Noteworthy Chemistry

July 16, 2012

Rhodium-catalyzed cyclization leads to polycyclic aromatics. Polycyclic aromatic compounds (PACs) have unique electronic structures and great potential for high-tech applications. J. Wang and co-workers at Peking University (Beijing) report a versatile method for synthesizing PACs that is based on Rh(II)-catalyzed intramolecular carbene dimerization.

To demonstrate the versatility of their method, the researchers prepared a series of substituted phenanthrenes (1) from biphenyl dialdehydes in yields of up to 95% (see figure). The reaction proceeds through the p-toluenesulfonylhydrazones of the aldehydes.

The one-pot synthesis is tolerant of electron-donating and -withdrawing functional groups. It can be used to prepare large PACs such as [4]- and [5]-helicenes (2 and 3), picene (4), and pentaphenes (5). The authors also used the technique to make the biomedically important natural product furostifoline (6), a carbazole alkaloid first isolated from the root bark of the Asian tree Murraya euchrestifolia Hayata. (Angew. Chem., Int. Ed. 2012, 51, 5714–5717; Ben Zhong Tang)

These antifouling coatings have strong “mussels”. P. Wilke and H. G. Börner* at Humboldt University of Berlin developed a new class of antifouling coatings that are based on interactions that mimic those in mussel glue. They synthesized a bioconjuagate of poly(ethylene oxide) (PEO) and the functional adhesive peptide sequence AKPSYPPTYK. Catalytic oxidation using the tyrosinase enzyme produced residues of 3,4-dihydroxylphenylalanine (DOPA)–quinone via a two-step process. The PEO segment enhances dissociation of the tyrosinase in the ninth position.

The authors measured the adhesive properties of this bioinspired coating on a stainless steel substrate and found that the enzymatically activated bioconjugate forms a conformable, stable ≈6-nm coating via noncovalent interactions. PEO at the coating surface greatly reduces protein adsorption. The authors discuss additional applications in antifouling and peptide-derived adhesion. (ACS Macro Letters 2012, 1, 871–875; LaShanda Korley)

Use phosphonium ionic liquids in metal separation. Existing ionic liquids (ILs) have not been used to remove metal ions from aqueous solution via liquid–liquid extraction because

  • the anion or cation of the IL can be lost to the aqueous phase,
  • the IL is unstable toward hydrolysis and may be toxic, or
  • an undesirable volatile hydrocarbon diluent may be required to reduce the IL’s viscosity.

Now S. Wellens, B. Thijs, and K. Binnemans* at the University of Leuven (Heverlee) and Umicore (Olen, both in Belgium) report the use of alkylphosphonium ILs to extract cobalt from aqueous solution and leave nickel in the aqueous phase.

The phosphonium IL n-tetradecyltri-n-hexylphosphonium chloride (1) can be used without dilution to separate CoCl2 from NiCl2. Cobalt migrates to the IL phase as the CoCl4 complex. The cobalt is recovered by washing the IL phase with water, and the IL can be reused.

Other metals such as magnesium and calcium also remain in the aqueous phase, but manganese is co-extracted with cobalt. Saturating the aqueous solution with HCl or NaCl facilitates the Co–Ni separation. In experiments scaled up to 250 mL, 35 g/L of cobalt was extracted from aqueous solution with an IL/aq distribution coefficient of 100.

Several other phosphonium ILs can also be used, but none were as efficient as 1. This method is “green” alternative for the liquid–liquid extraction of metals ions. (Green Chem. 2012, 14, 1657–1665; JosÉ C. Barros)

Optimize a titanium–Grignard-mediated cyclopropanation. In the synthesis of 1-(2-pyridinyl)cyclopropylamine-L-alanine dipeptide, the reaction of EtMgBr with 2-cyanopyridine in the presence of Ti(O-i-Pr)4 produces 1-(2-pyridinyl)cyclopropylamine as the major product, along with 2-pyridinyl ethyl ketone and some dimeric impurities. To optimize the reaction conditions for scale-up, W. Li and co-workers at Boehringer Ingelheim Pharmaceuticals (Ridgefield, CT) minimized the formation of the ketone by adding bis[2-(N,N-dimethylaminoethyl)] ether, which reduces the reactivity of the Grignard reagent. The other impurities can be minimized by adding 2-cyanopyridine and EtMgBr simultaneously to a heated solution of Ti(O-i-Pr)4 and a diamine ligand in THF.

The authors also modified the workup to avoid lowering the water solubility of the product in the presence of inorganic salts. The reaction mixture was neutralized by adding HOAc and water. The product was converted directly to the amide by adding N-Boc-alanine that had been pre-activated with carbonyldiimidazole. (Boc is tert-butoxycarbonyl.) The overall isolated yield from the sequence is 48–50% on a multikilogram scale. (Org. Process Res. Dev. 2012, 16, 836–839; Will Watson)

Here’s a way to generate near-infrared chemiluminescence. Chemiluminescence is light emission in which the excited state is produced in a chemical reaction. Most chemiluminescence luminophores emit visible light; only a few emit in the near-IR (NIR) range. NIR emitters are important for telecommunications, information security displays, bioimaging, and optical sensing.

G. Qian, J. P. Gao, and Z. Y. Wang* at Carleton University (Ottawa) and the Changchun Institute of Applied Chemistry (Jilin, China) developed a NIR chemiluminescence system with tunable emission in the wavelength region that is important for biological and telecommunications applications.

NIR chemiluminescence that is tunable from 900 to 1700 nm is produced from the reactions of a series of narrow–band gap luminophores (15) with (COCl)2 and H2O2. Each of the five luminophores has a different band gap and produces chemiluminescence at a specific wavelength. During the process, H2O2 reacts first with (COCl)2 to generate high-energy dioxetanedione, which then excites the luminophores by a chemically initiated electron-exchange mechanism.

The NIR chemiluminescence can be generated repeatedly as long as high-energy dioxetanedione is mixed with the luminophore—an indication that the luminophores are stable under the reaction conditions. Running the reactions in solution at low temperature or in polymer gels at room temperature prolongs the chemiluminescence process. (Chem. Commun. 2012, 48, 6426–6428; Ben Zhong Tang)

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