Temporary Intramolecular Generation of Pyridine Carbenes in Metal‐Free Three‐Component C?H Bond Functionalisation/Aryl‐Transfer Reactions

Nucleophilic addition of pyridines to benzyne generates zwitterionic adducts that evolve by a rapid intramolecular proton shift to produce the corresponding pyridine carbenes, N‐phenyl pyrid‐2‐ylidenes. In the presence of electrophilic ketones (isatin derivatives), the pyridylidenes can further react by an original bis‐arylation reaction of the carbonyl compounds involving a formal pyridine C? H bond functionalisation. The overall transformation is an unprecedented three‐component reaction featuring a carbene intermediate. The mechanism of this transformation was examined in detail by using both experimental and theoretical approaches. It was found that the generation of N‐phenyl pyrid‐2‐ylidene from pyridine and benzyne is energetically favoured, and that the corresponding carbene dimer can also form easily. Under the three‐component reaction conditions, the pyridylidene preferentially adds to the ketone group of the isatin derivative to produce a zwitterionic adduct amenable to an intramolecular aryl transfer reaction by a concerted nucleophilic aromatic substitution. This peculiar reactivity for a carbene was compared to possibly competitive known reactions of stable carbenes with carbonyl compounds, and the reaction was found to be under thermodynamic control. The reported method of generation of N‐phenyl pyrid‐2‐ylidenes and their reactivity with carbonyl compounds unlock new perspectives in organic synthesis.     

Structural diversity in a series of alkyl-substituted cesium aryloxides

A family of cesium aryloxides [Cs(OAr)](n) were synthesized and structurally characterized from the reaction of 1:1 or 1:excess stoichiometry of Cs(0) and the appropriate alkyl-substituted phenol: 2-alkylphenol [alkyl = methyl (H-oMP), isopropyl (H-oPP), and tert-butyl (H-oBP)] and 2,6-dialkylphenol [alkyl = methyl (H-DMP), isopropyl (H-DIP), tert-butyl (H-DBP), and phenyl (H-DPhP)]. The products were structurally identified as [Cs(oMP)(H-oMP)(2)](n) (1), [Cs(5)(oPP)(5)](n) (2), [Cs(4)(oBP)(4)(H-oBP)(6)](n) (3x, shown), [Cs(3)(DMP)(3)](n) (4), [Cs(2)(DIP)(2)](n) (5), [Cs(DIP)(H-DIP)](n) (5x), and [Cs(DPhP)](n) (7). Compounds 1-7 were found to adopt complex polymeric structures employing π interactions from the neighboring pendant phenoxide ligands. The solution behavior of these compounds was studied using solution (133)Cs NMR spectroscopy, and for each compound, a single (133)Cs NMR resonance was observed, with chemical shift values found to be strongly solvent-dependent. This implies that monomeric cesium salt species involving solvent interactions exist in solution.     

Multicomponent reactions and ionic liquids: a perfect synergy for eco-compatible heterocyclic synthesis

The efficiency of a chemical synthesis can be nowadays measured, not only by parameters like selectivity and overall yield, but also by its raw material, time, human resources and energy requirements, as well as the toxicity and hazard of the chemicals and the protocols involved. The development of multicomponent reactions (MCRs) in the presence of task-specific ionic liquids (ILs), used not only as environmentally benign reaction media, but also as catalysts, is a new approach that meet with the requirements of sustainable chemistry. The aim of this tutorial review is to highlight the synergistic effect of the combined use of MCRs and ILs for the development of new eco-compatible methodologies for heterocyclic chemistry.     

"In silico" mechanistic studies as predictive tools in microwave-assisted organic synthesis

Computational calculations can be used as a predictive tool in Microwave-Assisted Organic Synthesis (MAOS). A DFT study on Intramolecular Diels-Alder reactions (IMDA) indicated that the activation energy of the reaction and the polarity of the stationary points are two fundamental parameters to determine "a priori" if a reaction can be improved by using microwave irradiation.     

Seeking the optimal LaTaO4:Eu phosphor

Lanthanum orthotantalate, LaTaO₄, is an excellent host lattice for rare-earth luminescent ions such as Eu³⁺ for red emission. However, there are multiple RETaO₄ (RE=rare earth) polymorphs, and the stability of these is controlled predominantly by the RE-radius. Thus it is difficult to obtain a pure phase of LaTaO₄:Eu as Eu concentration and consequently the RE radius is varied. We recently reported a ‘soft-chemical’ route that allows crystallization of pure-phase LaTaO₄:Eu at temperatures as low as 800 °C. In the current report, we investigate polymorph evolution and Eu emission as a function of Eu concentration and annealing temperature. We obtain a maximum quantum yield (QY) of 83% at the highest Eu substitution (25%) for which the low temperature orthorhombic (Pbca) polymorph is stable. Therefore, QY is not limited necessarily by concentration quenching; rather it is limited by polymorph stability as the RE-radius decreases with increasing Eu substitution.     

Phase separation and superconductivity in Fe(1+x)Te(0.5)Se(0.5)

Fe(1+x)Te(0.5)Se(0.5) is the archetypical iron-based superconductor. Here we show that the superconducting state is controlled by the stacking of its anti-PbO layers, such that homogeneous ordering hinders superconductivity and the highest volume fractions are observed in phase separated structures as evidenced by either a distribution of lattice parameters or microstrain.     

Synthesis and Unexpected Rearrangement of a Hydroxyphostone (1)

Abstract

During our investigations on the wide range of stereoselective alkylations of 6-membered ring phostones,¹ we uncovered a novel rearrangement of 3-(diphenylhydroxymethyl)-2-ethoxy-2-oxo-1,2-oxaphosphorinane (1). The cis/trans diastereomers of 1 were prepared by the reaction of benzophenone with the appropriate ylide of the parent phostone (3).² When trans- 1 was left unattended at rt in CH₂Cl₂ a new compound, 2, was isolated which showed a significant downfield ³¹P shift and a higher melting point. Upon heating to the melting point 2 decomposed to give 4, which suggests that 2 may be an intermediate in the conversion of 1 to the Wittig-like product 4. The IR, ¹H, ¹³C, ³¹P, and 2-D NMR spectral data along with independent synthesis confirmed the identity of 2. Subsequently, 2 was also produced in 70% yield when cis- 1 was treated with CH₂Cl₂/ether-HC1_aq at 50–60°C for 2 weeks, but this product was contaminated with 30% of the exocyclic alkene 5. No rearrangement was observed when 1 was treated with TsOH/EtOH or HPF₆; only 5 was produced. The stereochemistry and mechanisms of these transformations are presented.     

Determination of colloidal gold nanoparticle surface areas, concentrations, and sizes through quantitative ligand adsorption

Determination of the true surface areas, concentrations, and particle sizes of gold nanoparticles (AuNPs) is a challenging issue due to the nanoparticle morphological irregularity, surface roughness, and size distributions. A ligand adsorption-based technique for determining AuNP surface areas in solution is reported. Using a water-soluble, stable, and highly UV–vis active organothiol, 2-mercaptobenzimidazole (MBI), as the probe ligand, we demonstrated that the amount of ligand adsorbed is proportional to the AuNP surface area. The equivalent spherical AuNP sizes and concentrations were determined by combining the MBI adsorption measurement with Au³⁺ quantification of aqua regia-digested AuNPs. The experimental results from the MBI adsorption method for a series of commercial colloidal AuNPs with nominal diameters of 10, 30, 50, and 90 nm were compared with those determined using dynamic light scattering, transmission electron microscopy, and localized surface plasmonic resonance methods. The ligand adsorption-based technique is highly reproducible and simple to implement. It only requires a UV–vis spectrophotometer for characterization of in-house-prepared AuNPs.