Two-in-One Strategy for the Pd(II)-Catalyzed Tandem C-H Arylation/Decarboxylative Annulation Involved with Cyclic Diaryliodonium Salts

Article abstract of DOI:10.1021/acs.orglett.9b02429

We report here a two-in-one strategy for the Pd(II)-catalyzed tandem C-H arylation/decarboxylative annulation between readily available cyclic diaryliodonium salts and benzoic acids. The carboxylic acid functionality can be used as both a directing group for the ortho-C-H arylation and the reactive group for the tandem decarboxylative annulation. By a step-economical double cross-coupling annulation procedure, the privileged triphenylene frameworks were efficiently constructed, which have potential applications in material chemistry.

Full text of DOI:10.1021/acs.orglett.9b02429

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Two-in-One Strategy for the Pd(II)-Catalyzed Tandem C−H
Arylation/Decarboxylative Annulation Involved with Cyclic
Diaryliodonium Salts
Tao Hu,† Kai Xu,† Zenghui Ye,† Kai Zhu,† Yanqi Wu,‡ and Fengzhi Zhang*,†
†College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, P. R. China
‡Institute of Information Resource, Zhejiang University of Technology, Hangzhou 310014, P. R. China
S
* Supporting Information
ABSTRACT: We report here a two-in-one strategy for the
Pd(II)-catalyzed tandem C−H arylation/decarboxylative
annulation between readily available cyclic diaryliodonium
salts and benzoic acids. The carboxylic acid functionality can
be used as both a directing group for the ortho-C−H arylation
and the reactive group for the tandem decarboxylative
annulation. By a step-economical double cross-coupling
annulation procedure, the privileged triphenylene frameworks were efficiently constructed, which have potential applications
in material chemistry.
he regioselective functionalization of aromatic C−H
directing and a reactive group to give 3 in a one-pot cascade
reaction.6
T
bonds using directing groups (DGs) has become a
prevalent strategy for the construction of new bonds in arenes.1
Nitrogen-containing heterocycles, amines, alcohols, carbonyl-
related groups, and so on have been employed as DGs for the
transition-metal (TM)-catalyzed direct C−H functionalization.
These DGs could be divided into the following four categories
(Scheme 1): (a) The DGs are usually hard to remove and thus
The aromatic carboxylic acids have been frequently
employed for TM-catalyzed reactions because of their ready
availability and chemical structure diversity.7 Previous research
has well demonstrated that the acid functionality can be
employed as a reactive group for ipso-decarboxylative cross-
coupling reactions;8 also, it can be used as a removable or
traceless DG for ortho-arene C−H functionalization.9 Although
a few classes of transformations by employing the carboxylic
acids as traceless DGs have been explored to date,10 only a
small range of substrates such as benzoic acid 7a are effective,
plus the carboxylic acid DGs were simply removed in situ or by
additional steps via protodecarboxylation from 8 to give 911
and had not been used as reactive groups for further
decaboxylative cross-coupling reactions (Scheme 2a). Re-
cently, by a decarboxylative annulation strategy, we reported
the synthesis of triphenylenes12 and dibnzo[f,h]quinolines
12,13 which are known as privileged structures for organic
light-emitting diode (OLED) materials.14 However, the
prefunctionalized ortho-halogenated (hetero)aromatic carbox-
ylic acid 10 is required (Scheme 2b).
Scheme 1. Arene C−H Bond Functionalization via
Directing Group Strategies
remain in product 2 after the arene C−H bond functionaliza-
tion of substrate 1.2 (b) The DG could be easily removed or
further modified to give 3 via intermediate 2.3 (c) The
traceless DGs were removed in one pot to give product 4 via
the functionalized product 5.4 (d) The new transient DGs in 6
were formed by adding a catalytic amount of external reagents
and reacting with functional groups (FGs) in substrate 1 (here
FG = DG), which can be reversibly removed in situ after the
C−H bond functionalization to give 2.5 Although the above
strategies have been successfully employed for the arene C−H
bond functionalization at a specific site within a molecule, a
more atom- and step-economical “two-in-one” strategy is of
high demand, in which the DG can be employed as both a
In continuation of our research interest15 in novel arylation
chemistry involved with diaryliodonium salts,16−19 we
envisioned that the valuable triphenylene skeleton 13 might
be constructed in an atom- and step-economical one-pot
reaction via a “two-in-one” strategy with simple substrates 7
and 11 as the starting materials (Scheme 2c), in which the
acids can be employed as both DGs for the palladium-
catalyzed ortho-arene C−H arylation and reactive groups for
Received: July 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02429
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Organic Letters
Letter
when the reaction was conducted in a more diluted solution
(Table 1, entry 13). The reactions were negative in the absence
of either base or catalyst (see the SI). Finally, a 10 mmol scale
reaction was carried out, and triphenylene 13aa was obtained
in 72% yield (1.64 g; see the SI).
Scheme 2. Acid-Directed Arene ortho-Arylation Chemistry
With the optimal conditions in hand, various commercially
available carboxylic acids were first examined (Scheme 3). In
Scheme 3. Substrate Scope of Carboxylic Acids
the tandem ipso-decarboxylative cyclization. The challenges for
this highly efficient cascade protocol are how to avoid the side
reactions, such as the protodecarboxylation,11 and how to
make the consecutive series of reactions proceed smoothly.
We started the condition optimizations by reacting acid 7a
with hypervalent iodine reagent 11a (Table 1). With the
a
Table 1. Optimization of the Reaction Conditions
b
entry
cat.
CuI
RuCl3
base
solvent
yield (%)
general, the carboxylic acids with alkyl (13ab and 13ak),
strong electron-donating (13ac, 13ad, 13ao, and 13ap) or
-withdrawing (13ag−aj) substituents, or halogen (13ae and
13af) all gave moderate to good yields. It is worth mentioning
that various FGs such as phenolic hydroxyl (13ac), halogen
(13ae and 13af), formyl (13ah), cyano (13ai), nitro (13aj),
and so on were untouched under the optimized reaction
conditions. The reaction went smoothly when there was an
electronic-donating substituent on the ortho position of the
acid functionality (13ak and 13am). However, with 2-
nitrobenzoic acid as the substrate, the yield was poor, probably
because the side reaction of protodecarboxylation happened. A
mixture of products (13an and 13an′) was obtained in 79%
yield with 2-naphthoic acid as the substrate. Pleasingly, the
heteroaromatic carboxylic acids were also effective and gave
the corresponding products in good yield (13aq−at).
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
KOtBu
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
DMAc
NMP
DMF
0
trace
15
30
46
13
43
55
36
44
23
67
81
PdCl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
9
10
11
12
c
c
d
13
DMF
a
Optimized reaction conditions: 7a (0.4 mmol), 11a (0.2 mmol),
Pd(OAc)2 (10 mol %), K3PO4 (0.4 mmol), DMF (4 mL), 145 °C, 16
h. Isolated yields. 7a (2 equiv). DMF (4 mL).
b
c
d
To further test the substrate scopes of this cascade one-pot
annulation process, various cyclic diaryliodoniums with
different substituents were prepared and treated under the
optimized conditions (Scheme 4).20 The symmetric hyper-
valent reagents (13ba−be) were examined first, and all gave
the desired products in moderate yield, respectively. Next,
various unsymmetric diaryliodoniums were tested, and the
reactions gave the corresponding products in moderate to
good yield (13ak−bj). Halogen-containing groups such as F,
Cl, and CF3 (13ba−be, 13bf, and 13bg), nitro (13al), and
ester (13bj) on cyclic diaryliodonium salts were untouched
under the optimized conditions.
benzoic acid as the limiting reagent, the TM catalysts were
initially screened (see the SI), and it was found that the
Pd(OAc)2 was the optimum catalyst with potassium carbonate
in dimethylformamide (DMF) (2 mL) at 145 °C (Table 1,
entries 1−4). The bases were then screened, and the reaction
with K3PO4 as the base gave the highest 55% yield (Table 1,
entries 5−8). There was no improvement in yield when other
solvents were screened (Table 1, entries 9−11) or the
additional ligand was added (see the SI). The yields were
dropped by decreasing the reaction temperature or the amount
of catalyst (see the SI). To our delight, the yield was increased
to 67% when an excess of benzoic acid 7a (2 equiv) was used
(Table 1, entry 12), and the yield was further improved to 81%
To demonstrate the utility of these triphenylene products, a
triphenylene-based electron-transport material (ETM) 16,
with high electron affinity and a coplanar structure, was
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Scheme 4. Substrate Scope of Cyclic Diaryliodoniums
Scheme 6. Mechanistic Experiments
prepared efficiently from 13bc by Suzuki−Miyaura coupling
reactions (Scheme 5). Previously, it took five steps to prepare
compound 16, and it has been used in OLEDs and showed
better performance than conventional ETMs.21
Scheme 5. Synthesis of Triphenylene-Based Electron-
Transport Material
To further understand this novel method, the following
experiments were conducted (Scheme 6). First, under the
optimized conditions, there is no reaction with methyl
benzoate 17 as the substrate, which demonstrates the vital
importance of carboxylic acid functionality (Scheme 6, eq 1).
The electronic effect of the substituents on the acids was then
examined (Scheme 6, eq 2). The product 13ad was obtained as
the major product when a mixture of 7d, 7j, and 11a was
treated under the optimized reaction conditions, which
indicated that the electron-rich acid reacted much faster.
With ortho-chloro benzoic acid 7u as the substrate, a mixture
of triphenylene products 13au with chlorine and 13aa without
chlorine was obtained in 54% yield, which formed from ortho
C−H arylation/decarboxylative annulation and dechlorinative
arylation/decarboxylative annulation respectively (Scheme 6,
eq 3).
triphenylene products 19a and 19b were obtained in moderate
yield which were formed via ortho-C−Ha activation. The other
product 20 formed via bay-C−Hb activation was not detected
(Scheme 6, eq 4). There was no triphenylene product 13aa
formed when 7a was treated under the optimized reaction
conditions alone in the absence of cyclic diaryliodonium salt
11a. Furthermore, by reacting 7a with 2-methylfuran, no
cyclized product 21 was obtained (Scheme 6, eq 5), which
showed that a benzyne intermediate did not form in this
cyclization reaction.22 The reaction still went smoothly with
the addition of TEMPO, which indicated that a radical
mechanism might not involve (Scheme 6, eq 6). An
intermolecular competition experiment with equimolar
amounts of 7a and [D5]-7a was then conducted (Scheme 6,
eq 7), and the reaction gave a crude mixture of 13bh/[D4]−
13bh with a ratio of 2:1 by 1H NMR analysis. Meanwhile, two
parallel reactions of 7a and [D5]-7a under the optimum
We further carried out the following experiments to better
understand the reaction mechanism. First, we prepared the
ortho-phenyl benzoic acid 18 and treated it with cyclic
diaryliodonium salt 11a under the optimized conditions. The
C
DOI: 10.1021/acs.orglett.9b02429
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Organic Letters
Letter
conditions were conducted (Scheme 6, eq 8). The reaction
rates of 7a and [D5]-7a provided a kinetic isotope effect (KIE)
of only kH/kD ≈ 1.3. These results indicated that the cleavage
of the C−H bond was not involved in the rate-determining
step. The difference between these two KIE values suggests
that the rate-determining step might happen before the
cleavage of the C−H bond, and the electrophilic metalation
is probably involved in the rate-determining step.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02429.
■
S
Experimental procedures and spectroscopic character-
ization data (PDF)
On the basis of the above experiments, this cascade one-pot
protocol might involve an acid-directed ortho-C−H arylation/
intramolecular decarboxylative annulation sequence (Scheme
7). First, a five-membered Pd(II) complex I may be generated
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhangfengzhi@zjut.edu.cn.
ORCID
Scheme 7. Possible Reaction Pathways
Fengzhi Zhang: 0000-0001-9542-6634
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
NSFC (no. 21871234) and Zhejiang Provincial NSFC for
Distinguished Young Scholars (no. LR15H300001) are
acknowledged.
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