August 22, 2019, 01:40:57 AM
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Topic: FMOC versus BOC under hi vacuum  (Read 396 times)

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Offline Babcock_Hall

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FMOC versus BOC under hi vacuum
« on: July 23, 2019, 07:17:46 PM »
I am working with the FMOC-protected and BOC-protected version of the same compound.  I have not noticed anything out of the ordinary with respect to the BOC derivative.  However, the FMOC derivative consistently wants to bubble up under high-vacuum during removal of solvents such as ethyl acetate.  I have found that I have to repeatedly turn on and turn off the stopcock to avoid the bubbling getting so severe that the compound starts to exit the flask.  One of my colleagues described something similar with respect to another FMOC-protected compound.  Is this a general property of FMOC-protected compounds?

Offline kriggy

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Re: FMOC versus BOC under hi vacuum
« Reply #1 on: July 24, 2019, 01:27:53 AM »
I think its not something FMOC-specific. I had compounds similarly bubgling and foaming under highvac to the point they almost left the flask. They had various structures.

Offline wildfyr

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Re: FMOC versus BOC under hi vacuum
« Reply #2 on: July 24, 2019, 07:54:14 AM »
Agreed with kriggy.

Offline Babcock_Hall

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Re: FMOC versus BOC under hi vacuum
« Reply #3 on: August 13, 2019, 10:03:45 AM »
I plan to run a column to purify a compound that always bubbles up.  Whether this is from the presence of FMOC or not, it means that putting it under a high vacuum takes about 40-60 minutes of repeatedly turning the vacuum on and off.  I want to divide the product into about four pools after the column and high vacuum each one separately (it is sticky, not free flowing).  Is there anything I can do during or after the rotary evaporative removal of EtOAc and hexane that will minimize the bubbling?  If my removal of solvent during or after rotary evaporation could be made more complete, is that likely to help?

Offline wildfyr

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Re: FMOC versus BOC under hi vacuum
« Reply #4 on: August 13, 2019, 03:04:59 PM »
I would just leave it on the rotovap longer, perhaps at higher temp.

Offline clarkstill

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Re: FMOC versus BOC under hi vacuum
« Reply #5 on: August 14, 2019, 10:12:35 AM »
An old colleague of mine used to just put a heavy-walled air condenser on the RBF and put the vacuum adaptor on top of the condenser. He just allowed it to foam up the condenser until all the solvent was gone, then used a spatula to knock it back down into the flask - seemed to work pretty well and much less labour intensive!

Offline Babcock_Hall

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Re: FMOC versus BOC under hi vacuum
« Reply #6 on: August 14, 2019, 10:46:21 AM »
I like that idea, but this stuff is pretty sticky and therefore is not easy to handle.

Offline TheUnassuming

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Re: FMOC versus BOC under hi vacuum
« Reply #7 on: August 14, 2019, 11:04:46 AM »
Not sure this is FMOC specific.  Maybe fmoc groups tend to form solids that like to hold onto solvents longer, making it more likely to "boil" under high vaccuo. 

I've had some success with rotovaping for extended period like @wildfyr suggested.  Can also try cooling it down in your dry ice bucket before putting it under high vac.  It slows down the rate of boiling under high vac to keep it from driving out of the flask.   

Random question, but is it actually sticky solid/low melting or is it sticky because it won't let go of the solvent?  If the later there are other tricks that can be tried.
When in doubt, avoid the Stille coupling.

Offline OrganicDan96

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Re: FMOC versus BOC under hi vacuum
« Reply #8 on: August 14, 2019, 11:32:07 AM »
if it likes holding on to solvent then add petrol or hexane then sonicate and rotovap this often works for removing solvent.

Offline Babcock_Hall

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Re: FMOC versus BOC under hi vacuum
« Reply #9 on: August 14, 2019, 11:40:09 AM »
TheUnassuming,

I am not sure.  I don't see a great deal of solvent by NMR, but there might be small amounts (I will have to double check this).  The chromatography solvent is 50/50 hexanes:ethyl acetate.  Cooling it down is an attractive option.
EDT
I have to redissolve and divide up my sample today.  I could try hexanes or another solvent.  I was thinking of toluene.
« Last Edit: August 14, 2019, 12:07:00 PM by Babcock_Hall »

Offline OrganicDan96

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Re: FMOC versus BOC under hi vacuum
« Reply #10 on: August 14, 2019, 11:55:32 AM »
things that i can't get to crystallize (even if there is not much solvent) i cool with liquid nitrogen until everything is solid then pull a vacuum works really well.

Offline Babcock_Hall

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Re: FMOC versus BOC under hi vacuum
« Reply #11 on: August 14, 2019, 09:34:01 PM »
I split my sample into four parts.  I tried rotary evaporation where I gradually increased the temperature.  Two of the four flasks bubbled a little bit.  I tried putting them on dry ice and applying high vacuum.  There was some additional bubbling, but not as severe as previously.  I will try the other two tomorrow.

Offline Enthalpy

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Re: FMOC versus BOC under hi vacuum
« Reply #12 on: August 15, 2019, 04:39:20 AM »
This may be irrelevant to chemistry, then apologies in advance.

When removing bubbles from glue or silicone with vacuum, the answer to foam is to use wide flat containers. I used a bell jar fit on a glass plate with silicone grease. The vacuum hose fit a hole in the plate with epoxy glue. The glue to degas was in an additional flat container.

Defoamers, or defoaming agents, are very efficient. They are usually formulated for aqueous compounds. Some exist for polyethylene glycol hydraulic fluids. Silicone oils have some chemical inertness.

Add some moving part that mechanically punches the bubbles.

Dilute the mixture with a runny nonvolatile liquid.

Offline kriggy

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Re: FMOC versus BOC under hi vacuum
« Reply #13 on: August 16, 2019, 10:51:53 AM »
You can also leave it overnight under flow of nitrogen, worked well for my buddy to get solids from oils

Offline OrganicDan96

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Re: FMOC versus BOC under hi vacuum
« Reply #14 on: August 16, 2019, 01:28:10 PM »
i wonder weather rotorvaping with high boiling solvent might work (like toluene) this should help to remove any lower boiling solvents.

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