[Jess_G]

I am kind of stuck understanding why e.g. a piperidine ion (cyclohexane with a protonated nitrogen) is not reactive towards DNA or nucleophiles, whereas an iminium ion is (cyclohexane with C=N and a positive charge on the N).

I don't understand why DNA reacts with the C of the C=N group of an iminium ion but not with the C of the piperidine ion.

Does it have to do with the electronegativity? Is the nitrogen then delta- and the adjacent C delta+ ? But then the C adjacent to the piperidine ion also would also be slightly delta +? Just like the C of the iminium ion is delta+ ??

Thus: I do not see the difference in a piperidine ion and an iminium ion towards reactivity with DNA regarding the adjacent C atom.

Thanks if you can help me out.. bugging me for a while now!

[Doc Oc]

The two functional groups you're talking about are actually very different, the only thing they share is the protonated nitrogen.  Piperidine you'll want to think of as a base, and protonated it loses its reactivity.  Imines, on the other hand, you want to think of like carbonyls.  And in that situation, protonating the amine makes it even more reactive, so it's the opposite effect that you see with piperidine.