Letter
Enhancing the Antiaromaticity of s‑Indacene through
ACCESS
*
ABSTRACT: Addressing the instability of antiaromatic compounds often
involves protection with bulky groups and/or fusion of aromatic rings, thus
decreasing paratropicity. We report four naphthothiophene-fused s-indacene
isomers, one of which is more antiaromatic than parent s-indacene. This
surprising result is examined computationally through nucleus-independent
chemical shift XY calculations and experimentally via nuclear magnetic
resonance spectroscopy, X-ray crystallography, ultraviolet−visible spectropho-
tometry, and cyclic voltammetry, with the latter two indicating that this
molecule possesses the lowest highest occupied molecular orbital−lowest
unoccupied molecular orbital energy gap observed for heterocycle-fused s-
indacene.
romaticity and antiaromaticity are foundational concepts
The unusual combination of aromatic and antiaromatic
characters means that s-indacene remains an interesting
synthetic target, and therefore, other substitution patterns
have been employed to make isolable, s-indacene-based
compounds. Over the past decade, our group has generally
1
,2
A
in modern organic chemistry. Although the definitions
3
−5
of aromaticity and antiaromaticity are debated,
they
ckel’s rule
defines a planar, conjugated, cyclic system with 4n + 2 π
6
−12
continue to be active areas of research.
Hu
̈
13
elections as aromatic. Later, Dewar and Breslow proposed
the concept of antiaromaticity, as a planar, cyclic system with
focused on the indeno[1,2-b]fluorene ([1,2-b]IF) scaffold,
29−33
which includes s-indacene at the core.
By fusing various
1
4−16
4
n π electrons;
however, neither rule clearly describes
carbocycles and/or heterocycles while simultaneously incor-
porating kinetically blocking functional groups, the highly
17
multi-ring systems. Because there is no broadly applicable,
physical measure of aromaticity or antiaromaticity, it is
generally defined for polycyclic hydrocarbons using three
criteria: structural, energetic, and magnetic. Applying only one
criterion can lead to misclassification; however, when applied
together, these criteria form a widely accepted basis of
30,31
antiaromatic s-indacene core can be isolated.
Unlike the
derivatized s-indacene 2 along with additional early exam-
25,26,34,35
ples,
which have the delocalized D2h geometry, the
compounds we have prepared show pronounced bond length
alternation in the s-indacene core, giving us the ability to access
the C2h symmetric conformation of s-indacene. One key study
was the preparation of syn- and anti-indacenodibenzothio-
1
,18
identification.
An interesting test case that pushes the limits of this
definition of aromaticity/antiaromaticity is s-indacene. s-
Indacene (1; Figure 1) is a conjugated molecule consisting
of a fused 5−6−5-membered ring system that has 12 π
31
phene (syn- and anti-IDBT, 3 and 6, respectively), because
we found that the fusion of benzothiophene units to parent s-
indacene came close to the paratropicity of the C structure of
1
4,15
electrons, making it formally antiaromatic.
indacene is expected to be an unstable molecule with
alternating bond lengths, leading to a C symmetric structure
As a result, s-
2h
compound 1 itself. If it was possible for compound 3 to
approximate the antiaromaticity of compound 1, we set out to
design and characterize molecules whose paratropicity would
exceed that of compound 1.
2h
14,19,20
and paratropic ring currents.
compound focused on its theoretical properties.
parent molecule was later synthesized in the early 1960s,
Most initial studies on this
21,22
The
23,24
although it was too unstable to cleanly isolate.
bulky substituents allowed 1,3,5,7-tetra-tert-butyl-s-indacene
TTBI, 2) to be fully characterized, but crystallographic
analysis showed the molecule to have a D delocalized
Inclusion of
Received: May 3, 2021
Published: June 15, 2021
(
2h
2
5,26
structure.
Continued research has since led to debate on its
classification as antiaromatic, aromatic, or non-aromatic within
20,27,28
the criteria described above.
©
2021 American Chemical Society
5
012
Org. Lett. 2021, 23, 5012−5017