An Approach to Tetraphenylenes via Pd-Catalyzed C-H Functionalization

Article abstract of DOI:10.1021/acs.orglett.6b00641

Tetraphenylenes not only are theoretically and experimentally interesting but also have potential applications in a variety of fields such as materials science, supramolecular chemistry, and asymmetric catalysis. A facile and efficient approach is reported for the syntheis of tetraphenylene and its derivatives from 2-iodobiphenyls via Pd-catalyzed C-H activation. A range of substituted tetraphenylenes can be synthesized using this method, and the reaction can be performed on gram scale with relatively high efficiency, demonstrating its practical utility. This novel approach provides easy access to tetraphenylenes and should facilitate research on the application of this type of fascinating molecules.

Full text of DOI:10.1021/acs.orglett.6b00641

Letter
pubs.acs.org/OrgLett
An Approach to Tetraphenylenes via Pd-Catalyzed C−H
Functionalization
Hang Jiang, Yu Zhang, Dushen Chen, Bo Zhou, and Yanghui Zhang*
Department of Chemistry, and Shanghai Key Lab of Chemical Assessment and Sustainability, Tongji University, 1239 Siping Road,
Shanghai, 200092, P. R. China
S
* Supporting Information
ABSTRACT: Tetraphenylenes not only are theoretically and
experimentally interesting but also have potential applications in a
variety of fields such as materials science, supramolecular chemistry,
and asymmetric catalysis. A facile and efficient approach is reported
for the syntheis of tetraphenylene and its derivatives from 2-
iodobiphenyls via Pd-catalyzed C−H activation. A range of
substituted tetraphenylenes can be synthesized using this method, and the reaction can be performed on gram scale with
relatively high efficiency, demonstrating its practical utility. This novel approach provides easy access to tetraphenylenes and
should facilitate research on the application of this type of fascinating molecules.
etraphenylene is composed of four benzene rings that are
T
ortho-annulated to form an eight-membered ring (Figure
1).1 It has a nonplanar saddle-shaped structure, with the two
Figure 1. Tetraphenylene and its saddle-shape structure.
opposite pairs of benzene rings oriented above or below the
average plane of the molecule.2 As a molecule with D2d
symmetry, tetraphenylene is achiral. However, substituted
tetraphenylenes can be chiral due to the high barrier for
inversion of the cyclooctatetraene ring.3,4 Owing to their unique
geometry, tetraphenylene and its derivatives are of great
theoretical interest. More importantly, there exist numerous
potential applications of these compounds in a variety of fields,2
such as materials science,5 supramolecular chemistry,6 and
asymmetric catalysis.7
Currently, there are three main methods for constructing the
skeleton of tetraphenylene (Figure 2).6b,8 (a) One is
homocoupling of 2,2′-dihalobiphenyl via lithiation to provide
2,2′-dimetalbiphenyl and subsequent transition-metal-mediated
homocoupling.9 This method requires harsh conditions. (b) The
second is homocoupling of biphenylene by transition-metal-
mediated C−C activation10 or pyrolysis.11 This approach has
very limited substrate scopes, because the synthesis of substituted
biphenylenes is often challenging. (c) The third method is
Diels−Alder cycloaddition of 1,2,5,6-dibenzocycloocta-3,7-diyne
and furan, followed by deoxygenation or hydrogenation/
dehydration.12 Notably, the formation of heterocyclic tetraphe-
nylenes incorporating pyridine via the coupling of 3-bromo-4-
phenylpyridines and biphenylene has been reported.13 The
Figure 2. Synthetic methods for tetraphenylene.
reactions are low-yielding and still require the use of biphenylene,
and the synthesis of normal tetrapheneylens via this strategy has
not been achieved. Intriguingly, the de novo synthesis of chiral
tetraphenylenes has been disclosed.14 Furthermore, tetrapheny-
lenes can be obtained via a double Suzuki reaction or double
Ullmann reaction.15
In the past decade, the development of novel synthetic
methods based on transition-metal-mediated C−H functional-
ization has attracted considerable interest.16 Compared to
traditional organic reactions that rely on the transformation
and interconversion of functional groups, C−H functionalization
reactions possess attractive features. The reactions can avoid the
use of prefunctionalized starting materials, reduce the formation
of unnecessary chemical waste, and provide novel synthetic
strategies with higher overall efficiency and fewer reaction steps.
Received: March 7, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00641
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Organic Letters
Letter
In pursuit of these attractive features, herein, we disclose a novel
protocol for the synthesis of tetraphenylenes via Pd-catalyzed
C−H activation/coupling of 2-iodobiphenyls (Figure 2d).
As a model system, the Pd-catalyzed homocoupling of 2-
iodobiphenyl (1) was first examined. Table 1 lists selected
temperature resulted in lower yields (entries 11 and 12).
Notably, the reaction was found to be rapid, reaching completion
in only 10 min (entry 13). A control experiment in the absence of
Pd(OAc)2 confirmed that it was necessary for formation of 1a
(entry 14). Attempts to perform the reaction in alternative acidic
solvents were low-yielding (entries 15 and 16).
Having developed an efficient protocol for Pd-catalyzed
homocoupling of 2-iodobiphenyl (1), we next investigated the
substrate scope of this reaction. A range of symmetrically
substituted 2-iodobiphenyls were examined. As shown in Table
2, substrates bearing two meta- or para-methyl groups (2 and 3)
Table 1. Optimization of the Reaction Conditions for Pd-
Catalyzed Synthesis of Tetraphenylene (1a)
Table 2. Substrate Scope for Pd-Catalyzed Synthesis of
Substituted Tetraphenylenes
Pd(OAc)2
(mol %)
a
a
entry
Ag (equiv)
TFA (mL)
1.0
1a
1b
1
2
3
4
5
−
10
10
10
10
10
0
0
AgOAc (1.0) 1.0
AgOTf (1.0) 1.0
44
38
46
53
21
9
Ag2O (0.5)
1.0
1.0
15
16
Ag2CO3
(0.5)
6
7
Ag2O (1.0)
1.0
1.0
10
10
0
0
22
27
Ag2CO3
(1.0)
8
Ag2CO3
(0.5)
2.5
10
15
5
58
12
b
9
Ag2CO3
(0.5)
2.5
70 (68 ) 11
10
Ag2CO3
(0.5)
2.5
50
48
40
69
0
14
8
c
11
Ag2CO3
(0.5)
2.5
15
15
15
0
d
12
Ag2CO3
(0.5)
2.5
10
8
e
13
Ag2CO3
(0.5)
2.5
14
15
16
Ag2CO3
(0.5)
2.5
0
Ag2CO3
(0.5)
CH3CO2H (1.0)
10
10
trace
22
9
Ag2CO3
(0.5)
CF2HCO2H
(1.0)
17
a
1
The yields were determined by H NMR analysis of crude reaction
b
mixture using CHCl2CHCl2 as the internal standard. Isolated yield.
c
d
e
120 °C. 80 °C. 10 min.
examples of conditions that were surveyed during reaction
optimization. The investigation commenced by treating 1 with
Pd(OAc)2 in trifluoroacetic acid (TFA) at 100 °C (entry 1).
However, the reaction failed to form the desired homocoupled
product, tetraphenylene (1a) (entry 1). Surprisingly, the
addition of 1.0 equiv of AgOAc promoted the reaction, providing
1a in 44% yield, along with a 21% yield of biphenyl (1b) as a
byproduct (entry 2).17 Whereas replacing AgOAc with AgOTf or
Ag2O led to a lower or similar yield (entries 3 and 4), the use of
0.5 equiv Ag2CO3 improved the yield to 53% (entry 5).
Interestingly, when the loading of Ag2CO3 or Ag2O was
increased to 1.0 equiv, the desired product 1a was not observed,
and only the byproduct 2a could be isolated (entries 6 and 7).
The reasons remain to be investigated. The yield of 1a improved
to 58% when the reaction was carried out under higher dilution
(2.5 mL TFA) (entry 8) and further increased to 70% when the
catalyst loading was raised to 15 mol % Pd(OAc)2 (entry 9). In
2.5 mL of TFA solvent, a 50% yield of 1a could still be obtained
when the catalyst loading was reduced to 5 mol % Pd(OAc)2
(entry 10). Conducting the reaction at a lower or higher
a
b
Isolated yields. 0.1 mmol scale in 1.0 mL of TFA.
were competent in the coupling reaction (entries 1 and 2).
Notably, the homocoupling of 2 took place selectively at the less
hindered ortho-C−H bond; as a consequence, 2 and 3 both
formed the same tetramethylated tetraphenylene product (2a).
Both fluoro and chloro groups were well-tolerated in the
reaction, giving desired homocoupled products in 62% and 41%
yields, respectively (entries 3 and 4). 3′,5-Diphenyl-2-iodobi-
phenyl (5) was also reactive (entry 5), affording a new method
for the synthesis of 6a, a tetraphenylene derivative bearing
multiple phenyl substituents. Substrates bearing ester, carbonyl,
nitro, or methoxy groups were examined. No desired products
were formed, or the yields were very low. The bromo group was
not tolerated in the reaction.
Next, we examined the cross-coupling of two different 2-
iodobiphenyls. Equimolar quantities of 2-iodophenyl and a 2-
iodobiphenyl derivative containing chloride or methoxy
B
DOI: 10.1021/acs.orglett.6b00641
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Letter
substituents were subject to the standard conditions (Table 3).
We targeted these two functional groups due to the possibility of
Scheme 2. Pd-Catalyzed Coupling of 2-Iodobiphenyl and
Iodobenzene
Table 3. Pd-Catalyzed Cross-Coupling of 2-Iodobiphenyls
a
1
The yields were determined by H NMR analysis of a mixture of
tetraphenylene and triphenylene.
ruled out as the pathway of formation of tetraphenylene, unless
the reaction of 2-iodobiphenyl with iodobenzene proceeds via a
different mechanism from the coupling of 2-iodobiphenyl.
While the detailed mechanism of this reaction remains the
topic of ongoing investigation in our laboratory, the above-
mentioned experimental results are consistent with the
mechanism shown in Scheme 3 involving the formation of
Scheme 3. Tentative Mechanism for the Formation of
Tetraphenylene via Pd-Catalyzed Coupling of 2-Iodobiphenyl
a
Isolated yields.
performing further downstream diversification. These reactions
proceeded to form the corresponding cross-coupled products in
yields ranging from 21% to 25%. The low yields in these cases are
primarily due to competitive formation of homocoupled
products (see Table S1 in the Supporting Information).
To gain insight into the reaction mechanism, additional
experiments were designed and carried out. Thus, 3′- and 5-
methyl-2-iodobiphenyl substrates were subjected to the standard
conditions (Scheme 1). Intriguingly, both reactions afforded an
a
Ligands are omitted for clarity.
Pd(2,2′-biphenyl) B. Therefore, 2-iodobiphenyl is transformed
into B, likely via the oxidative addition of the iodide to Pd
followed by subsequent C−H activation. The resulting complex
B reacts with a second molecule of 2-iodobiphenyl to generate
tetraphenylene. Alternatively, Pd(2,2′-biphenyl) B may react
with complex A to form 1a. At this stage, the oxidation states of
palladium during the catalytic cycle have not been determined.
Furthermore, byproduct biphenyls should result from the
protonation of complex A or B. When the reaction of 3 was
carried out in TFA-d, 2-deutero-4,4′-dimethylbiphenyl and 2,2′-
dideutero-4,4′-dimethylbiphenyl were formed, which indicated
that both complex A and B are responsible for the formation of
biphenyl.
To test whether the reaction was amenable to scale-up for
generating preparatively useful quantities of material (Scheme
4), we attempted a reaction using 1.0 or 3.0 g of 2-iodobiphenyl
(1). Gratifyingly, 1 was thus converted into tetraphenylene (1a)
in 50% or 48% yield with only 5 mol % Pd(OAc)2.
In summary, a simple and efficient protocol for the synthesis of
tetraphenylenes from 2-iodobiphenyls has been developed via
Pd-catalyzed C−H functionalization. Compared to alternatives,
this approach is advantageous in terms of simplicity, operational
Scheme 1. Pd-Catalyzed Coupling of Unsymmetrically
Substituted 2-Iodobiphenyls
a
Conditions: 15 mol % Pd(OAc)2, 0.5 equiv of Ag2CO3, 2.5 mL of
TFA, 100 °C, 1 h.
inseparable mixture of two isomeric products, 10a and 10a′.
Fortunately, the isomers could be separated following oxidation
to the corresponding tetraphenylene dicarboxylic acids with
KMnO4.9d Notably, the reactions with 10 and 11 not only
formed the same two products but also resulted in nearly
identical yields and isomeric ratios. In addition, in the cross-
coupling reaction of 1 with 7 or 8 (Table 3, entries 1 and 2), the
homocoupled product of 7 or 8 was observed. More importantly,
the homocoupling reactions of 7 and 8 also yielded the same
products based on 1H NMR analysis. These results are consistent
with the two reactions proceeding via a common intermediate,
which should be Pd(2,2′-biphenyl) (Scheme 3, B).
Scheme 4. Gram-Scale Synthesis of Tetraphenylene via Pd-
Catalyzed Coupling of 2-Iodobiphenyl
When equimolar amounts of 2-iodophenyl and iodobenzene
were stirred under the standard conditions, tetraphenylene was
isolated in 37% yield, along with 8% of triphenylene (Scheme 2).
Therefore, the homocoupling of Pd(2,2′-biphenyl) should be
C
DOI: 10.1021/acs.orglett.6b00641
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00641.
Detailed experimental procedures, spectroscopic data, and
characterization of products (PDF)
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Iverson, C. N.; Muller, C.; Satoh, T.; Jones, W. D. J. Mol. Catal. A: Chem.
̈
AUTHOR INFORMATION
Corresponding Author
■
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*E-mail: zhangyanghui@tongji.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The work was supported by the National Natural Science
Foundation of China (No. 21372176), Tongji University 985
Phase III funds, the Program for Professor of Special Appoint-
ment (Eastern Scholar) at Shanghai Institutions of Higher
Learning, and Shanghai Science and Technology Commission
(14DZ2261100). We thank Prof. Keary M. Engle at The Scripps
Research Institute for helpful discussions in the preparation of
this manuscript.
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