GREEN CHEMISTRY GETS GREENER
Catalysis, agriculture are part of myriad efforts to expand environmentally benign practices
STEPHEN K. RITTER, C&EN WASHINGTON
In just over 10 years, green chemistry has grown from a grassroots idea initiated by a few chemists to a concept that has permeated all of chemistry. Evidence of this rapid growth was clear by the number of talks last month at the American Chemical Society's national meeting in Orlando, Fla., that focused on developing environmentally benign laboratory and industrial practices.
Most of the green chemistry presentations were made in symposia organized by the Division of Industrial & Engineering Chemistry (I&EC). Topics included green catalysis, separations using supercritical CO2, agricultural green chemistry, and green chemistry in the pharmaceutical industry (C&EN, April 22, page 30). The Society Committee on Education also sponsored a general session that provided an introduction to green chemistry.
In the catalysis arena, meeting sessions covered homogeneous catalysis; heterogeneous catalysis; biocatalysis; and alternative catalysis involving photolysis, sonication, and ultrasound. Chemistry professor Chao-Jun Li of Tulane University discussed his group's efforts to apply the principles of green chemistry to make chemistry not only environmentally benign but also more efficient.
The greenest course of action is to minimize the number of reactions, Li stipulated. His group has become expert in demonstrating this reaction-elimination principle through CH bond activation of organic substrates using an array of metal-mediated and -catalyzed syntheses. Typically, these syntheses are carried out in open air under aqueous conditions or with no solvent. In many of the reactions, protection of hydroxyl, carboxylic acid, and amine groups isn't necessary, Li noted.
In total, these modifications offer potentially large savings in time, energy, and organic reagents in addition to reducing or eliminating by-products, he explained. Li's efforts in this area earned him a 2001 Presidential Green Chemistry Challenge Award.
ONE OF SEVERAL example reactions described during talks by Li and by postdoc Chunmei Wei was the direct addition of a terminal alkyne (phenylacetylene) to various imines [Chem. Commun., 2002, 268]. The imines are first generated in situ from an aldehyde and aniline, followed by addition of phenylacetylene and a RuCl3-CuBr cocatalyst.
The proposed mechanism involves simultaneous CH activation of the alkyne by ruthenium and the imine by copper. The ruthenium intermediate is thought to undergo a Grignard-type addition to the activated imine to give the acetylenic amine product and regenerate the cocatalyst. The reactions can be carried out in water or under solventless conditions, Li noted, with yields generally above 85%.
Li and Wei first tried RuCl3 and CuBr separately as the catalyst but obtained low yields. They later determined that the combination of the two metal salts gives optimal results. During their investigations they further discovered that the alkyne-imine additions can be carried out enantioselectively using copper bis(oxazolinyl)pyridine complexes [J. Am. Chem. Soc., 124, 5638 (2002)].
The reactions in water lead to yields of chiral amines generally above 65% with enantiomeric excesses of 7891%. In toluene, the conversions are generally above 80% with enantiomeric excesses of 8296%, Li said, although using the organic solvent is less desirable. "These addition reactions have many possible applications in the synthesis of pharmaceuticals, fine chemicals, and agricultural chemicals," Li stated.
As the number of chemists practicing greener chemistry continues to grow, so do the opportunities to form collaborations drawing on the strengths of different research groups. One such example is a new collaborative effort between Li's group at Tulane and the groups of Rajender S. Varma of the Clean Processes Branch of EPA's National Risk Management Research Laboratory, Cincinnati, and Luc Moens of the National Renewable Energy Laboratory, Golden, Colo.
One area of focus for this collaboration is the development of new catalyst systems in ionic liquids. Graduate student Charlene C. K. Keh in Li's group discussed some of the initial results: a Prins-type cyclization of homoallylic alcohols with aldehydes in an ionic liquid for the direct preparation of a series of tetrahydropyranols.
The reaction includes 1-butyl-3-methylimidazolium hexafluorophosphate as the ionic liquid and cerium triflate as a catalyst. A small amount of benzoic acid is added to assist the Lewis acid catalysis, Keh noted, which optimizes yields at about 80%. The ionic liquids, supplied by the Varma and Moens groups, can be used up to three times without any loss in reaction yield, she said.
Varma's group has been exploring the use of ionic liquids as both a solvent and a cocatalyst. One example is the palladium-catalyzed oxidation of styrene to acetophenone (Wacker reaction) under solventless conditions using H2O2 as the oxidant [Green Chem., 4, 170 (2002)]. The addition of a small amount of an ionic liquid, such as 1-butyl-3-methylimidazolium tetrafluoroborate, significantly enhances the reaction yield with high selectivity for acetophenone over benzaldehyde.
Postdoc Vasudevan V. Namboodiri in Varma's group described a microwave-assisted solventless synthesis of ionic liquids in high purity that should aid the collaborative research. Ionic liquids are generally prepared by refluxing an alkyl halide and an imidazole in an organic solvent for several hours, Namboodiri said.
In the new method, the ionic liquid can be prepared simply by heating the reactants together in a conventional microwave oven without solvent in a process that takes only a few minutes [Chem. Commun., 2001, 643]. The method provides an opportunity to generate ionic liquids in situ and carry out a subsequent reaction all in one pot, Namboodiri said.
Chemistry professor Daryle H. Busch of the University of Kansas discussed his group's work with Kansas chemical engineering professor Bala Subramaniam's group on "CO2-expanded" solvents for catalytic oxidations. Supercritical CO2 has long been considered a green solvent, Busch noted, and it has been shown to be useful in a number of oxidation reactions. But as many green chemistry researchers are finding, pure supercritical CO2 can be limiting because of low reaction rates, inadequate catalyst solubilities, and high process pressures.
A mixture of organic solvent with CO2, a system termed a CO2-expanded solvent, is a way to complement supercritical CO2 as a reaction medium, Busch explained. In one example, the Kansas researchers use acetonitrile with CO2 in a 1:1 volume ratio to oxidize 2,6-di-tert-butylphenol to the corresponding 1,4-benzoquinone [J. Am. Chem. Soc., 124, 2513 (2002)]. The reaction provides better than 80% selectivity using O2 as the oxidant and a cobalt(II) catalyst.
Carbon dioxide increases the solubility of O2 nearly two orders of magnitude compared with neat acetonitrile, Busch said, while the acetonitrile aids in catalyst solubility and allows a catalyst turnover rate that is nearly two orders of magnitude greater than in pure supercritical CO2. The reaction pressure needed is only 60 to 90 bar compared with 200 bar or greater normally needed to solubilize the catalyst. The catalyst can be conveniently recovered by increasing the CO2 pressure until the catalyst precipitates, he added.
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