Rational design of galactopyranoside-substituted N-heterocyclic carbene palladium(ii) complexes. Stable and efficient catalyst for C-C coupling in aqueous media

Following a rational design, three novel palladium(ii) complexes bearing galactopyranoside-based N-heterocyclic carbene ligands have been synthesized via transmetalation of the corresponding Ag(i) complexes. Palladium(ii) complexes have been characterized by NMR, FT-IR and elemental analysis. Catalytic studies, using the Stille and Suzuki-Miyaura cross-coupling reactions as model C-C coupling, reveal that the complexes are active and reusable. The best results in terms of TON values were achieved in aqueous medium using either the in situ deacetylation of the catalyst or the previously deacetylated catalyst. The catalytic condition using in situ deacetylation was more efficient because it avoids an additional deprotection step.

Synthesis and Antiparasitic Activity of New Trithiolato-Bridged Dinuclear Ruthenium(II)-arene-carbohydrate Conjugates

Eight novel carbohydrate-tethered trithiolato dinuclear ruthenium(II)-arene complexes were synthesized using CuAAC ‘click’ (Cu(I)-catalyzed azide-alkyne cycloaddition) reactions, and there in vitro activity against transgenic T. gondii tachyzoites constitutively expressing β-galactosidase (T. gondii β-gal) and in non-infected human foreskin fibroblasts, HFF, was determined at 0.1 and 1 µM. When evaluated at 1 µM, seven diruthenium-carbohydrate conjugates strongly impaired parasite proliferation by >90%, while HFF viability was retained at 50% or more, and they were further subjected to the half-maximal inhibitory concentration (IC50) measurement on T. gondii β-gal. Results revealed that the biological activity of the hybrids was influenced both by the nature of the carbohydrate (glucose vs. galactose) appended on ruthenium complex and the type/length of the linker between the two units. 23 and 26, two galactose-based diruthenium conjugates, exhibited low IC50 values and reduced effect on HFF viability when applied at 2.5 µM (23: IC50 = 0.032 µM/HFF viability 92% and 26: IC50 = 0.153 µM/HFF viability 97%). Remarkably, compounds 23 and 26 performed significantly better than the corresponding carbohydrate non-modified diruthenium complexes, showing that this type of conjugates are a promising approach for obtaining new antiparasitic compounds with reduced toxicity.

Carbohydrate‐functionalized triazolylidene iridium complexes: hydrogenation catalysis in water with asymmetric induction

Two sets of carbohydrate‐NHC hybrid iridium complexes were synthesised in order to combine properties of carbohydrates and triazolylidene (trz) ligands in organometallic catalysis. One set features a direct trz linkage to the anomeric carbohydrate carbon, while the second set is comprised of an ethyl linker between the two functional units. Deprotection of the carbohydrate afforded hybrid complexes that efficiently catalyse the direct hydrogenation of ketones in water. The catalytic activity of the hybrid complexes was influenced by the pH of the aqueous medium and surpassed the activity of carbohydrate‐free or acetyl‐protected analogues (>90 % vs 13 % yield). While no enantiomeric induction was observed for the ethyl‐linked hybrids, a moderate enantiomeric excess (ee) was induced by the directly linked systems. Moreover, these carbohydrate‐trz hybrid complexes displayed mixed inhibitory activity towards a glycosidase from H. orenii that contain a glucose binding site.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Simple synthesis of [Ru(CO3)(NHC)(p-cymene)] complexes and their use in transfer hydrogenation catalysis

A novel, efficient and facile protocol for the synthesis of a series of [Ru(NHC)(CO₃)(p-cymene)] complexes is reported. This family of Ru-NHC complexes was obtained from imidazol(in)ium tetrafluoroborate or imidazolium hydrogen carbonate salts in moderate to excellent yields, employing sustainable weak base. The ruthenium complexes were successfully utilized in the transfer hydrogenation of ketones as highly active multifunctional catalysts.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Dual utility of a single diphosphine-ruthenium complex: a precursor for new complexes and, a pre-catalyst for transfer-hydrogenation and Oppenauer oxidation

The diphosphine-ruthenium complex, [Ru(dppbz)(CO)₂Cl₂] (dppbz = 1,2-bis(diphenylphosphino)benzene), where the two carbonyls are mutually cis and the two chlorides are trans, has been found to serve as an efficient precursor for the synthesis of new complexes. In [Ru(dppbz)(CO)₂Cl₂] one of the two carbonyls undergoes facile displacement by neutral monodentate ligands (L) to afford complexes of the type [Ru(dppbz)(CO)(L)Cl₂] (L = acetonitrile, 4-picoline and dimethyl sulfoxide). Both the carbonyls in [Ru(dppbz)(CO)₂Cl₂] are displaced on reaction with another equivalent of dppbz to afford [Ru(dppbz)₂Cl₂]. The two carbonyls and the two chlorides in [Ru(dppbz)(CO)₂Cl₂] could be displaced together by chelating mono-anionic bidentate ligands, viz. anions derived from 8-hydroxyquinoline (Hq) and 2-picolinic acid (Hpic) via loss of a proton, to afford the mixed-tris complexes [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], respectively. The molecular structures of four selected complexes, viz. [Ru(dppbz)(CO)(dmso)Cl₂], [Ru(dppbz)₂Cl₂], [Ru(dppbz)(q)₂] and [Ru(dppbz)(pic)₂], have been determined by X-ray crystallography. In dichloromethane solution, all the complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows redox responses within 0.71 to -1.24 V vs. SCE. [Ru(dppbz)(CO)₂Cl₂] has been found to serve as an excellent pre-catalyst for catalytic transfer-hydrogenation and Oppenauer oxidation.

Anion-cation synergistic metal-free catalytic oxidative homocoupling of benzylamines by triazolium iodide salts

Triazolium iodide salts are excellent catalysts for the selective oxidative coupling of benzylamines to yield imines. This metal-free reaction proceeds in quantitative spectroscopic yields when run in refluxing 1,2-dichlorobenzene and open to the air. No catalytic activity was observed with related triazolium tetrafluoroborate salts. Variation of catalyst and reaction atmosphere provides mechanistic insights, and revealed dioxygen as the terminal oxidant and the iodine/iodide couple as key redox component in the catalytic dehydrogenation pathway. While molecular iodine is competent as a catalyst in its own right, the triazolium cation triples the reaction rate and reaches turnover frequencies up to 30 h-1, presumably through beneficial interactions of the electron-poor azolium π system and I2, which facilitate the electron transfer from the substrate to iodine and concomitant formation of I-. This acceleration is specific for triazolium cations and represents a hybrid anion/cation catalytic process as a simple and straightforward route towards imine products, with economic advantages over previously reported metal-based catalytic systems.

Synthesis of glucoside-based imidazolium salts for Pd-catalyzed cross-coupling reaction in water

Sugar-based imidazolium salts (IMSs) represent an outstanding type of material making them eye-catching for a wide variety of applications. Herein, a series of glucoside-based IMSs (Glu-IMSs) combining glucoside and imidazolium head groups with different substituents were synthesized. The catalytic activities of these Glu-IMSs were evaluated by Pd-catalyzed Heck-Mizoroki and Suzuki-Miyaura reactions in water. Among them, the Glu-IMSs contain both -OH and NHCs coordination sites was found to be the most efficient ancillary ligand in comparison with other Glu-IMSs with just single NHCs coordination site. The HR-TEM analysis showed that the palladium nanoparticles stabilized by the Glu-IMSs with an average size of ~4.0 nm was formed in the reaction system, which may be act as an efficient real catalytic species. Under the optimized reaction conditions, a series of novel fluorine-cored organic small molecule functional materials were synthesized with favorable yields.

Pseudo-enantiomeric carbohydrate-based N-heterocyclic carbenes as promising chiral ligands for enantiotopic discrimination

The practical synthesis of carbohydrate-based NHC-Rh complexes bearing C1 or C3 sterically differentiated positions, accessed by glycosylation or SNAr strategies, is reported. These catalysts exhibit pseudo-enantiomeric behaviour in the hydrosilylation of acetophenone. We show that steric bulk at C1 gives preference for (S)-phenyl-1-ethanol, while bulk at C3 leads to the (R)-enantiomer. These results represent the first example of pseudo-enantiomeric carbohydrate-based NHC ligands leading to enantiotopic discrimination.