Ionic liquids have been called many things: green, recyclable and non-volatile, an answer to many of our wows in organic synthesis. I liken the interest to nanoparticles or carbon nanotubes…..very interesting, and everyone scrambles to make and use, and publish (oh how many Buchwald amine-couplings did I perform after those first couple of publications). All of that said, I have mixed feelings on what can be done…or which one do you use? But hey maybe I waited long enough to jump in the deep end.

Since  I am not going to do an exhaustive literature dump on all of you, I thought I would impress upon the audience how these buggers can be used to enhance microwave chemistry. For starters, if you have a reaction that is normally performed in a non-polar solvent (an a kicker would be conventionally at high temp in a non-polar or non-microwave absorbing medium), the addition can help make the method a microwave method instantly. The following are a few examples of this idea:

Materials Research

Chem. Commun., 2010,46, 3866-3868

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Microwave Reaction Help with IL

Organic Reactions: Intech Open-Source Jan 2013

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IL added to non-absorbing solvent

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IL in Toluene under MW conditions

OK, so you can look at these and get the idea. These authors are on the money, but the idea of a susceptor is not a new concept for helping a non-absorbing reaction work under MW irradiation (they use the terms doping agents), but it certainly does get us to think how we might use ILs. Additional examples can be found in Rafael Palou’s review of IL/MAOS processes, both as a comparison and with some synergism. Among the many examples in the review, Kay Brummond’s [2+2] intramolecular methods employ doped-IL in toluene to the desired fused bicycle. There is also a table of references of named reactions that have utilized this approach.

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[2+2] intramolecular cycloaddtion IL-doped-MAOS

The other advantage of ILs as a medium for a microwave process has to do with the low vapor pressure these materials give during heating. They do require some getting used to on the work-up (isolating product away from the IL), but the advantage of mixing a solvent with a high vapor pressure with an IL can help the overall process in low-to-medium pressure vessels used in some of the microwave reactors.

My thought process is a little different (and I have detailed a new way to think of MW reactions in a single-reaction chamber SRC)– I want to take full advantage of the solvents used in traditional approaches but utilize the ILs as a liquid absorbing pool in a single reaction chamber where several different reactions can be done under MW conditions in their typical solvents (THF, toluene, DCE) and use the IL to absorb the MW surrounding the reactions. One of the great things about this would be that the system will be pre-pressurized to raise the b.p.s of the solvents and the IL can be recycled from one reaction to another, because it isn’t really a part of the reaction other than to absorb the MW energy. So don’t scoop my idea, LOL!