Green Chemistry Blog

French scientists report a new method to generate the oxidative homocoupling of N-phenylpyrazole using  Ru(OAc)₂(p-cymene) catalyst.

Traditionally, the transition metal-catalysed Heck reaction has proven to be a useful method to synthesize unsaturated molecules and conjugated materials viacross-coupling and C-C bond formation.  However, a potentially more efficient, greener route to the same family of compounds is through the catalytic oxidative alkenylation of aromatic C-H bonds.  This process would avoid the prehalogenation of the substrate as in the Heck reaction and therefore would be more atom economical.

The team of scientists here, led by Christian Bruneau and Pierre Dixneuf, report the direct dehydrogenative alkenylation of N-aryl pyrazoles with styrene and alkyl acrylates, catalysed by Ru(OAc)₂(p-cymene) in the presence of Cu(OAc)₂·H₂O and acetic acid in air.  The authors show that the acetic acid plays an important role, and believe its action lies in C-H bond cleavage as an autocatalytic process.

This article is free to access until the 20th October 2011!  To read more, just click below:

Ruthenium diacetate-catalysed oxidative alkenylation of C–H bonds in air: synthesis of alkenyl N-arylpyrazoles, Percia B. Arockiam, Cedric Fischmeister, Christian Bruneau and Pierre H. Dixneuf, Green Chem., 2011, DOI: 10.1039/C1GC15875A

A new recyclable copper catalyst has been developed which could be reused 20 times in the Diels-Alder reaction in ionic liquids without any obvious loss of activity or enantioselectivity.

Homogeneous metal-catalysed reactions are widely employed in organic synthesis.  However, because most catalysts are expensive and because the metal and the ligand can often higly toxic to the environment, the recycling of the these catalysts is very important. 

Zhi-Ming Zhou and colleagues from China report a series of imidazolium-tagged C2-symmetric bis(oxazoline) copper catalysts.  These catalysts were applied to the Diels-Alder reaction and achieved high levels of activity and enantioselectivity.  Due to the nature of the ligand, these catalysts were soluble in ionic liquids, including the hydrophobic [Bmim]NTf2, which enabled good recyclability.  In fact, these catalysts could be recycled at least 20 times without showing an obvious loss in activity or enantioselectivity.

To read more, please click on the link below:

Recyclable copper catalysts based on imidazolium-tagged C₂-symmetric bis(oxazoline) and their application in D–A reactions in ionic liquids, Zhi-Ming Zhou, Zhi-Huai Li, Xiao-Yan Hao, Xiao Dong, Xin Li, Li Dai, Ying-Qiang Liu, Jun Zhang, Hai-feng Huang, Xia Li and Jin-liang Wang, Green Chem., 2011, DOI: 10.1039/C1GC15788D

This article is free to access until the 14th October!

A four-component reaction catalysed by L-proline has been developed to efficiently synthesize a series of complex ortho-quinones.

Quinoline-detrived ortho-quinones are very important biological compounds and play crucial roles as cofactors, being involved in several biochemical reactions.  Therefore there is interest in providing economical routes to these compounds.  One strategy that could be employed are multi-comnponent reactions.  However, little attention has been focused on the development of these reactions in aqueous conditions. 

Scientists from India and Spain have now developed a L-proline catalysed four-component sequential reaction performed “on water” for the synthesis of 7-(aryl)-8-methyl-10-phenyl-5H-benzo[h]pyrazolo-[3,4-b]quinoline-5,6(10H)-diones.  This procedure gives high atom economy and leads to the generation of two rings together with two C-C, one C-N and two C=N bonds in a single operation.  Together with the short reaction times, excellent yields, easy work-up and without the need for chromatography purification, this methods show several environmental advantages over other protocols.

To read more, please click on the click below:

L-Proline-catalysed sequential four-component “on water” protocol for the synthesis of structurally complex heterocyclic ortho-quinones, Stephen Michael Rajesh, Balasubramanian Devi Bala, Subbu Perumal and J. Carlos Menéndez, Green Chem., 2011, DOI: 10.1039/C1GC15794A

This article is free to access until the 14th October 2011!

Pentose-based surfactants could be obtained enzymatically from hydrothermally pretreated wheat bran.

Wheat is an important agricultural cereal in Europe and the waste generated annually from this industry could produce 115 billion litres of bioethanol.  In view of this there is a lot of interest in producing fine chemicals from this lignocellulosic biomass. 

In this work, Caroline Rémond and colleagues from France, report the enzymatic synthesis of pentyl and octyl-β-D-xylosides and oligoxylosides first from xylan and then from hydrothermally pretreated wheat bran.  As well as thoroughly investigating the various parameters that affect the nature and yield of the final product, the pentyl and octyl oligoxylosides obtained from the hydrothermally pretreated wheat bran exhibited good surface properties, demonstrating that the production of green surfactants can be envisaged from renewable resources using biotechnological methodologies.

This article is free to access until 5th October!  To read more, click on the links below:

Enzymatic synthesis of alkyl β-D-xylosides and oligoxylosides from xylans and from hydrothermally pretreated wheat bran, Marjorie Ochs, Murielle Muzard, Richard Plantier–Royon, Boris Estrine and Caroline Rémond, Green Chem., 2011, 13, 2380-2388

UK scientists have conducted a study to show how waste carbon dioxide can become an exploitable resource. The work could kick idle supercritical processing plants back into life.

High pressure supercritical carbon dioxide (scCO₂) can be an effective solvent for a range of catalytic reactions, but the cost of compressing CO₂ is high. Martyn Poliakoff, Mike George and Trevor Drage have been collaborating at the University of Nottingham to assess the possibility of using scCO₂ from carbon capture and storage (CCS) – in which considerable quantities of pressurised liquid CO₂ will be available – in the hope of turning a waste product into a resource. 

It is hoped that CCS will achieve large reductions in CO₂ emissions from power plants, but as the cost of compression is incurred at the plant, using this captured CO₂ as a source of scCO₂ for chemistry would be a cheaper and greener route. ‘The first scCO₂ chemical plant used less than 0.5 ton of scCO₂ per hour, but a power station running at full blast produces more than 0.5 ton of CO₂ per second, so you could produce enough high pressure CO₂ during a football match to run the plant for a year without needing to recycle the CO₂,’ says Poliakoff.

However, one of the concerns is that captured CO₂ will contain impurities. So, the team investigated what effects these impurities might have on a reaction that has previously been carried out commercially in scCO₂ – the hydrogenation of isophorone to trimethylcyclohexanone catalysed by Pd on SiO₂/Al₂O₃. They used an automated continuous reactor and monitored the products using gas liquid chromatography. They found that none of the impurities caused problems that couldn’t be overcome.

‘I’m delighted to see some encouraging science that may bring supercritical processing back towards commercial reality,’ says Harry Swan, managing director of Thomas Swan & Co. Ltd, who built the world’s only continuous phase scCO₂ reactor in 2002. But, owing to economic challenges and lack of legislation to reduce the use of volatile organic solvents, which would have created demand for scCO₂ as an environmentally friendly alternative, the plant is sitting idle. ‘This work brings us a step closer to my long term objective of getting our plant back up and running,’ he adds.

‘The obvious next step is to try other reactions,’ says Poliakoff. ‘Although hydrogenations are interesting reactions, it may well be that the real future of CO₂ lies in oxidation reactions that are dangerous in conventional solvents’.

Reproduced from a Chemistry World story written by Tegan Thomas.

Could the energy cost of using supercritical fluids be mitigated by using CO₂ from carbon capture and storage (CCS)? James G. Stevens, Pilar Gómez, Richard A. Bourne, Trevor C. Drage, Michael W. George and Martyn Poliakoff, Green Chem., 2011, DOI: 10.1039/c1gc15503b

A facile and green method to generate reduced graphene oxide nanosheets in supercritical alcohols had been developed.

Graphene has attracted much attention as a potential material for a wide range of applications due to its superior electrical, optical, chemical, mechanical, thermal and catalytic properties.  In view of this, a large scale and cost effective method for the production of graphene is necessary in order for it to be used in industry. There are currently four different methods to synthesize graphene, the most promising of which is the chemical reduction route – but this method still has several limitations which hinder its use in commercial implementation. 

Jaehoon Kim and coworkers from the Republic of Korea have developed a method to produce reduced graphene oxide (RGO) in supercritical fluids.  This fast, facile and green route results in RGO with a high carbon-to-oxygen ratio, high electronic conductivity and high lithium storage capacity.  This process is applicable for large scale production of RGO using a continuous flow-type reactor, providing opportunities for its production on industrial scale.

This article is free to access until the 30th September 2011!  To read more, please see:

Facile synthesis of reduced graphene oxide in supercritical alcohols and its lithium storage capacity, Eduardus Budi Nursanto, Agung Nugroho, Seung-Ah Hong, Su Jin Kim, Kyung Yoon Chung and Jaehoon Kim, Green Chem., 2011, DOI: 10.1039/C1GC15678K

A magnetically separable nanomaterial has been developed which showed good photocatalytic activity under visible light in the selective transformation of malic acid in aqueous solution.

Titanium dioxide (TiO₂) is well known as a photocatalyst, but its application in various processes is hindered due to several limitations.  The most important of these limitations is its lack of photocatalytic activity under visible light.  In addition, efficient separation of the catalyst once the reaction is complete can be difficult, with additional filtration steps and/or a tedious work-up required. 

However, in this work scientists from Spain and the USA led by Rafael Luque and Rajender Varma have developed a novel nanomaterial which overcomes these problems.  The TiO₂-guanidine-(Ni,Co)Fe₂O₄nanomaterial was shown to have good activity in the selective transformation of malic acid.  Not only did the reaction proceed well under visible light, the material was also shown to have superior activity over the commercial catalyst Degussa P25.  The catalyst could be separated from the reaction mixture simply by using a magnet and the catalyst could be reused several times, preserving most of its activity.

To read more, please click on the link below.  This article is free to access until the 30th September 2011!

Magnetically separable nanocomposites with photocatalytic activity under visible light for the selective transformation of biomass-derived platform molecules, Alina M. Balu, Babita Baruwati, Elena Serrano, Jaume Cot, Javier Garcia-Martinez, Rajender S. Varma and Rafael Luque, Green Chem., 2011, DOI: 10.1039/C1GC15692F

Click here to read Anna Simpson’s recent interview with Rafael Luque!