Just got a call from colleague looking at microwave pyrolysis — and like my first look into inorganic hydrothermal reactions — I thought, well we are going from something traditionally done at higher temperature to a lower temperature in the microwave — my horse is pushing the cart again. Anyway, I put my hook in the water too….and although this is not a well established area, it is certainly doable, and fascinating.

Let’s back up a bit — most of these reactions I am speaking of are centered around aromatic cyclizations and rearrangement reactions….lots of dancing and not enough N atoms for me, but I do appreciate ring construction. A recent publication (JOC 2009)has perked me up to a whole host of things that can be tried — and quick too.I have included the scheme below how to thing of moving from FVP to MFP:

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Azulene thermal rearrangement

Looking at the scheme is like taking in the development of Pd catalysts and ligand optimization for the first time– we all kind of get the idea that graphite and nanotubes might be effective, but who’s going to figure it out. Since there are a number of examples of graphite applications and modifications: exfloliation, intercalation and doping strategies — some fine tuning to fit the application. If you have ever used graphite in a synthetic reaction — as a “susceptor” then you have experienced how quickly it absorbs the microwave radiation and how quickly you can get to the desired temperature…so if we add to that CNTs and SiC there are a number of things that can be done. But for this exercise — this group performed these reactions in a solid form — or solventless.

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Cyclizations on graphite — very fast! 450C may be just a tad much

As I read through the article looking for signs on how I could use this — I noticed several critical points the group noted throughout their process: 1) open vs. closed-vessel, 2) nitrogen atmosphere 3) power applied at short intervals (not pulsing folks), 4) in open vessel glass wool plug applied to prevent “ejection of material,” (and sealed tubes to prevent sublimation or losing material) 4) the addition of a thermocouple to look closely at the temperature rather than relying on an outside IR measurement — for verification and because the IR sensor in their unit topped out at 300C and 5) the lack of a build-up of pressure — very good. What I also noticed was the lack of agitation in the mixture so there is a possibility of creating hot-spots. I can see thee chemists in the lab now.

I included a few of their successful schemes below to show the utility of the chemistry and to note that there is plenty of unexplored territory out there. Take a look at their improvements over taking these reactions to 900C.

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Thermal rearrangement of 2-ethynylbiphenyl on graphite

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Elbs Cyclizations and related reactions

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Continued

Nice development of some microwave pyrolysis reactions and a great starting point for further study. Kudos to this group and enjoy the paper.

 

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