Molecularly engineered lignin to polyphenol via organocatalysis as an active sunscreen ingredient

Phenolation is one of the effective strategies to synthesize lignin-based polyphenols,improve lignin’s properties,and extend its value-added applications in biological,medicinal and cosmetic fields.Herein,by taking the structural feature advantage of lignin,an effective and green strategy was developed to molecularly engineer lignin into a robust lignin-3-(2-hydroxyphenyl)propionate ester(LPPE)derivative via a transesterification reaction between 3,4-dihydrocoumarin(DHC)and the aliphatic hydroxyls in lignin under organocatalysis.The strategy is optimized and the novel derivative was systematically characterized by ^(1)H,^(13)C and ^(31)P nuclear magnetic resonance(NMR)and Fourier transform infrared(FT-IR)spectroscopy.The findings indicated that the successful introduction of 3-(2-hydroxyphenyl)propionate groups using a OH groups/DHC/organic base mo-lar ratio of 1꞉1꞉0.3 at 120℃ for 6 h increased the content of phenolic hydroxyl groups from 1.7931 to 3.0179 mmol/g,and the LPPE exhibited excellent ultraviolet-absorbing and antioxidant per-formance with up to 90%free radical scavenging activity within 20 min using 5 mg/mL of LPPE.In addition,good biocompatibility and a high Sun protection factor(SPF)value of 40.9 were achieved at 5%(w)dosage of LPPE in the cream,indicating its significant application potential in sunscreen.

The development of MOFs-based nanomaterials in heterogeneous organocatalysis

Metal-organic framework(MOF) is a class of inorganic-organic hybrid material assembled periodically with metal ions and organic ligands. MOFs have always been the focuses in a variety of frontier fields owing to the advantageous properties, such as large BET surface areas, tunable porosity and easyfunctionalized surface structure. Among the various application areas, catalysis is one of the earliest application fields of MOFs-based materials and is one of the fastest-growing topics. In this review, the main roles of MOFs in heterogeneous organocatalysis have been systematically summarized, including used as support materials(or hosts), independent catalysts, and sacrificial templates. Moreover, the application prospects of MOFs in photocatalysis and electrocatalysis frontiers were also mentioned.Finally, the key issues that should be conquered in future were briefly sketched in the final parts of each item. We hope our perspectives could be beneficial for the readers to better understand these topics and issues, and could also provide a direction for the future exploration of some novel types of MOFs-based nanocatalysts with stable structures and functions for heterogeneous catalysis.

Perfectly Green Organocatalysis： Quaternary Ammonium Base Triggered Cyanosilylation of Aldehydes

Quaternary ammonium bases, such as aqueous （CH3）4NOH, were found to be an extraordinarily efficient cata-lyst for cyanosilylation of aldehydes. The addition reaction of trimethylsilyl cyanide （TMSCN） to equivalent alde-hydes could proceed smoothly with turnover frequency （TOF） up to 3000000 h l and in near 100% yield under solvent-free conditions. These organic catalysts also tolerated various aldehydes including aromatic, aliphatic and a,fl-unsaturated aldehydes. This process perfectly conforms to the features of green chemistry： no waste regarding side-products and unconverted reactants, solvent-free, excellent catalytic activity, and no requirement for separa- tion.

Nucleophilic Polymer-supported Tertiaryphosphine Organocatalysis：[3＋2] Annulation Reaction of Alkyl 2-Butynoates with Activated Alkenes

JandaJel resin-supported triphenylphosphine（JJ-TPP）,was used as an effective organocatalyst for the [3＋2] annulation reaction of alkyl 2-butynoates with electron-deficient alkenes to afford the corresponding cyclopentenes in 40％-68％ isolated yields.JJ-TPP was reused after simple filtration and vacuum drying.

Porous aromatic framework(PAF-1) as hyperstable platform for enantioselective organocatalysis

High density of phenyl rings makes PAF-1 have robust structure and highly lipophilic pore, which make it very suitable for organocatalysis. However, there is no report about using PAF-1 as platform for enantioselective organocatalysis.In this paper, using PAF-1 as the platform, a chiral prolinamide catalytic site was introduced onto the framework of PAF-1 via a series of stepwise post-synthetic modifications,obtaining a novel PAF-supported chiral catalyst named PAF-1-NHPro. Then its enantioselective catalytic performance was studied by subjecting it to catalyze the model Aldol reaction between p-nitrobenzaldehyde and cyclohexanone.PAF-1-NHPro showed good diastereoselectivity and enantioselectivity with excellent and easy recyclability.

Functionalization of Carbonyl Compounds via Photoredox Organocatalysis

In recent years, a methodology merging photoredox catalysis with organocatalysis termed ＂photoredox organo- catalysis＂ has emerged to allow the direct, selective and efficient functionalization of the α/β-C of carbonyl com- pounds under mild reaction condition. In this review, photophysics background of photoredox catalysis is intro- duced, followed by a report on recent advances in direct α- and β-functionalization of carbonyls with photoredox organocatalysis methodology. With this different reaction modes, valuable synthetic targets including different a/fl-functionalized carbonyls are accessible.

Transformations and Tautomeric Equilibrium among Different Intermediates in Proline-Catalyzed Reactions of Aldehydes or Ketones

The enamines, iminium ions, and oxazolidinones are thought to be the key intermediates in the proline-catalyzed reactions of aldehydes or ketones, but there is an extensive contro- versy about their roles. Here, the corresponding transition states connecting any two of the three kinds of species are located at the wB97XD/6-311＋＋G＊＊ level of theory. The calcula- tions demonstrate that the oxazolidinones are the predominant species in both the gas phase and solvents; there exists tautomeric equilibrium among these species and the equilibriums are controlled by the employed solvents and temperature in the reaction. These results demonstrate that the concentration and role of the mentioned species are controlled by the employed solvent and temperature. A new reaction pathway is presented herein for the trans- formation between iminium ions and oxazolidinones through iminium ion-water complex and oxazolidinone-water complex. The calculations demonstrate that the rate-limiting step in proline-catalyzed Mannich reaction between acetaldehyde/keteones and N-Boc imines is the formation of the C-C bond rather than the intermediates tautomerization. These calculations rationalize the available experimental observations and can be valuable in optimizing the experimental conditions of asymmetric organic-catalyzed reactions of aldehydes or ketones.

Selective aerobic oxidation promoted by highly efficient multi-nitroxy organocatalysts

Selective oxidation with molecular oxygen as the sole oxidant under mild conditions is of crucialimportance for the long‐term sustainable exploitation of available feedstocks and the formation ofrequired intermediates for organic synthesis and industrial processes.Among the developed oxidationprotocols,innovative strategies using hydroxyimide organocatalysts in combination with metallicor metal‐free cocatalysts have attracted much attention because of the good activities andselectivities of such catalysts in the oxo functionalization of hydrocarbons.This method is based onthe reaction using N‐hydroxyphthalimide,which was first reported by Ishii’s group in the1990s.Although the important and wide‐ranging applications of such catalysts have been summarizedrecently,there are no reviews that focus solely on oxidation strategies using multi‐nitroxy organocatalysts,which have interesting properties and high reactivities.This review covers the concisesynthetic methods and mechanistic profiles of known multi‐nitroxy organocatalysts and summarizessignificant advances in their use in efficient aerobic oxidation.Based on a combination of experimentaland theoretical results,guidelines for the future rational design of multi‐nitroxy organocatalystsare proposed,and the properties of various model multi‐nitroxy organocatalysts are predicted.The present overview of the advantages,limitations,and potential applications of multi‐nitroxyorganocatalysts can provide useful tools for researchers in the field of selective oxidation.

Dialkyl imidazolium acetate ionosilica as efficient and recyclable organocatalyst for cyanosilylation reactions of ketones

We report new heterogeneous organocatalyst based on silica hybrid supported N-heterocyclic carbene(NHC-)species.The organocatalyst is formed from an imidazolium iodide based ionosilica material,followed by iodide/acetate anion exchange.The imidazolium acetate generates the organocatalytic carbene via partial deprotonation of the imidazolium ring in situ.As monitored via EDX,solid state NMR and ion chromatography measurements,the iodide/acetate exchange involving the imidazolium ionosilica material took place only in small extent.Despite the fact that the exchanged material contains only a very small amount of acetate,we observed good catalytic activity and recyclability in cyanosilylation reactions of ketones with trimethylsilyl cyanide.The versatility of the catalyst was highlighted via reaction with several substrates,yielding the corresponding cyanohydrins in good yields.In recycling experiments,the material showed decreasing catalytic activity starting from the third reaction cycle,but high catalytic activity can be regenerated via another acetate treatment.Our work is important as it highlights the possibility to combine carbene chemistry and silica,which are antagonistic at a first glance.We show that imidazolium acetate based ionosilicas are therefore heterogeneous'proto-carbenes',and that there is no need to form strongly basic silica supported NHCs to obtain heterogeneous NHC-organocatalysts.This work therefore opens the route towards heterogeneous and re-usable NHC-organocatalysts from supported ionic liquid imidazolium acetates.

Relative Stability of the Key Enamine,Oxazolidinone,and Imine Intermediates in Some Proline-catalyzed Asymmetric Reactions

Many proline-catalyzed asymmetric addition reactions with ketones as substrates were assumed to involve a key intermediate, an enamine, produced by the condensation of proline and ketone. In this paper, the key intermediate enamines derived from L-proline and cyclohexanone （or acetone） as well as the corresponding oxazolidinone and imine tautomers have been investigated by means of density functional calculations at the B3LYP/6-311＋G^＊＊ level. The predicted order of stability for these tautomers is oxazolidinones 〉 enamines 〉 imines in gas phase and oxazolidinones 〉 imines 〉 enamines in aprotic THF solvent. This prediction explains why enamine intermediate can not be observed experimentally. The predicted energy/enthalpy difference between the formal oxazolidinone structure and the zwitterionic imine structures is very small in THF solvent, suggesting the oxazolidinone-to-imine tautomerization can be readily induced in solvent. ^13C NMR chemical shifts of the oxazolidinone and imine structures have been computed and used to explain the experimental NMR spectra observed in oxazolidinone-to-imine tautomerization induced by protic solvent.

Practical and Efficient Acylation and Tosylation of Sterically Hindered Alcohols Catalyzed with 1-Methylimidazole

Acylation of sterically hindered alcohols is frequently encountered in synthetic chemistry. An efficient and mild procedure for tosylation and esterification of sterically hindered hydroxyl groups with p-TsCl, Ac20 and Bz20 in the presence of l-methylimidazole is described. It was observed that auxiliary base, triethylamine, accelerates the acylation reaction.

Chiral Diamine-catalyzed Asymmetric Aldol Reaction

A highly efficient catalytic system composed of a simple and commercially available chiral primary diamine（1R,2R）-cyclohexane-1,2-diamine（6） and trifluoroacetic acid（TFA） was employed for asymmetric Aldol reaction in i-PrOH at room temperature.A loading of 10%（molar fraction） catalyst 6 with TFA as a cocatalyst could catalyze the Aldol reactions of various ketones or aldehydes with a series of aromatic aldehydes,furnishing Aldol products in moderate to high yields（up to 99%） with enantioselectivities of up to 99% and diastereoselectivities of up to 99：1.

Recyclable Cyclohexanediamine Derivatives as Organocatalysts: Organocatalytic Reduction with Trichlorosilane and Aldol Reaction

Reduction of ketimine with trichlorosilane was carried out using bisformamide catalyst 1a derived from cyclohexanediamine to give the corresponding product in 81% yield with 39% ee. Deprotection of the formyl groups of the catalysts 1 gave the corresponding diamines 2 which were utilized in aldol reaction of acetone with 4-nitrobenzaldehyde. The reaction using 2b in brine afforded the aldol adduct in 81% yield with 29% ee.

Organocatalyzed Decarboxylation of Naturally Occurring Cinnamic Acids: Potential Role in Flavoring Chemicals Production

The mechanism and the final outcome of the Knoevenagel-Doebner reaction are discussed. The condensation reaction between different hydroxy-substituted aromatic aldehydes and malonic acid is performed using piperidine as organocatalyst. The key role of the catalyst is clearly pointed out during the decarboxylation of ferulic acid, without the use of a strong decarboxylating agent, leading to a 4-vinylphenol derivative. Based on the results obtained, the studied pathway may be important in the understanding of vinylphenol production during malting and brewing of wheat and barley grains. Finally, changing the solvent of the reaction from pyridine to water in the Knoevenagel-Doebner reaction of 4-hydroxybenzaldehydes, dimerization of resulting styrene derivatives is observed. These results can be of interest also in the field of food chemistry, since cinnamic acids are frequently found in fruits and vegetables used for human consumption.

N-Heterocyclic carbene-catalyzed enantioselective(dynamic) kinetic resolutions and desymmetrizations

N-heterocyclic carbene-catalyzed enantioselective kinetic resolutions,dynamic kinetic resolutions,and desymmetrization reactions are systematically reviewed.The content is organized according to the activation modes involved in these transformations.Future advances within this highly active research field are discussed from our perspectives on the topic.

An Assembly of Pyrano[3,2-b]indol-2-ones via NHC-Catalyzed[3+3]Annulation of Indolin-3-ones with Ynals

We report herein an unprecedented N-heterocyclic carbene-catalyzed formal[3+3]annulation of ynals with N-Ts indolin-3-ones under the oxidation condition affording the functionalized pyrano[3,2-b]indol-2-ones.The alkynyl acylazoliums via the combination of a carbene with ynals in the presence of oxidate proved to be the important intermediates for the success of this transformation.This method features a broad substrate scope and mild conditions,including axially chiral skeletons with suitable substitutions.

(Parallel) kinetic resolution of 3,3-disubstituted indolines via organocatalyzed reactions with azodicarboxylates

A novel kinetic resolution(KR) method has been developed for 3,3-disubstituted indolines, whose catalytic asymmetric synthesis remains a significant challenge in organic synthesis. The key to the success of this KR protocol lies in the utilization of chiral phosphoric acid-catalyzed triazane formation reaction with azodicarboxylates, which enables the enantioselective synthesis of various substituted indolines bearing C3-quaternary stereocenters with good to high enantioselectivities(with sfactors up to 70). Moreover, an intriguing parallel kinetic resolution(PKR) has been developed by combining triazane formation and dehydrogenation reactions using different azodicarboxylates. Experimental studies have provided insight into the mechanism of this PKR reaction, demonstrating stereoselectivity in both triazane formation and dehydrogenation steps, favoring the opposite enantiomers. The large-scale synthesis and diverse derivatizations of the products, particularly the imine groupcontaining 3H-indoles, demonstrate the value of these(P)KR methods.

Catalytically generated noncovalent ammonium dienolate:a versatile platform for the development of organocatalytic asymmetric cascade reactions

Organocatalytic cascade reactions represent a powerful strategy for the rapid construction of complex chiral molecules with multiple stereocenters from simple substrates under mild conditions. The intriguing structural feature and diverse reactivity of catalytically generated dienolate species render them competent and versatile intermediates for the development of practical and valuable cascade reactions. Over the past years, a plethora of innovative and pioneering noncovalent ammonium dienolatemediated cascade reactions have been designed and implemented under the catalysis of chiral organocatalysts, making dienolate activation a general, robust, and complementary method for the functionalization of unsaturated carbonyl compounds and related substances. This review illustrates the recent advances in organocatalytic noncovalent ammonium dienolate-mediated cascade reactions(mainly from 2010 to 2023), including the cascade transformations of ammonium dienolates directly generated from unsaturated ketone/aldehyde, ester/lactone/azlactone, amide/lactam/pyrazolone/oxindole, and alkylidene nitrile compounds. The contents are arranged based on the reaction types of the ammonium dienolates, with an emphasis on cascade 2,5-, 3,5-, and 4,5-difunctionalizations of these intermediates. Furthermore, other cascade reactions involving the 1,3-, 2,3-, and even more complex 3,4,5-reactivities of ammonium dienolates were also discussed. The reaction pathway, reaction stereoinduction, and synthetic applications of the ammonium dienolate-mediated cascade reactions were highlighted throughout the article. As a stimulating and ever-growing research area, the organocatalytic noncovalent ammonium dienolate-mediated cascade reactions are expected to continue demonstrating their magic power for constructing chiral targets in the future and further expanding the boundaries of asymmetric catalysis.

Carbene-catalyzed enantioselective seleno-Michael addition as access to antimicrobial active Se-containing heterocycles

Organoseleniums exhibit a diverse set of biological activities that are pivotal for drug discovery and are widely explored in synthetic chemistry and material science.While many transformations have been developed for non-enantioselective C–Se bond formations,the catalyst-controlled stereoselective preparation of chiral organoseleniums continues to be of considerable challenge.In particular,there are limited studies on the enantioselective seleno-Michael addition reactions to access chiral selenium functional molecules.Here,we disclose a carbene-catalyzed highly enantioselective nucleophilic C–Se bond construction through formal[3+3]annulations between selenocarboxamides and bromoenals,affording seleno-thiazinone products with good yields and excellent enantioselectivities.The choice of a weak inorganic base was pivotal to suppressing the unproductive racemization and decomposition of the selenium products.Notably,the catalytically generated chiral selenium-containing heterocyclic products feature remarkable antimicrobial activities that could serve as promising lead scaffolds for further agrochemical development.

Catalytic asymmetric(3+3)cycloaddition between different 2-indolylmethanols

The catalytic asymmetric(3+3)cycloaddition between different 2-indolylmethanols has been established,in which a series of chiral indole-fused six-membered heterocycles were constructed in high yields with excellent enantioselectivities.This work not only has established the first catalytic asymmetric cycloaddition between different 2-indolylmethanols but also provided a powerful strategy for constructing enantioenriched indole-fused six-membered rings.Additionally,biological evaluation discovered some products with promising antitumor activities.Notably,theoretical calculations performed on the reaction pathway and activation mode provide an in-depth understanding of this catalytic asymmetric(3+3)cycloaddition among different 2-indolylmethanols,which will advance the understanding of the chemistry of indolylmethanols.