Highly flexible π-expanded cyclooctatetraenes: Cyclic thiazole tetramers with head-to-tail connection

Article abstract of DOI:10.1002/anie.201201265

Flipping fast: Head-to-tail cyclic thiazole tetramers representing flexible π-expanded cyclooctatetraenes were designed and synthesized (see picture). The presence of fused thiazole rings in a head-to-tail arrangement leads to a strong columnar stacking of the shallow saddle-shaped π-conjugated molecules, an unusually low inversion barrier of 6.8kcal mol^-1, and reduction-induced complete planarization to an aromatic ring system. Copyright

Full text of DOI:10.1002/anie.201201265

Angewandte
Chemie
DOI: 10.1002/anie.201201265
Cyclooctatetraene
Highly Flexible p-Expanded Cyclooctatetraenes: Cyclic Thiazole
Tetramers with Head-to-Tail Connection**
Kazuhiro Mouri, Shohei Saito, and Shigehiro Yamaguchi*
Flexible p-conjugated skeletons have been the focus of
considerable interest because their dynamic molecular
motions can be the basis of interesting molecule-based
functions. Representative examples of dynamic p systems
include the corannulenes[1] and the sumanenes,[2] which
exhibit a bowl-to-bowl inversion behavior, helicene deriva-
tives that undergo flipping,[3] twisted bay-substituted perylene
bisimides,[4] and expanded porphyrins[5] of various topologies.
Cyclooctatetraene (COT), an 8p annulene, is one of the
classical flexible p-conjugated skeletons that has long been
studied from both theoretical and experimental points of
view.[6] The instability of its planar conformation, is attribut-
able to its angle strain and antiaromaticity,[7] and causes this
skeleton to have a nonplanar tub structure in the ground
state; notably, this structure undergoes a tub-to-tub inversion
in solution (Figure 1a).[8] Additionally, in response to reduc-
tion or oxidation, this tub-shaped skeleton can undergo
a structural change into a planar form because of the
aromaticity of the reduced 10p dianion[9] or the oxidized 6p
dication.[10] Various fascinating COT materials that make use
of these dynamic behaviors have been developed, and include
cavity-size-controlled cage molecules,[11] electromechanical
actuators,[12] buckycatchers,[13] and molecular tweezers.[14]
More sophisticated COT materials need to be thermally
stable and this property can be ensured through a design that
incorporates arene rings fused to the COT structure. How-
ever, this approach destabilizes the planar conformation of
the COT skeleton because of the steric repulsion between
neighboring fused arene rings, thereby increasing the tub-to-
tub inversion barrier. A completely planar COT can be
accessed by preparing
a polyarene-fused structure, as
reported by Nishinaga, Iyoda, and co-workers.[15] In contrast,
we were interested in accessing a flexible and planarizable
COT in the form of a tetraarene-fused structure, specifically
a COT skeleton containing four thiazole rings arranged in
a head-to-tail manner (Figure 1b). We believed that, despite
the tetraarene-fused structure of this COT skeleton, the
alternating arrangement of the nitrogen and sulfur atoms,
À
which take the place of the peripheral C H bonds in phenyl-
fused derivatives, should lead to a low level of steric repulsion,
thereby enhancing the flexibility of the COT skeleton, and
allow it to take a planar conformation upon reduction or
oxidation. Moreover, we envisioned that the introduction of
aryl groups on the 2-position of the four thiazole rings of the
head-to-tail skeleton would allow for effective expansion of
the p conjugation. Herein, we describe the synthesis of cyclic
thiazole tetramers 1 and discuss their structural characteristics
in comparison to those of the head-to-head isomer 2.
The head-to-tail cyclic tetramers 1 were prepared using
À
two different routes, each employing a Pd-catalyzed C H
arylation of thiazole rings:[16] route A) intramolecular ring
closure of acyclic thiazole tetramers 6 under high-dilution
À
conditions; route B) intermolecular double C H arylation of
thiazole dimers 4 (Scheme 1). The final ring-closing steps in
both routes proceeded with acceptable yields, 62% yield of
1a (from 6a; route A) and 36% yield of 1a (from 4a;
route B). The high efficiencies of these transformations may
be attributable to the geometries of the acyclic thiazole
tetramer precursors. Based on DFT calculations,[17]
a
U-shaped bent conformation of 5a is more stable than its
Z-shaped conformation by 8.0 kcalmolÀ1, and thereby the
ring-closure reaction should be more favorable than poly-
merization (see the Supporting Information). The U-shaped
conformation of 5a was confirmed by the crystal structure
(Figure 2).[18] It is also worth noting that the use of the
thiazole dimers 4 is crucial for the preparation of cyclized
Figure 1. a) Dynamic conformational change of COT and b) molecular
structures of head-to-tail cyclic thiazole tetramer 1 and its head-to-
head isomer 2.
[*] K. Mouri, Dr. S. Saito, Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science
Nagoya University, and
À
product 1 using route B. Whereas the intermolecular C H
arylation of 4 gave the cyclized products 1, the reaction of the
regioisomers 3 did not yield 1, presumably owing to the lower
CREST, Japan Science and Technology Agency
Furo, Chikusa, Nagoya 464-8602 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
À
reactivity of the C H bond at the 4-position of the thiazole
toward the direct arylation. A head-to-head cyclic thiazole
tetramer 2a (Ar= p-Tol) was prepared by the oxidative
homocoupling of a dilithiated precursor according to the
method reported for the synthesis of a cyclic thiophene
tetramer (see the Supporting Information).[19]
[**] This work was supported by CRESTand JST. K.M. thanks the JSPS for
a Research Fellowship for Young Scientists. S.S thanks the Noguchi
Institute, Japan Association for Chemical Innovation, and The
Kurata Memorial Hitachi Science and Technology Foundation for
grants.
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Figure 3. X-ray crystal structures of a) 1a and b) 2a. Molecular struc-
tures are shown on the left wherein thermal ellipsoids are shown at
50% probability; crystal packing is shown on the right. Hydrogen
atoms are omitted for clarity.
Scheme 1. Synthesis of cyclic thiazole tetramers with a head-to-tail
connection. Reagents and conditions a) 1. nBuLi, THF, À788C;
2. ZnCl2(tmeda), À788C then RT; b) [Pd2(dba)3]·CHCl3, 558C; c) NBS,
CHCl3/DMF, 608C; d) Pd(OAc)2, XPhos, Cs2CO3, m-xylene (0.01m),
1408C, 4 days; e) Pd(OAc)2, PPh3, PivOH, K2CO3, DMA (0.25m),
1008C, 1 day. dba=dibenzylideneacetone, DMA=N,N’-dimethylacet-
amide, DMF= N,N’-dimethylformamide, NBS=N-bromosuccinimide,
Piv=pivaloyl, tmeda=N,N,N’,N’-tetramethylethylenediamine, Tol=
tolyl, XPhos=2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl.
almost coplanar, which is indicative of an expanded p
conjugation. Importantly, the tub structure of the COT
moiety in the head-to-tail tetramer 1a is shallower than that
in the head-to-head isomer 2a. The bent angle q[15] of the
COT skeleton in 1a is 298, which is much smaller than that in
2a (408), and is the smallest value among those of known
tetraarene-fused COT molecules (38–498).[12a,20] The rela-
tively shallow tub geometry of 1a is stabilized by the
following structural features: 1) the thiazole rings, which do
not experience the same repulsive interactions with their
neighbors as do phenyl rings because of the absence of
À
peripheral C H bonds; 2) the alternating arrangement of the
nitrogen and sulfur atoms in the head-to-tail tetramer
skeleton. The size of the bent angles may affect the electronic
characters of the COT moieties. To assess the aromaticity of
these compounds, their NICS (nucleus-independent chemical
shift)[21] values were examined. The NICS values at the mean
coordinate of the eight-membered ring in the optimized
geometries are + 6.2 ppm for 1a and + 3.3 ppm for 2a at the
B3LYP/6-311 + G** level of theory.[17] These results suggest
that the shallow tub COT structure in 1a slightly perturbs the
antiaromatic character, although both are essentially non-
aromatic compounds.[15]
Figure 2. X-ray crystal structure of 5a. Hydrogen atoms are omitted for
clarity and thermal ellipsoids are shown at 50% probability.
A thermogravimetric analysis (TGA) revealed the high
thermal stability of 1a: the decomposition temperature for
a 5% weight loss (Td5) was 4218C. This high stability is
attributable to its molecular structure, in which the central
COT moiety is fused with aromatic thiazole rings, the reactive
2-positions of which are capped by aryl substituents. Single-
crystal X-ray diffraction analysis of 1a and 2a revealed their
saddle-shaped structures wherein the central COT moieties
were in the tub conformation (Figure 3 and the Supporting
Information).[18] In all the compounds, the thiazole rings and
the aryl substituents have conformations that make them
In their crystal forms, both 1a and 1b pack together
through columnar stacking, despite their nonplanar struc-
tures. In the structure of 1a, the intermolecular distance
between the mean planes of the adjacent arylthiazole units is
2
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3.60 ꢀ and the tub-shaped COT cores are stacked with an
intermolecular distance of 3.91 ꢀ (Figure 3a). The large
contact area between the adjacent saddle-shaped molecules
results in a strong van der Waals interaction, which may be
responsible for the columnar stacking. In line with these
structural features, 1a and 1b have poor solubilities in
common organic solvents. In light of the fact that regioisomer
2a does not stack in a columnar form, it seems that the head-
to-tail arrangement of thiazole moieties in 1a is crucial for
realizing these stacking structures (Figure 3b).
The shallow tub structure of 1 prompted us to investigate
the tub-to-tub inversion behavior of this molecular system.
DFT (B3LYP/6-31G*) calculations revealed that the tub-to-
tub inversion barrier of COT 1 has the unusually low value of
DG° = 6.8 kcalmolÀ1 at 298 K.[17] This inversion should pro-
ceed along a simple bell-shaped energy profile via a planar
transition state; this hypothesis was confirmed by the IRC
(intrinsic reaction coordinate) calculation (Figure 4a).[22]
Notably, this inversion barrier is the lowest value among
We then focused on the origin of the significantly low
inversion barrier of 1. It can be rationalized by the following
= À
three considerations. Firstly, the small C C S angles of the
thiazoles, which are fused to the COT framework, cause
a rehybridization of the carbon atoms within the COT eight-
membered ring; this hybridization leads to wider inner angles
of the COT ring.[7] As the inner angle of a regular octagon
(1358) is larger than the ideal bond angle for a sp2 hybridized
carbon atom (1208), the widening of the inner angles results in
a shallower tub-shaped COT structure. Accordingly, the bent
angle q (34.58) of 1 in the optimized structure is smaller than
that of the nonsubstituted COT 7 (39.48).[23] It is known that
the annulation of ring skeletons having small inner bond
angles reduces the bent angle of COT.[7c,d,i] This structural
effect should also lead to reduced angle strain in the planar
COT structure of the transition state, thus leading to the
lower inversion barrier (Figure 4b). Secondly, the low level of
steric hindrance between neighboring thiazole rings in the
head-to-tail arrangement should contribute substantially to
the relative stability of the planar transition-state structure of
1. Indeed, the head-to-head regioisomer 2, which has more
significant levels of steric hindrance between neighboring
thiazoles, has a far higher inversion barrier of 22.4 kcalmolÀ1.
A comparison of the inversion barriers among a series of
dithiazole-fused COT skeletons that included 8 (6.6 kcal
molÀ1),
9
(7.3 kcalmolÀ1), 10 (10.3 kcalmolÀ1), and 11
(11.0 kcalmolÀ1) clearly demonstrates that the head-to-tail
arrangement engenders a lower inversion barrier. Thirdly, the
presence of fused thiazole rings in 1 should mean that in the
transition state, in which the COT core takes on a planar
conformation, the antiaromaticity of the COT core of 1 is
relatively low; this lower antiaromaticity would result in
a smaller antiaromatic destabilization of the transition state
for 1 than for the nonsubstituted COT 7.[7e,g] The NICS(0)
values of the tetrathiazole-fused COT 1, dithiazole-fused
COT 8, and nonsubstituted COT 7 were estimated to be
+ 17.2, + 24.2, and + 42.1 ppm, respectively, in the transition
state (see the Supporting Information), thus demonstrating
the significant difference in the antiaromaticity. These
considerations lead to the conclusion that the head-to-tail
arrangement of thiazole rings is optimal for achieving
a flexible COT skeleton.
We next investigated the structural changes in the
tetrathiazole-fused COT upon its reduction or oxidation. We
first obtained cyclic voltammetry responses of 1a in CH2Cl2 or
THF and compared them to those of the noncyclic congener
5a (Figure 5). The cyclic 1a showed reversible redox waves at
the first oxidation potential of Eox =+ 0.67 V and at the first
Figure 4. a) Relative energy diagram of the tub-to-tub inversion of
1 based on the IRC calculation and b) inversion barriers of the parent
COT and thiazole-fused COT derivatives calculated at the B3LYP/6-
31G* level of theory.
those of known COT derivatives, including the nonsubstituted
parent COT skeleton 8 (10.9 kcalmolÀ1),[8] and is even lower
than the bowl-to-bowl inversion barriers of corannulene
(11.5 kcalmolÀ1)[1b] and sumanene (18.8 kcalmolÀ1),[2c] and
the flipping barrier of the bay-substituted perylene bisimide
(14.3 kcalmolÀ1).[4] The value of DG° = 6.8 kcalmolÀ1 corre-
sponds to a frequency of 6.4 ꢁ 107 sÀ1 at 298 K. Although it is
difficult to experimentally confirm such a low energy barrier,
for example, by means of variable temperature NMR experi-
ments, this structural feature should render this skeleton
a unique flexible p-conjugated system.
and second reduction potentials of
E
red = À1.93 V and
À2.19 V (versus Fc/Fc+), respectively; the noncyclic tetramer
5a only showed irreversible processes during both oxidation
and reduction with a more positive anodic peak potential of
E
pa =+ 0.92 V and a more negative first cathodic peak
potential of Epc = À2.46 V. These results demonstrate that
the oxidation and reduction of the thiazole tetramer p
skeleton is facilitated when the skeleton is in the cyclic
form, and that the central COT moiety endows this skeleton
with high redox stability.
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than the sum of the van der Waals radii (3.35 ꢀ).[24] The
bonding orbital interaction is observed between the adjacent
nitrogen and sulfur atoms and involves the occupied molec-
ular orbitals HOMO-9 and HOMO-10, although a bond
critical point (BCP) was not found by the atoms-in-molecules
(AIM) calculations (see the Supporting Information).[25]
The aromaticity of 1a2À was assessed in terms of both
magnetic and structural criteria. The large negative NICS(0)
and NICS(1) values[21] (À14.8 and À12.5 ppm, respectively) of
the central COT moiety in 1a2À supports the high aromaticity;
1H NMR analysis is not an appropriate experimental diag-
nostic in this case because no protons are located near the
central COT moiety. More direct evidence of the high
aromaticity was obtained in the form of the observed small
À
C C bond length alternation in the central eight-membered
ring. The bond lengths of the planar COTunit are significantly
Figure 5. Cyclic voltammograms of 1a and 5a in CH2Cl2 for oxidation
and in THF for reduction. Fc=ferrocene.
deformed from those of the noncharged species 1a. Upon
À
reduction, the C C bonds connecting the two adjacent
thiazole rings are shortened from 1.47 to 1.41 and 1.42 ꢀ,
Consistent with the redox stability, the chemical reduction
of 1a led to a dianion species. The addition of potassium metal
to a THF suspension of 1a resulted in a dramatic color change
from light yellow to deep purple together with the obvious
À
while the thiazole-fused C C bonds are elongated from 1.38
to 1.43 and 1.44 ꢀ (Figure 6b). The planarized COT unit in
1a2À has a substantially higher HOMA (harmonic oscillator
model of aromaticity)[26] value (+ 0.63) than 1a (+ 0.17). This
result clearly demonstrates the high aromaticity of the 10p
COT dianion.
1
changes in the H and 13C NMR spectra (see the Supporting
Information). Upon the addition of [2.2.2]cryptand to the
solution, single crystals of the dianion salt 1a2À·2[K-
(cryptand)]+ were obtained. The X-ray crystal structure
revealed that the reduced COT moiety indeed has a com-
pletely planar geometry (q = 1.58), and is sandwiched
between two K+(cryptand) counter cations (Figure 6a).[18]
This is the first structural analysis of the tetraarene-fused
COT dianion (see below).[9] The planar structure of 1a2À is
stabilized by contribution of the 10p aromaticity of the COT
dianion. The low level of steric hindrance between the
adjacent thiazole units is also believed to contribute to its
complete planarization. Short intramolecular N—S contacts
(2.72 ꢀ) are observed between the adjacent nitrogen and
sulfur atoms (Figure 6b); this distance is significantly shorter
In summary, we have synthesized head-to-tail cyclic
thiazole tetramers, which represent a p-expanded COT with
a remarkable molecular flexibility. The fusion of thiazole
rings to the cyclooctatetraene (COT) structure in such a way
that they are linked in a head-to-tail fashion results in 1)
a shallow saddle-shaped structure with columnar stacking in
crystals, 2) a low tub-to-tub inversion barrier of DG° =
6.8 kcalmolÀ1 at 298 K, and 3) a compound that can be
reduced to the planar aromatic dianion. Most strikingly, the
low tub-to-tub inversion barrier of 6.8 kcalmolÀ1 is only
slightly higher than the umbrella inversion barrier of NH3
(5.5 kcalmolÀ1).[27] In light of the fact that the inversion of
amines can occur even in the crystalline state,[28] we are
interested in applications that may be based on such
molecular flexibility of 1 in the solid-state. Future investiga-
tions will explore dynamic molecular functions using this new
COT core skeleton.
Received: February 15, 2012
Published online: && &&, &&&&
Keywords: aromaticity · cyclooctatetraenes ·
.
molecular dynamics · thiazoles · redox chemistry
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!

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Angewandte
Communications
Communications
Cyclooctatetraene
Flipping fast: Head-to-tail cyclic thiazole
tetramers representing flexible p-
expanded cyclooctatetraenes were
designed and synthesized (see picture).
The presence of fused thiazole rings in
a head-to-tail arrangement leads to
a strong columnar stacking of the shallow
saddle-shaped p-conjugated molecules,
an unusually low inversion barrier of
6.8 kcalmolÀ1, and reduction-induced
complete planarization to an aromatic
ring system.
K. Mouri, S. Saito,
S. Yamaguchi*
&&&& — &&&&
Highly Flexible p-Expanded
Cyclooctatetraenes: Cyclic Thiazole
Tetramers with Head-to-Tail Connection
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!