Noteworthy Chemistry

Make an aziridine in situ to overcome an impurity problem. M. Michida and co-workers at Daiichi Sankyo (Kanagawa, Japan) developed a route for scaling up the synthesis of a chiral aziridine intermediate. The process worked well until it reached the step for converting the bromo nitrobenzenesulfonate (Ns) precursor (1) to the aziridine (2). Attempts to convert 1 to 2 gave modest yields at best, and a dimeric impurity was formed during the reaction.

The authors came up with an alternative strategy: directly converting 1 to 3, the ring-opened compound that follows 2 in the synthetic sequence. In this process, aziridine 2 is formed in situ. Treating 1 with amine 4 and K2CO3 gave 3 with low levels of the dimeric impurity.

Na2CO3 can also be used as the base, but only if a small amount of water is added. Product 3 was isolated in 67.4% yield. It contained ≈7.5% of a diastereomer that can be removed by crystallization after the next step in the sequence. (Org. Process Res. Dev. 2013, 17, 1430–1439; Will Watson)

Stormwater runoff gives chemical clues to its origin and history. The concentration and characteristics of dissolved organic matter (DOM) in runoff water are determined by the original source of the organic material and biogeochemical processing that occurs during transport through watersheds. This information can serve as important indicators of the health of an ecosystem. Although the types and amounts of DOM from forests and agricultural land runoff have been studied extensively, recent studies indicate that these results are probably not applicable to runoff from urban areas.

S. P. McElmurry*, D. T. Long, and T. C. Voice at Wayne State University (Detroit) and Michigan State University (MSU, East Lansing) evaluated the effects of land cover, solution chemistry, climate, and other factors on DOM characteristics in runoff water from urban and suburban areas. They collected 146 samples from 48 locations in central Michigan’s Grand River watershed within 24 h of runoff events such as heavy rainstorms. Of these sites, 29 receive more than 95% of their runoff from ground sources with a single type of land cover.

The authors compared runoff from forested and agricultural lands with that from golf courses, parking lots, and storm sewers on MSU's main campus. The sewer water came primarily from parking lots, rooftops, and landscaped areas dominated by grass lawns. Environmental data, including precipitation and solar radiation, were collected from a weather station on the MSU campus. They used statistical analysis to identify correlations among the various examined factors and to determine which factors had the most influence on DOM concentrations and characteristics.

The authors conclude that urban landscapes produce DOM with lower molecular weights and higher aromaticity than more natural landscapes. Urban and suburban landscapes have significantly different hydrologic flow paths, which are likely to influence the characteristics of DOM in the respective runoff waters. Land cover and environmental factors combine to determine DOM characteristics, so studies of runoff from natural environments should not be extrapolated to draw conclusions about runoff from paved surfaces and storm sewers.

The authors note that significant changes to urban infrastructure and land use practices will be required if urban runoff water is to be restored to a state that more closely resembles natural runoff water. (Environ. Sci. Technol. 2014, 48, 45–53; Nancy McGuire)

Is lithosphere carbon exchange really slow? The global carbon cycle is a process that encompasses the circulation of carbon through the atmosphere, the oceans, and the lithosphere. The cycle has significant implications on Earth’s biosphere and human activities. The lithosphere is the largest carbon reservoir, but the movement of carbon to and from the other reservoirs (mostly in the form of sedimentary carbonate) via weathering and metamorphic events is considered to occur on a multimillion-year time scale.

S. Ishihara, N. Iyi, and co-workers at the National Institute for Materials Science (Ibaraki, Japan) studied the carbon exchange of a layered double hydroxide (LDH). They found that CO2 “breathing” in the LDH is dynamic and might indicate a rapid process in carbon cycles that involve the lithosphere.

LDHs are a type of hydrotalcite in which carbonate anions are intercalated between the hydroxide layers. The authors prepared a 13C-labeled LDH, Mg3Al(OH)8(13CO3)0.5·mH2O (13C-LDH), from Mg3Al(OH)8Cl·mH2O by using an anion-exchange reaction with 13C-labeled Na2CO3 in water. The isotopic shift from 12C-LDH to 13C-LDH in the C–O stretching vibration frequency allowed the authors to use IR spectroscopy to quantitatively monitor the CO2 exchange between 13C-LDH and the atmosphere.

When the sample of 13C-LDH was placed in 24 ºC air at a relative humidity of 50%, the half-life for CO2 exchange was estimated to be ≈24 h (see figure). Non-intercalated Na213CO3, however, did not appreciably exchange CO2 with its surroundings even under 100 kPa CO2 at 150 ºC for 3 days. The exchange rate of 13C-LDH decreased as the relative humidity increased, most likely because of the interlayer vacancies that formed at low relative humidity.

Dynamic exchange of LDH carbonate anions with atmospheric CO2

These results suggest that various naturally occurring hydrotalcite-like clay minerals may make the lithospheric carbon cycle more dynamic than previously thought. They may change the current perception of the global carbon cycle. In addition, LDHs may have uses such as the separation and catalytic conversion of CO2. (J. Am. Chem. Soc. 2013, 135, 18040–18043; Xin Su)

Porous, cellularized scaffolds are not quite ready for ligament repair. A. S. Goldstein and coauthors at Virginia Tech (Blacksburg), Virginia Tech/Carilion School of Medicine (Roanoke), Rensselaer Polytechnic Institute (Troy, NY), and Vanderbilt University (Nashville) prepared composite tissue engineering scaffolds (acellular and cellular) with enhanced porosity with the objective of repairing damaged ligaments. They infiltrated electrospun meshes of poly(lactic-co-glycolic acid) (PLGA) or poly(ester-urethane urea) (PEUUR) with a cross-linkable, biocompatible, functionalizable poly(ethylene glycol) diacrylate (PEGDA). Cells were evenly distributed into the elastomeric scaffolds via a hybrid electrospinning–electrospraying process.

The acellular elastomeric hydrogel composites required the addition of ≈5 wt% PEGDA into the electrospinning solutions to achieve uniformity. A 21% increase in tensile modulus accompanied PEGDA infiltration into the PEUUR mesh, but in the PLGA composites the modulus decreased by 40%.

The authors observed significant hysteresis in the PLGA and PEUUR scaffolds that tapered with repeated mechanical cycling. PEUUR-based composites performed better under physiological conditions.

Unfortunately, significant cell proliferation was not achieved, possibly because the cells had limited access to nutrients, and their migration and adherence were impeded. In addition, the cellularized mesh–hydrogel composites were not mechanically matched to the intended ligaments of interest. (Biomacromolecules 2014, 15, 75–83; LaShanda Korley)

These electronic nanodevices are orthogonally modulated. Single external stimuli often are used to control the performance of a molecular device. Few molecular devices, however, take advantage of synergetic modulation by different stimuli. S. Rigaut at the University of Rennes 1 (France), X. Chen at Nanyang Technological University (Singapore), and coauthors designed and synthesized a family of organoruthenium fragment–modified dithienylethylenes (DTEs). They used the DTEs to develop the first orthogonally photo- and electro-modulated molecular devices by covalently incorporating the functional molecules into chemically synthesized nanogaps with precise size control.

The photochromic DTE units determine the molecular switching by isomerization between a π-conjugated closed state and a nonconjugated open state. The ruthenium moieties provide metal-promoted electrocyclization of the DTEs at low potentials, which ensures the stability of the Au–S bonds between the functional molecules and two gold nanoelectrodes.

The unique electronic structure of the long molecule that consists of two DTE units and three ruthenium fragments offers stepwise control of molecular transport junctions. These photo–electro cooperative molecular devices may provide a suitable platform for conducting Boolean computation. (Nat. Commun. 2014, 5, No. 3023 DOI: 10.1038/ncomms4023; Ben Zhong Tang)

Why is star fruit neurotoxic? In 2008, several individuals with chronic kidney disease (CKD) were poisoned by eating a tropical fruit called star fruit (Averrhoa carambola). The symptoms were hiccups, mental confusion, and prolonged seizures.

N. Garcia-Cairasco, N. P. Lopes, and co-workers at the University of São Paulo (Brazil) used a bioguided method to isolate the toxin in star fruit, a molecule they named caramboxin (1). It has a phenylalanine-like structure with two carboxylic acid groups. The structure was characterized by standard techniques (NMR and MS).

The authors microinjected crude star fruit extract or isolated caramboxin intracerebrally into the hippocampus of Wistar rats, obtained electroencephalograms (EEGs), and prepared hippocampal slices. The results showed that the behavioral, EEG epileptiform, and neurotoxic effects of ingesting star fruit are not caused only by the increase of oxalate concentrations in the brain, as first thought; the effects also are caused by the caramboxin, which has strong glutamatergic receptor agonist properties that produce brain hyperexcitability. The epileptiform effect of caramboxin could be significantly attenuated by injecting the animals with diazepam. (Angew. Chem., Int. Ed. 2013, 52, 13067–13070 DOI: 10.1002/anie.201305382; José C. Barros)

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