Suzuki-Miyaura coupling reaction by PdII-catalyzed aromatic C-H bond activation directed by an N-alkyl acetamino group

Article abstract of DOI:10.1002/anie.200700590

(Chemical Equation Presented) A seamless join: An efficient method to construct a C(sp²)-C(sp²) bond has been developed by using a Suzuki-Miyaura-type coupling of N-alkyl acetanilides with boronic acids. The reaction was catalyzed by

Full text of DOI:10.1002/anie.200700590

Communications
DOI: 10.1002/anie.200700590
Palladium Catalysis
II
À
Suzuki–Miyaura Coupling Reaction by Pd -Catalyzed Aromatic C H
Bond Activation Directed by an N-Alkyl Acetamino Group**
Zhangjie Shi,* Bijie Li, Xiaobing Wan, Jiang Cheng, Zhao Fang, Bin Cao, Changming Qin, and
Yang Wang
À
À
C C bond formation is an important process in organic
synthesis. The Suzuki–Miyaura coupling is one of the most
methods,[5] processes to form C C bonds by Suzuki–Miyaura-
type coupling were rarely reported. Very recently, the
research group of Yu reported the alkylation of sp3 or
sp C H bond directed by pyridinyl and carboxylic groups.
À
useful methods to construct C C bonds and has been broadly
2
[6]
À
applied in syntheses of natural products, synthetic drugs, and
materials.[1] In a traditional palladium-catalyzed Suzuki–
A number of research groups, including that of Sames, have
3
À
À
Miyaura coupling, C X (X = Br, I, OTf; Tf = trifluorometha-
reported the arylation of sp C H bonds catalyzed by
nesulfonyl) groups were required as substrates to initiate the
oxidative addition of Pd0 to produce PdII-containing inter-
mediates. Recent advances have allowed the use of the
relatively cheap and readily available organic chlorides,[2] but
an ideal and environmentally friendly method to construct C
C bonds would be the direct functionalization of C H bonds.
ruthenium complexes with an aromatic boronic ester and by
using a heterocyclic directing group.[7] Herein we report a
À
palladium-catalyzed C C bond formation to afford highly
regioselective ortho arylation of aromatic C H bond in a
À
À
Suzuki—Miyaura-type coupling reaction with aromatic bor-
onic acids directed by an acetyl amino group.
We previously developed a highly regioselective halogen-
ation of acetanilides by using a PdII-catalyzed reaction
[Eq. (1)].[8] A Suzuki–Miyaura coupling reaction mediated
À
Over the past several decades, extensive efforts have been
À
made to develop a transition-metal-catalyzed C H function-
alization.[3] Direct arylation of aromatic C H bonds has been
À
developed with or without directing groups to construct a
biaryl structural motif, which is an important scaffold in many
pharmaceutically relevant and biologically active com-
pounds.[4] In many cases, C H bond activation by transition-
À
À
metal complexes can offer a direct route to C M (M = metal)
intermediates that are also accessible through the oxidative
addition of a low-valent transition-metal complex to organic
halides. However, the direct activation of C H bonds could
lead to the same valuable products but avoid the requirement
of the presence of halides in the starting material. Although
by Pd could occur from this halide under basic conditions
[Eq. (2)].[9] In our halogenation reaction, we proposed the
palladacycle 5 as a key intermediate, identical to that
À
À
the C C bond could be constructed by transition-metal-
À
catalyzed C H bond activation of arenes by using other
[*] Dr. Z. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
Fax: (+86)10-6276-0890
E-mail: zshi@pku.edu.cn
Homepage: http://www.shigroup.cn/
generated from the oxidative addition of 4 to Pd0 in the
traditional Suzuki–Miyaura coupling. This intermediate 5, if
formed, will undergo transmetalation with boronic acids and
Dr. Z. Shi, B. Li, Dr. X. Wan, Prof. Dr. J. Cheng, Z. Fang, B. Cao,
Y. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS)
PKU Green Chemistry Centre and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education, College of Chemistry
Peking University, Beijing 100871 (China)
À
reductively eliminate to form the C C bond. Thus, we
À
hypothesized that a Suzuki–Miyaura coupling by using C H
functionalization of acetanilides with aryl boronic acids might
proceed under the proper conditions (Scheme 1).
However, the reaction conditions of the halogenation
reaction and Suzuki–Miyaura coupling may not be compat-
ible because, although both may go through the same
intermediate 5, they were catalyzed by different palladium
species. The halogenation was carried out under acidic
conditions to enhance the electrophilic ability of the PdII
species that is important for cyclopalladation. However,
most Suzuki–Miyaura couplings proceed under basic condi-
tions.[1,2] This difference makes it difficult to combine these
Prof. Dr. J. Cheng, C. Qin
College of Chemistry and Material Sciences
Wenzhou University, Wenzhou, Zhejiang 325027 (China)
[**] Support of this work by a starter grant fromPeking University and
the grant fromthe National Sciences of Foundation of China
(No. 20542001, 20521202, 20672006) is gratefully acknowledged.
We thank Professor Qinghua Fan at ICCAS for compounds 1c, 1d,
and 1e.
À
À
two processes and construct a C C bond from a C H bond.
We have to overcome this problem if we want to directly
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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coupling reactions, several different additives were tested in
our reaction system.[2,10] Gratifyingly, Ag2O was found to be
the most efficient additive in our system and may work as
either a co-oxidant, base, or both (Table 1, entry 6).
With other copper salts, such as Cu(OAc)2 and [Cu-
(acac)2], the reaction did not proceed well (Table 1, entries 7
and 8). Other common inorganic and organic oxidants were
not efficient for this transformation (Table 1, entries 9–11).
Different palladium species were also tested. Pd(OTFA)2 had
a comparable catalytic reactivity for this transformation
(Table 1, entry 12). The efficiency with [Pd(PhCN)2Cl2],
PdCl2, or [Pd(PPh3)2Cl2] was much lower than that with
Pd(OAc)2 (Table 1, entries 6 and 13–15). We hypothesized
that the catalytic efficiency of the PdII species arose from both
its electrophility and solubility. Pd0 species, such as [Pd-
(dba)2], could also act as a catalyst with good efficiency, and is
presumably oxidized to PdII under the oxidation conditions
employed (Table 1, entry 16). Of the solvents surveyed, only
toluene was found to be successful for the transformation.
Interestingly, the standard conditions reported by the
research group of Yu for the alkylation directed by a pyridinyl
group was also applied into this system, but only the
homocoupling biphenyl product was observed.[6]
After screening of the reaction conditions, the substrate
scope was studied. Different aromatic boronic acids were
submitted to the reaction with 1a (Table 2). We found three
trends: 1) Different substituents on the aromatic ring of the
boronic acids were compatible with the arylation. Monosub-
stituted electron-donating groups helped increase the effi-
ciency, and the products 3 were produced in good to excellent
yields (Table 2, entries 1 and 3–6), whereas electron-with-
drawing groups were found to decrease the yields (entries 10–
12). 2) The position of the substituents on the phenyl ring of
the boronic acids played a vital role and meta or para
substituents provided the best yields. When ortho-substituted
boronic acids (for example, ortho-tolylboronic acid) were
used, the coupling reaction did not proceed well and mainly
the starting materials were recovered (Table 2, entry 2).
3) The use of disubstituted aryl boronic acids was found to
decrease the efficiency of the transformation, which may be
due to steric hindrance (Table 2, entries 7 and 8).
Various N-alkyl acetanilides were investigated (Table 3).
An alkyl group could be introduced into either the phenyl or
aliphatic ring of 1,2,3,4-tetrahydroquniline, and all of the
coupling reactions performed well. The desired products 3ba–
3 fa were isolated in excellent yields (Table 3, entries 2–6).
Notably, arylation only occurred on the scaffold of 1,2,3,4-
tetrahydroquniline when there was a phenyl group in the side
chain (Table 3, entry 5). N-Acetyl-2,3-dihydroindole and its
derivatives were also good substrates for this transformation
(Table 3, entries 7–10). The relatively hindered N-acetyl-2-
methyl-2,3-dihydroindole could be used with 2a to give the
coupling products 3ha and 3ja in moderate to excellent yields
(Table 3, entries 8 and 10). Notably, the relatively stable
chloride group can survive the mild reaction conditions and is
available for further functionalization (Table 3, entry 10).[2]
Moreover, the acetyl directing group is key for the coupling
reaction, and when the non-acetylated 2,3-hydroindole was
used, the desired coupling product was not observed.[11]
Scheme 1. Design of a direct Suzuki–Miyaura coupling reaction with a
À
Pd-catalyzed C H bond activation.
À
À
construct C C bonds by functionalization of C H bonds with
palladium.
We first attempted the coupling reaction with phenyl-
boronic acid 2a as a reagent. The substrate 1a was chosen
because of its high reactivity in the halogenation reaction, and
the chlorination was first surveyed with Cu(OAc)2 as the
oxidant. Unfortunately, the desired coupling product was not
observed and only biphenyl was produced as a homocoupling
product of phenylboronic acid (Table 1, entries 2 and 3).
However, Cu(OTf)2 was found to be a sufficient oxidant for
this transformation and the desired product 3aa was observed
in 37% yield (calculated from GC) with a large amount of
biphenyl as by-product (Table 1, entry 4). Since additives are
important to inhibit the homocoupling in Suzuki–Miyaura
[a]
À
Table 1: Suzuki reaction under various conditions with C H activation.
Entry Pd cat.
(mol%)
CuII salt
(equiv)
Additive
(equiv)
3aa
[%][b]
1
2
3
4
Pd(OAc)2 (5.0)
–
–
–
<5
<5
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OAc)2 (5.0)
Pd(OTFA)2 (5.0)
PdCl2 (5.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OTf)2 (1.0)
Cu(OTf)2 (1.0)
Cu(OTf)2 (1.0)
Cu(OAc)2 (1.0)
[Cu(acac)2] (1.0)
PhI(OAc)2 (1.0)
1,4-benzoquilone (1.0) Ag2O (1.0) <5
oxone (0.5)
Cu(OTf)2 (1.0)
Cu(OTf)2 (1.0)
Cu2O (1.0) <5
– 37
5
Cu2O (1.0) 37
Ag2O (1.0) 85
Ag2O (1.0) <5
Ag2O (1.0) <5
Ag2O (1.0) <5
6[c]
7
8
9
10
11
12
13
14
15
16
17
18
Ag2O (1.0) <5
Ag2O (1.0) 71
Ag2O (1.0)
Ag2O (1.0) 12
Ag2O (1.0) 16
Ag2O (1.0) 59
Ag2O (1.0) <5
Ag2O (1.0) <5
6
[Pd(PhCN)2Cl2] (5.0) Cu(OTf)2 (1.0)
[Pd(PPh3)2Cl2] (5.0) Cu(OTf)2 (1.0)
[Pd(dba)2][d]
–
Pd(OAc)2 (5.0)
Cu(OTf)2 (1.0)
Cu(OTf)2 (1.0)
–
[a] All the reactions were carried out with 1a (0.2 mmol) with phenyl-
boronic acid (0.4 mmol) under standard conditions; acac=acetylaceto-
nate, OTFA=trifluoroacetate, Tf=trifluoromethanesulfonyl. [b] Yields
calculated fromGC using n-dodecane as an internal standard. [c] Yield of
isolated product. [d] dba=trans,trans-dibenzylideneacetone.
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Communications
Table 2: Suzuki–Miyaura coupling of 1a with various boronic acids 2.[a]
Table 3: Suzuki–Miraura coupling of various acetyl amides with 2a.[a]
Entry
2
3
Product
Entry
1
3
Product
(yield [%])[b]
(yield [%])[b]
1
2
3aa (85)
1
2
3
3aa (85)
3ba (56)
3ca (78)
3ab (20)[c]
3
4
3ac (82)
3ad (74)
4
5
6
7
3da (76)
3ea (70)
3 fa (70)
3ga (84)
5
6
7
3ae (80)
3af (75)
3ag (35)[c]
8
9
3ha (92)
3ia (82)
8
3ah (33)[c]
3ai (58)
3aj (62)
3ak (48)
9
10
3ja (41)
10
11
11[c]
3ka (66)
3kb (72)
12[c,d]
12
3al (31)[c]
[a] All the reactions were carried out with 1 (0.2 mmol) and 2a
(0.4 mmol) in the presence of Pd(OAc)2 (5 mol%), Cu(OTf)2
(1.0 equiv), and Ag2O (1.0 equiv) in toluene (4 mL) at 1208C, unless
otherwise stated. [b] Yields of isolated products are the average of at
least two experiments. [c] 1,2,4-Trimethylbenzene was used as a solvent.
[d] Para-tolylboronic acid 2d was used.
[a] All the reactions were carried out with 1a (0.2 mmol) and boronic acid
2 (0.4 mmol) in the presence of Pd(OAc)2 (5.0 mol%), Cu(OTf)2
(1.0 equiv), and Ag2O (1.0 equiv). [b] Yields of isolated products are
the average of at least two experiments. [c] Some starting material 1a
was also recovered during these reactions.
The coupling reaction may be initiated by electrophilic
attack of a PdII center to the aromatic ring with the assistance
of the acetamino group, followed by transmetalation and
reductive elimination to produce the expected product
(Scheme 2, path a). The other possibility is the transmetala-
tion of the boronic acid with PdII salts to form initially an
arylated PdII species,[12] which may attack in an electrophilic
manner to the aromatic ring to form a diaryl palladium
species, which then would undergo reductive elimination to
give the final product (Scheme 2, path b).
To further understand this transformation, we carried out
some preliminary mechanistic studies. An intramolecular
À
isotopic effect (KH/D = 2.3) indicated that the cleavage of a C
H bond was involved in the rate-determining step [Eq. (3)]. If
the reaction mixture with no added boronic acid 2a but with a
stoichiometric amount of Pd(OAc)2 was allowed to react for
one hour, after the addition of 2a, the coupling product was
isolated in good yield. In contrast, similar treatment of the
reaction mixture without acetanilide 1a only gave the
homocoupling product with no observation of the desired
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Experimental Section
General: All reactions were carried out in a stoppered
Schlenk flask under air. All the chemicals were pur-
chased from Alfa Aesar and Acros Chemical, and used
without further purification. Toluene was freshly dis-
tilled over sodium with the use of diphenyl ketone as an
indicator. 1H NMR (300 MHz) and 13C NMR (75 MHz)
spectra were recorded on Varian 300m spectrometers
with CDCl3 as the solvent and tetramethylsilane as the
internal standard. Chemical shifts are reported in ppm
by assigning TMS resonance in the 1H NMR spectrum as
0.00 ppm and CDCl3 resonance in the 13C spectrum as
77.0 ppm. All coupling constants (J values) were
reported in Hertz (Hz). Column chromatography was
performed on silica gel 200–300 mesh. IR, X-ray, GC,
MS, and HRMS were performed at the State-Author-
ized Analytical Center at Peking University.
General procedure: Pd(OAc)2 (2.2 mg, 0.01 mmol),
Cu(OTf)2 (72.4 mg, 0.2 mmol), Ag2O (46.4 mg,
0.2 mmol), and freshly distilled toluene (4 mL) were
Scheme 2. Proposed mechanism of the reaction. L=ligand.
added to
a dry Schlenk tube. The acetanilide 1
(0.2 mmol) was then added, followed by the boronic
acid 2 (0.4 mmol). The reaction mixture was heated and stirred for
24 h at 1158C in an oil bath and the progress of the reaction
monitored by thin-layer chromatography. Purification of the crude
mixture by flash chromatography (petroleum ether/ethyl acetate)
gave the desired product 3.
Received: February 8, 2007
Revised: May 10, 2007
Published online: June 20, 2007
product. We hypothesized that the electrophilic ability of the
intermediate 7 from the direct transmetalation was inhibited
by increased electron density on the PdII center with an aryl
group. A second transmetalation with an additional molecule
of boronic acid occurred to form the homocoupling by-
product.[8] Another plausible pathway for this transformation
involves the formation of the key intermediate 6 from the
À
À
Keywords: C C coupling · C H activation ·
electrophilic substitution · palladium· Suzuki–Miyaura reaction
.
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In summary, we have developed an efficient and clean
À
method to construct C C bonds from N-alkyl acetanilides
II
À
and aryl boronic acids catalyzed by a Pd species by direct C
H bond functionalization. This development also offers a new
strategy to construct complicated structures by Suzuki–
Miyaura coupling under a halogen-free process. Further
applications of this transformation to organic syntheses is
underway in our group.
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