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Vilsmeier without POCl3

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Nicodem:
I'm sorry I did not UTFSE prior posting. That discovery made me a little to impulsive :-[ . And today again I did not USTFE :P , 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 POCl3?”

The thing is that I don’t belive SOCl2/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 COCl2/DMF) Someone might say “look at the Patent US4157333!” but they use N-alkylformanilides that form the appropriate electrophile much easier than DMF.

Also, the triflic anhydride with DMF is reputably somewhat more reactive formylation combinations than POCl3/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, Tf2O 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). Tf2O is made by distilling it out of the mixture of P2O5/TfOH and I had this thought of possibly using TfOH in catalytic amounts in P2O5/DMF for Vilsmeier-Haack formylations. If the temperature would be high enough the Tf2O should be constantly recycled and should react with a new molecule of DMF. The P2O5 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:

P2O5 + Tf-OH ==> Tf2O + polyphosphoric acids
Tf2O + Me2N-CHO ==> Me2N+=CH-OTf + TfO-
Me2N+=CH-OTf + ArH ==> Ar-CH=N+Me2 + Tf-OH
The Tf-OH goes back in to the first reaction…

However if the T is to high the formed Ar-CH=N+Me2 might react with Ar-H but I don’t know if at the normal T used in the Vilsmeier the P2O5 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 ;) . Comments are very welcome.

Vitus_Verdegast:
The thing is that I don’t belive SOCl2/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.

In Patent GB607920, "Improvements in or relating to the preparation of tertiary amino aromatic aldehydes", the authors state that POCl3 can be substituted with PCl3, SOCl2, SO2Cl2 and S2Cl2. I found the last one very interesting, as it is easily prepared.

So I can assume that this will not work on anything but (N,N-dialkyl-) anilines and maybe phenols?


Well forgive my constant and long theorizations but I almost love that more than the preparations themselves

I find them very interesting really..
At least I didn't stink up my neighbourhood for nothing..  ;)

Nicodem:
So I can assume that this will not work on anything but (N,N-dialkyl-) anilines and maybe phenols?

I'm afraid so, but I don't know exactly why the patent is limited to N,N-disubstituted anilines while we know that POCl3/DMF work with alkoxybenzenes as well. Maybe exactly because of the other presumably less efective reagents that might not work on other substrates. Or maybe other substrates were alredy covered by other patents by the time of issue. I don't know, but it would be still interesting to find out some more examples that where they use PCl3, SO2Cl2 and S2Cl2 with DMF.

carl:
Stumbled upon this thread.
I researched that a while ago because I did not want to buy POCl3, and apparently you can even use SOCl2 or oxalyl chloride to create the vilsmeier reagent from DMF.

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