On message #15, I suggested that geminal polyamines are uncommon because they split to an imine and ammonia. It may be already known, and is inspired by geminal alcohols that are uncommon because they split to a ketone or aldehyde and water. This explanation is consistent with the easy synthesis of geminal tertiary amines while primary ones are scarce.
It is also consistent with the stability of urea, guanidine and the like. As the sketch illustrates, if the diaminated carbon has already a double bond, the expulsion of ammonia would create cumulated double bonds, which is energetically unfavourable. A polymer would be unfavourable too as it would contain hydrazines.
I wish I had credible heats of formation for imines and geminal polyamines.
Pushing the similarity with geminal alcohols, where cyclopropanone reacts with water to make the diol because a carbon unfavourably cumulates a small ring ans a double bond, the hypothetical stability of gem-diaminocyclopopane would be a test for this explanation. As on the other sketch, just react dihalocyclopropane with excess ammonia, for instance as warm gases.
Exaggerating further, the hypothetical hexaaminocyclopropane might be interesting for rockets. With H=4×C, it would need little additional hydrogen (mix with a guanidine) to decompose without soot and produce heat, and may also burn efficiently in oxygen.
Marc Schaefer, aka Enthalpy