Palladium-catalyzed direct arylation of electron-deficient polyfluoroarenes with arylboronic acids

Article abstract of DOI:10.1021/ol901200g

The palladium-catalyzed direct arylation of electron-deficient arenes that contain two or more fluorine groups with arylboronic acids was realized. The key to achieving a broad substrate scope with respect to both polyfluorobenzenes and arylboronic acids

Full text of DOI:10.1021/ol901200g

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3346-3349
Palladium-Catalyzed Direct Arylation of
Electron-Deficient Polyfluoroarenes with
Arylboronic Acids
Ye Wei, Jian Kan, Min Wang, Weiping Su,* and Maochun Hong
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
wpsu@fjirsm.ac.cn
Received May 28, 2009
ABSTRACT
The palladium-catalyzed direct arylation of electron-deficient arenes that contain two or more fluorine groups with arylboronic acids was
realized. The key to achieving a broad substrate scope with respect to both polyfluorobenzenes and arylboronic acids is the choice of bases
depending on acidities of polyfluorobenzenes.
The prevalence of the biaryl substructure in biologically
active molecules and functional materials has prompted
synthetic chemists to seek efficient methods for the construc-
tion of this class of molecules.1 Consequently, the transition
metal-catalyzed cross-coupling reactions between aryl halides
and organometallics have evolved into the most powerful
synthetic tools for generation of biaryl compounds.2 The
recent advances in metal-catalyzed C-H bond functional-
ization suggest that it is possible to achieve similar trans-
formations without the need for the preactivation of coupling
partners (e.g., preparations of aryl halides and organometallic
reagents).3,4 Strategies for functionalization of C-H bonds
not only offer the most efficient synthetic route to the target
compounds,5 but also in some cases provide a solution to
the problem arising from the use of unstable or hard to
prepare organometallic reagents in traditional cross-coupling
(4) For selected examples of C-H bond arylation, see: (a) Deprez, N. R.;
Kalyani, D.; Krause, A.; Sanford, M. S. J. Am. Chem. Soc. 2006, 128, 4972.
(b) Lafrance, M.; Fagnou, K. J. Am. Chem. Soc. 2006, 128, 16496. (c)
Stuart, D. R.; Villemure, E.; Fagnou, K. J. Am. Chem. Soc. 2007, 129,
12072. (d) Stuart, D. R.; Fagnou, K. Science 2007, 316, 1172. (e) Lane,
B. S.; Brown, M. A.; Sames, D. J. Am. Chem. Soc. 2005, 127, 8050. (f)
Wang, X.; Lane, B. S.; Sames, D. J. Am. Chem. Soc. 2005, 127, 4996. (g)
Phipps, R. J.; Grimster, N. P.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130,
8172. (h) Phipps, R. J.; Gaunt, M. J. Science 2009, 323, 1593. (i) Yang, S.;
Li, B.; Wan, X.; Shi, Z. J. Am. Chem. Soc. 2007, 129, 6066. (j) Chiong,
H. A.; Pham, Q.-N.; Daugulis, O. J. Am. Chem. Soc. 2007, 129, 9879. (k)
Lebrasseur, N.; Larrosa, I. J. Am. Chem. Soc. 2008, 130, 2926.
(1) Hassan, J.; Se´vignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem.
ReV. 2002, 102, 1359.
(5) For selected examples of C-H bond functionalization in synthesis,
see: (a) Beck, E. M.; Hatley, R.; Gaunt, M. J. Angew. Chem., Int. Ed. 2008,
47, 3004. (b) Hunma, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510.
(c) Baran, P. S.; Corey, E. J. J. Am. Chem. Soc. 2002, 124, 7904. (d) Garg,
N. K.; Caspi, D. D.; Stoltz, B. M. J. Am. Chem. Soc. 2005, 127, 5970. (e)
Read, S. A.; White, M. C. J. Am. Chem. Soc. 2008, 130, 3318. (f)
Fraunhoffer, K. J.; White, M. C. J. Am. Chem. Soc. 2007, 129, 7274.
(6) (a) Campeau, L.-C.; Rousseaux, S.; Fagnou, K. J. Am. Chem. Soc.
2005, 127, 18020. (b) Leclerc, J.-P.; Fagnou, K. Angew. Chem., Int. Ed.
2006, 45, 7781. (c) Lafrance, M.; Rowley, C. N.; Woo, T. K.; Fagnou, K.
J. Am. Chem. Soc. 2006, 128, 8754. (d) Lafrance, M.; Shore, D.; Fagnou,
K. Org. Lett. 2006, 8, 5097.
(2) (a) Metal-Catalyzed Cross-coupling Reactions; Meijere, A., Dieder-
ich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (b) Nicolauo, K. C.;
Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442.
(3) For recent reviews, see: (a) Dyker, G. Angew. Chem., Int. Ed. 1999,
38, 1698. (b) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (c)
Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731. (d) Kakiuchi,
F.; Chatani, N. AdV. Synth. Catal. 2003, 345, 1077. (e) Campeau, L.-C.;
Fagnou, K. Chem. Commun. 2005, 1253. (f) Dick, A. R.; Sanford, M. S.
Tetrahedron 2006, 62, 2439. (g) Alberico, D.; Scott, M. E.; Lautens, M.
Chem. ReV. 2007, 107, 174. (h) Seregin, I. V.; Gevorgyan, V. Chem. Soc.
ReV. 2007, 36, 1173. (i) Godula, K.; Sames, D. Science 2006, 312, 67
.
10.1021/ol901200g CCC: $40.75
Published on Web 07/15/2009
 2009 American Chemical Society

reactions.6-8 In this regard, Fagnou6 and Daugulis7 have
successfully developed the direct arylation of electron-
deficient polyfluoroarenes or heterocycles for synthesis of
fluorobiaryl or heterocycle-containing biaryl compounds.
Recently, efforts made by Yu,9 Shi,10 and others11 have
led to the development of palladium-catalyzed C-H bond
arylation with organoboron reagents, and Murai and Kakiuchi
disclosed a ruthenium-catalyzed ortho C-H arylation of
acetophenones with boronic esters.12 Copper-13 and iron-
based catalysts14 have successfully been exploited as a cost-
efficient replacement for palladium-based catalysts for the
direct arylation of arenes with arylboronic acids. These
reported reactions are limited to directing group-containing
or electron-rich arenes. However, to date, no example has
been reported that involves the direct arylation of electron-
deficient arenes with organoboron reagents, except for a few
substrates bearing directing groups.9b In view of the impor-
tance of electron-deficient fluorobiaryls in medicinal15 and
materials chemistry,16 the discovery of new reactions for the
synthesis of these valuable compounds remains highly
desirable. Herein, we report a palladium-catalyzed method
for the direct arylation of electron-deficient polyfluoroarenes
with organoboron reagents and also present the result of the
preliminary mechanistic study on this reaction.
the transmetalation between the arylboronic acid and the
Pd(II) species, whereas the fast transmetalation would readily
lead to the undesired homocoupling of arylboronic acid if
the C-H bond cleavage is relatively slow,17 thus hampering
the generation of the desired cross-coupling product. Our
strategy to solve this problem is to simultaneously add a weak
base, which we believe promotes the C-H bond cleavage,
and a weak acid, which we believe reduces the transmeta-
lation rate of arylboronic acid, and therefore suppresses the
undesired homocoupling.10b
A variety of reaction parameters were screened with the
reaction of pentafluorobenzene (1) with 1.5 equiv of phe-
nylboronic acid (2) as a model reaction system. Selected
results that illustrate the effect of acid and base on this
reaction are listed in Table 1. In the presence of 5 mol % of
Table 1. Selected Screening Results for Pd-Catalyzed Direct
Arylation of Pentafluorobenzene with Phenylboronic Acida
entry
base (equiv)
acid (0.3 equiv)
yield (%)b
To achieve the direct arylation of fluoroarenes with
arylboronic acids, we need to address the following issue:
the cleavage of the C-H bond of electron-deficient fluoro-
arene generally requires basic conditions that also accelerate
1
2
3
4
5
6
7
8
trace
75
70
7
K2CO3 (2)
Na2CO3 (2)
Cs2CO3 (2)
K3PO4 (2)
80
69
71
55
72
84
80
79
77
88
89
85
90c
(7) (a) Do, H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2007, 129, 12404.
(b) Do, H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2008, 130, 1128. (c) Do,
H.-Q.; Kashif Khan, R. M.; Daugulis, O. J. Am. Chem. Soc. 2008, 130,
KOAc (2)
K2HPO4 (2)
p-Me-C6H4-CO2K (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
KF (2.5)
K3PO4 (2)
15185
.
9
tBuCO2H
(8) Cho, S. H.; Hwang, S. J.; Chang, S. J. Am. Chem. Soc. 2008, 130,
9254
.
10
11
12
13
14
15
16
17
p-Me-C6H4-CO2H
o-MeO-C6H4-CO2H
o-O2N-C6H4-CO2H
p-Me-C6H4-CO2H
p-Me-C6H4-CO2H
p-Me-C6H4-CO2H
p-Me-C6H4-CO2H
p-Me-C6H4-CO2H
(9) (a) Giri, R.; Maugel, N.; Li, J.-J.; Wang, D.-H.; Breazzano, S. P.;
Saunders, L. B.; Yu, J.-Q. J. Am. Chem. Soc. 2007, 129, 3510. (b) Wang,
D.-H.; Mei, T.-S.; Yu, J.-Q. J. Am. Chem. Soc. 2008, 130, 17676. (c) Wang,
D.-H.; Wasa, M.; Giri, R.; Yu, J.-Q. J. Am. Chem. Soc. 2008, 130, 7190.
(10) (a) Shi, Z.; Li, B.; Wan, X.; Cheng, J.; Fang, Z.; Cao, B.; Qin, C.;
Wang, Y. Angew. Chem., Int. Ed. 2007, 46, 5554. (b) Yang, S.; Sun, C.;
Fang, Z.; Li, B.; Li, Y.; Shi, Z. Angew. Chem., Int. Ed. 2008, 47, 1473. (c)
Li, B.-J.; Yang, S.-D.; Shi, Z.-J. Synlett 2008, 949.
K2CO3 (0.5)
K3PO4 (0.5)
K2CO3 (0.5)
(11) (a) Kirchberg, S.; Vogler, T.; Studer, A. Synlett 2008, 2841. (b)
Kirchberg, S.; Fro¨hlich, R.; Studer, A. Angew. Chem., Int. Ed. 2009, 48,
4235. (c) Ge, H.; Niphakis, M. J.; Georg, G. J. J. Am. Chem. Soc. 2008,
130, 3708. (d) Deicamp, J. H.; White, M. C. J. Am. Chem. Soc. 2006, 128,
15076.
a 0.2 mmol scale, 2 mL of DMA (0.1 M). b Isolated yields. c Pd(OAc)2
(2 mol %) and phenylboronic acid (1.2 equiv).
(12) (a) Kakiuchi, F.; Usui, M.; Ueno, S.; Chatani, N.; Murai, S. J. Am.
Chem. Soc. 2004, 126, 2706. (b) Kakiuchi, F.; Kan, S.; Igi, K.; Chatani,
N.; Murai, S. J. Am. Chem. Soc. 2003, 125, 1698. (c) Kakiuchi, F.; Matsuura,
Y.; Kan, S.; Chatani, N. J. Am. Chem. Soc. 2005, 127, 5936.
(13) Ban, I.; Sudo, T.; Taniguchi, T.; Itami, K. Org. Lett. 2008, 10,
3607.
Pd(OAc)2 as a catalyst and 2 equiv of Ag2CO3 as an oxidant,
the reaction carried out in dimethylacetamide (DMA) at 110
°C in the absence of any additive did not afford the desired
product in appreciable quantities. Instead, it produced the
undesired biphenyl from homocoupling of phenylboronic
acid (entry 1, Table 1). The addition of 2 equiv of K2CO3
led to the formation of the desired arylation product in 75%
yield (entry 2, Table 1). Both Na2CO3 and KOAc gave a
slightly lower yield (entries 3 and 6, Table 1), but Cs2CO3
was almost ineffective for this reaction (entry 4, Table 1).
K3PO4 was found to be superior to K2CO3 (entry 5, Table
1). As expected, the addition of carboxylic acids efficiently
(14) Wen, J.; Zhang, J.; Chen, S.-Y.; Li, J.; Yu, X.-Q. Angew. Chem.,
Int. Ed. 2008, 47, 8897.
(15) (a) Zahn, A.; Brotschi, C.; Leumann, C. J. Chem. Eur. 2005, 11,
2125. (b) Russell, M. G. N.; Carling, R. W.; Atack, J. R.; Bromidge, F. A.;
Cook, S. M.; Hunt, P.; Isted, C.; Lucas, M.; McKernan, R. M.; Mitchinson,
S.-A.; Wafford, K. A.; Castro, J. L. J. Med. Chem. 2005, 48, 1367.
(16) (a) Saknmoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.;
Taga, Y. J. Am. Chem. Soc. 2000, 122, 1832. (b) Hwang, D. H.; Song,
S. Y.; Ahn, T.; Chu, H. Y.; Do, L. M.; Kim, S. H.; Shim, H. K.; Zyung, T.
Synth. Met. 2000, 111, 485. (c) Montes, V. A.; Li, G.; Pohl, R.; Shinar, J.;
Anzenbacher, P. AdV. Mater. 2004, 16, 2001. (d) Tsuzuki, T.; Shirasawa,
N.; Suzuki, T.; Tokito, S. AdV. Mater. 2003, 15, 1455. (e) Kitamura, T.;
Wada, Y.; Yanagida, S. J. Fluorine Chem. 2000, 105, 305. (f) Weck, M.;
Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E. B.; Grubbs,
R. H. Angew. Chem., Int. Ed. 1999, 38, 2741. (g) Nitschke, J. R.; Tilley,
T. D. J. Am. Chem. Soc. 2001, 123, 10183.
(17) Adamo, C.; Amatore, C.; Ciofini, I.; Jutand, A.; Lakmini, H. J. Am.
Chem. Soc. 2006, 128, 6829.
Org. Lett., Vol. 11, No. 15, 2009
3347

inhibited the homocoupling of phenylboronic acid in the
presence of 2.0 equiv of bases. Screening several carboxylic
acids and their loadings revealed that 0.3 equiv of p-toluic
acid in combination with 2.0 equiv of K3PO4 or K2CO3
provided excellent yields (entries 9-14). We can rule out
the possibility that the beneficial effect of p-toluic acid arises
from potassium p-toluate or K2HPO4 formed in situ from
the reaction of p-toluic acid with K3PO4 since neither
K2HPO4 nor potassium p-toluate gave a comparable yield
in the absence of acid (entries 7 and 8, Table 1). Reducing
the amount of K2CO3 to 0.5 equiv further improved the
reaction in the presence of 0.3 equiv of p-toluic acid (entry
15, Table 1). Gratifyingly, 2 mol % of Pd(OAc)2 in
conjunction with 0.5 equiv of K2CO3 and 0.3 equiv of
p-toluic acid effected the reaction of pentafluorobenzene with
1.2 equiv of phenylboronic acid with 90% yield (entry 17,
Table 1). This reaction also occurred in other polar solvents
such as DMSO to give somewhat lower yields than that in
DMA, but did not occur in nonpolar or less polar solvents
such as toluene and dioxane. The other common oxidants
for the Pd(0)/Pd(II) system such as a variety of Cu(II) salts
and oxygen were ineffective for this reaction, and other silver
sources such as Ag2O and AgOAc were less efficient than
Ag2CO3.
transformation (3b-d,f) except for the case involving the
dimethylamino group, in which 5 mol % of Pd(OAc)2, 1.5
equiv of arylboronic acid, and 2 equiv of K2CO3 were
required to obtain a good yield (3g). Although electron-
deficient arylboronic acids suffered from their relatively poor
reactivities, using 2.5 equiv of KF in place of 0.5 equiv of
K2CO3 gave rise to synthetically useful yields (3h and 3i).
This transformation can be scaled up, as illustrated by the
reaction on 5 mmol scale that offered a yield comparable to
that on 0.2 mmol scale (3a, Scheme 1). When the arylboronic
acid bearing the ortho-substituent on the aromatic ring was
used as a coupling partner, neither the cross-coupling product
nor the homocoupling product was formed, indicative of the
sensitivity of this transformation to steric hindrance (3j).
The substrate scope with respect to polyfluorobenzenes
was also investigated and is presented in Scheme 2. Under
Scheme 2. Pd-Catalyzed Direct Arylation of Various
Polyfluoroarenes with Arylboronic Acidsa
The scope of this reaction with respect to organoboron
reagents was further explored by employing the conditions
of entry 17 in Table 1. As shown in Scheme 1, a range of
Scheme 1
.
Pd-Catalyzed Direct Arylation of Pentafluorobenzene
with Various Arylboronic Acidsa
a Reaction conditions: 3k-q: polyfluoroarene (0.2 mmol), arylboronic
acid (1.2 equiv), Pd(OAc)2 (2 mol %), K2CO3 (0.5 equiv), Ag2CO3 (2 equiv),
p-Me-C6H4-CO2H (0.3 equiv), DMA (2 mL, 0.1 M), 110 °C, 10 h. 3r-z:
polyfluoroarene (3 equiv), arylboronic acid (0.2 mmol), Pd(OAc)2 (5 mol
t
%), K3PO4 (0.5 equiv), Ag2CO3 (2 equiv), BuCO2H (0.6 equiv), DMA (2
mL, 0.1 M), 120 °C, 10 h. b Pd(OAc)2 (5 mol %), arylboronic acid (1.5
equiv) and K2CO3 (2 equiv). c p-Me-C6H4-CO2K (0.5 equiv) as base instead
of K3PO4.
a Reaction conditions: pentafluorobenzene 1 (0.2 mmol), arylboronic
acid (1.2 equiv), Pd(OAc)2 (2 mol %), K2CO3 (0.5 equiv), Ag2CO3 (2 equiv),
p-Me-C6H4-CO2H (0.3 equiv), DMA (2 mL, 0.1 M), 110 °C, 10 h.
b Pd(OAc)2 (5 mol %) and 4-methylbenzeneboronic acid neopentyl ester
(1.5 equiv). c Pd(OAc)2 (5 mol %), arylboronic acid (1.5 equiv), and K2CO3
(2 equiv). d KF (2.5 equiv) as base instead of K2CO3. e 5 mmol scale.
the standard conditions, both 2,3,5,6-tetrafluoroanisole and
2,3,5,6-tetrafluorotoluene smoothly underwent direct aryla-
tion with moderate to good yields obtained (3k-q). How-
ever, these reaction conditions are not suitable for other
polyfluorobenzenes such as 1,3,5-trifluorobenzene. It has
been established that the reactivity of the C-H bond in
electron-deficient arenes depends directly on its acidity. In
light of this, we speculated that the identification of the
proper base for a specific polyfluorobenzene might be crucial
for obtaining efficient transformation. Moreover, the acid
required by this reaction system to suppress the homocou-
organoboron reagents can serve as arylating reagents for the
direct arylation of pentafluorobenzene, affording the desired
products in moderate to excellent yields. The reactivity of
organoboron reagents was observed to rely on the electronic
nature of substituents on the aromatic rings. Electron-
donating substituents were generally beneficial for this
3348
Org. Lett., Vol. 11, No. 15, 2009

pling must match the base used for deprotonation of the
polyfluorobenzene. We were pleased to achieve the direct
arylation of more C-H acidic polyfluorobenzenes such as
2,3,5,6-tetrafluoropyridine and 2,3,5,6-tetrafluorobenzonitrile
by using 0.5 equiv of potassium p-toluate as a base in
conjunction with 0.6 equiv of pivalic acid and increasing
the amount of polyfluorobenzene (3:1 ratio of polyfluoroben-
zene to arylboronic acid) (3r and 3s), and to realize the direct
arylation of less C-H acidic polyfluorobenzenes by using
0.5 equiv of K3PO4 combined with 0.6 equiv of pivalic acid
(3t-z). As a result, a wide range of polyfluorobenzenes,
some of which contain bromo, cyano, keto, and methoxy
groups, can be arylated with arylboronic acids. Although
more than one potential site for reaction is available in some
of the polyfluorobenzenes, the reactions of these polyfluo-
robenzenes mainly generated monoarylated product, as
exemplified by the reaction of 1,3,5-trifluorobenzene (3x).
The direct arylation of 1,3-difluorobenzene occurred pref-
erentially at the most acidic C-H bond, which is flanked
by two C-F bonds (3y). In contrast, the reaction of 2,4-
difluorobenzophenone occurred exclusively at the position
that is meta to the benzoyl group and para to a fluoride group
(3z). The palladium-catalyzed meta-selective C-H bond
olefination directed by the benzoyl group has been observed
by Yu recently.18
2,4,5-trimethoxyphenylpalladium trifluoroacetate complex
421 with pentafluorobenzene in DMSO gave the arylated
product in 28% yield as well as a lot of the homocoupling
product from the palladium complex 4 [eq 1], indicating that
an arylpalladium complex is able to arylate electron-deficient
polyfluorobenzene via C-H bond cleavage. Further experi-
mentation showed that 5 mol % of palladium complex 4 as
a catalyst efficiently catalyzed the direct arylation of pen-
tafluorobenzene with phenylboronic acid in 84% yield,
suggesting that the direct arylation of polyfluorobenzene most
likely involves the formation of an arylpalldium intermediate
and subsequent reaction of the palladium species with
fluorobenzene. As shown in Scheme 1, when sterically
demanding o-tolylboronic acid was used, neither cross-
coupling nor homocoupling took place. No homocoupling
product from o-tolylboronic acid reveals that the transmeta-
lation that generates arylpalladium species was inhibited
owing to steric factors.17 If the transmetalation precedes the
palladation of fluorobenzene in a catalytic cycle, the inhibi-
tion of transmetalation could be the reason why o-tolylbo-
ronic acid did not react with 1. Although path B cannot be
ruled out, these observations favor path A. The intermolecular
kinetic isotope effect (kH/D ) 1.7) in the direct arylation of
2,3,5,6-tetrafluoroanisole with 2 indicated that the C-H bond
cleavage was involved in the rate-limiting step.
Two different mechanisms are possible for the direct
arylation of polyfluorobenzenes with arylboronic acids
(Scheme 3). Path A involves the initial formation of
Scheme 3. Two Possible Mechanisms of This Reaction
In conclusion, we have demonstrated a palladium-
catalyzed method for the direct arylation of electron-deficient
polyfluorobenzenes with arylboronic acids. This method
exhibits a broad substrate scope with respect to both
polyfluorobenzenes and arylboronic acids, which is attributed
to the choice of bases depending on the acidities of the
polyfluorobenzenes. The preliminary mechanistic studies
suggested that this reaction may involve initial transmeta-
lation from boron to palladium, followed by palladation of
fluorobenzene and reductive elimination. Further efforts are
underway to expand the substrate scope of this method to
other electron-deficient arenes.
arylpalladium species via the transmetalation from boron to
palladium, followed by the palladation of fluorobenzene via
concerted metalation-deprotonation6c,19,20 and reductive
elimination. Path B begins with the formation of fluoro-
arylpalladium species via the cleavage of the C-H bond of
fluorobenzene, followed by the transmetalation from boron
to the fluoroarylpalladium intermediate and reductive elimi-
nation.
Acknowledgment. Financial support from the 973 Pro-
gram (2006CB932903, 2009CB939803), NSFC (20821061,
20671089), NSF of Fujian Province (2007J0001), “The
Distinguished Oversea Scholar Project”, “One Hundred
Talent Project”, and Key Project from CAS is greatly
appreciated.
Supporting Information Available: Detailed experimen-
tal procedures and characterization for products. This material
is available free of charge via the Internet at http://pubs.acs.org.
To distinguish between path A and path B, we carried out
preliminary mechanistic studies. Stoichiometric reaction of
(18) Zhang, Y.-H.; Shi, B.-F.; Yu, J.-Q. J. Am. Chem. Soc. 2009, 131,
5072.
OL901200G
(19) Garc´ıa-Cuadrado, D.; Braga, A. A. C.; Maseras, F.; Echavarren,
A. M. J. Am. Chem. Soc. 2006, 128, 1066
(20) Li, B.-J.; Tian, S.-L.; Fang, Z.; Shi, Z.-J. Angew. Chem., Int. Ed.
2008, 47, 1115
.
(21) Tanaka, D.; Romeril, S. P.; Myers, A. G. J. Am. Chem. Soc. 2005,
127, 10323.
.
Org. Lett., Vol. 11, No. 15, 2009
3349