Noteworthy Chemistry

August 19, 2013

 

Make nanotubes from polysiloxanes. Carbon nanotubes (CNTs) are a class of 1-D (i.e., linear) nanostructures that are useful in a wide range of applications. Inorganic nanotubes made from boron nitrides, metals, silica, and other substances are structurally analogous to CNTs but have distinct chemical and physical properties. S. Seeger and colleagues at the University of Zurich prepared polysiloxane-based nanotubes that are made from trifunctional organosilane precursors.

The authors previously prepared superhydrophobic silicone nanofilaments from polysiloxane (Artus, G. R. J., et al. Adv. Mater. 2006, 18, 2758–2762). They chose ethyltrichlorosilane (EtSiCl3) and methyltriethoxysilane [MeSi(OEt)3] as precursors for synthesizing polysiloxane nanotubes at room temperature via chemical vapor deposition (CVD) and liquid-phase coating, respectively.

EtSiCl3 was deposited on glass substrates in a reaction chamber at room temperature in air with various levels of humidity. When the water content was in the range 4.5–5.0 mmol H2O, tapered nanotubes (see figure) were obtained with lengths as great as 12 μm. The base diameter ranged from 200 to 700 nm, and the tip diameter measured between 60 and 200 nm.

Polysiloxane nanotubes prepared from EtSiCl3 at moderate humidity.

Lower humidity (2.5 mmol H2O) favored the formation of nanofilaments (20–40 nm diam), whereas higher humidity led to larger nanostructures such as microtubes (5.1–5.4 mmol H2O) and rings (5.5–5.6 mmol H2O).

The liquid-phase synthesis from MeSi(OEt)3 in toluene was initiated by a trace amount of HCl; it proceeded more slowly than the CVD method. The resulting 5-μm-long nanotubes, however, had 80–100-nm inner diam and a very high aspect ratio of 800:1.

Polysiloxane nanotubes made from EtSiCl3 and MeSi(OEt)3 are very hydrophobic, with contact angles of >130° and >175°, respectively, and sliding angles of 0 ± 1° and <4°, respectively. (Chem. Mater. 2013, 25, 2787–2792; Xin Su)

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Strong phase segregation leads to small copolymer features. C. J. Hawker, U. S. Schubert, L. M. Campos, and collaborators at Friedrich Schiller University of Jena (Germany), the University of California, Santa Barbara, Aberdeen Proving Ground (MD), Korea University (Seoul), Ghent University (Belgium), Columbia University (NY), and the Dutch Polymer Institute (Eindhoven) designed highly phase-segregated polystyrene-b-poly(2-ethyl-2-oxazoline) (PS-b-PEtOx) block copolymers with particle sizes <20 nm for lithographic applications.

The authors obtained PS-b-PEtOx by click-coupling atom-transfer radical polymerized polystyrene with azide functionalities to microwave-assisted, cationic ring-opened PEtOx with alkyne end groups. Hydrophobic polystyrene–hydrophilic PEtOx block copolymers with PEtOx volume fractions (f) between 0.16 and 0.30 and polydispersities from 1.10 to 1.24 were obtained with this efficient coupling reaction.

The authors demonstrated that a long-range cylindrical morphology (f > 0.12) oriented normal to the thin-film surface was obtained by room-temperature toluene vapor annealing and quenching in 90% humidity for ≈24 h. At lower PEtOx content, they observed a spherical microstructure with a lower degree of ordering.

This strategic approach toward block-copolymer lithography provides high-resolution, sub–20-nm features without the need for surface treatments. The polymers have low overall molecular weights as a result of the high degree of phase segregation of the polystyrene and PEtOx blocks. (ACS Macro Lett. 2013, 2, 677–682; LaShanda Korley)

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Beckmann rearrangement “catalysts” are actually initiators. The Beckmann rearrangement of an oxime to an amide is well known in organic synthesis and an important industrial reaction for manufacturing ε-caprolactam from cyclohexanone. The reaction is usually catalyzed by strong acids under harsh conditions, but mild, efficient organic promoters have recently been developed. The mechanism of the reactions that use the newer promoters, however, is not completely understood.

L. A. Eriksson and coauthors at the National University of Ireland–Galway, East China University of Science and Technology (Shanghai), and the University of Gothenburg (Göteborg, Sweden) used density functional theory to study three pathways for the reaction of acetophenone oxime that use the additives shown in the figure. The pathways are a self-propagating cycle and two mechanisms based on Meisenheimer complexes. They focused on whether a Meisenheimer complex is energetically more favored than a self-propagating cycle.

Beckmann rearrangement “catalysts” that are really initiators

The results showed that the pathways for additive 5 are slightly different from the others because of the stability of the carbocation generated from 5. In addition, and independently of the promoter, the results indicated that the self-propagating cycle is energetically favored over the Meisenheimer pathways. The authors conclude that all organic promoters of the Beckmann rearrangement are initiators rather than catalysts. (J. Org. Chem. 2013, 78, 6782–6785; José C. Barros)

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A type III dynamic kinetic asymmetric transformation forms a chiral morpholinone. D. W. Piotrowski and colleagues at Pfizer Worldwide Research and Development (Groton, CT) developed a multigram-scale synthesis of a mineralocorticoid antagonist. In one step, the reaction of (R)-2-phenylglycinol with rac-2-chloropropionyl chloride gave a mixture of diastereomeric amides that can be cyclized with NaH in THF or KO-t-Bu in t-BuOH to give a 90:10 mixture of diastereomeric morpholinones, with the desired cis isomer as the major product.

Further investigation showed that carrying out the cyclization reaction at reflux for 2 days gives the expected 50:50 mixture of diastereomers, whereas if the reaction is stopped after 1 h at room temperature, a 96:4 ratio in favor of the cis isomer is obtained. This is an example of a type III dynamic kinetic asymmetric transformation in which a diastereomeric mixture of enantiomeric pairs occurs is de-epimerized.

The authors explain this result by considering the transition state required for cyclization. The presence of the amide group means that the cis isomer is formed through a lower energy half-chair transition state, whereas the trans isomer must be derived from a higher energy half-boat transition state. (Org. Process Res. Dev. 2013, 17, 934–939; Will Watson)

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A V-shaped luminogen turns into fluorescent nanostructures. Many π-conjugated organic molecules have been synthesized and used as building blocks for making 1-D nanostructures. Most of these molecules, however, lose their ability to emit light when they assemble into the nanostructures. The design and synthesis of π-conjugated molecules that can be used to form 1-D luminescent nanostructures have been difficult.

S. Jiang and co-workers at Jilin University (Changchun, China) developed a V-shaped cyanostilbeneamide derivative (1) that can serve as building block for preparing highly luminescent organogels and nanowires.

V-shaped luminogen

Luminogen 1 is an excellent molecular gelator. It readily self-assembles into organogels with nanofibrous structures in a variety of organic solvents through the cooperative effects of intermolecular hydrogen bonding, π–π stacking, and nitrile interactions. Size-controllable, well-defined 1-D nanowires as long as several millimeters can be easily made by a slow evaporation process.

The individually dispersed nanowires with uniform morphologies are independent of substrates and solvents. Compared with the corresponding solutions, the organogels and nanowires are more luminescent because of aggregation-induced emission. (J. Mater. Chem. C 2013, 1, 4472–4480; Ben Zhong Tang)

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Simplify 1H NMR spectra with instant homonuclear decoupling. 1H NMR spectroscopy is probably the most widely used NMR technique for determining the structures and chemical environments of molecules. Despite the method’s vast popularity, 1H NMR spectra can have deficiencies such as low resolution and severe peak overlap as a result of significant proton scalar coupling (J-coupling).

Among the methods for obtaining pure-shift spectra, one of the most useful is the Zangger–Sterk (ZS) method, in which all spins are decoupled with the use of slice-selective excitation by a weak linear gradient field. But the drawbacks of the ZS method are poor selectivity and prolonged acquisition time.

N. H. Meyer and K. Zangger* at Karl Franzens University of Graz (Austria) improved the ZS method by developing an instant homonuclear broadband decoupling modification based on pulse sequences. The revised method avoids complicated process schemes and significantly reduces acquisition time.

In the presence of a weak field gradient, a band-selective 90° pulse is applied for slice-selective excitation. At the same time, at every 1/3 (3JHH), a ZS-decoupling block, consisting of a hard 180° pulse and band-selective 180° pulses, is applied. The acquisition time for the first fraction is half as long as the subsequent one, so that decoupling is completely achieved in the middle of all fractions.

By using this instant homo-decoupling method, the authors obtained a pure-shift spectrum of azithromycin in 20 min instead of the 10 h necessary with the traditional ZS method. They also completely resolved the crowded region between 1.4 and 2.2 ppm.

The pulse-sequence technique can be integrated into nuclear Overhauser enhancement spectroscopy (NOESY) and total correlation spectroscopy (TOCSY) experiments with improved resolution and reduced acquisition time. (Angew. Chem., Int. Ed. 2013, 52, 7143–7146; Xin Su)

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