Noteworthy Chemistry

September 19, 2011

Directly isolate orange essential oil with ionic liquids. Essential oils are complex mixtures of substances from natural sources used in products such as perfumes, cosmetics, nutraceuticals, and pharmaceuticals. Essential oils are usually obtained from crude materials by steam distillation, solvent extraction, or cold pressing. These processes can consume high amounts of energy, require toxic solvents, or thermally decompose the oils’ constituents.

K. Bica,* P. Gaertner, and R. D. Rogers* at the Vienna University of Technology and the University of Alabama (Tuscaloosa) used ionic liquids (ILs) as solvents for extracting essential oils. The ILs have negligible vapor pressures and can dissolve lignocellulosic plant biomass. The authors chose ILs 1‍–‍3 (see figure), which were previously reported to dissolve biomass, and focused on the isolation of limonene from orange peels.

The authors suspended 10 g of grated orange peel in 40 g of the IL at 80 °C for 24 h, after which they vacuum distilled the solution using a Kugelrohr apparatus. The distillate contained two layers: water and limonene, the major component of orange oil. The researchers obtained the best results with IL 3: The orange peel yielded as much as 5% of its mass as limonene. This is higher than yields from steam distillation or extraction.

GC–NMR analysis of the extracted oil shows that limonene is obtained in high purity. Six minor components are also present, but there are no traces of IL or degradation products. The IL can be recycled by adding water to precipitate the biomass, filtering, and evaporating the water.

The researchers also evaluated extraction of the IL solution with EtOAc, but this experiment proved unsuccessful because of emulsion formation, the large volume of solvent required, and the low purity of the essential oil obtained. The method developed in this study shows promise for biomass processing in the fragrance and flavor industries. (Green Chem. 2011, 13, 1997–1999; JosÉ C. Barros)

Oxidize the polyarene sumanene to form ketone derivatives. The bowl-shaped, π-conjugated fused polyaromatic compound sumanene (1) features three benzylic positions capable of being functionalized. Structure 1 also exhibits high electron mobility, which suggests the possibility for enhancing its electronic properties by functionalizing it to increase π-conjugation lengths and provide a stronger electron acceptor capability.

T. Hirao and co-workers at Osaka University (Japan) found that simple peroxide oxidation of 1 at the benzylic sites provides monooxosumanene 2 or the more highly oxidized trioxosumanene 3. These compounds, like sumanene, have intriguing bowl-shaped conformations. X-ray crystallographic analysis showed that 2 has a bowl depth of 1.11–1.13 Å. This sumanene oxide exhibits columnar stacking in the crystalline state.

A comparison of the absorption spectra of 1, 2, and 3 in solution showed significant differences. The high end of sumanene absorption is at ≈375 nm, whereas 2 and 3 have end absorptions at ≈495 and ≈550 nm, respectively. In the solid state, 2 is yellow and 3 is orange, in contrast to 1, which is white. These observations suggest that the carbonyl groups expand the π-conjugation.

Compound 3 easily undergoes stereospecific Grignard trimethylation by 1,2-addition at its carbonyl groups to form triol 4 as a single isomer. On the basis of chemical shift values from 1H NMR spectra and agreement with values calculated by density functional theory, the authors suggest that the methyl groups in 4 are in the exo positions. (J. Org. Chem. 2011, 76, Article ASAP DOI: 10.1021/jo2012412; W. Jerry Patterson)

Accurately control a simultaneous addition reaction through metering by weight, not volume. Indoles can be N-aminated on a large scale by simultaneously adding a solution of indole and hydroxylamine-O-sulfonic acid in N-methylpyrrolidone (NMP) and a solution of KO-t-Bu in NMP to a reactor. The presence of a slight excess of base at all times ensures the formation of the indolyl anion. The molar addition rates must be controlled within ≈2.5% deviation—preferably within 1%.

F. J. Weiberth and co-workers at sanofi-aventis U.S. (Bridgewater, NJ) found that the additions can be sufficiently controlled by using mass flow meters rather than volume flow meters, which are more susceptible to fluctuations in temperature, pressure, viscosity, and even composition because of entrained bubbles. The best mode of operation was to add the reactant solutions below the surface of the reaction mixture and 180° apart to ensure good mixing and to avoid simply neutralizing the acid and the base. (Org. Process Res. Dev. 2011, 15, 704–709; Will Watson)

A heptazethrene with a singlet open-shell ground state is stabilized by carboximide and bulky aromatic groups. Singlet diradical polycyclic aromatic hydrocarbons (PAHs) have intriguing structures and unique properties. Because of their open-shell character, however, most singlet diradical species, including heptazethrene, are difficult to prepare and are unstable.

K.-W. Huang* at King Abdullah University of Science and Technology (Thuwal, Saudi Arabia) and Z. Sun and J. Wu* at the National University of Singapore report the first synthesis of a stable, soluble heptazethrene derivative (1) with remarkable singlet diradical character in the ground state (right-hand structure in the figure).

The researchers used an intramolecular coupling pathway to synthesize a key intermediate, octadehydronaphthoannulene; its transannular cyclization forms 1 as a dark green solid. Because they designed the molecule to contain electron-withdrawing dicarboximide groups and bulky diisopropylphenyl groups, 1 is stable and soluble in common organic solvents. These attributes allow it to be isolated, purified, and spectroscopically identified and make it a good candidate for practical applications in materials science, molecular electronics, and information processing. (J. Am. Chem. Soc. 2011, 133, 11896–11899; Ben Zhong Tang)

Make structured poly(ethylene oxide)–cellulose nanofibers. C. Weder, P. Supaphol, and coauthors at Chulalongkorn University (Bangkok, Thailand), the University of Fribourg (Marly, Switzerland), and Case Western Reserve University (Cleveland) devised an electrospinning technique to prepare uniaxially oriented poly(ethylene oxide) (PEO)–cellulose nanowhisker (CNW) composites. They dissolved high–molecular weight PEO (3000 kDA) and high–aspect ratio (≈100:1) tunicate-derived CNWs in water in concentrations ranging from 0 to 20 wt% CNWs relative to ≈2.5% (w/v) PEO.

The authors modified the electrospinning setup to produce large-scale, uniaxial alignment of the PEO–CNW nanocomposites in the form of smooth electrospun nanofibers with 430–460 nm diam, depending on concentration. Transmission electron microscopy of thinner (≈100 nm) PEO–CNW nanofibers showed that the CNWs are aligned along the axis and within the core of the fiber, suggesting that the nanocomposite is organized hierarchically. The mechanical properties of the electrospun nanofiber arrays demonstrated the reinforcing effect of CNWs on the PEO matrix. One example can be found in the twofold increase in storage modulus at 15 wt% CNWs relative to PEO concentration. (Macromol. Rapid Commun. 2011, 32, 1367–1372; LaShanda Korley)

Poly(aryl ether)s contain sulfonamide as an activating group. Poly(aryl ether)s are an important class of engineering polymers with desirable thermal and mechanical properties. These structures are typically formed by the condensation polymerization of an aromatic diol with an activated aryl dihalide via an SNAr mechanism.

N. T. Rebeck and D. M. Knauss* at the Colorado School of Mines (Golden) note that the sulfonamide group is strongly electron-withdrawing at the ortho and para positions of the phenyl ring and should activate fluorine substituents at those positions. The researchers developed a polymerization strategy that uses 2,4-difluoro-N,N-dimethylbenzenesulfonamide (1) as an activated monomer. Condensation polymerization of 1 with a series of diphenols such as 2 leads to a new family of poly(aryl ether sulfonamide)s (3).

Adding K2CO3 to the reaction allows the phenoxide nucleophile to form in situ. The activating effect of the sulfonamide groups produces polymers with molecular weights as high as 446 kDa (Mw) and 109 kDa (Mn) when bisphenol A is the diol. The polymers are typically completely soluble in DMF, DMSO, and N-methylpyrollidine (NMP).

An important feature of the polymers is the presence of the latent sulfonamide group, which can be converted to the corresponding sulfonic acid. This modification may provide another type of functionalized poly(aryl ether) with applications in fuel-cell membrane and other technologies. (Macromolecules 2011, 44, 6717–6723; W. Jerry Patterson)

Make “dry ionic liquids”. Ionic liquids (ILs) maintain the liquid phase over wide temperature ranges. They have a large variety of applications (see “Directly isolate orange essential oil with ionic liquids” above), and their low vapor pressures allow them to be used as “green” solvents. They are useful as inert solvents and dispersion media, and they can dissolve polar and nonpolar molecules.

The use of ILs in solid formulations has not been reported until now. K. Shirato and M. Satoh* at the Tokyo Institute of Technology report a method for transforming ILs to powders by mechanical shearing. They call the powdered solids “dry ionic liquids”.

The authors used ILs 1‍–‍4 in their experiments. They mixed the ILs with hydrophobic fumed silica and subjected the mixtures to vigorous stirring. ILs 1 and 2 failed to produce powders; instead, the mixtures formed “soufflÉ-like” materials. The researchers attribute this to the low surface tensions of the liquids.

The other two ILs have higher surface tensions, and mixing them with silica conserves its powder form rather easily. The authors believe that the microscale IL droplets contained in the silica particles may have applications as “micro–reaction vessels”. (Soft Matter 2011, 7, 7191−7193; Sally Peng Li)

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