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Noteworthy Chemistry

June 17, 2013

What gives blue sapphires their color? The mineral corundum consists mainly of α-Al2O3. In its pure form, it is a transparent ceramic material; but its crystals become blue sapphires in the presence of iron and titanium impurities. The effect of iron and titanium has long been known, but the optical origin of the color remains unexplained. A. Walsh and colleagues at the University of Bath (UK) and University College London used atomistic modeling to clarify the optical process in blue sapphires.

According to previous work, there are two possible optical mechanisms:

  • intervalence charge transfer (IVCT) between Ti(IV) and Fe(II) [derived from Ti(III) and Fe(III)] and
  • intra-atomic d–d orbital transitions in the metal impurities.

The authors used a polarizable interatomic potential to model the charge transfer process and the angular overlap model to probe crystal field transitions.

The authors calculated that for the reaction Fe(III) + Ti(III) → Fe(II) + Ti(IV) to proceed, the optical absorption energy of the face-sharing nearest-neighbor pairs of iron and titanium should be 1.85 eV (equivalent to a wavelength of 670 nm). This value is consistent with results measured with absorption spectroscopy. The IVCT pathway is also supported by the difference in binding energies between the Ti(IV)–Fe(II) and Ti(III)–Fe(III) ion pairs.

In contrast, the authors showed that the d–d transitions in the metal impurities are outside the appropriate spectrum range or too weak to cause the intense coloration in blue sapphires. This finding excludes the possibility of intra-atomic d–d transitions. (Chem. Commun. 2013, 49, 5259–5261; Xin Su)

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Use human hair as a raw material. Human hair is not readily biodegradable. It consists of a hydrophobic lipid coating (18-methyleicosanoic acid) attached via thioester groups to a proteinaceous epicuticle at the outer surface of the hair fibers. The core is composed of a medulla surrounded by a cortex.

R. A Boulos and coauthors at the University of Western Australia (Crawley) and Flinders University (Bedford Park, Australia) transformed human hair into a nanomaterial by treating it with an ionic liquid (IL). They also report industrial and medical uses for this material.

The authors subjected human hair (1) to a choline chloride–urea IL to remove the lipid layer and separate the core (2) from cuticle cells (3). Other ILs such as [bmim][Cl] and [bmim][BF4] can also be used; but choline chloride–urea is less expensive, environmentally benign, and easier to separate from disassembled hair (bmim is 1-butyl-3-methylimidazolium).

IL decomposition of hair into potentially useful materials

The core material was subjected to cultures of the green microalga Chlorella vulgaris (4) to test its ability to remove nitrate from waste water. A comparison of untreated hair, nitrogen-treated hair, and IL-treated hair showed that the IL-treated hair best removes nitrate because the IL removes cuticle cells and creates pockets on the surface that improve the efficiency of entrapment of microalgal cells.

The authors subjected the cuticle cells separated from the hair core to 1,3,5-tris[(1E)-2′-(4″-benzoic acid)vinyl]benzene (5). The thiol groups on the cells’ surface stabilize this inexpensive, fluorescent, nontoxic antimicrobial compound. A fluorescence analysis showed that the cuticle–5 composite 6 is stable at pH 2.5 (stomach acidity) but liberates 5 at pH 7.5 (intestinal basicity). This means that the hair-derived material is suitable for oral administration and drug release. (Green Chem. 2013, 15, 1268-1273; José C. Barros)

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Make sulfur-containing polymeric flame-retardant coatings one layer at a time. J. C. Grunlan and coauthors at Texas A&M University (College Station) and the University of Dayton Research Institute (OH) prepared layer-by-layer–assembled chitosan–poly(vinylsulfonic acid) sodium salt (PVS) composites for use as sulfur-containing flame-retardant coatings for polyurethane foams. At an assembly pH of 6, the chitosan-PVS bilayers grew linearly and were deposited uniformly on polyurethane.

In qualitative flammability tests, a 10-bilayer chitosan-PVS–coated foam stopped flame propagation after the flame source was removed. The foam underwent structural changes but did not melt and drip.

The authors used cone calorimetry to measure the heat release rate and show that the chitosan-PVS conformal coating significantly reduces the flammability and fire growth rate of the foam. They also noted that the flame-retardant coatings bind some of the fuel in the form of char, and combustion in the vapor phase is inhibited. (ACS Macro Lett. 2013, 2, 361–365; LaShanda Korley)

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A solid-state organogold complex is luminescent and thermochromic. Many luminogenic organic molecules have aggregation-induced emission (AIE) characteristics, but AIE luminogens based on transition-metal complexes are rare. G.-A. Yu, S. H. Liu, and co-workers at Central China Normal University (Wuhan) synthesized an isocyanide–Au(I) complex (1) that exhibits an unusual combination of AIE effects and thermochromic behavior.

Gold complex with AIE properties

Complex 1 is almost nonluminescent in solution. Adding >3 parts of a poor solvent for 1 (water) to <7 parts of its solution in a common organic solvent (EtOH) causes the complex to aggregate. The aggregate photoluminesces with λmax values of 402 and 425 μm—typical AIE behavior.

The emission color of 1 in the solid state can be switched between blue and yellow-green by heating and cooling it. The authors believe that changes in intermolecular interactions (e.g., Au–Au, π–π, C–H···F, and C···F) are responsible for the complex’s unique light-emitting properties. (Chem. Commun. 2013, 49, 3567–3569; Ben Zhong Tang)

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Work up a Mitsunobu reaction without using chromatography. To develop a scalable synthesis of an oxadiazole-based S1P1 receptor inhibitor, K. Lufkin*, V. Inshore, and T. Gordon at GPRD Process R&D (North Chicago, IL) and GPRD Medicinal Chemistry (Worcester, MA) had to prepare an ether of trans-3-hydroxycyclobutanecarboxylic acid. The synthesis requires a Mitsunobu epimerization reaction because the reduction of 3-ketocyclobutanecarboxylic acid gives mainly the cis isomer.

Under standard Mitsunobu reaction conditions, byproducts Ph3PO and diethyl hydrazodicarboxylate (the reduced form of diethyl azodicarboxylate) are difficult to remove. The authors developed a workup procedure in which all of the impurities are precipitated from solution and removed by filtration. The procedure is based on reports that Ph3PO forms an insoluble complex with MgCl2 and that removing the Ph3PO–MgCl2 complex is most efficient when toluene is the solvent. Diethyl hydrazodicarboxylate is also insoluble in toluene.

The reaction is carried out in toluene instead of the original THF solvent. Upon completion, MgCl2 is added; and the mixture is heated at 50–60 °C for 1 h. It is then filtered to leave a toluene solution of the epimerized product. (Org. Process Res. Dev. 2013, 17, 666–671; Will Watson)

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Use TLC to determine absolute configurations. Competing enantioselective conversion (CEC), a method for determining the absolute configurations of enantiomers, takes advantage of the difference in rates when two enantiomers react with chiral kinetic reagents. A variety of chiral compounds, including secondary alcohols, oxazolidinones, lactams, and thiolactams, can by differentiated by using this method in conjunction with 1H NMR spectroscopy.

To simplify the CEC method, A. J. Wagner and S. D. Rychnovsky* at the University of California–Irvine developed a qualitative thin-layer chromatography (TLC) method that makes the results of CEC experiments visible. They applied the method to determining the absolute configuration of secondary alcohols.

The authors esterified a wide range of enantioenriched secondary alcohols (85:15–99:1 er) with 3 equiv (EtCO)2O in the presence of 4 mol% Birman's chiral homobenzotetramisole catalysts [(R)-1 and (S)-1] in CDCl3. The authors expected alcohol substrates (2) with the configuration shown in the figure to react with (R)-1 more slowly than with (S)-1. The lower conversion rate of 2 with (R)-1 was established by previous NMR studies.

Determining the absolute configuration of secondary alcohols with Birman’s catalysts and TLC

The reactions were quenched after 30 min by adding CD3OD. Aliquots were sampled from the reaction mixtures and spotted on TLC plates. After the TLC plates were run, dried, and stained, two spots appeared for each reaction. The high- and low-Rf spots corresponded to the ester product and the alcohol substrate, respectively.

For a faster reaction, the ester had a greater spot density than the alcohol; the reverse occurred for a slower reaction. The assignments of absolute configuration based on this qualitative method are consistent with the results of NMR experiments. (J. Org. Chem. 2013, 78, 4594–4598; Xin Su)

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