Microwave‐assisted Reactions on Graphite

On Scale Up of Organic Reactions in Closed Vessel Microwave Systems

“…For reaction mixtures that are poorly microwave absorbing, susceptors can be added. Such materials include salts, graphite-impregnated Teflon, , ionic liquids, , SiC elements, ,, and graphite alone . These materials absorb the input energy more effectively than the bulk and transfer heat to it mainly by conduction.…”

Microwave Flash Pyrolysis

“…Graphite-sensitized microwave reactions were first developed by Laporterie and co-workers . Many types of graphite-sensitized chemistry have been reported earlier, ,,− but the upper limits of this method remained to be explored. We have found that with higher initial power, graphite sensitization extends easily to reactions that have previously been accomplished at high temperature (>500 °C) by flash vacuum pyrolysis − even though the bulk graphite temperature remains much lower.…”

“…Our first method was “open vessel”, i.e., ambient pressure, with mixtures of substrate and graphite irradiated under a nitrogen atmosphere with use of 100−300 W of power in quartz tubes for periods of 1−5 min. In general, higher power and temperatures differentiates our approach from earlier graphite-sensitized experiments. , A loose plug of glass wool above the graphite minimized ejection of material from the reaction zone. As noted earlier by Laporterie, quartz glassware is desirable because of the potentially high surface temperatures.…”

“…In a microwave oven, graphite and carbon nanotubes − absorb radiation with high efficiency and heat rapidly to high surface temperatures. Applications of graphite as a thermal susceptor or “sensitizer” in microwave chemistry have been reviewed by Laporterie, who developed this method, and more recently by Besson . At present, few reported graphite sensitized reactions venture toward the more extreme regime of flash vacuum pyrolysis. − Some notable examples (Scheme ) include the ene cyclization of citronellal ( 1 ), “microwave assisted pyrolysis” of urea to give cyanuric acid ( 2 ), and ketodecarboxylation of adipic acid ( 3 ) .…”

“…Substrates were mixed with graphite, carbon nanotubes, or silicon carbide by light grinding prior to reaction. Although some earlier studies employed precoating of graphite by solvent evaporation, , we prefer to avoid residual solvent impurities and find that reactants are rapidly redistributed during pyrolysis.…”