Thermodynamically Stable o-Quinodimethane: Synthesis, Structure, and Reactivity

Article abstract of DOI:10.1002/chem.202004510

Thermal isomerization of cyclobutaphenanthrene to o-quinodimethane was investigated. Sterically congested substituents or electron-donating substituents on the four-membered ring promoted the ring-opening, affording o-quinodimethane in a relatively stable form. Isolation of the newly prepared o-quinodimethane allowed its structural elucidation and investigation of its potential reactivities. Dual [4+2] cycloaddition of an aryne and o-quinodimethane afforded tetrabenzopentacene, demonstrating the synthetic application of the isolated compound.

Full text of DOI:10.1002/chem.202004510

Accepted Article
Accepted Article
Title: Thermodynamically Stable o-Quinodimethane: Synthesis,
Structure, and Reactivity
Authors: Toshiyuki Hamura, Kazuhiko Adachi, Shunsuke Hirose,
Yasuyuki Ueda, and Hidehiro Uekusa
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To be cited as: Chem. Eur. J. 10.1002/chem.202004510
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01/2020

10.1002/chem.202004510
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Thermodynamically
Stable
o-Quinodimethane:
Synthesis,
Structure, and Reactivity
Kazuhiko Adachi,[a] Shunsuke Hirose,[a] Yasuyuki Ueda,[a] Hidehiro Uekusa,[b] and Toshiyuki Hamura*[a]
Dedication ((optional))
Abstract: Thermal isomerization of cyclobutaphenanthrene to o-
quinodimethane was investigated. Sterically congested substituents
or electron-donating substituents on the four-membered ring
biradical characters, small band gaps, and excellent
electrochemical behaviors have been observed (Scheme 2).[4]
However, steric protection by the sterically congested
substituents and/or fused ring in the butadiene moiety of these
compounds prevents their isomerization into the four-
membered ring. Due to this, their synthetic use as a quinoidal
building block is virtually difficult. Moreover, the steric protection
renders these quinodimethanes kinetically stable.
promoted the ring-opening, affording o-quinodimethane in
a
relatively stable form. Isolation of the newly prepared o-
quinodimethane allowed its structural elucidation and investigation
of its potential reactivities. Dual [4+2] cycloaddition of an aryne and
o-quinodimethane afforded tetrabenzopentacene, demonstrating
the synthetic application of the isolated compound.
Ar1 Ar1
Ar2 Ar2
Ar1
Ar1
Thermal isomerization of cyclobutene to 1,3-diene (Scheme 1)
is one of the most fundamental and useful reactions, since the
ring-opened isomer can further react with dienophiles to give
polycyclic compounds, which are key intermediates in the
syntheses of various natural and synthetic products.[1] The first
step (I ® II) in this pericyclic cascade reaction is exothermic
owing to release of the inherent ring strain in the four-
membered ring (Eq. 1). An opposite behavior is observed for
the case of benzocyclobutene IV, the benzo-analogue of
cyclobutene I, due to the loss of aromaticity of the benzene ring
in IV (Eq. 2). Consequently, o-quinodimethane V, the ring-
2 (Ar2 = Me3C6H2)
1 (Ar1 = p-MeOC6H4)
Br
Br
Ar2 Ar2
Ar2Ge
GeAr2
O
O
O
O
4 (Ar2 = Me3C6H2)
3 (Ar2 = Me3C6H2)
opened isomer of IV, is obtained only as
a transient
intermediate, thereby limiting the potential utility of this
structurally attractive molecule.[2,3]
Scheme 2. Representative examples of isolable o-quinodimethanes.
Considering this, we adopted an approach to prepare isolable
o-quinodimethanes that were not kinetically stabilized (Scheme
3). To access the ring-opened structure, we introduced extra
aromatic rings onto the parent structure IV to reduce the
destabilization caused by the ring opening (A ® B). In this p-
extended system, if the energy loss due to dearomatization of
the benzene ring in A is smaller than the ring strain of the four-
membered ring, the ring-opened isomer B would become
energetically more favorable than the ring-closed form A, thus,
producing thermodynamically stable quinodimethane B.[5]
(1)
I
II
III
(2)
VI
IV
V
Scheme 1. Pericyclic cascade reaction of cyclobutene
benzocyclobutene IV.
I
and
π
π
π
π
?
π
π
Thermally stable o-quinodimethanes (e.g., 1–4) have been
synthesized previously, and several unique properties including
A
B
Ring strain
vs
Loss of aromaticity
[a]
[b]
Dr. K. Adachi, Mr. S. Hirose, Dr. Y. Ueda, Prof. Dr. T. Hamura
Department of Applied Chemistry for Environment, Kwansei Gakuin
University, 2-1 Gakuen, Sanda 669-1337, Japan
E-mail: thamura@kwansei.ac.jp
OR
OR
OR
OR
fast
slow
Prof. Dr. H. Uekusa
Department of Chemistry, School of Science, Tokyo Institute of
Technology, Ookayama 2, Meguro-ku, Tokyo 152-8551, Japan
D
E
F
C
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Scheme 3. Synthetic strategy to access to isolable o-quinodimethane.
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Scheme 4. Thermodynamic preference (DEring-opened–ring-closed
cyclobutaarenes and their ring-opened isomers calculated at the B3LYP-
D3/6-311G(d,p) level with Grimme’s dispersion correction.
) between
The basis for this strategy lies in our related work for the
synthesis of p-extended triphenylenes by the successive
cycloaddition
of
tricyclobutabenzene.[6]
Specifically,
cyclobutaphenantrene E, obtained by double cycloaddition and
acid-promoted aromatization of tricyclobutabenzene, had higher
(DE14a–13a = +1.9 kcal/mol). This indicates that among mono-
and di-annulated cyclobutabenzenes 5–9, phenanthrene-based
condensations most efficiently promote the formation of o-
quinodimethane. In addition to this p-extension, introduction of
the hydroxy group on the four-membered ring strongly
facilitates the ring opening, rendering o-quinodimethane 14b a
thermodynamically stable product (DE14b–13b = –3.8 kcal/mol).[9]
Based on these calculations, cyclobutaphenanthrenes 23
bearing an electron donating aryloxy group on the four-
propensity
for
ring-opening
than
the
non-aromatic
cyclobutabenzene C and gave the Diels–Alder cycloadduct at
lower temperatures by the facile generation of quinodimethane
F.
We thus focused on the syntheses of cyclobutaarenes
containing extra aromatic rings on the parent benzocyclobutene.
Although the interconversion between cyclobutabenzene IV and
o-quinodimethane V has been thoroughly studied, the poly-
aromatic analogues with various potential functionalities have
remained unexplored, presumably due to the poor availability.
Herein, we demonstrate that cyclobutaphenanthrene E,
efficiently prepared by the (2+2) cycloaddition of an aryne and
ketene silyl acetal (KSA), underwent thermal isomerization to
afford the corresponding ring-opened isomer F as a relatively
stable product, provided the hydroxy group on the four-
membered ring in E is protected using a suitable substituent.[7,8]
Moreover, structure of the isolated o-quinodimethane was
characterized by X-ray crystallographic analysis, and its
reactivity was investigated through the trapping reaction with a
transiently generated aryne to afford the structurally attractive
tetrabenzopentacene.
membered ring were prepared as
a
precursor of o-
quinodimethane (Scheme 5). Reaction of bromo tosylate 16,
which is easily accessible from phenanthrene-9,10-dione 15,[10]
with n-BuLi in the presence of KSA 17 gave the (2+2)
cycloadduct (structure not shown) via the formation of aryne
18;[11] acid-promoted hydrolysis of the cycloadduct afforded
cyclobutenone 19. Further transformations, including the
reduction of the carbonyl group in 19 and the SN2 reaction of
iodide 22 with sodium phenoxide gave cyclobutaphenanthrene
23a. Other aryl ethers, 23b–23k (Table 1), could also be
accessed through the SN2 reaction of iodide 22 with ArONa.
We initially performed density functional theory calculations to
O
Br
EtO OTBDMS
1) n-BuLi, –78 °C
NaBH4
determine
the
thermodynamic
preference
between
THF, MeOH
quant.
2) aq. HF
OTs
cyclobutaarene and the corresponding ring-opened isomer and
to estimate the effect of p-extension on the ring-opening
(Scheme 4). Energy gap between the two isomers of the parent
system (DE10–5) was +13 kcal/mol. When an additional benzene
ring was introduced at the para position of the parent
benzocyclobutene 5, energy gap between the cyclic and ring-
opened product increased significantly; that is, energy of
quinodimethane 11 was much higher than that of
17
44%
(2 steps)
16
19
I
OH
OMs
MsCl, Et3N
CH2Cl2
NaI
MeCN
68%
(2 steps)
20
21
22
cyclobutanaphthalene
6 (DE11–6 = +23 kcal/mol). Further
OPh
addition of a benzene ring at the para position resulted in an
even larger energy gap (DE12–7 = +29 kcal/mol). This trend can
be explained by the increased quinoidal character owing to the
fusion of the benzene ring at the para position.
O
PhOH, NaH
DMF
O
quant.
23a
15
18
In contrast, an opposite trend was observed for the meta-fused
cyclobutaarenes.
The
energy
gap
between
Scheme 5. Synthesis of cyclobutaphenanthrene via (2+2) cycloaddition of
cyclobutanaphthalene 8 and its ring-opened isomer 13 was 5.2
kcal/mol, and the energy gap further decreased for
dibenzoannulated compounds 9a and 14a
aryne and KSA.
The thermal reaction of cyclobutaphenanthrene 23a was
monitored by VT NMR spectroscopy (toluene-d8, 90 °C),
and the spectra are shown in Figure 1. Heating 23a at
90 °C afforded the ring-opened product 24a, as evident
from the appearance of two sets of doublets and one
singlet corresponding to 24a (spectrum (b)). After 1 h, the
R
6
5
8
7
9a (R = H)
9b (R = OH)
R
interconversion reached equilibrium, and the 23a/24a
product ratio was 76:24 (spectrum (c)).[12] This indicates
that the ring-opened form was thermodynamically less
stable than the ring-closed form. After removing the
solvent, the reaction mixture was separated by preparative
TLC to give quinodimethane 24a in 29% yield as a single
10
12
11
13
14a (R = H)
14b (R = OH)
+13
+23
+1.9 (R = H)
–3.8 (R = OH)
ΔE
(kcal/mol)
+29
+5.2
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isomer.[13] Quinodimethane 24a was determined to be an E
isomer based on the X-ray analysis of a related compound
(vide infra). Importantly, product 24a could be stored in a
refrigerator for several months.[14]
Introduction of an electron-donating group at the para
position increased the proportion of the ring-opened form
relative to the proportion of 23a (entries 1–3). Indeed, the
ratio of aryl ether 23d, bearing a dimethylamino group, to
o-quinodimethane 24d after isomerization was 52:48. In
sharp contrast, introduction of electron-withdrawing groups
at the para position decreased the proportion of the ring-
opened form with increasing electron-withdrawing ability of
the substituents (entries 4–7). For substrate 23h having a
nitro group, the corresponding ring-opened isomer 24h
was obtained only in 8% yield (NMR yield).[15]
O
O
90 °C
toluene-d8
23a/24a = 76 : 24
24a
23a
Moreover, it is interesting to note that the ring-opened
isomer was produced predominantly when substrates 23j
and 23k bearing sterically congested aryloxy group on the
four-membered ring were heated (entries 9 and 10).
Gratifyingly, o-quinodimethane 24k having a terphenyloxy
group could be purified by preparative TLC, and slow
crystallization of 24k gave single crystals suitable for X-ray
analysis.
Figure 2 shows the X-ray structure of 24k [16]
The twisted
.
nature of 24k in the solid state was a striking feature; the
twist angle of the biphenyl backbone was 17.2°, and the
torsion angle of the two exocyclic double bonds was 41.7°.
The C–C bonds around the central six-membered ring
exhibited bond length alternation (C1–C2 = 1.335 Å, C2–
C3 = 1.478 Å, C3–C4 = 1.408 Å, C4–C5 = 1.480 Å, C5–C6
= 1.412 Å, C6–C7 = 1.476 Å, C7–C8 = 1.342 Å, C7–C2 =
1.484 Å), which is similar to that observed in the structure
Figure 1. Thermal isomerization of 23a monitored by VT 1H NMR
spectroscopy (500 MHz, [D8]toluene, 90 °C). a) 23a (RT), b) reaction mixture
after 5 min (90 °C), c) reaction mixture after 1 h (90 °C).
of o-quinodimethane 1.
(a)
(b)
Aryl ethers 23b–23k with various substitution patterns
were also heated under similar conditions, and the ratios of
the ring closed form 23 and ring opened form 24 were
determined (Table 1).
Table 1. Thermal isomerization of cyclobutaphenanthrenes 23
to o-quinodimethanes 24.
R1
(c)
R3
O
R1
OAr
R2
8
R3
7
2
6
3
5
4
O
R2
90 °C
1
toluene-d8
24k
24
23
Entry
Cyclobutene
R1
R2
R3
Molar ratio
23 : 24
Figure 2. X-ray structure of o-quinodimethane 24k. a) top view, b) side view,
c) the number of carbon atoms in 24k.
1
2
3
4
5
6
7
8
9
23b
23c
23d
23e
23f
23g
23h
23i
Me
OMe
NMe2
F
H
H
H
H
H
H
H
Me
Me
Ph
H
H
H
H
H
H
H
H
Me
Ph
66 : 34
63 : 37
52 : 48
82 : 18
81 : 19
89 : 11
92 : 8
65 : 35
15 : 85
33 : 67
Further investigation of the thermal isomerization of
cyclobutaphenanthrene revealed that the siloxy group on the
four-membered ring contributed substantially to promote the
ring opening, affording o-quinodimethane as an almost
exclusive product. The VT-NMR spectrum of silyl ether 23l
clearly shows that the peaks of starting compound 23l gradually
disappeared upon heating in toluene-d8 at 90 °C (spectrum (b)
Cl
CHO
NO2
H
H
H
23j
23k
10
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in Figure 3). After 20 min, the starting compound was almost
completely converted to o-quinodimethane 24l (spectrum
(c)).[17] The electron-donating ability of the substituent on the
four-membered ring is an important factor in controlling the ring-
opening step. Thus, in addition to facilitating the ring opening,
which is known as torquoselectivity,[9] the higher electron
donating ability of the siloxy group compared to the aryloxy
group[18] may stabilize the quinodimethane.[19]
cyclobutaphenanthrene by introducing an electron-
donating substituent on the four-membered ring. The o-
quinodimethanes obtained through this strategy can be
used as a reactive platform to access novel polycyclic
aromatic compounds with interesting properties. Further
synthetic applications, including the derivatization to more
p
-extended o-quinodimethanes with various substitution
patterns, are being investigated in our laboratory.
OTIPS
OTIPS
90 °C
Acknowledgements
toluene-d8
This work was supported by JSPS KAKENHI Grant Number
JP15H05840 in Middle Molecular Strategy and JST ACT-C
Grant Number JPMJCR12YY, Japan.
23l
24l
Keywords: • quinodimethanes • cyclobutaarenes •
cycloaddition • aryne • polycyclic aromatic hydrocarbon
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[2]
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Figure 3. Thermal reaction of 23l monitored by 1H NMR spectroscopy (500
MHz, toluene-d8, 90 °C). a) 23l (RT), b) reaction mixture after 5 min (90 °C),
c) reaction mixture after 20 min (90 °C).
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Interestingly, the discovery of a method to prepare isolable o-
quinodimethane provided a new opportunity for the reaction of
this compound with another reactive intermediate. Such
reactions are otherwise difficult, because o-quinodimethane is
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usually
Tetrabromobenzene
obtained
as
(25)
a
transient
as a
intermediate.
bis-benzyne
served
equivalent,[20] and its treatment with n-BuLi in the presence of
quinodimethane 24l resulted in dual [4+2] cycloaddition to give
tetrabenzopentacene 26, after the acid-promoted aromatization
of the dual cycloadduct (Scheme 6).[21–24]
[5]
[6]
Based on the Clar’s aromatic sextet, we can also say that extra
benzoannulation can make the ring-opened form more favorable than
the parent system (IV®V) in view the difference of number of Clar’s
aromatic sextet before and after the isomerization.
OTIPS
1) n-BuLi
–15 °C
Br
Br
Br
Br
2) TsOH, rt
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Chem. Soc. 2006, 128, 3534–3535; b) T. Hamura, Y. Ibusuki, H.
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Tozawa, T. Kakuda, K. Adachi, T. Hamura, Org. Lett. 2017, 19, 4118–
4121.
12%
25
24l
26
Scheme 6. Dual [4+2] cycloaddition of aryne and o-quinodimethane 24l for
the synthesis of tetrabenzopentacene 26.
In summary, we demonstrated that o-quinodimethane, a
thermodynamically stable
be prepared via the thermal ring-opening of
p-extended diene system, could
[7]
Some parts of this work were presented at the annual meeting of the
Chemical Society of Japan during 2012–2019, see: a) S. Hirose, T.
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10.1002/chem.202004510
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Hamura, Abstracts of Papers, 92nd Annual Meeting of The Chemical
Society of Japan, The Chemical Society of Japan, Tokyo, 2012, 3M1–
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(SHELXS97) and refined by the full-matrix least-squares on F2
(SHELXL97). A total of 110653 reflections were measured, and 17435
were independent. Final R1 = 0.0457, wR2 = 0.1264 (14799 refs; I >
2s(I)), and GOF = 1.065 (for all data, R1 = 0.0519, wR2 = 0.1308).
[17] Although clean formation of 24l was evident from the 1H NMR analysis,
it was highly prone to hydrolysis during chromatography on silica-gel,
affording 24l in 29% isolated yield. Due to this, the unpurified product
was used for further functionalization.
[8]
[9]
Recently, an efficient synthetic route to benzo-annulated
quinodimethanes was developed via intramolecular reductive
cyclization of a diyne with a low-valent zirconocene reagent. In these
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[19] Origin of the ring-opening selectivity depending on the substituent on
the four-membered ring is thoroughly investigated, which will be
described in the forthcoming paper.
[20] Tetrabromobenzene (25) can serve as a 1,4-benzdiyne equivalent.
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[10] For details, see supporting information.
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Takaki, Synlett, 2012, 23, 1725–1732; f) A. Bhunia, S. R. Yetra, A. T.
Biju, Chem. Soc. Rev. 2012, 41, 3140–3152; g) A. V. Dubrovskiy, N. A.
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[12] Quinodimethane 24a thus formed was gradually decomposed upon
further heating at the same temperature.
[21] The dual cycloadduct consisted of regio- and stereoisomers with
respect to the siloxy group, which were directly converted to
tetrabenzopentacene 26.
[13] Quinodimethane 24a was prone to hydrolysis, owing to the lability of
the enol ether moiety.
[22] Although 26 is obtained in moderate yield, the formation of four C–C
bonds in a single process through dual-cycloaddition and the rapid
access to a highly condensed structure show the potential utility of this
reaction.
[14] Retro-cyclization of the isolated o-quinodimethane 24a to cyclobutene
23a was observed by heating 24a in toluene-d8 at 90 °C, although the
decomposed products were also detected.
[23] Due to the low solubility of tetrabenzopentacene 26 in common
organic solvents, the 13C NMR spectra could not be obtained.
[24] For the previously reported synthesis of tetrabenzopentacene 26, see
a) E. Clar, W. Kely, W. G. Niven, J. Chem. Soc. 1956, 1833–1836; b)
K. G. Upul, R. Kumarasinghe, F. R. Fronczek, H. U. Valle, A. Sygula,
Org. Lett. 2016, 18, 3054–3057.
[15] Due to the instability of o-quinodimethanes 24f and 24h and the low
yields of the reactions, the 13C NMR spectra could not be obtained.
[16] C34H24O, MW = 448.53, 0.20 x 0.10 x 0.10 mm, Monoclinic, space
group P21/n, Z = 16, T = 173(2) K, a = 23.0307(4) Å, b = 17.9972(3) Å,
c = 23.4198(4) Å, b = 92.5300(10)°, V = 9697.8(3) Å3, l(CuKα) =
1.54186 Å, µ = 0.557 mm–1. Intensity data were collected on a Rigaku
RAXIS-RAPID IP. The structure was solved by direct methods
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10.1002/chem.202004510
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
R
Kazuhiko Adachi, Shunsuke Hirose,
O
O
R
Yasuyuki Ueda, Hidehiro Uekusa, and
Toshiyuki Hamura*
Page No. – Page No.
Enhanced stabilization by
1) π-extension
2) RO group
Thermodynamically
Quinodimethane:
Stable
o-
Synthesis,
Structure, and Reactivity
Isolable o-quiondimethanes were prepared by thermal ring-opening of
cyclobutaphenanthrenes. In addition to benzo-annulation on parent
benzocyclobutene, sterically congested aryloxy or siloxy group on the four-
membered ring promoted the ring-opening, affording a thermodynamically stable p-
extended o-quinodimethane.
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