Hot Articles – Page 31 – Chemical Communications Blog

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Green hydrogenation of cyclic di-esters to diols

02 Nov 2011

Hydrogenation of esters under mild conditions is difficult at best, however David Milstein and his team push the boundaries by hydrogenating biomass products, glycolide and lactide, to afford the corresponding 1,2-diols. Their synthesis, which utilises ruthenium pincer complex catalysts, offers an atom-economical, green alternative to existing methods for producing 1,2-diols, which currently rely on petroleum-derived ethylene and propylene feedstocks.

The team say that the cyclic di-esters can be selectively and efficiently hydrogenated under very mild conditions, whilst producing no waste, and even discovered that optically pure diols could be produced from chiral lactide.

Download the ChemComm article today to find out how Milstein and his team did this.

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Further insight into aromatic borylation chemistry

31 Oct 2011

Thorough characterisation of arene–borane Lewis acid contacts is essential for understanding the mechanistic pathway for aromatic borylation chemistry and by extension, electrophilic aromatic substitution. However, until now, structural determination of such arene–borane compounds has been somewhat ambiguous.

Simon Aldridge and his team have set out to fill this knowledge gap by reacting a sterically encumbered pyridine donor with BBr₃ to yield a cationic tri-coordinate borane–arene complex. Employing crystallography and computational studies, the team found that a weak electrostatic interaction is responsible for the short contacts between the positively charged boron centre and the arene π system.

Read the ChemComm article to find out more…

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Metal evasion still leads to nitration

31 Oct 2011

Researchers from Germany and China have designed a simple method for forming nitroarenes.

Aromatics substituted with nitro groups are ubiquitous motifs in important molecules required by the textiles, materials and pharmaceutical industries.

However, installing nitro groups can be tricky because it requires the use of strong acids or dinitrogen peroxide. These harsh conditions are not commonly tolerated by sensitive functionality present in molecules, and problems of selectivity or over-nitration commonly arise. Alternate methods use large quantities of silver salts or expensive palladium catalysts.

Now Xiao-Feng Wu (Zhejiang Sci-Tech University, China), Matthias Beller (Leibniz Institute for Catalysis, Germany) and colleagues have designed a metal-free method for nitrating aryl boronic acids.

The team used an alkyl nitrite at 80 ˚C to convert the boronic acid group to a nitro group. The reaction works in moderate to good yields for electron-rich and electron-neutral aryl boronic acids. Electron-poor aromatics are more challenging substrates, as the boronic acid are less reactive.

While this method is not yet suitable for vinyl or hydroxy substituted aryl boronic acids, it offers an economically and operationally attractive method for synthesising nitro compounds.

Researcher’s perspective:Our initial investigations started with the use of a rhodium catalyst, because of rhodium’s known ability to transmetalate with arylboronic acid. Our target product, nitrobenzene, was formed but we found that the rhodium catalyst is not necessary for this transformation.

On one hand, we were disappointed, as our aim is to develop catalytic reactions; on the other hand, we are happy, because this reaction works so nicely.

Xiao-Feng Wu

Using this method, nitroarenes can be easily prepared from arylboronic acids without using catalyst and hazardous reagents.

More information can be found in the ChemComm communication, free to download for a limited period.

Posted on behalf of Alice E. Williamson, ChemComm web writer.

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Supramolecular assembly of polyoxometalate triangles

31 Oct 2011

Polyoxometalates (POMs) are a diverse class of inorganic materials that are of great interest due to their exciting range of redox, conducting, magnetic and catalytic properties. Recent collaborative work from Professor Garry Hanan in Montreal and Professor Bernold Hasenknopf in Paris reports the inclusion of a Lindqvist-type hexavanadate POM as a component of a self-assembling supramolecular framework.

They designed ligand 1 utilising a triol motif to complex the POM in combination with pyridyl groups to serve as binding sites for a transition metal. The reaction of 1 with a suitable decavanadate yielded complex 2, a hexavanadate POM complex that is itself a structurally rigid and spatially well-defined bi-pyridyl ligand. The 60° angle between the coordination vectors of the pyridyl groups led the authors to predict that the coordination of a trans-PdCl₂ unit by 2 would result in a supramolecular triangle.

Initially the reaction of 2 with [PdCl₂(CH₃CN)₂] in DMAc yielded a complex mixture of products; however, heating to 80 °C for 48 hours led to just a single assembly. This was shown by a number of analytical techniques to be the predicted triangular assembly 3.

This work elegantly uses a classical motif for self-assembly to create a multi-component supramolecular architecture. It is a great step towards the goal of creating functional supramolecular arrays, integrating the desirable properties of POMs into a new framework and bridging the gap between solid state oxides and coordination chemistry.

Researcher’s perspective:

The obtention of discrete coordination-driven POM-based arrays had so far eluded the community, with only a few well characterised coordination polymers. We started this challenging project of a molecular triangle by a simple drawing based on geometrical considerations. A range of conditions were explored, but most lead to complex mixtures except one. This identification of optimal conditions was our first satisfaction. However, we needed solid evidence of the triangular nature of the obtained array to validate our approach as a rational design: a combination of techniques confirmed the successful and selective synthesis of the triangle. As a student, I learnt a lot from the complementary competencies provided by the two groups on a project at the interface of inorganic, organic, supramolecular and coordination chemistry. Dealing with the specific difficulties of each domain has been an exciting challenge that has re-confirmed my choice in academic research. Marie-Pierre Santoni, a student in the research collaboration

To read more about Hanan and Hasenknopf’s work, download their ChemComm article.

Posted on behalf of Cally Haynes, ChemComm web writer.

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Highlights in Chemistry: short reviews from the brightest scientists

21 Oct 2011

As the nights are drawing in here in Cambridge, it is nice to bring a little light in to our lives in the form of Highlights in Chemistry.

Commissioned to celebrate the International Year of Chemistry 2011, these short reviews cover the most significant advances in the chemical sciences since the millennium. The topics include photocatalytic hydrogen production, nucleic acid sequencing, antibiotic resistance mechanisms, molecular machines and metathesis, to name just a few.

We’ve nearly published all the Highlights in Chemistry so take a look at the collection on the ChemComm website. There are some fun graphical abstracts to accompany the articles – can you guess who supplied the images I’ve used in this blog? Click on the images to find out.

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Worm-like peptides form gels at physiological pH

18 Oct 2011

Salt can induce gelation of functionalised dipeptides at high pH, say a team of scientists from the UK and France. The finding will significantly expand the utility of hydrogels, they claim.

Hydrogels are formed when molecules assemble into nanofibres, which then entangle and form a gel matrix. They can be used for a variety of applications, including drug delivery and scaffolds for tissue engineering. Peptide-based gelators are of particular interest at present, in part because they are potentially biocompatible.

Gelation is commonly triggered by pH, temperature or solvent composition. Dave Adams, at the University of Liverpool, and colleagues discovered that functionalised dipeptides that adopt a worm-like micelle structure at high pH form gels when salts are added to them. High pH is normally outside the window for gelation so the finding could be very useful for the field. Importantly, now that these gels can be made at physiological pH, they should find increased use in bioapplications.

Graphical abstract: Salt-induced hydrogelation of functionalised-dipeptides at high pH

Read more in Adams’ ChemComm communication, free to access for a limited period.

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Synthetic DNA synthesises RNA by transcription: ‘click’ here for more

05 Oct 2011

What’s better than a short synthetic DNA sequence? That’s right: a long synthetic DNA sequence, able to take part in biological processes.

DNA and RNA are routinely synthesised using solid phase synthesis, connecting the bases through phosphodiester linkages. This method, however, imposes limitations on the length of the strand produced. Current methods employ enzymes to achieve larger oligonucleotides, which have disadvantages such as poor quality and stability and can be laborious.

Tom Brown and Afaf El-Sagheer at the University of Southampton, UK, have now demonstrated that these limitations can be overcome by the flick of a ‘click’ – they chemically ligated oligonucleotides functionalised with chemoselective azide and alkyne moieties using click chemistry to produce long nucleic acid sequences. After personally struggling to conjugate biological moieties using click chemistry for my Ph.D., I can assure you this is no mean feat!

Previous work by the group has shown that oligonucleotides with triazole-modified backbones do not inhibit replication and so they were interested to see if the same would be true for transcription – a fundamentally different process. The primary concern was how modification of the phosphodiester backbone would affect the template strand’s ability to participate in the transcription process.

By synthesising two triazole-modified DNA template strands, one with the triazole unit in the coding sequence and the other with the modification in the T7 RNA promoter region, El-Sagheer and Brown were able to evaluate the impact of changing the template strand backbone in comparison with the equivalent non-modified templates. Both modified strands successfully took part in transcription to produce RNA in good yield. The result: fully-synthetic, biologically-active DNA templates successfully synthesising RNA. This is not only an elegant demonstration of the abilities of click chemistry but could also be fundamental in changing approaches to the synthesis of biological constructs.

To read more about Brown’s research, download the ChemComm article today.

Posted on behalf of Sarah Brown, web science writer for ChemComm.

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Fullerene ‘hide and seek’ in lipid membranes

05 Oct 2011

Japanese researchers have experimentally proven that fullerenes reside in the hydrophobic core of liposome membranes.

Liposomes are vesicular structures made up of a phospholipid bilayer. Due to their ability to encapsulate a range of different substances and target numerous cell types, they have great potential as drug delivery agents. Recently, liposomes have been modified with fullerene for a number of uses such as DNA photocleavage, anti-bacterial agents, and photodynamic therapy. Although theoretical simulations have been performed to characterise these fullerene–liposomes, up until now there has been no experimental proof of where the fullerene units end up in the lipid membranes.

Using differential scanning calorimetry and ¹³C NMR spectroscopy, Atsushi Ikeda and colleagues have determined that the fullerenes self-aggregate in the core of the bilayer, separating themselves from the alkyl chains. The fullerene units are expected to be located in a similar fashion in a cell membrane, say the team. This new insight means efforts can commence on bringing the fullerenes up to the surface of the lipid which may help improve the photoactivity of the fullerenes whilst reducing any deleterious effects they may have.

Download Ikeda’s ChemComm article to find out more…

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Modelling reveals interconnect between SERS enhancement mechanisms

03 Oct 2011

Surface enhanced Raman scattering (SERS) uses noble metal substrates to enhance the inherently weak Raman signal from analytes. In some cases, the enhancement stems from two different mechanisms: a physical enhancement that comes from excitation of the metal surface plasmons and a chemical enhancement arising from charge transfer between the metal surface and an adsorbed molecule.

Up until now, theoretical SERS models have tended to treat these mechanisms separately. However, Duan and colleagues point out that this simplification is not always valid since plasmonic properties can be significantly affected by adsorbed molecules. Using state-of-the-art density functional models, they have revealed the connection between the two mechanisms, allowing more accurate modelling of SERS spectra.

To learn more about how Duan’s team have implemented this model, download the ChemComm article.

Also of interest:

Evidence of a surface plasmon-mediated mechanism in the generation of the SERS background, C. Farcau and S. Astilean, Chem. Commun., 2011, 47, 3861.
Scanned chemical enhancement of surface-enhanced Raman scattering using a charge-transfer complex, J. R. Lombardi et al., Chem. Commun., 2011, 47, 2426.

Visit the ChemComm Surface Enhanced Raman Spectroscopy web theme issue for more articles.

Posted on behalf of Iain A. Larmour, ChemComm web writer.

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Extension of Hirsch’s rule opens the door on spherical aromaticity

30 Sep 2011

Miquel Solà and Jordi Poater working University of Girona in Spain have put forward an extension of Hirsch’s rule to open-shell spherical species.

The famous Hückel rule allows one to estimate whether or not a planar ring molecule would have aromatic properties. When the molecule has 4N + 2 π-electrons then it follows Hückel’s rule. In 2000 Andreas Hirsch found a rule to predict the aromaticity of fullerenes, known as the 2(N +1)²rule and now Solà has extended this rule to spherical systems with an open-shell.

They found that spherical compounds with a half filled last energy level, e.g. those with 2N² + 2N + 1 electrons, are aromatic. This was backed up with computational evidence and showed that for example C₆₀¹⁹⁺and C₆₀^1- both have strong aromatic character.

Solà speculates that this finding could be an important tool for those working in the in stable high spin molecules such as molecular magnets.

Interested in finding out more? Then download the full ChemComm article for free today.

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Chemical Communications Blog

Archive for the ‘Hot Articles’ Category

Green hydrogenation of cyclic di-esters to diols

Further insight into aromatic borylation chemistry

Metal evasion still leads to nitration

Supramolecular assembly of polyoxometalate triangles

Highlights in Chemistry: short reviews from the brightest scientists

Worm-like peptides form gels at physiological pH

Synthetic DNA synthesises RNA by transcription: ‘click’ here for more

Fullerene ‘hide and seek’ in lipid membranes

Modelling reveals interconnect between SERS enhancement mechanisms

Extension of Hirsch’s rule opens the door on spherical aromaticity

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