Copper-catalyzed decarboxylase cross-coupling of potassium polyfluorobenzoates with aryl iodides and bromides

Article abstract of DOI:10.1002/anie.200904916

Chemical Equitation Presentation For copper only: The decarboxylative cross-coupling of readily accessible and nonvolatile potassium polyfluorobenzoates with aryl iodides and bromides using a copper catalyst provides poly-fluorobiaryls and polyfluorostilb

Full text of DOI:10.1002/anie.200904916

Communications
DOI: 10.1002/anie.200904916
À
C C Coupling
Copper-Catalyzed Decarboxylative Cross-Coupling of Potassium
Polyfluorobenzoates with Aryl Iodides and Bromides**
Rui Shang, Yao Fu, Yan Wang, Qing Xu, Hai-Zhu Yu, and Lei Liu*
Transition-metal-catalyzed decarboxylative cross-coupling
using carboxylic acids as aryl sources is of contemporary
interest.[1] This method does not use expensive and sensitive
organometallic reagents, and generates CO2 instead of toxic
metal halides. Early studies showed that a stoichiometric
quantity of copper could promote the decarboxylative
coupling of aromatic carboxylic acids with aryl iodides.[2]
Recently, Goossen et al. reported Pd/Cu-catalyzed decarbox-
ylative coupling of benzoic acids and a-oxo carboxylates with
aryl halides and triflates.[3] Related studies by the groups of
Myers,[4] Forgione,[5] and others[6–9] showed that palladium by
itself could also catalyze the decarboxylative coupling of
aromatic carboxylic acids. We reported a palladium-catalyzed
decarboxylative coupling of oxalate monoesters with aryl
halides,[10] and related decarboxylative reactions were also
reported recently by the groups of Tunge, Li, Chruma, and
others.[11]
Goossen et al. reported the copper-catalyzed protodecarbox-
ylation of aromatic carboxylic acids.[18] Nonetheless, there has
not been any example for copper-catalyzed decarboxylative
coupling of acids with aryl halides. Therefore, the reactions
reported herein represent a novel type of copper-catalyzed
cross-coupling reactions.[19,20]
Our work started with the decarboxylative coupling of
C6F5COOK with PhI. When 10 mol% of CuI/1,10-phenan-
throline was used as the catalyst, decarboxylation proceeded
rapidly in NMP at 1608C but the yield of the desired product
was only 10%. To improve the yield we lowered the reaction
temperature and found that the best compromise between the
reaction rate and yield is achieved at 1308C. However, the
optimal yield remained at about 40% after we examined
many combinations of solvents and ligands. A breakthrough
was then made when diglyme was used as the solvent, and the
optimal yield obtained was 99% with or even without the
ligand.
Herein we describe the first, copper-only systems that
catalyze the decarboxylative coupling of potassium polyfluor-
obenzoates with aryl iodides and bromides [Eq. (1)].[12] The
A possible explanation for the outstanding performance
of diglyme is that diglyme can coordinate to K+, thereby
À
facilitating the complexation between CuI and C6F5CO2
.
Extending the model reaction to other substrates showed that
both electron-rich and electron-poor aryl iodides could be
successfully converted and a range of functional groups were
tolerated (Table 1, entries 2–12). The reaction yields range
from good to excellent. Importantly, ortho substitution can be
well tolerated in the transformation (Table 1, entries 13–16).
In addition to phenyl iodides, other aryl substrates including
naphthyl, perfluorophenyl, and heteroaryl iodides can also be
used to produce the corresponding polyfluorobiaryls (Table 1,
entries 18–24).
importance of the study is twofold: 1) The new reactions can
replace the use of expensive but often less reactive[13]
fluorobenzene organometallics in the synthesis of polyfluor-
obiaryls, which are important molecules in medicinal chemis-
try[14] and material science.[15] They also provide a method
complementary to that reported by Fagnou and co-workers[16]
and Daugulis and co-workers[17] for fluorobiaryl synthesis
The above protocol can be applied to aryl iodides but not
aryl bromides. This problem can be solved by using 1,10-
phenanthroline as a ligand. As shown in Table 2, copper-
catalyzed decarboxylative cross-couplings between potassium
pentafluorobenzoate and a variety of aryl bromides display
high yields ranging from 88 to 99%. These coupling reactions
can tolerate both electron-rich and electron-poor substrates
(Table 2, entries 1–9) and can also tolerate ortho substitution
(Table 2, entries 14–16). In addition, heteroaryl bromides are
acceptable substrates in the reaction (Table 2, entries 17–20).
Notably, copper-catalyzed decarboxylative cross-coupling of
potassium pentafluorobenzoate with 1-bromoadamantane
À
through C H arylation of a polyfluoroarene. 2) Recently
[*] R. Shang, Prof. Dr. Y. Fu, Y. Wang, Q. Xu, H.-Z. Yu, Prof. Dr. L. Liu
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (China)
and
Tsinghua University
Beijing 100084 (China)
Fax: (+86)10-6277-1149
E-mail: lliu@mail.tsinghua.edu.cn
Homepage: http://chem.tsinghua.edu.cn/liugroup/
3
2
À
produces a C(sp ) C(sp ) bond [Eq. (2)]. Moreover, the
method can be used to synthesize polyfluorostilbene in high
yields from vinyl bromides [Eq. (3)].[24]
The scope of the reaction with respect to fluoroarene is
presented in Table 3. Although diglyme is important for the
reactions with potassium pentafluorobenzoate, dimethyl
acetamide (DMA) affords better results for fluoroarenes
[**] This work was supported by NSFC (20832004, 20802040), and the Li
Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904916.
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Table 1: Copper-catalyzed decarboxylative cross-coupling between
Table 2: Copper-catalyzed decarboxylative cross-coupling between
potassium pentafluorobenzoate and aryl bromides.[a]
potassium pentafluorobenzoate and aryl iodides.[a]
Entry
Product
Yield
[%]
Entry
Product
Yield
[%]
Entry
Product
Yield
[%]
Entry
Product
Yield
[%]
1
2
3
99
99
99
13[c]
14[c]
15
96
97
94
1
2
3
4
5
88
95
97
89
99
11
12
13
14
15
96
99
93
92
94
4
5
95
98
16
17
99
99
6
99
99
18[c]
19[b]
99
80
6
94
16
91
7
8
9
92
94
92
17
18
19
91
94
97
7[b]
8
99
96
98
92
99
20[b]
21
94
99
99
61
89
9
10
95
20
88
[a] Yields of isolated products were calculated based on the amount of
aryl bromide used. phen=1,10-phenanthroline.
10
11[b]
12
22
23
efficiently converted, unless an ortho-CF3 group is added
(Table 3, entries 1–3). Once two F atoms are placed at each of
the ortho positions, the decarboxylative coupling of potas-
sium bis(fluorobenzoate) can proceed smoothly with both
electron-rich and electron-poor aryl iodides (Table 3,
entries 4–7). Similar reactions are also observed with tri-
and tetrafluorobenzoates having two ortho-F atoms (Table 3,
entries 10–18). In entries 14 and 15 of Table 3 some diarylated
by-products are also observed. This means that the direct
24[d]
[a] Yields of isolated products were calculated based on the amount of
aryl iodide used. [b] 1.2 equiv C6F5CO2K was used. [c] 20 mol% CuI was
used. [d] 1,4-diiodobenzene was used as substrate.
[16–17]
À
containing fewer fluorine atoms. Under the optimized reac-
tion conditions, potassium monofluorobenzoate cannot be
arylation of acidic C H bonds of polyfluoroarenes
side reaction in the copper-catalyzed decarboxylative cou-
pling of polyfluorobenzoates.
is a
Goossen et al. previously conducted a
theoretical study on the mechanism of
copper-mediated decarboxylation of benzoic
acids.[21] For the copper-catalyzed decarboxy-
lative coupling described herein, there is a key
mechanistic question as to whether decarbox-
ylation occurs on copper(I) before oxidative
addition, or at the copper(III) stage. To solve
the problem DFT calculations were per-
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+ 20.3 kcalmolÀ1. This step produces a new
copper(I) complex 3 which can react with PhBr
through oxidative addition (transition state
TS(3À3)).[23] The energy barrier of oxidative addi-
tion is + 30.0 kcalmolÀ1 and therefore, oxidative
addition constitutes the rate-limiting step in
pathway I. Finally, reductive elimination is
found to be a facile step and it finishes the
Table 3: Copper-catalyzed decarboxylative cross-coupling between aryl iodides
reaction producing C6F5Ph.
and other polyfluorobenzoates.[a]
In pathway II, oxidative addition takes places first on the
complex 1. This step has a relatively low energy barrier of
+ 18.9 kcalmolÀ1. After oxidative addition the resulting
copper(III) species is pentacoordinated and therefore, decar-
boxylation at copper(III) has to pass through a hexacoordi-
nated transition state. As a result of the strong steric repulsion
in the hexacoordinated species, the energy barrier for
decarboxylation is calculated to be + 51.1 kcalmolÀ1. There-
fore, decarboxylation constitutes the rate-limiting step in
pathway II. By comparing pathways I and II we conclude that
decarboxylation likely occurs on copper(I) before oxidative
addition. This conclusion is in line with the observation by
Sheppard et al.[23a,b]
In summary, the decarboxylative cross-coupling of potas-
sium polyfluorobenzoates with aryl iodides and bromides
mediated by a copper-only system was discovered. This
reaction represents both a new type of copper-catalyzed
cross-coupling reaction and a new type of transition-metal-
catalyzed decarboxylative coupling reaction. The reaction is
practical for the synthesis of polyfluorobiaryls from readily
accessible and nonvolatile polyfluorobenzoate salts. In con-
trast to the previously reported decarboxylative coupling
reactions, palladium is not required for the present trans-
formation, meaning that both the decarboxylation and cross-
coupling steps in the newly discovered process are catalyzed
solely by copper. Theoretical analyses suggest that decarbox-
ylation should occur at first on copper(I) to generate a
polyfluorophenylcopper(I) intermediate, which then reacts
with aryl halides through oxidative addition and reductive
elimination to produce the coupling products.
Entry
Product
Yield
[%]
Entry
10
Product
Yield
[%]
1
2
trace
trace
68
73
95
80
21
48
69
97
95
82
11
3
4
5
6
7
8
9
12
73
13
78
14[b]
15[c]
16
86
83
89
17
Experimental Section
Representative procedure (Table 1): CuI (0.05 mmol, 9.5 mg), an
appointed amount of potassium pentafluorobenzoate (0.60–
0.75 mmol), and aryl iodide (0.50 mmol) (if solid) were placed in an
oven-dried 10 mL Schlenk tube. The reaction vessel was evacuated
and filled with argon, a process which was repeated three times. Then
aryl iodide (0.50 mmol) (if liquid) and diglyme (0.5 mL) were added
with a syringe under a counterflow of argon. The vessel was sealed
with a screw cap, stirred at room temperature for 10 min, and then
connected to the Schlenk line filled with argon. The reaction was
stirred at 1308C for the appointed time (24 h). Upon completion of
the reaction, the mixture was cooled to room temperature and diluted
with ethyl acetate or petroleum ether (20 mL). The mixture was
filtered through a short silica gel column to remove the deposition.
The organic layers were washed with water (3 ꢀ 20 mL) and then with
brine. The combined organic layers were dried over Na2SO4 and
filtered. The solvents were removed under vacuum. Purification of
88
18
[a] Yields of isolated products were calculated based on the amount of aryl
iodide used. [b] 46% of diarylated product was also isolated. [c] 29% of
diarylated product was also isolated. See the Supporting Information for more
details. DMA=N,N-dimethyl acetamide.
formed to compare the two plausible mechanisms (Fig-
ure 1a).[22] In pathway I, the initial copper(I) complex 1
reacts with perfluorobenzoate to form 2, which then under-
goes decarboxylation via the four-membered-ring transition
state TS(2À3) (Figure 1b), which has an energy barrier of
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Figure 1. Comparison of two plausible mechanisms for copper-catalyzed decarboxylative cross-coupling (B3LYP method. SDD basis set for I, 6-
31G(d) for C, H, O, F, Cu, and Br. Solvation=CPCM/Bondi). All energies (in parentheses) are given in kcal mol-1. a) Energy diagram for the two
plausible mechanisms. b) Minimized structure of the proposed transitions states showing some bond lengths and distances (in ꢀ).
the residue by column chromatography on silica gel (EtOAc/n-
hexane 1:1) yielded the corresponding fluoroarene.
g) L. J. Goossen, F. Rudolphi, C. Oppel, N. Rodriguez, Angew.
Chem. 2008, 120, 3085; Angew. Chem. Int. Ed. 2008, 47, 3043.
[4] a) A. G. Myers, D. Tanaka, M. R. Mannion, J. Am. Chem. Soc.
2002, 124, 11250; b) D. Tanaka, A. G. Myers, Org. Lett. 2004, 6,
433; c) D. Tanaka, S. P. Romeril, A. G. Myers, J. Am. Chem. Soc.
2005, 127, 10323.
Received: September 2, 2009
Published online: November 5, 2009
[5] P. Forgione, M.-C. Brochu, M. St-Onge, K. H. Thesen, M. D.
Bailey, F. Bilodeau, J. Am. Chem. Soc. 2006, 128, 11350.
[6] a) J. Moon, M. Jeong, H. Nam, J. Ju, J. H. Moon, H. M. Jung, S.
Lee, Org. Lett. 2008, 10, 945; b) J. Moon, M. Jang, S. Lee, J. Org.
Chem. 2009, 74, 1403.
Keywords: copper · cross-coupling · homogeneous catalysis ·
reaction mechanisms · polyfluoroarenes
.
[7] a) J.-M. Becht, C. Le Drian, Org. Lett. 2008, 10, 3161; b) J.-M.
Becht, C. Catala, C. Le Drian, A. Wagner, Org. Lett. 2007, 9,
1781.
[8] a) Z. Y. Wang, Q. P. Ding, X. D. He, J. Wu, Org. Biomol. Chem.
2009, 7, 863; b) Z. Y. Wang, Q. P. Ding, X. D. He, J. Wu,
Tetrahedron 2009, 65, 4635; c) A. Voutchkova, A. Coplin, N. E.
Leadbeater, R. H. Crabtree, Chem. Commun. 2008, 6312.
[9] C. Wang, I. Piel, F. Glorius, J. Am. Chem. Soc. 2009, 131, 4194.
[10] R. Shang, Y. Fu, J. B. Li, S. L. Zhang, Q. X. Guo, L. Liu, J. Am.
Chem. Soc. 2009, 131, 5738.
[11] a) S. R. Waetzig, D. K. Rayabarupu, J. D. Weaver, J. A. Tunge,
Angew. Chem. 2006, 118, 5099; Angew. Chem. Int. Ed. 2006, 45,
4977; b) S. R. Waetzig, J. A. Tunge, J. Am. Chem. Soc. 2007, 129,
14860; c) W. H. Fields, A. K. Khan, M. Sabat, J. J. Chruma, Org.
Lett. 2008, 10, 5131; d) S.-F. Pi, B.-X. Tang, J.-H. Li, Y.-L. Liu, Y.
Liang, Org. Lett. 2009, 11, 2309; e) H.-P. Bi, L. Zhao, Y.-M.
Liang, C.-J. Li, Angew. Chem. 2009, 121, 806; Angew. Chem. Int.
[1] a) O. Baudoin, Angew. Chem. 2007, 119, 1395; Angew. Chem.
Int. Ed. 2007, 46, 1373; b) L. J. Goossen, N. Rodriguez, K.
Goossen, Angew. Chem. 2008, 120, 3144; Angew. Chem. Int. Ed.
2008, 47, 3100.
[2] a) M. Nilsson, Acta Chem. Scand. 1966, 20, 423; b) C. Bjꢁrklund,
M. Nilsson, Acta Chem. Scand. 1968, 22, 2585; c) A. Cairncross,
J. R. Roland, R. M. Henderson, W. A. Shepard, J. Am. Chem.
Soc. 1970, 92, 3187; d) T. Cohen, R. A. Schambach, J. Am. Chem.
Soc. 1970, 92, 3189.
[3] a) L. J. Goossen, G. Deng, L. M. Levy, Science 2006, 313, 662;
b) L. J. Goossen, N. Rodriguez, B. Melzer, C. Linder, G. Deng,
L. M. Levy, J. Am. Chem. Soc. 2007, 129, 4824; c) L. J. Goossen,
B. Melzer, J. Org. Chem. 2007, 72, 7473; d) L. J. Goossen, T.
Knauber, J. Org. Chem. 2008, 73, 8631; e) L. J. Goossen, B.
Zimmermann, T. Knauber, Angew. Chem. 2008, 120, 7211;
Angew. Chem. Int. Ed. 2008, 47, 7103; f) L. J. Goossen, N.
Rodriguez, C. Linder, J. Am. Chem. Soc. 2008, 130, 15248;
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Communications
Ed. 2009, 48, 792; f) H.-P. Bi, W.-W. Chen, Y.-M. Liang, C.-J. Li,
Chem. Soc. 2001, 123, 7727; b) R. A. Altman, A. M. Hyde, X.
Huang, S. L. Buchwald, J. Am. Chem. Soc. 2008, 130, 9613; c) D.
Ma, Y. Zhang, J. Yao, S. Wu, F. Tao, J. Am. Chem. Soc. 1998, 120,
12459; d) D. Ma, S. Xie, P. Xue, X. Zhang, J. Dong, Y. Jiang,
Angew. Chem. Int. Ed. 2009, 48, 4222; e) M. B. Thathagar, J.
Beckers, G. Rothenberg, J. Am. Chem. Soc. 2002, 124, 11858;
f) V. del Amo, S. R. Dubbaka, A. Krasovskiy, P. Knochel,
Angew. Chem. 2006, 118, 8002; Angew. Chem. Int. Ed. 2006,
45, 7838; g) X. Chen, X.-S. Hao, C. E. Goodhue, J.-Q. Yu, J. Am.
Chem. Soc. 2006, 128, 6790; h) J. M. Villalobos, J. Srogl, L. S.
Libeskind, J. Am. Chem. Soc. 2007, 129, 15734; i) H. Prokopcova,
C. O. Kappe, Angew. Chem. 2008, 120, 3732; Angew. Chem. Int.
Ed. 2008, 47, 3674; j) P. H. Fuller, J.-W. Kim, S. R. Chemler, J.
Am. Chem. Soc. 2008, 130, 17638; k) A. E. King, T. C. Brunold,
S. S. Stahl, J. Am. Chem. Soc. 2009, 131, 5044; l) R. J. Phipps,
M. J. Gaunt, Science 2009, 323, 1593.
Org. Lett. 2009, 11, 3246; g) Z. Duan, S. Ranjit, P. Zhang, X. Liu,
Chem. Eur. J. 2009, 15, 3666; h) A. A. Yeagley, M. A. Lowder,
J. J. Chruma, Org. Lett. 2009, 11, 4022; i) P. Hu, J. Kan, W. Su, M.
Hong, Org. Lett. 2009, 11, 2341; j) W.-Y. Yu, W. Sit, Z. Zhou,
A. S.-C. Chan, Org. Lett. 2009, 11, 3174; k) M. Miyasaka, A.
Fukushima, T. Satoh, K. Hirano, M. Miura, Chem. Eur. J. 2009,
15, 3674.
[12] There are two literature examples for palladium-catalyzed
decarboxylative coupling of perfluorobenzoic acid with 4-
iodoanisole; see references [7b,8c].
[13] For a special protocol for the Suzuki–Miyaura coupling reaction
of pentafluorophenylboronic acid, which is an inactive substrate
under normal conditions, see: T. Korenaga, T. Kosaki, R.
Fukumura, T. Ema, T. Sakai, Org. Lett. 2005, 7, 4915.
[14] a) R. E. Mewshaw, R. J. Edsall, Jr., C. Yang, E. S. Manas, Z. B.
Xu, R. A. Henderson, J. C. Keith, Jr., H. A. Harris, J. Med.
Chem. 2005, 48, 3953; b) M. de Candia, F. Liantonio, A. Carotti,
R. De Cristofaro, C. Altomare, J. Med. Chem. 2009, 52, 1018.
[15] a) J. R. Nitschke, T. D. Tilley, J. Am. Chem. Soc. 2001, 123,
10183; b) P. Zacharias, M. C. Gather, M. Rojahn, O. Nuyken, K.
Meerholz, Angew. Chem. 2007, 119, 4467; Angew. Chem. Int. Ed.
2007, 46, 4388.
[16] a) M. Lafrance, C. N. Rowley, T. K. Woo, K. Fagnou, J. Am.
Chem. Soc. 2006, 128, 8754; b) M. Lafrance, D. Shore, K. Fagnou,
Org. Lett. 2006, 8, 5097.
[17] a) H.-Q. Do, O. Daugulis, J. Am. Chem. Soc. 2007, 129, 12404;
b) H.-Q. Do, O. Daugulis, J. Am. Chem. Soc. 2008, 130, 1128;
c) H.-Q. Do, R. M. Kashif Khan, O. Daugulis, J. Am. Chem. Soc.
2008, 130, 15185.
[21] L. J. Goossen, W. R. Thiel, N. Rodriguez, C. Linder, B. Melzer,
Adv. Synth. Catal. 2007, 349, 2241.
[22] Some previous studies on the mechanism of copper-catalyzed
cross-couplings: a) A. Ouali, J.-F. Spindler, A. Jutand, M.
Taillefer, Adv. Synth. Catal. 2007, 349, 1906; b) S.-L. Zhang, L.
Liu, Y. Fu, Q.-X. Guo, Organometallics 2007, 26, 4546; c) A.
Ouali, M. Taillefer, J.-F. Spindler, A. Jutand, Organometallics
2007, 26, 65; d) J. W. Tye, Z. Weng, A. M. Johns, C. D. Incarvito,
J. F. Hartwig, J. Am. Chem. Soc. 2008, 130, 9971; e) L. M.
Huffman, S. S. Stahl, J. Am. Chem. Soc. 2008, 130, 9196; f) H.
Kaddouri, V. Vicente, A. Ouali, F. Ouazzani, M. Taillefer,
Angew. Chem. 2009, 121, 339; Angew. Chem. Int. Ed. 2009, 48,
333; g) E. R. Strieter, B. Bhayana, S. L. Buchwald, J. Am. Chem.
Soc. 2009, 131, 78.
[18] L. J. Goossen, F. Manjolinho, B. A. Khan, N. Rodriguez, J. Org.
Chem. 2009, 74, 2620.
[23] Earlier studies already showed that pentafluorophenylcopper
can react with aryl iodides producing coupling products in high
yield. See: a) A. Cairncross, W. A. Sheppard, J. Am. Chem. Soc.
1968, 90, 2186; b) W. A. Sheppard, J. Am. Chem. Soc. 1970, 92,
5419.
[24] Thermal ellipsoids shown at 50% probability. CCDC-752845
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data re-
quest/cif.
[19] Reviews for copper-catalyzed cross-couplings: a) S. V. Ley,
A. W. Thomas, Angew. Chem. 2003, 115, 5558; Angew. Chem.
Int. Ed. 2003, 42, 5400; b) W. Deng, L. Liu, Q.-X. Guo, Chin. J.
Org. Chem. 2004, 24, 150; c) D. Ma, Q. Cai, Acc. Chem. Res.
2008, 41, 1450; d) F. Monnier, M. Taillefer, Angew. Chem. 2008,
120, 3140; Angew. Chem. Int. Ed. 2008, 47, 3096; e) G. Evano, N.
Blanchard, M. Toumi, Chem. Rev. 2008, 108, 3054.
[20] For selected examples of copper-catalyzed cross-couplings, see:
a) A. Klapars, J. C. Antilla, X. Huang, S. L. Buchwald, J. Am.
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