Noteworthy Chemistry

September 23, 2013


Use copper coordination to tune fluorogen aggregates. Light emission from inorganic semiconductor nanoparticles such as quantum dots (QDs) red-shifts with increasing dot size. How emission from organic nanoparticles changes with particle size, however, is an unanswered question

Adding Cu+ causes this fluorogen to shine brighter

Q. Zhu, S. Liu, and co-workers at Southern Medical University (Guangzhou, China) have an answer. They manipulated the particle sizes of aggregates of a tetrahydropyrimidine racemate (1) and studied how the luminescence behavior of the nanoparticles changes with size.

Adding Cu2+ to suspensions of 1 in EtOH–H2O solutions changes the aggregate size of 1. Particle size decreases with increasing Cu2+ concentration. This is the basis of the authors’ method for controlling the size of organic nanoparticles.

In contrast to inorganic QDs, the luminescence color of these organic nanoparticles is unaffected by particle size. Emission intensity, however, is enhanced by adding Cu2+. There is a linear relationship between fluorescence intensity and ion concentration in the range 0–80 μM.

The authors believe that the decrease in particle size and the increase in emission intensity result from coordination-induced dissociation of the intermolecular hydrogen bonds that connect the nonemissive groups of fluorogen 1. (RSC Adv. 2013, 3, 13286–13292; Ben Zhong Tang)

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Use a laser to speed up heterogeneously catalyzed reactions. Metal nanoparticles are often used as heterogeneous catalysts in organic chemical reactions. Gold nanoparticles are widely used to catalyze amide formation, an important reaction in drug discovery and industrial processes.

R. Luque and colleagues at the Universities of Córdoba and Zaragoza (Spain) irradiated silica-supported gold nanoparticles with a 532-nm laser to accelerate amidation reactions. This technique generates photons that are converted to phonons, which produce lattice vibrations, bubbles, and shockwaves in the vicinity of the nanoparticles. Heat is generated only near the catalyst and does not change the bulk reaction temperature.

The authors’ first model reaction was the amidation of morpholine with benzaldehyde in the presence of H2O2 to oxidize the aldehyde. The catalyst was spherical silica–supported gold nanoparticles. Laser irradiation reduced the reaction time from 12 h to 4–5 h. The authors expanded the method to amidate another amine (α-methylbenzylamine) and to esterify benzyl alcohol.

The atom efficiency of the laser-induced process is ≈0.60. The catalyst can be used at least three times without detectable gold leaching. The method is energy-efficient and potentially can be applied to other reactions. (Green Chem. 2013, 15, 2043–2049; José C. Barros)

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Upgrade ethanol to 1-butanol—an advanced biofuel. The bioethanol industry has been productive for years; but ethanol is not an ideal biofuel because of its low energy density (70% of that of gasoline), corrosivity, and propensity to absorb water. In contrast, 1-butanol (n-BuOH) is a better candidate for an alternative fuel source because it has higher energy density (90% of that of gasoline), is noncorrosive, and is immiscible with water.

Neither biological production nor upgrading from ethanol, however, provides a satisfactory solution for making bulk n-BuOH. D. F. Wass and co-workers at the University of Bristol (UK) tackled this challenge by developing a selective catalytic pathway for converting EtOH to n-BuOH in good yield.

By using the Guerbet reaction, in which C–C bonds can be formed between nonactivated alcohols, the authors screened a variety of ruthenium-based catalysts under various conditions. They found that n-BuOH is obtained in 10.5% conversion and 94.1% selectivity in the presence of 0.1 mol% trans-[RuCl2L2], where L is ligand 1 shown in the figure. When the reaction time is increased from 4 to 24 h, conversion increases to 45.8% under otherwise identical conditions, but selectivity drops to 84.6%.

Ruthenium-catalyzed conversion of ethanol to 1-butanol

The authors propose a Guerbet-type mechanism for this reaction, in which MeCHO formed by the dehydrogenation of EtOH undergoes aldol condensation and then rehydrogenation to n-BuOH. They also believe that an “on-metal” aldol coupling causes the extraordinary selectivity to the C4 dimer and suppression of the formation of higher oligomers (i.e., C8+). (Angew. Chem., Int. Ed. 2013, 52, 9005–9008; Xin Su)

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Compare actual purging values with theoretical calculations for removing genotoxic impurities. D. P. Elder*, G. Okafo, and M. McGuire at GlaxoSmithKline (Ware, UK, and King of Prussia, PA) studied the removal of five genotoxic impurities from pazopanib hydrochloride, a multikinase inhibitor. They used a semiquantitative risk-assessment tool to calculate theoretical purging figures based on the reactivity, solubility, volatility, and ionizability of the compounds and the various chemical steps that they undergo during the synthesis.

In all cases, the calculated figures agreed well with the actual figures. For example, (MeO)2SO4 had a theoretical (calculated) purge factor of 30,000, compared with an actual purge factor of 29,411. Four of the five compounds had high purge factors, but one did not. This required the imposition of a control strategy and specification limit for that compound. (Org. Process Res. Dev. 2013, 17, 1036–1041; Will Watson)

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Allow homochiral supramolecular cages to “self-sort”. Self-sorting is regarded as a major driving force for forming supramolecular assemblies and other complex nanostructures. Much research in this area is directed toward making “smart” materials. According to A. Lützen and co-workers at the University of Bonn (Germany),

Successful approaches to high-fidelity self-sorting usually employ geometrical complementarity of size and shape to predetermine the outcome of the self-assembly process. In this sense, chiral self-sorting processes are a true challenge, because neither the size nor the shape varies, but only the relative spatial orientation. If there is any self-sorting at all, chiral recognition between enantiomers can, in principle, either lead to self-recognition or self-discrimination resulting in homo- or heterochiral assemblies, respectively.

These authors report a high-fidelity, self-sorting process that creates homochiral metallo-supramolecular cages from 1,1’-binaphthyl–based bispyridine ligands.

The authors designed a bispyridine platform based on an axially chiral 1,1’-binaphthyl scaffold. Ligand 3 can be prepared in enantiomeric and racemic forms by using a twofold Sonogashira coupling reaction between 3,3’-diiodo-2,2’-bis(methoxymethyl)-1,1’-binaphthyl (1) and 3-ethynylpyridine (2) in 97% yield. MOM is methoxymethyl; dba is dibenzylideneacetone; dppf is 1,1’-bis(diphenylphosphino)ferrocene.

Formation of ligand 3 that leads to self-sorting

Coordinating (M)-, (P)-, and rac-3 to Pd2+ in 2:1 stoichiometry forms [Pd234] exclusively. Whereas (M)- and (P)-3 form only homochiral complexes of [Pd234], rac-3 can theoretically yield six distinct assemblies. The 1H NMR spectrum obtained for rac-3 with Pd2+, however, is identical to that of [Pd2(M)-34], indicating that two homochiral complexes, [Pd2(M)-34] and [Pd2(P)-34] are formed by a “narcissistic” self-sorting process.

As defined by the quotient P0/P of the number of all possible assemblies P0 and the number of formed assemblies P, the degree of self-sorting for this system is 3. (Chem. Eur. J. 2013, 19, 10890–10894; Xin Su)

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Controllable hydrogel networks have enhanced mechanics and self-repair capability. H-P. Cong*, P. Wang, and S-H. Yu* at the University of Science and Technology of China, Hefei (Anhui, China) and Hefei University of Technology designed a dual-network hydrogel that consists of graphene oxide (GO) and poly(acryloyl-6-aminocaproic acid) (PAACA). The hydrogel has enhanced mechanical properties and pH-triggered self-repair capability.

The authors prepared the GO-PAACA composite by polymerizing a GO–acryloyl-6-aminocaproic acid dispersion with an (NH4)2S2O8 initiator. The porous hydrogel consists of a network formed by hydrogen-bonding interactions between the PAACA side chains and GO oxygen-rich functional units and between pairs of PAACA side chains. This network is intertwined with a second network that is formed by Ca2+ bridges between GO sheets.

The GO-PAACA composite hydrogel is highly extensible (≈1200% elongation) and dissipates energy well. Because of their roles in network formation, the amounts of GO and Ca2+ used to make the composite influence its mechanical function. The pH-tunable swelling characteristics of the robust double-network hydrogel allows extended release of the drug doxorubicin.

An interesting characteristic of the PAACA-GO composite is its ability to reversibly self-repair damage as a function of structural modifications induced by changing pH. The structural organization in these dual-network hydrogels makes it possible to tune the hydrogen bonding and coordination mechanisms for tailored responses in areas such as drug delivery and structural repair. (Chem. Mater. 2013, 25, 3357–3362; LaShanda Korley)

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