Noteworthy Chemistry

July 22, 2013


Colorful colloid crystals combat counterfeiters. Anticounterfeiting efforts often rely on materials whose colors arise from physical rather than chemical properties. Iridescence, dichroism, diffraction, and other color properties are difficult to imitate convincingly. Colloidal crystal films, thin films formed from monodisperse colloidal particles self-assembled into periodic lattices, would seem to be ideal candidates for this type of application. Difficulties in production and the fragility of the resulting films, however, present obstacles to practical applications. Freestanding films and micropatterns often require complicated preparation processes. Strong light scattering makes the films milky or opaque, limiting the ability to mix colors using multiple layers.

S.-M. Yang, S.-H. Kim, and co-workers at the Korea Advanced Institute of Science and Technology (Daejeon) developed a practical method for creating highly transparent colloidal photonic crystal films in a variety of colors. They dispersed colloidal silica particles with the desired size in a photocurable ethoxylated trimethylolpropane triacrylate resin. To reduce light scattering and increase the transparency of the resulting film, the authors chose a resin with a similar refractive index to the particles.

The suspension was drawn between two glass plates by capillary forces and then photopolymerized under UV light. The resulting crystalline films can be micropatterned by using conventional photolithography.

Because the films are transparent, multicolored patterns can be made by superimposing two or more colored layers. Combining red, green, and blue structural colors produces the secondary colors cyan, magenta, yellow, and white. The colors produced lie within a narrow spectral range to provide a high degree of specificity, and they vary with the viewing angle and the size of the colloidal particles. The films are highly transparent under ambient light, but when the incident angle of the light is the same as the angle of observation, the films appear to be brightly reflective.

Examples of how colloidal crystal pattern colors strongly depend on the angle of incidence of the light

In the figure, the optical images (b) of the colloidal crystal pattern composed of letter Ks show a reflection color that strongly depends on the angle of incidence of the light, which is given in each image. Optical image (c) is a freestanding film that contains a pattern of Ks. Image (d) is a patterned photonic crystal film on a Korean bank note. The pattern exhibits a bright green color under normal reflection (main image), whereas it displays a blue color for high incident and reflection angles (top right). The film is highly transparent and difficult to distinguish when the viewing angle is different from the angle of incidence of the light (bottom right).

The authors note that their colored films can also be used in decorative coatings, paints, or cosmetics. (Chem. Mater. 2013, 25, 2684–2690; Nancy McGuire)

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An α-oxonium ion sets a new record for carbon–oxygen bond length. Oxatriquinanes are ideal candidates for creating extremely long, stable C–O bonds because of their unique steric and electronic characteristics. After pushing the covalent C–O bond length record to 1.622 Å in an oxatriquinane derivative (Nature Chem. 2012, 4, 1018–1023), M. Mascal and colleagues at the University of California, Davis, improved that record to 1.658 Å by introducing a hydroxyl group at the α-position of the oxatriquinane framework (2 in the figure).

Synthesis of α-hydroxy- and α-methoxyoxatriquinane

The authors reasoned that in an O–C–O bonding motif, a σ*-donor oxygen atom could elongate the C–O bond of its corresponding acceptor. Therefore, they sought to synthesize a series of hydroxy- and methoxyoxatriquinanes from the corresponding ketones.

Treating ketone 1 with CF3SO3H in MeCN produces α-hydroxyoxatriquinane (2). To obtain α-methoxyoxatriquinane (4), the authors first treated ketone 1 with p-toluenesulfonic acid and (MeO)3CH in MeOH to make dimethoxy-substituted compound 3. Protonating 3 with CF3SO3H gives the desired α-methoxyoxatriquinane 4. The crystal structures of 2 and 4 showed that the α-substituted C–O+ bond lengths are 1.658 and 1.619 Å, respectively.

Similarly, the authors synthesized the dihydroxy and dimethoxy counterparts of 2 and 4, but attempts to make trihydroxy and trimethoxy oxatriquinanes failed, probably because the products are unstable. (J. Am. Chem. Soc. 2013, 135, 8173–8176; Xin Su)

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Take a photochemical route to wrinkle control. C. J. Kloxin* and collaborators at the University of Delaware (Newark) addressed spatial control of wrinkle patterns on elastomeric surfaces with a two-step sequence. They first used thiol–ene chemistry to polymerize acrylate and tetrathiol monomers in a slight stoichiometric imbalance to form an elastomeric film with excess acrylate functionality. Then, under slight strain (20%), the unpolymerized acrylate groups were photopolymerized to form a surface skin layer. The polymerization conditions can be varied to tune the thickness and modulus of the surface layer.

Oxygen plays a critical role in the depth of the skin layer and in wrinkle formation on the microscale. The authors show that this process can form elaborate photomasked wrinkle structures with sequential patterning steps. Using this method with initiators that are photoactive at specific wavelengths gives wrinkled skin shape-memory behavior. (ACS Macro Lett. 2013, 2, 474–477; LaShanda Korley)

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Form a polytriazole organogel in the presence of cuprous ion. Intermolecular interactions can induce organic molecules to cluster into noncovalently bonded networks that exhibit gel-like properties. Organogels are usually formed via hydrogen bonding and van der Waals interactions. F. Huang and co-workers at East China University of Science and Technology (Shanghai) developed a polymeric gelator system that forms an organogel in the absence of these noncovalent forces.

Gel-forming polytriazole

The authors used Cu(I)-catalyzed diazide–diyne polymerization to synthesize the polytriazole polymer 1. Polymer 1 forms an organogel in the presence of Cu(I) because of the coordination interaction between the metal ion and the polytriazole backbone. The catalyst residue must be removed completely  to obtain a processible polytriazole gel from the Cu(I)-catalyzed click polymerization. (Polym. Chem. 2013, 4, 3444–3447; Ben Zhong Tang)

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Convert carbohydrates to lactic acid with an alkaline hydrothermal process. Biomass is a promising alternative to fossil fuel sources, but upgrading biomass to fine chemicals is challenging because traditional hydrothermal methods produce complex product mixtures. D. Esposito* and M. Antonieti at the Max Planck Institute of Colloids and Interfaces (Potsdam, Germany) report that the alkaline hydrothermal conversion of glucose and cellulose biomass to lactic acid proceeds smoothly with yields of up to 57%.

The authors first studied the hydrothermal reaction between sugars and alkali-metal and alkaline-earth hydroxides with glucose as the model substrate. At 220 ºC in an autoclave, NaOH gave only 15–17% yields of lactic acid, but alkaline-earth hydroxides Ca(OH)2, Sr(OH)2, and Ba(OH)2 increased the yield to 40–53%. Divalent cations were crucial for the conversion: The lactic acid yield in the presence of a combination of NaOH and BaCl2 was close to yields with alkaline-earth hydroxides.

The authors also found that excess base favors the formation of lactic acid. In the presence of 8 equiv Ba(OH)2, 0.025 M glucose was converted to lactic acid at 250 °C in an microreactor in <3 min with a yield of 57%. On a 10-g scale, cellulose was transformed directly into lactic acid in 50% yield at 220 °C for 1h. Ba(OH)2 can be recycled by precipitating it with carbonate.

Proposed mechanism for base-driven conversion of glucose to lactic acid

In the authors’ proposed mechanism (see figure), glucose (1) isomerizes to fructose (2), and a retro-aldol reaction splits the fructose into two C3 fragments, glyceraldehyde (3) and dihydroxyacetone (4). Upon dehydration, the C3 compounds form pyruvaldehyde (5), which subsequently undergoes a 1,2-hydride shift to produce the metal lactate salt (6). (ChemSusChem 2013, 6, 989–992; Xin Su)

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What is the best method to form an acyl aryl ketone? During the scale-up of a synthesis of a pyridazin-3-one histamine H3 receptor antagonist, R. C. Roemmle and co-workers at Teva Pharmaceuticals (Malvern and West Chester, PA) investigated several protocols for converting tert-butyl 6-bromospiro[4H-1,3-benzodioxine-2,4′-piperidine]-1′-carboxylate (BBBPC) to tert-butyl 6-(4-hydroxy-3,3-dimethyl-4-oxobutanoyl)spiro[4H-1,3-benzodioxine-2,4′-piperidine]-1′-carboxylate. Anionic chemistry that used lithium–halogen exchange (with n-BuLi or s-BuLi) or Grignard formation, followed by the reaction with 2,2-dimethylsuccinic anhydride or monomethyl 2,2-dimethylsuccinyl chloride gave 35–38% product yield at best.

The most successful approach was a palladium-catalyzed cross-coupling reaction between the boronic acid derived from BBBPC and monomethyl 2,2-dimethylsuccinyl chloride. Unexpectedly, the reaction worked better with the crude boronic acid (80–87% pure) than with the boronic acid that had been purified by column chromatography. The crude material was amorphous and had much higher solubility in the reaction medium than the crystalline purified material. (Org. Process Res. Dev. 2013, 17, 846–853; Will Watson)

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