Noteworthy Chemistry

January 28, 2013

A continuous-flow reactor catalyzes the Sonogashira coupling reaction. Continuous-flow tubular reactors are important tools for achieving “green” chemical reactions. Their advantages are high energy and atom efficiency. C.-L. K. Lee and co-workers at Singapore Polytechnic and Singapore University of Technology and Design took these reactors one step farther by developing a durable continuous-flow Pd–Cu dual-tube reaction platform that catalyzes Sonogashira coupling reactions to give high yields of coupled products and uses minimal amounts of palladium.

The dual reactor consists of a palladium-coated copper tubular reactor, a second copper reactor, and a metal-scavenging resin column. The authors used this setup to catalyze Sonogashira coupling between 2-ethynyl-1,4-dimethylbenzene (1) and iodobenzene in quantitative yield, identical with the results from a homogeneous palladium catalyst (1 mol%) in a copper reactor.

The authors believe that palladium leached from the first reactor and copper in the second catalyze the reaction, and that the amount of leached palladium is significantly smaller than the homogeneous palladium loading in a conventional reactor. When they reversed the order of the reactors, the yield was only 60%. Carrying out the reaction in the palladium reactor alone decreased the yield to 47%. The continuous-flow setup is robust: The yield was >90% for 10 reaction cycles.

The authors also explored the effect of iodobenzene substituents on the reaction. The reactor system gave moderate-to-high yields (75–100%) in the presence of a variety of substituents. An exception was 4-iodoaniline (41% yield), possibly because palladium is deactivated by coordinating with the amine group. (Org. Lett. 2013, 15, 65–67; Xin Su)

Solvent polarity and crystallization influence fluorescence behavior. Tetrabenzofluorene (TBF) was first synthesized in 1960, but its photophysical properties have not been studied. H. Ikeda, T. Kawase, and coauthors at the University of Hyogo (Japan), Osaka Prefecture University, and Osaka University designed and synthesized a series of TBF derivatives and investigated their fluorescence properties. They found that the light-emitting behavior of one of the derivatives (1) is dramatically affected by solvent polarity and the crystallization process.

The emission color of 1 is red-shifted with increasing solvent polarity. Theoretical calculations suggest that intramolecular charge transfer causes this unusual solvent effect. Adding a large amount of water (>70 vol%) to a THF solution of 1 enhances the orange-red emission.

The fluorogen also exhibits equally unusual crystallization-induced emission. Its crystals emit orange-red light. (Chem. Asian J. 2013, 8, Early View; Ben Zhong Tang)

“Print” reversibly on polycarbonate films. A. Nelson and co-workers at the IBM Almaden Research Center (San Jose, CA) used dynamic covalent chemistry—thermally induced Diels–Alders reactions—to make polycarbonate films that are suitable for reversible nanoimprinting. They prepared these biodegradable aliphatic polycarbonates by using living ring-opening polymerization of cyclic carbonate monomers to form low-polydispersity random copolymers with furanyl and protected maleimido side chains.

After deprotection at 130 °C, thermal initiation induced the formation of reversibly cross-linked cast polycarbonate films. The authors created sub-micrometer scale patterns on the cross-linked films at elevated temperatures. The films are stable under ambient conditions. Reheating the patterned films reverses the covalent linkage and leaves a pattern-free film surface. (ACS Macro Lett. 2013, 2, 19–22; LaShanda Korley)

Gold surfaces are grafted with visible-light irradiation. Surfaces of metals and other materials can be modified by using a variety of physical, chemical, and electrochemical methods. UV-light photografting has recently come into use, but visible-light grafting has been much less studied. A. Deronzier, J. Pinson, and colleagues at ESPCI Paris Tech, the University of Paris Diderot, and Joseph Fourier University (Grenoble, France) studied the use of visible light to graft diazonium salts to gold and poly(vinyl chloride) (PVC) surfaces in the presence of photosensitizers.

In the Pschorr photocyclization reaction, a stilbenediazonium salt undergoes sensitized dediazonation−cyclization via radical intermediates initiated by excited Ru(bipy)32+; bipy is 2,2’-bipyridine. The authors believed that they could use radical intermediates generated by dediazonation for photografting. They studied visible-light grafting of a variety of substituted phenyldiazonium salts on gold surfaces in several organic solvents (e.g., MeCN, DMF, and DMSO) and water, with Ru(bipy)32+ or eosin Y as the sensitizer. They characterized the grafted layers as thin films (monolayers) <10 nm thick. The method is also applicable to PVC surfaces.

The proposed mechanism is similar to that of the Pschorr photocyclization (see figure). Aryl radicals produced from diazonium salts by excited Ru(bipy)32+ form bonds with the surface. Additional aryl radicals can interact further with the surface or attack the grafted aryl groups to give biaryls.

When MeCN is the solvent, cyanomethyl radicals are likely to be involved in the grafting process. This method provides an alternative for grafting aryl monolayers onto a variety of conducting and nonconducting surfaces. (Chem. Mater. 2013, 25, 90–97; Xin Su)

Use metathesis reactions to modify essential oils. Widely used olefin cross-metathesis can be used to change the substituents on internal C=C bonds in fats and oils. D. Fogg and coauthors at the University of Ottawa (ON) and the Federal University of Minas Gerais (Brazil) used this reaction to transform phenylpropenoid essential oils to antioxidants and other personal-care product ingredients.

The authors first treated anethole (1) with methyl acrylate in the presence of a ruthenium Hoveyda catalyst. They found that the best conditions for this reaction are 0.5 mol% catalyst loading and 6 equiv methyl acrylate. Product 2 was obtained in 99% yield and contained 99% of the E isomer.

The method was expanded to other phenylpropenoids (isoeugenol and isosafrole) and acrylates (ethyl and 2-ethylhexyl acrylate); all reactions gave yields >98% (>83% isolated yields). The authors scaled up the method to the 1-g scale with similar yields.

The reaction of anethole with 2-ethylhexyl acrylate produces the sunscreen ingredient octyl methoxycinnamate. This study represents a paradigm shift in the use of biomass because it adds to the complexity of natural structures instead of simplifying them. (J. Am. Chem. Soc. 2012, 134, 18889–18891; José C. Barros)

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