Non-coordinating-Anion-Directed Reversal of Activation Site: Selective C?H Bond Activation of N-Aryl Rings

Article abstract of DOI:10.1002/chem.201600293

An Rh-catalyzed selective C?H bond activation of diaryl-substituted anilides is described. In an attempt to achieve C?H activation of C-aryl rings, we unexpectedly obtained an N-aryl ring product under non-coordinating anion conditions, whereas the C-aryl

Full text of DOI:10.1002/chem.201600293

DOI: 10.1002/chem.201600293
Full Paper
&
CÀH Activation
Non-coordinating-Anion-Directed Reversal of Activation Site:
Selective CÀH Bond Activation of N-Aryl Rings
[a]
[a]
[b]
[a]
[a]
[a]
Dawei Wang,* Xiaoli Yu, Xiang Xu, Bingyang Ge, Xiaoli Wang, and Yaxuan Zhang
Abstract: An Rh-catalyzed selective CÀH bond activation of
diaryl-substituted anilides is described. In an attempt to ach-
ieve CÀH activation of C-aryl rings, we unexpectedly ob-
tained an N-aryl ring product under non-coordinating anion
conditions, whereas the C-aryl ring product was obtained in
the absence of a non-coordinating anion. This methodology
has proved to be an excellent means of tuning and adjust-
ing selective CÀH bond activation of C-aryl and N-aryl rings.
The approach has been rationalized by mechanistic studies
and theoretical calculations. In addition, it has been found
and verified that the catalytic activity of the rhodium catalyst
is obviously improved by non-coordinating anions, which
provides an efficient strategy for obtaining a highly chemo-
selective catalyst. Mechanistic experiments also unequivocal-
ly ruled out the possibility of a so-called “silver effect” in this
transformation involving silver.
The motivation for chemists to develop new methods is to
find effective solutions for challenging chemical transforma-
tions, particularly those with good chemo-, regio-, and stereo-
[
1]
selectivities. Among these selective transformations, CÀH
bond activation has attracted considerable attention due to its
unique atom-economical properties and circumvention of the
[2]
need for functionalized starting materials. Anilides are effec-
tive as directing groups for CÀH activation and therefore have
been the focus of much attention in recent decades. In 2002,
de Vries and van Leeuwen developed Pd-catalyzed ortho CÀH
bond activation of anilides with benzoquinone (BG) as an oxi-
[
3]
dant (Scheme 1). Liu and Guo screened and improved the
[4]
same reaction with oxygen as oxidant. Shi demonstrated
highly regioselective halogenation and arylation of acetanilide
[5]
through palladium-mediated CÀH functionalization. Recently,
Ackermann and co-workers reported Ru-catalyzed alkenyla-
[6]
tions of anilides and benzamides in water. Related research
[
7]
[8]
[9]
[10]
[11]
has been reported by Yu, Ma, Chang, Fagnou, Glorius,
Scheme 1. Non-coordinating-anion-directed selective CÀH activation with
[12]
[13]
[14]
[15]
[16]
Cui,
Ackermann,
Shi,
Ge,
and many others
anilide as a directing group.
(
Scheme 1A). In 2010, the Rovis group developed Rh-catalyzed
oxidative cycloaddition of benzamides and alkynes through
ortho CÀH activation in the absence of a silver salt. Their work
provides a good example of functionalization of the C-aryl ring
scribed activation of the ortho CÀH bonds of C-aryl rings to
[
18]
give the vinylation products. Chatani et al. reported Ni-cata-
lyzed oxidative cycloaddition of aromatic amides with al-
[17]
rather than the N-aryl ring of anilides. Li and co-workers de-
[
19]
kynes. Tanaka and co-workers developed the oxidative annu-
lation of Ac-protected anilides with internal alkynes using a di-
nuclear electron-deficient rhodium complex as catalyst. Later,
they found that the same rhodium complex could be applied
to alkene substrates, giving the desired products in moderated
[
a] Prof. Dr. D. Wang, X. Yu, B. Ge, X. Wang, Y. Zhang
The Key Laboratory of Food Colloids and Biotechnology
Ministry of Education, School of Chemical and Material Engineering Jian-
gnan University, Wuxi 214122 (China)
[
20]
E-mail: wangdw@jiangnan.edu.cn
yields at mild temperatures.
volved five- or six-membered ring transition states
Scheme 1B), which led to N-aryl or C-aryl ring activation prod-
These examples typically in-
[
b] Prof. Dr. X. Xu
College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural
University
(
ucts. However, to the best of our knowledge, there is no gen-
eral method for obtaining one or other of the products in
Qingdao 266109 (China)
Supporting information for this article can be found under
http://dx.doi.org/10.1002/chem.201600293.
[
21]
a controlled manner.
Chem. Eur. J. 2016, 22, 8663 – 8668
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The chemistry of non-coordinating anions (NCAs) and very
weakly coordinating anions (WCAs) is of fundamental impor-
[a,b]
Table 1. Screening of reaction conditions
[22]
tance in many areas, yet is often overlooked. The application
of NCAs and WCAs, such as in gold-catalyzed, silver-mediated,
and hydrogen-bond related reactions, has attracted much at-
tention. Indeed, many new chemical reactions rely on the assis-
[23]
tance of NCAs and WCAs, and furthering their application is
a worthwhile goal. In the present work, we have found that
NCAs play a key role in the selective CÀH bond activation of
anilides (Scheme 1C).
Entry
Catalyst
Oxidant
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
[{Rh(cod)Cl}
[{Cp*RhCl
Pd(OAc)
none
2
]
]
K
K
K
2
2
2
S
S
S
2
2
2
O
O
O
8
8
8
8
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
THF
18
35
<5
<5
27
11
57
68
85
30
21
8
<5
15
24
9
<5
<5
<5
29
20
<5
38
2
}
2
2
Our previous research was mainly focused on the competi-
2 2
K S O
[
24]
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
p-quinone
Na Cr
Ag
AgOAc
Ag CO
CuCl
Cu(OAc)
CuSO
tive reactions of CÀH bond activation and CÀF activation.
2
2 7
O
Our results indicated that the selective CÀH bond activation of
2
O
[
25]
ferrocenyl anilides is an important issue (Scheme 2). When
2
3
10
1
1
2
1
1
1
1
1
1
1
1
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
4
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
Ag
air
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
3
3
3
dioxane
DMF
DME
H
2
O
toluene
benzene
DMSO
DMA
MeOH
DCE
Scheme 2. Selective CÀH activation by a non-coordinating anion.
[
(
a] Conditions: 1a (0.5 mmol, 1.0 equiv), 2a (0.75 mmol, 1.5 equiv), [Rh]
5 mol%), AgOTf (20 mol%), Ag CO (1.5 equiv), DCE (3 mL), 24 h. [b] Iso-
2
3
ferrocenyl anilide (1a) was coupled with methyl acrylate (2a),
the N-aryl ring activation product 3a was obtained in 85%
yield in the presence of AgOTf, whereas the C-aryl ring activa-
tion product 3a’ was obtained in 42% yield in the absence of
AgOTf (Scheme 2). Herein, we report oxidative coupling reac-
tions of anilides with alkenes through Rh-catalyzed selective
CÀH bond activation. This methodology has proved to be an
excellent means of selectively activating N-aryl rings rather
than C-aryl rings, and has been supported by mechanistic stud-
ies and theoretical calculations.
lated yields.
AgOTf as an additive (Table 2). The reactions were carried out
in DCE at 908C under air. As illustrated in Table 2, high yields
were achieved with nearly all of the substrates. Very interest-
ingly, reaction occurred at the C-aryl ring when there were two
substituents on the N-aryl ring (3o), which may be attributed
to a steric effect. This suggested an alternative method for se-
We first carried out the oxidative coupling reaction of ferro-
cenyl anilide (1a) with methyl acrylate (2a) in order to deter-
mine the CÀH bond activation induced by the rhodium cata-
lyst. The desired product was obtained in only 18% yield, but
this nevertheless implied the feasibility of the selective CÀH
bond activation of anilides. Next, the reaction conditions were
screened to maximize the yield, and the results are shown in
Table 1. The effects of different catalysts and oxidants on the
reactivity were studied. With [{Cp*RhCl } ] as catalyst and
[a]
Table 2. Substrate scope of ferrocenyl amides.
2
2
Ag CO3 as oxidant in 1,2-dichloroethane (DCE), the product
2
was obtained in 85% yield (Table 1, entry 9). The reaction
proved to be highly dependent on the solvent, and the best
results were achieved in DCE. A blank experiment showed that
no reaction occurred in the absence of a rhodium catalyst
(
Table 1, entry 4). When the reaction was carried out with air as
oxidant, only a low yield of the product was obtained (Table 1,
entry 23).
[
[
1
a] Conditions:
{Cp*RhCl ] (5 mol%), AgOTf (20 mol%), Ag
2 h.
1
(0.5 mmol, 1.0 equiv),
2
CO
(0.75 mmol, 1.5 equiv),
(1.5 equiv), DCE (3 mL),
2
}
2
2
3
Having established the optimal conditions, we explored the
scope of the Rh-catalyzed oxidative coupling reaction with
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Next, we consider expansion of the reaction scope
in terms of benzamide substrates (Table 3). Generally,
all of the substrates were converted into the desired
products. Moderate to good yields were obtained in
most cases. Notably, a substrate bearing a hydroxyl
group gave the desired product in 54% yield (5h,
Table 3).
Moreover, we explored the Rh-catalyzed selective
CÀH bond activation of anilides in the absence of
a silver salt (see the Supporting Information for de-
tails). Gratifyingly, the C-aryl ring activation products
5
(
’ were obtained in good yields in most cases
Table 4). This methodology thus offers a convenient
route to C-aryl ring products with excellent chemose-
Scheme 3. Deuteration experiments.
[
18a]
lectively activating the C-aryl rings. Indeed, with this methodol-
ogy in hand, both C-aryl ring products and N-aryl ring products
were obtained in high yields.
lectivity and yields; it is supported by Li’s results,
different substrates.
albeit with
Although NCA-directed reversal of the activation site has
been overlooked in previous selective CÀH bond activation re-
actions, we decided to seek evidence for this in the relevant lit-
erature reports. Tanaka developed the oxidative annulation of
anilides with internal alkynes, and obtained both C-aryl and N-
aryl ring activation with different catalysts, although one exam-
To further test this result, deuterated anilide d -4a was sub-
jected to the reaction, whereupon d -5a was obtained in 37%
yield as the sole product, as determined by NMR (see the Sup-
5
4
porting Information for details). It should be noted that the C-
aryl ring activation product d -5a was not observed in this
transformation (Scheme 3A). Furthermore, this cationic rhodi-
5
[
20]
ple with low yield was found, which provides a power proof
to support CÀH activation of N-aryl ring. Li and co-workers re-
ported the activation of ortho CÀH bonds on C-aryl rings to
um complex could lead to selective ortho H/D exchange on
the N-aryl ring when D O was added to the reaction mixture
2
[
18a]
instead of ethyl acrylate (Scheme 3B).
give vinylation products,
further demonstrating the prefer-
As illustrated by the reaction profiles (Figure 1), the reaction
rate and yield were both strongly affected by the presence of
silver salts. Higher yield and shorter reaction time were ob-
tained when AgOTf was added to the mixture, whereas other
silver salts, such as AgBF and AgSbF , gave only moderate
ence for this process in the absence of non-coordinating
anions. Other studies, such as those involving Ac-protected
[
6,11a,16b,i,17]
anilides,
could give part of the supporting to CÀH ac-
[10]
tivation of N-aryl ring.
In order to gain a better understanding of this transforma-
tion, competition experiments to assess the electronic effect
were conducted. It was found that an electron-rich amide was
preferentially consumed, implying that substrates bearing an
4
6
yields. It should be noted that the C-aryl ring activation prod-
uct was obtained in the absence of a silver salt, which is con-
[18]
[11]
sistent with the results of Li and Glorius. These experi-
ments implied that the silver salt plays a decisive and crucial
role in this transformation.
[a]
Table 3. Substrate scope experiments of N-aryl ring activation.
[
[
1
a] Conditions:
{Cp*RhCl ] (5 mol%), AgOTf (20 mol%), Ag
2 h.
4
(0.5 mmol, 1.0 equiv),
2
CO
(0.75 mmol, 1.5 equiv),
(1.5 equiv), DCE (3 mL),
2
}
2
2
3
Figure 1. Reaction process profile.
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[
a]
Table 4. Substrate scope experiments of C-aryl ring activation.
[
2 2 2 3 3
a] Conditions: 4 (0.5 mmol, 1.0 equiv), 2 (0.75 mmol, 1.5 equiv), [{Cp*RhCl } ] (5 mol%), Ag CO (1.5 equiv), CH CN (3 mL), 12 h.
electron-donating group react more readily than those bearing
an electron-withdrawing group (Scheme 4A). Since the elec-
tronic effect has a great influence on the reaction, it may also
govern the selectivity for reaction at the N-aryl ring. To test
this hypothesis, two classical substrates were prepared (Sche-
me 4B,C). However, only N-aryl ring products were obtained
under the above conditions for both reactions, seemingly dis-
favoring the hypothesis.
fore, a control reaction was performed in the presence of one
equivalent of TEMPO. The corresponding product was ob-
tained in almost the same yield, indicating that no radical in-
termediate was involved in the transformation (Scheme 5A).
Next, kinetic isotope effect (KIE) experiments were conducted
to elucidate the mechanism (Scheme 5B). When a 1:1 mixture
of 4a and d -4a was subjected to the reaction, an intermolecu-
5
lar KIE of K /K =4.32 was determined. This indicated that CÀH
H
D
2
,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) is widely used
bond cleavage was involved in the rate-determining step and
further confirmed that the reaction mechanism was a CÀH acti-
vation process.
as a radical scavenger to probe reaction mechanisms. There-
In addition,
a
computational study was performed
(
Scheme 6). Two structures were fully optimized using the
[
26]
B3LYP functional within the Gaussian 03 program, and vibra-
tional frequencies were calculated to confirm that they were
stable minima on the potential energy surface. For C, O, N, and
H, the 6-311+ +G(d, p) basis set was used; for Rh, the
Lanl2DZ basis set with effective core potential (ECP) was used.
Intermediate
C was calculated to be lower in energy
Scheme 4. Intermolecular competition and electronic effect studies.
Scheme 5. Control experiment and isotope effect experiment.
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Scheme 6. The two possible structures of intermediates.
(
À1591.111355 hartree), and hence more stable, than inter-
mediate C’ (À1590.531387 hartree). All energies were corrected
for zero-point vibrational energies (ZPE), and the optimized
structure of C is shown in Scheme 6. This clearly provided
a direct corroboration of the observed selective CÀH bond ac-
tivation of N-aryl rings over C-aryl rings adjacent to an amide
functional group in this system.
Scheme 8. The proposed possible mechanism.
Furthermore, we determined the Hammett correlation for
the process, which is a very useful method in mechanistic stud-
ies. We selected several typical substrates to build the equa-
tion, and the plot is shown in Scheme 7. The results showed
that electron-rich substrates reacted more rapidly, indicating
that the rate-determining transition state is more stabilized by
electron-donating substituents.
volving silver salts can actually be classified as bimetallic catal-
ysis or silver-assisted metal catalysis. Silver nanoparticles or
silver chloride are typically invoked in this context. To assess
whether such an effect might be operative in the present case,
a series of “silver effect” experiments was conducted. It was
found that the C-aryl ring activation product was obtained
with only [{Cp*RhCl } ] as the catalyst, whereas the N-aryl ring
2
2
activation product was obtained when a silver salt was added
to the mixture. After removing silver chloride by filtration
III
through Celite, the new catalyst [Cp*Rh OTf ] showed almost
2
the same catalytic effect and yield for this transformation
(
Table 5, entries 1, 6, and 7). These investigations indicated
that: i) non-coordinating anions greatly improve the catalytic
activity of a rhodium catalyst, effectively rendering it chemose-
lective; ii) silver does not play a role and the active catalyst is
only rhodium in this transformation.
In summary, Rh-catalyzed chemoselective CÀH bond activa-
tion of anilides with alkenes has been realized by introducing
non-coordinating anions. This methodology provides an effi-
cient means of selectively achieving N-aryl or C-aryl ring activa-
tion. Mechanistic studies and theoretical calculations have
[a,b]
Table 5. “Silver effect” test experiments
Scheme 7. Hammett plot for various anilides.
Taking together the above mechanistic evidence, a possible
[b]
Entry
Catalyst
Conditions
5a:5a’[%]
reaction pathway for this transformation may be proposed
1
2
3
4
5
6
7
8
9
[{Cp*RhCl
[{Cp*RhCl
none
2
}
}
2
]
]
–
–
–
–
–
0:58
0:93
0:0
0:0
III
(
Scheme 8). Initially, the substrate coordinates with Rh to give
[c]
2
2
III
the complex B. Next, ortho electrophilic attack by the Rh
cation with the assistance of the amide group forms complex
C under basic conditions, and subsequent insertion leads to in-
termediate D. Through b-H elimination, intermediate D releas-
AgOTf
Ag
2
CO
3
0:0
[{Cp*RhCl
[{Cp*RhCl
[{Cp*RhCl
2
2
2
}
}
}
2
]/AgOTf
2
]/AgOTf
2
]/AgSbF
no filtration
after filtration
no filtration
after filtration
no filtration
after filtration
88:0
85:0
35:0
31:0
48:0
45:0
I
III
es the coupling product. The Rh cation is oxidized back to Rh
6
6
I
[{Cp*RhCl } ]/AgSbF
by Ag to complete the catalytic cycle and generate the cata-
lyst A.
2
2
2
2
2
2
1
11
0
[{Cp*RhCl
[{Cp*RhCl
}
}
]/AgBF
]/AgBF
4
4
Recently, Shi et al. investigated the so-called “silver effect” in
gold catalysis, which is another frequently overlooked impor-
[
[
a] Optimal conditions. [b] AgCl was removed by filtration through Celite.
c] CH CN was used.
[27]
3
tant factor in transition metal catalysis. Many reactions in-
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shown that lower energy is needed to activate N-aryl rings
than C-aryl rings under non-coordinating anion conditions. Fur-
ther mechanistic studies and experiments with silver salts have
clearly indicated that the activity of the rhodium catalyst is ob-
viously improved by non-coordinating anions, which rules out
the possibility of a “silver effect” in this silver-mediated reac-
tion.
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Typical procedure for the reaction of an anilide and an
alkene
A mixture of ferrocenyl anilide (1a) (0.50 mmol), methyl acrylate
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(
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2
2
Ag CO (1.5 equiv) in DCE (3.0 mL) was stirred at 908C under air for
1
trated to dryness. The residue was purified by column chromatog-
raphy on silica gel (ethyl acetate/petroleum ether, 1:5) to afford
the corresponding product (3a) as a white solid.
2
3
2 h. Upon completion of the reaction, the mixture was concen-
7
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We gratefully acknowledge financial support of this work by
the National Natural Science Foundation of China (21401080),
the Natural Science Foundation of Jiangsu Province
[
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BK20130125), and MOE & SAFEA for the 111 Project (B13025).
[
Keywords: CÀH activation
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chemoselectivity
·
non-
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Received: January 21, 2016
Published online on May 9, 2016
Chem. Eur. J. 2016, 22, 8663 – 8668
www.chemeurj.org
8668
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