Bottom-up Construction of π-Extended Arenes by a Palladium-Catalyzed Annulative Dimerization of o-Iodobiaryl Compounds

Article abstract of DOI:10.1002/anie.201803603

A straightforward method was developed for construction of aromatic compounds with a triphenylene core. The method involves Pd-catalyzed annulative dimerization of o-iodobiaryl compounds by double C?I and C?H bond cleavage steps. Simple reaction conditions are needed, requiring neither a ligand nor an oxidant, and the reaction tolerates a wide range of coupling partners without compromising efficiency or scalability. Significantly, the tetrachloro-substituted synthon, 1,6,11-trichloro-4-(4-chlorophenyl)triphenylene, can be generated and used to prepare a series of fully fused, small graphene nanoribbons by a late-stage arylation with arylboronic acids and a subsequent Scholl reaction. The synthetic strategy enables bottom-up access to extended π-systems in a controlled manner.

Full text of DOI:10.1002/anie.201803603

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Accepted Article
Title: Bottom-up Construction of Graphene Segments Based on a
Palladium-Catalyzed Annulative Dimerization of o-Iodobiaryl
Compounds
Authors: Zhuangzhi Shi, Chendan Zhu, Di Wang, Dingyi Wang, Yue
Zhao, and Wei-Yin Sun
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803603
Angew. Chem. 10.1002/ange.201803603
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10.1002/anie.201803603
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
π-Extension
Bottom-up Construction of Graphene Segments Based on a Palladium-
Catalyzed Annulative Dimerization of o-Iodobiaryl Compounds
Chendan Zhu, Di Wang, Dingyi Wang, Yue Zhao, Wei-Yin Sun and Zhuangzhi Shi*
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday
Abstract: A straightforward method based on a Pd-catalyzed
annulative dimerization of o-iodobiaryl compounds via a double C-I
and C-H bond cleavage for the construction of aromatic compounds
with a triphenylene core has been developed. The protocol features
simple reaction conditions that require neither a ligand nor an
oxidant, and it tolerates a wide scope of coupling partners without
compromising its efficiency or scalability. More importantly, the
generated TCCT synthon can be used to prepare a series of fully
fused, small graphene nanoribbons by a late-stage arylation with
arylboronic acids and subsequent Scholl reaction. Thus, our
synthetic strategy allows bottom-up access to these extended π-
systems in a controlled manner.
Fused polycyclic aromatic hydrocarbons (PAHs), atomically precise
graphene segments, have attracted tremendous attention because of
their high electron density, narrow HOMO–LUMO gaps, low redox
potentials, strong π–π stacking abilities, aromaticity, and excellent
chemical stability.[1] The degree of π-extension, the topology of the
ring fusion, and the periphery are crucial not only for the electronic
properties at a molecular level but also for the packing of the PAHs
in two and three dimensions.[2] Therefore, bottom-up synthetic
Figure 1. Development of a protocol for the bottom-up construction of
routes for preparing these graphene segments in a controlled fashion
extended π-systems.
have received much attention in synthetic chemistry and materials
(2.5 mol% of Pd(OAc)2, 1.2 equiv of NaHCO3, DMF, 130 °C, open
to air), the reaction of 2-iodobiphenyl (1a) occurred preferentially
with regioselective construction of the eight-membered ring (2a'),
and another product with a triphenylene core, 2a, which was
generated via the formation of a benzene ring, was only detected in
trace amounts. As a natural extension of this chemistry, we became
interested in the synthesis of this byproduct. Intriguingly, with a
subtle variation of the former reaction conditions (2.5 mol% of
Pd(OAc)2, 1.1 equiv of KHCO3, DMA, 150 °C, under an argon
atmosphere), we observed an unprecedented switch in
regioselectivity (Figure 1a).
science.[3] The traditional approaches to preparing PAHs include
Diels–Alder,
cyclotrimerization,
cross-coupling,
and
cyclodehydrogenation reactions. Recently, C-H activation
techniques have also received significant attention as a reliable
strategy for rapidly accessing PAHs from small arenes in an
efficient manner.[4] Here, we reported that a series of aromatic
compounds with triphenylene cores,[5] which are precursors to many
valuable π-extended and fused aromatic materials, could be accessed
directly through a Pd-catalyzed annulative dimerization of o-
iodobiaryl compounds.
The dimerization of aryl halides has been an active field of
research for over 100 years,[6] and the classic Ullmann reaction,
discovered in 1901, remains one of the most powerful and versatile
routes to symmetric biaryl compounds.[7] In the past 20 years, there
have been numerous advances in dimerization reactions ranging
from the discovery of milder, catalytic Ullmann processes[8] to the
development of methods based on selective cross-coupling
reactions.[9] However, the diversity of the dimerization products has
increased only slowly. In 2016, we[10a] and Zhang[10b] have
developed the Pd-catalyzed annulative dimerization of o-iodobiaryl
compounds respectively to synthesize tetraphenylenes[11] through
double head-to-tail direct arylation.[12] In our optimized conditions
During the preparation of this work, Itami, Murakami and
colleagues reported that the use of PdCl2 as a catalyst in the presence
of a phosphine ligand and Cs2CO3 led to the annulative dimerization
of chlorophenylenes and allowed the construction of various fused
aromatic compounds with triphenylene cores (Figure 1b).[13] In their
strategy, various chlorophenylene substrates were synthesized from
1,4-dibromo-2-chlorobenzene and an arylboronic acid by Suzuki-
Miyaura cross-couplings. Then, these products were elegantly
converted to the partially fused polyaromatics in a single-step
annulative π-extension (APEX)[4a] via palladium catalysis, and these
products can be further transformed into fully fused, small graphene
nanoribbons by a Scholl reaction.[14] Unlike their system, our
dimerization is triggered by selective C-I bond cleavage; thus, less-
reactive Cl atoms can be retained in products during the reaction,
highlighting the potential of our products for used in further
conventional cross-coupling transformations. Therefore, we can
construct various fused polycyclic aromatics in a controllable
bottom-up manner by late-stage arylation of the generated 1,6,11-
[]
Chendan Zhu, Di Wang, Dingyi Wang, Yue Zhao, Prof. Dr.
Wei-Yin Sun and Prof. Dr. Zhuangzhi Shi
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, China
E-mail: shiz@nju.edu.cn
Homepage: http://hysz.nju.edu.cn/zzshi/
Supporting information and the ORCID identification
number(s) for the author(s) for this article can be found under
http://www.angewandte.org.
trichloro-4-(4-chlorophenyl)triphenylene
(TCCT)
synthon.
Significantly, the products generated by these two strategies are
complementary, and the numbers of possible connected benzenes (N)
in their edge structure are (2n+1) and (2n+2), respectively,
depending on the number of benzene units in the arylboronic acids
(Figure 1c).
1
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10.1002/anie.201803603
Angewandte Chemie International Edition
While the exact mechanism of this switchable transformation
remains unclear, a tentative assumption can be made based on some
experimental investigations. As shown in Figure 2, oxidative
addition of 1a to a Pd(0) species appears to trigger the reaction to
afford intermediate A; a neighboring C-H bond in that species is
activated to produce five membered palladacycle B.[15] Then,
oxidative addition of another molecule of 1a to B generates
intermediate C,[16] which can undergo reductive elimination and a
second C-H activation to deliver palladacycle E,[17] and E ultimately
generates product 2a by reductive elimination with concomitant
regeneration of the Pd(0) species. To better understand the
mechanism, we carried out the reaction of 1A under two sets of
reaction conditions. We found that only product 2a could be
detected in both systems, which not only supported the above
assumptions but also excluded the possibility of intermediate D in
the formation of tetraphenylene. Based on these results, we
speculate that these two slightly different systems may control the
formation of species D and D', which should be the key
intermediates in the formation of their respective products.
To examine the feasibility of this strategy, various reaction
parameters were surveyed, as shown in Table 1. We found that the
dimerization product 2a only resulted from C-I bond cleavage, and
substrates bearing Br (1a') and OTf (1a'') groups didn’t show
reactivity in our conditions. 2-Iodo-4,4'-dimethyl-1,1'-biphenyl (1b)
afforded desired product 2b in excellent yield. Notably, 2-iodo-3',5-
dimethyl-1,1'-biphenyl (1c) and 2-iodo-3',5-diisopropyl-1,1'-
biphenyl (1d), which each have two possible positions for C-H
activation, only generate one isomer, indicating that the C-H
metallation occurs at the less sterically hindered position. This
annulative dimerization also allows access to structurally diverse
substrates containing functional groups such as OMe (2e), OCF3 (2f),
F (2g), CF3 (2h) and Cl (TCCT) at the termini of the triphenylene-
cored aromatic species. In addition, multi-substituted o-iodobiaryl 1i
reacted smoothly under the optimal conditions, affording
octamethyltriphenylene 2i in 65% yield. Dibenzo[fg,op]tetracenes
3a, 3b, 3d and 3e were prepared in 56-75% yields by the Scholl
reaction of the corresponding substrates using FeCl3 as the oxidant.
Moreover, the structures of triphenylenes 2g, TCCT and
dibenzo[fg,op]tetracene 3b were unambiguously determined by X-
ray crystallographic analysis.
Using TCCT as a powerful synthon, our synthetic approach to a
wide range of graphene segments consists of two main steps: late-
stage arylation of TCCT by a Suzuki-Miyaura cross-coupling and
oxidative cyclodehydrogenation to an all-benzenoid hydrocarbon
(Figure 3). With of Pd(OAc)2, Davephos as the ligand and Cs2CO3
as the base, we first performed the reaction of TCCT with PhB(OH)2
(4a, n = 1) in dioxane at 80 °C in sealed pressure tubes to produceπ-
extended triphenylene 5a in 72% yield. Precursor 5a was then
Figure 2. Proposed catalytic cycle.
Table 1. Substrate Scope of the Palladium-Catalyzed Annulative Dimerization of o-Iodobiaryls.[a]
[a] Reaction conditions: 2.5 mol% of Pd(OAc)2, substrate 1 (0.4 mmol), KHCO3 (0.44 mmol) in 1 mL of DMA at 150 °C for 36 h
under argon; isolated yields are reported. [b] Using DMF as the solvent.
2
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10.1002/anie.201803603
Angewandte Chemie International Edition
Figure 3. Bottom-up Construction of Atomically Precise Graphene Segments Based on a TCCT Synthon. Reagents and conditions: (a) 10.0
mol% of Pd(OAc)2, 20.0 mol% of Davephos, 12.0 equiv of Cs2CO3, 6.0 equiv of arylboronic acids 4a-b, dioxane, 80 °C, 24 h; (b) 5.0
mol% of Pd(OAc)2, 10.0 mol% of Sphos, 8.0 equiv of K3PO4, 6.0 equiv of arylboronic acids 4c-d, toluene, 80 °C, 24 h; (c) FeCl3,
DCM, 0 °C.
subjected to a Scholl reaction with FeCl3 in DCM to afford small
graphene nanoribbon segment 6a (N = 4) in 64% yield by simple
filtration and washing. To extend the π-system of the formed
graphene nanoribbon segment, compound 5b was prepared by late-
stage arylation of TCCT with an inexpensive 4-biphenylboronic acid
(4d, n = 2) in 82% yield, and 5b can then undergo a Scholl reaction
with FeCl3 to afford 6b (N = 6) in good yield. Our strategy can also
be used to obtain PAHs with various peripheries, which is critical
for controlling their electronic properties and two- and three-
dimensional superstructures. For example, the reaction between
TCCT and 1-naphthaleneboronic acid (4c) in the presence of
Pd(OAc)2, Sphos and K3PO4 could introduce four naphthalen-1-yl
groups onto the triphenylene backbone (5c), and a Scholl reaction
produced compound 6c in 84% yield. Using a similar procedure, the
Scholl reaction of product 5d from the cross-coupling of TCCT with
2-biphenylboronic acid (4d) could afford “cove”-type graphene
nanoribbon 6d in 87% yield.
In summary, we have reported an efficient method for building
aromatic compounds with triphenylene cores via a Pd-catalyzed
annulative dimerization of o-iodobiaryl species. This reaction
sequence involves double direct arylation by cleavage of two C-H
bonds and two C-I bonds as well as the formation of a benzene ring.
Using TCCT as a building block, the synthetic route to the fully
fused, small graphene nanoribbons follows a two-step protocol
sterically possible. Given the functional π-systems of interest in
chemistry and materials science, we anticipate that our strategy will
simplify the synthesis and structural elaboration of graphene
nanoribbons.
Acknowledgments
We thank the “1000-Youth Talents Plan”, the “Innovation &
Entrepreneurship Talents Plan” of Jiangsu Province, and the NSF of
China (Grant 2167020084) for their financial support.
Keywords: coupling ·arylation ·dimerization ·graphene · PAHs
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Angewandte Chemie International Edition
Entry for the Table of Contents (Please choose one layout)
Layout 2:
π-Extension
Chendan Zhu, Di Wang, Dingyi Wang,
Yue Zhao, Wei-Yin Sun and Zhuangzhi
Shi* ___ Page – Page
Bottom-up Construction of Graphene
Segments Based on
a
Palladium-
Catalyzed Annulative Dimerization of o-
Iodobiaryl Compounds
Bottom-up: A palladium-catalyzed system has been established for the annulative
dimerization of o-iodobiaryl species via C-H activation to build various aromatics
with triphenylene cores. Based on the generated 1,6,11-trichloro-4-(4-
chlorophenyl)triphenylene synthon, a series of fully fused, small graphene
nanoribbons were be prepared by late-stage arylation and subsequent Scholl
reaction.
5
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