups and other substituents on the cyclobutane ring is also operative.

Recent Progress of Asymmetric Catalysis from a Chinese Perspective

The precise construction of chiral molecules is a core research area in chemical science and technology.Among the plethora of reported synthetic methods,asymmetric catalysis has emerged as one of the most efficient and direct ways to obtain optically pure chiral compounds.This article mainly reviews the recent progress of asymmetric catalysis made by Chinese chemists from the following aspects:the development of new chiral ligands,transition metal-catalyzed asymmetric reactions,asymmetric organocatalytic reactions,and new strategies for asymmetric catalysis.This review does not intend to be comprehensive;only representative examples are selected to highlight the important progress made in this field.Finally,a brief outlook is given on the development of novel chiral ligands/catalysts,new enantioselective reactions,and their applications in chiral drugs and agricultural chemicals.

Designed Bifunctional Ligands in Cooperative Homogeneous Gold Catalysis

Over the past two decades,homogeneous gold catalysis has experienced exponential development and contributed a plethora of highly valuable synthetic methods to the synthetic toolbox.Metal–ligand cooperative catalysis is a versatile strategy for achieving highly efficient and/or novel catalysis but has seldom been explored in gold chemistry.This minireview summarizes the progress we have made in developing remotely functionalized biaryl-2-ylphosphine ligands and employing them in cooperative gold catalysis that achieves excellent catalytic efficiency or realizes previously unknown reactivities.This approach also provides new venues for implementing asymmetric gold catalysis.

Asymmetric cooperative catalysis in the conjugate addition of pyrazolones to nitroolefins and subsequent dearomative chlorination

Over the past decade many bifunctional amine-thioureas have been developed as active metal-free organocatalysts. Coopera-tive catalysis of these amino-thioureas allows high reaction rates and excellent transfer of stereocbemical information. Despite these impressive advances, the design of new high-performance catalysts for applications in asymmetric catalytic reactions is of ongoing interest in organic chemistry. Herein we describe a cooperative catalyst system consisting of a chiral amine thiourea and an achiral organic acid that promotes the conjugate addition of 4-nonsubstituted pyrazolones to nitroolefins and subsequent dearomative chlorination. The corresponding adducts and the subsequent products were obtained in high to excel lent yields （up to 99%） and high stereoselectivities （up to 99/1 dr, 98% ee） under mild reacton conditions. These transforma tions provide an easy access to enantio-enriched pvrazole derivatives, which could possess Potential oharmaceutical activity.

Chiral Spirocyclic Phosphoric Acids and Their Growing Applications

Chiral spirocyclic phosphoric acids(SPAs)are introduced in 2010 and have been versatile catalysts capable of promoting a wide range of asymmetric organocatalytic reactions,such as multi-component reactions,Friedel–Crafts reactions,Pictet–Spengler reactions,Fischer indolizations,cycloaddition reactions,desymmetrization reactions,dearomatization reactions,conjugate addition reactions and rearrangement reactions,etc.Moreover,a diverse range of applications in metal-organic cooperative catalysis,organic photoredox catalysis,total synthesis,materials science and molecular recognition are beautifully illustrated.This account summarizes the past decade's advances in this field and highlights the selected but not comprehensive significant achievements.

A bio-inspired O2-tolerant catalytic CO2 reduction electrode

The electrochemical reduction of CO2 to give CO in the presence of O2 would allow the direct valorization of flue gases from fossil fuel combustion and of CO2 captured from air. However, it is a challenging task because O2 reduction is thermodynamically favored over that of CO2. 5% O2 in CO2 near catalyst surface is sufficient to completely inhibit the CO2 reduction reaction. Here we report an O2-tolerant catalytic CO2 reduction electrode inspired by part of the natural photosynthesis unit. The electrode comprises of heterogenized cobalt phthalocyanine molecules serving as the cathode catalyst with >95% Faradaic efficiency(FE) for CO2 reduction to CO coated with a polymer of intrinsic microporosity that works as a CO2-selective layer with a CO2/O2 selectivity of $20. Integrated into a flow electrolytic cell, the hybrid electrode operating with a CO2 feed gas containing 5% O2 exhibits a FECOof 75.9% with a total current density of 27.3 mA/cm^2 at a cell voltage of 3.1 V. A FECO of 49.7% can be retained when the O2 fraction increases to 20%. Stable operation for 18 h is demonstrated. The electrochemical performance and O2 tolerance can be further enhanced by introducing cyano and nitro substituents to the phthalocyanine ligand.

Predictable site-selective radical fluorination of tertiary ethers

In this communication,we disclose the first example of metal-free and site-selective radical fluorination of readily available tertiary alkyl ethers,enabled by synergistic photocatalysis and organocatalysis.This catalytic combination allows for exclusive fluorination of tertiary C–O bonds under mild conditions even in the presence of competing reaction sites.The excellent functional group tolerance affords valuable access to sterically hindered alkyl fluorides through late-stage modification of complex molecules.The successful use of tertiary alkyl ethers in radical fluorination enhances the structural diversity of aliphatic fluorides that can be derived from naturally abundant alcohols.

Stereodivergently asymmetric synthesis of chiral phosphorus compounds by synergistic combination of ion-pair catalyst and base

Stereodivergently constructing the designed products having adjacent multi-stereocenters via a given reaction,with excellent control of both absolute and relative configurations,presents one of the substantial hurdles in asymmetric catalysis.Herein,we report a precisely stereodivergent asymmetric protocol by synergistic combination of phosphonium-involved ion-pair catalysis and base for accessing to chiral phosphorus compounds bearing two adjacent chiral centers particularly containing an acidic protonated enantioenriched carbon atom,having broad functional group compatibility in both dynamic and thermodynamic processes under mild reaction conditions.Two keys for the success in constructing these stereoisomers with high levels of regio-,diastereo-,and enantioselectivities were contained:firstly,the precise stereo-control in providing dynamic products was enabled by bifunctional phosphonium salt catalyst with semi-enclosed cavity;secondly,the readily stereospecific transformation of adducts from dynamic to thermodynamic version was initiated by achiral base.All four stereoisomers could be readily accessed even in gram-scale in high yields with maintaining excellent stereoselectivities,illustrating the potential of this synergistic catalytic methodology in organic synthesis.Moreover,mechanistic studies including density functional theory(DFT)calculations and control experiments provide insights into the mechanism.

Foam-like boron-doped g-C_(3)N_(4) as acid-base cooperative catalyst for efficient nucleophilic addition in water

Intramolecular acid-base cooperative catalysis is a powerful strategy to promote the nucleophilic additions in synthetic chemistry,since both substrates and nucleophiles can be activated simultaneously,yielding a lower energy reaction pathway.However,the systems often suffer from catalyst quenching due to the mutual neutralization of acid/base sites.Herein,we doped the boron(B)atoms in the tri-s-triazine ring of graphite-like carbon nitride(g-C_(3)N_(4))framework,to develop the acid-base cooperative catalyst for nucleophilic addition.Characterization results demonstrated a foam-like hierarchical porous structure,as well as the coexistence of Lewis acid and base sites in the as-synthesized samples.As a result,the B-doped g-C_(3)N_(4) acted as the efficient acidbase cooperative catalysts,significantly promoting the Knoevenagel condensation whose key step involves in nucleophilic addition.A wide range of α,β-unsaturated carbonyl compounds with remarkably high yields(>90%)was achieved even within 1 h in water,whereas the acid-free or base-free counterpart was far less efficient.In addition,the bifunctional catalysts exhibited excellent reusability with simple treatment.The novel B-doped g-C_(3)N_(4) catalytic system offers several advantages,such as operational simplicity,mild reaction conditions,high efficiency,and facile reuse of catalyst.All these advantages make it a benign protocol from the sustainability point of view,and inspired us to design the other bifunctional cooperative catalysts for a wide range of sustainable industrial applications.

Iridium and B(C_(6)F_(5))_(3) co-catalyzed chemoselective deoxygenative reduction of tertiary amides:application to the efficient synthesis and late-stage modification of pharmaceuticals

The Vaska’s complex—tris(pentafluorophenyl)borane combination was found to be a highly efficient cooperative catalysis system for the hydrosilylative reduction of tertiary amides to yield amines under mild conditions.The reaction shows high chemoselectivity,tolerating halide,phenolyl,alkenyl,nitro,nitrile,ester,azido,ketone,and enone functional groups.For unsubstituted cyclohexanone carboxamide,two variations were established to achieve either catalytic concomitant reduction of the two carbonyl groups or selective reduction of the amide carbonyl.The protocol was applied to the efficient synthesis and latestage modification of several pharmaceuticals and derivatives.Importantly,we showed that by simply prolonging reaction time to 24–28 h,the reaction can reach an exceptionally high efficiency with turnover number(TON)up to 9.8×10^(6) and turnover frequency(TOF)up to 408,333 at a quite low catalyst loading of 0.00001 mol%(S/C(Ir)=10,000,000).

Asymmetric Three-Component Propargyloxylation for Direct Assembly of Polyfunctionalized Chiral Succinate Derivatives

An enantioselective three-component propargyloxylation reaction of propargyl alcohols,pyridotriazoles,and imines has been realized by cooperative catalysis with dirhodium complex and chiral phosphoric acid under mild conditions.This is the first example of a catalytic asymmetric three-component propargyloxylation reaction,which provides practical access to chiral polyfunctionalized succinate derivatives with adjacent quaternary and tertiary stereocenters in good to high yields with excellent enantioselectivity.In addition to the alkyne motif,pyridyl,alkoxy,amino,and alkenyl species are all tolerated under the reaction conditions.Notably,the utility of the current method is demonstrated by catalytic cyclization of the product alkyne into a variety of heterocyclic structures without loss of enantiomeric purity.

Cooperative Dual Organocatalytic Asymmetric Decarboxylative[4+3]Annulations with Benzoxazinanones

The cooperative catalysis of an achiral Lewis base and a chiral N-heterocyclic carbene(NHC)enables a highly enantioselective[4+3]annulation of benzoxazinanones with isatin-derived enals.DMAP serves as a nucleophilic Lewis base to promote decarboxylation of benzoxazinanones,which leads to azaortho-xylylene intermediates that undergo[4+3]annulations with NHC-bound homoenolates.This method breaks the substrate scope limitation of transition-metal-catalyzed variants,thus a broader range of benzoxazinanones are tolerated,and provides a straightforward entry to enantioenriched spirooxindoles in high structural diversity.