Thursday, November 09, 2006

How I learnt to love oxy-mercurations

Well maybe I'm pushing it a bit with the title. I'm not exactly loving oxy-mercurations but I have now added it to the list of synthetic transformations worth remembering. Now I have had a long standing disrespect for the goode olde oxy-mercuration. I have always erroneously assumed that you needed stoichiometric mercury for oxy-mercurations to work. However, oxy-mercurations work really well on paper and I teach the stuff to my undergrads when we do electrophilic addition to alkenes, I do however tell them that it has no place in the lab. Well not any more! Today I completed my first oxy-mercuration and it worked really well and guess what...it's catalytic in mercury! Now I hear that many Universities don't teach this stuff to their undergrads anymore so here's a quick little scheme if you don't have a clue what I'm talking about:

I've shown the reaction for an alkyne starting material but it also works for alkenes to give alcohols rather than ketones.

So why did I end up doing an oxy-mercuration? Well as it turns out we needed to get hold of a serious quantity of acetoxymethyl vinyl ketone and a quick search resulted in the following result:

Now these Scandos did a thorough job and wrote a fairly detailed experimental procedure:
A few small additions to the experimental procedure would be:
1) During the acetylation the reaction is indeed somewhat exothermic and unintentionally I proceeded to boil the crap out of it (boils at ~150 oC) for about 5 minutes.
2) If reactions go jet black and shite starts precipitating from them this should be included in the experimental procedure. This is exactly what happens when you start adding the alkyne to the mercury cocktail. At this stage I was convinced that I had fried my alkyne but apparently black crap means that the reactions is working smoothly! This is what it looks like after the addition of a small amount of the alkyne:3) Removing "part of the acetic acid" should be changed to: "remove most of the acetic acid". Otherwise, you'll end up doing nothing but adding sodium bicarbonate and filtering bucket loads of sodium acetate off for an entire day (just like I did!).

Anyway, to finish this oxy-mercuration business the chemistry works very well but man I was working my ass off for two days to get it all done. There's quite a few time consuming steps in this prep such as the adjusting of the pH to 8 followed by removal of endless quantities of sodium acetate not to mention a vacuum distillation at the very end. D!

2 comments:

Canuck Chemist said...

Hmmm...that looks like a pretty useful method (though the conditions would be too harsh for most functionalized substrates. I like how the enone is generated via beta-elim. of the acetate from the vinyl mercury intermediate.

Thanks for the info!

Anonymous said...

This kind of chemistry can be very useful, but you should remind people to treat organo-mercury compounds like they will kill you, because they will. I think that's why they don't teach this reaction to undergraduates anymore.