Palladium-Catalyzed Synthesis of Heteroarene-Fused Cyclooctatetraenes through Dehydrogenative Cyclodimerization

Article abstract of DOI:10.1002/anie.201707515

Arene-fused cyclooctatetraenes (COTs) possess unique structural and electronic properties that originate from their saddle-shaped π-conjugated architectures. Considerable attention has been paid to the transition-metal-mediated synthesis of these cyclic compounds; however, there have been limited achievements to date in the efficient construction of heteroarene-fused COTs. In this contribution, we report a novel Pd-catalyzed dehydrogenative cyclodimerization of biheteroarenes through four-fold C?H activation toward the synthesis of a series of heteroarene-fused COTs. A set of mechanistic investigations indicated the involvement of high-valent Pd species prior to the dimerization event in the catalytic cycle. The redox behavior of the obtained COTs is also described briefly.

Full text of DOI:10.1002/anie.201707515

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Accepted Article
Title: Palladium-Catalyzed Synthesis of Heteroarene-Fused
Cyclooctatetraenes via Dehydrogenative Cyclodimerization
Authors: Masahiro Miura, Keita Fukuzumi, and Yuji Nishii
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707515
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Palladium-Catalyzed Synthesis of Heteroarene-Fused
Cyclooctatetraenes via Dehydrogenative Cyclodimerization
Keita Fukuzumi,[a] Yuji Nishii,*[b] and Masahiro Miura*[a]
Abstract: Arene-fused cyclooctatetraenes (COTs) possess unique
structural and electronic properties originated from their saddle-
shaped -conjugated architectures. Considerable attention has
been paid for the transition-metal-mediated synthesis of these cyclic
compounds; however, limited achievements have been represented
to date in the efficient construction of heteroarene-fused COTs. In
this contribution, we report a novel Pd-catalyzed dehydrogenative
cyclodimerization of biheteroarenes via four-fold C–H activation
toward the synthesis of a series of heteroarene-fused COTs. A set
of mechanistic examinations indicated the involvement of high-valent
Pd species prior to the dimerization event in the catalytic cycle. The
redox behavior of the obtained COTs is also described briefly.
catalyzed cyclodimerization of biphenyl derivatives based on
Suzuki-Miyaura coupling[8] and Ullman-type coupling[9] (Scheme
1b and 1c) has emerged as an effective tool for preparing
tetraphenylenes. Shi et al.[10] and Zhang et al.[11] independently
developed a novel reaction system involving the catalytic C–H
functionalization[12] of 2-iodo-1,1’-biphenyl derivatives (Scheme
1d). It is noteworthy that biphenylene can be catalytically
converted into tetraphenylene, while the synthetic utility of this
reaction is limited by the narrow structural diversity of
biphenylene derivatives.[13] Furthermore, an elegant study on
the Rh-catalyzed enantioselective [2+2+2] cycloaddition of
triynes was reported by Shibata and coworkers.[14]
Tetraphenylene and related arene-fused cyclooctatetraenes
(COTs) have saddle-shaped three-dimensional architectures
where the four arene rings are alternately oriented upward and
downward.[1]
Although tetraphenylene itself is an achiral
molecule due to its D2d symmetry, an installation of appropriate
substituents or replacement of the benzene rings by
heteroaromatics will break the symmetry, providing versatile
chiral -conjugated scaffolds. In another perspective,
a
a
structural change of the central COT moiety into a planar form
can be triggered in response to redox stimuli because of the
aromaticity in the corresponding oxidized 6 dication or reduced
10 dianion.[2] Such a flexible redox behavior is one of the
significant characteristics of COTs, particularly, those
constructed by five-membered heteroaromatics where the tub-
to-plane interconversion of the saddle-shaped skeleton is
sufficiently facilitated.[3] Additionally, an antiaromatic character
can be invoked if the COT core is planarized by an additional
annulation while maintaining its neutral 8 system.[4] Owing to
the unique structural and electronic properties, there exist
potential applications in various fields including asymmetric
synthesis[5] and supramolecular chemistry.[1,6]
As a conventional synthetic method for these arene-fused COTs,
the transition-metal-mediated homocoupling of a 2,2’-dihalo-1,1’-
biphenyl-derived Grignard reagent, organolithium, or organozinc
has been widely used (Scheme 1a).[7] Such a reaction, however,
often gives a complicated mixture of oligomers, and the desired
product is obtained in low to moderate yields. Recently, the Pd-
Scheme 1. Conventional synthetic methods for tetraphenylene derivatives.
Despite the significant research interest in this field, limited
achievements[15] have been represented to date in the efficient
construction of heteroarene-fused COTs besides those relevant
to Scheme 1a.[16] In this context, we focused our attention on
the dehydrogenative coupling strategy due to the simplicity of
starting materials,[17] and we herein describe a Pd-catalyzed
construction of thiophene-fused COT frameworks through the
dehydrogenative cyclodimerization of 3,3’-bithiophenes. The
protocol is applicable to various heteroaromatics, achieving the
facile synthesis of “composite” heteroarene-fused COTs (vide
infra).[18] A set of mechanistic examinations suggested the
involvement of palladacycle intermediates as well as high-valent
Pd species in the catalytic cycle.
Firstly, we examined the oxidative cyclodimerization of 5,5'-
diphenyl-3,3'-bithiophene (1a) as a representative substrate
using 10 mol% of a Pd(OAc)2 catalyst in combination with a
[a]
[b]
Prof. Dr. M. Miura, Mr. K. Fukuzumi
Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Suita, Osaka 565-0871 (Japan)
E-mail: miura@chem.eng.osaka-u.ac.jp
Dr. Y. Nishii
Frontier Research Base for Global Young Researchers,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871 (Japan)
stoichiometric oxidant and an additive (Table 1).
The
corresponding coupling product 2a was obtained in 63% yield in
the presence of Ag2CO3 and PhCO2H (entry 1). Sodium
benzoate as the additive gave a comparable product yield to the
parent benzoic acid (entry 2), indicating that these additives
acted as carboxylate sources in the present catalytic system.
E-mail: y_nishii@chem.eng.osaka-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201707515
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This is consistent with the fact that AgOPiv prompted the
catalytic turnover without any additive, whereas no coupling
product was formed by using Ag2CO3 alone (entries 3 and 4).
Consequently, we examined several oxidants and the highest
yield was achieved with the combination of AgOPiv and PhCO2H
(entries 5–7). Exclusion of any oxidant resulted in a minimal
yield of the desired product (entry 8).
obtained from the first oxidation waves reasonably corresponded
with the computational values.[20]
Table 2: Pd-catalyzed cyclodimerization of 3,3’-bithiophenes.[a]
Table 1: Optimization study for Pd-catalyzed cyclodimerization of 1a. [a]
entry
R
yield [b]
61%
64%
59%
37%
41%
56%
n.d.
4-nHexC6H4 (1b)
4-tBuC6H4 (1c)
4-CF3C6H4 (1d)
Me (1e)
1
2
3
4
5
6
7
entry
oxidant
additive
PhCO2H
PhCO2Na
--
yield [b]
63%
60%
53%
n.d.
Ag2CO3
1
2
3
4
5
6
7
8
CO2Et (1f)
CONnBu2 (1g)
H (1h)
Ag2CO3
AgOPiv (2.0 equiv.)
Ag2CO3
--
[a] Reaction conditions: 1 (0.1 mmol), Pd(OAc)2 (0.01 mmol), AgOPiv (0.2
mmol), and PhCO2H (0.05 mmol) in toluene was heated at 120 °C for 16 h.
AgOAc (2.0 equiv.)
AgOPiv (2.0 equiv.)
Cu(OAc)2•H2O
--
PhCO2H
PhCO2H
PhCO2H
PhCO2H
49%
78%
15%
trace
[b] Isolated yield.
[a] Reaction conditions: 1a (0.1 mmol), Pd(OAc)2 (0.01 mmol), oxidant, and
additive (0.05 mmol) in toluene was heated at 120 °C for 16 h.
[b] Isolated yield.
Under the optimized conditions, we examined the substrate
scope with respect to the substituent on the C5 and C5’
positions (Table 2). Aryl-substituted bithiophenes 1b-d reacted
smoothly to furnish the desired cyclodimers in good yields
Scheme 2. Cyclodimerization using other heteroaromatics.
(entries 1–3).
5,5’-Dimethyl-3,3’-bithiophene (1e) gave a
somewhat lower yield, and in this case a significant amount of
oligomers was detected in MS analyses (entry 4). Ester and
amide functionalities were tolerated in the catalytic system
(entries 5 and 6). Exclusion of the C5 and C5’ substituents
resulted in the formation of a complicated mixture of oligomers,
and no desired product was obtained (entry 7).
Bibenzo[b]thiophene derivatives were not applicable (not shown).
The cyclodimerization protocol could be extended to other
heteroarenes including furan, thiazole, and oxazole to give the
corresponding products 4, 6,[3b] and 8, respectively (Scheme 2),
while further optimization studies to improve the catalytic
turnover for these heterocycles are needed.
The cyclic voltammogram of 2a in MeCN/o-dichlorobenzene
(1:10) exhibited reversible two oxidation waves at E1/2 = +0.60 V
and +0.90 V (vs. Fc/Fc+),[19] and the other COTs 4, 6, 8 showed
quasi-reversible two-electron oxidation waves. These flexible
redox behaviors can be attributed to their dynamic
conformational change, where the tub-to-plane interconversion
occurs upon the redox cycles. The HOMO energy levels
Notably, an unsymmetrical biheteroaryl 9 was converted into the
corresponding COT 10 as
a single regioisomer and no
thiophene-furan linkage was formed (eq. 1). The structure was
confirmed by X-ray diffraction measurement and a smaller
dihedral angle between two furan rings (42.62°) compared to
that between two thiophenes (50.45°) is consistent with the
connectivity (Figure 1).[21] In a similar manner, a thiophene-
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thiazole cyclodimer 12 was obtained (eq. 2).[21] Construction of
these “composite” arene-fused COTs has been difficult with
conventional methods, demonstrating the utility of the present
dehydrogenative coupling protocol.
obtained in 43% yield.[24] These results indicate that high-valent
palladium species[25] are involved in the reaction mechanism and
carboxylate sources are essential not only for the C–H activation
but also in the dimerization stage. Furthermore, the isolation of
13 implies the intermediacy of a nine-membered palladacycle
complex, which is similar to Pt(PEt3)2(2,2’-tetraphenyl).[13d,22]
Figure 1. Molecular structure of 10 (left) and 12 (right) with 50% thermal
probability. Hydrogen atoms and solvent molecules are omitted for clarity.
Scheme 3. Reaction of complex 14 in the presence of AgOPiv.
To shed light on the reaction mechanism, a number of controlled
experiments were conducted. A linear thiophene tetramer 13,
which is
a
possible intermediate supposed that the
cyclodimerization proceeds through the independent two-fold
dehydrogenative coupling, was treated under the standard
conditions (eq. 3). Interestingly, a negligible amount of the
cyclized product 2a was formed and an inseparable mixture of
oligomers was obtained. This result clearly ruled out the
intermediacy of 13 in the catalytic cycle and prompted us to
perform further mechanistic examinations.
Scheme 4.
A
proposed reaction mechanism for the Pd-catalyzed
cyclodimerization of bithiophenes.
It should be cited that a series of transition-metal-catalyzed
transformation reactions of biphenylene into tetraphenylene
were found to involve the corresponding metallacycle complexes
as the key intermediates. Eisch et al. described the dimerization
of Ni(PEt3)2(2,2’-biphenyl) to form a dinuclear Ni(I)–Ni(I) complex,
and its thermal decomposition resulted in the rapid formation of
tetraphenylene along with metallic nickel.[13b] Thereafter, a
transient species bearing formal Ni(III)–Ni(I) valencies was
reported by Johnson and coworkers.[13f] For the platinum
catalysis, Jones et al. successfully characterized a nine-
membered complex Pt(PEt3)2(2,2’-tetraphenyl) which was
probably formed via reductive elimination from the parent Pt(IV)
complex Pt(PEt3)2(2,2’-biphenyl)2.[13d,22] It was proposed that
similar reaction mechanisms could be drawn for the related Pd-
catalyzed tetraphenylene formation, while a limited experimental
evidence was provided.
Scheme 5. KIE measurement for the Pd-catalyzed cyclodimerization.
On the basis of these observations, a plausible reaction
mechanism for the catalytic cyclodimerization is illustrated in
Scheme 4. Bithiophene 1 is reacted with the catalytically active
Pd(II) species and converted into the corresponding five-
membered complex A via two-fold C–H activation. This complex
is then oxidized by the Ag(I) salt added into a high-valent Pd
complex B. While the exact mechanism of this oxidation step
remains unclear, the indispensability of a carboxylate anion
Accordingly, we prepared a palladium complex 14 by treating
PdCl2(dcpe)
with
2,2'-dilithio-3,3'-bithiophene
and its
(dcpe
potential
=
bis(dicyclohexylphosphino)ethane),[23]
intermediacy for the present catalytic system was examined
(Scheme 3). The complex was remained intact after heating at
80 °C in toluene and no dimerization of the bithiophene fragment
was detected. Addition of benzoic acid or Ag2CO3 did not
resulted in the formation of any dimers. In sharp contrast,
cyclodimer 2a was isolated in 7 % yield along with 13 when the
complex was heated at 80 °C in the presence of AgOPiv. At 120
oC, formation of the liner dimer 13 was suppressed and 2a was
suggests the involvement of
a carboxylate-bridged Pd(III)
dinuclear species[26] prior to the formation of the Pd(IV)
intermediate.[27] To gain insight into the nuclearity of Pd, a
kinetic study was carried out. A reaction order of 1.12 with
respect to Pd(OAc)2 was obtained and, additionally, a primary
kinetic isotope effect (KIE: kH/kD = 2.96) was observed between
two separate reactions using 1a and 1a–d2 (Scheme 5). These
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[7]
For
a
review on metal-mediated synthesis of cyclooctatetraene
results suggest that the initial formation of A is a turnover-
limiting step. Reductive elimination affords a nine-membered
complex C and then liberate the cyclodimer 2. In certain
instances, acyclic products will be given through protonation of
the intermediate C. The catalytic cycle is completed by the
reoxidation of thus formed Pd(0) species into Pd(II). The
observed high regioselectivity for the formation of thiophene-
furan cyclodimer 10 as well as thiophene-thiazole cyclodimer 12
might be attributed to the conformation of Pd(IV) intermediate
that corresponds to B; preliminary DFT calculations for the
complex similar to B exhibited the lowest energy content when
the two furan subunits occupy the trans positions[20] that might
interrupt the thiophen-furan bond formation.
derivatives, see: C. Wang, Z. Xi, Chem. Commun. 2007, 5119. See the
Supporting Information for a set of references relevant to Scheme 1a.
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Y. Zhang, J. Han, Z.-J. Liu, J. Org. Chem. 2016, 81, 1317.
[8]
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2032.
[12] Yamaguchi and co-workers reported a cyclic thiazole tetramer through
Pd-catalyzed direct arylation (ref. 3b).
[13] a) C. Perthuisot, B. L. Edelbach, D. L. Zubris, D. L.; N. Simhai, C. N.
Iverson, C. Mꢀller, T. Satoh, T.; W. D. Jones, J. Mol. Catal. A: Chem.
2002, 189, 157; b) J. J. Eisch, A. M. Piotrowski, K. I. Han, C. Krüger, Y.
H. Tsay, Organometallics 1985, 4, 224; c) H. Schwager, S. Spyroudis,
K. P. C. Vollhardt, J. Organomet. Chem. 1990, 382, 191; d) B. L.
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In
dehydrogenative cyclodimerization of biheteroaryls for the facile
construction of heteroarene-fused COT derivatives. The
obtained cyclic tetramers show flexible redox behaviors that can
be attributed to their conformational changes. Controlled
summary,
we have
developed
a
Pd-catalyzed
[14] T. Shibata, T. Chiba, H. Hirashima, Y. Ueno, K. Endo, Angew. Chem.
2009, 121, 8210; Angew. Chem. Int. Ed. 2009, 48, 8066.
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Angew. Chem. 2013, 125, 12051; Angew. Chem. Int. Ed. 2013, 52,
11835.
experiments using the palladacycle complex 14 clearly support
the intermediacy of high-valent Pd species in the catalytic cycle.
Acknowledgements
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This work was supported by JSPS KAKENHI (Grant Nos. JP
24225002 and JP 17H06092).
Keywords: palladium • dehydrogenative coupling • metallacycle
• cyclooctatetraene • thiophene
[17] For recent reviews on catalytic dehydrogenative bi(hetero)aryl coupling,
see: a) S. H. Cho, J. Y. Kim, J. Kawak, S. Chang, Chem. Soc. Rev.
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See the Supporting Information for a list of selected references.
[18] As part of mechanistic study on the “crossover Heck” process, a
catalytic formation of tribenzocycloocta[1,2-c]pyridines was reported: D.
Masselot, J. P. H. Charmant, T. Gallagher, J. Am. Chem. Soc. 2006,
128, 694.
[1]
[2]
Reviews on tetraphenylenes and related compounds, see: a) I. Hisaki,
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[19] Wang et al. reported a single irreversible oxidation wave for the cyclic
voltammogram of 2a in CH2Cl2 (ref. 16f).
[20] For details, see the Supporting Information.
[21] CCDC 1563086 (10) and 1563088 (12): these data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[3]
[4]
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[23] The structure of 14 was confirmed by X-ray crystallographic analysis:
CCDC 1563089.
For recent examples, see: a) R. Rathore, S. H. Abdelwahed,
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[24] A comparable yield of 2a (46%) was obtained when 10 mol% of
bis(dicyclohexylphosphino)ethane is added to the optimal reaction
condition (Table 1, entry 6).
[25] For a review, see: K. J. Bonney, F. Schoenebeck, Chem. Soc. Rev.
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[6]
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[27] Direct reductive elimination from a dinuclear Pd(III) species is unlikely
to occur because this is symmetry forbidden: G, Trinquier, R. Hoffmann,
Organometallics 1984, 3, 370.
For reviews, see reference 1. See also the Supporting Information for
selected references.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Keita Fukuzumi, Yuji Nishii,* and
Masahiro Miura*
Page No. – Page No.
Palladium-Catalyzed Synthesis of
Heteroarene-Fused
A Pd-catalyzed construction of thiophene-fused COT frameworks through
dehydrogenative cyclodimerization of 3,3’-bithiophenes (X = S, Y = CH) has been
developed. The protocol is applicable to various heteroaromatics, achieving the
facile synthesis of “composite” arene-fused COTs. Mechanistic study indicates the
involvement of palladacycle intermediates as well as high-valent Pd species in the
catalytic cycle.
Cyclooctatetraenes via
Dehydrogenative Cyclodimerization
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