Noteworthy Chemistry

December 12, 2011

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These amphiphilic phospholes are mesomorphic and luminescent. Heteroatomic π-conjugated molecules often have useful properties that are difficult, if not impossible, to obtain with their all-carbon counterparts. The phosphole ring is a phosphorus-based π-conjugated structure with features not seen in other heteroatomic systems. Because they are synthetically challenging, however, few examples of amphiphilic phospholes are known; and their mesomorphic and luminescent properties have not been systematically studied.

A research team led by T. Baumgartner at the University of Calgary (AB) synthesized a series of “phosphole-lipids” that combine the conjugated phosphole skeleton with amphiphilic lipid substituents. They then studied their liquid-crystalline and light-emitting behavior. They found that intermolecular ionic interactions are effective driving forces for stabilizing lamellar liquid crystals and soft crystal phases in the ionic phospholes. Their well-preserved, highly ordered organization under ambient conditions makes the phospholes promising candidates for organic electronics applications.

The phospholes’ dynamic structural feature induces unusually intense luminescence in the solid state—up to 40 times as high for compound 1 in the figure than its emission in solution. The authors attribute this result to aggregation-induced enhanced emission. The flexible structure also makes the systems’ solution emissions as thermally responsive as the red shifts obtained by chemically modifying the phosphorus center. In the solid state, compound 2 displays a mechanically responsive emission that can be reversed by thermal annealing. (J. Am. Chem. Soc. 2011, 133, 17014–17026; Ben Zhong Tang)

Prepare antiaromatic dications without magic acid. The study of antiaromatic dications was recently advanced by “magic acid” (SbF5·FSO3H)–mediated ionization of diol precursors at –78 °C (Dahl, B. J.; Mills, N. S. J. Am. Chem. Soc. 2008, 130, 10179–10186; Org. Lett. 2008, 10, 5605–5608). This synthetic method, however, requires SO2ClF in addition to SbF5; both reagents can generate highly dangerous HF when exposed to trace moisture.

As part of their research on antiaromatic indenyl dications, S. P. McClintock and N. S. Mills* at Trinity University (San Antonio, TX) prepared more stable dicationic species at room temperature without the need for magic acid reagents. A key to their method is the protonation of diol 1 with FSO3H and trifluoroacetic anhydride (TFAA) in SO2Cl2 solvent to form difluorenyl dication 2.

In the SO2Cl2 reaction medium, the reaction produces a deep red solution that indicates the presence of dications. Adding NaPF6 to the reaction mixture provides the preferred PF6 counterion. It appears that TFAA acts as a drying agent for water eliminated during the reaction. The authors note that TFAA promotes the stability of the dications and the resolution of their NMR spectra.

Dication 2 shows no evidence of decomposition after 8 h at room temperature, and it remains the primary component in the solution after 4 days. These are the most stable, longest lasting antiaromatic dications reported to date.

The authors extended their method to preparing 9-phenylfluorenyl cation 3 at room temperature. No decomposition of 3 was observed after 24 h. When SO2ClF is the solvent instead of SO2Cl2, and the reaction temperature is –78 °C, dication 4 is formed from the corresponding diol. This transformation does not occur in magic acid, and it is the first example of an indenyl cation formed by ionizing an alcohol precursor. (J. Org. Chem. 2011, 76, Article ASAP DOI: 10.1021/jo201512n; W. Jerry Patterson)

Use design of experiments to improve a reaction workup. P. D. de Koning*, D. J. McManus, and G. R. Bandurek at Pfizer Global Research and Development (Sandwich, UK) ran into significant problems when they attempted to scale up the synthesis of a nonsteroidal progesterone receptor antagonist. One step in the sequence, N-alkylation of a pyrazole with ClCH2SMe, produced material that contained an unexpectedly high amount of starting material and a regioisomeric impurity that carried through the subsequent stages and led to out-of-specification final product.

The two regioisomeric products degraded during the reaction workup—specifically during the formation of the hydrogen sulfate salt. The authors conducted a statistical study using design of experiments to examine the effects of temperature, H2SO4 charge, water content, and time of addition. The results showed that temperature was the dominant factor.

Adding H2SO4 at 20 °C instead of 50 °C and aging for 3 h instead of 6 h reduce contamination by starting material. The remaining reactant is purged during an MeCN slurrying step. In addition to a purer product, the modifications increase the isolated yields of the alkylated pyrazole. (Org. Process Res. Dev. 2011, 15, 1081–1084; Will Watson)

Household chemicals can destroy chemical warfare agents. Chemical warfare agents (CWAs) are normally decontaminated with products for military use that contain H2O2. The decomposition mechanisms of CWAs VX (1), GD (2), and HC (3) are shown in the figure. Despite the environmental friendliness of H2O2, the concentrations used are so high that their use is generally restricted to trained personnel. G. W. Wagner at the US Army Edgewood Chemical Biological Center (Aberdeen Proving Ground, MD) describes combinations of household materials that could be made available to the public for decontaminating CWAs.

The author first tested an ammonia-based floor-cleaner against the three CWAs. He added the decontaminant to the CWA in an NMR tube and analyzed the mixture by using 31P and 1H NMR. The results indicated that only water-soluble 2 is converted to harmless materials. Alkaline hydrolysis of 1 converts it to toxic compound 4.

Next, the author formulated readily available 3% H2O2 solution with NaHCO3, Na2CO3, and/or i-PrOH and tested them against the CWAs. A H2O2–NaHCO3i-PrOH mixture was the best combination for decontaminating 3, whereas H2O2–Na2CO3 was the best for 1 and 2. A “universal” solution—H2O2 with 5% NaHCO3—decomposes all three CWAs if a sufficient amount is used. These mixtures can be easily prepared by the public to decontaminate domestic property and personal possessions. (Ind. Eng. Chem. Res. 2011, 11, 12885-12887; JosÉ C. Barros)

[This article contains a disclaimer that it reflects only the author’s views and is not the policy or position of the Department of the Army, the Department of Defense, or the US Government. It is odd that an employee of the military would communicate to the public in this way.—Ed.]

Use atomic force microscopy to study block copolymer surfaces. Atomic force microscopy (AFM) is a high-resolution imaging technique for characterizing polymer surfaces by moving a sharp probe along the sample. Another AFM application measures responses of surfaces to applied stimuli. Now, K. Nakajima and co-workers at Tohoku University (Sendai, Japan) report the use of AFM to explore the behavior of copolymer membranes under mechanical force and compare the results under static and dynamic conditions.

The authors’ test material was a triblock copolymer, poly(styrene-b-ethylene-co-butylene-b-styrene). The rigid polystyrene and the soft poly(ethylene-co-butylene) blocks are immiscible, and their phase separation produces patterns on the membrane surfaces.

In dynamic amplitude-modulation AFM, the tip of the probe operates in a tapping mode, touching the sample and disengaging completely in an oscillating cycle. The applied force varies, and the frequency is constant. The relaxation of the polymer chains is a function of the interaction force.

Force-volume AFM operates in a static mode: The tip is in close contact with the surface and moves along it with constant force. The two polymeric blocks exhibit different mechanical behavior in both operating modes. The authors conclude that the two types of AFM are useful for mechanically investigating copolymer surfaces and that the results are quantitatively comparable. (Macromolecules 2011, 44, 8693−8697; Sally Peng Li)

Hydrophosphonate olefins to prepare Z-selective Wittig reagents. One common method for preparing the diverse, valuable class of phosphine reagents is hydrophosphination, the addition of a P–H bond to a C–C multiple bond. S. D. Daeffler and R. H. Grubbs* at Caltech (Pasadena, CA) now report alternative methods: radical-mediated or photochemically mediated addition of triarylphosphonium tetraborate to olefins. This concept is termed hydrophosphonation; it involves radical-mediated P–H bond addition of [HPPh3][BF4] to unactivated olefins.

The authors first assessed standard radical initiators for the hydrophosphonation of olefin 1 to form the corresponding phosphonium salt 2. 1,1’-Azobis(cyclohexanecarbonitrile) (ACN) is the most effective initiator, producing 2 in up to 94% conversion.

The authors then developed an alternative to the more standard radical technique—a photochemical reaction that occurs under mild conditions. A typical example is the photochemically mediated hydrophosphonation of 1-hexene (3) to give hexyltriphenylphosphonium tetraborate (4) with almost complete conversion. The photochemical technique can be readily scaled up to prepare product 2 in 1-g batches.

A practical feature of this method is the use of products such as 2 as Z-selective Wittig olefination reagents. Treating 2 with base forms the intermediate phosphorus ylide, which reacts with aldehyde 5 to give Wittig olefin product 6 in high yield and Z/E selectivity as high as 7:1. Crude product 2 can be used in the Wittig reaction to give the same yield and selectivity of 6 as those obtained from purified 2, thus making the purification step unnecessary. (Org. Lett. 2011, 13, Article ASAP DOI: 10.1021/ol202790n; W. Jerry Patterson)

Make supramolecular networks from ternary copolymer blends. A. R. A. Palmans and coauthors at Eindhoven University of Technology (The Netherlands) and Colorado State University (Fort Collins) developed elastomeric networks via the orthogonal self-assembly of supramolecular motifs into nanorods. The team used 2-ureido-4[1H]-pyrimidinone (UPy) groups, which stack laterally by dimer formation into nanofibrils, and benzene-1,3,5-tricarboxamide (BTA), which assembles into helical columns.

Using a poly(ethylene-co-butylene) (pEB) core, the authors synthesized monosubstituted pEB-UPy and pEB-BTA and a difunctional pEB compatibilizer with UPy and BTA motifs (Figure 1). Spectroscopic studies of a mixture of the oily pEB-UPy and pEB-BTA confirmed that the native self-assembled structure BTA is unaffected by the presence of UPy units.

Mixing pEB-UPy, pEB-BTA, and elastomeric, microphase segregated UPy-pEB-BTA in a 42.5:42.5:15 mol ratio produces a ternary blend with enhanced elastomeric behavior (Figure 2). The UPy-pEB-BTA compatibilizer allows this behavior to be maintained after the UPy nanofibrils are heated to melting. (ACS Macro Lett. 2012, 1, 105–109; LaShanda Korley)

[This is one of the first articles published in ACS’s newest journal, ACS Macro Letters.—Ed.]

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