Expanded N‑Confused Phlorin: A Platform for a Multiply Fused Polycyclic Ring System via Oxidation within the Macrocycle

Novel interrupted
π-conjugated macrocycles derived from expanded
porphyrinoids were synthesized, and their unique reactivity was investigated
in this work. The specific porphyrin analogs, so-called phlorins and
isoporphyrins, possess a meso-sp3 methylene
moiety, showing inner 3NH and 1NH pyrrolic cores, respectively, and
extended near-infrared (NIR) absorption. Expanded N-confused pentapyrrolic
phlorin analog 1 bears an interrupted cyclic π-conjugated
system that is featured by a distinct higher HOMO and a lower LUMO.
Oxidation of 1 allowed structural transformations through
the expanded isoporphyrin-like species 2. One of the
representative products is a spiro-carbon-bridged multiply N-fused
product 3 comprising a fused [5.6.5.7.6.5]-hexacyclic
ring obtained by oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone. When magic blue was used as the oxidant, an aromatic
N-confused pentaphyrin 4 was obtained via migration of
one of the meso-phenyl groups to the β-position
of the neighboring pyrrolic ring. By employing the flexible cavity
of 1 for metal coordination, Pd­(II) complexation occurred
with a specific meso oxygenation to give a bimetallic
complex 5. In contrast to the rich oxidation reactions,
reduction of 1 with NaBH4 resulted in the
regioselective nucleophilic hydrogen substitution reaction at the
para position of one of the meso-C6F5 groups. These results provide a practical approach for synthesizing
novel interrupted or aromatic π-conjugated frameworks showing
NIR absorptions.