Noteworthy Chemistry

February 13, 2012

Low applied voltages actuate polymeric ionic gels. At Yokohama National University (Japan), S. Imaizumi, H. Kokubo, and M. Watanabe* prepared ionic gels that consist of a monondisperse ABA block copolymer, polystyrene-b-poly(methyl methacrylate)-b-polystyrene (SMS), in an ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. The authors examined two polystyrene weight fractions: 0.183 (SMS-18) and 0.482 (SMS-48).

The PMMA block forms a soluble continuous pathway, whereas the polystyrene blocks phase-separate to form cross-link junctions. Dynamic rheological studies showed that the gelation point occurs at 9 wt% and 8 wt% for SMS-18 and SMS-48, respectively. SMS-18 exhibits a concentration- or frequency-dependent sol–gel transition temperature, but SMS-48 forms stable, aggregated polystyrene structures and does not have a sol–gel transition between 0 and 120 °C.

The authors’ examination of the nonlinear regime of the viscoelastic response to strain showed that SMS-48 has a higher cross-link density than SMS-18, which contains a significant sol fraction. The IL content influences the morphology of SMS-48: A single glass-transition temperature (Tg) of ≈100 °C is observed at low IL concentrations. As the IL concentration increases, an additional, lower temperature Tg emerges.

Atomic force microscopy shows a weakly ordered ionic gel. SMS-48 has a better-defined morphology than SMS-18, as manifested by its higher ionic conductivity. The authors demonstrated that the soft ionic gels can be actuated at low applied voltages (≤3.0 V). (Macromolecules 2012, 45, 401–409; LaShanda Korley)

Use Ugi–Smiles couplings to make potential antimalarials. Aminoquinoline is a key motif in antimalarial drugs such as chloroquine, primaquine, and amodiaquine. L. El Kaïm*, L. Grimaud*, and P. Pravin at ParisTech (France) developed an aminoquinoline synthesis with the Ugi–Smiles reaction as a key step.

The Ugi–Smiles four-component coupling of an amine, an isonitrile, an aldehyde or ketone, and a hydroxylated pyridine or quinoline (1a) produces amides (2a) that can be reduced to aminoquinolines that have biological activity. When the authors used 4-hydroxypyridine or 4-hydroxyquinolines as reactant 1a, they obtained good yields of the corresponding amides. Reduction with BH3, however, failed to produce any of the desired amines (3).

The researchers replaced the hydroxyl group in the starting N-heterocyclic compounds with mercaptan (1b). The thioamides (2b) obtained from the coupling reaction could be reduced with BH3·SMe2 or Raney nickel to give chloroquine analogues 3 in overall yields of ≈50%. This is an attractive method for synthesizing new antimalarial pharmacophores. (Org. Lett. 2012, 14, 476–478; JosÉ C. Barros)

Will cellulosic biofuels be a secure energy source? It is widely acknowledged that automotive biofuels will at least partially replace fossil fuels such as gasoline and diesel in the not-too-distant future. Current biofuel production in the United States comes mostly from corn and soybeans; 40% of the US corn crop already goes to ethanol production. Growing these crops for fuel on a larger scale would severely hamper food production. Cellulosic ethanol, obtained from crop residues, wild grasses, wood shavings, and household waste is a potential replacement for crop-derived ethanol.

Unfortunately, research on methods to produce biofuels from cellulose has not yet been successful. J. L. Schnoor of the University of Iowa (Iowa City) laments that, whereas ethanol production from corn has succeeded, the development of cellulosic biofuel processes has floundered. (Environ. Sci. Technol. 2011, 45, 7099)

B. E. Dale at Michigan State University (Lansing) takes a more optimistic view. He discusses three roadblocks that can and must be overcome if the country is to achieve energy security from cellulosic biofuels:

  • The ethanol “blend wall”. Federal policy limits the amount of ethanol that can be blended with gasoline to low levels. Flex-fuel vehicles must be developed that can run on any combination of ethanol, methanol, and gasoline.
  • Biofuel production costs. The only way to reduce capital requirements for cellulosic biofuel plants is to build and test commercial-scale plants. Fortunately, at least six companies are in the process of doing this.
  • Patience and persistence. Many decades were required to make fuels from petroleum and ethanol from Brazilian sugarcane inexpensive and abundant. It will be the same for cellulosic biofuels, and government policy and corporate will must be consistent over the long run to achieve environmentally sound energy independence.

(Environ. Sci. Technol. 2011, 45, 9823; Sally Peng Li)

Silk triggers solid-state emission in two-photon fluorophores. Organic fluorophores that produce two-photon fluorescence (TPF) commonly possess a rigid planar structure and have strong tendencies to aggregate because of molecular stacking. This phenomenon is believed to be the reason that their emissions are reduced in the solid state. X. Y. Liu, H. Xu, and coauthors at Donghua University (Shanghai) and the National University of Singapore devised an effective strategy to revitalize the quenched emissions of TPF fluorophores.

The authors chose styrylfluorene dyes end-capped by nitro groups for the TPF fluorophores and Bombyx mori silk, one of the oldest biomaterials used by humans, as the substrate. Powders of the dyes have fluorescence quantum yields (η) as low as 1% as a result of stacking-driven aggregation in the solid state. But because of the specific recognition through hydrogen bonding between the dye’s nitro groups and the silk’s amide units, the fluorophore molecules are decoupled, and their η values increase by as much as 32%. Similarly, TPF fluorophores decoupled by the silk substrate exhibit much larger two-photon absorption cross-sections than those of the solid powders. (Adv. Funct. Mater. 2012, 22, 361–368; Ben Zhong Tang)

How does a particle translocate through a nanopore? W.-J. Lan and H. S. White* at the University of Utah (Salt Lake City) used a pressure-reversal technique to answer this question. They made a glass nanopore membrane (GNM) with a single conical nanopore embedded in 50–100-μm–thick glass and submerged it in an electrolyte solution containing carboxylated polystyrene nanoparticles. Pressure or vacuum can be applied to the GNM by moving a syringe; particles can be captured by lowering the pressure or released by increasing it (A in the figure). The nanoparticles generate resistive pulses as they pass through the nanopore; the pulses are monitored by an electric circuit with two Ag/AgCl electrodes.

Without stochastic influences, the release time τr of one particle should theoretically be equal to the capture time, given equal driving pressures. The same rule should apply to the time sequence of the capture–release process of a group of particles (B). The authors found that, despite the stochastic broadening of τr caused by diffusion motion, the sequence of particle capture is largely preserved and can be detected by measuring pulses during the release process.

Finite-element simulations provided quantitative evidence that the capture–release process probably occurs according to a diffusion–convection model. This study helps to explain certain migration phenomena on the mesoscopic scale. (ACS Nano 2012, 6, Article ASAP DOI: 10.1021/nn2047636; Xin Su)

Here are two ways to remove palladium from pharmaceuticals. Two Organic Process Research & Development articles describe methods for removing residual palladium catalysts from pharmaceutical products and intermediates. L. Wang and co-workers at Merck (Rahway, NJ) used the combination of a chelating agent such as ethylenediamine or N-acetylcysteine and a solid support such as activated carbon or silica. Both types of materials are relatively inexpensive, and their synergistic effect is far more efficient than either used alone. The order in which they are added does not seem to matter, but the optimum solid-to-chelating agent ratio depends on the substrate, the metal species, the solvent, and other conditions. (Org. Process Res. Dev. 2011, 15, 1371–1376)

G. Reginato*, P. Sadler, and R. D. Wilkes at AstraZeneca R&D Charnwood (Loughborough, UK) and PhosphonicS (Abingdon, UK) describe the use of solid-supported reagents for the scale-up of metal scavenging operations in pharmaceutical plants. They investigated three modes of operation and found that standard batch adsorption, as used on the laboratory scale, involves more unit operations than the fixed-bed recirculation or the semicontinuous single-pass method. In addition, the recirculation and single-pass processes are not affected by the mass-transfer properties of the medium. (Org. Process Res. Dev. 2011, 15, 1396–1405; Will Watson)

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