2-Hydroxy-1,10-phenanthroline vs 1,10-Phenanthroline: Significant ligand acceleration effects in the Palladium-Catalyzed Oxidative Heck reaction of arenes

Article abstract of DOI:10.1021/ol4033804

A series of bidentate monoanionic nitrogen ligands were designed and applied in the Pd-catalyzed oxidative Heck reaction of arenes with alkenes. Significant ligand-accelerated effects were observed, and direct C-H functionalized products were formed in high yields with meta-selectivity.

Full text of DOI:10.1021/ol4033804

Letter
pubs.acs.org/OrgLett
2‑Hydroxy-1,10-phenanthroline vs 1,10-Phenanthroline: Significant
Ligand Acceleration Effects in the Palladium-Catalyzed Oxidative
Heck Reaction of Arenes
Cheng-Hao Ying, Shao-Bai Yan, and Wei-Liang Duan*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
LingLing Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A series of bidentate monoanionic nitrogen ligands
were designed and applied in the Pd-catalyzed oxidative Heck
reaction of arenes with alkenes. Significant ligand-accelerated effects
were observed, and direct C−H functionalized products were formed
in high yields with meta-selectivity.
Scheme 1. Basic Concept for Designing Bidentate
Monoanonic Nitrogen Ligands in the C−H Activation
Reactions of Arenes
igand-accelerated catalysis is widely applied in various
1
Lreactions, such as epoxidation, addition reactions, etc. In
such processes, efficiency and selectivity can be adjusted and
controlled by appropriate ligand selection. By contrast, only a
few suitable ligands can be utilized in palladium-catalyzed C−H
functionalization reactions,2−4 particularly for substrates with-
out chelating groups. Usually, Pd(OAc)2 or other simple
palladium salts are the most widely used catalysts in such
processes, and the addition of nitrogen ligands inhibits the
catalytic activity of palladium catalyst. Only recently, Yu et al.
and Sanford et al. reported that the use of monodentate
pyridine-type ligands can increase the activity of the palladium
catalysts in the C−H bond functionalization reactions of simple
arenes.5,6 Palladium ligated only to one nitrogen ligand is also
reportedly important to the success of C−H activation
reactions. Despite such impressive progress, the development
of new ligands suitable for Pd-catalyzed C−H activation
reactions remains challenging.
site can be generated with relative ease and may be able to act
as a catalyst for the C−H bond activation reaction (Scheme 1,
eq 2). In the present work, we design and apply bidentate
monoanionic nitrogen ligands for the Pd-catalyzed oxidative
Heck reaction of simple arenes.
Fujiwara and Moritani reported the first Pd-catalyzed
oxidative Heck reaction of simple arenes. Since then, major
progress had been made.9 However, most studies are still
limited to electron-rich arenes.10,11 A simple Pd salt without a
ligand is inefficient for the direct olefination of electron-
deficient arenes. Hence, we initially examined the oxidative
Heck reactions of benzene and fluorobenzene with n-butyl
acrylate in the presence of Pd(OAc)2 and a series of nitrogen
ligands (4−10). For the benzene substrate, the use of palladium
acetate without ligand only afforded the product in 10% yield in
the presence of Cu(OAc)2 as oxidant (Table 1, entry 1).
Interestingly, the addition of ligand 4, 2-OH-1,10-phen,
significantly increased the product yield to 97% (entry 2). In
contrast, the use of 1,10-phen totally inhibited the progress of
reaction with no formation of the desired product (entry 3). A
Bidentate nitrogen ligands such as 1,10-phenanthroline
(1,10-phen) are seldom used in the palladium-catalyzed C−H
activation reactions,6 although it is a widely useful ligand for
catalysis and for the preparation of various metal complexes.7
1,10-Phen coordinates with palladium acetate with square
planar geometry, in which a Pd atom bonds covalently with two
acetate anions.8 We hypothesize that the inability of Pd/1,10-
phen for C−H activation reaction may be due to the difficult
dissociation of the acetate anion from the Pd atom to generate
a vacant coordination site for an incoming simple arene (low
coordinate ability without strong chelating groups) (Scheme 1,
eq 1).6 Based on this hypothesis, we envisioned that
introducing a hydroxyl group at the 2-position of 1,10-phen
to give a bidentate monoanionic nitrogen ligand 2-OH-1,10-
phen would solve this problem. The complex of 2-OH-1,10-
phen with Pd(OAc)2 only has one acetate anion on the Pd
atom. Given that the binding ability of acetic anion to Pd is not
strong, a palladium intermediate having a vacant coordination
Received: November 21, 2013
© XXXX American Chemical Society
A
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Letter
(entry 11). Ligand 10 bearing a methyl group on the oxygen
atom compared with ligand 4 only afforded the product in low
yield (entry 12). This result seems to suggest the importance of
the bidentate monoanionic framework of the ligand 4. For
comparison, monopyridine ligands 115a and 129f were
examined under the current reaction conditions. Both of
them afforded the ortho-functionalized compound preferen-
tially (entries 13 and 14). Notably, Boc-protected L-valine
featured as the L-X type ligand to transition metals4 was
demonstrated to afford the desired product in moderate yield,
also with ortho-selectivity (entry 15). Using ligand 4/Pd(OAc)2
under Yu’s conditions5a for the reaction of trifluorotoluene with
2a did not produce the corresponding Heck product (entry
Table 1. Palladium-Catalyzed Oxidative-Heck Reaction of
Arenes with n-Butyl Acrylate
a
b
yield (%)/(ratio of m/p/o-
c
entry substrate
ligand
oxidant
isomers)
1
2
3
4
5
6
7
8
9
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
10
97
0
Table 2. Palladium-Catalyzed Oxidative-Heck Reaction of
Arenes with Alkenes
4
a
1,10-Phen
6
d,e
4
93 (4.9/1/1.6)
0
1,10-Phen
5
41 (3.7/1/1.2)
0
6
7
45 (3.6/1.3/1)
0
10
11
12
13
14
15
8
e
9
52 (1/0.9/5.8)
19 (2.1/1/3.2)
17 (1/2.4/6)
47 (1/1.9/7.5)
63 (1/3.0/6.5)
e
e
e
e
10
11
12
Boc-L-
valine
f
16
17
18
19
20
21
22
23
24
1c
1b
1b
1b
1b
1a
1a
1a
1a
4
4
4
4
4
4
4
4
4
O2
0
g
h
i
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
BQ
0
69 (4.6/1/1.3)
28 (3.7/1/1.9)
j
26 (5/1/3.2)
1
4
3
0
Tempo
Oxone
CuCl2
a
All the reactions were conducted with 0.20 mmol of alkenes in 3.0
b
1
mL of arenes. Yields were determined by H NMR analysis of the
c
crude product using CH2Br2 as the internal standard. The ratio of m/
p/o-isomers was determined by 1H NMR analysis of the crude
d
e
product. Isolated yield. The ratio of m/p/o-isomers was determined
f
by 19F NMR analysis of the crude product. Reaction conditions: 1 mL
of trifluorotoluene, 0.20 mmol of 2a, and 0.20 mmol of Ac2O in the
presence of 10 mol % of Pd(OAc)2 and 20 mol % of 4 under 1 atm of
g
h
O2 at 120 C, 15 h. Without Pd(OAc)2. 5 mol % Pd(OAc)2 used.
i
j
The reaction was conducted at 100 °C. 10 mol % of PdCl2 used.
similar ligand-accelerated effect was also observed for the
reaction of fluorobenzene with n-butyl acrylate (entries 4−6),
and a meta-product was preferentially formed. The more
flexible 2-OH-1,1′-bipyridine 5 also exhibited moderate
catalytic activity (entry 7). By contrast, ligand 612 that contains
two hydroxyl groups at the 2,9-positions of 1,10-phen did not
catalyze this reaction (entry 8). Moreover, the easy-to-prepare
ligand 7 bearing an acetyl group derived from 8-amino
quinoline skeleton was also found to exhibit moderate
accelerating activity (entry 9). However, ligand 8 that contains
a benzoyl moiety was not effective (entry 10). In addition,
FMOC-protected amino-oxazoline-based bidentate ligand 913
also exhibits certain catalytic activity, but with ortho-selectivity
a
The reactions were conducted with 0.20 mmol of alkenes in 3.0 mL
b
c
of arenes unless otherwise specified. Isolated yield. The ratio of m/
p/o-isomers was determined by 1H or 19F NMR analysis of the isolated
d
e
f
product. 10 mol % of ligand 7 used. The ratio of β/α-isomers. The
ratio of β/α/α′-isomers. 1.0 mL of toluene used. 1 mL of anisole
and 1 mL of EtOAc used. 0.20 mmol of N-methylindole, 0.60 mmol
g
h
i
j
k
of 2a in 1 mL of dioxane. Only the indole 3-isomer was formed. 1
l
mL of 2-methylthiophene was used. Only the β-isomer was formed.
m
Only the meta-isomer was formed.
B
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Letter
16). These results indicated that ligand 4 possesses some
particular properties for the current reaction with regard to the
catalytic activity and regioselectivity. Finally, the screening of
various oxidants, palladium precursors, temperature, and metal-
to-ligand ratio revealed that 10 mol % Pd(OAc)2 and 4 with 1
equiv of Cu(OAc)2 at 120 °C were the optimum conditions
(entries 17−24).
Scheme 2. Experiments with Isotopically Labeled
Compounds
With the optimum conditions known, the substrate scope
was then examined, and the results are shown in Table 2.
Substrates having various electron-withdrawing groups, such as
nitro-, ester-, nitrile-, chloro-, and bromo-moieties, can be
tolerated to afford the corresponding products with good yields
(Table 2, entries 1−11). With respect to the alkene
counterparts, the acrylate, aryl-substituted ethylene also reacted
with fluorobenzene smoothly, thereby furnishing the corre-
sponding alkenes with good yields (entries 12−16). Fur-
thermore, ethylcinnamic acid ester and chalcone containing
substituted groups at the β-position can also be used in this
protocol, furnishing β,β-diaryl-substituted acrylates or enones
in moderate yields (entries 17 and 18). Electron-rich arenes
have also been examined under the current conditions (entries
19−24). In sharp contrast to the reported system,9d,e the Heck
products of toluene and anisole were obtained also with meta-
selectivity (entries 19 and 20), which have not been reported to
date. However, N-methylindole and 2-methylthiophene were
functionalized at more electron-rich positions (entries 22 and
23). Notably, 1,3-dimethoxybenzene and pyridine6a were found
to afford single regioisomers in moderate yields (entries 24 and
25). Molecular oxygen is regarded as an ideal oxidant in
catalysis that enables a cleaner and more economical process.14
Therefore, in the current catalytic system, oxygen (1 atm) was
examined as an oxidant in the presence of 20 mol % of
Cu(OAc)2. An array of arenes can also be efficiently
functionalized with alkenes with good yields (Table 3). It is
palladium complex of ligand 4 with Pd(OAc)2 were
unsuccessful due to the very low solubility of the formed
product in various solvents.16 Alternatively, the ligand 7 reacted
with palladium acetate smoothly, affording a palladium complex
existing in dimeric form.17 The use of the isolated complex for
the reaction of fluorobenzene with n-butyl acrylate afforded the
product in 73% yield (Scheme 3), which is higher than the yield
Scheme 3. Preparation of a Palladium Complex of Ligand 7
and Use in the Oxidative-Heck Reaction
obtained through in situ method. Competition experiments
were also conducted between arenes having different electronic
densities. Electron-rich arenes such as anisole or electron-poor
arenes such as fluorobenzene, were found to both have lower
reactivities than benzene (Scheme 4). Such trends in the
Table 3. Palladium-Catalyzed Oxidative-Heck Reaction of
Arenes with n-Butyl Acrylate Using Oxygen as the Oxidant
Scheme 4. Intermolecular Competition Experiments with n-
Butyl Acrylate
a
b
entry
R
R′
yield (%)/(ratio of m/p/o-isomers)
1
2
3
4
5
6
CF3
CO2-n-Bu
CO2-n-Bu
CO2-n-Bu
CO2-n-Bu
CO2-n-Bu
CO2-n-Bu
81 (1.5/1/0)
72 (5.5/2.5/1)
86 (2/1/2.6)
61 (1.7/1/1)
73 (2.1/1/1.3)
80 (4.3/1.6/1)
CO2Me
C(O)Me
NO2
CN
Br
a
b
Isolated yield. The ratio of m/p/o-isomers was determined by 1H or
19F NMR analysis of the isolated product.
worth mentioning that regioselectivities with catalytic Cu-
(OAc)2 are different from that with stoichimetric Cu(OAc)2.
This observation may indicate that different oxidants or
additives could be examined for further increasing the reaction
selectivity.
To gain further insight into the reaction mechanism in this
catalytic system, the reaction of deuterated benzene with n-
butyl acrylate was carried out to determine the kinetic isotope
effect under standard conditions. A KH/KD ratio of 2.5 was
observed (Scheme 2, see the Supporting Information), and this
value indicated that the C−H bond cleavage was involved in
the rate-limiting step.15 Attempts to identify the structure of
reactivity of arenes could not be explained solely by concerted
metalation-deprotonation (CMD)18 or electrophilic aromatic
substitution (SEAr) pathways,19 and the mechanism for the
current system remains unclear.
In summary, we have designed a series of bidentate
monoanionic nitrogen ligands and used them in the Pd-
catalyzed oxidative Heck reaction of simple arenes with mainly
meta-selectivity in high yields. The characteristics of Pd catalyst
coordinated with bidentate monoanionic ligands (i.e., having
readily available vacant coordinated sites for incoming
substrates) are the key factor affecting the success of C−H
activation reactions. Attempts to further understand the
C
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Letter
Angew. Chem., Int. Ed. 2011, 50, 9409. (c) Kubota, A.; Emmert, M. H.;
Sanford, M. S. Org. Lett. 2012, 14, 1760.
reaction mechanism and applications of this catalytic system in
other reactions are ongoing.
(6) For examples using 1,10-phenanthroline as the ligand in the Pd-
catalyzed C−H activation reaction of pyridines, see: (a) Ye, M.; Gao,
G.-L.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 6964. (b) Ye, M.; Gao, G.-
L.; Edmunds, A. J. F.; Worthington, P. A.; Morris, J. A.; Yu, J.-Q. J. Am.
Chem. Soc. 2011, 133, 19090. (c) Liu, B.; Huang, Y.; Lan, J.; Song, F.;
You, J. Chem. Sci. 2013, 4, 2163. (d) Sharma, U.; Naveen, T.; Maji, A.;
Manna, S.; Maiti, D. Angew. Chem., Int. Ed. 2013, 52, 12669. (e) Dai,
F.; Gui, Q.; Liu, J.; Yang, Z.; Chen, X.; Guo, R.; Tan, Z. Chem.
Commun. 2013, 49, 4634.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization of the products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
(7) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994, 33,
497.
(8) (a) Milani, B.; Alessio, E.; Mestroni, G.; Sommazzi, A.; Garbassi,
F.; Zangrando, E.; Bresciani-Pahora, N.; Randaccio, L. J. Chem. Soc.,
Dalton Trans. 1994, 1903. (b) Ragaini, F.; Gasperini, M.; Cenini, S.;
Arnera, L.; Caselli, A.; Macchi, P.; Casati, N. Chem.Eur. J. 2009, 15,
8064.
*E-mail: wlduan@mail.sioc.ac.cn.
Notes
The authors declare no competing financial interest.
(9) (a) Moritani, I.; Fujiwara, Y. Tetrahedron Lett. 1967, 8, 1119.
(b) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y.
Science 2000, 287, 1992. (c) Jia, C.; Lu, W.; Kitamura, T.; Fujiwara, Y.
Org. Lett. 1999, 1, 2097. (d) Yokota, T.; Tani, M.; Sakaguchi, S.; Ishii,
Y. J. Am. Chem. Soc. 2003, 125, 1476. (e) Dams, M.; Vos, D. E.; De
Celen, S.; Jacobs, P. A. Angew. Chem., Int. Ed. 2003, 42, 3512.
(f) Ferreira, E. M.; Stoltz, B. M. J. Am. Chem. Soc. 2003, 125, 9578.
(g) Cai, G.; Fu, Y.; Li, Y.; Wan, X.; Shi, Z. J. Am. Chem. Soc. 2007, 129,
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ACKNOWLEDGMENTS
■
This work was financially supported by the National Basic
Research Program of China (973 Program 2010CB833300),
NSFC (20902099, 21172238), and SIOC. We thank Dr. Hao-
Yang Wang for helping with MS analysis of the palladium
complexes.
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