Palladium-catalyzed aerobic dehydrogenative cross-coupling of polyfluoroarenes with thiophenes: Facile access to polyfluoroarene-thiophene structure

Article abstract of DOI:10.1021/om200873j

We report a Pd-catalyzed aerobic dehydrogenative cross-coupling of polyfluoroarenes with thiophenes via 2-fold C-H functionalization. The advantages of this reaction are its high reaction efficiency, excellent functional group compatibility, and use of mo

Full text of DOI:10.1021/om200873j

Article
pubs.acs.org/Organometallics
Palladium-Catalyzed Aerobic Dehydrogenative Cross-Coupling of
Polyfluoroarenes with Thiophenes: Facile Access to
Polyfluoroarene−Thiophene Structure
Chun-Yang He,‡ Qiao-Qiao Min,† and Xingang Zhang*,†
†Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, China
‡College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620,
China
S
* Supporting Information
ABSTRACT: We report a Pd-catalyzed aerobic dehydrogenative cross-coupling of polyfluoroarenes with thiophenes via 2-fold
C−H functionalization. The advantages of this reaction are its high reaction efficiency, excellent functional group compatibility,
and use of molecular O2 as terminal oxidant. This reaction provides a useful and facile protocol for the preparation of
polyfluoroarene−thiophene structure of interest in functional materials. Mechanistic studies revealed that the reaction is initiated
by the C−H bond cleavage of polyfluoroarenes.
electronic devices, such as organic light-emitting diodes
(OLEDs) and field-effect transistors (FETs).9,10 Hence, it is
of great synthetic interest to develop efficient reactions to
access them. Very recently, we successfully developed an
efficient method for the synthesis of polyfluoroarene−
thiophene structure through Pd-catalyzed dehydrogenative
cross-coupling of polyfluoroarenes with thiophenes.11a One
drawback of this reaction is the use of excess Ag2CO3 as
oxidant. In view of the importance of polyfluoroarene−
thiophene structure in the functional materials, the use of
molecular O2 as oxidant is appealing,12 because only water is
produced as a byproduct in this Pd-catalyzed overall process.
However, replacement of stoichiometric transition-metal
oxidants with O2 represents a significant fundamental challenge
that has important implications for practical applications of
these methods.12 To the best of our knowledge, only limited
success has been achieved thus far.7 Herein, we describe a
convenient method for the Pd-catalyzed aerobic dehydrogen-
ative cross-coupling of polyfluoroarenes with thiophenes,13
which provides a concise and useful protocol for the
preparation of polyfluoroarene−thiophene structures of interest
in functional materials (Scheme 1).
INTRODUCTION
■
The formation of aryl−aryl bonds is an important and
fundamental transformation in organic synthesis since the
biaryl structural motif is found in many natural products,
pharmaceuticals, agrochemicals, and functional materials.1 The
most common method used to prepare biaryls relies on
transition-metal-catalyzed cross-coupling of aryl halides with
aryl metals.2 These reactions are extremely useful and are one
of the most reliable synthetic methods. However, this
“prefunctionalization” process suffers from an intrinsic
limitation in terms of atom and step economy. In recent
years, the direct C−H arylation in which the C−H bond is used
as a functional group has attracted considerable attention.3 In
particular, the carbon−carbon bond formation obtained via 2-
fold C−H bond functionalization (i.e., the dehydrogenative
cross-coupling approach) is the ideal strategy from the point of
view of synthetic simplicity.4−8 But such methods face great
challenges, including the difficulties in controlling the selectivity
of the two C−H activation steps and in suppressing undesired
homocoupling of the substrates and identification of an atom-
economical oxidant for the reaction. In view of the fact that the
majority of Pd-catalyzed methods for dehydrogenative cross-
coupling between two different arenes require stoichiometric
transition-metal oxidants, such as Cu(II) salt or Ag(I) salt,6
which decrease their atom economy, new catalytic systems to
overcome this intrinsic limitation for widespread synthetic
application are highly desirable.
Special Issue: Fluorine in Organometallic Chemistry
On the other hand, molecules containing polyfluoroarene−
thiophene structure play a primary role as active materials in
Received: September 17, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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also serve as a base cannot be ruled out. The replacement of
Ag2O with other bases, such as K2CO3 and K3PO4, could also
furnish 2a, albeit in low yields (Table 1, entries 17, 18). The
absence of Pd-catalyst did not give any desired product (Table
1, entry 14), thus indicating that Pd-catalyst was involved in the
catalytic cycle. The reaction concentration also had a critical
impact on the reaction efficiency. The best yield of isolated
product (81%) can be obtained by increasing the reaction
concentration with utilization of only 5 mol % of Pd(OAc)2,
0.05 equiv of Ag2O, and 0.4 equiv of pivalic acid (PivOH)
(Table 1, entry 13).
Scheme 1. Pd-Catalyzed Dehydrogenative Cross-Coupling of
Polyfluoroarenes with Thiophenes
A variety of pentafluorophenyl-substituted thiophenes were
generated by the present method, and good to high yields were
obtained (Table 2). Importantly, substrates bearing functional
groups, such as ester, amide, methyl ketone, aldehyde, chloride,
and nitro, all showed good tolerance to the reaction conditions.
Although for chloride-substituted thiophene 3e, a slightly lower
yield (60%) was obtained compared to previous work (65%
yield),11a the homocoupling of 2-chlorothiophene was sup-
pressed, thus making the purification process easier (Table 2,
3e). It should be pointed out that the access to 3g is more
efficient and can afford 10% higher yield through the present
strategy than using excessive silver salts as oxidant, thus
providing a facile protocol to prepare symmetrical bis-
pentafluorophenyl-substituted thiophene structures10,16 (Table
2, 3g). 2-Aryl-substituted thiophenes are also suitable substrates
for the reaction, and the nature of the substituents on the
aromatic ring does not influence the reaction efficiency (Table
2, 3h−3j). In addition, benzo[b]thiophene and indole are also
suitable substrates for the reaction (Table 2, 3k and 3l).
However, for the benzofuran, a complex was obtained (Table 2,
RESULTS AND DISCUSSION
■
Initially, the dehydrogenative cross-coupling of pentafluoro-
benzene (1) with 1-(thiophen-2-yl)ethanone (2a) was
investigated in the presence of Pd(OAc)2 (10 mol %) and a
catalytic amount of Ag2CO3 (0.1 equiv) in anhydrous DMSO14
under 1 atm O2 at 120 °C (Table 1, entry 1). Fortunately, this
reaction provided 56% NMR yield of 3a without observation of
decomposed thiophene substrate. Further switching DMSO
with a mixed solvent system of DMSO/DMF (1:1) was found
to benefit the reaction (Table 1, entry 2). Encouraged by this
result, a survey of silver salts and additive was conducted (Table
1, entries 3−12). We found that Ag2O is the best choice, and
0.4 equiv of pivalic acid (PivOH)15 is also beneficial to the
reaction, while the absence of either Ag2O or O2 failed to
provide the desried product (Table 1, entries 15, 16), thus
demonstrating that silver salt and O2 play pivotal roles in the
catalytic cycle. However, the possibility that silver salt could
Table 1. Optimization of Aerobic Dehydrogenative Cross-Coupling of Pentafluorobenzene (1) with 1-(Thiophen-2-yl)ethanone
a
(2a)
b
entry
x (mol %)
Ag(I) (equiv)
additive (equiv)
yield (%)
c
1
10
10
10
10
10
10
10
10
10
10
5
Ag2CO3 (0.1)
Ag2CO3 (0.1)
AgOAc (0.2)
AgF (0.2)
56
2
67
3
69
4
61
5
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.1)
Ag2O (0.05)
Ag2O (0.05)
Ag2O (0.05)
80 (72)
80
6
PivOH (0.1)
AdOH (0.1)
PivOH (0.2)
PivOH (0.4)
PivOH (0.6)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
7
71
8
81
9
85 (76)
80
10
11
12
70 (67)
74 (69)
88 (81)
N.D.
trace
trace
29
5
d
13
5
d
14
d
15
5
5
5
5
de
,
16
17
18
Ag2O (0.05)
Cs2CO3 (0.05)
K3PO4 (0.05)
d
d
29
a
Reaction conditions (unless otherwise specified): 1 (3.0 equiv), 2a (0.3 mmol), DMF/DMSO (v/v = 1:1, 2 mL, DMSO must be stored with the
b
powder of 4 Å molecular sieves), O2 (1 atm), 120 °C, 9 h. Yield determined by 19F NMR spectroscopy using fluorobenzene as the internal
standard, and the yield of the isolated product is in parentheses. DMSO (2 mL) was used as solvent. DMF/DMSO (v/v = 1:1, 1 mL) were used as
solvent. Reaction run in the absence of O2.
c
d
e
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a
Table 2. Aerobic Dehydrogenative Cross-Coupling of Pentafluorobenzene (1) with Heteroarenes 2
a
Reaction conditions (unless otherwise specified): 1 (3.0 equiv), 2 (0.3 mmol), DMF/DMSO (v/v = 1:1, 1 mL), 120 °C, 9 h. All reported yields are
b
c
isolated yields. Reaction run in 2 mmol scale. Reaction run at 140 °C.
a
Table 3. Aerobic Dehydrogenative Cross-Coupling of Fluoroarenes 4 with Thiophenes 2
a
Reaction conditions (unless otherwise specified): 4 (3.0 equiv), 2 (0.3 mmol), DMF/DMSO (v/v = 1:1, 1 mL), 120 °C, 9 h. All reported yields are
b
isolated yields. 4 (0.3 mmol, 1.0 equiv) and 2 (2.0 equiv).
3m). The usefulness of this method can also be featured by
rapid access of 3b (92%) in a 2 mmol scale synthesis, thus
indicating the good reliability of the process (Table 2, 3b).
To further probe the applicability of this catalytic system,
couplings of various fluoroarenes 4 bearing three or four
fluorines with thiophenes were also tested (Table 3). To our
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Scheme 2. Kinetic Isotope Effect Studies
delight, comparable yields of compounds 5a and 5b were
provided at 120 °C with use of 3.0 equiv of fluoroarenes, while
140 °C and 5.0 equiv of fluoroarenes were required under our
previous reaction conditions (Pd(OAc)2 (5 mol %), Ag2CO3
(1.5 equiv), and HOAc (1.0 equiv) in DMF + DMSO (5%))11a
(Table 3, 5a and 5b). Furthermore, higher yields of 5c and 5i
could also be obtained (Table 3, 5c and 5i), which
demonstrated high reaction efficiency of this catalytic system,
by using an inexpensive and environmentally benign oxidant
(O2). Another fluoroarene, 1,2,4,5-tetrafluoro-3-
(trifluoromethyl)benzene, is also a good coupling partner
under the present reaction conditions; thus good yields are
obtained as well (Table 3, 5d and 5e). It should be mentioned
that for the vinyl- and aryl-substituted fluoroarenes (Table 3,
5f−5h), the use of 1.0 equiv of fluoroarene still can provide
moderate to good yields, thus providing an efficient way for
using these unusual substrates.17
Scheme 3. Homocouplings of Pentafluorobenzene (1) and
Benzo[b]thiophene (2k)
To investigate the working mode of the present catalytic
system, kinetic isotope effect (KIE) experiments for both
coupling partners were conducted (Scheme 2). An intermo-
lecular competition reaction between pentafluorobenzene and
its deuterated derivative does not exhibit a kinetic isotope effect
(kH/kD = 1.12) (Scheme 2, eq 1), which implies that the C−H
bond cleavage of polyfluoroarenes is not involved in a rate-
determining step. However, a primary KIE of 2.86 was observed
by coupling of pentafluorobenzene (1) with 2k/d-2k, indicating
that the C−H bond cleavage of benzo[b]thiophene (2k) is the
rate-determining step in the overall catalytic process. This
finding further implied that the C−H bond cleavage of
thiophenes may proceed via a concerted metalation−deproto-
nation pathway,18 as the typical electrophilic aromatic
substitution pathway does not involve C−H cleavage in the
rate-determining step.19
Scheme 4. H/D Exchange Experiments of
Pentafluorobenzene (1) and Benzo[b]thiophene (2k)
Further experiments by comparison of the homocoupling
reactions of 1 and 2k revealed that both pentafluorobenzene
(1) and benzo[b]thiophene (2k) failed to afford homocou-
plings (Scheme 3). These findings indicate that the formation
of homocouplings can be suppressed under the optimized
reaction conditions; thus only desired cross-couplings were
obtained.
To further understand the selectivity of the cross-coupling,
the following H/D exchange experiments were conducted
(Scheme 4). When pentafluorobenzene (1) was treated with
D2O under optimized conditions, almost a quantitative yield of
deuterated d-1 was observed (determined by 19F NMR, Scheme
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Scheme 5. Experiment for Mechanistic Study
4, eq 1). The absence of Pd(OAc)2 led to a comparable yield as
well (determined by 19F NMR, Scheme 4, eq 2). However, no
deuterated d-1 was detected, when the reaction was performed
without Ag2O (Scheme 4, eq 3). These results indicated that
the present cross-coupling initiated from polyfluoroarene is
reasonable, and the silver salt may serve as a base for
deprotonation of highly acidic polyfluoroarene, which is
consistent with our previous results (Table 1, entries 15, 17,
18). The H/D exchange experiment of 2k did not afford
deuterated d-2k (Scheme 4, eq 4); thus the possibility that the
reaction starts from thiophenes can be ruled out. This finding
further indicated that an electrophilic attack of Pd(II) on the
thiophenes is not involved in the reaction.
To further confirm that the reaction is initiated from
polyfluoroarene, a reaction of Pd(pentafluorophenyl) complex I
with 2k was conducted, providing 3k in 64% yield (Scheme 4
eq 1). Hence, the present cross-coupling that starts from
polyfluoroarene is reasonable. Intermediate I was prepared in
two steps by desilylation of compound 8 with AgF in DMF,20
followed by metal exchange with Pd(II). The formation of
intermediate I was further confirmed by ESI-MS analysis.21
On the basis of the preliminary studies, we proposed the
following mechanism for the dehydrogenative cross-coupling of
polyfluoroarenes with thiophenes (Scheme 6). The reaction is
and affords high yields and excellent functional group
compatibillity. Hence, it is a useful and facile protocol for the
preparation of polyfluoroarene−thiophene structure. The
mechanistic studies revealed that this tandem oxidation of
C−H bonds is initiated by the C−H cleavage of polyfluoroar-
enes.
EXPERIMENTAL SECTION
■
General Procedure for Pd-Catalyzed Aerobic Dehydrogen-
ative Cross-Coupling of Polyfluoroarenes with Thiophenes 2.
To a septum-capped 25 mL sealed tube were added Pd(OAc)2 (5 mol
%) and Ag2O (3.5 mg, 0.015 mmol, 0.05 equiv). After the sealed tube
was refilled with O2 three times, DMSO (0.5 mL), DMF (0.5 mL),
and PivOH (15 μL, 0.12 mmol, 0.4 equiv) were added with stirring.
Polyfluoroarene (0.9 mmol, 3.0 equiv) and thiophene 2 (0.3 mmol 1.0
equiv) were then added. The sealed tube was screw capped and heated
to 120 °C (oil bath). After stirring for 9 h, the reaction mixture was
cooled to room temperature, diluted with ethyl acetate, washed with
H2O and brine, dried over Na2SO4, filtered, and concentrated. The
residue was purified with silica gel chromatography to provide pure
product.
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures and characterization data for
new compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
Scheme 6. Mechanistic Proposal for Pd-Catalyzed Aerobic
Dehydrogenative Cross-Coupling of Polyfluoroarenes with
Thiophenes
AUTHOR INFORMATION
Corresponding Author
*E-mail: xgzhang@mail.sioc.ac.cn.
■
ACKNOWLEDGMENTS
■
The 973 Program (2012CB821602), the NSFC (Nos.
20902100, 20832008, 21172242), and SIOC are greatly
acknowledged for funding this work. Dr. Hao-Yang Wang
and Prof. Yin-Long Guo are warmly thanked for mass
spectrometric analyses.
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■
In conclusion, we have developed a straightforward method for
Pd-catalyzed aerobic dehydrogenative cross-coupling of poly-
fluoroarenes with thiophenes via 2-fold C−H functionalization.
The reaction makes use of molecular O2 as terminal oxidant
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(14) DMSO must be stored with the powder of 4 Å molecular sieves.
(15) There might be two roles for the PivOH in the reaction. One is
to function as a pivalato ligand and assist deprotonation at a transition
state; see: Ackermann, L. Chem. Rev. 2011, 111, 1315. The other is
to suppress undesired homocoupling in Pd-catalyzed oxidative cross-
coupling reactions; for details, see: Yang, S.; Sun, C.; Fang, Z.; Li, B.;
Li, Y.; Shi, Z. Angew. Chem., Int. Ed. 2008, 47, 1473, and references
therein.
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dx.doi.org/10.1021/om200873j | Organometallics 2012, 31, 1335−1340