Water-Soluble Pd-Imidate Complexes as Versatile Catalysts for the Modification of Unprotected Halonucleosides

Modification of unprotected nucleosides has been attracting continuous interest, since these building blocks themselves and their phosphate-upgraded corresponding nucleotides have shown a plethora of uses in fields like biochemistry or pharmacy. Pd-catalyzed cross-coupling reactions, conducted in water or its mixtures with polar organic solvents, have frequently been the researchers' choice for the functionalization of the purine/pyrimidine base of the unprotected nucleosides. In this scenario, the availability of hydrophilic ligands and its water-soluble palladium complexes has markedly set the pace of the advances. The approach of our group to the synthesis of such complexes, Pd-imidates specifically, has faced critical stages, namely the jump to synthesize water soluble complexes from our experience working in conventional solvents, the preparation of phosphine free complexes and the overall goal of getting catalytic systems able to work close to room temperature. The continuous feedback with Kapdi's group, experienced in the chemistry of nucleosides, has produced over the last decade the interesting results in both fields presented here.

Self-assembled monolayer protection of chiral-imprinted mesoporous platinum electrodes for highly enantioselective synthesis

In modern chemistry, chiral (electro)catalysis is a powerful strategy to produce enantiomerically pure compounds (EPC). However, it still struggles with uncontrollable stereochemistry due to side reactions, eventually producing a racemic mixture. To overcome this important challenge, a well-controlled design of chiral catalyst materials is mandatory to produce enantiomers with acceptable purity. In this context, we propose the synergetic combination of two strategies, namely the elaboration of mesoporous Pt films, imprinted with chiral recognition sites, together with the spatially controlled formation of a self-assembled monolayer. Chiral imprinted metals have been previously suggested as electrode materials for enantioselective recognition, separation and synthesis. However, the outermost surface of such electrodes is lacking chiral information and thus leads to unspecific reactions. Functionalising selectively this part of the electrode with a monolayer of organosulfur ligands allows an almost total suppression of undesired side reactions and thus leads to a boost of enantiomeric excess to values of over 90% when using these surfaces in the frame of enantioselective electrosynthesis. In addition, this strategy also decreases the total reaction time by one order of magnitude. The study therefore opens up promising perspectives for the development of heterogeneous enantioselective electrocatalysis strategies.

Quadrol-Pd(II) complexes: phosphine-free precatalysts for the room-temperature Suzuki-Miyaura synthesis of nucleoside analogues in aqueous media

Commercially available Quadrol, N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine (THPEN), has been used for the first time as a N^N-donor neutral hydrophilic ligand in the synthesis and characterization of new water soluble palladium(II) complexes containing chloride, phthalimidate or saccharinate as co-ligands. [PdCl₂(THPEN)] (1) [Pd(phthal)₂(THPEN)] (2), [Pd(sacc)₂(THPEN)] (3) and the analogous complex with the closely related N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (THEEN) [Pd(sacc)₂(THEEN)] (4) were efficiently prepared in a one-pot reaction from [PdCl₂(CH₃CN)₂] or Pd(OAc)₂. Structural characterization of 1 and 3 by single crystal X-ray diffraction produced the first structures reported to date of palladium complexes with Quadrol. The resultant palladium complexes are highly soluble in water and were found to be effective as phosphine-free catalysts for the synthesis of functionalized nucleoside analogues under room-temperature Suzuki-Miyaura cross-coupling conditions between 5-iodo-2'-deoxyuridine (& 5-iodo-2'-deoxycytidine) with different aryl boronic acids in neat water. This is the first report of the coupling process performed on nucleosides in water at room temperature.

Kinetic effects of molecular clustering and solvation by extended networks in zeolite acid catalysis

Reactions catalyzed within porous inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, collectively referred to as "solvent effects". Transition state theory treatments define how solvation phenomena enter kinetic rate expressions, and identify two distinct types of solvent effects that originate from molecular clustering and from the solvation of such clusters by extended solvent networks. We review examples from the recent literature that investigate reactions within microporous zeolite catalysts to illustrate these concepts, and provide a critical appraisal of open questions in the field where future research can aid in developing new chemistry and catalyst design principles.

Palladium nanoparticles anchored on amphiphilic Janus-type cellulose nanocrystals for Pickering interfacial catalysis

Developing green and sustainable Pickering interfacial catalysts for organic reactions in water is of great importance to both the environment and human health. In this study, Janus-type amphiphilic cellulose nanocrystals (CNCs) were synthesized by the surface modification of hydrophilic CNCs with hydrophobic alkyl chains for efficient Pickering emulsion stabilization. Further deposition of palladium nanoparticles on amphiphilic CNCs provides catalytic activity for organic reactions in water, which occur at the interface of water and the organic reactant phase. Different reactions, hydrogenation and C-C coupling, were performed using the obtained Pickering interfacial catalyst. Excellent results were achieved in both reactions. The catalyst developed in our study is expected to advance the field of environment-friendly catalyst systems for green chemistry.

Stereoselective Double Reduction of 3-Methyl-2-cyclohexenone, by Use of Palladium and Platinum Nanoparticles, in Tandem with Alcohol Dehydrogenase

The combination of metal nanoparticles (Pd or Pt NPs) with NAD-dependent thermostable alcohol dehydrogenase (TADH) resulted in the one-flask catalytic double reduction of 3-methyl-2-cyclohexenone to 3-(1S,3S)-methylcyclohexanol. In this article, some assumptions about the interactions between a chemocatalyst and a biocatalyst have been proposed. It was demonstrated that the size of the NPs was the critical parameter for the mutual inhibition: the bigger the NPs, the more harmful for the enzyme they were, even if the NPs themselves were only moderately inactivated. Conversely, the smaller the NPs, the more minimal the TADH denaturation, although they were dramatically inhibited. Resuming, the chemocatalysts were very sensitive to deactivation, which was not related to the amount of enzyme used, while the inhibition of the biocatalyst can be strongly reduced by minimizing the NPs/TADH ratio used to catalyze the reaction. Among some methods to avoid direct binding of NPs with TADH, we found that using large Pd NPs and protecting their surfaces with a silica shell, the overall yield of 3-(1S,3S)-methylcyclohexanol was maximized (36%).

Ultrasound-assisted facile one-pot sequential synthesis of novel sulfonamide-isoxazoles using cerium (IV) ammonium nitrate (CAN) as an efficient oxidant in aqueous medium

A series of novel 3,5-disubstituted isoxazoles have been synthesized, using a new, green, and versatile "one-pot three-steps" methodology. The key step is an oxidative 1,3-dipolar cycloaddition under ultrasonic irradiation, occurring in aqueous media, and mediated by cerium (IV) ammonium nitrate (CAN). CAN is a one-electron oxidant, highly soluble in water, slightly toxic and inexpensive, that allows the in situ conversion of the intermediate aldoximes into nitrile oxide. The syntheses are highly regioselective, as illustrated by the structures of the final compounds, which have been fully assessed by spectral analyses (¹H and ¹³C NMR, MS). This study illustrates the potency of the ultrasound activation to synthesize a set of highly functionalized heterocycles, with potential applications in biology, in short reaction times and following an eco-friendly process.

Catalytic Organic Reactions in Water toward Sustainable Society

Traditional organic synthesis relies heavily on organic solvents for a multitude of tasks, including dissolving the components and facilitating chemical reactions, because many reagents and reactive species are incompatible or immiscible with water. Given that they are used in vast quantities as compared to reactants, solvents have been the focus of environmental concerns. Along with reducing the environmental impact of organic synthesis, the use of water as a reaction medium also benefits chemical processes by simplifying operations, allowing mild reaction conditions, and sometimes delivering unforeseen reactivities and selectivities. After the "watershed" in organic synthesis revealed the importance of water, the development of water-compatible catalysts has flourished, triggering a quantum leap in water-centered organic synthesis. Given that organic compounds are typically practically insoluble in water, simple extractive workup can readily separate a water-soluble homogeneous catalyst as an aqueous solution from a product that is soluble in organic solvents. In contrast, the use of heterogeneous catalysts facilitates catalyst recycling by allowing simple centrifugation and filtration methods to be used. This Review addresses advances over the past decade in catalytic reactions using water as a reaction medium.

Quinones as novel chemiluminescent probes for the sensitive and selective determination of biothiols in biological fluids

Altered plasma aminothiol concentrations are thought to be a valuable risk indicator and are interestingly utilized for routine clinical diagnosis and for the monitoring of various metabolic disorders and human diseases, and accordingly there is a need for an accurate and reliable assay capable of simultaneously determining aminothiols including glutathione (GSH), N-acetylcysteine (NAC), homocysteine (Hcys), and cysteine (Cys) in human plasma. Herein, a highly sensitive, selective, and very fast HPLC-chemiluminescence (HPLC-CL) coupled method is reported, exploiting for the first time the strong nucleophilicity and high reactivity of aminothiols toward quinones for a CL assay. The unique redox-cycling capability of quinone and/or Michael addition adducts, thioether-quinone conjugates, was utilized to establish a novel analytical method based on the reaction of adducts with dithiothreitol (DTT) to liberate reactive oxygen species (ROS), which are detected by using a luminol-CL assay. Specimen preparation involved the derivatization of aminothiols with menadione (MQ) for 5 minutes at room temperature. A unique green chemistry synthesis of thioether-quinones in HEPES buffer (pH 8.5) was introduced by using our reaction methodology without needing any hazardous organic solvent or catalyst. The aminothiol-MQ adducts were separated using solid-phase extraction followed by isocratic elution on an ODS column. Linearity was observed in the range of 2.5-500, 5-500, 10-1500, and 20-2000 nM with detection limits (S/N of 3) of 3.8, 4.2, 8, and 16 (fmol per injection) for GSH, NAC, Hcys, and Cys, respectively. The method was successfully applied for the selective determination of aminothiols in human plasma from healthy people and patients with rheumatic arthritis and diabetes mellitus. The obtained results postulated the usefulness of our method for investigating the relationship between aminothiol metabolism and related human disorders.

Organocatalysis by p-sulfonic acid calix[4]arene: a convenient and efficient route to 2,3-dihydroquinazolin-4(1H)-ones in water

An efficient and eco-friendly method is reported for the synthesis of 2,3-dihydroquinazolin-4(1H)-ones using p-sulfonic acid calix[4]arene as a recyclable organocatalyst in excellent yields in water at room temperature.

Pyridine-grafted graphene oxide: a reusable acid–base bifunctional catalyst for the one-pot synthesis of β-phosphonomalonates via a cascade Knoevenagel–phospha Michael addition reaction in water

Py–GO as a new acid–base bifunctional catalyst was synthesized and employed for the one-pot synthesis of β-phosphonomalonates in water.