Noteworthy Chemistry

September 24, 2012

Deoxydehydration converts biomass to polyolefins. One of the obstacles to converting cellulosic biomass to fuels is the low reactivity of oxygen-rich polyol saccharides. Deoxydehydration removes adjacent hydroxyl groups to produce alkenes, but the reaction rarely works on polyols.

M. Shiramizu and F. D. Toste* at the University of California, Berkeley, optimized the process for polyol sugars. They first tested several rhenium catalysts and found that methyltrioxorhenium, [MeReO3], is the most useful because it is a solid and does not require the use of ligands. Studies of the catalyst on polyol deoxydehydration resulted in good yields of polyalkene products, which are potential feedstocks for fuels and polymers.

The adjacent hydroxyl groups must be cis to each other for the reaction to give high yields. Despite this limitation, the deoxydehydration process is promising, and further optimization, such as catalyst recycling, is under study. (Angew. Chem., Int. Ed. 2012, 51, 8082–8086; Chaya Pooput)

A single hybrid controllably emits multiple colors of visible light. Luminescence color can be manipulated by changing material preparation conditions or by device assembly, for example, by varying the ratio of components in a mixture or adjusting layer thicknesses in a light-emitting diode. Y. Zhao and coworkers at Nanyang Technological University (Singapore) and KTH Royal Institute of Technology (Stockholm) report that the emission colors of a single material can be reversibly tuned by using photochemical and thermal controls.

The material is an organic–inorganic hybrid prepared by the self-assembly of a thiol-functionalized cyanostilbene luminogen on the surfaces of CdSe quantum dots (QDs). The Z-isomer of the luminogen (1) has a twisted conformation and emits no light because its photoluminescence is quenched by the dynamic intramolecular rotation of its aromatic rings. The hybrid emits a greenish-yellow color typical of CdSe QDs.

When the hybrid is irradiated with 254-nm UV light for 3 h, the Z-isomer of 1 is partially transformed to the E-form (2), which emits a purplish-blue light because the intramolecular rotation of the aromatic rings in the planar isomer becomes restricted. The complementarity of the emission colors from the QD core and the luminogen shell results in light emission with a color coordinate close to that of standard white light.

Irradiating the hybrid for another 3 h completely converts 1 to 2. The intense emission of 2 overwhelms that of the QD core and changes the color of the hybrid’s emission to purplish blue. The emission can be changed back to white light by heating the hybrid to isomerize a portion of 2 to 1. (Adv. Mater. 2012, 24, 4020–4024; Ben Zhong Tang)

Use natural resources in nanofibrous mats for wound healing. L. Jiang and collaborators at North Dakota State University (Fargo) made electrospun mats from soy protein isolate (SPI) with polyethylene oxide (PEO, 200 kDa) to improve spinnability. They used 1,1,1,3,3,3-hexafluoro-2-propanol as the solvent.

SPI is attractive because of its low-cost, abundant supply from plant sources and its potential for biomaterial applications. Increasing the PEO content balanced the charge distribution within the spinning dope and gave smooth, homogenous, uniform nanofibers.

The authors showed that the SPI–PEO electrospun mats are amorphous because of the nature of the electrospinning process and the miscibility of SPI–PEO. The surfaces of the nanofiber mats are superhydrophilic, which suggests that these materials may absorb sufficient moisture to be used for wound healing. (ACS Appl. Mater. Interfaces 2012, 4, 4331–4337; LaShanda Korley)

Make hindered secondary amides from isocyanates. The only method for preparing almost all amides is to condense carboxylic acids or their derivatives with amines. This method, however, cannot be used with electron-deficient amines or for making hindered amides.

G. Schäfer, C. Matthey, and J. W. Bode* at ETH Zurich report a method to prepare secondary amides by using the reaction of Grignard reagents and isocyanates. They made several “difficult-to-obtain” amides from extremely hindered, electron-deficient, or electron-rich aromatic and aliphatic starting materials.

The reactions are performed in ether, and the products are obtained in good yields after 30 min. The workup consists of extraction and solvent evaporation; no chromatography is necessary. The authors observed no over-reaction of the Grignard reagents with the product amides. The reaction is tolerant to functional groups such as esters or ketones.

In one example, amide 1 was prepared on a 15-mmol scale in 2-methyl tetrahydrofuran (2MeTHF), an “industry-friendly” solvent. This method can be used to prepare secondary amides that may have applications in pharmaceutical or materials synthesis. (Angew. Chem., Int. Ed. 2012, 51, 9173–9175, JosÉ C. Barros)

Boost NMR sensitivity with a hyperpolarization technique. NMR spectroscopy is an extremely powerful characterization tool in chemistry, but its inherent insensitivity is a drawback. Several polarization techniques have been developed to populate nonequilibrium nuclear spin states and enhance NMR signals by several orders of magnitude. Most of these methods, however, use analytes that have been chemically modified to allow polarization.

Signal amplification by reversible exchange (SABRE), however,, uses the polarization transfer capability of hydride-containing labile complexes and thus makes chemical modification unnecessary. S. B. Duckett and coauthors at the University of York (UK), AstraZeneca (Loughborough and Macclesfield, UK, and Mölndal, Sweden), and York Neuroimaging Center (UK) report a SABRE-based hyperpolarization technique that involves a polarization transfer catalyst and parahydrogen for fast NMR measurements at low concentrations.

The instrumental setup consists of a polarization chamber equipped with a transfer line for purging parahydrogen. The operation can be controlled by the spectrometer software. The polarization transfer catalyst precursor [Ir(COD)(IMes)Cl] (1) can be activated by hydrogen with quinoline and MeCN. [IMes is 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene.] MeCN exchanges with the chloride in 1; the complex then coordinates with quinoline.

For a 2-μmol quinolone sample purged with parahydrogen, the signal was enhanced by >60-fold over one achieved by thermal polarization. The hydrogen signal-to-noise (S/N) ratio was 730:1. A 20-nmol nicotinamide sample reached an S/N ratio of 60:1.

The authors applied this protocol to other NMR techniques, including 1-D nuclear Overhauser effect, 2-D correlation spectroscopy, and especially 13C{1H} insensitive nuclei enhanced by polarization transfer NMR. It is very fast and succeeds at low concentrations for the 1–quinoline–MeCN system. Because SABRE can exhibit up to 8600-fold 1H signal enhancement, this method has the potential of being used for rapid NMR analysis of trace compounds. (J. Am. Chem. Soc. 2012, 134, 12904–12907; Xin Su)

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