Divergent Synthesis of Densely Substituted Arenes and Pyridines via Cyclotrimerization Reactions of Alkynyl Triazenes

Densely substituted
fused aromatic triazenes can be prepared by
[2 + 2 + 2] cyclotrimerization reactions of 1-alkynyl triazenes. The
Cp*Ru-catalyzed cyclization proceeds well with both simple alkynyl
triazenes and tethered 1-diynyl triazenes. Attractively, the methodology
can be extended to pyridine synthesis by replacing an alkyne with
a nitrile. The reaction is regioselective and yields the sterically
more hindered product. The triazene group precisely installed on the
synthesized aryl and pyridyl ring is a highly versatile moiety, which
is effortlessly converted into the most important and frequently used
functional aryl substituents, including fluorides. It is also suited
for intramolecular transformations to afford a variety of valuable
heterocycles. The coordination chemistry of alkynyl triazenes and
Cp*RuCl was studied and led to the structural characterization of
a Cp*RuCl­(η2-alkyne) complex, a Cp*RuCl­(η4-cyclobutadiene) complex, and an unusual dinuclear Ru complex
with a bridging tetramethylfulvene ligand. Complexes of this type
are potentially involved in catalyst deactivation pathways.