Double trouble — the art of synthesis of chiral dimeric natural products

Double or nothing! Recently the total ynthesis of secalonic acids A and D was reported. This work and other natural product syntheses with a dimerization step as a common feature are featured in this highlight. The significant biological activity of the secalonic acids and the fact that their synthesis has fascinated synthetic chemists for the past forty years make this work a milestone in natural product synthesis.

On the importance of an acid additive in the synthesis of pyrido[1,2-a]benzimidazoles by direct copper-catalyzed amination

Pyrido[1,2-a]benzimidazoles1, 2a are interesting compounds both from the viewpoint of medicinal chemistry2–7 (solubility,7 DNA intercalation3) and materials chemistry8 (fluorescence). Of note among the former is the antibiotic drug Rifaximin,5 which contains this heteroaromatic core. The classical synthetic approach for the assembly of pyrido[1,2-a]benzimidazoles is by [3+3] cyclocondensation of benzimidazoles containing a methylene group at C2 with appropriate bielectrophiles.2a However, these procedures are often low-yielding, involve indirect/lengthy sequences, and/or provide access to a limited range of products, primarily providing derivatives with substituents located on the pyridine ring (A ring, Scheme 1).2–4 Theoretically, a good alternative synthetic method for the synthesis of pyrido[1,2-a]benzimidazoles with substituents in the benzene ring (C ring) should be accessible by intramolecular transition-metal-catalyzed CN bond formation in N-(2-chloroaryl)pyridin-2-amines, based on chemistry recently developed in our research group.9 These substrates themselves are easily available through SNAr or selective Pd-catalyzed amination10 of 2-chloropyridine with 2-chloroanilines.11 If a synthetic procedure that eliminated the need for preactivation of the 2-position of the 2-chloroarylamino entity could be developed, this would be even more powerful, as anilines are more readily commercially available than 2-chloroanilines. Therefore the synthesis of pyrido[1,2-a]benzimidazoles (4) by a transition-metal-catalyzed intramolecular CH amination approach from N-arylpyridin-2-amines (3) was explored (Scheme 1).

One-step synthesis of titanium oxide with trilayer structure for dye-sensitized solar cells

Titanium oxide films with trilayer structure grown on fluorine doped tin oxide substrate were prepared from one-step hydrothermal process. The trilayer structure consists of microflowers, nanorod array and compact nanoparticulates, which is expected to possess the merits of good light harvesting, a high electron transport rate, while avoiding the issues of electron shunting. The photovoltaic performance was comprehensively studied and a 60% enhancement in short circuit photocurrent density was found from microflowers contribution as a light scattering layer. This unique trilayer structure exhibits great potential application in future dye-sensitized solar cells.

In-situ combustion synthesis of ultrafine TiB2 particles reinforced Cu matrix composite

Due to the lower strength of pure copper (Cu), ceramic particulate or whisker reinforced Cu matrix composites have attracted wide interest in recent years [1–3]. These materials exhibit a combination of excellent thermal and electrical conductivities, high strength retention at elevated temperatures, and high microstructural stability [3]. The potential applications include various electrodes, electrical switches, and X-ray tube components [4].

Mechanochemical synthesis of aluminium nanoparticles and their deuterium sorption properties to 2 kbar

A mechanochemical synthesis process has been used to synthesise aluminium nanoparticles. The aluminium is synthesised via a solid state chemical reaction which is initiated inside a ball mill at room temperature between either lithium (Li) or sodium (Na) metal which act as reducing agents with unreduced aluminium chloride (AlCl3). The reaction product formed consists of aluminium nanoparticles embedded within a by-product salt phase (LiCl or NaCl, respectively). The LiCl is washed with a suitable solvent resulting in aluminium (Al) nanoparticles which are not oxidised and are separated from the byproduct phase. Synthesis and washing was confirmed using X-ray diffraction (XRD). Nanoparticles were found to be ∼25–100nm from transmission electron microscopy (TEM) and an average size of 55nm was determined fromsmall angle X-ray scattering (SAXS) measurements. As synthesised Al/NaCl composites, washed Al nanoparticles, and purchased Al nanoparticles were deuterium (D2) absorption tested up to 2 kbar at a variety of temperatures, with no absorption detected within system resolution.

One-pot synthesis of imines from benzyl alcohol and amines on Au/ZrO2 catalyst

Imines were synthesized from benzyl alcohol and amines by using catalysts of gold nanoparticles supported on ZrO2 (Au/ZrO2). The effects of reaction time, temperature, gold loadings and base were investigated. High yields were achieved under moderate conditions (60 °C) in the presence of KOCH3. For instance, the yield of N-benzylidenebenzylamine produced from benzyl alcohol and benzylamine on 3 wt% Au/ZrO2 is 87 %. The synthesis of imine involves two reaction steps: selective oxidation of benzyl alcohol to benzaldehyde and the coupling reaction of amines with benzaldehyde. In the first step, the base promotes the selective oxidation. The reactions of benzyl alcohol with three different amines, aniline, n-butylamine and benzylamine, were conducted to produce corresponding imines. The results show that the amine with stronger nucleophilicity has better ability to react with benzaldehyde in the second step, resulting in higher yield of the corresponding imine. We proposed a tentative mechanism for the synthesis process.

Synthesis of MgB2 at low temperature and autogenous pressure

High quality, micron-sized interpenetrating grains of MgB2 with high density are produced at low temperatures (~420oC < T < ~500oC) under autogenous pressure by pre-mixing Mg powder and NaBH4 and heating in an Inconel 601 alloy reactor for 5−15 hours. Optimum production of MgB2 with yields greater than 75% occurs for autogenous pressure in the range 1.0 MPa to 2.0 MPa with the reactor at ~500oC. Autogenous pressure is induced by the decomposition of NaBH4 in the presence of Mg and/or other Mg-based compounds. The morphology, transition temperature and magnetic properties of MgB2 are dependent on the heating regime. Significant improvement in physical properties accrues when the reactor temperature is held at 250oC for >20minutes prior to a hold at 500oC.

‘Click’ functionalised polymer resins : a new approach to the synthesis of surface attached bipyridinium and naphthalene diimide [2]rotaxanes

Herein we describe the design and synthesis of a series of solid-tethered [2]rotaxanes utilising crown ether-naphthalene diimide or crown ether- bipyridinium host guest interactions. TentaGel polystyrene resins were initially modified in a two-stage procedure to azide functionalised beads before the target supramolecular architectures were attached using a copper catalysed “click” procedure. The final assembly was examined using IR spectroscopy and gel-phase 1H High Resolution Magic Angle Spinning (HR MAS) NMR spectroscopy. The HR MAS technique enabled a direct comparison between the solid-tethered architectures and the synthesis and characterisation of analogous solution-based [2]rotaxanes to be made.

New approaches to the synthesis of strapped porphyrin containing bipyridinium [2]rotaxanes

Inspired by the interesting photo- and electrochemical properties observed in bipyridinium and porphyrin containing interlocked catenanes, herein we describe new approaches towards the synthesis of related rotaxanes. Previous efforts in this domain had been hampered by the limited range of chemical reactions that are compatible with these motifs, however the use of a “click” methodology, together with a better understanding of the size of these strapped porphyrin macrocycles, resulted in the synthesis of a bipyridinium porphyrin [2]rotaxane in modest yields. X-ray crystallography of the zinc metalloporphyrin macrocycle used in this study revealed that in the solid state, these strapped porphyrins adopt a 1-dimensional coordination polymer, in which an oxygen atom in the strap of one macrocycle is coordinated to the zinc metal center in an adjacent porphyrin ring

Chemo-enzymatic synthesis of unnatural amino acids

A general chemo-enzymatic process has been developed to prepare enantiomerically pure L- and D-amino acids in high yield by deracemisation of racemic starting materials. The method has been developed from initial academic studies to be a robust, scalable industrial process. Unnatural amino acids, in high optical purity, are a rapidly growing class of intermediates required for pharmaceuticals, agrochemicals and other fine chemical applications. However, no single method has proven sufficiently adaptable to prepare these compounds generally at large scale. Our approach uses an enantioselective oxidase biocatalyst and a non-selective chemical reducing agent to effect the stereoinversion of one enantiomer and can result in an enantiomeric excess of > 99 % from a starting racemate, and product yields over 90 %. The current approach compares very favourably to resolution methods which have a maximum single pass yield of 50 %. Efficient methods have been developed to adapt the biocatalyst used in this process towards new target compounds and to optimise key factors which improve the process efficiency and offer competitive economics at scale.

Fast, energy-efficient synthesis of luminescent carbon quantum dots

A simple, fast, energy and labour efficient, carbon dot synthesis method involving only the mixing of a saccharide and base is presented. Uniform, green luminescent carbon dots with an average size of 3.5 nm were obtained, without the need for additional energy input or external heating. Detection of formation moment for fructose-NaOH-produced carbon dots is also presented.

Plasma functionalization of SiO2 nanoparticles for the synthesis of polymer nano-dielectrics

In this study, we improve the insulation performance of polymeric nano-dielectrics by using plasma pre-treatment on the filled nanoparticles. Non-equilibrium atmospheric-pressure plasma is employed to modify a commercial type of silane-coated SiO2 nanoparticles. The treated nanoparticles and the synthesized epoxy-based nanocomposites are characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The plasma-treated SiO2 nanoparticles can disperse uniformly and form strong covalent bonds with the molecules of the polymer matrix. Moreover, the electrical insulation properties of the synthesized nanocomposites are investigated. Results show that the nanocomposites with plasma-treated SiO2 nanoparticles obtain improved dielectric breakdown strength and extended endurance under intense electrical ageing process.

Simulation of carbon arc discharge for the synthesis of nanotubes

Arc discharge ablation with a catalyst-filled carbon anode in a helium background was used for the synthesis of graphene and carbon nanotubes. In this paper, we present the results of the numerical simulation of the distribution of various plasma parameters in discharge, as well as the distribution of carbon flux on the nanotube surface, for the typical discharge with an arc current of 60 A and a background gas pressure of 68 kPa.

Minimizing the Gibbs–Thomson effect in the low-temperature plasma synthesis of thin Si nanowires

An advanced combination of numerical models, including plasma sheath, ion- and radical-induced species creation and plasma heating effects on the surface and within a Au catalyst nanoparticle, is used to describe the catalyzed growth of Si nanowires in the sheath of a low-temperature and low-pressure plasma. These models have been used to explain the higher nanowire growth rates, low-energy barriers, much thinner Si nanowire nucleation and the less effective Gibbs–Thomson effect in reactive plasma processes, compared with those of neutral gas thermal processes. The effects of variation in the plasma sheath parameters and substrate potential on Si nanowire nucleation and growth have also been investigated. It is shown that increasing the plasma-related effects leads to decreases in the nucleation energy barrier and the critical nanoparticle radius, with the Gibbs–Thomson effect diminished, even at low temperatures. The results obtained are consistent with available experimental results and open a path toward the energy- and matter-efficient nucleation and growth of a broad range of one-dimensional quantum structures.