Noteworthy Chemistry

January 30, 2012

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An old reagent can be used as an inexpensive carbene source. N-Heterocyclic singlet carbenes (NHCs) have become workhorses in catalysis and organometallic synthesis. Commercially available triazolylidenes such as 1, however, are expensive. U. Siemeling and co-workers at the University of Kassel (Germany) discovered that an inexpensive older compound can be used to generate an NHC.

Nitron (1,4-diphenyl-3-phenylamino-4H-1,2,4-triazolium, 2) is a low-cost, readily available analytical reagent that has been known for more than a century. When the authors treated it with elemental sulfur in THF, the 13C NMR chemical shift of the C=S carbon atom in the product was 164.5 ppm, close to 168.5 ppm for the equivalent carbon in the sulfide of 1. The reaction with sulfur is typical for nucleophilic carbenes, and the researchers conclude that 2 has the tautomeric carbene structure 3 in concentrations too low to be detected in its NMR spectra.

The authors also treated Nitron with the complex [{Rh(μ-Cl)(CO)2}2] in THF. (COD is 1,5-cyclooctadiene.) Analysis of the Tolman electronic parameter of the product showed that Nitron is an NHC that has moderate donor strength. X-ray diffraction spectroscopy of the Nitron–rhodium complex confirmed that the NHC-type tautomer 3 is a minor component of Nitron, but that it is important for the reactivity of the compound in solution. Nitron thus becomes the least expensive NHC available. (Chem. Commun. 2012, 48, 227–229; JosÉ C. Barros)

Surfaces control polymer nanocomposite dielectric properties. Z.-M. Dang and coauthors at Beijing University of Chemical Technology and the University of Science and Technology Beijing functionalized the surfaces of multiwall carbon nanotubes (MWNTs) to alter the interfaces of polymer nanocomposites and potentially their dielectric properties. They applied a uniform ≈5-nm thickness of emeraldine base (EB) to the surfaces of the MWNTs by oxidatively polymerizing aniline. The EB-functionalized MWNTs (MEBs) were blended and hot-molded with poly(vinylidene fluoride) (PVDF).

The PVDF–MEB nanocomposites had a higher percolation limit than PVDF–MWNT nanocomposites because of the nonconducting EB coating. At or above the percolation threshold, the dielectric permittivity was higher, and the loss tangent and AC conductivity were lower, in the PVDF–MEB nanocomposites compared with PVDF–MWNT at lower frequencies.

The authors attribute this result to the minimization of leakage loss and intimate contact of the MWNT core in the MEB–PVDF materials. The enhancement, however, did not occur at higher frequencies. At a frequency of 100 Hz and above the percolation limit, the dielectric constant increased with increasing temperature for both nanocomposites. The dielectric loss tangent was higher for MEB–PVDF at higher temperatures, whereas the MWNT–PVDF systems produced the opposite result. The authors believe that the conductive MWNT network is disrupted at >100 °C, increasing the dielectric loss tangent. (ACS Applied Materials & Interfaces 2011, 3, 4557–4560; LaShanda Korley)

Chemical stimuli trigger aggregated fluorescent probes. Many fluorescent probes based on photophysical processes such as photo-induced electron transfer, internal charge transfer, and fluorescence energy transfer have been developed. Using a luminescent process called aggregation-induced emission (AIE), which does not involve energy or charge transfer, G. Zhang, S. Liu, and co-workers at the University of Science and Technology of China (Hefei) developed a new strategy for making fluorescent probes that respond to multiple analytes.

A charge-generation polymer (CGP) can undergo stimulus-triggered transformations from the neutral state to the charged state in the presence of a specific analyte, whereas an AIE fluorogen becomes emissive when its molecules aggregate. The researchers believe that specific combinations of oppositely charged CGPs and AIE fluorogens will help in developing stimulus-responsive fluorescent probes.

The researchers synthesized a series of CGPs with caged amine substituents, then transformed them into cationic polyelectrolytes (shown schematically as 1) by chemoselectively cleaving their carbamate protecting groups with biologically relevant reagents such as H2O2 and thiols. This analyte-triggered transformation causes a negatively charged AIE fluorogen (e.g., 2) to aggregate as a result of electrostatic interactions and dramatically intensifies its light emission. (Angew. Chem., Int. Ed. 2012, 51, 455–459; Ben Zhong Tang)

Here’s a case that stresses the importance of order of addition. J. P. Scott and co-workers at Merck (Hoddesdon, UK, and Rahway, NJ) report a practical synthesis of a hepatitis C virus polymerase inhibitor. They found that the only feasible way to convert a chiral eight-membered heterocyclic alcohol to the corresponding amine with inversion of configuration requires the formation of an intermediate azide. Instead of hazardous NaN3, they used a Mitsunobu inversion reaction with diphenylphosphoryl azide (DPPA).

The order in which the reagents are added is critical in this process because PPh3 reacts with DPPA to form the corresponding iminophosphorane with release of nitrogen. To prevent this, the authors activated the secondary alcohol with diisopropyl azodicarboxylate (DIAD) before they added PPh3. DPPA was added only after the PPh3 fully reacted with DIAD.

In addition, they added i-Pr2EtN at the beginning of the reaction to ensure basic conditions throughout and prevent HN3 formation. Adding more PPh3 avoids the need to isolate the intermediate azide, which is completely consumed via a Staudinger reduction. (Org. Process Res. Dev. 2011, 15, 1116–1123; Will Watson)

Triarylamine groups transform silicone polymer properties. Compounds such as triarylamines are well-known organic hole-transporting materials. Many conventional polymers have been modified with triarylamines or their analogues to improve electrooptic performance in applications such as organic light-emitting diodes (OLEDs) and photorefractive materials.

T. P. Bender and coauthors at the University of Toronto and McMaster University (Toronto) recognized the usefulness of reported triarylamine-modified silicone polymers for these applications. They now describe the use of Piers–Rubinsztajn conditions to prepare silicone–triarylamine hybrid polymers cleanly and easily. The figure illustrates the synthesis of a triarylamine and its subsequent grafting onto a conventional poly(phenylmethylsiloxane).

The synthesis begins by treating aniline derivative 1 with bromoanisole 2 under Buchwald–Hartwig coupling conditions to produce intermediate diarylamine 3. (The ligand dba is dibenzylideneacetone.) Further coupling with bromobiphenyl 4 forms the triarylamine 5, which contains a reactive anisole unit that acts as a substrate for the subsequent Piers–Rubinsztajn coupling reaction. The coupling “decorates” polysiloxane structure 6 with triarylamine moieties.

The authors chose a moderate molecular weight version of 6 (Mn 1.39 kDa) that contains reactive in-chain silicon hydride groups. Approximately 60% of the available Si–H bonds participate in the coupling reaction under optimum conditions to form target polymer 7. The reaction conditions were chosen to quantitatively incorporate 5 into the polymer.

The only purification of 7 is treatment with alumina to remove the remaining boron catalyst. No redistribution or metathesis of the modified silicone polymer occurs. The coupling reaction produces C–O–Si bonds that are normally susceptible to hydrolytic cleavage. Structure 7, however, demonstrates excellent hydrolytic stability in air—perhaps the result of significant steric shielding by the bulky triarylamine groups.

Polymer 7 has electronic and optical properties that are comparable with those of the parent triarylamines, as verified by cyclic voltammetry, optical absorption, and fluorescence spectroscopy. It retains silicone polymers’ physical properties such as high thermal stability and low glass-transition temperature.

The authors comment that the remaining Si–H bonds in 7 do not appear to affect the polymer’s electrooptic properties. If needed, the hydride functionality can be rendered inert with the proper choice of terminating groups. (Macromolecules 2011, 44, 723–728; W. Jerry Patterson)

Supramolecular interactions induce magnetic alignment over a large area in a block copolymer. C. O. Osuji and colleagues at Yale University (New Haven, CT) tailored the magnetic alignment of a block copolymer by making use of supramolecular interactions. They selectively incorporated an imidazole mesogen with 3.3-nm monolayer spacing into the polyacrylate block of a low–molecular weight (9 kDA), lamellar-forming polystyrene-b-poly(acrylic acid) (PS-b-PAA) via hydrogen bonding.

The liquid-crystalline side-chain block copolymer forms a highly ordered, lamellar structure, but it has smaller domain spacing than neat PS-b-PAA when the ratio, R, of biphenyl ligands to acrylic acid binding sites is ≤0.2:1. At these low R values, the copolymer does not align in a magnetic field.

When R is 0.33:1 or 0.5:1, applying a 5-T magnetic field produces a lamellar-within-lamellar morphology that is aligned perpendicular to the field. The authors believe that above the binding limit of 0.3:1 additional imidazole mesogens associate with the bound mesophase and respond to temperature changes up to 180 °C. The increased association enhances mobility and aids magnetic alignment.

At R ≥ 0.5:1, the copolymer forms a hierarchical arrangement of polystyrene cylinders surrounded by a smectic liquid crystal mesophase. Magnetic field alignment orients the cylindrical polystyrene long axis parallel to the applied field at R = 0.5. The authors believe that a shift from homeotropic to planar anchoring of the biphenyl mesogens as R increases from 0.4:1 to ≥0.5:1 is likely the result of an abundance of unbound mesogenic ligands. Polarized optical microscopy shows that the authors’ technique achieves magnetic alignment over a large area. (ACS Macro Letters 2012, 1, 184–189; LaShanda Korley)

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