Archive for the ‘Organic’ Category

Synthesizing Polymers Using CO2

Ring-opening polymerizations produce commercial polymeric materials including epoxy resins, but they usually liberate small molecules such as the greenhouse gas, CO2. In the context of climate change, it is urgent to reduce CO2 emissions. Recently, a group of UK researchers led by Prof. Charlotte K. Williams at the University of Oxford developed a step-growth polymerization method that self-consumed CO2. The work has been published in a recent issue of Chemical Communications.

The synthesis involved two catalytic cycles (Figure 1). The first cycle polymerized L-lactide-O-carboxyanhydride into poly(L-lactide acid) (PLLA) via a ring-opening polymerization and released one CO2 molecule per polymer repeat unit. In the second cycle, epoxide molecules (cyclohexeneoxide) combined with the CO2 generated in the first step and grew into poly(cyclohexene carbonate) (PCHC) from the terminal ends of the PLLA chains. A di-zinc-alkoxide compound catalyzed both cycles and coupled the two processes together. The product is PLLA-b-PCHC block copolymers, which are composed of PLLA and PCHC covalently tethered together.

Figure 1. The two catalytic cycles are joined by a zinc-based catalyst, [LZn2(OAc)2]. The CO2 gas produced in the first step serves as a reactant in the second step. OCA: O-carboxyanhydride; ROP: ring-opening polymerization; CHO: cyclohexeneoxide; ROCOP: ring-opening copolymerization.

The two reactions resulted in block copolymers with few byproducts. In-situ 1H NMR revealed that the reactants in the first step (LLAOCA) were rapidly consumed during the first four hours (Step I, Figure 2a), and the concentration of PLLA increased notably. The concentration of PCHC only markedly increased after the concentration of PLLA saturated (Step II, Figure 2a). The byproduct of the second step, trans-cyclohexene carbonate, exhibited consistently low concentrations. The pronounced single peak in each size-exclusion chromatogram of the corresponding product confirmed the presence of block copolymers, instead of polymer mixtures (Figure 2b). Although the authors did not fully elucidate the origin of the excellent selectivity towards the block copolymer, they speculated that the change in CO2 partial pressure played a role. Significantly, nearly all CO2 molecules were consumed in the second step, with 91% incorporated into the block copolymer, and 9% converted to the byproduct.

Figure 2. (a) The evolution of the concentrations of PLLA, PCHC, and trans-CHC (the byproduct of the second step) with reaction time. (b) Size-exclusion chromatograms of the products at different reaction times. Mn: number-average molecular weight; Đ: polydispersity.

The authors are investigating the detailed polymerization mechanism, as well as identifying new catalysts to expand the polymerization scheme to other polymers.

 

To find out more, please read:

Waste Not, Want Not: CO2 (Re)cycling into Block Copolymers

Sumesh K. Raman, Robert Raja, Polly L. Arnold, Matthew G. Davidson, and Charlotte K. Williams

Chem. Commun., 2019, 55, 7315-7318

 

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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Guiding Light with Molecular Crystals

We’re all used to communications and computing happening at high, and seemingly ever-increasing speeds. Continuing on this trajectory requires the development of materials capable of acting as micro/nanoscale waveguides that don’t experience interference effects from strong external electromagnetic fields. Molecular crystals represent an exciting but relatively under-explored materials class due to their inherently limited emission and absorption properties. However, an international group of researchers recently combined two different crystalline materials with complementary optical properties in a filled-hollow crystal architecture, involving no binding materials or polymer matrices.

Figure 1. Spectra and structure of DCA (left) and PDI (right).

The group used 9,10-dicyanoanthracine (DCA) as the hollow outer crystal, with a perylene diimide derivative (PDI) as the interior compound (Figure 1). When combined, these two compounds exhibit fluorescence that covers the visible and near-IR portions of the electromagnetic spectrum. The researchers grew hollow crystals of DCA with diameters ranging from 50-400 μm in diameter with pores of 10-200 μm and filled them with 1-50 μm PDI crystal fibrils manually by hand(!) (Figure 2) (I honestly can’t imagine how many crystals ended up broken during that experimental learning curve!). The assembled structure for study had a single hollow DCA crystal filled with 18 individual PDI fibrils to create the waveguide.

Figure 2. Schematic of hollow crystal architecture (top) with demonstration of construction (bottom).

When the researchers excited the full structure with a 365 nm continuous wavelength LED, both crystal components emitted light that was guided down to the opposite end. The specific makeup of the spectrum depends on the point of illumination; only the excited compounds emit. This supports the active waveguiding capabilities of the materials. The emissive properties can also be controlled by changing the excitation wavelengths to exclude the absorbance of one of the molecular crystals. PDI can be selectively excited using light above 550 nm and both PDI and DCA act simply as passive waveguides for light in the infrared region of the spectrum, of particular importance for wireless communication. This study represents an exciting next step for organic molecular materials as optical waveguides with a new architecture for devices.

To find out more please read:

A filled organic crystal as a hybrid large-bandwidth optical waveguide

Luca Catalano, Patrick Commins, Stefan Schramm, Durga Prasad Karothu, Rachid Rezgui, Kawther Hadef and Panče Naumov

Chem. Commun, 2019, 55, 4921-4924.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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Synthesis of Maleimide Dyes with Colourful Fluorescent Emissions

A group of researchers based at universities spanning the UK, China and Spain have synthesised a diverse library of fluorescent maleimide dyes with the aim of developing a structure-function relationship, relating substituent effects to the optical properties of such molecules. This work is not only important to build upon fundamental understanding of the fluorescence mechanism, but to develop knowledge that may be used to guide the synthesis of organic fluorophores which demand particular optical properties.

Organic fluorescent molecules are used as tools in many areas such as forensics, genetic analysis, DNA sequencing and biotechnology. Maleimides are commonly used as fluorescent labels for proteins, as they can couple with the thiol groups of cysteine residues. They are suited to this purpose as they are stable, easily functionalised, give strong emissions and do not perturb the protein structure to a large extent.

Molecules fluoresce upon absorption of UV or visible light, elevating an electron from a ground state orbital to a higher-energy orbital and resulting in a singlet excited state. Relaxation to the ground state occurs rapidly (~ 10 ns) with concomitant emission of a photon – this is what we observe as ‘fluorescence’. The emitted photon almost always has a longer wavelength than the absorbed light, a phenomenon known as the ‘Stokes shift’.

 

Structures of selected aminohalomaleimides and alkoxyhalomaleimides

Structures of selected amino-halo-maleimides and alkoxy-halo-maleimides synthesised for the study

With three dihalomaleimide precursers in hand (Cl, Br and I) the researchers assembled a library of amino-halo-maleimides, amino-alkoxy-maleimides, and amino-thio-maleimides. They varied the R groups bound to the N, O and S heteroatoms to include aliphatic, phenyl and benzyl examples.

The optical properties of the amino-halo-maleimides in diethyl ether were examined and the emission wavelengths were measured to be 461-487 nm, giving green-blue fluorescence. The fluorescence quantum yields, a measure of the quantity of emitted photons compared to absorbed photons and an indication of emission brightness, decreased with the electronegativity of the halide (Cl: 37%, Br: 30%, I: 8%). Like many fluorescent molecules in solution the compounds exhibited solvafluorochromism: when the polarity of the solvent alters the optical properties. In protic solvents (methanol and water) the fluorescence quantum yields decreased to below 1% and the emission wavelengths increased by 73-109 nm. On the other hand, in non-polar solvents (cyclohexane) the fluorescence quantum yield increased, up to 56% for the chloro analogue.

a) The UV and emission spectra of fluorescent maleimides bearing amino (2a-c) and alkoxy (3a, 3b) substituents. b) The quantum yields of selected amino and alkoxymaleimides. c) The solvafluorochromism effect for three aminomaleimides (2a-c) in increasingly non-polar solvents.

a) The UV and emission spectra of fluorescent maleimides bearing amino (2a-c) and alkoxy (3a, 3b) substituents. b) The quantum yields of selected amino and alkoxymaleimides. c) The solvafluorochromism effect for three maleimides (2a-c) in various solvents.

Compared to their amino-substituted counterparts, alkoxy-halo-maleimides have lower quantum yields (reduction of 20-25%), indicating the increased electron-donating capacity of the amine substituent is important for fluorescence intensity. Furthermore, the slight decrease in the emission wavelengths of alkoxy-halo-maleimides (458-465 nm) gives them blue fluorescent emissions. Amino-thio-maleimides, with greater electron-donating capacity than both the amino and alkoxy analogues, have increased emission wavelengths (526-564 nm), thus yellow fluorescent emissions.

This study is a worthwhile read for anyone who uses fluorescent molecules in their work, those wishing to understand a little more about the practical principles of fluorescence and all those curious minds who like to form their own hypotheses.

To find out more please read:

Rational design of substituted maleimide dyes with tunable fluorescence and solvafluorochromism

Yujie Xie, Jonathan T. Husband, Miquel Torrent-Sucarrat, Huan Yang, Weisheng Liu, Rachel K. O’Reilly.
Chem. Commun., 2018, 54, 3339 – 3342
DOI: 10.1039/C8CC00772A

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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Synthesis of Oxazolines with a Triple Activation Twist

A group of researchers based in Valencia, Spain, have developed an enantio- and diastereoselective method to synthesize oxazolines, using a combination of silver and an organocatalyst that is only one degree of separation away from a classic British cocktail (to get that one, you might have to keep reading). This method complements the current literature by employing unactivated ketone substrates to prepare products with two adjacent chiral centres, one of which is quaternary.

Oxazolines are a versatile functional group, found in synthetic precursors and functional molecules and materials. They are a common structural feature in many natural products and in medicinal chemistry they are incorporated into synthetic drug candidates as bioisosteres of other functional groups, such as thiazoles and imidazoles. Oxazolines can also be used to prepare poly(2-alkyl/aryl-oxazoline) polymers, which have utility in biomedical fields to prepare hydrogels, nanoparticles for drug delivery and imaging, and polymer-protein conjugates. In an organic synthesis laboratory oxazolines are encountered as ligands in asymmetric catalysis, such as the bis(oxazoline) ‘BOX’ family of compounds.

Synthesis of oxazolines from ketones and isocyanaoacetate esters via a formal [3+2] cycloaddition reaction catalysed by a multicatalytic system of silver and a dihydroquinine squaramide organocatalyst

Synthesis of oxazolines from ketones and isocyanaoacetate esters via a formal [3+2] cycloaddition reaction.

The researchers optimised the reaction with acetophenone and tert-butyl isocyanoacetate, and found that Ag2O (2.5 mol%) and a dihydroquinine squaramide organocatalyst (5 mol%) promoted the reaction to obtain a quantitative yield of the cis-oxazoline (80:20 cis/trans) in 24 hours with excellent enantiomeric excess (99/93% e.e.). The researchers tested the reaction scope and found that acetophenone derivatives substituted with electron-withdrawing and donating groups (NO2, Cl, Br, Me) gave the cis-oxazoline products in reliable yields (60 – 99%), fair to good diastereoselectivity (56:44 – 95:5 cis/trans) and excellent enantioselectivities (91 – 99% e.e.). Other examples tested included cyclohexanone, 2-acetylthiophene, deoxybenzoin and acetone as well as aliphatic ketones with a combination of methyl, isopropyl and cyclopropyl groups (63-99% yield, 45:55 – 98:2 cis/trans, 56 – 98% e.e.).

Diagram showing the triple activation of the substrates achieved using a dihydroquinone squaramide organocatalyst combined with a silver lewis acid.

Diagram showing the triple activation of substrates using a dihydroquinine squaramide organocatalyst combined with a silver lewis acid.

The dihydroquinine squaramide organocatalyst used is a composite structure comprising a rigid cyclobutendione ‘squaramide’ and a reduced quinine analogue. The former makes strong hydrogen bonds with carbonyl derivatives while the latter confers chirality on the catalyst and has a Brønsted basic quinuclidine moiety. Quinine is an alkaloid isolated from the cinchona plant, and is the same stuff in anti-malarial tonic that is better washed down with a measure of gin. The reaction proceeds via silver-coordination to the terminal carbon of the isocyanate, accelerating deprotonation of the alpha protons by the quinuclidine base and forming a nucleophilic enoate. The electrophilicity of the ketone is enhanced by the formation of two hydrogen bonds with the N-H bonds of the squaramide, shaping a doubly activated system which rapidly undergoes a formal [3+2] cycloaddition to give the products.

At first glance this seems to be a simple and straightforward transformation, but in reality the authors have succeeded in optimising a reaction for unactivated substrates which has stereocontrol over two chiral centres, one of which is quaternary. Furthermore, the reaction uses a combination of two catalysts reacting via three modes of activation.

To find out more please read:

Enantioselective synthesis of chiral oxazolines from unactivated ketones and isocyanoacetate esters by synergistic silver/organocatalysis

Pablo Martínez-Pardo, Gonzalo Blay, M. Carmen Muñoz, José R. Pedro, Amparo Sanz-Marco and Carlos Vila.
Chem. Commun., 2018, 54, 2862 – 2865
DOI: 10.1039/c8cc00856f

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

 

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An Industrial Revolution on the Nanoscale

“What are the possibilities of small but movable machines? They may or may not be useful, but they surely would be fun to make”. In December 1959 Richard Feynman addressed the annual meeting of the American Physical Society at Caltech with a talk entitled ‘There’s Plenty of Room at the Bottom’, imploring the scientific community to start thinking small, like ‘entire 24 volumes of the Encyclopaedia Britannica on the head of a pin’ kind of small. Many quote this lecture as when the notion of nanomachines first entered the scientific sphere – with talk of miniature cars, injectable molecular ‘surgeons’ and machines that place atoms side by side to synthesize any molecule imaginable. The lecture reads like the description of the futuristic setting in ‘Back to the Future’, an exploration of possibilities at a time when we fundamentally lacked the tools to make them a reality.

As in ‘Back to the Future’, which predated yet predicted the emergence of mobile-banking technology, video calling and personal drones, Richard Feynman’s plea for scientists to prepare molecular-scale machines has also become a reality, and for their successes in this field Jean-Pierre Sauvage, Sir Fraser Stoddart and Ben Feringa were jointly awarded the Nobel Prize in Chemistry in 2016.

A group of researchers based in London and Singapore have written a feature article introducing both the foundational work in this field and state-of-the-art examples. Nanomachines are single molecules or molecular assemblies on the nanoscale (this review defines a 1 – 100 nm scope) that have the ability to perform ‘useful work’ upon application of an external energy source. To extract work (often in the form of controlled mechanical movement) molecular machines are designed to operate at a thermodynamically far-from-equilibrium state, maintained by an energy input, with movement occurring as the system relaxes towards equilibrium. At the synthetic level, molecules are designed with components which have restricted translational and rotational movements with respect to each other, and the ability to control these movements is key to obtaining the desired function.

A catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine , moving along a polybutadiene polymer

The catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine, moving along a polybutadiene polymer

One of the first advances towards the synthesis of nanomachines was by the research group of Jean-Pierre Sauvage, who achieved the templated synthesis of catenanes; structures with two circular molecules that are interlocked like two links in a chain. It was subsequently shown that a catenane motor could be prepared, with one ring rotating with respect to the other in a controlled manner. Fraser Stoddart further contributed to the field with ‘rotaxanes’, composite molecules comprising a ring threaded onto an axle. Nanomachines based on rotaxanes have been developed and include switches, shuttles and ‘molecular elevators’. A state-of-the-art example of a catalytically active rotaxane synthesised by Nolte and co-workers in 2003 demonstrates the potential of nanomachines to revolutionise organic synthesis. The rotaxane is constructed with a magnesium-bound porphyrin, which threads onto a polybutadiene polymer (300 kDa, 98% cis) and catalyses the epoxidation of the double bonds (turnover number: 140, cis/trans ratio of the polyepoxide: 1:4).

Ben Feringa's electric nano-car, a single molecule with four fluorene 'wheels' capable of driving across a copper surface

Ben Feringa’s electric nano-car, a single molecule with four fluorene ‘wheels’ capable of driving across a copper surface

In 2011 Ben Feringa and co-workers synthesized the worlds tiniest electric car using the same design principles they had used to create a spinning motor in 1999. The car is a single molecule with the ability to propel itself across a crystalline copper surface upon activation by a voltage pulse, with 10 pulses moving the car 6 nm across the surface. The car itself is comprised of a central diyne strut bonded at each end to carbazole ‘axles’. Each axle is bound through alkenes to two fluorene ‘wheels’. The key design elements are the alkenes and two chiral methyl substituents on each axle which forces each wheel to twist out of the plane. For one wheel rotation: an electronic excitation induces transcis isomerisation of the alkene causing a quarter turn of the wheel such that it sits adjacent to the methyl group. Next, a vibrational excitation induces helical inversion, allowing the wheel to push past the methyl group another quarter turn. Another isomerisation and helical inversion completes a full rotation. Research achievements like these demonstrate mechanical work on the nanoscale, with the vision of achieving movement on the macroscale via synchronised motion.

These examples represent a small subset of those discussed in the feature article review, which not only spans the current scope of molecular-scale machines, but reviews the design principles guiding their development and the possibilities nanomachines represent in the future of scientific research.

To find out more please read: 

Artificial molecular and nanostructures for advanced nanomachinery

Elizabeth Ellis, Suresh Moorthy, Weng-I Katherine Chio and Tung-Chun Lee.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c7cc09133h

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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Hiding Carbon Dioxide in Oxazolidinones

Sometimes it feels as though the pinnacle of synthetic achievement is represented by 20 step total syntheses (with 10 contiguous stereocentres and 5 fused rings…). The level of chemical complexity that can be fashioned from simple building blocks is undoubtedly impressive, but amid such feats it is important not to lose sight of the elegance and worth of simple chemistry, especially when it aims to play a part in resolving profound challenges. One such challenge, which will increasingly confront future generations, is how to reduce the load of carbon dioxide in the atmosphere. One solution is to ‘fix’ carbon dioxide by integrating it into chemical building blocks of added complexity in a sustainable way.

The porosity and high surface area of metal organic frameworks (MOFs), a class of three-dimensional coordination networks, proffers them as ideal materials for capture and storage of carbon dioxide. A team of researchers have designed a MOF which consumes carbon dioxide in a different way: by transformation into value-added chemicals. The group have developed a catalytic MOF embedded with lewis-acidic copper centres capable of converting aziridines to oxazolidinones by the addition of carbon dioxide. Oxazolidinones are used as auxiliaries in chiral synthesis, and are structural components of some antibiotics.

The MOF, termed MMPF-10, is a metal-metalloporphyrin framework constructed from a copper-bound porphyrin ring chemically modified to incorporate 8 benzoic acid moieties, generating an octatopic ligand. These carboxylic acids groups form a second complex with copper in situ, termed a ‘paddlewheel’ for its appearance, with the formula [Cu2(CO2)4]. The resulting network contains hexagonal channels measuring 25.6 x 15.6 Å flanked by four of each of the two copper complexes. With 0.625 % of the catalyst at room temperature, 1 bar CO2 pressure, and in a solvent free environment, MMPF-10 catalyses the transformation of 1-methyl-2-phenylaziridine to yield 63% of the product.

metal-metalloporphyrin MOF catalyses catalyzes carbon dioxide fixation aziridines to oxazolidinones

Topology of MMPF-10 showing hexagonal channels in a) and c), and pentagonal cavities in b). Turquoise: copper, red: oxygen, grey: carbon, blue: nitrogen.

This work, a simple reaction to prepare oxazolidinones, shows that carbon dioxide can be fixed in specialised synthetic building blocks in a sustainable way. This is the way the first paragraph ended, ‘in a sustainable way’, because the challenge of developing such a reaction is two-fold: it must use carbon dioxide, and the reaction conditions must be sustainable. There will be no beneficial offset if the reaction uses a lot of energy, requires many resources, or generates larges quantities of waste. In this reaction the researchers have remained mindful of developing a mild, solvent-free reaction with low catalyst loading employing an earth abundant metal, reflecting an earnest aim to develop practical and sustainable chemistry.

To find out more please read:

A metal-metalloporphyrin framework based on an octatopic porphyrin ligand for chemical fixation of CO2 with aziridines

Xun Wang, Wen-Yang Gao, Zheng Niu, Lukasz Wojtas, Jason A. Perman, Yu-Sheng Chen, Zhong Li, Briana Aguila and Shengqian Ma
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c7cc08844b

About the Author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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A warm welcome to Sandeep Verma, our new ChemComm Associate Editor

We are excited to welcome new Associate Editor Sandeep Verma (Indian Institute of Technology Kanpur) to the ChemComm Editorial Board

Professor Sandeep Verma

Sandeep Verma holds the positions of Professor of Chemistry and Shri Deva Raj Endowed Chair Professor at the Department of Chemistry, Indian Institute of Technology Kanpur, which he joined in 1997. His work has been recognized by numerous awards such as Swarnajayanti Fellowship (2005), Shanti Swarup Bhatnagar Prize in Chemical Sciences (2010), Department of Atomic Energy-Science Research Council Outstanding Investigator Award (2012), Ranbaxy Research Award in Pharmaceutical Sciences (2013), J C Bose National Fellowship (2013), Silver Medal, Chemical Research Society of India (2017), and National Prize for Research on Interfaces between Chemistry and Biology (2017).

His main research interests include peptide/protein assemblies for disease modeling, soft biomaterials, bioimaging, and surface chemistry of metal complexes. In particular, his group focuses on heterogeneous catalysts designed by developing polymeric templates based on nucleobase frameworks for application to interesting chemical and biochemical reactions. His work also focuses on the construction of architectures mimicking biological assemblies and metal-organic frameworks.

As a ChemComm, Sandeep will be handling submissions to the journal in the above areas. Why not submit your next paper to his Editorial Office?

Read Professor Verma’s recent articles published in ChemComm and its sister journals:

Chemical sensing in two dimensional porous covalent organic nanosheets
Gobinda Das, Bishnu P. Biswal, Sharath Kandambeth, V. Venkatesh, Gagandeep Kaur, Matthew Addicoat, Thomas Heine, Sandeep Verma and Rahul Banerjee
Chem. Sci., 2015, 6, 3931-3939

Organostannoxane-supported nucleobase arrays: synthesis and supramolecular structures of polymeric and molecular organotin complexes containing guanine, uracil and 2-aminopurine
Subrata Kundu, N. Nagapradeep, Balaram Mohapatra, Sourav Biswas, Sandeep Verma and Vadapalli Chandrasekharn
CrystEngComm, 2016, 18, 4807-4817

Assembly, postsynthetic modification and hepatocyte targeting by multiantennary, galactosylated soft structures
Anisha Thomas, Akansha Shukla, Sri Sivakumarb and Sandeep Verma
Chem. Commun., 2014, 50, 15752-15755

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Bioinspired catalysis for eco-friendly chemical transformations in water

One challenge that today’s chemists face is making large-scale processes more economical and environmentally friendly. Within this area, there has been a surge of interest in the development of bioinspired catalytic systems which, relative to traditional catalysis, have the potential to reduce chemical waste by 85% by performing efficient reactions in pure water.

Prof. Normand Voyer and coworkers from Laval University have recently published an eco-friendly methodology for the preparation of chiral a,b-epoxyketones in pure water using the supramolecular catalyst, homo-oligopeptide poly-L-leucine (PLL).

Achieving enantioselectivity in organic reactions carried out in water poses challenges but peptide derived catalysts have shown great promise in this regard. The best example of this is the Juliá-Colonna epoxidation which has been studied and improved since its discovery in the early 1980’s. While significant progress in this area has been made, most transformations using natural homo-oligopeptides have required the use of an organic co-solvent to improve reaction efficiency.

Professor Voyer shows the new, eco-friendly process begins with several homo-oligopeptides being synthesised from their corresponding amino acid N-carboxyanhydrides and used to catalyse the Juliá-Colonna epoxidation of an electron deficient olefin in water. Of all the catalysts, PLL provided the highest conversion and enantioselectivity (Table) however, the generality of the reaction appeared to be dependent on the sterics and electronics of the substrates.

Computational analysis was used to model the PLL supramolecular catalyst and rationalise the observed reaction trends. PLL adopts a helical conformation with hydrophobic grooves distributed along the helical axis. When modelled with substrate 1 (Table), it was observed that the chalcone moiety fits perfectly within the PLL groove and forms a stable complex. It is this complexation that also aids in solubility of the ketone, removing the need for an organic co-solvent.

Epoxidation is proposed to take place through a “groove sliding” mechanism, where the substrate slides into the hydrophobic pocket generated by the leucine side chains until it reaches the N-terminal of PLL where a hydroperoxide anion is waiting (Figure). This mechanistic proposal lends to the enantioselectivity of the reaction and explains the observed electronic and steric constraints.

While the scope of PLL remains limited, this study underscores the fact that conformation and the hydrophobic nature of the oligopeptide catalysts are critical for carrying out environmentally benign organic reactions and has set a precedent for the development of future biomimetic supramolecular catalysts.

To find out more see:

Revisiting the Juliá–Colonna enantioselective epoxidation: supramolecular catalysis in water
Christopher Bérubé, 
DOI:10.1039/C7CC01168G


Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

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Opening the door to poly(ionic liquid)s with enhanced properties

Poly(ionic liquid)s, or PILs, are polyelectrolytes whose potential uses are being investigated for a variety of technologies, such as batteries, membranes, solar cells and switchable surfaces. In this ChemComm communication, Professor Eric Drockenmuller and co-workers at the Université de Lyon, University of Liège and the Institut Universitaire de France describe a new family of PILs based on poly(vinyl ester 1,2,3-triazolium)s, which should give rise to new properties and application possibilities. 

The materials are prepared from a multistep route making use of `click chemistry´(copper(I) catalysed azide alkyne Huisgen cycloaddition reaction), palladium catalyzed vinyl group exchange, and cobalt mediated radical polymerisation. This route yields a neutral polymer, which is transformed into the poly(ionic liquid) using N-methyl bis[(trifluoromethyl)sulfonyl]imide. This useful reagent alkylates the triazole group present, and delivers the bis[(trifluoromethyl)sulfonyl]imide counterion in one step. 

Synthetic route used to yield new poly(vinyl-ester 1,2,3-triazolium)s

The ionic conductivity for the PIL reported is slightly lower than for other types of PIL. To tune this property, a variety of alkynes and azides are being tested in the ring forming step of the reaction, which will result in different substituents on the triazolium ring and on the spacer group between the polymer backbone and triazolium ring.  Changes in thermal properties in the the neutral precursor-to-PIL stage of the reaction were measured using broadband dielectric spectroscopy. Significant changes in solubility, and a 9⁰C rise in glass transition temperature to -16⁰C, were observed. 

The molecular variety introduced by this new synthetic approach offers large scope for fine tuning the electronic and mechanical material properties of these polyelectrolytes, further enabling their use in important technological applications. 

Read this Chemical Communication today – it’s free to access until 3rd April*: 

Poly(vinyl ester 1,2,3-triazolium)s: a new member of the poly(ionic liquid)s family
M. M. Obadia, G. Colliat-Dangus, A. Debuigne, A. Serghei, C. Detrembleurb and E. Drockenmuller
DOI: 10.1039/c4cc08847f 

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Accessing Chiral Space with Visible Light

Researchers have made tremendous efforts to unlock stereoselective, catalytic organic transformations. In this recent ChemComm Feature Article, Professor Eric Meggers, one of the pioneers in the field of photoredox catalysis, provides a comprehensive review of the recent advances in asymmetric catalysis driven by visible light.

Asymmetric catalysis has been one of the most attractive yet challenging areas of organic chemistry for the synthesis of unique, biologically active natural products such as Taxol, Rapamycin, or Vinblastine that possess numerous stereocenters.

C4CC09268F gaRecently, visible light, a sustainable and affordable energy resource, gained substantial interest with its capability to selectively access chiral molecules from prochiral substrates without undesirable by-products. Transformations including aldehyde α-functionalization and [2+2] cycloadditions demonstrate the potential of visible light in the presence of a photosensitizer.

These photosensitizers are typically ruthenium or iridium complexes that can facilitate electron/energy transfer upon photoinduction. In most cases, a photoredox catalyst has to be coupled with a chiral co-catalyst to introduce stereocenters in the products.

Notable advances in the Meggers, Melchiorre, and MacMillan research groups have recently demonstrated that photoactivation can be achieved with a single chiral photosensitizer to provide products of high enantiomeric excess and good yield.

This inspirational review was just published in Chemical Communications as a Feature Article. I recommend reading “Asymmetric catalysis activated by visible light” (DOI: 10.1039/c4cc09268f) by Professor Eric Meggers to learn more about the recent advances with mechanistic details and his forecast for one of the rapidly-growing research topics in organic chemistry.

This article is free to access until 17th March.* Download it here:
Asymmetric catalysis activated by visible light
Eric Meggers �
Chem. Commun., 2015, Advance Article
DOI: 10.1039/C4CC09268F, Feature Article


Dr. Tezcan Guney is a guest web writer for Chemical Communications. Dr. Guney received his Ph.D. from the Department of Chemistry at Iowa State University with Prof. George Kraus, where he focused on the synthesis of biologically active polycyclic natural products and multifunctional imaging probes. Currently, he is a postdoctoral research scholar at the Memorial Sloan-Kettering Cancer Center in New York with Prof. Derek Tan, contributing to the efforts to access biologically active small molecules using the diversity-oriented synthetic approach.

*Access is free through a registered RSC account

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