Noteworthy Chemistry

August 12, 2013

 

Make a porphyrin–calixarene hybrid. Porphyrins are among the most important macrocycles because they have applications in materials science, biological systems, and medicine. These pyrrole-based heterocycles have 16-membered rings and 18 delocalized π-electrons, making them aromatic according to the Hückel rules.

D. Jacquemin, O. Siri, and coauthors at the Universities of Aix-Marseille, Nantes, and Strasbourg (all in France) created a new ring structure called azacalixphyrin that has characteristics of porphyrins and azacalix[4]arenes. Azacalixphyrin (3) is easily obtained by the reaction between 1,2,4,5-tetraaminobenzene (1) and 1,5-difluoro-2,4-dinitrobenzene (2), followed by reduction of the nitro groups to amines and air oxidation at room temperature. Macrocycle 3 is obtained in 14% yield.

Synthesis of azacalixphyrin

The researchers characterized compound 3 by NMR and high-resolution MS. Based on the X-ray diffraction pattern of its 2HCl salt, they concluded that the compound adopts a nonplanar saddle conformation in which the eight diaminobenzoquinonediimine nitrogen atoms are coplanar.

Azacalixphyrin is stable for months under air and for days in aqueous solution because of its unusual zwitterionic structure. It absorbs in the near-IR and has a very low HOMO–LUMO gap (1.01 eV), making it suitable for technological applications that require macrocyclic ligands. (Angew. Chem., Int. Ed. 2013, 52, 6250–6254; José C. Barros)

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Light releases the cargo from a metal–organic framework. Metal–organic frameworks (MOFs), a well-known class of porous materials, can be used to store and release guest molecules, mainly on the basis of diffusion. To make the guest-release process more controllable in MOFs, O. M. Yaghi and co-workers at the University of California (Los Angeles and Berkeley), Northwestern University (Evanston, IL), Lawrence Berkeley National Laboratory, and KAIST Institute (Daejeon, Korea) used light irradiation to demonstrate pore-size control in an azobenzene-containing MOF.

Azobenzene MOF strut

The authors first functionalized the organic strut (1) with an azobenzene unit. The solvothermal reaction between 1 and Mg(NO3)26H2O in a mixture of DMF, EtOH, and water at 120 °C yields the MOF “azo-IRMOF-74-III” in 24 h. Azo-IRMOF-74-III features hexagonal 1-D pores built on helical Mg−O−C rods connected by 1. The MOF is 19.5 Å in diam and has a surface area of 2410 m2/g. The authors estimate the aperture sizes to be 8.3 and 10.3 Å when the azobenzene units adopt trans and cis configurations, respectively.

When it is irradiated at 377 nm, the photostationary state of azo-IRMOF-74-III in DMSO is reached in 30 min, with a trans/cis ratio of 0.43:1. The “guest cargo” propidium iodide dye is added to Azo-IRMOF-74-III to a loading capacity of 0.4 wt%. Propidium iodide release is triggered by a 50-mW laser beam at 408 nm and lasts for 40 h. Removing the laser source slows the cargo release rate. (Chem. Sci. 2013, 4, 2858–2864; Xin Su)

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Find the correct asymmetric reduction route to the desired enantiomer. M. D. McLaws and coauthors at AMRI (Albany and North Syracuse NY) and Resolvyx Pharmaceuticals (Cambridge, MA) developed a practical synthesis of a five-carbon segment of the endogenous metabolite resolvin E1. They evaluated the asymmetric reduction of (E)-1-iodopent-1-en-3-one to (R,E)-1-iodopent-1-en-3-ol with a variety of hydride reducing agents. Unfortunately, the highest enantiomeric ratio (97:3) they obtained—with the reducing agent (–)-diisopinocampheylchloroborane—favored the undesired (S)-enantiomer.

The best route to the desired (R)-enantiomer, a Corey–Bakshi–Shibata (CBS) reduction catalyzed by (S)-2-methyl-CBS-oxazaborolidine, gave an 86:14 er. Attempts to improve the ratio by chemical kinetic resolution by using a selective Sharpless epoxidation were problematic.

Biocatalytic approaches via selective acetylation were more successful: Candida antarctica lipase B gave the best results. The (R)-acetate and the (S)-alcohol can be separated by silica gel plug filtration or by reaction with maleic anhydride followed by extraction of the (S)-enantiomer into aqueous base. (Org. Process Res. Dev. 2013, 17, 915–920; Will Watson)

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Mechanical force ionizes an amphoteric molecule, changes its color, and intensifies its fluorescence. Mechanochromic dyes change colors when mechanical forces are applied to them; they are promising candidates for stimuli-responsive “smart” materials. Most mechanochromic systems developed to date function by force-induced conformational and morphological changes. M. Li, S. X.-A. Zhang, and coauthors at Jilin and Northeast Normal Universities (both in Changchun, China) developed an “intelligent” mechanochromic system that operates by a different mechanism.

The researchers used stress to trigger intramolecular acidification, which activates the mechanochromic process via proton transfer in amphoteric molecule 1. Upon grinding, the yellow color of 1 changes to red, with a bathochromic shift of as much as 100 nm in its absorption spectrum, the result of the stress-induced neutral-to-zwitterionic transition.

Neutral molecule becomes zwitterionic when mechanical force is applied

Whereas 1 is nonfluorescent, its ground form (2) emits red light at 652 nm. The red color of 2 reverts to the original yellow color when it is heated or wetted with an organic solvent such as EtOH. The red light emission also disappears. (Chem. Commun. 2013, 49, 6587–6589; Ben Zhong Tang)

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Phosphorus in flame retardants: More is not always better. For many years, scientists assumed that aryl phosphate flame retardants require a high phosphorus content to perform effectively. These compounds tend to be volatile, and they can be contact allergens and neurotoxins. An ideal flame retardant should be used in small enough quantities to minimize its effect on the material properties of the flammable polymer matrix while being effective in reducing char formation. It would release a negligible amount of volatile material at room temperature.

K. Shin, B. J. Sung, and co-workers at Sogang University, Samsung Cheil Industries, the Korea Institute of Science and Technology (all in Seoul), and Samsung Electronics (Suwan, Korea) investigated the dynamics and mechanisms of aryl phosphates in polycarbonate matrices to assist in designing environmentally friendly flame retardants. They demonstrated that flame retardants with relatively low phosphorus contents can perform effectively if the dynamics and molecular interactions are tuned correctly.

Highly volatile flame retardants, including the widely used triphenyl phosphate (TPP), capture hydrogen and hydroxyl radicals in the vapor phase. Less-volatile oligomeric phosphates are more active in the condensed phase and provide thermally stable char layers that protect the underlying polymer. Mixtures of volatile and nonvolatile phosphates are more effective than either type alone.

The researchers conducted flammability tests that showed that 2,4-di-tert-butylphenyl diphenyl phosphate (DDP) should be as efficient as TPP, even though DDP contains much less phosphorus. On the other hand, 2-tert-butylphenyl diphenyl phosphate, with a phosphorus content intermediate between TPP and DDP, is less efficient as a flame retardant than either of those compounds.

The researchers suggest that the bulky tert-butyl groups play a greater role than simply decreasing the relative phosphorus content. Dynamic secondary ion MS (DSIMS) and molecular dynamics simulations (used at higher temperatures, where DSIMS is impractical) show that the diffusion of DDP through a polycarbonate matrix at low temperatures is slower by an order of magnitude than that of TPP, so more DDP is retained in the polymer over time rather than being lost to the atmosphere. Thermogravimetric analysis confirms that DDP is much less volatile than TPP at low temperatures. DDP diffusion becomes comparable to that of TPP at the ignition temperature (360 °C), enabling it to enter the vapor phase and capture free radicals.

Previous studies indicate that char formation by thermal decomposition of polycarbonates is a result of Fries rearrangement of the carbonate linkages, which produces phenolic groups that transesterify with aryl phosphate flame retardants. In the present study, potential mean force computations, supported by Fourier transform IR spectroscopy, show that DDP suppresses Fries rearrangement and char formation better than TPP. (J. Phys. Chem. B 2013, 117, 8571–8578; Nancy McGuire)

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Grow carbon nanotubes from carbon nanorings. Carbon nanotubes (CNTs) are useful in numerous applications, but no method has been developed to produce them with structural uniformity. K. Itami and coauthors at Nagoya University and FUJIFILM (Kanagawa, both in Japan) developed a bottom-up strategy for synthesizing structurally uniform CNTs by using synthetic carbon nanoring templates.

Carbon nanorings are essentially cycloparaphenylenes ([n]CPPs, where n denotes the number of phenylene groups). Improving the syntheses of CPPs has made it possible to prepare [6]–[16]CPPs. The authors formed CNTs by heating [12]CPP (1)–coated C-plane sapphire wafers in EtOH gas flow at 500 °C for 15 min under ≈1 torr vacuum. The CNT diam range was 1.3–1.7 nm, consistent with the [12]CPP diam (1.7 nm). The [12]CPP template and EtOH carbon source are both prerequisites for forming CNTs.

Lower temperatures (400–450 °C) decrease the efficiency of CNT formation, and higher temperatures (550–650 °C) decompose [12]CPP. C-plane sapphire substrates are ideal; A-, M-, and R-plane sapphires and silicon give few or no CNTs.

The authors also synthesized CNTs from [9]CPP with excellent structural uniformity. The authors believe that the growth-from-template process likely undergoes a CPP radical mechanism in which a CPP radical formed via hemolytic C–H cleavage couples with reactive C2 species from EtOH. (Nature Chem. 2013, 5, 572–576; Xin Su)

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[12]CPP precursor to CNTs