SYNTHESIS
MATERIALS
▸ Tropylium cation now serves as an organocatalyst
Polymer ribbons make waves in response to light
The tropylium ion is a curious seven-membered aromatic ring (C7H7+) used in organic synthesis and as a ligand for metal complexes. Chemists led by T. Vinh Nguyen of the University of New South Wales Sydney have shown for the first time that tropylium salts can act as organic Lewis acid catalysts (Green Chem. 2017, DOI: 10.1039/ c7gc01519d). Carbonyl compounds used in multistep reactions are often temporarily functionalized via acetalization reactions to mask their reactivity. This protection step typically relies on a metal salt Lewis acid catalyst. But residual metal can pose problems in purifying pharmaceutical products, leading to excessive use of solvents and generating waste. Organocatalysts that avoid metals are often a greener option. +
H R1
BF4-
H
Tropylium catalyst
O
ROH or HC(OR)3
R1
OR OR
C R E D I T: A DA PT E D FRO M NAT U RE ( R I B BO N ) ; CO R N I S H LA B ( BI O S EN S OR )
R = alkyl, R1 = various groups
Building on their prior research on tropylium chemistry, Nguyen and coworkers reasoned that the carbocation with one positive charge delocalized over the conjugated seven-membered ring could serve as a “soft Lewis catalyst” for protective acetalizations of aldehydes with alcohols and esters. The team also tested the method in a flow reactor system to save time and facilitate catalyst recycling, further reducing the environmental impact of the reaction. “This is one of the most interesting papers I have read in a long time,” says James H. Davis Jr. of the University of South Alabama, who studies ionic liquids and molecular salts. “I find the simplicity of the catalyst and its broad, metal-free scope of activity especially exciting.”—STEVE RITTER
ANALYTICAL CHEMISTRY
▸ Sensing fungus among us for less than a penny Inexpensive biosensors made from engineered yeast can detect 10 different fungi, including human pathogens, and can potentially identify other fungi, bacteria,
Researchers have developed a polymer ribbon that adopts continuous mechanical wavelike motions when illuminated with a fixed source of ultraviolet light. The wave motion can eject tiny objects, such as sand particles, from the film surface and transport a larger object, such as a rod, along the length of the film. The waves move toward light with the ribbon in one orientation and away from light when it is reversed. Azobenzene molecules in a polymer And the film can “walk” when atribbon isomerize to cis form in tached to a lightweight frame. Dirk light, shrinking the material, and J. Broer of Eindhoven University of isomerize back to trans in shade, causing expansion. The result is Technology and coworkers devised the new material (Nature 2017, DOI: directional (red arrow) wavelike motions (blue arrows). 10.1038/nature22987). A possible application is self-cleaning solar cells that remove sand and dust particles that accumulate on their surfaces. The film, a liquid-crystal network, contains a light-sensitive azobenzene dye that isomerizes from trans to cis conformation in response to UV light, shrinking the material, and relaxes quickly back to the trans isomer in shade, causing expansion. Together, the isomerizations lead to directional wave motion. The liquid-crystal molecules are perpendicular to the surface on one side of the ribbon and parallel to it on the other, making the two sides respond to light differently and allowing wave direction to switch when the film is flipped.—STU BORMAN
and viruses. The biosensors could be put to work monitoring microorganisms that cause health problems, damage crops, or cause food to spoil (Sci. Adv. 2017, DOI: 10.1126/sciadv.1603221). Virginia W. Cornish, Nili Ostrov, Miguel Jimenez, and Sonja Billerbeck of Columbia University and coworkers, who developed and tested the biosensors, point out that current fungal tests, such as antibody and nucleic acid assays, often require specialized labs, refrigerated reagents, expensive instruments, and highly trained personnel. The new biosensor tests could cost less than a penny when mass-produced, the researchers estimate. The devices are dry, do not require refrigeration, and can be used
by anyone. To create the biosensors, the researchers removed the surface receptor protein that yeast use to detect pheromones from mating partners and replaced it with receptors responsive to pheromones from other fungi, such as candida, which causes human yeast infections. They engineered the yeast so activation of the receptor turns on a biosynthetic pathway for lycopene, the compound that makes tomatoes red. In the presence of candida pheromone, the yeast turns red, a visual signal of the fungus.—STU BORMAN
Dipstick yeast-based biosensors (left) are a few pennies in size and might cost less than a penny to make. The paper test strip (left, top) absorbs solution containing a pathogen’s pheromone, causing a square containing yeast engineered to detect the pathogen to turn red compared with a control square (right, bottom); scale bars = 0.5 cm. JULY 3, 2017 | CEN.ACS.ORG | C&EN
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