Noteworthy Chemistry

January 21, 2013

Use gold nanoparticles to visualize fingerprint “negatives”. Fingerprints on paper are traditionally detected by precipitating silver on sebaceous material. This process, however, is influenced by sweat composition, and a considerable portion of the latent fingerprints is not detected. D. Mandler, J. Almog, and co-workers at the Hebrew University of Jerusalem developed a method for visualizing fingerprint “negatives” on paper, even after the paper is soaked in water.

The authors’ method consists of applying modified gold nanoparticles (AuNPs) to paper, then preferentially precipitating silver on the nanoparticles instead of on the sebaceous material. They synthesized linker 1 with an acylpyrazine group that has a high affinity for paper on one end and a metal-binding sulfur atom on the other. They then treated (n-C8H17)4NBr-stabilized AuNPs (2) with 1 to form modified AuNPs (3).

When paper with fingerprints is treated with 3, it binds to the area not covered by sebaceous material. The paper is then immersed in a silver physical developer (Ag-PD) bath (a freshly prepared mixture of AgNO3, a redox solution, and a detergent) for 40–60 s. Silver precipitates rapidly on the AuNPs. If the paper is left in the Ag-PD solution for longer periods, silver eventually precipitates on the lipid ridges of the fingerprints, reducing the contrast of the image.

Fingerprints that contain little tallow also develop well, but eccrine (sweat-containing) fingerprints cannot be visualized. [The authors do not define “tallow” in this context, but it presumably means sebaceous material.—Ed.] This process provides an alternative way for law-enforcement personnel to visualize latent fingerprints. (Angew. Chem., Int. Ed. 2012, 51, 12224–12227; José C. Barros)

Produce nanorods via directed topochemical photo-cross-linking. J.-F. Morin and colleagues at the University of Laval (Quebec) previously prepared organic nanorods via the supramolecular assembly of phenylacetylene macrocycles (PAMs) (Org. Biomol. Chem. 2011, 9, 4440–4443). This report focuses on the topochemical reaction of a PAM that contains two terminal butadiyne groups on the macrocycle’s exterior as cross-linking sites.

The PAM gels in a range of organic solvents. In EtOAc (10 mg/mL), it forms a fibrillar network structure with significant bundling of individual strands. The gel has the orientation and proximity needed for assembly via topochemical polymerization.

UV-irradiating the EtOAc organogel for 24 h produces a blue product. After exchange with CHCl3 and purification, soluble cross-linked nanorods are obtained in low yield (≈15%). Irradiation produces the micrometers-long, ≈2 nm–wide nanorods by polymerizing diyne units in the exterior and interior of the macrocycle.

The authors believe that the nanorods are covalently linked in 1-D stacks. They plan to investigate the porous properties of these architectures for gas storage and separation. (J. Am. Chem. Soc. 2013, 135, 110–113; LaShanda Korley)

Make amides from aldehydes and tertiary amines. In addition to traditional methods, amides can be formed by condensing primary or secondary amines with aldehydes. Readily available tertiary amines, however, have not been used to make amides in one step. Y. Li, F. Jia, and Z. Li* at Renmin University of China and China University of Petroleum (both in Beijing) developed an iron-catalyzed oxidative amidation of aldehydes with tertiary amines.

On the basis of their earlier findings that secondary amines are produced by oxidatively dealkylating tertiary amines (Li, H. et al. Org. Lett. 2009, 11, 4176–4179; Liu, W. et al. Org. Lett. 2011, 13, 6272– 6275), the authors believed that amide bonds might be formed from tertiary amines and aldehydes via a similar pathway. They treated amines (e.g., N,N-dimethylaniline, 1) with aldehydes (e.g., butyraldehyde, 2) in the presence of metal salts as catalysts and various oxidizers to produce amide 3. They optimized the reaction to prepare 3 in 90% yield by using 10 mol% FeCl2 as the catalyst and 3 equiv t-BuOOH as the oxidant. The reaction between 2 and N-methylaniline, which the authors believe is generated in situ in the dimethylaniline reaction, produced only 26% 3 under the same conditions.

This protocol is compatible with a variety of substrates, including aliphatic amines, and it can be used to synthesize amides that are difficult to make by traditional methods. Aromatic aldehydes are more reactive than aliphatic aldehydes; and dealkylation of unsymmetrical tertiary amines favors the least sterically hindered aliphatic substituent.

Preliminary mechanism studies showed that intermediate 5 forms from α-amino radical 4. After t-BuOO is eliminated in the presence of the iron catalyst, 5 is converted to cation 6, which subsequently hydrolyzes to 8 via intermediate 7. Secondary amine 8 readily condenses with excess aldehyde 2 to give amino alcohol 9, which oxidizes to 3. (Chem. Eur. J. 2013, 19, 82–86; Xin Su)

Organoiridium complexes selectively recognize the copper(II) ion. Many completely organic fluorogens have aggregation-induced emission (AIE) characteristics because of restriction of intramolecular rotation (RIR) of their rotating components in the aggregated state. A team led by Z.-N. Chen at the Chinese Academy of Sciences (Fuzhou and Shanghai) prepared two organometallic complexes, 1 and 2, that exhibit AIE phosphorescence by the RIR mechanism.

The cyclometallated Ir(III) complexes do not emit when they are dissolved in “good” solvents; but when they aggregate in “poor” solvents or the solid state, they show behavior typical of AIE luminogens by becoming phosphorescent. The complexes are sensitive, selective Cu2+-responsive chemosensors because copper impedes the rapid isomerization of the C=N bond in the 2,2-bipyridine–acylhydrazone ligand. (Analyst 2013, 138, 894–900; Ben Zhong Tang)

Can alkylating agents contaminate glycopyrrolate? T. Allmendinger and co-workers at Novartis Pharma (Basel, Switzerland) studied the impurities that may contaminate glycopyrrolate, a muscarinic receptor antagonist. All of the possible routes for synthesizing the two starting materials were considered; they and intermediates in the syntheses are potential impurities that might carry through to the final product.

One starting material, 1-methylpyrrolidin-3-ol, can be synthesized by several routes. Many of the precursors are alkylating agents such as epoxides, 1,4-dichloro-2-butene, 1,4-dibromo-2-butanol, ethyl acrylate, benzyl halides, and ethyl haloacetates. Because the intermediate is a tertiary amine, however, it is likely to react with alkylating agents to form salts that will be removed during processing.

The authors treated 1-methylpyrrolidin-3-ol with the alkylating agents and showed that all react with it to form salts that decline to zero concentration after workup. Therefore, they do not need to be considered as possible contaminants of the final glycopyrrolate. (Org. Process Res. Dev. 2012, 16, 1754–1769; Will Watson)

Use NMR to measure polymer molecular weights. The molecular weight (MW) of a polymer affects its physical properties. Although various techniques have been developed for polymer MW measurements, most of them require expensive instrumentation. Even gel permeation chromatography (GPC), the most commonly used method for determining MW, has drawbacks such as prolonged measurement time, high consumption of organic solvents, and high maintenance costs.

Diffusion-ordered NMR spectroscopy (DOSY) is used widely to characterize polymers. J. A. Johnson, R. H. Grubbs, and coauthors at MIT (Cambridge, MA) and Caltech (Pasadena, CA) expanded the use of DOSY to determine MWs of products from a variety of polymerization reactions.

Starting from the Stokes−Einstein equation, the authors developed a linear relationship between log D (diffusion coefficient) and log Mw (weight-average molecular weight), given that other variables are constrained (approximately constant). In the equation, ρ is the density of the liquid; η is the solvent viscosity; k is the Boltzmann constant; T is the absolute temperature; and NA is the Avogadro constant.

The authors then plotted a calibration curve between log D and log Mw by using commercial polystyrene standards that had Mw values from 9 to 200 kDa. The calibration curve showed excellent linear correlation, and the predicted Mw values are consistent with those determined by GPC.

After obtaining the calibration curve, the authors used this relationship to monitor common polymerization reactions, including atom-transfer radical polymerization (ATRP), reversible addition−fragmentation chain transfer (RAFT), and ring-opening metathesis polymerization (ROMP). In addition to accurately measuring Mw in the polymer products, the method was used to evaluate the efficiency of ROMP catalysts by constructing kinetic profiles. Further development of this method may potentially simplify several analyses that are based on polymer MWs. (Macromolecules 2012, 45, 9595–9603; Xin Su)

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