Diels–Alder reactions in ionic liquids: A safe recyclable alternative to lithium perchlorate–diethyl ether mixtures
Martyn J. Earle, Paul B. McCormac and Kenneth R. Seddon
Green Chem., 1999, 1, 23 - 25, DOI: 10.1039/a808052f
http://www.rsc.org/Publishing/Journals/GC/article.asp?doi=a808052f
The Abstract:
"Diels–Alder reactions in neutral ionic liquids (such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methyl-imidazolium lactate) are reported. Rate enhancements and selectivities similar to those of reactions performed in lithium perchlorate–diethyl ether mixtures have been observed. As the ionic liquids used have no measurable vapour pressure, are thermally robust, will tolerate impurities such as water, and are recyclable, it is envisaged that these systems could be used on an industrial scale."
What it means:
Diels-Alder reactions are a cornerstone of organic synthesis. The reaction joins a conjugated diene (a molecule with two carbon-carbon double bonds with a single carbon-carbon bond in between them) with a molecule that has a carbon-carbon double bond, referred to as the dieneophile, to form cyclohexene, a six-membered carbon ring with a single double bond. Cyclohexene and other Diels-Alder products are used as building blocks for many different chemicals. Reaction mixtures that allow this step to occur selectively and efficiently in an industrial setting are vital.
Lithium perchlorate–diethyl ether mixtures are often used for this very reason, but have the downside of producing toxic lithium perchlorate-derived wastes and requiring high pressures to get the reaction going. The researchers here looked at a wide range neutral ionic liquids to use in these reactions instead of the lithium mixture. The ionic liquids have very low vapor pressure and can be recovered from the reaction afterwards and recycled. They found that the ionic liquids produced comparable amounts of the desired products verses the traditional mixture.
"Ionic liquids have several advantages over lithium perchlorate–diethyl ether systems, in that they can be used over a wide temperature range (240 °C to 200 °C), can be used under pressure, are not oxidising or explosive, are easily recycled and can be used in conjunction with Lewis acids."
Why it's cool:
Green chemistry is an interesting field, combining the ethics of sustainable industry with the science of chemistry. Green chemists focus on finding ways to reduce waste and perform reactions under more normal conditions. This article is a great example of the research happening lately: finding ways to do what we are doing now, with less impact on the world at large.
