Mysterious Reddish-Brown Color in Acylation Reaction

[Karl Schilke]

Hi,

As part of my thesis research, I am trying to acetylate solid-supported secondary amines using acetic anhydride. I am doing this by a literature method, using 20% acetic anhydride in DMF with 1 mg/mL dimethylaminopyridine (DMAP) as a catalyst. The reaction takes place over a few hours at 100C (boiling water bath). The acetic anhydride is ACS, newly-opened bottle. The DMF is Sigma anhydrous grade, in a SureSeal bottle stored and transferred under argon. The headspace of the reaction is also purged with argon before sealing and immersion in the water bath.

During the reaction (even in the absence of amines), I observe formation of a very deep reddish-brown color reminiscent of truck-stop coffee. I have also seen this with solutions of succinic anhydride in acetone, without catalyst. Although it's a little disconcerting, the color doesn't seem to affect the capping reaction -- I am no longer able to label the surface with amine-reactive dyes (e.g. FITC).

I can't for the life of me figure out where the chromophore is coming from. I assume it's some sort of polymerization or a side-reaction with the DMF, but I can't come up with a feasible mechanism. Anyone have an idea? Am I the only one seeing this?

Thanks in advance,

-Karl "Rat" Schilke, Oregon State University

 

[Karl Schilke]

A quick followup of one thing I forgot to mention -- the same reagents (Ac2O/DMAP) in toluene remain clear after 2 hours at 100C, while the DMF solution has become brown.

Either this is an effect of the solvent polarity (no color in toluene vs. brown in DMF or acetone), or reaction of the solvent and the DMAP or acetic anhydride.

Thanks for any ideas!

-Karl Schilke, Oregon State University