April 12, 2010
Synthesize self-assembled poly(styrenesulfonic acid)-based copolymer membranes. Fuel cells are an attractive technology for converting chemical energy directly into electrical power and are a major focus of energy research. Most fuel cell research has concentrated on proton-exchange membrane fuel cells (PEMFCs). A polyelectrolyte membrane between the cathode and the anode is an essential component for conducting protons.
Sulfonated polystyrene–based copolymers are often used for these membranes. Postsulfonation of polystyrene-based copolymers is most often used to obtain the sulfonated block copolymers. The polystyrene segment is only partially sulfonated, however, and the sulfonation reaction is not well controlled. K.-Y. Baek and co-workers at the Korea Institute of Science and Technology (Seoul) developed a way to synthesize poly(styrenesulfonic acid)-based copolymers directly.
The authors protected the styrenesulfonic acid monomer with neopentyl groups before running the polymerization reaction. They then used atom-transfer radical polymerization, a living radical polymerization technique, to produce a well-defined poly(n-butyl acrylate)-b-poly(neopentyl styrenesulfonate) copolymer (1). Annealing at 120 °C induced self-assembly of the diblock copolymer with lamellar morphology. Heating to 150 °C completely removed the alkyl groups and formed an ionic membrane of copolymer 2 without changing the morphology. (Macromol. Chem. Phys. 2010, 211, 613–617; Sally Peng Li)
How do you separate (R)-2-methylpentanol after a kinetic resolution? Racemic 2-methylvaleraldehyde can be kinetically resolved by treating it with an evolved ketoreductase enzyme. At the end of the reaction, the mixture contains the desired product, (R)-2-methylpentanol, along with unreacted (S)-2-methylvaleraldehyde, water, i-PrOH (the stoichiometric reducing agent), and acetone. Isolating the pure product by fractional distillation is not feasible, and base treatment to convert the aldehyde to higher molecular weight aldol products results in poorer enantiomeric enrichment of the desired (R)-2-methylpentanol.
O. W. Gooding and coauthors at Codexis (Redwood City, CA) and Pfizer (Groton, CT) discovered a binary azeotrope between i-PrOH and 2-methylvaleraldehyde that allows the aldehyde content of the reaction mixture to be reduced. The remaining aldehyde is removed by forming its hydrogen sulfite adduct. (Org. Process Res. Dev. 2010, 14, 119–126; Will Watson)
Here’s an efficient synthesis of cycloheptenones. P. A. Wender and co-workers at Stanford University report an intermolecular [5 + 2] cycloaddition between vinylcyclopropanes and alkynes that is mediated by cationic Rh(I) complex 1. Catalyst 1 is exceptionally reactive; it provides the desired cycloadducts (2) in 5–15 min at room temperature with high yields (91–97%) from most of the reactants studied.
This method offers a convenient way to populate the cycloheptenone ring with a variety of functional groups. The scope of the reaction is broad, including sterically and electronically diverse internal and terminal alkynes. The authors reduced the catalyst loading to as low as 0.2 mol% with no diminution of the high product yields. Complex 1 significantly outperformed all other common neutral and cationic Rh(I) catalysts evaluated.
The authors showed that 1 is easily prepared from [RhCl(cod)2] by treating it with the appropriate silver salt, filtering, and introducing the naphthalene ligand; cod is 1,5-cyclooctadiene. Alternatively, the active catalyst can be generated in situ from a commercially available Rh(I) precatalyst. Catalyst 1 is air- and moisture-stable and retains its high activity even after storage for >6 months.
By changing the structures of the reactants, the authors extended this reaction to form cycloheptene rings with two internal double bonds. (Org. Lett. 2010, 12, 1604–1607; W. Jerry Patterson)
Block-copolymer–patterned graphene has semiconductor applications. M. S. Arnold, P. Gopalan, and co-workers at the University of Wisconsin–Madison developed a block-copolymer patterning strategy to assemble large-area nanoperforated graphene suitable for semiconductor technology. A vertically oriented, hexagonally packed cylindrical morphology of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA, 67 kDa) was patterned onto highly oriented pyrolytic graphite (HOPG) after the surface was primed with a thin wetting layer of silicon
oxide and a neutral layer of polystyrene-r-poly(methyl methacrylate)-r-poly(glycidyl methacrylate) (PS-r-PMMA-r-PGMA, 46.7 kDa).
The PMMA cylinders were removed by UV irradiation; sequential plasma exposure and HF treatment produced nanostructured graphene with preserved, controlled dimensions. Raman spectroscopy confirmed the controlled HOPG patterning and showed the presence of perforated edges and unperturbed crystalline regions. Nanostructured, mechanically exfoliated graphene on a silicon wafer was used to construct field-effect transistor devices to examine the electronic properties of monolayers.
The authors report a substantial reduction in charge transport behavior upon deposition of a silicon oxide wetting layer and an on/off conductance ratio of 6.1 ± 0.3:1. This ratio was maintained during the PS-b-PMMA patterning and UV etching steps. Complete removal of the template and wetting oxide layers enhanced the on/off conductance ratio to 39 ± 7:1 at room temperature because an effective gap (~102 meV) opened as a result of quantum confinement effects. When one of the devices was cooled, its on/off conductance ratio increased from 41:1 at room temperature to ~207:1 at 105 K. The free carrier mobility of >1 cm2/(V∙s) at <10 mV and room temperature was comparable with existing thin-film technology. (Nano Lett. 2010, 10, Article ASAP DOI: 10.1021/nl9032318; LaShanda Korley)
Mechanically racemize hindered-rotation stereiosomers. Atropisomers are molecules with sterically congested bonds that impede rotation, making the molecules potentially chiral. Molecules such as BINOL and BINAP are used in catalysis and organic synthesis; their isomerization barriers usually exceed 30 kcal/mol.
C. W. Bielawski and coauthors at the University of Texas at Austin and at the University of Illinois at Urbana–Champaign present a new method for overcoming this energy barrier and forcing the reconfiguration of the stereoisomers. They envisaged that a tensile force could be used to induce a planar intermediate that results in racemization.
Using copper-mediated single-electron-transfer living polymerization, the authors prepared poly(methyl acrylate) (PMA) from the bifunctional initiator dimethyl (S)-1,1’-binaphthyl-2,2’-bis-(2-bromoisobutyrate). The resulting (S)-polymer (1) was characterized by gel permeation chromatography and circular dichroism (CD). When polymer 1 was sonicated in MeCN solution, the CD signals of aliquots withdrawn from the solution decreased over time, and racemization was 95% complete after 24 h. The authors believe that the sonication applies tensile forces to create planar intermediate 2. Similar results were found when (R)-polymer 3 was sonicated.
Substituting the sonochemical conditions with refluxing in Ph2O at 257 °C did not produce racemic product. Sonicating BINOL with nonpolymeric substituents did not result in racemization, indicating that the mechanically induced transformations are strongly influenced by substituent size. A study of the racemization of several polymers showed that CD signal intensities decay with first-order kinetics; the authors determined the racemization rate as function of polymer molecular weight. No chain scission was detected during the sonication process. This method may provide new techniques for toggling atropisomer configurations. (J. Am. Chem. Soc. 2010, 132, 3256–3257; José C. Barros)
Use both isomers from an epoxide ring opening to produce a chiral pyrrolidine intermediate. A. Ohigashi*, T. Kikuchi, and S. Goto at Astellas Pharma (Ibaraki, Japan) found that the ring opening of (R)-styrene oxide with 3-(benzylamino)propionitrile gives a 68:32 mixture of regioisomers in favor of the linear product. Treating the crude mixture with MeSO2Cl and Et3N in toluene produces a single product, 3-{benzyl-[(2R)-2-chloro-2-phenylethyl]amino}propanenitrile, presumably via a common aziridium ion intermediate.
When the amino alcohol mixture is chlorinated with SOCl2 instead of MeSO2Cl–Et3N, the same β-chloroamine is produced, but in racemic form. Treatment of the chloroamine with base gives a mixture of cyanopyrrolidine epimers that form a single enantiomer of the carboxylic acid on hydrolysis and epimerization under alkaline conditions. (Org. Process Res. Dev. 2010, 14, 127–132; Will Watson)
Vanadium complexes can be used in living radical polymerizations. Several living radical polymerization techniques rely on transition-metal complexes to control the whole reaction by mediating the radical concentration, minimizing bimolecular termination, and preserving the reversible equilibrium between the reactive centers and dormant species. Atom-transfer radical polymerization (ATRP) is a process in which a reduced transition metal complex maintains the reversible equilibrium between a halogen-terminated polymer chain and its related radical. Organometallic-mediated radical polymerization (OMRP) depends on the thermal or photolytic decomposition of metal–carbon bonds in the dormant species.
Many late–transition-metal complexes are used in ATRP and OMRP, but applications of early transition metals are scarce. M. P. Shaver*, M. E. Hanhan, and M. R. Jones at the University of Prince Edward Island (Charlottetown) have for the first time used a vanadium complex for controlling the radical polymerization of vinyl acetate.
The authors believe that V(III) complex 1 controls the radical polymerization of vinyl acetate by two mechanisms. The catalyst first donates a chlorine atom to a radical to form a reduced vanadium complex (2) and an organic chloride; this exchange facilitates controlled polymer chain growth via ATRP. V(II) complex 2 then reversibly bonds with a polymer radical, leading to an OMRP. The “living” aspect of these polymerizations is shown by a linear increase of molecular weight with monomer conversion and first-order monomer kinetics. The identities of the polymer chain end groups, however, were not determined; and the authors plan to investigate the mechanism of this reaction sequence. (Chem. Commun. 2010, 46, 2127–2129; Sally Peng Li)
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