Green Chemistry Blog

UK scientists have developed a continuous photo-oxidation procedure of an allylic alcohol in supercritical carbon dioxide.

Malaria is one of the most infectious diseases in the world.  Some strains of malaria are becoming resistant to the traditionally used quinine based antimalarials, and so there is a need to develop new antimalarial drugs.  A common feature of some newly developed antimalarial compounds is the trioxane moiety, and a convenient way to introduce this group is via photochemically generated singlet oxygen species (¹O₂).  However the highlight reactive nature of this species can introduce problems when trying to scale-up the synthesis of these groups for industrial production, particularly in terms of identifying acceptable solvents (non-flammable and in-efficient ¹O₂ quenchers).

In this work, Martyn Poliakoff, Michael George and colleagues from the University of Nottingham, UK, have developed a continuous process for the sustainable synthesis of trioxones with  ¹O₂in supercritical CO₂.  The team also examined the remaining two steps in the synthesis of antimalarial trioxanes from readily available starting materials, and hope that this approach could lead to the exploration of libraries of different trioxanes as potential antimalarial agents.

Read the full article for free until the 9th January 2013!

Synthesis of antimalarial trioxanes via continuous photo-oxidation with ¹O₂ in supercritical CO₂, Jessica F. B. Hall, Richard A. Bourne, Xue Han, James H. Earley, Martyn Poliakoff and Michael W. George, Green Chem., 2013, DOI: 10.1039/C2GC36711D

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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, 13, 2727-2733

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Biocatalysis is becoming an increasingly attractive method to achieve enantiopure chemicals, in a cleaner and more environmentally sustainable way.  This is in part due to the fact that in most of these reactions, water is used as the solvent primarily because the enzyme catalysts are most stable in this medium.  However, some substrates are insoluble or only sparingly soluble in water which can limit the use of these enzymes.

In this work, Lasse Greiner and colleagues from Germany have developed a procedure whereby sparingly water-soluble long-chain ketones can be reduced in a continuous process.  The authors used an ionic liquid as a detergent to increase the solubility of these substrates and products, and ultrafiltration in an enzyme membrane reactor was employed to increase the enzyme utilization.  A cascade of two enzyme membrane reactors was configured which could run for more than 1000 hours with high turnover numbers and 99.9% enantioselectivity.

Finally, downstream adsorption of the resulting alcohols allowed 90% recycling of the aqueous buffer solution, reducing the E-factor of the process to 13.

This article is currently free to access until the 3rd January 2013!

Enantioselective reduction of sparingly water-soluble ketones: continuous process and recycle of the aqueous buffer system, Susanne Leuchs, Shukrallah Na’amnieh and Lasse Greiner, Green Chem., 2013, DOI: 10.1039/C2GC36558H

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Utilising hardly-water soluble substrates as a second phase enables the straightforward synthesis of chiral alcohols, Christina Kohlmann, Nora Robertz, Susanne Leuchs, Lasse Greiner and Shukralla Na’amnieh, Green Chem., 2011, 13, 3093-3095

Continuous biocatalytic synthesis of (R)-2-octanol with integrated product separation, Christina Kohlmann, Susanne Leuchs, Lasse Greiner and Walter Leitner, Green Chem., 2011, 13, 1430-1436

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Scientists from Germany employed a mechanochemical approach to cleave β-O-4-linkages in lignin.

In recent years, plant biomass has come to the fore due to it’s potential to replace fossil-fuel derived chemicals.  Lignin is one of the three main constituents of biomass, but it’s use is hampered by its poor solubility and structural complexity. The β-O-4-linkage is the most abundant linkage found in lignin, and attempts to cleave them currently employ harsh reaction conditions.

Here, Carsten Bolm and colleagues have developed a base-assisted ball milling process for the degradation of lignin and wood.  The process is transition metal- and solvent-free and is tolerant of standard reagent impurities and water.  The authors hope that further work into optimising this reaction can reduce the current quantities of base required.

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Mechanochemical degradation of lignin and wood by solvent-free grinding in a reactive medium, Tillmann Kleine, Julien Buendia and Carsten Bolm, Green Chem., 2013, DOI: 10.1039/C2GC36456E

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Mechanocatalysis for biomass-derived chemicals and fuels, Sandra M. Hick, Carolin Griebel, David T. Restrepo, Joshua H. Truitt, Eric J. Buker, Caroline Bylda and Richard G. Blair, Green Chem., 2010, 12, 468-474

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Chinese scientists have developed a new approach to synthesize acridones viacopper-catalyzed C–C bond cleavage and intramolecular cyclization.

C–C and C–H bond cleavage/activation has become a very important area of research in recent years due to the variety of possible applications.  Presently, the cleavage of C–C bonds generally involves the use of noble metal catalysts such as Rh, Ry, Pd and Pt among others.  However, there are very few examples in the current literature of cheaper metals such as copper or iron being employed in these transformations.

In this work, Wang Zhou and colleagues from Xiangtan University, China, used a copper catalyst and air as the oxidant to synthesize acridones through a C–C bond cleavage and intramolecular cyclization, under neutral conditions.  This reaction provides an alternative strategy for C–C cleavage.

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Copper-catalyzed C–C bond cleavage and intramolecular cyclization: an approach toward acridones, Wang Zhou, Youqing Yang, Yong Liu and Guo-Jun Deng, Green Chem., 2012, DOI: 10.1039/C2GC36502B

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Copper(II) catalysis provides cyclohexanone-derived propargylamines free of solvent or excess starting materials: sole by-product is water, Conor J. Pierce and Catharine H. Larsen, Green Chem., 2012, 14, 2672-2676

An efficient copper-catalyzed formation of highly substituted pyrazoles using molecular oxygen as the oxidant, Mamta Suri, Thierry Jousseaume, Julia J. Neumann and Frank Glorius, Green Chem., 2012, 14, 2193-2196

A highly efficient Cu-catalyst system for N-arylation of azoles in water, Deping Wang, Fuxing Zhang, Daizhi Kuang, Jiangxi Yu and Junhua Li, Green Chem., 2012, 14, 1268-1271

Microwave-assisted solvent- and ligand-free copper-catalysed cross-coupling between halopyridines and nitrogen nucleophiles, Zhen-Jiang Liu, Jean-Pierre Vors, Ernst R. F. Gesing and Carsten Bolm, Green Chem., 2011, 13, 42-45

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Scientists from India have developed a tandem N-alkylation-reduction-condensation route to synthesize N-arylmethyl-2-substituted benzimidazoles.

1,2-Disubstituted benzimidazoles exhibit a broad range of biological activities, which make them a very popular synthetic target.  The regio-specific synthesis of these compounds is the main synthetic challenge, with the desired regioselectivity of the final compounds determined at the aryl-amination stage.  However, this traditionally requires a transition metal catalyst, expensive ligands and an appropriate base.

In this work, Asit Chakraborti and colleagues from the National Institute of Pharmaceutical Education and Research, Punjab, India, present a one-pot strategy to produce N-arylmethyl-2-substituted benzimidazoles in water.  The procedure does not require a transition metal catalyst or base, with water directing the regioselectivity of the reaction through hydrogen bond mediated ‘electrophile-nucleophile dual activation’.

Read the full article for free until the 4th December 2012!

“All-water” chemistry of tandem N-alkylation–reduction–condensation for synthesis of N-arylmethyl-2-substituted benzimidazoles, Damodara N. Kommi, Dinesh Kumar, Rohit Bansal, Rajesh Chebolu and Asit K. Chakraborti, Green Chem., 2012, DOI: 10.1039/C2GC36377A

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Catalytic procedures for multicomponent synthesis of imidazoles: selectivity control during the competitive formation of tri- and tetrasubstituted imidazoles, Dinesh Kumar, Damodara N. Kommi, Narendra Bollineni, Alpesh R. Patel and Asit K. Chakraborti, Green Chem., 2012, 14, 2038-2049

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