Iron/Photosensitizer Hybrid System Enables the Synthesis of Polyaryl-Substituted Azafluoranthenes

Photosensitization of organometallics is a privileged
strategy
that enables challenging transformations in transition-metal catalysis.
However, the usefulness of such photocatalyst-induced energy transfer
has remained opaque in iron-catalyzed reactions despite the intriguing
prospects of iron catalysis in synthetic chemistry. Herein, we demonstrate
the use of iron/photosensitizer-cocatalyzed cycloaddition to synthesize
polyarylpyridines and azafluoranthenes, which have been scarcely accessible
using the established iron-catalyzed protocols. Mechanistic studies
indicate that triplet energy transfer from the photocatalyst to a
ferracyclic intermediate facilitates the thermally demanding nitrile
insertion and accounts for the distinct reactivity of the hybrid system.
This study thus provides the first demonstration of the role of photosensitization
in overcoming the limitations of iron catalysis.