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Noteworthy Chemistry

November 18, 2013

Natural wetland remediates acidic mine drainage. A. P. Dean and co-workers at the University of Manchester (UK) conducted a 14-year evaluation of a natural wetland along the southern Afon Goch river system (Anglesey, North Wales, UK) between the Parys Mountain copper mine and the Irish Sea (see map). They measured its ability to remediate highly acidic mine drainage (AMD). This well-established wetland has existed since the 1890s or earlier. 

Map of Parys Mountain mine and the southern Afon Goch showing sampling sites

The Parys Mountain mine was in continuous operation from the mid-18th century until 1911. It continues to discharge more copper and zinc to the Irish Sea than any other source. The discharges also account for the largest proportion of the total iron release nationally.

Natural wetlands may be highly effective for remediating AMD because they have adapted to the low-pH, high-metal environment of an AMD-polluted system over a long time. There are, however, no reported long-term evaluations of natural wetland remediation performance; and the biological composition and diversity of natural AMD wetlands are poorly understood.

Previous studies of natural wetlands indicated that they might release metal contamination downstream, but those studies were shorter in duration than this one. Although seasonal changes were not examined in this study, changes in flow rates were incorporated. Limitations in data collection prevented a full mass balance analysis in this study, but the data show that this wetland acts as a net sink for all dissolved metals before and after drainage diversion.

Between 1997 and 2010, the researchers sampled one upstream site, four downstream sites, and a control site. They sampled six sites in the wetland region (including two of the earlier sites) twice in the autumn of 2011.

Before 2003, the wetland retained 55, 64, and 37% of the dissolved iron, zinc, and copper runoff from the mine. In 2003, to reduce flood risk, the drainage from the mine was diverted from the southern Afon Goch to the northern Afon Goch, thereby decreasing the load on the wetland and allowing it to reduce the downstream metal content by 83−94%.

An associated pH increase from 2.7 to 5.5 (compared with 6.2 at the control site) provided long-term improvements to the downstream benthic invertebrate community. Invertebrate fauna sampling from 2008 on showed an increase in diversity, including pollution-intolerant taxa, at sites downstream from the wetland.

Although plant roots played a part in retaining dissolved metals, the main metal accumulation was in the sediment. Plant and microbial activity increases groundwater pH, which promotes precipitation of metal salts and complexation of metal ions with organic ligands in the sediment. Thus, multiple interacting processes created an efficient, self-sustaining AMD remediation site in this wetland region. (Environ. Sci. Technol. 2013, 47, 12029–12036; Nancy McGuire)

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Select the right solvent for a two-phase biocatalytic reaction. (S)-1-(5-Fluoropyrimidin-2-yl)ethylamine is an intermediate in the synthesis of a Janus kinase 2 inhibitor. R. E. Meadows, K. R. Mulholland, M. Schürmann, and coauthors at DSM Innovative Synthesis BV (Geleen, The Netherlands) and AstraZeneca (Macclesfield, UK) prepared this compound via an enzyme-catalyzed transamination reaction with (S)-α-methylbenzylamine as the amine donor.

Attempts to increase the volume efficiency of the reaction by increasing the starting ketone concentration from 140 mM to 350 mM, however, led to incomplete conversion. The authors showed that stoichiometric byproduct acetophenone significantly inhibits the enzyme.

The authors investigated the use of an organic solvent in a biphasic system to solve this problem. Most solvents, including toluene, heptane, isooctane, n-butyl ether, and tert-butyl acetate, led to an increased reaction rate. But 2-methyltetrahdyrofuran and ethyl acetate inhibited the enzyme and completely quenched the reaction. (Org. Process Res. Dev. 2013, 17, 1117–1122; Will Watson)

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This maleimide luminogen emits efficiently in the solid state. Organic molecules with many structural motifs (e.g., silole, tetraarylethylene, bisstyrylanthracene, biarylethylene, diaryldistyrylbenzene, diphenyldibenzofulvene, borondipyrromethene, and triazole) exhibit the aggregation-induced emission (AIE) effect. M. Nagaraj, S. Muthusubramanian, and coauthors at Madurai Kamaraj University (Madura, India) and Texas A&M University (College Station) added a new member to the AIE family.

Maleimide derivative that shows AIE

The “AIEgen” is maleimide derivative 1, which is soluble in THF but insoluble in water. In THF solution, 1 emits weakly, with a quantum yield of 0.024. Its emission intensifies when water is added to the THF solution to form aggregates of 1

In a 1:9 THF/H2O mixture, the luminogen’s emission is enhanced by 240-fold. Crystal structure analysis shows that the AIE activity is caused by the restriction of intramolecular rotation of the phenyl rings attached to the nitrogen atoms. (RSC Adv. 2013, 3, 22246–22252; Ben Zhong Tang)

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Pure inorganic salts make highly porous gels. Gels are highly dilute, cross-linked supramolecular networks formed by noncovalent interactions. Their primary forms are hydrogels and organogels. Gels are generated when solvent molecules are trapped inside matrices formed from fibrils. Most hydrogels have carbon in their skeletons; however, T. Pal and colleagues at the Indian Institute of Technology (Kharagpur) produced highly porous inorganic gels from silver salts.

The authors first prepared a gel formed from AgNO3, with NH4VO3 as the gelator. About 1100 water molecules were trapped per gelator molecule at the minimum gelation concentration of 5 × 10–3 M. This gel was highly thermally stable and retained its morphology up to 150 ºC under hydrothermal conditions. The dried xerogel had an X-ray diffraction pattern consistent with that of monoclinic β-AgVO3, a total surface area of 523 m2/g, and a total pore volume of 3.966 cm3/g.

The authors generated similar gels from AgOAc and Ag2SO4 with Na3VO4 as the gelator. Other transition metal salts, however, failed to form vanadate-induced hydrogels. The authors believe that Ag–Ag interactions and the formation of polymeric metavanadate are responsible for forming the interlaced fibrous network that can accommodate large volumes of water. (Chem. Commun. 2013, 49, 9428–9430; Xin Su)

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Transform azides into diazo compounds in water. Diazo compounds are versatile intermediates in organic chemistry. Few methods, however, are available for preparing this class of compounds; and the preparation usually requires harsh conditions.

H.-H. Chou and R. T. Raines* at the University of Wisconsin–Madison developed a procedure to prepare diazo compounds from azides in water at neutral pH. The procedure was based on an earlier method for preparing diazo compounds by the reaction between azides and phosphino esters followed by the decomposition of the resulting acyltriazenes. They prepared several phosphino esters, including ester 1 with methoxyethoxymethyl (MEM) substituents, which is soluble and stable in water. 

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Diazotization of a model azide

The authors then studied the reaction of a model azide, α-azido-N-benzylacetamide (2) and phosphino ester 1 at several pH levels in water–organic solvent mixtures. They obtained the best results at neutral pH with no organic solvent. The diazo product was isolated in 91% yield.

Several glycine-derived azides were converted to diazo compounds with good yields despite their potential instability under the reaction conditions. These azides contained

  • acetal groups (prone to hydrolysis);
  • styrene groups (susceptible to polymerization); or
  • electrophilic groups such as aldehydes, α-chloro esters, disulfide bonds, and epoxides that are in principle prone to nucleophilic attack by the phosphine ester.

The authors also tested their technique in reaction media that contained oxidized L-glutathione or bovine pancreatic ribonuclease A to illustrate the potential for biological applications. The biological activity of the substrates was preserved in the products. The method is simple, tolerant to functional groups, and suitable for widespread use in organic synthesis and chemical biology. (J. Am. Chem. Soc. 2013, 135, 14936–14939; José C. Barros).

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Porphyrin synthesis goes green in ionic liquids. Porphyrins, a class of organic compounds that have biological and chemical significance, are usually prepared with acids in halogenated solvents that produce large amounts of chemical waste. Also, the use of propionic acid as the catalyst in the Adler synthesis, one of the most common porphyrin preparation methods, generates acid waste and consumes large amounts of energy.

S. Kitaoka and coauthors at Kinki University (Higashihiroshima, Japan) and Oita University (Japan) report a synthetic protocol for producing tetraphenylporphyrin (TPP) that uses recyclable ionic liquid (IL) media. The authors used imidazolium-based acidic ILs to synthesize TPP. The procedure minimizes waste production without reducing the yields obtained by conventional methods.

The authors screened three types of ILs by heating PhCHO and pyrrole at 120 ºC for 1 h (see figure). With ILs [HC1im][CF3CO2] and [HC4im][CF3CO2], TPP was produced in yields of 8 and 15%, respectively. The 15% yield was the same as obtained from the Adler synthesis. Other ILs, except for [HC4im][BF4] (9% yield), failed to catalyze the reaction.

TPP synthesis with ILs

The authors also demonstrated the recyclability of the IL media. For example, [HC4im][CF3CO2] can be used up to four times with a recovery rate of 92%. The yield drops only to 14% in the last cycle.

Purifying TPP requires only extraction and flash column chromatography. This protocol capitalizes on ILs’ properties to provide a green synthetic route to porphyrins, with high energy efficiency after optimization. (Chem. Lett. 2013, 42, Advance Publication; Xin Su)

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