January 4, 2010
Make switchable nanostructures by tailoring the macrocycle aspect ratio. Researchers at Seoul National University and Yonsei University (Seoul) report the nanostructured, aqueous assembly of amphiphilic macrocycles that contain dendritic oligoether end units. By varying the aspect ratio of the elliptical macrocycle, M. Lee and coauthors observed a shift in self-assembly behavior in water that ranges from spheres to helices to vesicles.
The structural transformation is a result of enhanced π–π stacking and hydrophobic interactions as the length of the elliptical macrocycle increases. For example, structure 1, with an intermediate molecular length, exhibits primarily helical nanostructures with a pitch of ~10 nm and displays a Cotton effect in 0.01% aqueous solutions. The authors attribute this behavior to stacking and synergistic rotation of the rigid macrocycle. The behavior is not observed in a more circular macrocycle analogue (2), which displays a cylindrical nanostructure.
The lower critical solution temperature (40 °C) of the oligoether dendrons controls the phase transition of the macrocycles and results in a change from helices to uniform micellar rods with heating to 50 °C. This transition is reversible, but it exhibits significant hysteresis. It takes 3 days to restore the helical nanostructures completely because of slow π–π interaction kinetics. The authors note that this thermoresponsive evolution in nanostructure parallels a conformational change of the macrocycle from a boat configuration to a planar arrangement. This study provides clues to the relationship between structural complexity and design and dynamic self-assembly behavior. (J. Am. Chem. Soc. 2009, 131, 17768–17770; LaShanda Korley)
Four- and six-component domino reactions form imidazoles and benzamides. The rapid increase in research on multicomponent domino reactions has led to easy access to new complex structures. In some cases, these structures were previously available only by tedious multistep syntheses. S. J. Tu, G. Li, and coauthors at Xuzhou Normal University (Jiangsu, China), Suzhou University (Jiangsu), and Texas Tech University (Lubbock) report efficient four-component reactions that lead to highly substituted 2-(2’-azaaryl)imidazoles (e.g., 1) and six-component reactions that form anti-1,2-diarylethylbenzamides (e.g., 2). Both microwave
irradiation–promoted reactions proceed quickly and provide good-to-excellent product yields. No organic solvents are needed.
The four-component reaction uses pyridine-2-carbonitrile as the model reactant. The authors’ 25 variants of the reaction differed primarily by the substituents on the phenyl ring of the aryl aldehyde component. Several products obtained by this method are very difficult to synthesize by known methods.
The authors added benzonitrile to a six-component reaction of aromatic aldehydes and NH4OAc under similar conditions. The reaction forms derivatives of product 2, generated as single isomers. When the reaction is repeated without benzonitrile, derivatives of 2 are again obtained, indicating that in this case the nitrile does not participate in the reaction. The authors formulated possible umpolung mechanisms to account for products 1 and 2. (J. Org. Chem. 2009, 74, 9486–9489; W. Jerry Patterson)
A luminogen’s emissivity depends on its location in a micelle. Luminescence behaviors of luminogenic molecules can be affected by many factors, including solvent, temperature, viscosity, pH, and polarity. N. Ali and S.-Y. Park* of Kyungpook National University (Daegu, Korea) show that location can also dramatically
influence light emission from a fluorogen.
Molecules of azo fluorogen 1 form supramolecular complexes with polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) chains by hydrogen bonding with the pyridine units in the PVP block. PS-b-P4VP–1 complexes form micelles in EtOH and toluene. In EtOH, the P4VP block and therefore 1 are located in the corona of the micelles, whereas in toluene, these components are in the core of the micelles. Compound 1 is nonemissive when it is in the corona, but it is highly luminescent in the core.
In the loosely packed corona, molecular collisions break some of the hydrogen bonds, and the increased conformational flexibility of 1 weakens its fluorescence. In the hard core, however, the molecules of 1 are compactly aggregated, which rigidifies the conformation of the fluorogen and significantly boosts its fluorescence efficiency. (Langmuir 2009, 25, 13426–13431; Ben Zhong Tang)
Use an NMR tool for biodiesel analysis. Biodiesel fuel is made by transesterifying triglycerides from vegetable oils or animal fats with an alcohol in the presence of a catalyst. The reaction produces monoalkyl esters of fatty acids that have diesel fuel–like properties. Byproduct glycerol, fatty acids, unreacted alcohol, and catalyst may be present as contaminants that change the fuel’s physical properties. Current methods of biodiesel analysis include chromatography (HPLC or GC) and wet chemical titrations such as potentiometry and iodometry.
M. Nagy, M. Foston, and A. J. Ragauskas* at Georgia Tech (Atlanta) and Chalmers University of Technology (Gothenburg, Sweden) developed an NMR technique for analyzing biodiesel fuels. They used 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (1) in the presence of DMF, pyridine, and CDCl3 to make derivatives of the hydroxyl groups of several biodiesel precursors, production line samples, and biodiesel samples. They then quantitatively analyzed the phosphited compounds (2) by 31P NMR with good signal resolution. The method uses Cr(acac)3 as a relaxation agent and cyclohexanol as an internal standard. The NMR pulse program was optimized to reduce analysis time from 50 min to 80 s.
By using 31P NMR, the authors were able to assign the chemical shifts of several components of biodiesel mixtures: glycerol, substituted glycerols, fatty acids, and residual alcohols. The method was used on industrial biodiesel samples to evaluate transesterification reaction yields and the efficacy of purification steps. (J. Phys. Chem. A 2009, 113, Article ASAP DOI: 10.1021/jp906543g, José C. Barros)
Metal nanoparticle arrays are templated by block copolymers for plasmonic applications. Researchers at the University of Massachusetts (Amherst) and Ulsan National Institute of Science and Technology (Korea) have developed a block copolymer (BCP)–directed nanofabrication technique to generate substrates for plasmonic applications. The team, led by M. Achermann and S. Park, used a spin-coated, solvent-annealed polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) with well-ordered, vertically oriented cylindrical morphology as a template.
Immersion of the BCP thin film (~28 nm) into a gold precursor solution and subsequent NaBH4 reduction produces gold nanoparticles selectively embedded in P4VP domains. Oxygen plasma etching completely removes the organic BCP template. The process achieves a gold surface coverage of ~14%. The ellipsoidal nanoparticles (~17 nm diam, ~13 nm high) are placed ~43 nm apart, corresponding to the center-to-center distances of the cylindrical P4VP domains in the BCP. An examination of the nanoarray with embedded nanoparticles revealed that the in-plane (0°) resonance exhibited a longer resonance wavelength (533 nm, than the out-of-plane (70°) wavelength of 520 nm.
By using a similar procedure and a template consisting of a blend of polystyrene-b-poly(2-vinylpyridine) and polystyrene-b-poly(ethylene oxide), the authors created a composite array of gold and silver nanoparticles. This composite thin film with embedded gold and silver nanoparticles exhibits two distinct resonance peaks at 411 and 578 nm. Plasma etching oxidizes the silver, as evidenced by the absence of a peak in the 400-nm range.
To prepare plasmonic nanostructured materials, the authors deposited a templated gold nanoparticle array on a 50-nm film of silver with a silicon oxide dielectric layer (6, 10, or 15 nm thick). Reflectivity measurements captured the interaction between the surface plasmon resonance of the gold nanoparticles and the surface plasmon polaritons (SPPs) of the silver layer and showed excitation at a higher SPP coupling angle (larger wave vectors) than a silver control film. Increasing the dielectric thickness enhances this effect. (ACS Nano 2009, 3, 3987–3992; LaShanda Korley)
Conversion of nitriles to amides uses aldoximes as the water source. The nitrile group is extremely important in organic synthesis partly because of its ready transformation to many other functional groups or intermediates. One of the important reactions is the hydration of nitriles to form amides—a process with broad industrial and pharmaceutical applications. This method is limited, however, by the need for a sequence of distinct steps that use strong inorganic acids or bases. Recently, transition-metal catalysis of nitrile hydrolysis has offered mild reaction conditions, but functional group tolerance is still a problem.
S. Chang, H.-Y. Lee, and co-workers at the Korea Advanced Institute of Science and Technology (Daejeon) describe a new approach to this reaction that involves the selective hydrolysis of nitriles with aldoximes under anhydrous and neutral conditions. Their method is a mild process that allows labile functional groups and protecting groups to remain intact. An important feature is that the aldoximes serve as the water source for nitrile hydration. Water is completely excluded from the reaction.
MOM is methoxymethyl. The conversion to amides uses an excess of the aldoxime and is mediated by a moderate amount of [RhCl(PPh3)3] catalyst. The study covered 15 aromatic or aliphatic nitriles and gave near-quantitative yields of the amide products in some cases. An added benefit of this protocol is that no racemization of chiral substrates occurs.
The authors chose acetaldoxime (1) as the standard hydrating agent for practical reasons. It is commercially available, and it simplifies product purification because the acetamide byproduct is easily soluble in water. (Org. Lett. 2009, 11, 5598–5601; W. Jerry Patterson)
Log In