[Wisemanleo]

I am trying to monitor the formation of a benzothiazolium carbene using H-NMR, by simply watching for the disappearance or decreased signal strength of the subject proton. The problems I've had in my previous attempts were first figuring out the best solvent to suspend NaH in. Deuterated DCM was too acidic, as I noticed the formation of quite a bit of bubbles. Next was THF-d8 (outrageously expensive stuff). While THF was good for the NaH and 15crown5 ether, it wasn't good enough to dissolve my thiazolium salt. So now I'm on the hunt for a solvent that'll solubilize my thiazolium salt (like DMSO), but won't react with my NaH, thus allowing me to deprotonate the acidic proton on my thiazolium to form a carbene stable enough to allow me to visualize formation by peak analysis. Any tips? I've drawn out the reaction scheme below.

[kiwi]

You could do the reaction in unlabelled solvents - most solvents don't have any signals up around 10, so just set your spectral width to reflect that. You can throw in a little d6-benzene to give a signal to lock and shim too, alternatively ask your local NMR guru how to shim off the FID.
If you want a deuterated system, you could preform a solution of deutero-dimsyl sodium  by premixing the NaH and d6-DMSO, then add the carbene precursor. Dimsyl sodium is a pretty strong base, but i'm not sure whether its strong enough.

[alphahydroxy]

How about dimethylformamide?

[Wisemanleo]

What do you mean by "shim off the FID"? And I always thought you *need* deuterated solvents to do NMR's, otherwise you won't be able to visualize anything?

DMF would work, but I'd have to do it at below 0C temperatures. I was hoping that I wouldn't have to go that route...

[alphahydroxy]

DMF would work, but I'd have to do it at below 0C temperatures. I was hoping that I wouldn't have to go that route...

not sure why this would be the case...? If you're talking about getting "runaway exotherms" from mixing DMF and NaH, as far as I am aware, this something related to the water content of the solvent and more of a problem for heating larger scale reactions. You can dry DMF by distilling from CaH under reduced pressure, and storing ovdr 4 A molecular sieves. Generally speaking, small scale, room temperature reactions using NaH in DMF are fine...

[kiwi]

What do you mean by "shim off the FID"? And I always thought you *need* deuterated solvents to do NMR's, otherwise you won't be able to visualize anything?

a common misconception. the deuterated solvents perform two functions, to provide a signal to lock/shim from, and provide a clean spectrum without solvent peaks.
Originally, these were big issues in NMR, but instrumentation is so good these days that you can get away with a few tricks.
For a lock signal, you only need a trace of something deuterated in the sample, i have run spectra of crude reaction mixtures in 9:1 CH2Cl2:CDCl3 no worries. In shimming, you are just trying to smooth the magnetic field over the sample, in order to gain the maximum signal. This is normally done by trying to maximise the 2H signal from the deuterated solvent, however the principle is general for any signal. In FID shimming, you collect and display a series of 1 scan 1H spectra, and optimise the signal of the FID by adjusting the shims to give a long, regular decay. Its actually pretty easy. And for the signal problem; it isn't really an issue. You can do work arounds, like collecting heteroatom spectra, or adjusting your spectral width to ignore solvent signals, and just collect the regions of interest (like you would do). Alternatively, in order to see sample signals amongst big solvent signals, your instrument only need be able to collect signals varying roughly over an intensity range of 1-1000. This is not a problem for any modern spectrometer, for example see this org lett paper, DOI: 10.1021/ol049979

[Wisemanleo]

interesting! so in my case, since NaH is in excess over my NHC, could I perhaps do something like 10:1 mixture of THF(undeteurated) and d-DMF or d-DMSO? The THF would be for suspending the NaH, and the d-DMSO for the NHC.

[kiwi]

interesting! so in my case, since NaH is in excess over my NHC, could I perhaps do something like 10:1 mixture of THF(undeteurated) and d-DMF or d-DMSO? The THF would be for suspending the NaH, and the d-DMSO for the NHC.

No, i'd use no THF in that case unless the NHC precursor requires it - the NaH will react with the DMSO forming dimsyl sodium in situ, which is very soluble (>1M from memory) in DMSO, so there will be no need for any other solvents to assist the NaH. You probably don't even need to use deuterated DMSO - crop your spectra to remove the DMSO peak, this will work because the area of interest, ~10 ppm, is way away from the DMSO methyl peak.
Either shim off the FID or add 10% of something like d6-benzene to lock to, alternatively run the spectrum unlocked.

[Wisemanleo]

Ah hah! Arduengo did report that the dimsyl anion actually catalyzes the reaction (I assume it's because a free, more reactive hydride is formed).

I imagine that this NMR reaction will allow me to monitor the nhc formation in one sitting.

[kiwi]

i imagine, but that depends on the kinetics involved. incidently, both bruker and varian instruments have good nmr kinetics commands built in (multizgdelay and pad respectively), so you can run it overnight if need be. If you don't mind me asking, whats the product carbene do? is it for a ligand, or something more mysterious?

[Wisemanleo]

I ran the nmr experiment yesterday, and I don't think it went. After adding the NaH mixture, the acidic proton peak did go down a bit, but it still integrated for 1 proton relative to other protons in the compound. I wonder if the dimsyl anion died before it got to do its thing...

But to answer your question, the carbene will be generated in situ and used as a nucleophilic catalyst for some reactions I'm doing  . The reason for the nmr experiment is just to simply prove that the thiazolium compound can be turned into a carbene.

[nj_bartel]

Spam bot bumps a post about my PI's work, cool!