An insight into the mechanism of the aerobic oxidation of aldehydes catalyzed by N-heterocyclic carbenes

Catalytic Activity of Water-Soluble Palladium Nanoparticles with Anionic and Cationic Capping Ligands for Reduction, Oxidation, and C-C Coupling Reactions in Water

The availability of water-soluble nanoparticles allows catalytic reactions to occur in highly desirable green environments. The catalytic activity and selectivity of water-soluble palladium nanoparticles capped with 6-(carboxylate)hexanethiolate (C6-PdNP) and 5-(trimethylammonio)pentanethiolate (C5-PdNP) were investigated for the reduction of 4-nitrophenol, the oxidation of α,β-conjugated aldehydes, and the C-C coupling of phenylboronic acid. The study showed that between the two PdNPs, C6-PdNP exhibits better catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride and the selective oxidation of conjugated aldehydes to conjugated carboxylic acids. For the latter reaction, molecular hydrogen (H2) and H2O act as oxidants for the surface palladium atoms on PdNPs and conjugated aldehyde substrates, respectively. The results indicated that the competing addition activities of Pd-H and H2O toward the π-bond of different unsaturated substrates promote either reduction or oxidation reactions under mild conditions in organic solvent-free environments. In comparison, C5-PdNP exhibited higher catalytic activity for the C-C coupling of phenylboronic acid. Gas chromatography-mass spectrometry (GC-MS) was mainly used as an analytical technique to examine the products of catalytic reactions.

Fully bio-based cellulose ester synthesis from natural aldehydes via aerobic oxidation

This study presents a novel synthetic method to synthesize fully bio-based cellulose esters from natural aldehydes in a multi-functional ionic liquid (as a solvent for cellulose and as an esterification and oxidation catalyst) in atmospheric oxygen. The aerobic oxidative esterification of cellulose with high atom economy proceeded under metal-free conditions without any additional catalysts or activators.

A Combined Experimental/Computational Study of Dicationic Ionic Liquids with Bromide and Tungstate Anions

A panel of dicationic ionic liquids (DILs) with different rigid xylyl (ortho, meta, para) spacers and different anions (bromide and tungstate) has been synthetised and characterised through different experimental and computational techniques. Differences and analogies between the systems are analysed using information derived from their DFT structures, semiempirical dynamics, thermal behaviour, and catalytic properties versus the well-known reaction of CO2 added to epichlorohydrin. A comparison between the proposed systems and some analogues that present non-rigid spacers shows the key effect displayed by structure rigidity on their characteristics. The results show an interesting correlation between structure, flexibility, properties, and catalytic activity.

N-Heterocyclic Carbene-Catalyzed Facile Synthesis of Phthalidyl Sulfonohydrazones: Density Functional Theory Mechanistic Insights and Docking Interactions

N-heterocyclic carbene catalysis reaction protocol is disclosed for the synthesis of phthalidyl sulfonohydrazones. A broad range of N-tosyl hydrazones react effectively with phthalaldehyde derivatives under open-air conditions, enabling the formation of a new C-N bond via an oxidative path. The reaction proceeds under mild reaction conditions with broad substrate scopes, wide functional group tolerance, and good to excellent yields. The mechanistic pathway is studied successfully using control experiments, competitive reactions, ESI-MS spectral analyses of the reaction mixture, and computational study by density functional theory. The potential use of one of the phthalidyl sulfonohydrazone derivatives as the inhibitor of β-ketoacyl acyl carrier protein synthase I of Escherichia coli is investigated using molecular docking.

Carbene-Catalyzed Tandem Imine Umpolung-Intramolecular Aza-Conjugate Addition-Oxidation to Access N-Substituted Isoindolinones

Herein, we have described a novel N-heterocyclic carbene (NHC)-catalyzed synthesis of N-substituted isoindolinone acetates. The presented transformation proceeds through NHC-catalyzed tandem imine umpolung-intramolecular aza-Michael addition followed by oxidation, while molecular oxygen in air acts as a sole oxidant. Atom efficiency, operational simplicity, large-scale syntheses, and mild reaction conditions are the salient features of this method. Mechanistic studies were indicative of the necessity of molecular oxygen in air as oxidant for the conversion of imine to amide.

Comment on "CO2 as oxidant: an unusual light-assisted catalyst free oxidation of aldehydes to acids under mild conditions" by S. R. Khan, S. Saini, K. Naresh, A. Kumari, V. Aniya, P. K. Khatri, A. Ray and S. L. Jain, Chem. Commun., 2022, , 2208

Experimental data supporting the claim that CO2 could be used as an oxidant for light-assisted oxidation of aldehydes to carboxylic acids recently published by S. L. Jain et al., Chem. Commun., 2022, 58, 2208 could be mistrusted.

The importance of four-membered NHCs in stabilizing Breslow intermediates on benzoin condensation pathway

N-heterocyclic carbenes (NHCs) have been established to be effective organocatalysts for facilitating the benzoin condensation and many other reactions. These reactions involve the formation of a Breslow intermediate (BI), which exhibits umpolung chemistry. To facilitate organocatalysis, several new cyclic carbenes are being introduced, four-membered NHCs are of special interest. Whether these NHCs can exhibit catalytic influence or not, can be evaluated by exploring the potential energy surface (PES) of the benzoin condensation reaction. Quantum chemical analysis has been carried out to compare the PES of these four-membered NHCs with that of standard five-membered NHCs to explore their catalytic ability. The barrier for the first step of the reaction for the formation of BI is comparable in all the cases. But the barrier for the second step of the reaction leading to the benzoin formation from BI is estimated to be very high for the four membered NHCs. These results indicate that the probability of identifying and isolating the BI is very high in comparison to the completion of benzoin condensation reaction in the case of the four-membered NHCs.

Oxidative N-Heterocyclic Carbene Catalysis

N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O2 /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHC-catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

N-Heterocyclic carbene (NHC)-catalyzed oxidation of unactivated aldimines to amides via imine umpolung under aerobic conditions

Herein, we disclose an NHC-catalyzed aerobic oxidation of unactivated aldimines for the synthesis of amides via umpolung of imines proceeding through an aza-Breslow intermediate. We have developed an eco-friendly method for the conversion of imines to amides by using molecular oxygen in air as the sole oxidant and dimethyl carbonate (DMC) as a green solvent under mild reaction conditions. Broad substrate scope, high yields and gram scale syntheses expand the practicality of the developed method.

A general NHC-catalyzed conversion of imines to amides proceeding through umpolung–oxidation under aerobic conditions in green solvent is reported.

Dioxygenation of unprotected mesoionic N-heterocyclic olefins

We report the dioxygenation of mesoionic N-heterocyclic olefins (mNHOs) using molecular dioxygen. For 1,2,3-triazole-derived mNHOs possessing a vinyl proton and at least one acidic C-H group, they are oxidized into the corresponding triazolium benzoate salts, whereas those without vinyl proton or an acidic C-H group are oxidized into triazolium oxide and ketones/aldehydes.

N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading

We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and β-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/β-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.

Charge-Tagged N-Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC-Catalysed Reactions through Electrospray Ionization Mass Spectrometry

N-heterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESI-MS) to the detection of charge-tagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the charge-tagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry.

N-Heterocyclic carbene (NHC)-catalyzed tandem imine umpolung–aza-Michael addition–oxidation of β-carboline cyclic imines

N-Heterocyclic carbene (NHC)-catalyzed tandem imine umpolung–aza-Michael addition–oxidation of cyclic imines for the synthesis of a wide range of biologically relevant β-carboline-1-one derivatives has been disclosed.

Aerobic oxidation of 5-hydroxymethylfurfural to 5-hydroxymethyl-2-furancarboxylic acid and its derivatives by heterogeneous NHC-catalysis

The application of the oxidative system composed of a heterogeneous triazolium pre-catalyst, iron(ii) phthalocyanine and air is described for the selective conversion of 5-hydroxymethylfurfural (HMF) into the added-value 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The disclosed one-pot two-step procedure involved sequential oxidative esterifications of HMF to afford a polyester oligomer having hydroxyl and carboxyl terminal groups (Mw = 389-1258), which in turn was hydrolyzed by a supported base (Ambersep 900 OH) to yield HMFCA in 87% overall yield. The same strategy was adopted for the effective synthesis of ester and amide derivatives of HMFCA by nucleophilic depolymerization of the oligomeric intermediate with methanol and butylamine, respectively. The utilization of the disclosed oxidative system for the direct conversion of HMF and furfural into their corresponding ester, amide, and thioester derivatives is also reported.

Intermediates of N-Heterocyclic Carbene (NHC) Dimerization Probed in the Gas Phase by Ion Mobility Mass Spectrometry: C-H⋅⋅⋅:C Hydrogen Bonding Versus Covalent Dimer Formation

N-Heterocyclic carbenes (NHCs, :C) can interact with azolium salts (C-H⁺ ) by either forming a hydrogen-bonded aggregate (CHC⁺ ) or a covalent C-C bond (CCH⁺ ). In this study, the intramolecular NHC-azolium salt interactions of aromatic imidazolin-2-ylidenes and saturated imidazolidin-2-ylidenes have been investigated in the gas phase by traveling wave ion mobility mass spectrometry (TW IMS) and DFT calculations. The TW IMS experiments provided evidence for the formation of these important intermediates in the gas phase, and they identified the predominant aggregation mode (hydrogen bond vs. covalent C-C) as a function of the nature of the interacting carbene-azolium pairs.

Systematic Synthesis and Properties Evaluation of Dicationic Ionic Liquids, and a Glance Into a Potential New Field

Dicationic ionic liquids (DILs), a subset of the ionic liquid (IL) family, have attracted growing interest in recent years, and the range of applications within which they are investigated is constantly expanding. However, data which allows structure to property correlation of a DIL is still limited, and thus selecting an appropriate salt to address a specific challenge can be problematic. In comparison to traditional ILs, DILs physico-chemical properties can be tuned by changing the length and type of spacer which connects the cationic heads as well as the type of cation. This in turn could give rise to symmetrical or asymmetrical DILs. In this work, a systematic study of a homogeneous class of 12 dibromide DILs and 12 di-carboxylate salts has been performed. The latter class of DILs were also compared to mono cation derivatives. The different traditional exchange methods to prepare carboxylate DILs have been evaluated and an insight into the drawbacks encountered is also presented. Prepared DILs were characterized (NMR, TGA, DSC) allowing the influence of the structure on their thermal stability to be understood. Most DILs were obtained as solid salts after careful drying. For some of these compounds, a new possible application was studied, namely their use as hydrogen bond acceptors (HBA) of deep eutectic mixtures, showing again some significant structural related effects.

Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions

Heterogeneous NHC-catalysis in batch and flow modes is an effective synthetic platform for the production of monoacylglycerols.

A Cooperative Ternary Catalysis System for Asymmetric Lactonizations of α-Ketoesters

A general and enantioselective N-heterocyclic carbene (NHC)-catalyzed lactonization of simple enals and α-ketoesters has been discovered using a new ternary cooperative catalytic system. The highly selective annulation was achieved by using a combination of a chiral NHC, a hydrogen-bond donor, and a metal salt, facilitating self-assembly of the reactive partners. A proposed model for this new mode of NHC chiral relay catalysis is supported by experimental and computational mechanistic studies.

Access to Amide from Aldimine via Aerobic Oxidative Carbene Catalysis and LiCl as Cooperative Lewis Acid

Herein, an efficient route to amides from aldimines via aza-Breslow intermediates through aerobic oxidative carbene catalysis with LiCl as a cooperative Lewis acid is described. Many of the obtained N-heteroarylamides feature biological activity. Ambient air was used as the sole oxidant and source of oxygen in this catalytically oxidative amidation. This method allows for a broad substrate scope and mild conditions. The aza-Breslow intermediate derivative was isolated and its crystal structure confirmed.

Formation, Oxidation, and Fate of the Breslow Intermediate in the N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aldehydes

The reaction paths and intermediate structures related to the formation of the Breslow intermediate and its oxidation along the oxidative/oxygenative lanes have been studied from a mechanistic point of view, with the support of gas-phase and computational studies. The results confirm the occurrence of a single-electron transfer from the Breslow intermediate to the molecular oxygen with formation of a radical couple that recombines either as a peroxide anion 7' to afford the aldehyde-to-carboxylic acid product or as a hydroperoxy derivative 7″ that evolved into an electrophilic acyl azolium, opening to the aldehyde-to-ester conversion. Steric factors enter into determining the different reactivity. All of the intermediates of both catalytic paths have been observed and characterized under mass spectrometric conditions. In particular, for the imidazoline catalyst, the (+)ESI-MS/(MS) detection of the genuine Breslow intermediate was made possible in virtue of its limited reactivity. Mechanistic aspects of the N-heterocyclic carbenes catalyzed aerobic oxidation of aldehydes shares important similarities with that one of the recently revisited benzoin condensation.

Cross-benzoin and Stetter-type reactions mediated by KOtBu-DMF via an electron-transfer process

The KOtBu-DMF system promotes benzoin- and Stetter-like reactions of alpha-diketone donors through a double electron-transfer-based mechanism.

Oxidative N-heterocyclic carbene catalyzed stereoselective annulation of simple aldehydes and 5-alkenyl thiazolones

A highly stereoselective annulation proceeded successfully to afford chiral thiazolo pyrones with excellent ee (up to 99%) and d.r. (up to >20 : 1). Both enantiomers can be obtained. In the presence of oxygen (O₂), the oxidant O-1 can be used in a catalytic amount (even low to 2 mol%).

A new chiral C1-symmetric NHC-catalyzed addition to α-aryl substituted α,β-disubstituted enals: enantioselective synthesis of fully functionalized dihydropyranones

A new C₁-symmetric biarylsaturated imidazolium exhibits a superior ability to enable previously unavailable transformation, and the corresponding fully functionalized dihydropyranones are efficiently synthesized.

N-heterocyclic carbene catalysed oxidative esterification of aliphatic aldehydes

Aliphatic aldehydes are readily transformed to the corresponding esters by oxidative carbene catalysis.