Closed-Shell and Open-Shell 2D Nanographenes - Polyarenes - page 210

Fig. 14 (a) Resonance structures of radical 42;(b) single-crystal structures of 42b [ 50 ]. Reprinted
with permission from [ 50 ]. Copyright 2011 American Chemical Society
Fig. 15 Resonance structures of zethrene and heptazethrene
molecular skeleton (Fig. 15 ). On the basis of the number of fused six-membered rings,
longitudinal homologues of zethrene are defined as heptazethrene and octazethrene
with two phenalenyl rings separated by benzene or naphthalene units, respectively, in a
similar Z-shape manner. The family of zethrenes is featured by the fixed double bond
structure in the central part and a singlet biradical character as illustrated by the
resonance structures shown in Fig. 15 . The biradical form of zethrene is destabilized
by losing one sextet ring, but for heptazethrene and higher homologues it will be
stabilized by gaining one more sextet ring as well as delocalization.
Although the attempt to make zethrene compounds was performed by Clar more
than 50 years ago, the low stability derived from existence of fixed double bond and
the biradical character obscures their practical applications. Very recently, stable
zethrene derivatives with the most reactive 7, 14-positions blocked by substituents
were prepared independently by Tobe [ 51 ] and Wu's group [ 52 ]. Tobe's synthesis
took advantage of iodine promoted transannular cyclization reaction from the
tetradehydro[10]annulene precursor 45, which gave the 7, 14-diiodozethrene inter-
mediate that could be subjected to Sonogashira coupling reaction (Fig. 16 ). How-
ever, the formation of zethrene by this strategy generally suffered from low yield. In
2010, Y.-T. Wu's group developed an even more straightforward and effective way