Noteworthy Chemistry

March 19, 2012

Hollow metallic microspirals are created from nature. J. Liu and co-workers at Beihang University (China) took inspiration from Spirulina unicellular algae to make metallic microspirals. They exposed the perfectly spiral organisms to a colloidal Pd–Sn catalyst, causing the catalyst to noncovalently adsorb onto the algae. The supported catalyst acted as a biological scaffold for the electroless deposition of copper.

Deposition formed hollow copper microspirals with rough surfaces (see figure). They had ≈10 µm i.d. and were 200–300 µm long. Nanoindentation measurements showed that their hardness was ≈0.65 GPa with elastic moduli of ≈12.5 GPa.

These values are significantly lower than those reported for bulk copper and copper electrodeposited on solid substrates. The authors attribute the lower values to surface defects and the hollow structure of the microspirals. They propose potential applications such as functional composite fillers and micro- and nanodevices. (Langmuir 2012, 28, 3690–3694; LaShanda Korley)

See the entire rainbow from a single viewing angle. Large-area, low-cost technologies are being sought to mimic the principles of structured colors found in nature. Artificial structures, however, can be prohibitively expensive. In addition, most reported micro- or nanophotonic structures cannot be made multispectral from the same viewing angle.

Q. Gan, A. N. Cartwright, and co-workers at the University at Buffalo (NY) developed a one-step method for preparing a graded holographic photopolymer reflection grating. The period of the polymer photonic crystal reflector at different lateral positions along the structure is varied gradually to provide a unique rainbow-colored reflection image at a given viewing angle. This technique provides a way to make graded optical elements by using lenses with curved surfaces.

These lenses can be easily extended to 2-D or 3-D patterns (e.g., cylindrical, plano-convex, positive meniscus, plano-concave, and biconcave) by using advanced optics. Compared with previously reported graded photonic structures prepared by using expensive focus-ion-beam milling or electron beam lithography techniques, this holographic photopatterning method is inexpensive and amenable to preparing large structures.

The photonic rainbow-colored filter can be integrated with detectors or imaging devices to produce compact, portable spectroscopic analyzers that could be used in miniaturized and more affordable multispectral or hyperspectral imaging applications. They are also desired in transformation optics, metamaterials, and bioinspired photonics. (Adv. Mater. 2012, 24, 1604–1609; Gary A. Baker)

Polyazomethines can be highly fluorescent and halochromic. The syntheses of “conventional” conjugated polymers such as polyfluorenes can require stringent reaction conditions and troublesome product purification procedures. High-purity polyazomethines can be prepared by using much simpler reaction and isolation procedures, but they are usually nonfluorescent (fluorescence quantum yields [ΦF] ≈ 1%). S. Barik, T. Bletzacker, and W. G. Skene* at the University of Montreal now report the synthesis of highly emissive polyazomethines.

Three of the authors’ polyazomethines are shown in Figure 1. All are emissive, with ΦF values as high as 26%. Figure 2 shows how their CH2Cl2 solutions appear when they are irradiated with a UV lamp at 350 nm.

The emission efficiencies can be enhanced by as much as ≈5-fold by cooling the solutions to –160 °C, suggesting that fluorescence at room temperature is partially quenched by bond rotation of the azomethine units. The polymers also exhibit halochromism: Simple exposure to CF3CO2H leads to a dramatic increase in their ΦF values—up to 86%. (Macromolecules 2012, 45, 1165–1173; Ben Zhong Tang)

Choose the best acylating agent for an enzymatic resolution. L. Murtagh and co-workers at Pfizer Global Research and Development (Sandwich, UK; Kalamazoo, MI; Holland, MI; and Groton, CT) needed large amounts of (R)-2-methylpentanol as the starting material for the large-scale synthesis of an α2δ ligand. A lipase-catalyzed kinetic resolution using Amano Lipase PS-C1 was the best method for obtaining the (R)-isomer.

The authors selected vinyl decanoate as the optimal acyl donor because the boiling point difference between the desired unreacted (R)-isomer and the ester is wide enough to separate them by fractional distillation. On scale, it is essential to limit reaction times and to filter the enzyme from the reaction mixture. This minimizes erosion of the enantiomeric excess of the product that presumably occurs via transesterification during the distillation.

(R)-2-Methylpentanol can also be produced by enantiospecific enzymatic reduction of 2-methylvaleraldehyde (Gooding, O. W., et al. Org. Process Res. Dev. 2010, 14, 119–126). (Org. Process Res. Dev. 2011, 15, 1315–1327; Will Watson)

A tetrahedral host captures the Walrafen water pentamer. Almost 50 years ago, G. E. Walrafen proposed a pentameric water structure that features a central water molecule surrounded by four others in a tetrahedral arrangement (J. Chem. Phys. 1964, 40, 3249–3256). Until recently, however, this structure had not been observed directly. Q.-Q. Wang, V. W. Day, and K. Bowman-James* at the University of Kansas (Lawrence) designed a tricyclic tetrahedral molecular cage (1) that can capture the Walrafen water pentamer and the isoelectronic tetrahydrated fluoride ion.

The authors synthesized structure 1 from tris(2-aminoethyl)amine (2) in three steps with an overall yield of 2.1%. Heating 2 in refluxing MeOH with excess dimethyl 2,6-pyridinedicarboxylate (3) gave tripodal fragment 4. A similar condensation between 4 and excess 2 yielded precursor 5. Finally, in the presence of 3 equiv 3, the periphery of 5 was closed to form compound 1.

The four tertiary nitrogen atoms in 1 form a tetrahedral cage. Its 12 amide hydrogen atoms and 10 amide nitrogen atoms are available as hydrogen-bonding donors and acceptors, respectively. In the presence of water, 1 forms the inclusion complex H2O·4H2O⊂1. The authors obtained crystals of the complex and showed that its structure contains a tetrahedrally arranged water network with one water molecule in the center. The central water forms four hydrogen bonds 2.649–2.751 Å long with the surrounding water molecules. Two of the water molecules are acceptors, and two are donors—in perfect accordance with Walrafen’s prediction.

The peripheral water molecules do not interact with one other. The authors observed hydrogen bonds between the tetrahedral water network and 1; they believe that these bonds are responsible for stabilizing the pentameric cluster. (Angew. Chem., Int. Ed. 2012, 51, 2119–2123; Xin Su)

Choose the right ligand for a double asymmetric hydrogenation. One of the key steps in a scaled-up synthesis of the core of the cyclic tripeptide desoxybiphenomycin B is the asymmetric reduction of two C=C bonds to create two chiral centers. The substrate contains one chiral center that has no directing effect on the asymmetric reduction. A nonchiral hydrogenation with Wilkinson’s catalyst produces a random mixture of isomers.

W. Jöntgen and co-workers at Bayer Pharma (Wuppertal, Germany) achieved the double chiral reduction with rhodium (S,S)-Et-DuPHOS in quantitative yield and high selectivity (>99% de). {The ligand (S,S)-Et-DuPHOS is (+)-1,2-bis[(2S,5S)-2,5-diethylphospholano]benzene.} They found that the success of the reduction depends on the quality of the starting material, which is related to details of the previous synthetic steps. The presence of inorganic salts, chloride in particular, shuts the reaction down. This problem can be avoided by using AgCF3CO2 in the asymmetric hydrogenation or, less expensively, by adding an aqueous wash as part of the workup procedure for preparing the substrate. (Org. Process Res. Dev. 2011, 15, 1348–1357; Will Watson)

What do you think of Noteworthy Chemistry? Let us know.