Phosphane-free palladium-catalyzed carbonylative suzuki coupling reaction of aryl and heteroaryl iodides

Article abstract of DOI:10.1002/ejoc.200900219

The carbonylative cross-coupling reaction of an arylboronic acid with an aryl iodide and a heteroaryl iodide to give unsymmetrical biaryl ketones was carried out by using Pd(tmhd)₂/Pd (OAc)₂ as a phosphane-free catalyst. Various aryl and heteroaryl iodides with different arylboronic acid derivatives provided, good, to excellent yields of the desired products under optimized reaction conditions. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.

Full text of DOI:10.1002/ejoc.200900219

FULL PAPER
DOI: 10.1002/ejoc.200900219
Phosphane-Free Palladium-Catalyzed Carbonylative Suzuki Coupling Reaction
of Aryl and Heteroaryl Iodides
[a]
[a]
[a]
[a]
Pawan J. Tambade, Yogesh P. Patil, Anil G. Panda, and Bhalchandra M. Bhanage*
Keywords: Carbonylation / Cross-coupling / Palladium / Boron
The carbonylative cross-coupling reaction of an arylboronic
acid with an aryl iodide and a heteroaryl iodide to give un-
symmetrical biaryl ketones was carried out by using
derivatives provided good to excellent yields of the desired
products under optimized reaction conditions.
Pd(tmhd)
2
/Pd(OAc)
2
as a phosphane-free catalyst. Various
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
aryl and heteroaryl iodides with different arylboronic acid
Introduction
Among carbonylative cross-coupling reactions, the Su-
zuki carbonylative coupling reaction is one of the most
The cross-coupling reaction is one of the most straight- promising methods for direct synthesis of biaryl ketones
forward and general methods for the formation of carbon– from carbon monoxide, aryl halides, and arylboronic acids.
[
1]
carbon bonds. In this area, the transition-metal-catalyzed, Several groups have reported a variety of palladium-based
[
7]
three-component, cross-coupling reaction between organo- catalytic systems such as PdCl (PPh ) , PdCl (PPh ) /
2
3 2
2
3 2
[
8]
[9]
metallic reagents, carbon monoxide, and organic halides is PdCl (dppf), Pd(OAc) –imidazolium salts, Pd(OAc)2/
2
2
now considered as a useful tool for the synthesis of ketones. N,N-bis(2,6-diisopropylphenyl)dihydroimidazolium chlo-
[
10]
Biaryl ketones are important moieties in many biologically ride,
active molecules, natural products, and pharmaceuticals.
and so on for this reaction. Usually, the catalyst is
[
2]
generated in situ from a palladium source in the presence
Traditionally, biaryl ketones are synthesized by the transi- of highly efficient N/P-containing ligands. In spite of the
tion-metal-catalyzed, three-component, cross-coupling re- significant advances in this area, there are very few reports
action between arylmetal reagents, carbon monoxide, and that employ phosphane-free Pd complexes as catalysts.
aryl electrophiles. Various arylmetal reagents like magne- Moreover, there are a limited number of reports on this
[
3]
[4]
[5]
[6]
sium, silicon, aluminum, and tin have been reported. reaction using heteroaryl halides together with different
However, the known protocols are limited due to the forma- phenylboronic acid derivatives.
tion of side products ensuing from direct coupling of the
aryl groups.
In continuation of our previous work on phosphane-free
carbonylation reactions, we report here a facile protocol
[
11]
Scheme 1. Carbonylative Suzuki coupling reaction of aryl and heteroaryl iodides with phenylboronic acid.
[
a] Department of Chemistry, Institute of Chemical Technology for the carbonylative Suzuki coupling of boronic acids with
(Autonomous),
aryl and heteroaryl iodides by using Pd(tmhd) /Pd(OAc)2
2
N. Parekh Marg, Matunga, Mumbai 400019, India
Fax: +91-22-24145614
(tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) as the cat-
E-mail: bhalchandra_bhanage@yahoo.com
alyst (Scheme 1). Excellent yields of the desired biaryl
3022
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 3022–3025

Phosphane-Free Pd-Catalyzed Carbonylative Suzuki Coupling Reactions
ketones were obtained by using only 2 to 2.5 mol-% of the iodobenzene, providing 96% yield of benzophenone. Elec-
catalyst under optimized reaction conditions.
tron-donating groups such as -CH3 and -OCH3 on the
iodoaryl partner gave excellent results. The reaction of steri-
cally hindered o-iodotoluene with phenylboronic acid also
provided good yield of the desired ketone under the opti-
mized reaction conditions (Table 2, Entry 4). The reaction
Results and Discussion
The reaction of iodobenzene with phenylboronic acid in also worked well with less-reactive p-nitroiodobenzene, pro-
the presence of Pd(tmhd) was chosen as a model reaction, viding 72% yield of p-nitrobenzophenone (Table 2, En-
2
and the influence of various reaction parameters such as try 5). Aryl bromides were not reactive under the carbon-
catalysts {Pd(OAc) , palladium bis(acetylacetonate) [Pd- ylation reaction conditions, and thus, p-bromoiodobenzene
2
(
acac) ], palladium bis(ethyl acetoacetonate) [Pd(eaa) ], and could be converted into a bromophenyl ketone selectively
2
2
PdCl } were tested (Table 1, Entries 1, 3–5). All these com- (Table 2, Entry 6). The carbonylation of bulky 1-iodonaph-
2
plexes showed a lower activity with respect to Pd(tmhd)2, thalene with phenylboronic acid yielded 88% yield of ben-
which provided a 96% yield of the desired product. The zoyl naphthalene (Table 2, Entry 7). We then investigated
influence of different solvents on the carbonylative Suzuki the effect of substituents on the phenylboronic acid, and
coupling reaction was then studied (Table 1, Entries 6–10). both electron-donating and electron-withdrawing groups
Solvents like anisole (96%), toluene (78%), N,N-dimeth- were well tolerated under the reaction conditions. Electron-
ylforamide (DMF, 56%), 1,4-dioxane (62%), THF (68%), donating groups like -CH and -OCH on the boronic acid
3 3
and water (34%) were screened. It was observed that the gave 84% yield of the corresponding ketones (Table 2, En-
reaction gave better results with the use of anisole as a sol-
vent. An inorganic base such as K CO led to a better yield Table 2. Carbonylative Suzuki coupling reaction of aryl iodides
2
3
with various boronic acids.[
a]
than did the use of Et N (Table 1, Entry 11). The influences
3
of temperature and CO pressure were also studied (Table 1,
Entries 12 and 13), and the optimum temperature and pres-
sure were used for further studies. Hence, the optimized re-
action conditions are: Pd(tmhd)2 (2 mol-%), anisole,
K CO , 80 °C, CO (100 psi), 6 h.
2
3
Table 1. Effect of reaction parameters on the carbonylative Suzuki
coupling reaction of iodobenzene with phenylboronic acid.[
a]
Entry
Catalyst
Solvent
Yield [%][b]
Effect of catalyst
1
2
3
4
5
Pd(OAc)
Pd(tmhd)
2
anisole
anisole
anisole
anisole
anisole
84
96
87
82
79
2
Pd(acac)
Pd(eaa)
PdCl
2
2
2
Effect of solvent
6
7
8
9
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
Pd(tmhd)
2
2
2
2
2
2
2
2
toluene
DMF
water
78
56
34
68
62
76
65
74
THF
10
11
12
13
dioxane
anisole
anisole
anisole
[
[
[
c]
d]
e]
[
(
6
a] Reaction conditions: iodobenzene (1 mmol), phenylboronic acid
1.2 mmol), catalyst (2 mol-%), K CO (3 mmol), solvent (10 mL),
h, 80 °C, CO (100 psi). [b] GC yield. [c] Et N was used as the base
instead of K CO . [d] T = 50 °C. [e] p(CO) = 50 psi.
2
3
3
2
3
The optimized reaction conditions were then applied to
the carbonylative Suzuki coupling reaction of various aryl
iodides with different phenylboronic acid derivatives
(Table 2, Entries 1–10). Various electron-donating and elec-
tron-withdrawing groups such as -CH , -OCH , -NO ,
3
3
2
and -Br on both aryl iodides and phenylboronic acids were
well tolerated to give the desired ketone in good to excellent
[
(
a] Reaction conditions: aryl iodide (1 mmol), arylboronic acid
1.1 mmol), Pd(tmhd) (2 mol-%), CO (3 mmol), anisole
2
K
2
3
yield. Phenylboronic acid was found to react smoothly with (10 mL), 6 h, 80 °C, CO (100 psi). [b] Isolated yields.
Eur. J. Org. Chem. 2009, 3022–3025
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P. J. Tambade, Y. P. Patil, A. G. Panda, B. M. Bhanage
FULL PAPER
tries 8 and 9), whereas p-bromophenylboronic acid gave
For the reaction of 2-iodothiophene with phenylboronic
70% yield of the desired product under the optimized reac- acid, a comparatively similar yield of the desired product
tion conditions (Table 2, Entry 10). was obtained with Pd(tmhd) and Pd(OAc) as catalysts in
2
2
For generality, we extended our protocol to heteroaryl 10 h, so the subsequent reactions were carried out with
iodides such as 3-iodopyridine, 2-iodopyridine, and 2-iodo- Pd(OAc) as a catalyst. The reaction of 2-iodothiophene
2
thiophene (Table 3, Entries 1–9). The reaction of 3-iodopyr- with phenylboronic acid by using Pd(OAc) as a catalyst
2
idine with phenylboronic acid was optimized with respect gave 93% of benzoyl thiophene (Table 3, Entry 6). Further,
to different parameters; the optimized reaction conditions the effect of various groups on the boronic acid partner
are: Pd(tmhd) (2.5 mol-%), toluene, K CO , 100 °C, CO demonstrated that electron-donating groups (-CH3 and
2
2
3
(200 psi), 12 h.
-OCH ) and electron-withdrawing groups (Br) gave good to
3
excellent yields (76–88%) of the desired products under the
optimized reaction conditions (Table 3, Entries 7–9).
Table 3. Carbonylative Suzuki coupling reaction of heteroaryl
iodides with various boronic acids.[
a]
Conclusions
In conclusion, we have developed an efficient protocol
for the carbonylative Suzuki coupling of aryl and heteroaryl
iodides with various boronic acids by using phosphane-free
palladium complexes viz. Pd(tmhd) /Pd(OAc) (2–2.5 mol-
2
2
%) as a catalyst. The reaction was optimized with respect
to various reaction parameters and enables carbonylative
coupling reactions of various electron-rich, electron-de-
ficient, and sterically hindered iodides with different bo-
ronic acids, affording excellent yields of the desired prod-
ucts; thus, the methodology has a broad application in this
field.
Experimental Section
Materials and Methods: All chemicals were obtained from Lancas-
ter (Alfa-Aesar) and used as is. Optimized yields were based on
GC (Chemito 1000) and GC–MS (Shimadzu QP 2010) analysis.
1
3
All the products were characterized by
1
2
C NMR (Varian
1
00 MHz), H NMR (Varian 400 MHz), GC–MS (Shimadzu QP
010), and FTIR (Perkin–Elmer).
General Procedure for the Carbonylative Suzuki Reaction of Aryl
Iodides: To a 100-mL autoclave vessel was added the aryl iodide
(
1.0 mmol), phenylboronic acid (1.1 mmol), Pd(tmhd)
anisole (10 mL), and K CO (3 mmol). The mixture was stirred for
0 min, CO (100 psi) was then introduced, and the reaction mixture
2
(2 mol-%),
2
3
1
was heated at 80 °C for 6 h. After completion of the reaction, the
reaction mixture was cooled to room temperature and the solvent
was removed under reduced pressure. The residue obtained was
purified by column chromatography (silica gel, 60–120 mesh; petro-
[a] Reaction conditions: aryl iodide (1 mmol), arylboronic acid
(
1.2 mmol), Pd(tmhd)
10 mL), 12 h, 100 °C, CO (200 psi). [b] Yield based on GC and
was used as catalyst, 10 h.
2
(2.5 mol-%), K
2
CO
3
(3 mmol), toluene leum ether/ethyl acetate, 60:80) to afford the desired carbonylated
(
product. All the prepared compounds are known and compared
GC–MS. [c] Pd(OAc)
2
1
13
with authentic samples and confirmed by GC–MS, H and
C
NMR, and FTIR analysis. Purity of the compounds was deter-
mined by GC–MS analysis.
The coupling reaction of 3-iodopyridine and 2-iodopyr-
idine with phenylboronic acid in the presence of carbon
monoxide gave 90 and 86% yield, respectively, of the de-
sired products. The effect of substituents on the boronic
acid partner with heteroaryl iodides was then explored.
Electron-donating group such as -CH and -OCH on the
boronic acid gave 92 and 84% yield, respectively, of the ex-
pected heteroaryl ketones (Table 3, Entries 3 and 4). The
reaction of 3-iodopyridine with p-bromophenylboronic acid
General Procedure for the Carbonylative Suzuki Reaction of Hetero-
aryl Iodides: To a 100-mL autoclave vessel was added the heteroaryl
iodide (1.0 mmol), phenylboronic acid (1.2 mmol), Pd(tmhd)
2
/
Pd(OAc) (2.5 mol-%), toluene (10 mL), and K CO (3 mmol). The
2
2
3
3
3
mixture was stirred for 10 min, CO (200 psi) was then introduced,
and the reaction mixture was heated at 100 °C for 10 h. After com-
pletion of the reaction, the reaction mixture was cooled to room
temperature and the solvent was removed under reduced pressure.
gave a satisfactory yield of the desired product (Table 3, En- The residue obtained was purified by column chromatography (sil-
try 5). ica gel, 60–120 mesh; petroleum ether/ethyl acetate, 60:80) to afford
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 3022–3025

Phosphane-Free Pd-Catalyzed Carbonylative Suzuki Coupling Reactions
the desired carbonylated product. All the prepared compounds
143.2, 135.5, 134.6, 134.0, 129.5, 129.2, 128.0, 21.8 ppm. GC–MS
1
13
+
were confirmed by GC–MS, H and C NMR, and FTIR analysis.
Purity of the compounds was determined by GC–MS analysis.
(EI, 70 eV): m/z (%) = 202 (100) [M ], 187 (45), 119 (90), 111 (80).
IR (KBr): ν˜ = 1624 cm .
–
1
(
4-Methoxyphenyl)(thien-2-yl)methanone (Table 3, Entry 8): Solid.
Phenyl(pyridin-3-yl)methanone (Table 3, Entry 1): Solid. Yield: 90%
1
164 mg). 1H NMR (400 MHz, CDCl
Yield: 82% (178 mg). H NMR (400 MHz, CDCl , 25 °C): δ =
7
1
(100 MHz, CDCl , 25 °C): δ = 187.0, 163.2, 143.9, 134.2, 133.6,
3
1
˜
= 218 (80) [M ], 187 (20), 135 (100). IR (KBr): ν = 1626 cm .
(
1
3
, 25 °C): δ = 9.03 (d, J =
.2 Hz, 1 H), 8.85–8.86 (dd, J = 4.8, 1.8 Hz, 1 H), 8.15–8.18 (dd,
3
.87–7.89 (d, J = 8.5 Hz, 2 H), 7.62–7.67 (m, 2 H), 7.14–7.16 (m,
1
3
H), 6.95–6.98 (d, J = 8.5 Hz, 2 H), 3.87 (s, 3 H) ppm. C NMR
J = 7.9, 1.8 Hz, 1 H), 7.80 (d, J = 7.9 Hz, 1 H), 7.36–7.79 (m, 4
H) ppm. 1 C NMR (100 MHz, CDCl
3
3
, 25 °C): δ = 194.6, 152.0,
31.8, 130.7, 127.9, 113.8, 55.6 ppm. GC–MS (EI, 70 eV): m/z (%)
1
50.2, 138.0, 133.7, 133.6, 130.1, 129.0, 128.4, 123.9 ppm. GC–MS
+
–1
+
(EI, 70 eV): m/z (%) = 183 (100) [M ], 105 (80), 77 (95), 51 (45).
–1
IR (KBr): ν˜ = 1663 cm .
(
4-Bromophenyl)(thien-2-yl)methanone (Table 3, Entry 9): Solid.
1
Yield: 76% (202 mg). H NMR (400 MHz, CDCl
7.73–7.75 (d, J = 6.95 Hz, 2 H), 7.62–7.65 (m, 2 H), 7.16–7.26 (m,
3 H) ppm. C NMR (100 MHz, CDCl , 25 °C): δ = 189.0, 143.3,
3
1
3
, 25 °C): δ =
Phenyl(pyridin-2-yl)methanone (Table 3, Entry 2): Solid. Yield: 86%
+
(
1
157 mg): GC–MS (EI, 70 eV): m/z (%) = 183 (40) [M ], 155 (80),
05 (65), 77 (100). R (KBr): ν˜ = 1660 cm .
1
3
–1
36.7, 134.9, 134.7, 131.8, 130.8, 128.2, 127.4 ppm. GC–MS (EI,
(
4-Methylphenyl)(pyridin-3-yl)methanone (Table 3, Entry 3): Solid.
+
7
0 eV): m/z (%) = 266/268 (20) [M ], 187 (20), 111 (100). IR (KBr):
1
Yield: 92% (183 mg). H NMR (400 MHz, CDCl
3
, 25 °C): δ = 8.97
–1
ν˜ = 1631 cm .
(s, 1 H), 8.80 (d, J = 3.4 Hz, 1 H), 8.11 (d, J = 1.4 Hz, 1 H), 7.73–
7
7
.71 (d, J = 7.8 Hz, 2 H), 7.43–7.46 (dd, J = 4.87, 4.87 Hz, 1 H),
.30–7.32 (d, J = 7.8 Hz, 2 H), 2.45 (s, 3 H) ppm. 1 C NMR
3
Acknowledgments
(100 MHz, CDCl
3
, 25 °C): δ = 194.5, 152.5, 150.7, 144.3, 137.3,
Financial assistance from the Technical Education Quality Im-
provement Programme (TEQIP), Government of India is kindly
acknowledged.
1
7
34.0, 133.7, 130.3, 129.4, 123.5, 115.4, 21.8· ppm. GC–MS (EI,
+
0 eV): m/z (%) = 197 (50) [M ], 182 (50), 119 (100), 91 (50). IR
–1
(KBr): ν˜ = 1656 cm .
(
4-Methoxyphenyl)(pyridin-3-yl)methanone (Table 3, Entry 4): So-
1
[1] a) J. Tsuji, Transition Metal Reagents and Catalysts, Wiley,
Chichester, 2000; b) R. F. Heck, Palladium Reagents in Organic
Synthesis, Academic Press, New York, 1985.
lid. Yield: 84% (178 mg). H NMR (400 MHz, CDCl
8
7
3
, 25 °C): δ =
.81 (s, 1 H), 8.55 (d, J = 4.2 Hz, 1 H), 7.83 (d, J = 7.6 Hz, 1 H),
.50–7.53 (d, J = 8.6 Hz, 2 H), 7.26–7.35 (dd, J = 5.13, 7.33 Hz, 1
[
2] N. De Kimpe, M. Keppens, G. Froncg, Chem. Commun. 1996,
1
3
H), 6.99–7.02 (d, J = 8.6 Hz, 2 H), 3.85 (s, 3 H) ppm. C NMR
100 MHz, CDCl , 25 °C): δ = 189.0, 159.7, 147.9, 147.7, 136.3,
33.9, 130.1, 128.2, 124.0, 123.5, 114.5, 55.4 ppm. GC–MS (EI,
635–636.
(
1
7
=
3
[3] M. Yamamoto, T. Kohara, A. Yamamoto, Chem. Lett. 1976,
1217–1220.
+
0 eV): m/z (%) = 213 (60) [M ], 198 (20), 135 (100). IR (KBr): ν˜
[4] Y. Hatanaka, S. Fukushima, T. Hiyama, Tetrahedron 1992, 48,
–
1
2113–2119.
1659 cm .
[
5] N. A. Bumagin, A. B. Ponomaryov, I. P. Beletskaya, Tetrahe-
(
4-Bromophenyl)(pyridin-3-yl)methanone (Table 3, Entry 5): Solid.
dron Lett. 1985, 26, 4819–4822.
1
Yield: 78% (203 mg). H NMR (400 MHz, CDCl
3
, 25 °C): δ = 9.04 [6] a) A. M. Echavarren, J. K. Stille, J. Am. Chem. Soc. 1988, 110,
(
7
(
1
1
s, 1 H), 8.90 (d, J = 4.4 Hz, 1 H), 8.18 (d, J = 7.2 Hz, 1 H), 7.83–
1557–1565; b) S. K. Kang, T. Yamaguchi, T. H. Kim, P. S. Ho,
J. Org. Chem. 1996, 61, 9082–9083; c) E. Morera, G. Ortar,
Bioorg. Med. Chem. Lett. 2000, 10, 1815–1818; d) S. Ceccarelli,
U. Piarulli, C. Gennari, J. Org. Chem. 2000, 65, 6254–6256.
7] a) S. Bonnaire, J. F. Carpentier, A. Mortreux, Y. Castanet, Tet-
rahedron Lett. 2001, 42, 3689–3691; b) S. Bonnaire, J. F. Carp-
entier, A. Mortreux, Y. Castanet, Tetrahedron 2003, 59, 2793–
.86 (d, J = 8.3 Hz, 2 H), 7.63–7.66 (d, J = 8.3 Hz, 2 H), 7.50–7.57
dd, 1 H) ppm. 1 C NMR (100 MHz, CDCl
3
3
, 25 °C): δ = 193.0,
51.5, 149.5, 138.3, 138.2, 135.4, 135.2, 135.0, 133.2, 131.9, 128.7,
[
+
24.8 ppm. GC–MS (EI, 70 eV): m/z (%) = 261/263 (10) [M ], 182
–1
(100), 155 (20). IR (KBr): ν˜ = 1667 cm .
2799.
Phenyl(thien-2-yl)methanone (Table 3, Entry 6): Solid. Yield: 93%
1
[8] T. Ishiyama, H. Kizaki, T. Hayashi, A. Suzuki, N. Miyaura, J.
Org. Chem. 1998, 63, 4726–4731.
(
2
(
1
174 mg). H NMR (400 MHz, CDCl
H), 7.48–7.73 (m, 5 H), 7.15–7.17 (m, 1 H) ppm. C NMR
100 MHz, CDCl , 25 °C): δ = 188.3, 143.7, 138.2, 135.0, 133.6,
32.4, 129.2, 129.0, 128.5, 128.0, 124.5 ppm. GC–MS (EI, 70 eV):
3
, 25 °C): δ = 7.85–7.87 (m,
13
[
9] E. Maerten, F. Hassouna, S. Bonnaire, A. Mortreux, J. F.
Carpentier, Y. Castanet, Synlett 2003, 12, 1874–1876.
3
[
10] M. B. Andrus, Y. Ma, Y. Zang, C. Song, Tetrahedron Lett.
+
m/z (%) = 188 (60) [M ], 111 (100), 77 (35). IR (KBr): ν˜ =
1
2002, 43, 9137–9140.
–
1
629 cm .
[11] a) P. J. Tambade, Y. P. Patil, N. S. Nandurkar, B. M. Bhanage,
Synlett 2008, 6, 886–888; b) P. J. Tambade, Y. P. Patil, M. J.
Bhanushali, B. M. Bhanage, Tetrahedron Lett. 2008, 49, 2221–
(4-Methylphenyl)(thien-2-yl)methanone (Table 3, Entry 7): Solid.
1
Yield: 88% (177 mg). H NMR (400 MHz, CDCl
7
1
3
, 25 °C): δ =
.77–7.79 (d, J = 8.0 Hz, 2 H), 7.69–7.70 (m, 1 H), 7.63–7.64 (m,
H), 7.28–7.30 (d, J = 7.6 Hz, 2 H), 7.14–7.16 (m, 1 H), 2.44 (s, 3
2
224; c) P. J. Tambade, Y. P. Patil, M. J. Bhanushali, B. M.
Bhanage, Synthesis 2008, 15, 2347–2352.
Received: March 3, 2009
Published Online: May 4, 2009
13
H) ppm. C NMR (100 MHz, CDCl
3
, 25 °C): δ = 188.0, 143.9,
Eur. J. Org. Chem. 2009, 3022–3025
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3025