Noteworthy Chemistry

May 30, 2013

This fluorescent oligomer is a superamplified chemosensor. In fluorescent polymers that have amplified responses, the target analyte quenches all of the fluorophore units by transporting excited states along the 1-D polymer chains. The synthesis of these polymers, which usually contain bulky side groups and specific receptors, often requires complicated multistep procedures. In a 1-D conjugated polymer, an exciton’s limited mobility and finite lifetime prevent it from traveling the length of the long chain.

X.-G. Li, M.-R. Huang, R. B. Kaner, and co-workers at Tongji University (Shanghai) and the University of California, Los Angeles, developed a procedure to synthesize an oligomer with a 3-D conic structure that exhibits superamplified responses to chemical analytes.

The authors prepared the oligomer in a one-step oxidative oligomerization of fluoranthene (1). The fluorescence of the oligofluoranthene (2) is quenched effectively by specific electron-deficient molecules and ions. It can be used to make inexpensive superamplified chemosensors for detecting ions such as Fe3+ and explosive compounds such as picric acid.

The chemosensor covers a wide concentration range (>9 orders of magnitude) down to extremely low detection limits (≈10–12 M). Sample enrichment is not needed, probably because of the synergistic effects of well-distributed π-conjugated electrons throughout the conical structure.

The system resists interferents: Fe+3 can be detected in tap and sea waters that contain many other metal ions. Picric acid can be detected at low concentrations even in the presence of inorganic acids. (Chem. Sci. 2013, 4, 1970–1978; Ben Zhong Tang)

Click your way to tailored, functional synthetic polypeptides. A. J. Rhodes and T. J. Deming* at the University of California, Los Angeles, devised a synthetic route to azide-containing N-carboxyanhydride (NCA) monomers that can be used to make functional synthetic polypeptides. They used a multistep protocol to transform Nα-carboxybenzyl–protected amino acids into stable NCA monomers with azide substituents in good yield.

The authors used living polymerization initiated by (PMe3)4Co to produce synthetic polypeptides from the NCA monomers L -azidonorvaline (Anv) and L -azidonorleucine (Anl). The azide groups remained intact during the polymerization. Investigation of the more soluble poly(Anl) showed that this reaction strategy facilitates controlled chain growth, low polydispersity, and high yields, as well as the formation of block-copolymer architectures.

The authors functionalized poly(Anl) homopolymers and block copolymers with various alkynes by using efficient copper-catalyzed azide–alkyne cycloadditions (click reactions). For example, a water-soluble, stable glycopolypeptide was obtained via a click reaction with a glycoside. This robust reaction scheme opens pathways to robust functional synthetic polypeptides. (ACS Macro Letters 2013, 2, 351–354; LaShanda Korley)

Can a fluoronium ion exist? In halonium ions, a positively charged halogen atom is bound to two carbon atoms via three-center bonds. The stability of these structures in solution decreases in the following order: iodonium > bromonium > chloronium. T. Lectka and co-workers at John Hopkins University (Baltimore) found evidence that supports the existence of solution-state fluoronium ions, which previously were detected only in the gas phase.

The researchers designed and synthesized compound 1, a precursor to the proposed fluoronium ion intermediate 2. Precursor 1 has a cagelike structure, in which a fluorine atom is close to an electrophilic carbon that bears a trifluoromethanesulfonate (OTf) leaving group. The hydrolysis of 1 follows an SN1 reaction path through 2 or an extended SN2 [SN2(e)] path through transition state 3. In both cases, product 4 is formed.

To evaluate which reaction occurs, the authors prepared deuterium-labeled 1 (5). When 5 is hydrolyzed, the product is a 1:1 mixture of two isomers of 7 or 8, depending on the solvent. R is H when the solvent is TFE–H2O; R is CF3CH2– when it the solvent is neat TFE. (TFE is 2,2,2-trifluoroethanol.)

This result, together with Grunwald–Winstein kinetic analysis and computational stability modeling, supports the existence of fluoronium intermediate 6 and the SN1 path. This study opens possibilities for using fluoronium intermediates in organic and medicinal chemistry. (Science 2013, 340, 57–60; José C. Barros)

This epoxide reduction can go three ways. R. N. Bream and colleagues at GlaxoSmithKline Medicines Research Center (Stevenage, UK) developed two synthetic routes to a muscarinic antagonist that is being investigated for treating chronic obstructive pulmonary disease. Depending on the reagent used, they found that reducing an exocyclic tropine epoxide gives one of two expected ring opening products: 8-[2-(benzyloxy)ethyl]-8-azabicyclo[3.2.1]octan-3-yl)methanol or 8-[2-(benzyloxy)ethyl]-3-methyl-8-azabicyclo[3.2.1]octan-3-ol.

Strong reducing agents such as LiAlH4 and NaAlH2(OCH2CH2OMe)2 (Red-Al) form the first (undesired) product; the desired product is made when the reduction is carried out with (i-Bu2AlH)2 (Dibal-H) in toluene. If the Dibal-H reduction is carried out in THF, however, the reaction follows another path in which skeletal rearrangement forms byproduct 3-{1-[2-(benzyloxy)ethyl]-4-methylenepyrrolidin-2-yl}-propan-1-ol in addition to the desired product. The results of a deuterium-labeling experiment showed how the byproduct is formed and made it possible to improve the reaction conditions. (Org. Process Res. Dev. 2013, 17, 641–650; Will Watson)

Photoelimination produces azaborines. Azaborines, or aromatic compounds that contain a B–N bond, have attractive electronic and photophysical properties; however, it is difficult to synthesize these compounds. S. Wang and coauthors at Queen’s University (Kingston, ON) and Kangwon National University (Chuncheon, Korea) describe a way to make azaborines from B–C-containing B,N-heterocycles via photoelemination.

The authors first prepared compounds 14 by lithiating the corresponding N-heteroaromatic compounds (phenylpyridine or benzothiazole) with n-BuLi in the presence of tetramethylethylenediamine (TMEDA). The lithiated species were treated with BMes2F (1, 2, and 4) or BMe2Br (3). (Mes is mesityl.) These precursors are colorless or lightly colored and air- and heat-stable.

The authors irradiated compounds 14 at 300 nm (13) or 350 nm (4) in dry toluene or benzene under a nitrogen atmosphere. Irradiation released methane from 3 and mesitylene from 1, 2, and 4; and the solution colors darkened. Spectral studies confirmed the formation of C=B bonds and the formation of 58.

Compounds 5, 6, and 8 are moderately stable in the solid state and in solution, whereas 7 readily decomposes under the same conditions. The bulky mesityl group helps stabilize 5, 6, and 8. Azaborines 58 emit green or yellow-green light in solution and in poly(methyl methacrylate) films. The quantum yields for 6 and 7 are as high as 1.00, an unprecedented value for B,N-phenanthrene compounds. (Angew. Chem., Int. Ed. 2013, 52, 4544–4548; Xin Su)

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