ATMOSPHERIC CHEMISTRY
▸ Fate of natural isoprene emissions clarified One of the key compounds emitted into the air by trees is isoprene. Once in the atmosphere, this common diene reacts with hydroxyl radicals and O2 to produce six peroxy radical isomers. Those peroxy species affect air quality through reactions with nitrogen oxides and affect climate through reactions that form organic species able to condense into sunlight-reflecting aerosol particles. A new lab study digs into the complex and dynamic chemistry of isoprene-derived species, Isoprene determining a comprehensive picture of their lifetimes and reaction rates (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.6b12838). Alexander P. Teng, John D. Crounse, and Paul O. Wennberg of Caltech used an environmental chamber with mass spectrometry measurements to track yields of isomer-specific reaction products. The researchers found that the amounts of the peroxy radicals depend on their thermodynamic stability and the rate of hydrogen-shift intramolecular chemistry for two of the isomers. The results suggest that in typical atmospheric conditions β-hydroxyl peroxy radical isomers compose 95% of the radical pool, even though they are not initially formed in that proportion. In most prior laboratory studies, the peroxy radical pool reflected the nascent isomer distribution.—JYLLIAN KEMSLEY
GREEN CHEMISTRY
Chemists get picky about organosulfur compounds When hearing someone describe the typical preparation of organosulfur compounds, it’s clear the approach is not a very sustainable one. Mixtures of sulfur compounds naturally found in crude oil are first treated with hydrogen in a catalytic refinery process to remove the sulfur—the goal is to pull out as much sulfur as possible to create cleaner transportation fuels. The hydrogen sulfide that forms in this hydrodesulfurization step is converted S( ) n
S
Starting mixture
to elemental sulfur. Chemists then pair this sulfur with selected reactants to reconstruct desired organosulfur compounds one product at a time. Overall, the energy-intensive, multistep process is costly and generates a significant amount of waste. A new strategy reported by Valentine P. Ananikov of the Russian Academy of Sciences’ Zelinsky Institute of Organic Chemistry and coworkers enables unprecedented direct C–H functionalization and separation of only one sulfur component at a time from mixtures of organosulfur compounds (ACS Omega 2017, DOI: 10.1021/acsomega.7b00137). The Russian researchers use palladium acetate along with oxygen and silver trifluoroacetate as a combination oxidant to couple an olefin, such as ethyl acrylate, to an aryl or heteroaryl sulfur compound. The reaction’s selectivity centers on the sulfur acting as a directing group to guide the C–H activation and alkenylation. Benzyl compounds provide the most favorable geometry for palladium binding, followed by compounds with longer alkyl chains, which leads to exclusive functionalization of one component at a time in a mixture.—STEVE RITTER
▸ Now we’re printing with glass
CREDIT: NEPTUNLAB/KIT
Single product, n = 1
n = 0–3
3-D PRINTING
A new silica nanoparticle-based ink makes it possible to create intricate glass reaction vessels and optical components with a three-dimensional printer (Nature 2017, DOI: 10.1038/nature22061). Although 3-D printing has been growing in popularity, the materials it’s been able to shape have been limited to polymers, ceramics, and metals, says Bastian E. Rapp of Karlsruhe Institute of Technology. Researchers have tried to 3-D print glass in the past, but the high melting temperature of the material has been a challenge. For instance, melting a glass filament in a 3-D printer has
R´
Palladium catalyst, oxidant, olefin (R´H)
This 3-D printed, honeycomb-shaped piece of fused silica glass can withstand harsh conditions, such as this 800 °C flame. resulted in parts with low resolution—on the order of millimeters—and rough surfaces not suitable for use in optics. To overcome these difficulties, Rapp and co-
workers adapted a “liquid glass” composite ink made from silica nanoparticles and UV-curable hydroxyethylmethacrylate monomers that they previously developed for making molded glass objects. To make the material compatible with a 3-D printing method called stereolithography, which builds objects layer by layer using UV light to turn monomers into polymers, they removed a solvent from the liquid glass to make it more transparent. The researchers also had to make sure the material could mechanically withstand the printing process. So they made it stiffer by adding higher molecular weight triacrylates. Using the modified ink with a 3-D printer, the researchers created structured objects, heated them once to burn off the polymer and then a second time up to 1,300 °C to sinter the nanoparticles, leaving pure, transparent glass parts with a resolution of tens of micrometers.—KATHERINE BOURZAC,
special to C&EN APRIL 24, 2017 | CEN.ACS.ORG | C&EN
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Science Concentrates BIOCHEMISTRY SYNTHESIS
HO
HO
The slimy mucus that coats the skin of an Indian frog contains peptides that destroy the H1 influenza virus, according to a team of researchers led by Joshy Jacob of Emory University. One of these flu-killing peptides appears to be harmless to human cells, making it a promising antiviral candidate to fight the flu (Immunity 2017, DOI: 10.1016/j. immuni.2017.03.018). The slimy surface of Hydrophylax bahuvistara and other amphibians could become a siren call for researchers searching for antimicrobial compounds. The mucus contains many molecules that protect the animals against bacterial and viral pathogens, and the slime is relatively easy to isolate: Researchers need to just give the frogs a small electric shock, collect the released mucus, and let the amphibians hop away. Jacob’s team focused on one 23-amino-acid peptide, which the researchers named urumin after a deadly whip sword found in the same region as the frog’s native habitat. Most antiviral drugs interfere in a biological process such as replication or infection; urumin appears The peptide urumin from the skin mucus to smash the viral capsid of an Indian frog kills the H1 influenza virus to smithereens.—SARAH by breaking apart its capsid (two examples
Olefins are important components in many organic compounds, such as fragrances, natural products, and drugs. And while chemists have many ways of introducing double bonds into molecules, most rely on chemistry developed more than 30 years ago. Researchers led by Phil S. Baran of Scripps Research Institute California have now come up with a new way to introduce olefins into organic compounds (Nature 2017, OH DOI: 10.1038/ nature22307). The O reaction—known OH as decarboxylative alkenylation— (+)-PGF2𝛂 starts with ubiquitous alkyl carboxylic acids. The chemists first transform the carboxylic acid into a redox-active ester, which then undergoes nucleophilic attack from an alkenyl zinc reagent in the presence of a nickel or iron catalyst. The whole process takes place in a single pot and can be used to make mono-, di-, tri-, and tetrasubstituted olefins. The reaction also allows exquisite control of olefin geometry because it is established by the geometry of the alkenyl zinc reagent rather than the C–C bond-forming event, as is the case in olefin metathesis. The chemists used the new reaction to prepare more than 60 different olefins, including the prostaglandin (+)-PFG2α (shown; red bonds made via decarboxylative alkenylation).—BETHANY HALFORD
EVERTS
shown, top to bottom).
SPECIALTY CHEMICALS plementing the industrial pathways that lead to corresponding branched alkyl arenes. The products could add to the diversity of compounds available for making soaps, detergents, fuel and lubricant additives, polymer precursors, and flavors and fragrances. The reaction was developed by T. Brent Gunnoe of the University of Virginia and coworkers (J. Am. Chem. Soc.
▸ New route to unbranched alkenyl and alkyl arenes A new catalytic reaction combines arenes with α-olefins to selectively make unbranched alkenyl and alkyl arenes, com-
CH2R´
CH2R´
R +
Rh catalyst
CH2R´
R = H, CH3, Cl, OCH3 R´= H, CH2CH3, C(CH3)3
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C&EN | CEN.ACS.ORG | APRIL 24, 2017
Hydrogenation
+
Cu(II) oxidant
R R Unbranched alkenyl arenes
2017, DOI: 10.1021/jacs.7b01165). The researchers use a rhodium salt catalyst without an added ligand and a Cu(II) oxidant to functionalize benzene, toluene, and other arenes with α-olefins such as propylene and 1-hexene to form unbranched alkenyl arenes. The products can be hydrogenated to make unbranched alkyl arenes, which are not accessible by the acid-catalyzed routes traditionCH2R´ CH2R´ ally used industrially to make alkyl arenes. One challenge for the reaction is the high cost of rhodium, but Gunnoe + believes recovering and recyR cling the metal could keep the R expense reasonable.—STU BORMAN Unbranched alkyl arenes
CREDIT: SANIL GEORGE & JESSICA SHARTOUNY (FROG), DAVID HOLTHAUSEN (VIRUS)
Frog mucus peptide kills flu virus
▸ Forging olefins via decarboxylation
