Thursday, September 27, 2012

Eschenmoser's Salt

On monday, just gonna start easy on the lab. Going to make some kind of alkylation using Eschenmoser's salt (N,N-Dimethylmethyleneiminium iodide), which is a hygroscopic crystalline substance having a melting point of 240 °C (although I have seen everything from 117 °C and 147 °C as melting points, but this is the one you find at Sigma and other serious producers).

Safety Information

Symbol GHS07  GHS07
Signal word Warning
Hazard statements H315-H319-H335
Precautionary statements P261-P305 + P351 + P338
Personal Protective Equipment dust mask type N95 (US)EyeshieldsGloves
Hazard Codes Xi
Risk Statements (Europe) 36/37/38
Safety Statements (Europe) 26-36

It is used to prepare compounds of the type RCH2N(CH3)2, and is very compatible with enolates, enolsilyl ethers and ketones. The mechanism of reaction is quite simple, and I have seen many different solvents and bases being used (like Et3N and Dichloromethane, or Et3N in acetonitrile).

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Now, the salt we have at the lab is quite old and dirty, so I have some different alternatives:

Of coure, I'll go for re-crystallization of the outdated compound. But, to synthesize some more should be quite easy:

Well, so I guess I will have to recrystallize then....


So, I said I was planning to recrystallize the salt. I did not manage to find a proper solvent for recrystallization so I opted to test the salt as it was, since I supposed I would get some product and then I could purify it. I was quite wrong. I did run the reaction (in dry Acetonitrile, reflux) and got a by-product in HUGE amounts. This by-product had a molecular mass of 12 a.u. higher than the expected compound. So, of course, a carbon was added to my molecule somehow.  

I got curious to know what this was and started to purify the stuff and run NMR (H, C, COSY, HSQC, HMBC) to see what the hell was happening. It turned out that there was a big amount of decomposed Eschenmoser's salt (i.e., it had decomposed to formaldehyde). So what was happening was that the formed product was subjected to a second enolization, which resulting in the attack of formaldehyde present in the mixture. Apparently, there is a publication about this reaction and would I have come around this problem 15 years ago, I would have invented a new methodology 8-)).

Reaction of arylketone with Eschemoser's salt

So, I really had to purify the salt, and the best way was actually to run a bulb-to-bulb. I crushed the salt (with a mortar) and put the temperature at 80 °C and the pressure at 8 mbar. It did the trick. All paraformaldehyde sublimated. Raising the temperature to 95 °C resulted in slow sublimation of the Eschenmoser salt. I did this quite quickly and I do not think those temperatures are optimal, but the product was pure enough for my purposes. Now the reaction is running and I hope I can get some pure product by tomorrow :))

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