Synthesis of condensed heteroaromatic compounds by palladium-catalyzed oxidative coupling of heteroarene carboxylic acids with alkynes

Article abstract of DOI:10.1021/ol900736s

The palladium-catalyzed oxidative coupling of indole-3-carboxylic acids with alkynes effectively proceeds in a 1:2 manner accompanied by decarboxylation to produce the corresponding 1,2,3,4-tetrasubstituted carbazoles, some of which exhibit solid-state fl

Full text of DOI:10.1021/ol900736s

ORGANIC
LETTERS
2009
Vol. 11, No. 11
2337-2340
Synthesis of Condensed Heteroaromatic
Compounds by Palladium-Catalyzed
Oxidative Coupling of Heteroarene
Carboxylic Acids with Alkynes
Mana Yamashita, Koji Hirano, Tetsuya Satoh,* and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity,
Suita, Osaka 565-0871, Japan
satoh@chem.eng.osaka-u.ac.jp; miura@chem.eng.osaka-u.ac.jp
Received April 7, 2009
ABSTRACT
The palladium-catalyzed oxidative coupling of indole-3-carboxylic acids with alkynes effectively proceeds in a 1:2 manner accompanied by
decarboxylation to produce the corresponding 1,2,3,4-tetrasubstituted carbazoles, some of which exhibit solid-state fluorescence. Pyrrole-,
benzofuran-, and furancarboxylic acids also undergo the decarboxylative coupling to afford highly substituted indole, dibenzofuran, and
benzofuran derivatives, respectively.
Polycyclic aromatic and heteroaromatic compounds have
attracted considerable attention due to their electro- and
photochemical and optoelectronic properties and their utility
as π-conjugated functional materials such as organic semi-
conductors and luminescent materials.1 Highly substituted
derivatives with condensed aromatic cores are of particular
interest because of their stability, solubility, enhanced ability
to transport charge, and their fluorescent properties in the
solid state.2 Among modern potential strategies to prepare
polysubstituted, condensed aromatics is the transition-metal-
catalyzed homologation, such as benzene to naphthalene and
naphthalene to anthracene, by the coupling of a given
aromatic substrate with two alkyne molecules (Scheme 1).3
Scheme 1
(1) Selected reviews: (a) Anthony, J. E. Angew. Chem., Int. Ed. 2008,
47, 452. (b) Watson, M. D.; Fethtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001,
101, 1267. (c) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471.
(d) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH: New
York, 1996.
Thus, the catalytic transformations of di- (X * H, Y * H)3,4
and more readily available monofunctionalized aromatic
substrates (X * H, Y ) H)5 have been developed. The latter
reaction involving regioselective C-H bond cleavage is also
attractive from the atom-economic point of view.6 In
particular, the decarboxylative coupling of benzoic acids with
(2) For recent example, see: (a) Kaur, I.; Stein, N. N.; Kopreski, R. P.;
Miller, G. P. J. Am. Chem. Soc. 2009, 131, 3424. (b) Ahmed, E.; Briseno,
A. L.; Xia, Y.; Jenekhe, S. A. J. Am. Chem. Soc. 2008, 130, 1118. (c)
Paraskar, A. S.; Reddy, A. R.; Patra, A.; Wijsboom, Y. H.; Gidron, O.;
Shimon, L. J. W.; Leitus, G.; Bendikov, M. Chem.sEur. J. 2008, 14, 10639.
(d) Cicoira, F.; Santato, C. AdV. Funct. Mater. 2007, 17, 3421.
10.1021/ol900736s CCC: $40.75
Published on Web 05/08/2009
 2009 American Chemical Society

alkyne molecules under iridium catalysis (X ) CO2H, Y )
H), which we recently succeeded in developing,5g seems to
be of considerable interest because of wide availability of
the acids as aryl sources.7
During our further study of the scope of this reaction, it
was found that heteroarene carboxylic acids such as 1-me-
thylindole-3-carboxylic acid hardly underwent the Ir-
catalyzed coupling with alkynes (Scheme 2, path a). Under
attractive synthesis targets in medicinal chemistry and
materials field, because of their biological activities as well
as photophysical and optoelectronic properties.9 Expectedly,
some of carbazoles obtained by this procedure have been
found to show solid-state fluorescence. The results obtained
with the new coupling reactions of not only indolecarboxylic
acids but also pyrrole-, benzofuran-, and furancarboxylic
acids are described herein.
We recently reported that 1-methylindole-3-carboxylic acid
(1a) smoothly underwent the oxidative coupling with alkenes
such as acrylates in the presence of Pd(OAc)2, Cu(OAc)2·
H2O, and LiOAc as catalyst, oxidant, and additive, respec-
tively.10 In an initial attempt, the reaction of 1a (0.8 mmol)
with diphenylacetylene (2a) (0.8 mmol) was conducted under
similar conditions, using Pd(OAc)2 (0.02 mmol), Cu(OAc)2·
H2O (0.8 mmol), LiOAc (1.2 mmol), and molecular sieves
(MS4A, 400 mg) in DMAc (10 mL) at 140 °C for 2 h under
N2 to afford 9-methyl-1,2,3,4-tetraphenyl-9H-carbazole (3a)
in 34% yield (entry 1 in Table 1). Other additives such as
Scheme 2
rhodium catalysis, meanwhile, a lactone was obtained as a
1:1 oxidative coupling product (path b).8 To our delight, the
desired 1:2 coupling has been observed to proceed efficiently
by the use of a palladium catalyst to produce the corre-
sponding 1,2,3,4-tetrasubstituted carbazole derivative selec-
tively (path c). Highly substituted carbazoles have been
Table 1. Reaction of 1-Methylindole-3-carboxylic Acid (1a)
with Diphenylacetylene (2a)a
entry 1a (mmol) additive T (°C) time (h) % yield of 3ab
(3) (a) Takahashi, T.; Li, S.; Huang, W.; Kong, F.; Nakajima, K.; Shen,
B.; Ohe, T.; Kanno, K.-I. J. Org. Chem. 2006, 71, 7967. (b) Takahashi, T.;
Li, Y.; Stepnicka, P.; Kitamura, M.; Liu, Y.; Nakajima, K.; Kotora, M.
J. Am. Chem. Soc. 2002, 124, 576. (c) Takahashi, T.; Kitamura, M.; Shen,
B.; Nakajima, K. J. Am. Chem. Soc. 2000, 122, 12876.
1c
2c
3c
4c
5
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.6
0.6
LiOAc
LiCl
140
140
140
140
140
120
100
120
120
120
2
4
2
2
2
6
8
6
9
8
34
21
4
24
64 (60)
99 (80)
61
86
99
Cs2CO3
NaOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
(4) X) Y ) halogen: (a) Huang, W.; Zhou, X.; Kanno, K.-I.; Takahashi,
T. Org. Lett. 2004, 6, 2429. X ) OTf, Y ) SiMe3: (b) Yoshikawa, E.;
Radhakrishnan, K. V.; Yamamoto, Y. J. Am. Chem. Soc. 2000, 122, 7280.
(c) Pen˜a, D.; Pe´rez, D.; Guitia´n, E.; Castedo, L. J. Org. Chem. 2000, 65,
6944. (d) Pen˜a, D.; Pe´rez, D.; Guitia´n, E.; Castedo, L. J. Am. Chem. Soc.
1999, 121, 5827. (e) Pen˜a, D.; Escudero, S.; Pe´rez, D.; Guitia´n, E.; Castedo,
L. Angew. Chem., Int. Ed. 1998, 37, 2659.
6
7
8d
9
(5) X ) CrPh2, Y ) H: (a) Whitesides, G. M.; Ehmann, W. J. J. Am.
Chem. Soc. 1970, 92, 5625. (b) Herwig, W.; Metlesics, W.; Zeiss, H. J. Am.
Chem. Soc. 1959, 81, 6203. X ) I, Y ) H: (c) Kawasaki, S.; Satoh, T.;
Miura, M; Nomura, M. J. Org. Chem. 2003, 68, 6836. (d) Wu, G.;
Rheingold, A. L.; Feib, S. L.; Heck, R. F. Organometallics 1987, 6, 1941.
(e) Sakakibara, T.; Tanaka, Y.; Yamasaki, T.-I. Chem. Lett. 1986, 797. X
) COCl, Y ) H: (f) Yasukawa, T.; Satoh, T.; Miura, M.; Nomura, M.
J. Am. Chem. Soc. 2002, 124, 12680. X ) CO2H, Y ) H: (g) Ueura, K.;
Satoh, T.; Miura, M. J. Org. Chem. 2007, 72, 5362. X ) CR2OH, Y ) H:
(h) Uto, T.; Shimizu, M.; Ueura, K.; Tsurugi, H.; Satoh, T.; Miura, M. J.
Org. Chem. 2008, 73, 298. X ) Y ) H: (i) Umeda, N.; Tsurugi, H.; Satoh,
T.; Miura, M. Angew. Chem., Int. Ed. 2008, 47, 4019. (j) Wu, Y.-T.; Huang,
K.-H.; Shin, C.-C.; Wu, T.-C. Chem.sEur. J. 2008, 14, 6697.
10e
71
a Reaction conditions: [2a]/[Pd(OAc)2]/[Cu(OAc)2·H2O]/[additive] ) 0.8:
0.02:0.8:1.2 (in mmol), MS4A (400 mg) in DMAc (2.5 mL) under N2. b GC
yield based on the amount of 2a used. Value in parentheses indicates yield
afterpurification.c InDMAc(10mL).d InDMF(2.5mL).e 1-Methylindole-2-carbo-
xylic acid (1b) was used in place of 1a.
LiCl, Cs2CO3, and NaOAc were found to be less effective
than LiOAc (entries 2-4). In these cases, significant amounts
(6) Selected reviews: (a) Kakiuchi, F.; Kochi, T. Synthesis 2008, 3013.
(b) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2008, 41,
1013. (c) Ferreira, E. M.; Zhang, H.; Stoltz, B. M. Tetrahedron 2008, 64,
5987. (d) Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41,
222. (e) Herrerias, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. ReV. 2007, 107,
2546. (f) Alberico, D.; Scott, M. E.; Lautens, M. Chem. ReV. 2007, 107,
174. (g) Godula, K.; Sames, D. Science 2006, 312, 67. (h) Satoh, T.; Miura,
M. J. Synth. Org. Chem. 2006, 64, 1199. (i) Conley, B. L.; Tenn, W. J.,
III; Young, K. J. H.; Ganesh, S. K.; Meier, S. K.; Ziatdinov, V. R.; Mironov,
O.; Oxgaard, J; Gonzales, J.; Goddard, W. A., III; Periana, R. A. J. Mol.
Catal. A 2006, 251, 8. (j) Miura, M.; Satoh, T. Top. Organomet. Chem.
2005, 14, 55. (k) Kakiuchi, F.; Chatani, N. AdV. Synth. Catal. 2003, 345,
1077. (l) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731.
(m) Miura, M.; Nomura, M. Top. Curr. Chem. 2002, 219, 211. (n) Kakiuchi,
F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (o) Dyker, G. Angew. Chem.,
Int. Ed. 1999, 38, 1698. (p) Kakiuchi, F.; Murai, S. Top. Organomet. Chem.
1999, 3, 47. (q) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879.
(7) For recent examples of the catalytic decarboxylation as well as
decarboxylative coupling reactions of carboxylic acids, see: (a) Miyasaka,
M.; Fukushima, A.; Satoh, T.; Hirano, K.; Miura, M. Chem.sEur. J. 2009,
15, 3674. (b) Huang, X.-C.; Wang, F.; Liang, Y.; Li, J.-H. Org. Lett. 2009,
11, 1139. (c) Duan, Z.; Ranjit, S.; Zhang, P.; Liu, X. Chem.sEur. J. 2009,
15, 3666. (d) Bi, H.-P.; Zhao, L.; Liang, Y.-M.; Li, C.-J. M. Angew. Chem.,
Int. Ed. 2009, 48, 792. (e) Sharma, A.; Kumar, R.; Sharma, N.; Kumar, V.;
Sinha, A. K. AdV. Synth. Catal. 2008, 350, 2910. (f) Goossen, L. J.;
Rodr´ıguez, N.; Linder, C. J. Am. Chem. Soc. 2008, 130, 15248. (g) Goossen,
L. J.; Zimmermann, B.; Knauber, T. Angew. Chem., Int. Ed. 2008, 47, 7103.
(h) Voutchkova, A.; Coplin, A.; Leadbeater, N. E.; Crabtree, R. H. Chem.
Commun. 2008, 6312. (i) Becht, J.-M.; Le Drian, C. Org. Lett. 2008, 10,
3161. (j) Nakano, M.; Tsurugi, H.; Satoh, T.; Miura, M. Org. Lett. 2008,
10, 1851. For a recent review, see: (k) Goossen, L. J.; Rodr´ıguez, N.;
Goossen, K. Angew. Chem., Int. Ed. 2008, 47, 3100.
2338
Org. Lett., Vol. 11, No. 11, 2009

(0.36-0.49 mmol) of 1-methylindole were also formed by
simple decarboxylation. However, under concentrated condi-
tions using DMAc (2.5 mL), the formation of 1-methylindole
was suppressed (0.16 mmol) and the yield of 3a was
improved to 64% (entry 5). At 120 °C, to our delight, 3a
was obtained almost quantitatively (entry 6), while the yield
was significantly reduced at 100 °C (entry 7). In DMF, the
yield decreased slightly (entry 8). Decreasing the amount of
1a to 0.6 mmol did not affect the reaction efficency (entry
9). Under similar conditions, 1-methylindole-2-carboxylic
acid (1b) also reacted with 2a, but the yield of 3a was
considerably lower (71%, entry 10).
anthracene, by a factor of 3.7 (λemis 393, 414 nm, A versus
B in Figure 1).
The reactions of 1a using various internal alkynes in place
of 2a were next examined. Under optimized conditions in
Table 1 (entry 6, condtions A), methyl- (2b), methoxy- (2c),
and chloro-substituted diphenylacetylenes (2d) underwent the
coupling with 1a to afford the corresponding 1,2,3,4-
tetraaryl-9- methylcarbazoles 3b-d in good yields (entries
1-3 in Table 2). Compared to these diarylacetylenes,
Figure 1. Fluorescence spectra of 3b (A) and anthracene (B) in
the solid-state upon excitation at 350 nm.
The reaction of 1a with 2 seems to be initiated via
fundamentally similar steps to those proposed for the
oxidative coupling of 1a with alkenes using Pd(OAc)2/
Cu(OAc)2·H2O/LiOAc system.10 Thus, as depicted in Scheme
3, coordination of the carboxyl oxygen to Pd(OAc)2 with
Table 2. Reaction of 1-Methylindole-3-carboxylic Acid (1a)
with Alkynes 2a
Scheme 3
entry
2
R
conditions time (h) 3, % yieldb
1
2
3
4
5
6
2b 4-MeC6H4
2c 4-MeOC6H4
2d 4-ClC6H4
2e Pr
A
A
A
A
B
B
8
8
12
6
10
10
3b, 98 (82)
3c, (75)
3d, (62)
3e, 32
3e, 62 (58)
3f, 70 (55)
2e Pr
2f
heptyl
a Reaction conditions: (A) [1a]/[2]/[Pd(OAc)2]/[Cu(OAc)2·H2O]/[LiOAc]
) 0.6:0.8:0.02:0.8:1.2 (in mmol), MS4A (400 mg) in DMAc (2.5 mL) at
120 °C under N2; (B) [1a]/[2]/[Pd(OAc)2]/[Cu(OAc)2·H2O]/[LiOAc]/
[LiOH·H2O] ) 0.4:1.6:0.02:0.8:2.4:1.2 (in mmol), MS4A (400 mg) in
DMAc (2.5 mL) at 100 °C under air. b GC yield based on the amount of 2
(conditions A) or 1a (conditions B) used. Value in parentheses indicates
yield after purification.
liberation of AcOH gives a palladium(II) carboxylate A and
directed palladation at the C2-position forms a palladacycle
intermediate B. Subsequent alkyne insertion and decarboxy-
lation occur to produce a five-membered palladacycle
intermediate D.11 Then, the second alkyne insertion and
reductive elimination steps take place to form carbazole 3.
The resulting Pd(0) species may be oxidized in the presence
4-octyne (2e) was found to be less reactive. Thus, under
conditions A, 9- methyl-1,2,3,4-tetrapropyl-9H-carbazole (3e)
was formed in 32% yield (entry 4). Some screening experi-
ments for reaction conditions led to the establishment of
conditions B, which are suitable for the reaction with
dialkylacetylenes. Thus, using an excess amount of 2e (1a/
2e ) 1:4) in the presence of LiOH·H2O (1.2 mmol) as well
as LiOAc (2.4 mmol) as additive at 100 °C under air, 3e
was obtained in 62% yield (entry 5). The reactions of 1a
with 8-hexadecyne (2f) proceeded similarly under conditions
B to give 1,2,3,4-tetraheptyl-9-methylcarbazole (3f) in 70%
yield (entry 6).
(8) Shimizu, M.; Hirano, K.; Satoh, T.; Miura, M. J. Org. Chem. 2009,
74, 3478.
(9) For recent examples, see: (a) Adhikari, R. M.; Neckers, D. C.; Shah,
B. K. J. Org. Chem. 2009, 74, 3341. (b) Jordan-Hore, J. A.; Johansson,
C. C. C.; Gulias, M.; Beck, E. M.; Gaunt, M. J. J. Am. Chem. Soc. 2008,
130, 16184. (c) Tsuchimoto, T.; Matsubayashi, H.; Kaneko, M.; Nagase,
Y.; Miyamura, T.; Shirakawa, E. J. Am. Chem. Soc. 2008, 130, 15823. (d)
Janosik, T.; Wahlstro¨m, N.; Bergman, J. Tetrahedron 2008, 64, 9159. (e)
Song, Y.; Di, C.; Wei, Z.; Zhao, T.; Xu, W.; Liu, Y.; Zhang, D.; Zhu, D.
Chem.sEur. J. 2008, 14, 4731. (f) Tsang, W. C. P.; Munday, R. H.; Brasche,
G.; Zheng, N.; Buchwald, S. L. J. Org. Chem. 2008, 73, 7603. (g) Lie´gault,
B.; Lee, D.; Huestis, M. P.; Stuart, D. R.; Fagnou, K. J. Org. Chem. 2008,
73, 5022. (h) Kuwahara, A.; Nakano, K.; Nozaki, K. J. Org. Chem. 2005,
70, 413. (i) Qiao, X.; Ho, D. M.; Pascal, R. A., Jr. J. Org. Chem. 1996, 61,
6748. (j) For review, see: (k) Kno¨lker, H.-J.; Reddy, K. R. Chem. ReV.
2002, 102, 4303.
Some of tetraarylcarbazoles obtained above showed solid-
state fluorescence in a range of 380-450 nm (see the
Supporting Information). Notably, 3b exhibited a relatively
strong emmission compared to a typical blue emitter,
Org. Lett., Vol. 11, No. 11, 2009
2339

of the copper(II) salt to regenerate Pd(OAc)2. During
palladium-catalyzed oxidative reactions, in general, the
regeneration of Pd(II) from Pd(0) is considered to be
the crucial step to determine catalyst efficiency.12 One of
the possible roles of added LiOAc is to provide acetate anions
as ligand to prevent the deactivation of Pd(0) to metallic
species.13
In the cases using dialkylacetylenes, the addition of
LiOH·H2O besides LiOAc improved the reaction efficiency.
Without this base, formation of 1:1 oxidative coupling
products together with 3 was detected by GC-MS. One of
its possible roles appears to be the trap of acids formed during
the reaction, which may induce ring-opening of palladacycle
intermediates such as C and D to form acyclic vinylpalladium
species. The latter are known to undergo ꢀ-hydrogen
elimination to produce allene derivatives.14
catalyst system of Pd(OAc)2/Cu(OAc)2·H2O/LiOAc. Expect-
edly, 1-(methoxymethyl)indole-3-carboxylic acid (1c) un-
derwent the coupling with 2a to produce the corresponding
carbazole 3g selectively in 80% yield (entry 1). In contrast,
the reaction of 1-phenylindole-3-carboxylic acid (1d) with
2a gave a mixture of 1:2 and 1:1 coupling products. Thus,
when these substrates were treated using Pd(OAc)2 (0.04
mmol) for 12 h, not only the expected homologation product,
1,2,3,4,9-pentaphenyl-9H-carbazole (3h) (21%) but also a
tetracyclic compound 4 were formed (36%) by vinyl bridging
(entry 2). From the reaction of 1-methylpyrrole-2-carboxylic
acid (1e), a highly substituted indole derivative 5 was
obtained in 63% yield (entry 3). In this case, increasing in
the amounts of the substrate 1e (0.8 mmol) and LiOAc (2.4
mmol) resulted in a better product yield.
Meanwhile, the reactions of benzofuran- (1f) and furan-2-
carboxylic acids (1g) with 2a were found to proceed effectively
under air with the addition of an appropriate acid. The reaction
of 1f using LiOH·H2O (1.2 mmol) and 2,2-dimethylsuccinic
acid (2.4 mmol) as additives in place of LiOAc afforded 1,2,3,4-
tetraphenyldibenzofuran (6) in 72% yield (entry 4). In the
reaction of 1g, once produced 1,2,3,4-tetraphenylbenzofuran (7)
was decomposed under the standard oxidative conditions.
Therefore, the amount of Cu(OAc)2·H2O was decreased to 0.05
mmol and pivalic acid (2.4 mmol) was added in place of LiOAc.
Under such modified conditions, 7 was obtained with a
substantial yield (entry 5).
Table 3 summarizes the results for the coupling reactions
of a series of heteroarene carboxylic acids with 2a using the
Table 3. Reaction of Heteroarenecarboxylic Acids 1c-g with 2aa
In summary, we have demonstrated that the palladium-
catalyzed oxidative coupling of indole-3-carboxylic acids with
two molecules of alkynes proceeds efficiently via directed C-H
bond cleavage and decarboxylation to give the corresponding
1,2,3,4-tetrasubstituted carbazoles. Other related heteroaren-
ecarboxylic acids also undergo the aromatic homologation. In
these reactions, the carboxyl function effectively acts as a
unique, removable directing group.5g,7a,j,8,10,15 Work is under-
way toward further development of relevant reactions around
the key functional group.
Acknowledgment. This work was partly supported by
Grants-in-Aid from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and the Kurata Memorial
Hitachi Science and Technology Foundation.
Supporting Information Available: Standard experimental
procedure and characterization data of products. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL900736S
(10) Maehara, A.; Tsurugi, H.; Satoh, T.; Miura, M. Org. Lett. 2008,
10, 1159.
(11) However, the participation of another sequence involving decar-
boxylation on a copper carboxylate cannot be excluded (see ref 7k).
(12) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400.
(13) Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254, and
references therein.
a Reaction conditions: [1]/[2a]/[Pd(OAc)2]/[Cu(OAc)2·H2O]/[LiOAc] )
0.6:0.8:0.02:0.8:1.2 (in mmol), MS4A (400 mg) in DMAc (2.5 mL) at 120
°C. b GC yield based on the amount of 2a used. Value in parentheses
indicates yield after purification. c Pd(OAc)2 (0.04 mmol) was used. d 1
(0.8 mmol) was used. e LiOAc (2.4 mmol) was used. f LiOH·H2O (1.2 mmol)
and 2,2-dimethylsuccinic acid (2.4 mmol) were used in place of LiOAc.
g Cu(OAc)2·H2O (0.05 mmol) was used. h Pivalic acid (2.4 mmol) was used
in place of LiOAc.
(14) For example, see: Pivsa-Art, S.; Satoh, T.; Miura, M.; Nomura,
M. Chem. Lett. 1997, 823.
(15) For recent examples of ortho-arylation, see: (a) Chiong, H. A.;
Pham, Q.-N.; Daugulis, O. J. Am. Chem. Soc. 2007, 129, 9879. (b) 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.
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