Noteworthy Chemistry

February 28, 2011

A detergent ingredient is an oxidant in acyloxylation reactions. Structures formed by allylic C–H acetoxylation are useful intermediates for synthesizing more complex molecules. L. T. Pilarski, P. G. Janson and K. J. Szabó* at Stockholm University evaluated various oxidants to promote this reaction. They observed that sodium perborate (NaBO3·H2O), a principal component of laundry detergents, is a weak oxidant that can be accelerated with Ac2O to form a useful oxidant for the palladium-catalyzed C–H acyloxylation of alkenes.

The reaction proceeds across a range of substrates to give fair-to-good product yields. The procedure is suitable for functionalizing terminal and internal alkenes, as shown in the figure. Allylic and benzylic C–H bonds can also be functionalized. Benzoyl-substituted products are readily formed by using (PhCO)2O in place of Ac2O.

The authors emphasize the selective nature of the reaction. It leads to only one regioisomer and one stereoisomer, and substitution always occurs at the allylic γ-position. The procedure has the potential for asymmetric induction, which would increase its utility. A particular advantage of NaBO3 is that it generates nontoxic byproducts H3BO3 and water. (J. Org. Chem. 2011, 76, Article ASAP DOI: 10.1021/jo1024199; W. Jerry Patterson)

Here’s a new take on thiol–ene chemistry. P. Espeel, F. Goethals, and F. E. Du Prez* at Ghent University (Belgium) developed a one-pot reaction scheme for thiol–ene chemistry that uses thiolactones and has applications in polymer synthesis. Their goal was to use a metal-free “click” approach by generating thiol groups in situ and combining multistep processes.

They first demonstrated the utility of this method by combining model compounds BnNH2, thiolactone 1, norbornene (2), and the nucleophilic catalyst 4-dimethylaminopyridine (DMAP) under UV irradiation to produce ring-opened product 3. They found that the presence of a photoinitiator may lead to undesired side products.

For polyaddition reactions, the authors synthesized an AB’-type monomer with a thiolactone group and a double bond. By using an excess of nucleophilic ethanolamine and 5 mol% photoinitiator under mild conditions, the authors formed a soluble poly(thioether)–polyurethane with 22 kDA Mn and 1.6 polydispersity after 1 h of photocuring. They extended the method to network systems with film formation predicated on the choice of diamine cross-linker and on solubility and steric considerations. (J. Am. Chem. Soc. 2011, 133, 1678–1681; LaShanda Korley)

How should you quench phosphorus oxychloride? M. M. Achmatowicz and co-workers at Amgen (Thousand Oaks, CA) encountered an exothermic reaction in the aqueous filtrates after workup of a Vilsmeier–Haack reaction. This occurrence led to an in-depth study of the hydrolysis of POCl3 during inverse quenching by MeCN–H2O-water at <5 °C. The initial hydrolysis to form phosphorodichloridic acid [P(OH)Cl2] is almost instantaneous, as is the final hydrolysis of phosphorochloridic acid [P(OH)2Cl] to form H3PO4; the rate-determining step is the conversion of P(OH)Cl2 to P(OH)2Cl.

The hydrolysis rate slows under strongly acidic conditions, but it increases with the addition of base. The authors developed a safe quenching method in which the reaction mixture is diluted with MeCN and added to 1:1 MeCN–H2O simultaneously with 1.5 equiv of 5 N NaOH solution between 15 and 25 °C. The mixture is aged at 15–25 °C for 1 h before being filtered. Adding excessive amounts of base phase-separates the quench mixture and lowers product yields, in this case a 2-chloroquinoline-3-carboxaldehyde. (Org. Process Res. Dev. 2010, 14, 1490–1500; Will Watson)

Cysteine makes metallabenzenes chiral. Metallabenzenes are unique metallacyclic complexes in which metal atoms are embedded in benzene rings. Their chemical properties lie between those of their aromatic and metallic components. The properties of materials made from them, however, are almost unknown. S. Li, H. Zhang, H. Xia, and coauthors at Xiamen University (China) and South China Normal University (Guangzhou) report a study of the optical activity of a series of chiral ruthenabenzenes.

The researchers found that achiral ruthenabenzene 1 selectively binds to L-cysteine (L-Cys) in an aqueous medium at physiological pH to form chiral adduct Δ-(R)-2 by a dynamic epimerization process. (The dinitrogen ligand is 1,10-phenanthroline.) The conformation and hence the ruthenium-centered chirality of the complex are reversibly tunable between the Δ and Λ forms by increasing the pH of the medium. Similar results are obtained when D-Cys is used, but the conformations of the complexes are reversed, indicating that the chirality is induced by the stereogenic center of the cysteine ligand. (Chem.–Eur. J. 2011, 17, 2420–2427; Ben Zhong Tang)

Purify water with chitosan–titania–magnetite microspheres. Wastewater purification is a challenge because of the cost and high energy consumption of membrane technology. A possible solution is the use of adsorbent particles to remove impurities.

Z. Liu*, H. Bai, and D. D. Sun at Nanyang Technological University (Singapore) developed a process to purify water that uses microspheres consisting of chitosan, TiO2, and Fe3O4. Chitosan is an inexpensive adsorbent with very high-capacity because of its large concentration of amino and hydroxyl groups. TiO2 is a photocatalyst used to reactivate the microspheres after adsorption. Fe3O4 allows the microspheres to be separated with a magnetic field.

The microspheres are prepared by using a simple electrospraying technology, in which a suspension of the reagents is subjected to high voltage. The resulting particles are almost spherical and consist of a chitosan matrix with embedded TiO2 and Fe3O4.

The authors show that the microspheres are superior to most activated carbons for adsorbing the dye acid orange 7. The magnetism of the microspheres is strong enough to separate the particles from the solution. To reuse the microspheres, the dye is decomposed by using UV irradiation aided by TiO2. The microspheres can be reused up to eight times.

The combination of adsorption and UV irradiation purified water better than adsorption alone. This process is simple, economical, and convenient to scale up. (New J. Chem. 2011, 35, 137–140; JosÉ C. Barros)

Prepare bicyclo[3.2.1]octenediones with a Dieckmann reaction. The bicyclic alkane scaffold is the core of many natural products with useful biological properties, including anticancer agent welwistatin and radical scavenger vitisinol D. I. N. Michaelides, B. Darses, and D. J. Dixon* at the University of Oxford (UK) developed an efficient three-step synthesis of aryl- and alkyl-substituted bicyclo[3.2.1]octenediones illustrated by structure 1.

A typical synthesis involves the base-promoted cyclization of commercially available alkyne ester 2 and keto ester 3 to give cyclic diester 4. Dealkoxycarbonylation of 4 produces a mixture of unsaturated keto esters represented by 5, which are isomeric substrates suitable for forming the desired bicyclic ring. A Dieckmann acid-promoted rearrangement of 5 produces target bicyclo[3.2.1]octenedione 1, whose structure was confirmed by single-crystal X-ray diffraction.

This procedure leads to numerous variations of 1 with a variety of substituent groups. The authors extended their method by using an eight-membered ring analogue of 5 to form ring-expanded bicyclo[3.3.1]nonenone 6. (Org. Lett. 2011, 13, 664–667; W. Jerry Patterson)

Exchange polymer architectures via chemical triggers. Most polymers are linear; but complex comblike, starlike, branched, and cross-linked configurations have emerged with the advent of living polymerization. These structures affect the physical properties and potential uses of polymeric materials. Controlling polymer architecture is appealing to polymer scientists, but multistep syntheses and specially designed monomers are major impediments.

Topology transition of linear structures after polymerization is an attractive alternative solution to these problems, but very little work has been done in this area. J.-J. Yan, C.-Y. Hong, and Y.-Z. You* at the University of Science and Technology of China (Hefei) proposed converting linear oligomers to 3-D architectures and tested the feasibility of their idea.

The authors used a living free-radical polymerization technique, reversible addition–fragmentation (RAFT) chain transfer polymerization, with poly(trithiocarbonate) (PTTC, 1). PTTC provides the polymer backbone functional trithiocarbonate groups that are later transformed to reactive thiol groups. PTTC is copolymerized with a vinyl monomer, N,N-dimethylacrylamide (DMA), which is incorporated into the backbone, in the presence of 2-(pyridin-2-yldisulfanyl)ethyl acetate (PDEA) with an active pyridine disulfide group, which is incorporated into the side chains.

Aminolysis of polymer chain 2 dissociates it into linear oligomers 3 that are terminated by thiol groups. Coupling reactions occur spontaneously among the functional groups to form branched architecture 4. Oxidation further cross-links the branches to make PDMA gel 5, which can be reversibly reduced to its components. (Macromolecules 2011, 44, Article ASAP DOI: 10.1021/ma102944k; Sally Peng Li)

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