Rhodium-catalyzed direct oxidative carbonylation of aromatic C#H bond with CO and alcohols

Article abstract of DOI:10.1021/ja807167y

A general protocol for the rhodium-catalyzed oxidative carbonylation of arenes to form esters has been developed. A broad substrate scope has been demonstrated allowing carbonylation of electron-rich, electron-poor, and heterocyclic arenes, and the reacti

Full text of DOI:10.1021/ja807167y

Published on Web 12/19/2008
Rhodium-Catalyzed Direct Oxidative Carbonylation of
Aromatic C-H Bond with CO and Alcohols
Zheng-Hui Guan,†,‡ Zhi-Hui Ren,† Stephen M. Spinella,‡ Shichao Yu,‡
Yong-Min Liang,*,† and Xumu Zhang*,‡
Department of Chemistry and Chemical Biology and Department of Pharmaceutical Chemistry,
Rutgers, The State UniVersity of New Jersey, Piscataway, New Jersey 08854-8066, and State
Key Laboratory of Applied Organic Chemistry, Lanzhou UniVersity, Lanzhou 730000,
People’s Republic of China
Received September 10, 2008; E-mail: xumu@rci.rutgers.edu
Abstract: A general protocol for the rhodium-catalyzed oxidative carbonylation of arenes to form esters
has been developed. A broad substrate scope has been demonstrated allowing carbonylation of electron-
rich, electron-poor, and heterocyclic arenes, and the reaction shows wide functional group tolerance and
excellent regioselectivities. Up to 96% yield of ortho-substituted aryl or heteroaryl carboxylic esters were
obtained with this methodology. The possible mechanism for the rhodium-catalyzed oxidative carbonylation
reaction was proposed in this article. Studies show that Oxone play an important role in the transformation.
Scheme 1. Transition Metal Catalyzed Carbonylation Reaction
Introduction
Aryl carboxylic acids and derivatives are valuable commodity
chemicals. Transition metal-catalyzed carbonylation1 of aryl
iodides, bromides and triflates is a well-known method for the
regioselective installation of carbonyl functional groups on
arenes.2 Recent advances have allowed the use of the readily
available aryl chlorides and aryl tosylates,3 but an ideal and
environmentally friendly method to construct aryl carboxylic
acids functional groups would be direct carbonylation of aryl
C-H bonds in regioselective manner (Scheme 1). There are a
few reports of Pd-catalyzed carbonylation of aromatic amines
to form benzolactams, however problems in regiocontrol of the
carbonylation reaction remain challenging.4
Activation5 and functionalization of C-H bonds is one of
the most challenging tasks in organic chemistry.6,7 Recently,
Pd-catalyzed direct functionalizations of aryl C-H to form C-C
and C-heteroatom bonds have been investigated intensively.8
Pd has proved to be one of the most effective catalyst in this
transformation.9-14 However, Pd-catalyzed ortho-selective C-H
bond carbonylation is exceedingly difficult because the depal-
† State Key Laboratory of Applied Organic Chemistry.
‡ Rutgers, The State University of New Jersey.
(1) (a) Schoenberg, A.; Bartoletti, I.; Heck, R. F. J. Org. Chem. 1974,
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Angew. Chem., Int. Ed. 2007, 46, 8460. (c) Cai, C.; Rivera, N. R.;
Balsells, J.; Sidler, R. R.; McWilliams, J. C.; Shultz, C. S.; Sun, Y.
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9
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2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 729–733 729

A R T I C L E S
Guan et al.
Table 1. Optimization of Direct Oxidative Carbonylation of Arenesa
ladation process is often complicated by the reduction of Pd(II)
to Pd(0) under CO atmosphere.15-17 In the past decade, Rh18
and Ru19 complexes have emerged as very effective catalysts
in the activation and functionalization of C-H bonds. And
Rh-CO complexes are the most active species for carbonylation
reactions, such as Monsanto acetic acid process20 and hydro-
formylation of alkenes.21 In connection with Rh and Ru-
catalyzed reductive coupling of aryl C-H bonds with alkenes
or CO/alkenes,22 we envision that it may be possible to conduct
direct oxidative carbonylation using a simple Rh-CO catalyst
under CO atmosphere. Prior to our work, mild, highly efficient
and regioselective carbonylation through direct C-H bond
functionalization with CO and alcohol has not been realized.
In this article, we report an unprecedented protocol for regi-
entry
catalyst
oxidant
solvent
yield (%)b
1
2
3
4
5
6
7
8
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
Cu(OAc)2
BQ
CAN
toluene
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
n-pentanol
DMF
48
ndc
32
K2S2O8
Oxone
Tempo
Oxone
Oxone
Oxone
Oxone
Oxone
Oxone
75
82
ndc
<5
<5
<5
15
9
(11) (a) Kawai, H.; Kobayashi, Y.; Qi, S.; Inoue, Y. Chem. Commun. 2008,
1464. (b) Chen, X.; Li, J.-J.; Hao, X.-S.; Goodhue, C. E.; Yu, J.-Q.
J. Am. Chem. Soc. 2006, 128, 78.
10
11
12
THF
toluene
toluene
d
Pd(OAc)2
<5
<5
d
Ru3(CO)12
(12) (a) Yang, S.; Li, B.; Wan, X.; Shi, Z. J. Am. Chem. Soc. 2007, 129,
6066. (b) Giri, R.; Chen, X.; Yu, J.-Q. Angew. Chem., Int. Ed. 2005,
44, 2112.
a Reaction conditions: 1a (0.1 mmol), n-pentanol (5 equiv), oxidant
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(3 equiv), and catalyst (2 mol %) in solvent (2 mL) under CO (2 atm) at
110 °C for
performed with 5 mol % catalyst.
8
h. b Isolated yield. c Not detected. d Reaction was
oselective Rh-catalyzed oxidative carbonylation to form esters
using a directed C-H bond activation coupled with an
inexpensive and environmentally benign terminal oxidant.
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Results and Discussion
Compounds containing heteroatoms are prevalent in nature.
The syntheses of these compounds have attracted much attention
in industrial and academic research due to desirable biological
and pharmaceutical properties. Our experiment was initially
conducted by treating 2-phenylpyridine 1a with n-pentanol (5
equiv), [Rh(COD)Cl]2 (2 mol %), Cu(OAc)2 (3 equiv) in toluene
under CO (2 atm) (Table 1, entry 1). We were pleased to find
that after 8 h at 110 °C, the reaction resulted in 48% yield of
the carbonylation product 2a. Further experiments showed that
Cu(OAc)2 is not a good oxidant for the C-H activation
carbonylation. We reasoned that the coordination of Cu(II) to
the pyridine moiety might prevent the carbonylation reaction,
which is in agreement with the results reported by Yu.16
Therefore, a variety of other oxidants were screened for better
efficiency in this transformation. To our delight, Oxone
(2KHSO5KHSO4K2SO4) was found to be a particularly effective
terminal oxidant in this Rh-catalyzed direct carbonylation
reaction. Inexpensive, safe, and environmentally benign Oxone
also makes this transformation more practical. After treatment
of 1a (0.1 mmol) with n-pentanol (5 equiv), Oxone (3 equiv),
and [Rh(COD)Cl]2 (2 mol %) in toluene at 110 °C under CO
(2 atm) for 8 h, 2a was obtained in 82% yield (Table 1, entry
5). Other oxidants such as BQ (benzoquinone), CAN (am-
monium cerium (IV) nitrate), K2S2O8, and TEMPO (2, 2, 6,
6-tetramethylpiperidine-N-oxyl radical) are less effective for this
Rh-catalyzed reaction (Table 1, entries 2-4, 6). We have also
tested a number of solvents; toluene was found to be most
effective (Table 1, entries 7-10). Low conversion was observed
when Pd(OAc)2 or Ru3(CO)12 was employed as the catalyst in
the oxidative carbonylation reaction (Table 1, entries 11-12).
Under the optimized conditions, for this direct carbonylation
process, we have explored the substrate scope (Table 2). This
new carbonylation procedure displayed good functional group
tolerance. Arenes with ester, trifluoromethyl, and ether groups
(15) (a) Dupont, J.; Consorti, C. S.; Spenser, J. Chem. ReV. 2005, 105,
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Rhodium-Catalyzed Direct Oxidative Carbonylation
A R T I C L E S
Table 2. Rh-Catalyzed Carbonylation of Aromatic C-H Bondsa
all gave high yields of corresponding esters (Table 2, entries 4,
5, 9-11). An aryl C-F bond was tolerated under the reaction
conditions; 2g and 2h were obtained in high yields and without
any products of C-F bond carbonylation (Table 2, entries 7,
8). For the electronic effects of this transformation, we found
that electron-rich arenes show more reactivity and gave slightly
higher yields than electron-deficient arenes (Table 2, entries 2,
3, 7, 9, and 11). Slightly lower yields were achieved for the
carbonylation reaction of 2-(4-methoxyphenyl)pyridine 1d and
2-(2-methoxyphenyl)pyridine 1e due to partial decomposition
of the substrates in the carbonylation reaction (Table 2, entries
4, 5). Hetero arenes exhibit higher reactivity than arenes. The
carbonylation product 2m was obtained in excellent yield from
2-(thiophen-2-yl)pyridine 1m (Table 2, entry 13). In addition,
the synthesis of ester derived from low molecular weight alcohol
was also achieved, albeit the yield was limited by the boiling
point of the alcohol.3a Ethyl 2-(pyridin-2-yl)-3-(trifluorometh-
yl)benzoate 2j was obtained in moderate yield with 56% of the
starting material 1j recovered (Table 2, entry 10).
Furthermore, different directing groups and different alcohols
were tested for this oxidative carbonylation reaction (Table 3).
Nitrogen heterocycles, such as pyrazole and quinoline, can serve
as efficient directing groups and generate the carbonylation
products in moderate to good yields under the optimal conditions
(Table 3, entries 1, 2). It is interesting to note that the
monocarbonylation products were obtained in all cases from
the corresponding substrates. Even with pyrimidine as the
directing group, the monocarbonylation product 2p was formed
exclusively from 2-p-tolylpyrimidine 1p (Table 3, entry 3). The
steric hindrance of a directing group played an important role
in the transformation. The carbonylation product 2q was
achieved in only 45% yield when 6-methylpyridyl group was
used as the directing group (Table 3, entry 4). Even when the
catalyst loading was doubled, the yield did not improve. It has
been recently reported that acetanilide shows a good reactivity
in Pd-catalyzed C-H activation.10b,12a However, only very low
conversion was observed when acetanilide was employed as
the substrate in this Rh-catalyzed oxidative carbonylation
reaction (Table 3, entry 5). An extensive investigation of the
reaction shows that both steric hindrance and boiling point of
the alcohol played the important role in the transformation.
Ethanol and 2-propanol gave lower yields of the carbonylation
products (Table 3, entries 6, 7). Only very low conversion was
observed when t-BuOH was employed (Table 3, entry 8).
Additionally, phenol exhibited no reactivity under the carbo-
nylation reaction conditions (Table 3, entry 10).
Although the exact mechanism of the reaction remains
unclear, two mechanisms were proposed on the basis of our
own observation and other related studies6a,d,18a for this oxidative
carbonylation reaction (Scheme 2). In one case, first, coordina-
tion of the ortho-directing group and oxidative addition of an
aromatic C-H bond to Rh(I) gives a Rh(III) complex A.23 Next,
insertion of CO in the resulting C-Rh bond to form an
acylrhodacycle intermediate B, followed by coordination of
alcohol to form intermediate C. The acylrhodacycle intermediate
C is assumed to be oxidized by Oxone to give the Rh(III)
complex D (path a) and undergoes a subsequent reductive
elimination to afford the active catalyst species Rh(I) F and
the carbonylation product 2. An alternative mechanism (path
b) is alcoholysis24 of acylrhodium species C to give the Rh-H
a Reaction conditions: all reactions were carried out with 1 (0.1
mmol), n-pentanol (5 equiv), Oxone (3 equiv), and [Rh(COD)Cl]2 (2
mol %) in toluene (2 mL) under CO (2 atm) at 110 °C for 8 h.
b Isolated yield. c Reaction was performed with ethanol (10 equiv).
(23) (a) Boyd, S. E.; Field, L. D.; Partridge, M. G. J. Am. Chem. Soc.
1994, 116, 9492. (b) Rosini, G. P.; Boese, W. T.; Goldman, A. S.
J. Am. Chem. Soc. 1994, 116, 9498.
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A R T I C L E S
Guan et al.
Table 3. Carbonylation of Aromatic C-H Bonds with Different
Scheme 2. Possible Mechanism of Rh-Catalyzed Oxidative
Carbonylation Reaction
Directing Groups and Different Alcoholsa
reaction does not occur at all in the absence of Oxone even
when the stoichiometric [Rh(COD)Cl]2 was used. This observa-
tion indicates that the path b is less likely.
Conclusion
In summary, we have developed a mild and general procedure
for the Rh-catalyzed oxidative carbonylation of arenes and
heteroarenes with carbon monoxide and alcohols. This Rh-
catalyzed oxidative carbonylation reaction shows high regiose-
lectivity and good functional group tolerance. Up to 96% yield
of ortho-substituted aryl or heteroaryl carboxylic esters were
obtained with this methodology. The use of Oxone as an
inexpensive and environmentally benign terminal oxidant makes
this unprecedented transformation attractive in organic synthesis.
A possible mechanism was proposed in this article, and the study
provides a new avenue for the direct carbonylation of aryl C-H
bonds. Current research is focused on extending the scope and
gaining more detailed information on the exact mechanism of
the reaction.
Experimental Section
General Methods. Column chromatography was carried out on
1
silica gel. H NMR spectra were recorded on 500 or 400 MHz in
CDCl3, and 13C NMR spectra were recorded on 125 or 100 MHz
in CDCl3. All new products were further characterized by HRMS;
copies of their H NMR, 13C NMR, and 19F NMR spectra are
provided in the Supporting Information. Unless otherwise stated,
all arenes and solvents were purchased from commercial suppliers
and used without further purification. Other substrates (1c, 1d, 1e,
1
a Reaction conditions: all reactions were carried out with 1 (0.1
mmol), alcohol (5 equiv), Oxone (3 equiv), and [Rh(COD)Cl]2 (2 mol
%) in toluene (2 mL) under CO (2 atm) at 110 °C for 8 h. b Isolated
yield.
(24) (a) van Leeuwen, P. W. N. M.; Zuideveld, M. A.; Swennenhuis,
B. H. G.; Freixa, Z.; Kamer, P. C. J.; Goubitz, K.; Fraanje, J.; Lutz,
M.; Spek, A. L. J. Am. Chem. Soc. 2003, 125, 5523, and references
cited therein. (b) Milstein, D. Acc. Chem. Res. 1988, 21, 428. (c)
Ozawa, F.; Kawasaki, N.; Okamoto, H.; Yamamoto, T.; Yamamoto,
A. Organometallics 1987, 6, 1640.
species E. The latter is assumed to be oxidized by Oxone to
afford the active catalyst species Rh(I) F.18f,25 If the reaction is
proceeded though path b, we will expect to observe the reaction
when a stoichiometric [Rh(COD)Cl]2 is used. Therefore, a study
was carried out to establish the fundamental steps of this
catalytic cycle and the role of Oxone therein. Indeed, this
(25) (a) Vogler, T.; Studer, A. Org. Lett. 2008, 10, 129. (b) Ueura, K.;
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Rhodium-Catalyzed Direct Oxidative Carbonylation
A R T I C L E S
1f, 1 g, 1 h, 1i, 1j, 1p, 1q, 1r) were prepared according to the
literature procedures.26
anhydrous Na2SO4 and evaporated in vacuo. The desired products
2 were obtained in the corresponding yields after purification by
flash chromatography on silica gel with hexane, ethyl acetate, and
triethylamine.
Typical Procedure for Carbonylation of 1 with CO and
Alcohol. [Rh(COD)Cl]2 (2 mol %, 1 mg,) was added to a solution
of arene 1 (0.1 mmol), alcohol (0.5 mmol), and Oxone (185 mg,
0.3 mmol) in toluene (2 mL) in a vial (5 mL). The resulting solution
was then transferred into a steel autoclave and charged with CO (2
atm). After stirring at 110 °C for 8 h, the CO was released carefully.
The suspension was filtered through a Celite pad and extracted with
CH2Cl2 three times. The combined organic layers were dried over
Acknowledgment. This paper is dedicated to Professor Xiyan
Lu on the occasion of his 80th birthday. We thank the NIH(GM
58832) and China scholarship council for financial support.
Supporting Information Available: Experimental procedures
and spectral data for all products. This material is available free
of charge via the Internet at http://pubs.acs.org.
(26) (a) Liu, D.; Gao, W.; Dai, Q.; Zhang, X. Org. Lett. 2005, 7, 4907. (b)
Kitamura, Y.; Sako, S.; Udzu, T.; Tsutsui, A.; Maegawa, T.; Monguchi,
Y.; Sajiki, H. Chem. Commun. 2007, 5069.
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