Organic Synthesis: Strategies make visible-light-driven reactions easier
Perylene diimide catalyst
Using visible light to drive catalytic reactions offers a simple, inexpensive, and green approach to functionalizing complex molecules under mild reaction conditions. Two new catalytic strategies could finally make visible-light photocatalysis practical for commercial use, particularly in the fine chemicals and pharmaceutical industries.
In one report, Eric Meggers of Philipps University, in Marburg, Germany, and coworkers coupled low-intensity visible light and the inherent chirality of an iridium complex to drive the enantioselective α-alkylation of carbonyl compounds with benzyl or phenacyl groups (Nature 2014, DOI: 10.1038/nature13892). The tasks of photoactivating the substrate and controlling the stereochemistry of the product are typically shared by two separate catalysts. Meggers and coworkers showed that their chiral catalyst can perform both tasks at the same time, via proposed enolate and radical intermediates.
In a second report, Burkhard Konig and coworkers of the University of Regensburg, in Germany, describe a perylene diimide organocatalyst that absorbs more light energy than typical photocatalysts. The catalyst can therefore functionalize less-reactive chemical bonds, a feat that until now has required using higher-energy ultraviolet light or harsher reaction conditions (Science 2014, DOI: 10.1126/science.1258232). Konig’s team found that the perylene diimide undergoes a two-photon electron-transfer process to form an excited radical anion with enough energy to reduce aryl halides. The aryl radicals that form can then be trapped by hydrogen atom donors such as amines to form aryl derivatives or pyrroles to form substituted aryl derivatives.
In the past, chemists relied on single catalysts, UV light, auxiliary reagents, and special equipment to pull off light-driven enantioselective reactions, note Kazimer L. Skubi and Tehshik P. Yoon of the University of Wisconsin, Madison, in a commentary accompanying the Meggers paper. Chemists have more recently discovered dual chiralphotoredox catalyst systems that use lower-energy visible light supplied by simple household lighting for the same reactions, they add. “The discovery of a single transition-metal catalyst that fulfills both roles is a crucial conceptual step forward.”
Chemical & Engineering News
Self-Assembly: Simple electrochemical reactions at microsized metal disks drive microparticles to form crystalline structure
A microscale pump uses a simple chemical reaction to coax silica beads in a fluid to self-assemble into a crystalline structure (Langmuir 2014, DOI: 10.1021/la503118t). Such a device could provide a tiny motor for nanomachines, pull contaminants out of a fluid, or place coatings on nanoscale devices.
The catalytic pump consists of a gold anode and a platinum cathode. Maria Jose Esplandiu of the Autonomous University of Barcelona and her team placed 20- to 50-µm-diameter platinum disks on top of gold films sitting on a wafer. They subjected the pumps to one minute of oxygen plasma cleansing to remove any residual contamination and activate the surface then placed a gasket on top of the patterned pumps. They then mixed negatively charged 1.5-µm-wide silica spheres in a solution of hydrogen peroxide and added the mixture onto the pumps.
Electrochemical reactions between hydrogen peroxide and a platinum-gold micropump (large gray disk) cause negatively charged silica spheres to first cluster at a distance from the pump (top). But as the spheres’ charge is neutralized by the chemical reactions, they begin to gather around the disk (middle) then arrange themselves into a crystalline structure (bottom). The disk is about 20 µm in diameter.
The hydrogen peroxide reacted with the metals, creating an electrical field oriented from the gold toward the platinum. This field triggers electroosmosis, causing the fluid containing silica spheres to flow toward the platinum disk. But because the spheres are negatively charged, the electrical field repels them, and they stop about 20 µm from the edge of the platinum. Gradually, however, protons produced by the chemical reactions bind to the spheres and neutralize their charge. The particles then begin to clump together, move toward the disk, and eventually build up into a crystalline structure surrounding the platinum.
Esplandiu says translating chemical energy into mechanical energy could be useful in nanofabrication, both by providing a power source and by guiding materials to self-assemble. She’d like to try replacing one of the metals in the pump with a semiconductor, which could absorb photons to create even stronger electrical fields.
Chemical & Engineering News
Catalysis: Graphitic carbon nitride looks promising as a catalyst to harness sunlight to produce hydrogen fuel from water
TWO-STEPPING
In a water-splitting system inspired by photosynthesis, first sunlight activates a metal oxide photocatalyst (WO3 or BiVO4), which oxidizes water into O2 and hydrogen ions (left). The catalyst transfers electrons absorbed during the reaction to graphitic carbon nitride (g-C3N4) through a redox mediator. Sunlight excites the now electronrich g-C3N4, which catalyzes the formation of H2 from hydrogen ions.
Inexpensive water-splitting catalysts could lower the costs of generating hydrogen for fuel cells. Towards that end, scientists in England now have integrated a stable, lowcost organic catalyst into a water-splitting system (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja506386e). The researchers made the catalyst, graphitic carbon nitride, from abundant, cheap urea.
Water-splitting techniques usually mimic plant photosynthesis, using photocatalysts to drive a two-step process to generate O2 and H2 from water. First, light activates a catalyst, such as a metal oxide, which then oxidizes water into O2 and hydrogen ions. The metal oxide absorbs electrons in the process and then transfers them to a redox mediator, such as sodium iodide, dissolved in the water. This mediator transfers the electrons to a second photocatalyst. Light excites the electrons in the second catalyst, which then reduces hydrogen ions to produce H2.
Junwang Tang of University College London says the trick lies in finding the right material to use as a catalyst in each half of the reaction. Organic semiconductors have high photocatalytic activity, he says, but they tend to be a lot less physically stable than inorganic semiconductors. But other researchers recently found that graphitic carbon nitride (g-C3N4), an organic semiconductor, remains stable in both acid and alkaline conditions.
So Tang’s team decided to test g-C3N4 as a catalyst on the hydrogen side of the process. To make g-C3N4, Tang’s group thermally decomposed urea, an inexpensive compound owing to its abundance. This approach, he says, increased the degree of polymerization of the g-C3N4 units. Greater polymerization means more surface sites for the reduction reaction, allowing electron transfer to take place quickly and easily.
The researchers tested the g-C3N4 with two metal oxides previously developed for the oxidation process, bismuth vanadate (BiVO4) and tungsten trioxide (WO3), and found they got the best results with the tungsten.
The reaction is still not very efficient; they reduced 200 mL of water in 10 hours, consuming less than 1% of the total water in their system but producing an ideal hydrogen to oxygen ratio of 2:1. Tang hopes the efficiency can be improved. For example, reducing the particle size of the WO3 would provide more surface area for reactions to take place.
Tang’s group is also working to increase the polymerization of the carbon nitride material.
Kazunari Domen of the University of Tokyo says carbon nitride is indeed a promising photocatalyst for largescale use because its elements are so abundant. He says Tang’s work provides a good example of how carbon nitride can be used, though the researchers should aim at improving the material’s efficiency at turning absorbed photons into usable electrons, “which appears to be considerably low for practical use.”
Chemical & Engineering News
Switchable catalysts are those whose activity can be turned on and off, or whose stereoselectivity can be reversed, as a result of structural changes triggered by external stimuli, such as chemicals and light. Although a number of switchable catalysts have been synthesized, few can go beyond the scope of one specific type of reaction.
Following their recent discovery of a rotaxane-based switchable organocatalyst for the thiol-Michael addition, David A. Leigh and colleagues continue to explore its capability in activating carbonyl compounds (J. Am. Chem. Soc., 2014, DOI: 10.1021/ja509236u).
The catalyst masks and exposes an amine catalytic center through a macrocycle by acid and base modulation, respectively, and shows distinct on/off activity in iminium, enamine, tandem iminium–enamine, and trienamine catalyses.
The researchers have for the first time demonstrated multiple activation modes of a switchable catalyst, providing valuable insights into behaviors of its different states in various types of reactions. This study also opens avenues for the design of advanced switchable catalytic systems, where multistep reaction sequences may be choreographed by programming the “on” and “off” states of catalysts.Journal of the American Chemical Society
The construction of acyclic all-carbon quaternary stereogenic centers is an ongoing challenge for synthetic chemists. One of the most common strategies is the chiral α-alkylation of carbonyl compounds, usually entailing coupling between electronrich enolate or enamine-based intermediates and electrophiles, but this strategy has disadvantages that must be overcome.
Now, Sanzhong Luo and coauthors report a creative modification to this approach by combining organocatalysis with radical addition, instead of conventional nucleophilic substitution (J. Am. Chem. Soc., 2014, DOI:10.1021/ja508605a). The transformation proceeds via a tandem primary aminephotoredox catalytic process where photogenerated openshell acyl radicals are added to β-ketocarbonyl-derived chiral enamine intermediates with high stereoselectivity. Even more impressively, spiro-γ-lactams with two well-defined nonadjacent quaternary stereocenters can be synthesized from appropriate substrates in single operations.
As a facile and powerful path to asymmetric α-alkylation, this method expands the tools available for the construction of chiral carbon centers. By facilitating access to molecular motifs that are otherwise intractable, it demonstrates potential utility in overcoming some of the challenges facing pharmaceutical and natural product chemistry.
Journal of the American Chemical Society
The manufacture of many chemicals and fuels depends on heterogeneous catalysis on a metal surface. Additionally, chemical production is often a process requiring a significant energy input. Therefore, an important step in the future of heterogeneous metal catalysis is the development of highly selective catalysts that can function at lower energies, ideally using sustainable resources.
In order to create a catalyst that has high selectivity for a particular reaction, researchers must be able to determine the relative concentrations of intermediates at the surface, which may depend on the relative binding strengths of the intermediates. A team led by Cynthia Friend and Robert Madix has found a way to better predict these binding strengths, using both experimental and computational methods to take into account the van der Waals interactions between the reactants and the surface (J. Am. Chem. Soc., 2014, DOI: 10.1021/ja506447y).
The authors find that, although these associations are weak, they affect the relative stability of the intermediates and, therefore, have an important impact on the conditions required for peak selectivity. The methods used are highly relevant to many catalytic applications. They give insight into the significant role of weak interactions in more complex chemical processes, and may lead to more efficient manufacture of important chemicals in the future.
Journal of the American Chemical Society
Redox-active ligands are useful compounds in catalysis, bond activation, and other metalloenzymatic transformations where they donate electrons to metal centers to which they are attached. However, ligands that can activate the substrate while leaving the metal unaffected are rare. This type of transformation could allow selective substrate activation through a controlled radical-type mechanism, combined with favorable metal–substrate coordination.
Now Jarl Ivar van der Vlugt and colleagues have synthesized such a ligand, and they present ligand-to-substrate single-electron transfer without metal redox changes (J. Am. Chem. Soc., 2014, DOI: 10.1021/ja502164f). The ligand binds with palladium(II) to form a paramagnetic complex that bears a ligand-centered radical, as evidenced by both experimental and computational data. Once reduced, the complex creates a diamagnetic Pd–Co complex that can bind an organic azide. Activation by ligand-induced electron transfer leads to H-atom abstraction and cyclization of the resulting Pd-bound radical to produce pyrrolidine.
The reaction is rare in that it goes through a radical-type pathway with a metal that ordinarily goes through two-electron processes, and it also proceeds very cleanly. “This concept is likely more broadly applicable with group 8–10 metals, including for cooperative bond activation processes and catalysis,” the authors say.
One well-known approach for speeding up a Diels–Alder reaction involves Lewis acid catalysis, which requires that the dienophile bear a heteroatom to serve as a coordination site. But the addition of a Lewis acid catalyst introduces steric effects, which makes it impossible to determine the exact contribution of the electronic effects to the rate increase. In order to tease apart these contributions, the team compares [60]fullerene (C60) with its Li+-encapsulated counterpart (Li+@C60) on the basis of each molecule’s ability to undergo a cycloaddition reaction with 1,3-cyclohexadiene.
Because the Lewis acid, Li+, is contained inside the soccer-ball-shaped molecule, its presence does not affect the molecule’s size, so any observed changes in reaction rate can be attributed fully to electronic effects. In this first-of-its-kind study, the team determines that the entrapped Li+causes the reaction to proceed 2400 times faster, a finding confirmed by both experimental and theoretical studies.
Journal of the American Chemical Society
Elsevier publishes 15 out of the top 20 journals within the ISI Catego-ry, Engineering, Chemical, both with regards to Impact Factors and total citations.
The top ranking Elsevier journals in terms of Impact Factor are Progress in Energy and Combustion Science (1st), Journal of Catalysis (4th), Applied Catalysis B: Environmental (5th), Applied Energy (6th), and Journal of Membrane Science (7th).
In terms of total citations, Journal of Membrane Science, Journal of Catalysis and Chemical Engineering Science rank respectively 2nd, 3rd and 4th.
Also noteworthy for their increase in Impact Factor are Desalination (+ 94% since 2009) and Chemical Engineering Research and Design (+ 86% since 2009).
| Journal title | 2013 Impact Factor |
| Applied Energy | 5.261 |
| Chemical Engineering Journal | 4.058 |
| Proceedings of the Combustion Institute | 3.828 |
| Combustion and Flame | 3.708 |
| Journal of Molecular Catalysis. A, Chemical | 3.679 |
| Applied Catalysis A, General | 3.674 |
| Dyes and Pigments | 3.468 |
| Fuel | 3.406 |
| Catalysis Communications | 3.320 |
| Catalysis Today | 3.309 |
| Microporous and Mesoporous Materials | 3.209 |
| Separation and Purification Technology | 3.065 |
| Fuel Processing Technology | 3.019 |
| Reactive and Functional Polymers | 2.822 |
| Journal of Molecular Catalysis. B, Enzymatic | 2.745 |
| Journal of Aerosol Science | 2.705 |
| Journal of the Taiwan Institute of Chemical Engineers | 2.637 |
| Chemical Engineering Science | 2.613 |
| Journal of Food Engineering | 2.576 |
| The Journal of Supercritical Fluids | 2.571 |
| Computers and Chemical Engineering | 2.452 |
| Biochemical Engineering Journal | 2.368 |
| Food and Bioproducts Processing | 2.285 |
| Powder Technology | 2.269 |
| Hydrometallurgy | 2.224 |
| International Journal of Adhesion and Adhesives | 2.216 |
| Journal of Process Control | 2.179 |
| Journal of Industrial and Engineering Chemistry | 2.063 |
| Chemical Engineering & Processing: Process Intensifi-cation | 1.959 |
| Process Safety and Environmental Protection | 1.829 |
| Minerals Engineering | 1.714 |
| Particuology | 1.648 |
| Advanced Powder Technology | 1.642 |
| Chinese Journal of Catalysis | 1.552 |
| International Journal of Mineral Processing | 1.461 |
| Journal of Loss Prevention in the Process Industries | 1.347 |
| Journal of Bionic Engineering | 1.333 |
| Chinese Journal of Chemical Engineering | 0.872 |
Лекция Н.З. Ляхова (ИХТТМ СО РАН, Новосибирск) была посвящена особенностям дисперсного наномодифицирования металлов, сплавов и полимеров. В докладе было отражено влияние механохимического воздействия на формирование интерметаллических сплавов с заданными магнитострикционными свойствами. Также был рассмотрен новый способ синтеза пенополиуретановых композитов на основе литьевого полиуретана с повышенным значением предела прочности на разрыв.
Н.З. Ляхов поделился с молодыми учеными информацией о методе механохимически стимулированного низкотемпературного синтеза нитрида алюминия для получения нанокерамики с высокой теплопроводностью. 
XII European Congress on Catalysis