MATERIALS
Liquid metals went to work Unusual properties of gallium alloys opened a door to stretchable electronics and soldering without heat Liquid metals, which have long been treated as scientific curiosities, drew renewed attention this year with novel applications, for example in flexible, stretchable electronics for wearable and implantable devices. This small group of materials mainly includes gallium and a few of its alloys. On exposure to air, the liquid spontaneously forms a thin oxide skin that mechanically stabilizes droplets and arbitrary patterns that researchers create. If the material is jostled, the skin breaks and the metal flows momentarily until the skin re-forms around the liquid. A team led by Michael D. Dickey of North Carolina State University took advantage of this unusual property to make 10-µm-wide polymer-encased wires of eGaIn, a eutectic mixture of gallium and
indium that’s a liquid at room temperature. Unlike ordinary wires, the ones made with eGaIn can easily be stretched, bent, and shaped while maintaining electrical conductivity (Extreme Mech. Lett. 2016, DOI: 10.1016/j.eml.2016.03.010). In another example from this year, Stéphanie P. Lacour and coworkers at the Swiss Federal Institute of Technology, Lausanne (EPFL), devised a method for making a two-phase material consisting of solid AuGa2 clusters interspersed with microscopic liquid gallium droplets. They used the material to fabricate stretchable devices containing stacked layers of LEDs and sensors embedded in gloves that can track the subtle motions of fingers (Adv. Mater. 2016, DOI: 10.1002/adma.201506234). Martin Thuo’s group at Iowa State Uni-
Dickey and coworkers create random patterns by dispensing a gallium-based liquid metal from a nozzle. The metal shapes are stable and free-standing thanks to a thin oxide shell that forms spontaneously in air. versity exploited the spontaneously forming oxide skin of bismuth-indium-tin and related alloys to keep microscopic liquid metal droplets from solidifying, even at temperatures below their melting points. The researchers showed that applying a gentle force to the droplets breaks the shells, causing the metal to briefly flow before the skin re-forms. They used that property to bond metal parts together at room temperature, in effect soldering without electricity or heat (Sci. Rep. 2016, DOI: 10.1038/srep21864).—MITCH JACOBY
C–H ACTIVATION
Methylene activation reached new heights An organic synthesis method that took flight this year could prove to have an unusually wide wingspan. Developed by Jin-Quan Yu and coworkers at Scripps Research Institute California after a 14-year effort, the reaction advances a long-standing goal: activating specific C–H bonds in organic compounds and converting them catalytically and enantioselectively into C–C bonds or other derivatives. Since there’s no shortage of C–H bonds in organic compounds, the technique has
lots of potential for widespread applicability. Specifically, the new reaction uses a palladium catalyst and quinoline-based ligand to convert β-methylenes—CH2 groups two carbon atoms away from amides or carboxylic acids—into chiral centers (Science 2016, DOI: 10.1126/science.aaf4434). Synthetic organic chemists had previously developed ways to activate several types of C–H bonds, such as by preinstalling activating groups in substrates. But the ability to target C–H bonds of unactivated methylene groups and In the new reaction, a palladium catalyst with a chiral then derivatize them cat-
ligand activates a 𝝱-methylene C–H bond in an amide starting material to enantioselectively form an aryl product. Ar
H H O R1
NHAr
ArI, Pd catalyst, ligand
R1 = alkyl or aryl, Ar = aryl
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R1
R2
N
O
AcHN
NHAr Ac = acetyl, R2 = H or ethyl
Ligand
alytically and enantioselectively had been largely unsolved. “Yu’s trailblazing group has managed to bring to reality what might have been considered seemingly impossible,” commented Erick M. Carreira, an expert on asymmetric synthesis at the Swiss Federal Institute of Technology (ETH), Zurich. Since the paper came out, Yu and his coworkers have been developing their approach further by extending it to create chiral centers near other functional groups, such as alkyl amines. A number of applications for the β-methylene reaction have already been identified at Bristol-Myers Squibb and another pharmaceutical company, “but optimization is needed to improve the yield for the complex substrates at hand,” Yu says. “We are negotiating to license this technology to a chemical development company.”—STU BORMAN
CREDIT: MICHAEL DICKEY/NCSU
Technique that enantioselectively derivatizes targeted C–H bonds could have widespread applications
