A Foldamer-Based Organocatalyst for Direct Arylations of Unactivated Arenes

Article abstract of DOI:10.1021/acs.orglett.7b00921

It was demonstrated that a simple yet well-folded pyridone dimer, possessing two convergently aligned electron-rich O atoms for potassium binding, can serve as a highly efficient organocatalyst for catalyzing transition-metal-free arylations of unactivated aromatic C-H bonds with aryl halides in the presence of t-BuOK. A wide range of aryl iodides could be cross-coupled with unactivated arenes in moderate to excellent yields. The experiments using radical-scavenging reagents confirm the participation of radicals in this catalytic transformation.

Full text of DOI:10.1021/acs.orglett.7b00921

Letter
pubs.acs.org/OrgLett
A Foldamer-Based Organocatalyst for Direct Arylations of
Unactivated Arenes
Huaiqing Zhao,† Jie Shen,‡ Changliang Ren,‡ Wei Zeng,§ and Huaqiang Zeng*,‡
†School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong, 255022, China
‡Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, 138669, Singapore
§Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou, 510641, China
S
* Supporting Information
ABSTRACT: It was demonstrated that a simple yet well-folded pyridone
dimer, possessing two convergently aligned electron-rich O atoms for
potassium binding, can serve as a highly efficient organocatalyst for
catalyzing transition-metal-free arylations of unactivated aromatic C−H
bonds with aryl halides in the presence of t-BuOK. A wide range of aryl
iodides could be cross-coupled with unactivated arenes in moderate to
excellent yields. The experiments using radical-scavenging reagents confirm
the participation of radicals in this catalytic transformation.
rganocatalysts as an efficient alternative to transition-
metal-based catalysts have attracted increasing attention
similarities in structure and in potassium binding between cyclic
P5b and acyclic F1−F5, we believed F1−F5 might be capable
of performing similar catalytic functions. We therefore decided
to investigate the possible use of these acyclic foldamers to
catalyze direct arylations of unactivated arenes with aryl halides
in the absence of transition metals.
O
and interest from synthetic chemists. Using these greener
catalysts,1 expensive transition metals and other organometallic
reagents can be avoided while providing a strategic solution to
construct pharmaceutically relevant molecular scaffolds, includ-
ing biaryls that could serve as important moieties in the
synthesis of pharmaceuticals, natural products, agrochemicals,
and functional materials.2 It is now believed that the potassium
tert-butoxide mediated catalytic arylation reactions, leading to
biaryl molecules, involve aryl radicals produced via a single-
electron transfer (SET) process, which is initiated by the
interaction of organocatalysts with t-BuOK.1
H-bonded aromatic foldamers constitute a rich family of
structurally diverse folding molecules with a rigid conformation
and versatile functions.3 In particular, over recent years, our
group has demonstrated a wide range of interesting functions,
including molecular recognition of small molecules,4a−c ions4d,i
and water,4m solvent gelation,4n reactive sieving,4o,p and water
transport across cell membranes.4l However, aromatic
foldamers have rarely been used in organic reactions as the
catalysts. We recently reported such a first example, i.e., a
macrocyclic H-bonded aromatic pentamer (P5b) made up of
five pyridone motifs that could efficiently catalyze direct
arylations of unactivated arenes (Figure 1a).1k This pentamer
contains an appropriately sized interior cavity of 1.4 Å in radius
that is decorated by five carbonyl O atoms, and can bind many
metal ions including the K+ ion.4f
The simplest arylation reaction between 4-iodoanisole (1a)
and benzene (2a) was used to gauge the catalytic potential of
foldamer-based catalysts (F1−F5) using the standard con-
ditions involving 3 equiv of t-BuOK at 100 °C for 24 h. With
respect to the control experiment that generated no desired
product 3a in the absence of catalysts (entry 1, Table 1), a
loading of 20 mol % of F1−F5 invariably produced 3a with F2
giving the highest yield of 76% (entries 2−6, Table 1).
Although we are not sure why F2 acts as the best catalyst
among F1−F5, the ability of these folding molecules to catalyze
the direct arylation should arise from their ability to bind the K+
ion, enabling s single-electron transfer process to take place. We
then further investigated the reactions using different bases
(e.g., t-BuOLi, t-BuONa, and KOH) and found that no product
could be obtained under the same conditions using F2 as the
catalyst (entries 7−9, Table 1). Given the fact that F2 binds K+
more strongly than Na+ or Li+,5 these results indicate that a
stronger base (t-BuO− vs OH−) and availability of such a base
(e.g., t-BuO−) in solution via potassium chelation by the ligand
are critically important to drive the reaction to completion.
Similar to P5b,1k a higher catalyst loading of F2 does not lead
to a higher catalytic efficiency, which is contradictory to many
other types of organocatalysts. Instead, the highest catalytic
Very recently, we found that acyclic pyridone-based aromatic
foldamers (F1−F5, Figure 1a) fold into crescent-shaped
structures to enclose a slightly enlarged interior cavity of
∼1.6 Å in radius. Interestingly, these acyclic foldamer molecules
also exhibit good binding toward the K+ ion.5 Given the
Received: March 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00921
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Organic Letters
Letter
a
Table 1. Optimization of Arylation Conditions
b
entry
catalyst (equiv)
base (equiv)
temp (°C)
yield (%)
1
−
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOLi (3.0)
t-BuONa (3.0)
KOH (3.0)
100
100
100
100
100
100
100
100
100
100
100
100
120
120
120
120
120
120
120
120
120
120
0
2
F1 (0.2)
F2 (0.2)
F3 (0.2)
F4 (0.2)
F5 (0.2)
F2 (0.2)
F2 (0.2)
F2 (0.2)
F2 (0.1)
F2 (0.05)
F2 (0.05)
F2 (0.05)
F2 (0.05)
F2 (0.02)
F2a (0.05)
F2b (0.05)
F2c (0.05)
F1 (0.05)
F3 (0.05)
F4 (0.05)
F5 (0.05)
64
76
25
42
5
3
4
5
6
7
0
8
0
9
0
10
11
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (2.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
t-BuOK (3.0)
75
84
99 (95)
99 (96)
51
80
92
93
93
25
91
86
92
cd
,
12
13
14
15
16
17
18
19
20
21
22
c
Figure 1. (a) Chemical structures of aromatic foldamers P5b and F1−
F5 and (b) crystal structure of a trimer molecule containing three
pyridone units.4g
a
Reaction conditions: 4-iodoanisole (0.2 mmol), benzene (3 mL),
b
base, and catalyst in a sealed Schlenk tube, 24 h. Yields were
determined by NMR analysis. Isolated yield in parentheses. Reaction
c
d
time: 48 h.
yield of 84% was achieved with the use of 5 mol % loading of
F2 (entries 3 and 10 vs entry 11, Table 1), likely due to the
weakened π−π stacking among aromatic molecules at lower
concentrations. Increasing the reaction time from 24 to 48 h
(entry 12, Table 1) or temperature from 100 to 120 °C (entry
13, Table 1) resulted in complete conversions with isolated
yields of 95% and 96%, respectively. Additional investigations
using a lesser amount of base (entry 14, Table 1) or catalyst
(entry 15, Table 1) show that 3 equiv of t-BuOK and 5 mol %
of F2 are necessary for achieving optimum catalytic efficiency.
Under these optimized conditions, oxygen-rich 18-crown-6
only gave a 25% yield. Using these optimized conditions, we
also have tested three analogs of F2, i.e., F2a−F2c that contain
COOH or NH2 at the ends, together with F1 and F3−F5.
Except for F1 that is not very stable at 120 °C under the basic
conditions (entry 19, Table 1), all the others gave rise to
excellent yields (entries 16−22, Table 1), albeit slightly worse
than F2. In view of its structural simplicity and ease in
synthesis, F2 was employed in all subsequent arylation
reactions.
To examine the generality of the optimized catalytic system
toward substrates of various types, we surveyed a range of aryl
iodides as well as bromides/chloride of limited types (Table 2).
The direct arylation reactions of benzene with various aryl
iodides including those bearing electron-donating or -with-
drawing groups proceeded readily with yields ranging from 71%
to 97% (entries 1−9, Table 2). Notably, a high yield of 89%
could be obtained when sterically hindered 2-iodoanisole (1c)
was used in the reaction (entry 3, Table 2). While a mixture
containing desired 3h in high yield of 85% and byproduct of p-
terphenyl in 10% yield was obtained for 1-fluoro-4-iodobenzene
carrying two possibly reactive functional groups (1h, entry 8,
Table 2), use of 1-chloro-4-iodobenzene (1i, entry 9, Table 2)
led to a mixture of 4-chlorobiphenyl, which is the expected
product, and p-terphenyl, both at low yield (data not shown).
Nevertheless, increasing t-BuOK from 3 to 5 equiv generated p-
terphenyl as the only product with an 85% yield (entry 9, Table
2). This is interesting since simple chlorobenzene is unreactive
under the coupling conditions (entry 14, Table 2). Naphthyl
and thienyl iodides also underwent coupling reactions with
benzene in good yields (entries 10 and 11, Table 2). Further
testing showed that aryl bromides expectedly reacted more
slowly than aryl iodides with lower yields (entries 12 and 13,
Table 2).
This foldamer-based catalytic protocol for C−H bond
activation was further applied to a series of arenes. Reactions
of 1a with monosubstituted arenes bearing either electron-
donating or -withdrawing groups such as 2b−2d yielded a
mixture of regioisomers in which the ortho-isomers always
predominated (entries 1−3, Table 3), and arylations seem to
favor electron-deficient arenes (entries 1 and 2 vs 3, Table 3).
When disubstituted arenes, i.e., p-xylene (2e) and 1,4-
difluorobenzene (2f), were used in the reaction, the desired
products were produced with 41% and 51% yields, respectively
(entries 4 and 5, Table 3).
Involvement of radicals in this type of arylation reactions has
been proposed with experimental support from early
investigations.1 Consistent with this mechanism and for F2-
catalyzed arylation reactions, addition of 1 equiv of radical
scavengers of either TEMPO (2,2,6,6-tetramethyl-piperidin-1-
yl)oxyl) or 1,1-diphenylethelene into the reaction indeed
completely aborted the arylations, and no desired products
B
DOI: 10.1021/acs.orglett.7b00921
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Organic Letters
Letter
Table 2. Direct Arylation Reactions of Benzene with Aryl
Halides
Table 3. Direct Arylation Reactions of Aryl Iodides with
a
a
Arenes
a
Reaction conditions: Ar−X (0.2 mmol), t-BuOK (3 equiv), F2 (5
mol %), and arenes (2 mL) in sealed Schlenk tube, 120 °C, 24 h.
b
c
Isolated yields. Ratio of products determined by NMR analysis.
Table 4. Participation of Radicals in Arylation Reactions
a
entry
additive (equiv)
yield (%)
1
2
3
TEMPO (1.0)
0
1,1-diyldibenzene (1.0)
0
no radical scavenger
99
a
1
Yields determined by H NMR.
Scheme 1. Kinetic Isotope Effect Experiment
a
Reaction conditions: Ar−X (0.2 mmol), t-BuOK (3 equiv), F2 (5
mol %), and benzene (3 mL) in a sealed Schlenk tube, 120 °C, 24 h.
b
c
d
Isolated yields. 1a at the 1.0 mmol scale. 10% p-terphenyl was
e
f
isolated. 5 equiv of t-BuOK were used. 48 h.
Scheme 2. A Possible Catalytic Cycle
were observed (entries 1 and 2, Table 4). A kinetic isotope
experiment involving 1a reacting with an equal molar mixture
of benzene and deuterated-benzene yielded 1.09 for the value
of kH/D, suggesting that cleavage of the aromatic C−H bond
does not constitute a rate-limiting step in the arylation (Scheme
1).
Based on the above observations and recent reports by
others,1,2e,6 a radical-mediated catalytic mechanism can be
proposed as shown in Scheme 2. The catalytic cycle is initiated
by encapsulating potassium tert-butoxide in the cavity of F2 via
the formation of two strong K+−O coordination bonds. This
complex then transfers one electron to the aryl halide, which
undergoes one-electron reduction to produce the aryl radical
anion after C−X bond cleavage. The generated radical anion
adds to benzene to form a biaryl radical intermediate from
which the biaryl radical anion was generated after deprotona-
tion by potassium tert-butoxide. Subsequent reaction involving
this radical anion and an aryl halide through a single electron
transfer affords the product as well as aryl radical that enables
the catalysis to continue to the next catalytic cycle.
In conclusion, we have demonstrated here a novel foldamer-
based organocatalyst F2 for efficient construction of biaryl
compounds using transition-metal-free direct arylations of
C
DOI: 10.1021/acs.orglett.7b00921
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Organic Letters
Letter
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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.or-
glett.7b00921.
Synthetic procedures and characterizations (1H and 13C
NMR) for arylation products (PDF)
́
Ferrand, Y.; Bao, C.; Kauffmann, B.; Grelard, A.; Jiang, H.; Huc, I.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hqzeng@ibn.a-star.edu.sg.
ORCID
Wei Zeng: 0000-0002-6113-2459
Huaqiang Zeng: 0000-0002-8246-2000
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Institute of Bioengineering and
Nanotechnology (Biomedical Research Council, Agency for
Science, Technology and Research, Singapore) and Shandong
Provincial Natural Science Foundation, China (ZR2016JL009).
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