I am conducting a synthesis involving Buchwald-Hartwig coupling reaction. The reagents are 2,3,5-tribromothiophene, 1-aza-18-crown-6, NaOtBu as base, $\ce{Pd_2(dba)_3}$ as palladium source, DavePhos as P ligand, and toluene as solvent. Because tribromothiophene is the limiting reagent in my case, so I monitored only the spot of it in TLC and concluded that the reaction went to completion. But some analysis didn't make sense, so I was advised to check the spot of aza crown also. Because aza crown doesn't absorb the 254 nm UV, I checked it through ninhydrin staining. Then I found that the stain of aza crown hardly fades as time goes by, even after 48 hrs. Initially I set the temperature to 100 ℃ and used 3 mol% of catalyst, and I have tried set the temperature to 80 ℃ and reduced the catalyst to 1 mol%, but it was not useful. Also, as far as I know, the ninhydrin stain only detects the amine compounds that have N-H bond. Because aza crown is secondary amine, any byproduct I expect cannot be detected in ninhydrin staining. The aza crown is stained with purple color with streak, but I found that some other spots are also stained in ninhydrin staining with no streak.

ninhydrin stained TLC 1

ninhydrin stained TLC 2

There are two possibilities:

  1. Ninhydrin also stains other compounds other than amines.
  2. There can be some byproducts that have N-H bond.

But I can't search any information about these two cases. Anyone who knows about this?



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