February 23, 2009
An elusive aromatic hybrid organic–inorganic benzene has been isolated. Borazine (1), benzene’s isoelectronic inorganic counterpart, exhibits the remarkable feature of aromaticity. Although borazine was reported in 1926, the corresponding hybrid structure 1,2-dihydro-1,2-azaborine (2) has eluded synthesis and characterization. D. A. Dixon, S.-Y. Liu, and coauthors at the University of Alabama (Tuscaloosa) and the University of Oregon (Eugene) report an innovative synthesis of 2. Their accomplishment demonstrates that 2 is not only isolable but has remarkable stability that is consistent with substantial aromatic character.
In the figure, TBS is tert-butyldimethylsilyl and Cy is cyclohexyl. In the first step of the synthesis, allylboron dichloride is coupled with a protected allylamine to yield a diene (3) that contains the critical B–N bond. The conformation of 3 provides the appropriate structure for ring-closing metathesis to give two isomers of heterocycle 4. Dehydrogenation of 4 followed by treatment with LiBHEt3 incorporates the desired B–H functionality (5).
Ordinarily, removing the N-protecting group would lead directly to 2. However, attempts to isolate 2 in this way were unsuccessful, and the authors were forced to treat 5 with [Cr(CO)3] as a temporary “protecting group” to yield complex 6. The N-protecting group could then be removed to install the necessary N–H bond in 7 as the precursor to 1. Decomplexation of 7 gives the target organic–inorganic hybrid 2.
A solution of 2 shows no appreciable degradation when heated at 60 °C for 5 days. The authors characterized 2 by using standard spectral techniques and found the data to be consistent with their proposed structure for 1,2-dihydro-1,2-azaborine.
An important part of this study was to determine the aromatic character of 2 because the all-inorganic heterocycle borazine 1 is considered to be less aromatic than benzene. Several facts and measurements support a considerable aromatic character for 2. Previously synthesized 1,2-azaborines underwent electrophilic aromatic substitution reactions readily. The authors recently provided structural data showing that 1,2-azaborines have delocalized structures consistent with aromaticity (Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250–7252).
Although the authors could not determine the crystal structure of 2, they obtained the structure of its [Cr(CO)3] complex 7, which showed similar bond lengths when compared with the corresponding benzene [Cr(CO)3] complex. They also determined the binding energy of the 1,2-azaborine ring to [Cr(CO)3] to be –54.4 kcal/mol—essentially the same as the value for the benzene complex.
As part of this study, the authors computed the resonance stabilization energy of 2 to be 21 kcal/mol, ~13 kcal/mol less than that of benzene. Based on the accumulated data, they consider 2 to be significantly aromatic. (Angew. Chem., Int. Ed. 2009, 48, 973–977; W. Jerry Patterson)
Ion association generates highly fluorescent organic nanoparticles. Luminescent nanoparticles are potentially useful for making light-emitting diodes, organic lasers, sensors, and the like. To realize this potential, versatile processes for producing such nanoparticles must be developed. H. Yao*, M. Yamashita, and K. Kimura at the University of Hyogo (Japan) developed an ion-association technique for preparing organic nanoparticles that fluoresce efficiently in the solid state.
When 2-[4-(dimethylamino)styryl]-1-ethylpyridinium (1), a cationic luminogen, is mixed with an anionic borate such as tetrakis(4-fluorophenyl)borate (2) in the presence of aqueous poly(vinylpyrrolidone), nanoparticles with ~30–100-nm diam are formed as a result of the electrostatic interaction between ion pairs. Whereas the iodide salt of 1 in water is virtually nonluminescent, the nanoparticles of the 1–2 pair are highly emissive; their fluorescence quantum yield is >20-fold higher than that of the iodide.
The researchers believe that the fluorescence comes from an intramolecular charge-transfer excited state stabilized by the matrix of 2. They also conjecture that the emission enhancement is caused by the high rotational resistance around the single bond in 1 and the effect of matrix polarity that suppresses nonradiative processes. (Langmuir 2009, 25, 1131–2237; Ben Zhong Tang)
Expeditiously extract biodiesel from sunflower seeds. Biodiesel is a diesel fuel derived from renewable natural resources such as biomass, vegetable oils, and waste cooking oil. Multiple processing steps are required to produce fatty acid methyl esters (FAME), the principal components of biodiesel, leading to low profits for manufacturers.
An improvement on the multistep method is the in situ transesterification process, which permits extraction and transesterification in one batch. MeOH is a necessary reactant in the transesterification process, but it is a far-from-ideal solvent for the product oil. This situation has prompted researchers to modify this technique.
Y. Wang and co-workers at the Chinese Academy of Sciences (Beijing) discovered that diethoxymethane (DEM) is miscible with MeOH and FAME; and they developed an easy method to obtain crude biodiesel from sunflower seeds as a solution in MeOH and DEM. Adding DEM does not significantly increase the yield of FAME, but it reduces the amount of MeOH required to complete the reaction. (Ind. Eng. Chem. Res. 2009, 48, 850–856; Sally Peng Li)
Silver–gold core–shell nanoparticles supported on porous solids are thermally stable and catalytically active. The reactivity of gold-based solid catalysts depends strongly on their particle size. This makes the preparation of solid-supported gold nanoparticles with appropriate sizes extremely important. T. Zhang and coauthors describe a two-step method for synthesizing bimetallic Au–Ag nanoparticles supported on SiO2. They believe that silver helps stabilize gold nanoparticles by forming a Au–Ag alloy.
Gold nanoparticles are first supported on amino-functionalized SiO2 (or Al2O3) by using NaBH4 reduction (a in the figure). In the next step, silver ion adsorbs on the negatively charged gold nanoparticles (b); this is followed by another NaBH4 reduction to form bimetallic nanoparticles with a gold core and a silver shell (c). These particles are very thermally stable; their particle size remains unchanged upon calcination in air at 500 °C (d) or hydrogen reduction at 550 °C (e). The Au–Ag core–shell alloy nanoparticles display excellent performance in CO oxidation catalysis, even in a hydrogen-rich environment. (Chem. Mater. 2009, 21, 410–418; George Xiu Song Zhao)
Clay fractionation leads to optical clarity in polymer nanocomposites. S. R. Raghavan and coauthors at the University of Maryland (College Park) and the National Institute of Standards & Technology (Gaithersburg, MD) combined fractionation and surface-modification methods to enhance the optical clarity and rheological behavior of polystyrene (PS)–clay nanocomposites. They centrifuged aqueous dispersions of commercial montmorillonite (MMT) to generate fractionated montmorillonite (FMT). Compared with MMT dispersions, the FMT dispersions are less turbid, exhibit exfoliated clay platelets devoid of large (>1 μm) aggregate structures, display gelation behavior at lower clay concentrations, and are more birefringent. These observations confirm the nanoscale, more monodisperse structure of FMT.
Organically-modified FMT (o-FMT) particles and organophilic, commercially available Cloisite 15A MMT, both with ~60 wt% organic content, were loaded into a high-molecular weight PS matrix at various concentrations. At 5 wt% organoclay loading, X-ray diffraction studies showed that the Cloisite 15A–PS and o-FMT–PS nanocomposites have intercalated morphologies. However, examination via transmission electron microscopy showed that ordered, nanoscale clay stacks or tactoids of a narrow size distribution exist in the o-FMT–PS nanocomposite, whereas a mixture of meso- and nanoscale structures exists in the Cloisite 15A–PS system.
The authors observed that transparency (measured as percentage of transmission) is higher for the o-FMT–PS nanocomposites because of the smaller, homogenous o-FMT clay structures compared with the more polydisperse Cloisite 15A. There is also a marked difference in rheological properties; the elastic modulus G’ and complex viscosity η* are significantly higher for o-FMT–PS because of the ability of the o-FMT organoclay to form effective, jammed networks that reinforce the PS matrix. For example, at 5 wt% clay, the low frequency G’ is a factor of 5 higher for the o-FMT–PS. This study illustrates the possibility of reinforcing matrix materials without sacrificing clarity. (ACS Appl. Mater. Interfaces 2009, 1, 130–135; LaShanda Korley)
A palladium–curcumin complex may help fight prostate cancer. Curcumin is a yellow pigment of turmeric, the powdered rhizome of the herb Curcuma longa L., which has long been associated with beneficial health effects such as anti-inflammatory activity and cancer prevention. This compound exhibits cytotoxic effects in human prostate cancer cell lines and acts as a free-radical scavenger and antioxidant to inhibit oxidative damage to DNA.
A. Valentini and coauthors at the University of Rome Tor Vergata and the University of Calabria (Italy) have developed a new palladium bifunctional ionic complex (1) that consists of the curcumin anion and a 2,2’-bipyridinium dication substituted at the 4- and 4’-positions with aliphatic chains. The new complex was evaluated against human prostate cancer cell lines using curcumin as a standard. The complex inhibits cell proliferation and is less toxic than curcumin itself. Whereas curcumin delays cell growth by inducing G2/M arrest, the palladium complex inhibition is the result of apoptosis induction. Also, the complex displays lower oxidant properties than curcumin in target cells, possibly by producing fewer reactive oxygen species.
The authors believe that the new complex may avoid prostate cancer–acquired resistance to electrophilic anticancer drugs by glutathione S-transferase π overexpression and induced apoptosis in cell lines mediated by c-Jun N-terminal kinase phosphorylation. (J. Med. Chem. 2009, 52, 484–491; José C. Barros)
Asymmetric synthesis of substituted oxetanes depends on sequential addition of sulfur ylides. Previously reported strategies for enantioselective synthesis of useful 2,2-disubstituted oxetanes are limited in scope. However, S. Matsunaga, M. Shibasaki, and co-workers at the University of Tokyo now report a strategy that uses double methylene transfer produced by sequential treatment of ketone substrates with dimethyloxosulfonium methylide.
The reaction proceeds through an intermediate epoxide and is mediated by an interesting chiral LaLi3tris(binaphthoxide) complex (1; the S-enantiomer is shown). All oxetanes were produced in at least 99% ee, and some products from alkyl methyl ketone substrates (e.g., 2) were obtained in essentially optically pure form (>99.5% ee).
A key additive to the reaction was a triarylphosphine oxide in which Ar = 2,4,6-trimethoxyphenyl. This ingredient dramatically improved reactivity and selectivity. Attractive features of this procedure include mild conditions (room temperature) and a one-pot process. The chiral amplification of the process in the second reaction of the epoxides with the sulfur ylide was the key to forming the oxetane products with high optical purity. (Angew. Chem., Int. Ed. 2009, 48, 1677–1680; W. Jerry Patterson)
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