[shrent]

I performed a reaction making an isocyanate using ORGSYN process of aq. bicarbonate & dichloromethane in biphasic media. The reaction goes nicely using 1/3 eq. of triphosgene giving 85-90% final yield.

However, I'm facing severe problem of the remaining excess triphosgene in the reaction media after the completion. It is not being removed at all with any sort...

I tried it giving multiple washes of aq. bicarbonate, caustic, but still it remains ! Even if I take some lower % of triphosgene in the reaction, then also it consequently gives lower yield with some excess remaining ! 

I even tried distilling my isocyanate with high vacuum & low vacuum but at this stage also triphosgene comes alongwith the isocyanate in the receiving flask..!!!!! 

I just fed-up with this situation....Pls try to suggest me some solution for this.


 

[njlgdxhy]

I have done the reasearch on this .So ,you can use the weak aqua ammonia to remove triphosgene.After reaction,drop the solution slowly

[lavoisier]

Going back to your original question, you say you're making the isocyanate of an amino acid.
I saw a procedure in orgsyn, but they actually start from an amino ester.

So is it the amino acid or ester you're using?

The difference might be important. Have a look at http://www.sigmaaldrich.com/aldrich/bulletin/al_techbull_al176.pdf, they say you don't get the isocyanate, but a cyclic 'anhydride'.

On the other hand, if you're using an amino ester, you could try two different solutions:
1 - changing the alcoholic moiety of the ester to a heavier of lighter one, so that you can have a sufficiently different bp wrt TP.
2 - starting from the amino acid and see if the cyclic anhydride is easier to separate from TP, while still reacting as you want afterwards.

By all means, it's a tricky problem, and even commercial websites selling TP admit that its separation is sometimes awkward.

[lavoisier]

Well, in this case I'm afraid I have no suggestions for you, IF you definitely have to use TP and not any other phosgene substitute.

However, if you can change your reagents, have a look at http://www.sigmaaldrich.com/Area_of_Interest/Chemistry/ChemFiles/Vol_2_No_7/Phosgene_and_Substitutes.html.

I particularly like CDI, but I'm not sure whether it works or not for making isocyanates.

[HP]

Hm interesting question: both your isocyanate product and the triphosgene regent are very reactive compounds so you cant chemically neutralize triphosgene without desactivating the isocyanate i think...I suggest you using some selective solvent for your product which is bad solvent for TP - is your product soluble in cyclohexane? Also TP decompose at 130C in heating so if your isocyanate is stable at this temperature then the formed phosgene will flow out the flask - be careful with such a procedure throw the gases true a flask with NaOH solution for safety....One more hint: phosgene is chemically absorbed by hexamethylene triamine (urotropin) forming insolublesalt and i am not sure if TP can also react with it or only at heating

[HP]

Well, if your isocyanate is stable at 130C then if heat mixture your product with not reacted TP disolved in high boiling aromatics as p- or m- xylene (b.p 139C) then TP will decompose to phosgen gas. Think about laboratory equipment you need for such technique. You may try to see if TP chemically react with urotropine to form salt compound as precipitate in the common solvent you choose. If so then isocyanate shouldnt react with urotropine at moderate temperatures so ...Have you think and about preparative TLC or flash chromatography?

[shrent]

Again, I can't use higher temp as the compound is highly unstable at higher temperatures & it decomposes readily at this stage.

Regarding urotropine, I'm afraid as it is an amine & can react with my isocyanate. Are you sure about its non-reactivity with my compound (i mean isocyanates) ? 

I can't use chromatography as this compound is dangerously toxic (severe eye-burning & smell ) & due to loss of material in big batches.

 

[HP]

Urotropine as tertiary amine cant react with its N-atoms with isocyanates but it have CH2 groups which has relatively mobile H-atoms. Thats why may be if you heat isocyanate with urotropine its possible reaction NCO + -N-CH2-N- --> NHCOCHN2=
But at room temperature i think such reaction doesn't occur..
I am thinking and about this possible route solving your problem: If add some Ag2O(excess) i think it will readily react with TP forming AgCl and may be CO2 formed from TP...think about it