[whmice]

i am looking for the mechanism of conversion of anhydrides to acyl halides using thionyl chloride and DMF as a catalyst.
in specific reaction of  pyromellitic anhydride to give pyromellitoyl chloride.
i have searched: organic chemistry books (Bruice, Streitwieser and March), references in the Encyclopedia of reagents for organic synthesis, "chemistry of acyl halides" and various articles by searching google scholar and scifinder
reaction is done only with mentioned reactants (no proton source)
source: Burma and Marvel, J. Polym. Sci. Polym. Chem. Ed., Vol. 14 p. 1 (1976)
any help would be appreciated

[orgopete]

I presume DMF opens the anhydride(?). The other syntheses use ZnCl₂. In that case, I expect that Lewis acid coordination is an important part of the mechanism. The higher the temperature a reaction is carried out at, the larger the number of mechanisms that can be operating, so it becomes more of a guess.

I am also guessing that simple aliphatic anhydrides may be (minor) intermediates in the synthesis of acid chlorides by thionyl chloride (or other reagents). Therefore, they too must become converted to acid chlorides. Because aliphatic acid chlorides are more reactive, I presume that chloride ion must open the anhydride or protonated anhydride to give an acid chloride. Because aromatic acid chlorides are much less reactive, this reaction may be slow. Phthalic anhydride would be even slower because it can revert to the anhydride entropically. DMF may be a better nucleophile in that case.

If I were to guess at an alternate mechanism, I would argue that DMF (which is also a very effective catalyst to form an acid chloride and the preferred method) will increase the chloride ion concentration as the iminium salt that readily forms. .

[whmice]

DMF is added only after 2 hr of reflux and refluxing continues for another hour. there are 2eq of thionyl chloride
according to the acid mechanism the hydroxy  turns to the carbonyl, in this case the ether and then i am left with a carbocation?

[stewie griffin]

Hmm, well I drew up a mechanism and then realized you said that DMF isn't added until 2 hrs into the reaction. That's certainly odd. I know that DMF is more nucleophilic than the anhydride (and an acid as well) and that in the DMF catalyzed reaction of an acid to an acid chloride the DMF attacks SOCl₂ first.
Well since I drew it up I'll post it and maybe it will arouse some further insight. Note that I got a bit lazy with some of my arrows (I got tired of drawing out the addition-elimination arrows).

[orgopete]

I don't know where this is supposed to go, but this is a good example of how difficult it can be to establish a mechanism. I have faced these kinds of decisions also. If you have an addition to a C=O, is it nucleophilic, protonation first, or concerted?

Let us discuss some chemistry. Phthalimides are remarkably acid stable. I presume that is because two carbonyl groups deplete the non-bonded electrons of the nitrogen to such an extent that very low concentrations of protonated phthalimide is present to allow competitive attack on a carbonyl group. If base is used, they are actually very easy to open (though difficult to hydrolyze an amide bond).

This suggests to me that protonation of phthalic anhydride would also be difficult and it would also be an unlikely nucleophile in a reaction with thionyl chloride. Will a chloro iminium salt be more reactive?

I thought it more likely that DMF would be the nucleophile. This would generate an iminium anhydride and a carboxylate. The iminium anhydride would be the normal intermediate in the formation of acid chlorides catalyzed by DMF.

If the procedure did not call for the addition of DMF until refluxing for 2 hrs, I would change that immediately. I don't see the purpose of the delay. I would do the reverse and start with a catalytic amount of DMF and slowly add the thionyl chloride. The reaction with DMF should be instantaneous because DMF is probably a better nucleophile that an acid. I presume that is part of the rational for the use of it as a catalyst. After all, if an acid reacted instantaneously with thionyl chloride, why add DMF?

Another possibility is to increase the chloride concentration in the reaction. In the absence of DMF, the HCl will outgas and be lost upon heating. DMF can form the iminium salt and hold chloride as a counter ion/nucleophile. If that were the case, then chloride would be the nucleophile in a reaction with the anhydride.

[stewie griffin]

Wow...somebody smack me on the head! I drew my DMF as not having methyls on the nitrogen.! 
Anyway, I like orgopete's mechanism idea there. I really didn't like the idea of having the anhydride serve as a nucleophile and your mechanism takes care of that issue. Clever idea.
I also agree that it doesn't make sense to add the DMF two hours into the reaction.