Synthesis of Fully Aliphatic Aziridines with a Macrocyclic Tetracarbene Iron Catalyst

A second-generation
aziridination catalyst supported by a borate-based
dianionic macrocyclic tetracarbene ligand has been synthesized. The
new macrocyclic tetracarbene iron­(II) complex catalyzed the aziridination
of alkyl azides and aliphatic alkenes showcasing the first fully aliphatic
version of this C2 + N1 reaction. High isolated
yields were obtained when no functional groups were present on the
organic azides and alkenes, while modest yields were achieved when
nonprotic functional groups were included. Even multiple functional
groups can be added to the azide and alkene fragments to produce the
most complex aziridines yet synthesized by this C2 + N1 catalytic reaction. The catalyst generated higher yields
for aziridination with aryl azides and alkenes than the previously
reported catalyst, [(Me,EtTCPh)­Fe­(NCCH3)2]­(PF6)2. The contrast is particularly apparent with functionalized aryl
azides where the second-generation catalyst now provides practical
yields for synthetic chemistry. Finally, catalytic intramolecular
aziridination was investigated since many natural products with aziridines
feature bicyclic tertiary aziridines. For five- and six-membered rings,
the bicyclic aziridines were formed catalytically, in contrast to
previously studied catalyzed and uncatalyzed reactions.