Dear Research Gate community,
I am currently attempting to install a formyl group using the Vilsmer-Haak reaction on a 2,4- substituted thiophene.
The reaction was adapted from a reference, using dry DMF and glassware. I followed the reaction by TLC and LCMS, but did not see any change after 6 h. I am currently working up the reaction. However, I am not confident in the reaction.
I did not notice any fuming of the phosphorous oxychloride upon opening the bottle. Is it general practice to distill POCl3 before use?
How are you sure that the DMF is dry?
It's definitely possible to use POCl3 without distillation first, but sometimes it's not--if you're reaction works then there's no need. Since your reaction is not working I'd suggest troubleshooting in other ways first and then going to distilling POCl3 (unless you have no problem carrying this out, of course).
POCl3 has a reputation for not being as reactive with water as you'd expect (until a certain temperature is reached, causing a runaway reaction and often a geyser erupting from your flask), so that it's not fuming isn't so surprising. Perhaps order some fresh just to be sure.
What are the 2 and 4 substituents? I'm wondering if this could be deactivating your thiophene or if there is some other problem intrinsic to your substrate.
Also, have you considered using the 2,4-DNP (2,4-Dinitrophenylhydrazine) TLC stain to monitor the formation of the aldehyde? This might be helpful for you as you troubleshoot this reaction.
Like Grant Simpson pointed out, if substituents (or one of them) is electron withdrawing the reaction will be difficult to achieve.
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