Rhodium:
Thanks for the ref-- interesting info! (where do you find this stuff? I thought that I had pretty much read everything in the literature concerning eugenol->4-allylcatechol)
L42L:
The microwave was a standard commercial model (i.e. multi-mode) with a hole drilled in the top to allow for a passage to a reflux condensor. In fact, I got the idea for the microwave setup from one of Rhodium's posts.
Microwave reactors are very easy to construct from domestic microwaves. I do not recall if anyone on this board has actually made one, but it took less than 2 hours to build, equip, and make the necessary modifications in order for the oven to work. The entire cost of the project was on the order of $40 ($10 for the actual modification, $30 for the microwave detector).
In my experience, the microwave leakage is almost negligible. Of course, it is always wise to check irradiation levels, but I noticed that within a five foot radius, there was essentially no radiation-- even
without the pipe shielding the condensor.
The main reason that I like microwave experiments is that they are very fast and easy to conduct- far more convenient than traditional reflux, which is about as easy as you can get in terms of orthodox methods. The irradiation time used in this particular instance was 90 minutes, but I think it was total overkill; I hypothesize that it could reduced to < 30 minutes without change in yield, and perhaps this would help reduce polymerization etc.
The two most important outcome of the experiments (thus far) were:
1) The substitution of triethylamine for pyridine in an aqueous reaction matrix. Hitherto, the type of amine-base used in microwave cleavage had been severly restricted by the melting point of the respective hydrochloride salt. But since the reflux point is lowered with the addition of water, product composition does not appear to occur (it is true that the reaction probably also occurs much slower, but experimentation suggests that it doesn't impact the yield).
I would like to point out that there are a lot of amine-bases available, including variations of pyridine (i.e. picoline etc.) as well as other aromatic/non-aromatics. Research into these could yield useful results not only for eugenol demethylation, but for ether cleavage in general (who really wants to mess with BBr
3 or pyridine when deprotecting aryl ethers?)
2) The mole ratio was bumped down from 5:1 all the way to 1.5:1 (triethylamine:eugenol) and triethylamine is still present in excess.
I haven't tried the methanesulfonic acid method-- it might be interesting for a future project, but I chose to stick with the amine-bases this time, mainly because there are so many of them.
I would suggest that if anyone is interested in the conversion of eugenol to safrole (or in methylenation reactions in general) that the second synthetic step (catechol -> benzodioxole) be first established. I have tried many of the methods, including PTC w/TBAB/wide variety of surfactants as well as DMSO, with poor results.
The reason I say this is that the result from the first (and only) reaction that I ran from 4-allylcatechol -> safrole resulted in a yield of 20% safrole (overall). This reaction was carried out with some no-name PTC that I had never heard of, but subsequent runs with established PTCs failed to improve the situation. So until a reliable method is found to methylenate m-sterically hindered catechols (and I mean by experimentation, literature can fudge a lot
![Sad :(](https://thevespiary.org/talk/Smileys/default/sad.gif)
) there will be a lot of guesswork involved.
As a side-note, neither pyridine*HCl nor triethylamine*HCl/triethylamine*aqueous HCl demethylated vanillin at all under microwave irradiation. But then again, if you want to synthesize protocatechualdehyde (isn't this available commercially?) you could always use amine-base/AlCl
3 in ethyl acetate, etc.