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Noteworthy Chemistry

May 6, 2013

Prepare aryliodonium ylides in one pot. Iodonium ylides, a subclass of zwitterionic iodonium compounds, contain a carbon atom that has carbanionic properties. They are used to prepare of fluoroarenes—notably 18F-labeled compounds.

Iodonium ylides are prepared by the reaction of (diacetoxyiodo)arenes with active methylene compounds, but most of these ylides are unstable. J. Cardinale and J. Ermert* at the Institute for Neuroscience and Medicine (JÜlich, Germany) devised a simple route to stable aryliodonium ylides (1) derived from Meldrum’s acid (2,2-dimethyl-1,3-dioxane-4,6-dione).

In the authors’ method, an iodoarene is treated with the oxidant m-chloroperbenzoic acid (mCPBA) to form the corresponding iodoso compound. A suspension of KOH and Meldrum’s acid in CH2Cl2 is added to the reaction mixture to produce the iodonium ylide.

Conversion of iodoarenes to aryliodonium ylides

The ylide yield from unsubstituted iodobenzene is 31%. Iodobenzenes with substituents at the 2- and 4-positions give yields ranging from 19 to 59%. 3-Substituted iodobenzenes give no or very little product, presumably because electronic factors interfere with the oxidation reaction or decrease the stability of the product.

This simple method can be used to produce precursors of [18F]fluoroarenes that are used in positron emission tomography. (Tetrahedron Lett. 2013, 54, 2067–2069; JosÉ C. Barros)

Sodium hydrazinidoborane may be ideal for chemical hydrogen storage. The difficulty of storing hydrogen is one of the greatest obstacles to using hydrogen energy efficiently. Among the materials considered for chemical hydrogen storage, boranes and their derivatives have been thoroughly evaluated because of their excellent hydrogen-storage capacities.

Hydrazine borane (N2H4BH3), in particular, is a stable, high-capacity storage compound. Its stability, however, prevents it from releasing hydrogen under mild conditions. To circumvent this problem, U. B. Demirci and coauthors at the University of Montpellier 2 (France), Hiroshima University (Hagati-Hiroshima, Japan), the Catholic University of Louvain (Louvain-la-Neuve, Belgium), and the University of Lyon 1 (Villeurbanne, France) prepared sodium hydrazinidoborane (NaN2H3BH3), the sodium salt of N2H4BH3. The hydrazone-borane salt improves the hydrogen-release properties of N2H4BH3.

The authors synthesized NaN2H3BH3 from N2H4BH3 and NaH in 1:1 stoichiometry by ball-milling them under argon at –30 °C. NaN2H3BH3 is a colorless solid with a hydrogen content of 8.85 wt%. Single-crystal X-ray diffraction spectroscopy confirmed that a hydrogen atom in N2H4BH3 was replaced by a sodium cation.

When NaN2H3BH3 is heated, dehydrogenation begins at <60 °C; 6 wt% hydrogen is liberated when the temperature reaches 100 °C. The dehydrogenation rate of NaN2H3BH3 at constant temperatures between 80 and 100 °C is greater than that of N2H4BH3. For example, NaN2H3BH3 releases 8 wt% hydrogen smoothly at a rate of 1.5 L/min at 100 °C, whereas N2H4BH3 releases hydrogen abruptly and rapidly at >100 °C, possibly by a different mechanism. The gas released by N2H4BH3 contains trace amounts of ammonia.

The authors identified the solid residues from NaN2H3BH3 as sodium-containing linear poly(aminoborane) [–(NH2BH2)n–]. (ChemSusChem 2013, 6, 667–673; Xin Su)

Make robust hydrogels from tetrazines and norbornenes. K. S. Anseth and collaborators at the University of Colorado at Boulder describe a synthetically robust, simple strategy for developing functional hydrogel systems. Their procedure uses the click reaction between tetrazine- and norbornene-substituted polymers.

The authors conjugated an amine-terminated four-arm star poly(ethylene glycol) (PEG) with a tetrazinecarboxylic acid derivative. The tetrazine-functionalized PEG (PEG-Tz) was stoichiometrically cross-linked with a cell-degradable dinorbornene-based synthetic peptide to yield hydrogels in <5 min. The reaction can be run at several concentrations and conditions because of the fast reaction kinetics and high cross-linking reactivities.

The equilibrium shear moduli can be tuned from ≈225 to 2350 Pa by adjusting the content of PEG-Tz and including of monofunctional peptides. Human mesenchymal stem cells (hMSCs) encapsulated in these hydrogel constructs, even for as long as 72 h, are highly viable, demonstrating the cytocompatibility of the materials.

By using a bifunctional peptide linker that consists of norbornene and pendant thiol groups, the authors showed that the biochemical environment of the hydrogels can be modified by photopatterning via thiol–ene click chemistry. (Biomacromolecules 2013, 14, 949–953; LaShanda Korley)

Chiral “nanocalipers” can distinguish among carbon nanotubes. An ongoing goal of supramolecular chemistry is to develop molecular devices that are as functional as their macroscopic counterparts. The precise sorting of single-walled carbon nanotubes (SWNTs) is challenging because they differ from each other in many ways.

N. Komatsu and colleagues at the Shiga University of Medical Science (Otsu, Japan), Osaka University, the University of Tokyo, and Bhabha Atomic Research Center (Mumbai) developed a series of diporphyrin-based chiral “nanocalipers” that simultaneously differentiate among SWNTs with different diameters, chiralities, and—surprisingly—metallicities.

The authors previously reported the synthesis of “nanotweezers” that selectively recognize (6,5)- and (9,4) SWNTs (Wang, F., et al. J. Am. Chem. Soc. 2010, 132, 10876–10881). But the nanotweezers can accommodate only SWNTs with diameters <1 nm. Believing that longer spacers and narrower dihedral angles can provide better (n,m) selectivity, they designed nanocalipers 1 with a carbazole-anthracene-carbazole spacer that spans >1.4 nm. They added long alkyl chains to the carbazole units to increase their solubility; and they installed two chiral porphyrin units on the spacer framework to make enantiomeric nanocalipers (R)-1 and (S)-1.

Chiral nanocalipers

The authors extracted commercial SWNT samples (HiPco, from Carbon Nanotechnologies [Houston]) with 1. They found that 1 selectively enriches SWNTs with larger diameters (>1 nm); for example, the abundance of (9,4)-, (10,2)-, and (8,6)-SWNTs (0.88–0.97 nm diam) drops from 59 to 8% after extraction, whereas the abundance of (10,5)-, (9,7)-, (10,6)-, and (13,3)-SWNTs (1.05–1.17 nm diam) increases from 30 to 82%.

(R)-1 and (S)-1 preferably form stable complexes with (P)- and (M)-SWNTs, respectively. Most surprisingly, when SWNTs with diameters <1.05 nm are extracted with 1, they are enriched in metallic SWNTs and depleted in semiconducting SWNTs. (J. Am. Chem. Soc. 2013, 135, 4805–4814; Xin Su)

Create optical patterns by cross-linking poly(phenylacetylene) films. Conjugated polymers are susceptible to external stimuli because their conjugated backbones are sensitive to perturbations. Future generations of chemical sensors based on conjugated polymers demand processing techniques that allow direct patterning of functional conjugated polymers on micro- and macroscopic length scales.

A. C. Pauly and P. Theato* at the Universities of Hamburg and Mainz (both in Germany) and Seoul National University (Korea) developed a photolithographic process for creating optical patterns from conjugated poly(phenylacetylene) (PPA) derivatives.

The PPAs contain pentafluorophenyl (PFP) ester units. The authors replaced varying amounts of the reactive PFP groups in the polymers with a mono o-nitrobenzyl–protected diamine. When thin films of the resulting copolymers are irradiated with UV light, the partially aminated copolymers cross-link, but PFP and completely aminated homopolymers do not. The polymers that are not cross-linked are washed off with CH2Cl2 to leave the desired pattern.

During the photo-cross-linking process, excess PFP ester groups remain intact, allowing subsequent postpolymerization modification with amines. Secondary modification with amines results in a blue shift in the absorption spectrum, making the patterned PPA films promising candidates for optical sensors. (Macromol. Rapid Commun. 2013, 34, 516−521; Ben Zhong Tang)

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