Dibenzopentalenes from B(C6F5)3-induced cyclization reactions of 1,2-bis(phenylethynyl)benzenes

Article abstract of DOI:10.1002/anie.201300871

'Lene' and mean: The strong Lewis acid B(C₆F₅) ₃ efficiently converts some bis(arylethynyl)benzenes into dibenzopentalenes through a series of Lewis acid induced cyclization reactions at room temperature. Thus the reaction has the potential to be useful in the synthesis of substituted dibenzopentalene derivatives which are difficult to make by conventional means. Copyright

Full text of DOI:10.1002/anie.201300871

Angewandte
Chemie
DOI: 10.1002/anie.201300871
Carbocycles
Dibenzopentalenes from B(C6F5)3-Induced Cyclization Reactions of
1,2-Bis(phenylethynyl)benzenes**
Chao Chen, Marcel Harhausen, Renꢀ Liedtke, Kathrin Bussmann, Aiko Fukazawa,
Shigehiro Yamaguchi,* Jeffrey L. Petersen, Constantin G. Daniliuc, Roland Frçhlich,
Gerald Kehr, and Gerhard Erker*
Dibenzopentalene derivatives are compounds with extended
p systems which have been of great interest in organic
materials science with regard to their electronic properties.[1]
A variety of synthetic entries to this class of compounds has
been described, including classical group transformation
chemistry,[2,3] radical coupling,[4,5] and metal-induced reac-
tions.[6–10] We have now found that dibenzopentalenes can be
formed by B(C6F5)3-induced intramolecular alkynyl coupling
reactions.
In a first experiment we treated 1,2-bis(phenylethynyl)-
tetrafluorobenzene (1) with B(C6F5)3 (one molar equiv) in
dichloromethane (ꢀ208C!RT, 2 d). Workup, including
column chromatography, eventually gave compound 2 as
a red solid in 58% yield (Scheme 1).[11] The product 2 was
characterized by X-ray diffraction (single crystals were
obtained from CH2Cl2; Figure 1). The data showed that
Figure 1. Molecular structure of the dibenzopentalene derivative 2.
Selected bond lengths [ꢀ] and angles [8]: C1A–C2A 1.349(3), C2A–C3A
1.470(3), C3A–C8A 1.413(3), C8A–C9A 1.503(2), C9A–C10A 1.365(2),
C10A–C11A 1.465(2), C11A–C16A 1.428(2), C1A–C16A 1.468(2), C2A–
C10A 1.457(2); C1A-C2A-C10A 111.2(2), C2A-C10A-C9A 110.6(2). Only
one of the two independent molecules in the asymmetric unit is
shown. Thermal ellipsoids are shown at 50% probability.
Scheme 1. Formation of the unsymmetric dibenzopentalene derivative
2.
1 had been isomerized to its dibenzopentalene isomer 2,
which features the typical alternating double and single bonds
[*] Dr. C. Chen, Dr. M. Harhausen, R. Liedtke, K. Bussmann,
at its central bicyclic core. In solution we observed the
1H NMR resonance of the single CH proton at the central
Dr. C. G. Daniliuc, Dr. R. Frçhlich, Dr. G. Kehr, Prof. G. Erker
Organisch-Chemisches Institut der Universitꢁt Mꢂnster
pentalene core at d = 6.60 ppm (JFH = 2.3 Hz). The 13C NMR
Corrensstrasse 40, 48149 Mꢂnster (Germany)
signals of the four core sp2-carbon atoms are found at d =
E-mail: erker@uni-muenster.de
118.0 (CH), 150.2, 139.0, and 146.7 ppm, respectively, and we
Dr. M. Harhausen, Prof. A. Fukazawa, Prof. S. Yamaguchi
Department of Chemistry Graduate School of Science
Nagoya University
Furo, Chikusa, Nagoya, 464-8602 (Japan)
E-mail: yamaguchi@chem.nagoya-u.ac.jp
observed four distinct 19F NMR signals for 2 (d = ꢀ131.0,
ꢀ159.5, ꢀ158.6, ꢀ146.0 ppm).
The reactions of the bis(phenylethynyl)benzene deriva-
tives 3a,b with B(C6F5)3 under similar reaction conditions had
a slightly more complex outcome. We isolated a mixture of
the three products, 4, 5, and 6; 4a, 5a, and 6a had a combined
yield of 68%, 4b, 5b, and 6b had a combined yield of 92%
(Scheme 2). The products were separated by chromatography
and each product was characterized by X-ray diffraction and
by spectroscopy (for details see the Supporting Information).
The compounds 4a[10] and 4b were identified as the dibenzo-
pentalene derivatives (for example, 4b: pentalene CH: d =
6.44 ppm (1H NMR), d = 126.1 ppm (13C NMR); see the
Supporting Information). The compounds 5a and 5b contain
the same dibenzopentalene framework as 4, but in addition to
Prof. S. Yamaguchi
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya
University, Furo, Chikusa, Nagoya, 464-8602 (Japan)
Prof. J. L. Petersen
Bennett Department of Chemistry, West Virginia University
P.O. Box 6045, Morgantown, WV 26506 (USA)
[**] Financial support from the Deutsche Forschungsgemeinschaft and
SusChemSys (which is co-financed by the Regional Development
Fund (Investing in Your Future) of the European Union and the state
of North Rhine-Westphalia) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300871.
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Scheme 2. Formation of 4, 5, and 6.
the single phenyl group at the core they have an additional
C6F5 substituent, attached at the central pentalene unit, which
obviously has been transferred from the B(C6F5)3 reagent in
the course of the synthetic sequence. The X-ray crystal
structure analysis of 5b shows the planar dibenzopentalene
framework to which the phenyl and the newly introduced
C6F5 groups are bonded (Figure 2).
Scheme 3. Proposed mechanism for the formation of 4 and 5.
during the workup procedure (for details see the Supporting
Information).
Scheme 3 provides a possible mechanistic rationalization
of the formation of the key compounds 4 (and consequently
also 2) and 5. We assume that initial B(C6F5)3 addition[12] to
a carbon–carbon triple bond of the starting material 3
generates the reactive intermediate 7, which then undergoes
an intramolecular electrophilic aromatic substitution reaction
to give 8. Deprotonation of this intermediate by means of
proton transfer to the starting material 3 would then give the
borylated product 10 and at the same time would open
a pathway to the formation of 4. The latter product in this
scheme is consequently suggested to be formed in a B(C6F5)3-
induced catalytic isomerization reaction of 3. Competing 1,2-
C6F5 migration reaction from a boron to carbon atom[13] opens
the stoichiometric reaction pathway to the observed co-
product 5. It features the reactive intermediate 9 which then
may lose HB(C6F5)2 in the rearomatization reaction to
eventually give 5.
The following experimental evidence may serve to sup-
port this suggested pathway. We found one case were the
isomerization was Brønsted acid catalyzed, namely the 1 to 2
isomerization was achieved by HN(SO2CF3)2 catalysis (36%
of 2 isolated).[14] The compound 1 also reacted with the Lewis
acid BCl3 to give a BCl2-substituted dibenzopentalene
derivative which was deborylated with acetic acid to even-
tually also yield 2 (see the Supporting Information). We were
able to prepare and isolate the salt 10a (and 10b, both with an
ammonium counterion) by reacting 3a with the B(C6F5)3/
tetramethylpiperidine (TMP) frustrated Lewis pair.[15] In this
case the alleged intermediate 8a was deprotonated by the
bulky amine base to give the TMPH+ salt of 10a. It was
isolated in 93% yield and characterized by an X-ray crystal
structure analysis (see Figure 3 and the Supporting Informa-
tion).
Figure 2. Molecular structure of the dibenzopentalene derivative 5b.
Selected bond lengths [ꢀ] and angles [8]: C1–C2 1.350(3), C2–C3
1.459(4), C3–C8 1.427(3), C8–C9 1.488(4), C9–C10 1.364(3), C10–C11
1.462(3), C11–C16 1.411(3), C1–C16 1.494(4), C2–C10 1.464(3); C1-
C2-C10 110.2(2), C2-C10-C9 109.8(2). Thermal ellipsoids are shown at
50% probability.
In solution, 5b exhibits the typical 19F NMR signals of the
C6F5 substituent (d = ꢀ137.0 (o), ꢀ154.4 (p), ꢀ162.0 ppm
(m)) and the 1H NMR resonances of the vicinal methyl
substituents (d1H = 2.12 and 2.10 ppm). The third set of
products in this series (6a, b) contains a geminal pair of C6F5
substituents and they do not contain the finished pentalene
nucleus. We could show that 6 is not a primary product of this
reaction, but likely originates from a hydrolysis reaction
We have obtained experimental evidence for the pro-
posed HB(C6F5)2 elimination step from the reaction of
bis(thienylethynyl)dimethylbenzene (11) with B(C6F5)3
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Figure 4. A view of the molecular structure of compound 12. Selected
Figure 3. Molecular structure of the dibenzopentalene derivative 10a.
Selected bond lengths [ꢀ] and angles [8]: C1–C2 1.367(6), C2–C3
1.467(5), C3–C8 1.426(5), C8–C9 1.486(5), C9–C10 1.350(6), C10–C11
1.461(5), C11–C16 1.419(6), C1–C16 1.506(5), C2–C10 1.491(5); C1-
C2-C10 111.0(3), C2-C10-C9 110.7(3). For clarity only the anion is
shown. Thermal ellipsoids are shown at 30% probability.
bond length [ꢀ] and angles [8]: B1–C3 1.590(3), C3–S4 1.876(2), B1–S4
1.988(2), C3–C7 1.509(3), C7–C6 1.329(3), C6–C5 1.501(3), C5–S4
1.840(2); C1-C2-C9 110.3(2), C2-C9-C8 111.2(2). Thermal ellipsoids are
shown at 30% probability.
in 12 is 354.78. Together with the MP2/6-31G* calculated
Wiberg bond index[19,20] of 0.72 (see the Supporting Informa-
tion), these data indicate sulfur coordination to boron.[21] The
unusual bonding situation in 12 might be realized by the
structural constraint resulting from the ring-fused structure.
The UV/Vis absorption spectra of the dibenzopentalene
derivatives 2, 4a, and 4b exhibit three characteristic absorp-
tion bands as already reported for dibenzopentalene deriva-
tives:[6] the longest and very weak absorption bands around
l = 500 nm (attributable to symmetry-forbidden HOMO–
LUMO transitions typical for 4n p-electron systems[6]), the
second longest absorption band with the maximum wave-
length of l = 387–452 nm, and the third longest absorption
bands at l = 250–300 nm. The introduction of a C6F5 group at
the 5-position of dibenzopentalene (i.e., going from 4a to 5a,
and from 4b to 5b) results in the significant bathochromic
shifts of the absorption bands. According to the results of
DFT calculations, C6F5 groups cause a pronounced decrease
in the LUMO energy level mainly because of their electron-
withdrawing effect (see Figure 5 and the Supporting Infor-
mation).
Our study shows that readily available 1,2-bis(arylethy-
nyl)benzenes can efficiently be doubly ring closed to yield
dibenzopentalene products upon treatment with the strong
Lewis acid B(C6F5)3 under mild reaction conditions. We have
found that the overall sequential reaction scheme follows two
competing pathways, thus leading to either the dibenzopen-
talene isomers of the starting materials (e.g. 2 or 4) or to the
C6F5-substituted dibenzopentalenes, which involves substitu-
ent transfer from the Lewis acid to the newly formed
annulated tetracyclic framework. We need to work out
measures for generally controlling this competing situation,
although we have found examples of highly selective product
formation (2, 12), wherein one of the pathways seems to be
Scheme 4. Formation of 12.
(1 molar equiv, pentane, RT, 12 h) from which we isolated the
product 12 in 90% yield (Scheme 4).[16] It constitutes the
HB(C6F5)2 addition product to the alleged pentalene deriv-
ative 14 which itself was probably formed by the elimination
of HB(C6F5)2[17] from the respective reactive intermediate 13,
the hetarene analogue of the proposed intermediate 9. The
compound 12 shows the 1H NMR signals of the geminal pair
of dihydrothiophene hydrogen atoms at d = 4.08 and
3.58 ppm (2J = 17.0 Hz).
Compound 12 was characterized by X-ray diffraction
(Figure 4). It shows that HB(C6F5)2 has 1,4-hydroborated the
annulated thiophene ring of 14. The angular B(C6F5)2
substituent features a strong interaction with the sulfur
ꢀ
atom of the dihydrothiophene subunit of 12. The B1 S4
ꢀ
distance (1.988(2) ꢀ) is even shorter than the B S bond
length in B(C6F5)3·SMe2.[18] The sum of the C-B-C bond angles
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Figure 5. Orbital energy diagram and pictorial representation of HOMO-1, HOMO, and LUMO for the series of dibenzopentalenes 2, 4a, 4b, 5a,
and 5b; calculated at the B3LYP/6-31G* level (isovalue: 0.03).
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4
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[11] Preparation of 2: B(C6F5)3 (256 mg, 0.5 mmol) was dissolved in
dichloromethane (ꢀ208C, 10 mL) and added to a solution of the
diyne 1 (175 mg, 0.5 mmol) in dichloromethane (ꢀ208C, 10 mL).
The resulting dark-green reaction mixture was allowed to warm
to room temperature and stirred for 2 days before ether (p.a.,
5 mL) was added. Then all the volatiles were removed under
vacuum and the residue was purified by column chromatography
(SiO2, cyclohexane:dichloromethane = 20:1). The product was
obtained as a red solid (102 mg, 0.29 mmol 58%).
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dry n-pentane (5 mL). Then a solution of B(C6F5)3 (80 mg,
157 mmol, 1 equiv) in dry n-pentane (5 mL) was added at room
temperature. The reaction mixture turned red-brown immedi-
ately. The mixture was stirred overnight. Subsequently the
supernatant of the suspension was decanted by pipette and the
obtained residue was washed with n-pentane (3 ꢃ 2 mL). After
drying in vacuo a brown solid was obtained (90%, 117 mg,
141 mmol).
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Communications
Carbocycles
C. Chen, M. Harhausen, R. Liedtke,
K. Bussmann, A. Fukazawa,
S. Yamaguchi,* J. L. Petersen,
C. G. Daniliuc, R. Frçhlich, G. Kehr,
’Lene’ and mean: The strong Lewis acid
B(C6F5)3 efficiently converts some bis-
(arylethynyl)benzenes into dibenzo-
pentalenes through a series of Lewis acid
induced cyclization reactions at room
temperature. Thus the reaction has the
potential to be useful in the synthesis of
substituted dibenzopentalene derivatives
which are difficult to make by conven-
tional means.
G. Erker*
&&&& — &&&&
Dibenzopentalenes from B(C6F5)3-
Induced Cyclization Reactions of 1,2-
Bis(phenylethynyl)benzenes
6
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