Microwave-assisted aminocarbonylation of aryl bromides at low carbon monoxide pressure

Article abstract of DOI:10.1055/s-0028-1087381

An efficient and rapid procedure for microwave-assisted aminocarbonylation of aryl bromides and iodides at low CO pressure, using (PPh₃) ₂PdCl₂ or Pd(OAc)₂ as catalysts is reported. Different reaction conditions

Full text of DOI:10.1055/s-0028-1087381

LETTER
47
Microwave-Assisted Aminocarbonylation of Aryl Bromides at Low Carbon
Monoxide Pressure
M
icrowave-
A
r
ssiste
d
A
a
minocarbonylation cesca Cardullo,a Daniele Donati,a Giancarlo Merlo,a Alfredo Paio,a Elena Petricci,*b Maurizio Taddeib
a
GlaxoSmithKline, Medicinal Research Centre, Via A. Fleming 4, 37135 Verona, Italy
Dipartimento Farmaco Chimico Tecnologico, Università degli Studi di Siena, Via A. Moro 2, 53100 Siena, Italy
Fax +39(057)7234275; E-mail: petricci@unisi.it
b
Received 1 August 2008
(over)stoichiometric use results in extreme metal waste, a
potential problem in scale-up.
Abstract: An efficient and rapid procedure for microwave-assisted
aminocarbonylation of aryl bromides and iodides at low CO pres-
sure, using (PPh3)2PdCl2 or Pd(OAc)2 as catalysts is reported. Dif-
ferent reaction conditions have been tested in order to carry out the
reaction even on less nucleophilic anilines such as 2-chloro-4-
nitroaniline, allowing a rapid access (in 20–30 min) to several di-
verse arylamides in good yields.
Recent improvements in design of commercial micro-
wave ovens for organic synthesis allow the controlled in-
troduction of gases inside the reaction tube that can be
considered as a potential small autoclave. With these kind
of apparatus hydroformylation,13 hydroaminomethy-
lation,14 hydrogenation15 and hydroxycarbonylation16 or
alkoxycarbonylation17 have been performed in mild con-
ditions.18
Key words: aminocarbonylation, microwave, carbon monoxide
Chemoselective acylation of amino groups is an important
synthetic reaction in organic chemistry. From the time of
Theodor Curtius1 and Emil Fisher2 the formation of car-
boxamides is based on the reaction of amines with an ac-
tivated carboxylic acid such as acyl chloride, mixed
anhydride, acyl azide, or ester,3 or by in situ activation
with coupling reagents, such as 2¢,3¢,-dideoxycytidine
(DDC), EDC, N-hydroxybenzotriazole (HOBt), BOP,
1,1¢-carbonyldiimidazole (CDI), or O-benzotriazole-
N,N,N¢,N¢-tetramethyluronium hexafluorophosphate (HBTU).4
Other methods include Schmidt’s reaction,5 Beckmann’s
rearrangement,6 decomposition of ammonium salts of car-
boxylic acids, direct attack of isocyanates on aromatic
rings7 and carbonylation reaction in the presence of amino
derivatives.8
Due to the potential interest in finding more new versatile
procedures, a microwave-assisted carbonylative way to
aryl amides was investigated.
Iodobenzene (1a), 1-bromo-4-fluorobenzene (1b) and p-
methoxybenzylamine (2) were used as model substrates
for the optimization of the carbonylation reaction condi-
tions (Scheme 1, Table 1).19
X
OMe
R
cat. (5 mol%), base (3 equiv)
solvent, CO, MW
1a,b
O
+
N
H2N
H
R
Palladium-catalyzed amino- and alkoxycarbonylation of
aryl halides and triflates is a valuable alternative to the
above methods especially regarding selectivity and atom
economy.9,10 Although appreciated in industrial chemis-
try, the use of gaseous carbon monoxide (sometimes em-
ployed at high pressure) may be problematic in a standard
organic synthesis laboratory. Microwave dielectric heat-
ing has been applied in order to circumvent these prob-
lems. Even if thermal decomposition of DMF in highly
basic conditions has been employed,11 Mo(CO)6 is the
most represented source of CO applied to the microwave-
assisted synthesis of amides, esters and carboxylic acids
starting from aryl iodides.12 Advantages of using
Mo(CO)6 as a replacement for gaseous CO include its sol-
id state for its use in small-scale monomode microwave
ovens. However, Mo(CO)6 is highly toxic and its
OMe
3a,b
2
a: R = H, X = I; b: R = F, X = Br
Scheme 1
Several Pd catalysts are commercially available and po-
tentially suitable for aminocarbonylation; amongst them,
Pd(OAc)2 and (Ph3P)2PdCl2 were selected for their avail-
ability, easy storage and sustainable price. Relying on the
assistance of microwave heating, no additional ligands
were used in the first attempts. Using ‘standard’ coupling
conditions [Pd(OAc)2, DMF and DIPEA] iodide 1a was
transformed into 3a in high yields in 20 minutes at 160 °C
in a tube for microwave reactions pressurized with 120 psi
of CO (entry 1 in Table 1). When the same conditions
were applied to bromide 1b no reaction occurred. Chang-
es in temperature, reaction time and solvent did not afford
the expected amide (entries 2–4 in Table 1). When
(Ph3P)2PdCl2 was used as catalyst (5% mol), bromide 1b
was carbonylated and transformed into amide 3b in high
yield (entry 5 in Table 1).8a,20 A decrease in temperature to
SYNLETT 2009, No. 1, pp 0047–0050
x
x.
x
x.
2
0
0
8
Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1087381; Art ID: D28908ST
© Georg Thieme Verlag Stuttgart · New York

48
F. Cardullo et al.
LETTER
130 °C was still supported by the catalytic system with
formation of expected product (entry 6 in Table 1). How-
ever, at 100 °C the yield of 3b dropped.21 THF was also a
suitable solvent for the reaction (entry 9 in Table 1)
whereas change in the nature of the base (K2CO3 in the
place of DIPEA) lowered the yields. Surprisingly,
(Ph3P)2PdCl2 was not able to catalyze the aminocarbony-
lation of iodide 1a (entry 11 in Table 1). The use of lower
pressure of CO (40 psi) caused an abrupt decrease in con-
version into 3a and even after a longer microwave irradi-
ation of the reaction mixture the amide was never
obtained in satisfactory yields (entries 12 and 13 in
Table 1).22
Pd(PPh3)2Cl2 (5 mol%),
DIPEA (3 equiv), CO (120 psi),
THF, MW 130 °C, 20 min
O
Br
R2
N
R3
R2
+ HN
R3
R1
1b–d
R1
O
2, 4–7
8–14
O
N
N
O
N
F
F
Ph
8 (90%)
9 (88%)
O
COOMe
O
N
H
N
H
Table 1 Optimization of Microwave-Assisted Aminocarbonylation
F
10 (70%)
11 (50%)
F
Entry ArX Reaction conditions
Catalyst
Yield
(%)a
O
O
N
1
2
1a 160 °C, 20 min, DMF, DIPEA, Pd(OAc)2
92
N
H
CO (120 psi)
O
Et
OMe
12 (88%)
Et
13 (85%)
1b 160 °C, 20 min, DMF, DIPEA, Pd(OAc)2
0
CO (120 psi)
OMe
NO2
O
3
1b 160 °C, 40 min, DMF, DIPEA, Pd(OAc)2
0
N
H
CO (120 psi)
OMe
4
1b 160 °C, 20 min, THF, DIPEA, Pd(OAc)2
0
14 (50%)
CO (120 psi)
Scheme 2
5
1b 160 °C, 20 min, DMF, DIPEA, (Ph3P)2PdCl2
90
92
40
90
95
70
0
CO (120 psi)
amino acids (amide 10) and aniline (amide 11) in good to
acceptable yields and in short reaction times.
6
1b 130 °C, 20 min, DMF, DIPEA, (Ph3P)2PdCl2
CO (120 psi)
When a less nucleophilic amine such as 2-nitro-5-chloro-
aniline (16) was submitted to aminocarbonylation with
aryl bromide 1c, the expected anilide was not obtained.
The synthesis of these kinds of amides is indeed particu-
larly difficult due to the low nucleophilicity of the
amine.24 The use of the corresponding nitroazides and
selenocarboxylates25 or N-Boc anilines with aryliodides26
at 100 °C for 5–24 hours has been reported in order to ob-
tain good yields. With the idea of applying aminocarbony-
lation procedure to the synthesis of 17, different reaction
conditions were investigated, and we found that changes
in the nature of base or solvent had a dramatic effect on fi-
nal result (Table 2).
7
1b 100 °C, 20 min, DMF, DIPEA, (Ph3P)2PdCl2
CO (120 psi)
8
1b 160 °C, 20 min, DMF,b
(Ph3P)2PdCl2
CO (120 psi)
9
1b 130 °C, 20 min, THF, DIPEA, (Ph3P)2PdCl2
CO (120 psi)
10
11
12
13
1b 160 °C, 20 min, THF, K2CO3, (Ph3P)2PdCl2
CO (120 psi)
1a 160 °C, 20 min, DMF, DIPEA, (Ph3P)2PdCl2
CO (120 psi)
1a 160 °C, 20 min, DMF, DIPEA, Pd(OAc)2
35
70
CO (40 psi)
Best reaction conditions for 16 were as follows: THF,
Cs2CO3 (3 equiv), Pd(PPh3)2Cl2 (10 mol%) under pres-
sure of CO (120 psi) at 120 °C for 30 minutes arising com-
pound 17 in 80% yield.27 It is worth noting that the
expected amide 17 was obtained using only one equiva-
lent of 16 with respect to 1c whereas the use of three
equivalents excess of the amine has been reported as a
condition required for microwave-assisted aminocarbon-
ylation with other CO sources.11, 12 When three equiva-
lents of aniline 16 were employed, a complete conversion
of the bromide was observed just after 15 minutes of irra-
diation. However, no trace of 17 was observed in DMF as
well as using a tertiary amine like DIPEA as base. More-
1a 160 °C, 90 min, DMF, DIPEA, Pd(OAc)2
CO (40 psi)
a Isolated yield.
b Three equivalents of 2 were used as base and nucleophile.
The best reaction conditions found on model substrates
were applied to different aryl bromides 1b–d to prepare
amides 8–14 (Scheme 2).23
The protocol allows the preparation of amides starting
from primary and secondary amines as well as from
Synlett 2009, No. 1, 47–50 © Thieme Stuttgart · New York

LETTER
Microwave-Assisted Aminocarbonylation
49
Table 2 Optimization of the Reaction Conditions with Aromatic
Amines
O
O
N
NO2
Cl
Cl
N
H
NO2
H
Br
+
Pd(PPh3)Cl2 (10 mol%),
base (3 equiv), additive,
solvent
Et
19 (82%)
O
Et
18 (85%)
Et
H2N
N
H
1c
CO (120 psi), MW
Et
O
Cl
O
NO2
NO2
17
N
H
NO2
16
N
H
NO2
Solvent, base
Additivesa
Temperature, time
Yield (%)
F
21 (80%)
20 (75%)
F
DMF, DIPEA
DMF, DIPEA
toluene, DIPEA
DMF, Cs2CO3
DMF, Cs2CO3
toluene, Cs2CO3
toluene, Cs2CO3
toluene, DIPEA
toluene, Cs2CO3
toluene, Cs2CO3
toluene, Cs2CO3
THF, Cs2CO3
THF, K2CO3
–
130 °C, 20–40 min
160 °C, 20–40 min
160 °C, 20–40 min
130 °C, 20–40 min
160 °C, 20–40 min
120 °C, 2 × 20 minb
130 °C, 20–40 min
130 °C, 20–40 min
120 °C, 30 minb
0
0
O
HN
N
O
imidazole
N
N
H
N
H
–
0
F
22 (70%)
23 (90%)
Et
–
0
O
S
imidazole
0
10c
0
N
N
–
H
Et
imidazole
24 (85%)
Xantphos
0
Figure 1
–
48d
14d
0
80 °C, 2 × 30 minb
120 °C, 2 × 30 min
120 °C, 30 min
Acknowledgment
–
This work was financially supported by GlaxoSmithKline, and the
Psychiatry Centre of Excellence for Drug Discovery (Verona,
Italy). The support of MIUR (PRIN Project 2006) is also acknow-
ledged.
4 Å MSe
80
43d
120 °C, 30 min
a For the use of additives, see refs. 11 and 12.
References and Notes
b Higher reaction temperatures (130 °C) were reached using not com-
pletely anhyd Cs2CO3 but with lower yield of 17.
c The product arising from dehalogenation of 1c was observed.
d HPLC yields.
(1) Curtius, T. Ber. Dtsch. Chem. Ges. 1902, 35, 3266.
(2) Fisher, E. Ber. Dtsch. Chem. Ges. 1905, 39, 530.
(3) (a) Smith, M. B.; March, J. In March’s Advanced Organic
Chemistry, 5th ed.; John Wiley & Sons, Inc.: New York,
2001, 506; and references cited therein. (b) Larock, R. C.
Comprehensive Organic Transformations; Wiley-VCH:
Weinheim, 1989, 963.
(4) Merrifield, B. in Peptides: Synthesis, Structures and
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(5) (a) Koldobski, G. I.; Ostrovskii, V. A.; Gidaspov, B. V.
Russ. Chem. Rev. (Engl. Transl.) 1978, 47, 1084. (b)Smith,
P. A. S. Org. React. (N.Y.) 1946, 3, 337. (c) Surabhi, P.;
Wu, X.; Hu, L. Tetrahedron Lett. 2006, 47, 4609.
(6) (a) Nguyen, M. T.; Raspoet, G.; Vanquickenborne, L. G.
J. Am. Chem. Soc. 1997, 119, 2552. (b) Costa, A.; Mesters,
R.; Riego, J. M. Synth. Commun. 1998, 28, 2275.
(c) Laurent, A.; Jacquault, P.; DiMarino, J.-L.; Hamelin, J.
J. Chem. Soc., Chem. Commun. 1995, 1101.
e A 20 psi absorption of CO by molecular sieves was observed.
over, in contrast with that previously observed, Cs2CO3
was the base of choice for the transformation.
The above procedure was applied to aminocarbonylation
of 1c,d with other aromatic and heteroaromatic amines
giving the corresponding amides 18–24 in only 30 min-
utes in good to excellent yields, showing the general ap-
plicability of this optimized procedure (Figure 1).29
In conclusion, a good methodology for microwave-assist-
ed synthesis of amides by aminocarbonylation of aryl bro-
mides at low carbon monoxide pressure has been
developed. A wide range of amides have been synthesized
in 20–30 minutes under 120 psi of CO, using easily avail-
able catalysts and no additional ligands. We hope that this
procedure could be considered as a practical alternative to
other existing methodologies for the synthesis of aryl
amides, especially in cases made difficult by the low nu-
cleophilicity of the amine.
(7) Piccolo, O.; Filippini, L.; Tinucci, L.; Valoti, E.; Citterio, A.
Tetrahedron 1986, 42, 885.
(8) (a) Schoenberg, A.; Heck, R. F. J. Org. Chem. 1974, 39,
3327. (b) Takács, A.; Jakab, B.; Petz, A.; Kollár, L.
Tetrahedron 2007, 63, 10372.
(9) Skoda-Földes, R.; Kollár, L. Curr. Org. Chem. 2002, 6,
1097.
(10) (a) El Ali, B.; Alper, H. Synlett 2002, 161. (b) Ma, S.; Wu,
B.; Jiang, X. J. Org. Chem. 2005, 70, 2588.
(11) Wan, Y.; Alterman, M.; Larhed, M.; Hallberg, A. J. Org.
Chem. 2002, 67, 6232.
Synlett 2009, No. 1, 47–50 © Thieme Stuttgart · New York

50
F. Cardullo et al.
LETTER
(12) (a) Wu, X. Y.; Larhed, M. J. Org. Chem. 2005, 70, 3094.
(b) Wannberg, J.; Larhed, M. J. Org. Chem. 2003, 68, 5750.
(13) Mann, A.; Petricci, E.; Rota, A.; Schoenfelder, A.; Taddei,
M. Org. Lett. 2006, 8, 3725.
(14) Petricci, E.; Salvadori, J.; Mann, A.; Taddei, M. Tetrahedron
Lett. 2007, 48, 8501.
(15) Piras, L.; Genesio, E.; Ghiron, C.; Taddei, M. Synlett 2008,
1125.
(16) Kormos, C. M.; Leadbeater, N. E. Synlett 2006, 1663.
(17) Kormos, C. M.; Leadbeater, N. E. Org. Biomol. Chem. 2007,
5, 65.
(26) Martinelli, J. R.; Clark, T. P.; Watson, D. A.; Munday, R. H.;
Buchwald, S. L. Angew. Chem. Int. Ed. 2007, 46, 8460.
(27) General Procedure for the Synthesis of Amides 18–24:
A solution of 1 (0.35 mmol) and the amine (0.23 mmol) in
anhyd THF (1 mL) was prepared in a 10-mL tube for
microwave reactions. Anhyd Cs2CO3 (225 mg, 0.69 mmol)
and PdCl2 (PPh3)2 (16 mg, 0.023 mmol) were added and the
solution was submitted to pressurized CO at 120 psi and
heated for 30 min at 120 °C by microwave at 200 W (as
previously described for amides 8–14). The tube was cooled
and the internal gas was released. The reaction mixture was
filtered, evaporated in vacuo and the crude mixture was
analysed by HPLC and/or purified by flash chromatography.
4-Ethyl-N-(3-nitrophenyl)benzamide (18): 1H NMR (400
MHz, CDCl3): d = 8.45 (s, 1 H), 7.85–7.94 (m, 4 H), 7.32 (m,
1 H), 7.27 (d, J = 8.1 Hz, 2 H), 2.72 (q, J = 7.6 Hz, 2 H), 1.26
(q, J = 7.6 Hz, 3 H). ES–MS: m/z = 271 [M + 1]+, 293 [M +
Na]+.
(18) For a review on the argument, see: Petricci, E.; Taddei, M.
Chem. Today 2007, 25, 40.
(19) All reactions were performed in a CEM Discover microwave
oven equipped with a 10-mL tube for reactions under
pressure (CEM Corporation). This glass vial, tested for
resisting up to 250 psi (17 bar, 1723 KPa), is provided with
a tube connection to an external pressure controlling system
equipped with a valve and an exit tube for venting the vial at
the end of the reaction. The exit tube was connected to a
cylinder containing CO through a three-way connector
equipped with two taps to pressurized the system before
microwave irradiation.13
(20) Schoenberg, A.; Bartoletti, I.; Heck, R. F. J. Org. Chem.
1974, 39, 3318.
(21) Surprisingly the use of an additional ligand such as 1,1-
bis(diphenylphosphino)ferrocene (dppf) had a negative
influence on the reaction yields.
4-Ethyl-N-(2-methyl-3-nitrophenyl)benzamide (19):
1H NMR (400 MHz, CDCl3): d = 7.70–7.91 (m, 4 H), 7.29
(d, J = 8.1 Hz, 2 H), 2.72 (q, J = 7.6 Hz, 2 H), 2.33 (s, 3 H),
1.26 (q, J = 7.6 Hz, 3 H). ES–MS: m/z = 285 [M + 1].
N-(2-Chloro-5-nitrophenyl)-4-fluorobenzamide (20): see
ref. 12b.
4-Fluoro-N-(2-methyl-5-nitrophenyl)benzamide (21):
1H NMR (400 MHz, CDCl3): d = 7.98–8.01 (m, 3 H), 7.41–
7.52 (m, 3 H), 2.09 (s, 3 H). ES-MS: m/z = 275 [M + 1]+.
4-Ethyl-N-(quinolin-3-yl)benzamide (22): see ref. 34.
4-Fluoro-N-(1H-imidazol-2-yl)benzamide (23): 1H NMR
(400 MHz, CDCl3): d = 7.99 (d, J = 7.9 Hz, 2 H), 7.39 (d,
J = 7.9 Hz, 2 H), 6.95 (s, 2 H). ES-MS: m/z = 228 [M + Na]+.
4-Ethyl-N-(1,3-thiazol-2-yl)benzamide (24): 1H NMR
(400 MHz, CDCl3): d = 7.91 (d, J = 8.0 Hz, 2 H), 7.32 (d,
J = 8.0 Hz, 2 H), 7.12 (d, J = 3.6 Hz, 1 H), 6.93 (d, J = 3.6
Hz, 1 H), 2.73 (q, J = 7.6 Hz, 2 H), 1.26 (q, J = 7.6 Hz, 3 H).
ES-MS: m/z = 235 [M + 1]+, 257 [M + Na]+.
(22) Working at 1 atm of CO at r.t., no reaction occurred after
24 h.
(23) General Procedure for the Synthesis of Amides 8–14:
A solution of the aryl bromide (0.46 mmol) and the amine
(0.46 mmol) in anhyd THF (1.5 mL) was placed in a 10-mL
tube for microwave reactions. DIPEA (240 mL, 1.38 mmol)
and PdCl2 (PPh3)2 (16 mg, 0.023 mmol) were added and the
solution was submitted to pressurized CO (120 psi) and
inserted in the cavity of a Discover System (CEM
Corporation).13,19 After heating for 20 min at 130 °C at 150
W (value previously set on the microwave oven), the tube
was cooled and the internal gas pressure was released. The
reaction mixture was filtered, evaporated in vacuo and the
amide was purified by flash chromatography.
(28) N-(5-Chloro-2-nitrophenyl)-4-ethylbenzamide (17):
A solution of 1c (33 mL, 0.23 mmol) and 16 (40 mg, 0.23
mmol) in anhyd THF (1 mL) was prepared in a 10-mL tube
for microwave reactions. Anhyd Cs2CO3 (225 mg, 0.69
mmol) and PdCl2 (PPh3)2 (16 mg, 0.023 mmol) were added
and the solution was submitted to pressurized CO at 120 psi
and heated for 30 min at 120 °C by microwave at 200 W (as
previously described for amides 8–14). The tube was cooled
and the internal gas was released. The reaction mixture was
filtered, evaporated in vacuo and the crude mixture was
analysed by HPLC and/or purified by flash chromatography.
1H NMR (400 MHz, CDCl3): d = 11.51 (br s, 1 H), 9.13 (s,
1 H), 8.25 (d, J = 7.8 Hz, 1 H), 7.88 (d, J = 8.0 Hz, 2 H), 7.34
(d, J = 8.0 Hz, 2 H), 7.13 (d, J = 7.8 Hz, 1 H), 2.72 (q, J = 7.6
Hz, 2 H), 1.26 (q, J = 7.6 Hz, 3 H). ES-MS: m/z = 319 [M +
1]+ (35Cl), 321 [M + 1]+ (37Cl).
4-[(4-Fluorophenyl)carbonyl]morpholine (8): see ref. 30.
1-[(4-Fluorophenyl)carbonyl]-4-phenylpiperazine (9):
see ref. 31.
(S)-Methyl 2-[(4-Fluorophenyl)formamido]-2-phenyl-
acetate (10): 1H NMR (400 MHz, CDCl3): d = 7.56 (d, J =
5.6 Hz, 2 H), 7.36–7.42 (m, 5 H), 7.09–7.11 (m, 2 H), 5.73
(d, J = 7.2 Hz, 1 H), 3.75 (s, 3 H). ES-MS: m/z = 289 [M +
1]+.
4-Fluoro-N-phenylbenzamide (11): see ref. 32.
4-Ethyl-N-[(4-methoxyphenyl)methyl]benzamide (12):
1H NMR (400 MHz, CDCl3): d = 7.37 (d, J = 8.0 Hz, 2 H),
7.27–7.31 (m, 5 H), 7.04 (d, J = 8.0 Hz, 2 H), 4.54 (d, J = 5.2
Hz, 2 H), 3.77 (s, 3 H), 2.58 (q, J = 8.2 Hz, 2 H), 1.20 (q,
J = 8.2 Hz, 3 H).
(29) Goldsmith, R. A.; Sutherlin, D. P.; Robarge, K. D.; Oliveo,
A. G. Int. Patent Appl., WO2007059157, 2007.
(30) Tillack, A.; Rudloff, I.; Beller, M. Eur. J. Org. Chem. 2001,
3, 523.
4-[(4-Ethylphenyl)carbonyl]morpholine (13): see ref. 33.
2-Methoxy-N-[(4-methoxyphenyl)methyl]-5-nitrobenz-
amide (14): 1H NMR (200 MHz, CDCl3): d = 8.52 (m, 2 H),
7.64 (d, J = 2.1 Hz, 1 H), 7.26 (d, J = 7.8 Hz, 2 H), 6.82 (d,
J = 7.8 Hz, 2 H), 4.48 (d, J = 3.5 Hz, 2 H), 3.84 (s, 3 H), 3.75
(s, 3 H). ES-MS: m/z = 339 [M + Na]+.
(31) Yu, M. S.; Curran, D. P.; Nagashima, T. Org. Lett. 2005, 7,
3677.
(32) Waisser K., Kunes J., Kubicova L., Budesinsky M., Exner
O.; Magn. Reson. Chem.; 1997, 35: 543.
(33) Jacobson, R. M. Eur. Patent Appl., EP 94-306464, 2005.
(34) Wannberg, J.; Larhed, M. J. Org. Chem. 2003, 68, 5750.
(35) Jensen, J. L.; Burek, P. P. Int. Patent Appl., WO 20040202,
2004.
(24) Surabhi, P.; Wu, X.; Hu, L. Tetrahedron Lett. 2006, 47,
4609.
(25) (a) Klapars, A.; Antilla, J. C.; Huang, X.; Buchwald, S. L.
J. Am. Chem. Soc. 2001, 123, 7727. (b) See also: Arora, R.;
Paul, S.; Gupta, R. Can. J. Chem. 2005, 83, 1137.
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