NEWS OF THE W EEK
METAL-FREE HYDROGENATIONS ORGANIC SYNTHESIS: Lewis acid-base pairs enable unprecedented reduction of anilines and other aromatics
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N A CHEMICAL FIRST, an international research
team has developed a metal-free reaction that hydrogenates aromatic rings to form cyclohexyl derivatives. The achievement could spark a broader range of applications for industrial hydrogenations, which are widely used for processing petroleum and foods. A metal-free aromatic hydrogenation is surprising, says team leader Douglas W. Stephan of the University of Toronto, because it’s exceedingly hard to overcome the additional stability a molecule gains from aromaticity, even with the best transition-metal catalysts. Stephan and his colleagues have done so by using a chemical construct known as a frustrated Lewis pair, which Stephan introduced in 2006. Lewis acid-base adducts are common in chemistry: An electron-deficient Lewis acid readily shares a Lewis base’s spare pair of electrons. However, when the Lewis
SINGLE-ATOM TRANSISTOR STM image shows a lithography mask used to incorporate a phosphorus atom at center pink spot and electrical leads at pink rectangular sites to create a single-atom transistor.
NANOELECTRONICS: Device’s performance bodes well for quantum computing
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N WORK THAT COULD ADVANCE the development
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of quantum computers, researchers have created a transistor that consists of a single atom positioned precisely between two electrodes in a silicon substrate. Quantum computers could perform some calculations not possible on current computers, such as solving the Schrödinger equation for large molecules. Quantum computing specialist Michelle Y. Simmons of the University of New South Wales, in Australia, and coworkers prepared the transistor. They used scanning tunneling microscopy, lithography, and phosphine chemistry to place, with single-lattice-site spatial accuracy, an individual phosphorus atom between electrodes in a silicon device (Nat. Nanotechnol., RT
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For the researchers’ take on the significance of their single-atom transistor, visit cenm.ag/trans. WWW.CEN-ONLIN E .ORG
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acid and base have bulky substituents, their ability to form a close relationship is denied, causing the pair to become “frustrated.” But the pair still garners penned-up reactivity, comparable with that of an organometallic catalyst. Several research groups have shown that frustrated Lewis pairs can intercept and split H2 during an electron tug-of-war and subsequently hydrogenate compounds such as imines, silyl ethers, and N-heterocyclic compounds. Stephan’s group in collaboration with computational chemist Stefan Grimme at the University of Bonn, in Germany, tackled the hydrogenation of aromatics by using B(C6F5)3 as the Lewis acid and various anilines as the Lewis base (J. Am. Chem. Soc., DOI: 10.1021/ ja300228a). When H2 is added, the frustrated Lewis pair splits H2 and then reductively adds hydrogens to aniline’s aromatic ring to form cyclohexylammonium borate salts. Stephan says the salts could be easily deprotonated to release the free cyclohexylamines. Princeton University’s David W. C. MacMillan, an expert in metal-free organocatalytic reactions, says the reactivity of frustrated Lewis pairs “is conceptually really intriguing” and that the new chemistry “certainly makes one think differently about the notion of aromatic hydrogenations. All in all, this work points to the exceptional fertility of this area for new reactivity discoveries and for mechanistic explorations.”—STEVE RITTER
DOI: 10.1038/nnano.2012.21). Such precise positioning hadn’t been achieved before. Single-atom transistors could be combined to give integrated circuits of unprecedented density. But creating such transistors is painstaking, and the feasibility of making devices that comprise millions or billions of them is not yet known. The transistor operates only at close to absolute zero, also limiting applications for now. Nevertheless, the phosphorus transistor represents a step toward quantum computers. Quantum computers would achieve greater power and speed by encoding information in qubits, which adopt more states than just the two (0 and 1) in conventional computers bits. Precise atom positioning would be required to interrogate the information in qubits accurately. Device modeler Asen Asenov of the University of Glasgow believes the experimentation is “groundbreaking.” Molecular device fabricator Robert A. Wolkow of the University of Alberta believes that Simmons’ group and others reported substantially similar results earlier. Some of the study’s simulations have technical deficiencies, Asenov adds. Quantum computing expert Bruce E. Kane of the University of Maryland notes that the one-atom transistor is not currently practical for conventional devices, nor does it carry out quantum operations. But he calls the work “an experimental and engineering tour de force” and believes Simmons’group now has the requisite tools to begin building quantum computers“that would go beyond the current state of the art.”—STU BORMAN
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