Iron-Catalyzed Trimerization of Terminal Alkynes Enabled by Pyrimidinediimine Ligands: A Regioselective Method for the Synthesis of 1,3,5-Substituted Arenes

The
development of pyrimidine-based analogues of the well-known
pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant
methodology for the catalytic trimerization of terminal aliphatic
alkynes. Remarkably, in contrast to established alkyne trimerization
protocols, the 1,3,5-substituted arenes are the main reaction products.
Preliminary mechanistic investigations suggest that the enhanced π-acidity
of the pyrimidine ring, combined with the hemilability of the imine
groups coordinated to the iron center, facilitates this transformation.
The entry point in the catalytic cycle is an isolable iron dinitrogen
complex. The catalytic reaction proceeds via a 1,3-substituted metallacycle,
which explains the observed 1,3,5-regioselectivity. Such a metallacycle
could be isolated and represents a rare 1,3-substituted ferracycle
obtained through alkyne cycloaddition.