Recent Advances in the Synthesis of 3,4-Dihydropyran-2-Ones Organocatalyzed by N-Heterocyclic Carbenes.

In recent years, N-heterocyclic carbenes (NHC) have gained recognition as versatile molecules capable of acting as organocatalysts in various reactions, particularly through the activation of aldehydes via Breslow-type adducts. This organocatalytic activation has enabled the production of numerous 3,4-dihydropyran-2-ones and related derivatives. In this review, we provide an overview of the production of 3,4-dihydropyran-2-ones and derivatives via organocatalytic processes involving NHCs over the past eight years. These processes involve the use of a diverse range of substrates, catalysts, and reaction conditions, which can be classified into [4+2]-and [3+3]-type cycloadditions, primarily aimed at synthesizing this skeleton due to its biological activity and multiple stereocenters. These processes are scaled up to the gram scale, and the resulting products are often directed towards epimerization and functionalization to produce more complex molecules with potential applications in the biological field. Finally, we provide a perspective and the future directions of this topic in organic synthesis.

Water-soluble NHC Pd/Ni bimetallic nanoparticles for H/D exchange in aromatic amino-acids.

Labelling of amino-acids is important for the production of deuterated proteins. However, aromatic amino-acid reduction is a common undesired process with noble-metal nanocatalysts. In this work, we describe a new NHC-stabilized water-soluble Pd/Ni system able to perform H/D exchange reactions in an enantiospecific fashion without reducing the aromatic ring of phenylalanine and tyrosine thanks to a synergetic Pd-Ni effect.

Tuning the catalytic activity and selectivity of water-soluble bimetallic RuPt nanoparticles by modifying their surface metal distribution.

Bimetallic ruthenium-platinum nanoparticles (RuPt NPs) of different surface distributions and stabilized by using a sulfonated N-heterocyclic carbene ligand (1-(2,6-diisopropylphenyl)-3-(3-potassium sulfonatopropyl)-imidazol-2-ylidene) were prepared from Ru(COD)(COT) (COD = cyclooctadiene and COT = cyclooctatriene), and platinum precursors having various decomposition rates (Pt(NBE)3, NBE = norbornene, Pt(CH3)2(COD) and Pt2(DBA)3, DBA = dibenzylideneacetone). Structural and surface studies by FT-IR and solid-state MAS NMR, using carbon monoxide as a probe molecule, revealed the presence of different structures and surface compositions for different nanoparticles of similar sizes, which principally depend on the decomposition rate of the organometallic precursors used during the synthesis. Specifically, the slower the decomposition rate of the platinum precursor, the higher the number of Pt atoms at the NP surface. The different bimetallic RuPt NPs, as well as their monometallic equivalents (Pt and Ru NPs), were used in isotopic H/D exchange through C-H activation on l-lysine. Interestingly, the activity and selectivity of the direct C-H deuteration were dependent on the NP surface composition at the α position but not on that at the ε position. Chemical shift perturbation (CSP) experiments revealed that the difference in reactivity at the α position is due to a Pt-carboxylate interaction, which hinders the H/D exchange.

Water-soluble platinum nanoparticles stabilized by sulfonated N-heterocyclic carbenes: influence of the synthetic approach.

The synthesis of metal nanoparticles (NPs) under controlled conditions in water remains a challenge in nanochemistry. Two different approaches to obtain platinum NPs, which involve the treatment of aqueous solutions of preformed sulfonated (NHC)Pt(ii) dimethyl complexes with carbon monoxide, and of (NHC)Pt(0) diolefin complexes with dihydrogen (NHC = N-heterocyclic carbene), are disclosed here. The resulting NPs were found to be highly stable in water under air for an indefinite time period. Coordination of the NHC ligands to the platinum surface via the carbenic carbon was monitored by solid-state NMR spectroscopy, and the presence of a platinum-carbon bond was unambiguously evidenced by the determination of a 13C-195Pt coupling constant (1106 and 1050 Hz for NPs containing 13C labeled-NHC ligands and prepared under CO and H2, respectively). The coordination of CO to the (NHC)Pt(ii) precursors prior to formation of the NPs was confirmed by NMR spectroscopy. When using a disulfonated NHC ligand, a second coordination sphere containing bis(NHC)Pt(ii) complexes is described. Under CO, the formation of NPs was found to be slower than in a previously reported thermal method (Angew. Chem., Int. Ed., 2014, 53, 13220-13224), but led to NPs of similar sizes, whereas under H2, the synthesis of platinum NPs progressed even more slowly and produced larger NPs. In addition to the influence of the synthetic approach, the present study highlights the importance of ligand design for NP stabilization.

Monitoring of nanoparticle reactivity in solution: interaction of l-lysine and Ru nanoparticles probed by chemical shift perturbation parallels regioselective H/D exchange.

Thanks to new water-soluble Ru nanoparticles (NPs) stabilized by sulfonated NHC ligands, we demonstrate that it is possible to monitor the catalyst/substrate interaction using NMR chemical shift perturbations (CSPs), under conditions that closely resemble those applied during the enantiospecific C-H deuteration of l-lysine. Correlating the pH dependence of the interaction of l-lysine with the surface of the RuNPs and its subsequent deuteration, our study underscores the importance of oriented binding to the surface as a critical factor for H/D exchange.

Surface-Engineering of Ultrathin Gold Nanowires: Tailored Self-Assembly and Enhanced Stability.

Gold nanowires with a mean diameter of 1.7 nm were synthesized by reduction of HAuCl4 in a solution of oleylamine (OY) in hexane. A bilayer of oleylammonium chloride/oleylamine at the surface of the raw nanowires was evidenced by NMR and diffusion ordered spectroscopy (DOSY) experiments. After washing a monolayer of oleylammonium chloride remained at the surface of the nanowires. The oleylammonium chloride layer could be progressively replaced by a phosphine shell as evidenced with NMR and DOSY experiments, which are in good agreement with the adsorption energies given by density functional theory calculations. The nanowires crystallize into hexagonal superlattices with a lattice parameter that can be tailored depending on the ligand shell. Small-angle X-ray scattering showed the following lattice parameters: Au@OY+Cl-(OY) (a = 7.2 nm) > Au@TOPO/OY (a = 6.6 nm) > Au@ OY+Cl- (a = 4.1 nm) > Au@TOP (a = 3.75 nm). This is one of a few examples of surface modification of ultrathin nanowires that does not alter their morphology. Moreover, the nanowires coated with phosphines exhibited long time stability (at the opposite of other ligands like thiols) opening the way to more complex functionalization.

Identifying short surface ligands on metal phosphide quantum dots.

The control and understanding of the chemical and physical properties of quantum dots (QDs) demands detailed surface characterization. However, probing the immediate interface between the inorganic core and the ligands is still a major challenge. Here we show that using cross-polarization magic angle spinning (MAS) NMR, unprecedented information can be obtained on the surface ligands of Cd3P2 and InP QDs. The resonances of fragments which are usually challenging to detect like methylene or methyl near the surface, can be observed with our approach. Moreover, ligands such as hydroxyl and ethoxide which have so far never been detected at the surface can be unambiguously identified. This NMR approach is versatile, applicable to any phosphides and highly sensitive since it remains effective for identifying quantities as low as a few percent of surface atoms.

Insights into the Ligand Shell, Coordination Mode, and Reactivity of Carboxylic Acid Capped Metal Oxide Nanocrystals.

A detailed knowledge of surface chemistry is necessary to bridge the gap between nanocrystal synthesis and applications. Although it has been proposed that carboxylic acids bind to metal oxides in a dissociative NC(X)2 binding motif, this surface chemistry was inferred from indirect evidence on HfO2 nanocrystals (NCs). Here, a more detailed picture of the coordination mode of carboxylate ligands on HfO2 and ZrO2 NC surfaces is shown by direct observation through solid-state NMR techniques. Surface-adsorbed protons are clearly distinguished and two coordination modes of the carboxylic acid are noted: chelating and bridging. It is also found that secondary ligands penetrate the ligand shell and have the same orientation with respect to the surface as the primary ligands, indicating that the ionic or hydrogen-bonding interactions with the surface are more important than the van der Waals interactions with neighboring ligands. During ligand exchange with amines, the chelating carboxylate is removed preferentially. Finally, it is shown that the HfO2 and ZrO2 NCs catalyze imine formation from acetone and oleylamine. Together with the previously reported catalytic activity of HfO2 , these results put colloidal metal oxide nanocrystals squarely in the focus of catalysis research.

Synthesis and behavior of novel sulfonated water-soluble N-heterocyclic carbene (η(4)-diene) platinum(0) complexes.

A series of water-soluble (NHC)Pt(0)(dvtms) and (NHC)Pt(0)(AE) complexes containing different sulfonated NHC ligands (dvtms = divinyltetramethyldisiloxane and AE = diallyl ether) are reported. The dvtms compounds have been found to be quite robust and to display some conformational rigidity, whereas their AE counterparts are less stable and more flexible. The catalytic evaluation of these complexes in the hydrosilylation of alkynes in water revealed no benefits in favor of the complexes containing the more labile spectator diene (AE), and a fairly regular catalytic behavior for all complexes that restricts the location of the sulfonate group to the proximity of the metal site.

Highly stable water-soluble platinum nanoparticles stabilized by hydrophilic N-heterocyclic carbenes.

Controlling the synthesis of stable metal nanoparticles in water is a current challenge in nanochemistry. The strategy presented herein uses sulfonated N-heterocyclic carbene (NHC) ligands to stabilize platinum nanoparticles (PtNPs) in water, under air, for an indefinite time period. The particles were prepared by thermal decomposition of a preformed molecular Pt complex containing the NHC ligand and were then purified by dialysis and characterized by TEM, high-resolution TEM, and spectroscopic techniques. Solid-state NMR studies showed coordination of the carbene ligands to the nanoparticle surface and allowed the determination of a (13)C-(195)Pt coupling constant for the first time in a nanosystem (940 Hz). Additionally, in one case a novel structure was formed in which platinum(II) NHC complexes form a second coordination sphere around the nanoparticle.