I was recently attempting cyclizations that are best done thermally but they did not work at 250C (unlike the literature precedent) so I investigated them after the Russian manner; in diphenyl ether and under pressure at 290 – 340C they confessed everything.
The temperatures needed were outside the range of a Biotage microwave reactor and oil bath. I did not have an alloy bath and I was looking for some cheap high-temperature bath liquid (I needed temperature reproducibility and the sand bath/solid metal block temperature readings are notoriously unreliable). I found that molten sodium nitrite worked well for the purpose: A 60mL beaker was filled with solid NaNO2 and buried in sand bath obtained by filling a heating mantle with sand (the mantle was controlled via Variac transformer). This setup was placed on top of a stirplate. When NaNO2 melted a magnetic stirbar rod was added (a common stirbar from which the Teflon coating was shaved off) and bath behaved nicely in range 280-340C. I also found out that I could extend the range downwards a little, approximately to 260C-340, by adding few spoons of KNO3. The bath starts bubbling a bit at 360C because NaNO2 begins to decompose slowly above 320C but a 20 min heating to 340 was without problems. My main worry was what might have happened if my reaction vial had ruptured and the organics got into the bath: molten NaNO2 is a pretty strong oxidant and tiny bits of paper burst into flames when fed into the bath…
If you have run high-temperature reactions like this – please what kind of bath did you use?
Addendum: A 2:1 (by weight) fused mixture of KNO3 and LiNO3 melts around 140-160C and solidifies at 125C. It should be far more heat resistant than the NaNO2-based mixtures and thus it should be applicable even for very high temperatures (>300C). LiNO3 is not expensive – but it is somewhat hygroscopic. Of course spilling a reaction mixture into the bath would still backfire :) Thanks Kai for the reference.