Noteworthy Chemistry

June 2, 2014



Photoreduce zinc ion to the metal. Photoreduction, the use of light to convert metal ions to their elemental forms, is rarely studied but potentially useful, especially for highly reactive metals such as zinc, aluminum, and lithium. These reactions have potential applications in synthetic chemistry, solar cells, and metal coatings.

A. C. Brooks, K. Basore, and S. Bernhard* at Carnegie Mellon University (Pittsburgh) previously showed that Zn(II) can be reduced to zinc metal with an Ir(III)-based visible-light catalyst. They have now replaced the iridium with an organic catalyst, an improvement that avoids using expensive, toxic noble-metal complexes.

After they reviewed the literature on zinc-based quinolate complexes, the authors postulated that Zn(II)–8-hydroxyquinoline (Hq) complexes could catalyze the reduction reaction. They screened a wide range of substituted Hq ligands before they identified 5,7-dichloro-8-hydroxyquinoline (1; see figure) as the best option. Under 465-nm LED light irradiation, 3 mmol ZnCl2, 0.1 mol% 1, and triethylamine (a sacrificial electron donor) in acetonitrile solvent yielded ≈140 μmol zinc metal in 68 h.

Catalyst for photoreduction of Zn(II)

Zn(II) salts must contain halide anions for the reaction to proceed. Salts with other counterions such as sulfate, acetate, and tetrafluoroborate failed to produce a significant amount of zinc metal. Acetonitrile and stoichiometric quantities of water are also required.

The authors propose a mechanism in which a Zn(II)–1–acetonitrile complex (2), formed in situ, is the catalytically active intermediate. LED irradiation energizes the complex to an excited state, which is reductively quenched by triethylamine. Then, in dark reactions, electrons supplied by triethylamine reduce Zn(II) to a chloride-bridged dinuclear Zn(I) intermediate, which disproportionates to elemental zinc and Zn(II).

This reaction may eventually be used in zinc-coating processes, especially galvanization. (Chem. Commun. DOI: 10.1039/c3cc47633b; Xin Su)

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These tellurophenes are phosphorescent in the solid state at room temperature. Developing materials with efficient phosphorescence in the solid state at room temperature is challenging, but important for advancing the state of organic LED (OLED) technology.  Two difficulties must be overcome: Molecules that emit light in solution do not usually luminesce in the solid state, and phosphorescence is normally inefficient at room temperature.

K. Shankar, E. Rivard, and co-workers at the University of Alberta (Edmonton) report a breakthrough in solid-state room-temperature phosphorescence. They “decorated” a tellurophene core with multiple pinacolboronate units to form products that have exotic photophysical behavior. For example, compound 1 in the figure is nonluminescent when it is dissolved in an organic solvent. But it becomes emissive when, under ambient conditions, its molecules aggregate in an aqueous medium or form solid films.

This is a new example of aggregation-induced emission (AIE). The lifetime of its excited state is long, which verifies that the AIE is phosphorescent. Similar phenomena are observed for compounds 2 and 3, indicating that AIE is a general feature of this type of luminogen.

By conducting a series of control experiments, the authors showed that the Te–B interaction in the tellurophenes plays an important role in the AIE process. They demonstrated the utility of these luminogens by using them to reversibly sense volatile organic compounds (VOCs). Future applications include integrating the film-forming luminogens into host-free optoelectronic devices such as OLEDs. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201307373; Ben Zhong Tang)

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Keep the next step in mind when you choose a peroxide quench agent. Glucokinase, which occurs naturally in the human body, triggers shifts in metabolism and cell function in response to rising or falling glucose levels. The synthesis process for one potent glucokinase activator compound requires an oxidation step, followed by the addition of a quench agent to remove excess peroxides after the oxidation is complete.

The oxidation of ethyl 4-(cyclopropylthio)phenylacetate by oxone (2KHSO5·KHSO4·K2SO4) gives the corresponding sulfone, which is a key intermediate in the synthesis of this glucokinase activator. T. Yamagami and co-workers at Mitsubishi Tanabe Pharma Corp. (Osaka, Japan) found that the choice of quench agent for the oxidation reaction was critical to the purity of the product from the subsequent alkylation step.

In the laboratory, the authors used sodium thiosulfate (Na2S2O3) as the quench agent. The longer contact times in the pilot-scale reactions, however, resulted in the formation of elemental sulfur from Na2S2O3 decomposition under the mildly acidic conditions. The sulfur remained as a contaminant in the sulfone, allowing sulfide impurities to form during the base-mediated alkylation step.

The researchers avoided this problem by using sodium hydrogen sulfite (NaHSO3) as the quench agent for the oxidation reaction. (Org. Process Res. Dev. DOI: 10.1021/op400354g); Will Watson

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Laser pulses make quick, economical surface holograms. Holograms in the visible and near-infrared region offer great promise in security, display, and data-storage applications. The lack of an easy-to-use, inexpensive, and quick method for producing them, however, has limited their practical application. The most commonly used methods for producing holograms often require multiple steps and processing chemicals.

F. da Cruz Vasconcellos, C. R. Lowe, and co-workers at the University of Cambridge (UK) used a single laser pulse to form holographic surface gratings in ink printed onto substrates. The entire hologram production process can be completed within a few minutes, and it can be used to produce two- and three-dimensional holograms on optically transmissive and opaque surfaces. Some examples are shown in the figure. The method requires no photochemical processing.

Phosphorescent boronated tellurophenes
Examples of surface holograms

The researchers printed or spin-coated permanent black ink (with blue, yellow, and red components) onto a flexible plastic substrate. They used a 6-ns pulse from a Nd:YAG laser, with a spot size 1 cm in diameter, to locally heat the coating, which evaporated from the surface in a holographic pattern. They also developed mathematical models to test the effects of various configurations before trying them in the lab.

Reducing thermal diffusion in the light-absorbing material produces greater resolution in the pattern, which can have features comparable in size to the wavelength of the laser light used to produce it. The light-absorbing material can be printed onto the substrate by using inkjet printing, spin coating, stamping, or screen printing. Holograms produced on a transparent substrate are visible from both sides of the substrate.

The authors note that their method can be used with laser light–absorbing inks, dyes, nanoparticles, metals, and biomolecules. They envision that this technology will be integrated into desktop printers or other widely available devices. (ACS Photonics DOI: 10.1002/ph400149m; Nancy McGuire)

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Inhibiting glutamine blocks vaccinia virus protein synthesis. Viruses use the metabolism of the cells they infect to acquire energy and synthesize the macromolecules required for their replication. Viral infection has been shown to alter host metabolic pathways; numerous viruses affect glycolysis activation, which suggests that glucose is a critical source of carbon during viral infections.

Glucose is typically the main carbon donor for cellular adenosine triphosphate (ATP) through its oxidation via glycolysis and the tricarboxylic acid (TCA) cycle. Yet in a process called anaplerosis, glutamine can replenish the TCA cycle if carbon atoms from glucose are unavailable. This process occurs in most cancer cells and in cells infected by human cytomegalovirus. Studies of the impact of glucose and glutamine deprivation on the production of three viruses found that both carbon sources are required for optimal viral replication.

K. A. Fontaine, R. Camarda*, and M. Lagunoff at the University of Washington (Seattle) examined alterations in host cellular metabolic pathways during vaccinia virus infections. Vaccinia virus is a member of the poxvirus family, large DNA viruses that uniquely replicate in the cytoplasm of infected cells rather than the nucleus.

The authors performed a global metabolic screen of infected primary human cells and were surprised by the results: Depriving infected cells of glucose had no significant impact on viral replication, but starving them of glutamine from outside sources reduced the production of infectious virus by ≈90%. When they compared this finding with the results of the uninfected experimental controls, it became clear that viral infection caused the differential metabolic changes, making glucose dispensable and glutamine imperative for virus production.

The authors’ data suggest that vaccinia virus alters glutamine utilization in infected cells. Through glutaminolysis, the virus induces full dependence on glutamine metabolism in the TCA cycle to generate ATP molecules and sustain efficient viral replication. The viral yield in glutamine-deprived cells was “rescued” by adding TCA cycle intermediates.

The authors observed that glutamine is not required for transcription or virion maturation, but it is needed for protein synthesis. The lack of glutamine leads to a decrease in infectious viral particles. This research suggests that glutaminolysis inhibitors may be viable therapeutics against poxvirus infection. (J. Virol. DOI:10.1128/JVI.03134-13; Abigail Druck Shudofsky)

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A pinch of salt helps convert fructose to 2,5-diformylfuran. 2,5-Diformylfuran (DFF) is an important renewable platform chemical. It can be used for the green manufacture of fine chemicals, plastics, and polymers. Currently, however, most DFF is produced by the direct oxidation of hydroxymethylfurfural (HMF), which is not cost-effective. All of the existing processes are limited by the need for large amounts of catalyst and low yields.

B. Estrine, J. Le Bras, and coauthors at Agro-industrie Recherches et Développements (Pomacle, France) and the University of Reims Champagne-Ardenne (France) streamlined the HMF oxidation process. They produced DFF in one pot by using NaBr as a catalyst.

Because fructose can be degraded to HMF in dimethyl sulfoxide (DMSO) at high temperature, the researchers first studied the oxidation of HMF to DFF in DMSO. Heating HMF in DMSO at 150 °C for 18 h gave a 30% DFF yield; but when they added HBr or NaBr the yield increased to 85% (75% isolated). The two-step procedure is shown in the first line in the figure; compound 1 is DFF.

One- and two-step procedures for converting fructose to DFF

Based on this finding, the authors explored the one-pot transformation of fructose directly to DFF (second line in figure). When the two steps were combined, the yield remained almost unchanged at 65–67% for the NaBr-catalyzed reaction, but it dropped to 13% with the HBr catalyst. They believe that the decrease in yield is a result of HBr inhibiting the dehydration of fructose to HMF.

The authors suspect that acids produced by DMSO thermolysis catalyze the fructose dehydration. But the mechanism for the oxidation of HMF to DFF was rather elusive. Given that halogenated HMFs can be converted to DFF with 57−81% yields upon heating in DMSO, they suggest that a bromo-substituted HMF intermediate is involved in the oxidation step. (ChemCatChem DOI: 10.1002/cctc.201400023; Xin Su)

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