I am doing a lot of peptide synthesis work and use the following reagents:
1) 20% piperidine in DMF
2) 0.65M HBTU in DMF
3) acetic anhydride in DMF
4) N-Methylmorpholine in DMF
I noticed that NMM seems to change color upon storage so I imagine its unstable in DMF. But what about other reagents? Can they be prepared in bulk and stored indefinitely and used during synthesis? Does preparing them in NMP (methylpyrrolidone) increase stability of any of these reagents?
Assuming that DMF itself is pure, I don't see why NMM should be particularly unstable in DMF solution (the color change might be due to the oxidation of NMM by oxygen from the air). For the same reason piperidine in DMF should be stable too (potential transformylation will be negligibly slow at normal conditions). By the way, keep in mind that the commercial DMF often contains traces of dimethylamine.
Acetic anhydride and HBTU solutions in DMF will not be very stable, in my opinion. These reagents are sensitive to water, and sooner or later they will deteriorate.
That being said, if you have to make these solutions for prolonged storage, use high quality anhydrous DMF and keep the solutions tightly closed, away from direct sunlight and at low temperature if possible.
HBTU solution is not stable in DMF and must be freshly prepared daily. For piperidine this is possible to keep it for a week but preferably not more
NMM does change color even on its own, due to oxidation. If you want to keep your solution stable for longer, you have to keep it under inert atmosphere. An issue with DMF solutions is that they do dissolve some oxygen, so oxidation in them frequently occurs faster.
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