Titanocene-Mediated Dinitrile Coupling: A Divergent Route to Nitrogen-Containing Polycyclic Aromatic Hydrocarbons

A general synthetic strategy for
the construction of large, nitrogen-containing
polycyclic aromatic hydrocarbons (PAHs) is reported. The strategy
involves two key steps: (1) a titanocene-mediated reductive cyclization
of an oligo­(dinitrile) precursor to form a PAH appended with di­(aza)­titana­cyclo­penta­diene
functionality; (2) a divergent titanocene transfer reaction, which
allows final-step installation of one or more o-quinone,
diazole, or pyrazine units into the PAH framework. The new methodology
enables rational, late-stage control of HOMO and LUMO energy levels
and thus photophysical and electrochemical properties, as revealed
by UV/vis and fluorescence spectroscopy, cyclic voltammetry, and DFT
calculations. More generally, this contribution presents the first
productive use of di­(aza)­metalla­cyclo­penta­diene
intermediates in organic synthesis, including the first formal [2
+ 2 + 2] reaction to form a pyrazine ring.