I'm sorry I did not UTFSE prior posting. That discovery made me a little to impulsive
![Embarrassed :-[](https://thevespiary.org/talk/Smileys/default/embarrassed.gif)
. And today again I did not USTFE
![Tongue :P](https://thevespiary.org/talk/Smileys/default/tongue.gif)
, but did go trough all the links given and attempted a small resume. Hey, at least this opened a new discussion that might clear some mistaken concepts, or in short: ”Did anybody ever had any success with the Vilsmeier-Haack with anything but POCl
3?”
The thing is that I don’t belive SOCl
2/DMF works with anything but a highly nucleophilic aromatic like dimethylaniline in the example given by
Post 207567 (missing)
(lunatic_asylum: "Re: Vilsmeier mechanism: the alternatives to POCl3", Chemistry Discourse). Must I remind that of all benzene derivatives that are not phenols or N-unsubstituted anilines the N,N-dimethylaniline is the most nucleophilic of them all. This makes it activated toward many electrophiles that would otherwise react only with the phenol/aniline type of aromatics.
Therefore, until somebody gives an example I will restrain on wasting my precious thionyl chloride. Well, maybe I will loose patience and at least give it a try on the most activated methoxybenzene viable (1,3,5-trimethoxybenzene) especially if someone comes up with some good news. (I also have similar reconsiderations about COCl
2/DMF) Someone might say “look at the
Patent US4157333
!” but they use N-alkylform
anilides that form the appropriate electrophile much easier than DMF.
Also, the triflic anhydride with DMF is reputably somewhat more reactive formylation combinations than POCl
3/DMF. Meaning that it might give (better) yields with some less nucleophilic aromatics (I have 4-halogeno-1,4-dimethoxybenzene in mind). Yeah, I know, Tf
2O is not laying around just anywhere and is also expensive as gold but triflic acid is at least two times cheaper (not to mention more common). Tf
2O is made by distilling it out of the mixture of P
2O
5/TfOH and I had this thought of possibly using TfOH in catalytic amounts in P
2O
5/DMF for Vilsmeier-Haack formylations. If the temperature would be high enough the Tf
2O should be constantly recycled and should react with a new molecule of DMF. The P
2O
5 and its products are unreactive toward the substrate and should not cause any side products. The idea can be formulated like the following all going on in the DMF solution:
P
2O
5 + Tf-OH ==> Tf
2O + polyphosphoric acids
Tf
2O + Me
2N-CHO ==> Me
2N
+=CH-OTf + TfO
-Me
2N
+=CH-OTf + ArH ==> Ar-CH=N
+Me
2 + Tf-OH
The Tf-OH goes back in to the first reaction…
However if the T is to high the formed Ar-CH=N
+Me
2 might react with Ar-H but I don’t know if at the normal T used in the Vilsmeier the P
2O
5 would dehydrate Tf-OH efficiently enough. Especially when more and more of the Tf-OH gets deprotonated during the second reaction. The polyphosphoric acid is nearly not acidic enough to protonate Tf-O
- but to compensate it is at least thirsty as hell anywhere above 50°C and will dehydrate just anything. Well forgive my constant and long theorizations but I almost love that more than the preparations themselves
![Wink ;)](https://thevespiary.org/talk/Smileys/default/wink.gif)
. Comments are very welcome.