Spotlights Cite This: J. Am. Chem. Soc. 2017, 139, 16997-16998
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Spotlights on Recent JACS Publications
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SURPRISING INSIGHTS INTO POLYETHYLENE CATALYST FROM IN SITU TECHNIQUES The Phillips catalyst is widely used for ethylene polymerization, a commercialized process for production of polyethylene. The active form of this catalyst is obtained upon reduction of the metal sites following exposure to ethylene at ca. 100 °C, while the nature of the reduced metal sites has been unclear. There has long been proposed to feature low-coordinate, low-valent chromium (Cr) and formaldehyde has been thought to be the major byproduct of ethylene oxidation. Caterina Barzan, Elena Groppo, and co-workers have turned this conventional wisdom on its head by demonstrating that the active site is divalent, 6coordinate Cr and that methylformate is a major byproduct, resulting from disproportionation of incipient formaldehyde (DOI: 10.1021/jacs.7b07437). A longstanding challenge in studying the active catalyst is that ethylene serves as both reductant and monomer. The authors overcome this difficulty by using operando spectroscopic techniques, coupled with theory, that enable in situ analysis of the active catalyst and subsequent onset of polymerization. Experimental results strongly suggest CrII, ligated by oxidation byproducts, mostly methylformate, is active in polymerization. This study highlights the overlooked relevance of Cr site structure and ligand sphere in polyethylene catalysis, which may be of particular significance in directing the design of improved olefin polymerization catalysts. Katie Meihaus, Ph.D.
SPECTROSCOPIC DETECTIVE WORK UNRAVELS RESPIRATORY MYSTERY One of the most important reactions in biologythe driving of protons across the mitochondrial membrane to create the motive force that makes ATPis also one of the most poorly understood. Mitochondrial complex I is a redox-coupled proton pumping enzyme that contributes to the proton motive force and is essential for respiration. Scientists still have not been able to pin down the “energy-coupling site,” the exact location where redox reactions initiate the transfer of protons across the membrane, despite the existence of detailed structures of the mitochondrial enzyme. One complicating factor is that there are seven electron-shuttling iron−sulfur clusters in this complex enzyme. Scientists have homed in on one of the Fe−S clusters in particular, called cluster N2, as a possible energy-coupling site. To investigate this possibility, Maxie Roessler and colleagues perform EPR spectroscopy on the complex, identifying two exchangeable protons in the vicinity of N2, one of which is coupled to a histidine residue (DOI: 10.1021/jacs.7b09261). The researchers find that the histidine protonation state is related to the redox state of the N2 cluster. However, the magnitude of the relationship is so weak that N2 cannot be the energy-coupling site. This work is an important step toward understanding how complex I facilitates proton translocation during respiration to store energy. Erika Gebel Berg, Ph.D.
BETTER CONTROL OVER FULLERENE FUNCTIONALIZATION Fullerenes are molecules of carbon (C60) that can take on many different hollow forms, such as sphere, tube, and ellipsoid. Researchers are interested in developing synthetic methods for the functionalization of fullerenes for possible applications in electronic devices and biomedicine. Conventional methods for creating highly functionalized fullerenes involve direct, sequential addition. Introducing functional groups at specified positions often requires the use of tethers/linkers, with varying degrees of success, due to the challenge of controlling the degree of functionalization and pattern. Yasujiro Murata and colleagues report a more efficient and reliable procedure for the functionalization of C60 in a controlled fashion (DOI: 10.1021/jacs.7b09459). The team uses palladium-catalyzed cyclization to afford C60 derivatives fused with several different arenes: naphthalene, pyrene and naphthalimide. They are able to demonstrate that by modification of the electronic structure of substrates, the regioselectivity of the π-functionalization of C60 can be controlled. The method may be applicable to the creation of additional π-functionalized C60 derivatives, which could lead to new structural motifs in nanocarbon chemistry. Christine Herman, Ph.D.
RAPID SYNTHESIS OF PANCRATISTATINS STARTING FROM BENZENE The pancratistatins are a family of alkaloids with bioactivity that makes them interesting clinical leads. These molecules kill cancer cells in vitro with minimal damage to health cells. They are also potent antivirals, particularly for Japanese encephalitis, a disease with no other known anti-infective small molecule. There has been tremendous interest to synthesize pancratistatins chemically. However, previous methods have only been able to prepare milligrams of the molecules. David Sarlah and colleagues report a scalable and concise synthesis of (+)-pancratistatin in merely 7 steps with yields up to several grams (DOI: 10.1021/jacs.7b10351). Sarlah and his team start with benzene and build the molecule by functionalizing each double bond. The key step is an asymmetric dearomative carboamination of benzene that establishes the trans relationship of two vicinal stereocenters in the aminocyclitol core and can produce greater than 10 g of the functionalized diene in a single batch. Five more steps produce (+)-7-deoxypancratistatin in gram amounts which could then be oxidized directly to (+)-pancratistatin. The strategy of functionalizing a double bond using dearomative
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© 2017 American Chemical Society
Published: November 29, 2017 16997
DOI: 10.1021/jacs.7b12155 J. Am. Chem. Soc. 2017, 139, 16997−16998
Journal of the American Chemical Society
Spotlights
carboamination could be applied to making other useful natural products as well. Melissae Fellet, Ph.D.
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CHIRAL ADDITIVE ENABLES QUANTIFICATION OF CARBON NANOTUBE ENANTIOMERIC PURITY The physical and electronic properties exhibited by single-wall carbon nanotubes (SWCNTs) have excited much interest toward their potential use in diverse technologies, although it is not yet possible to synthesize these nanostructures with the required enantiomeric purity (EP). Though progress has been made in the development of techniques for enantiomer separation and differentiation, it remains a challenge to quantify sample EP, because any two enantiomers will generally exhibit indistinguishable optical transitions. Hiromichi Kataura and colleagues report an effective method for quantifying EP based on the circular dichroism (CD) signal of samples containing two SWCNT enantiomers, which are distinguished by their interaction with chiral flavin mononucleotide (FMN) (DOI: 10.1021/jacs.7b09142). The authors find that the two enantiomers in FMN solution exhibit distinctions in their absorption, emission, and CD spectra, due to their differing interactions with the chiral additive. Further analysis shows a linear relationship between the normalized CD signal intensity and the enantiomer ratio, which enables evaluation of EP of any mixture of these two enantiomers with high accuracy. The method should be broadly applicable to establishing EP for other pairs of SWCNT enantiomers. Katie Meihaus, Ph.D.
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DOI: 10.1021/jacs.7b12155 J. Am. Chem. Soc. 2017, 139, 16997−16998
