[karlschilke]

Hi,

As part of my thesis research, I am trying to acetylate silica gel-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 with triethylamine (a sort of purplish-brown). Although it's a little disconcerting, the color doesn't seem to affect the capping reaction -- the silica remains white, and I am no longer able to label the surface with amine-reactive dyes (e.g. FITC).

The same reactants/conditions using toluene yielded a clear gel/floc after 100'C for several hours, but I think this is because the toluene swelled and permeated through the polypropylene tubes I have been using -- the reaction volume dropped from 1 mL to ~250uL. DMF does not appear to permeate the tubes in this manner, and does not create a gel.

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