Imidazolium-based frameworks: Imidazolium salt-oxazoline/palladium(II) acetate in situ catalyst systems

Article abstract of DOI:10.1055/s-2007-965938

New simple imidazolium-based bidentate ligand precursors were prepared by a three-step protocol and bis(oxazoline) ligands were prepared in two steps. A Suzuki-Miyaura reaction was utilized to ascertain the activity of a palladium(II) catalyst system gene

Full text of DOI:10.1055/s-2007-965938

PAPER
865
Imidazolium-Based Frameworks: Imidazolium Salt–Oxazoline/Palladium(II)
Acetate In Situ Catalyst Systems
Imidazolium
S
a
r
lt–Oxaz
m
oline/Palladium(II)
i
A
cetate
t
I
n
Sit
a
uCatalyst
S
s
ystems Alcalde,* Immaculada Dinarès, Neus Mesquida,* Sandra Rodríguez
Laboratori de Química Orgànica, Facultat de Farmàcia, Universitat de Barcelona, Avda. Joan XXIII s/n, 08028 Barcelona, Spain
E-mail: ealcalde@ub.edu
Received 16 October 2006; revised 2 January 2007
O
O
N
Abstract: New simple imidazolium-based bidentate ligand precur-
sors were prepared by a three-step protocol and bis(oxazoline)
ligands were prepared in two steps. A Suzuki–Miyaura reaction was
utilized to ascertain the activity of a palladium(II) catalyst system
generated in situ from an oxazolinyl-imidazolium cation, a bis(ox-
azoline), and palladium(II) acetate. Among them, two sterically hin-
dered N-arylimidazolium cations demonstrated their efficiency with
both aryl bromides and chlorides in low catalyst-system loading.
N
N
N
N
R2
R3
N
N
N
R1
O
R1
R2
N
R3
R1
R2
A
B
C
R2
R3
O
Key words: bidentate ligands, carbenes, cross-coupling, catalysis,
N-heterocyclic carbenes
N
N
O
N
O
N
R1
N
N
N
N
N
R2 R3
R2
R1
R1
A broad array of imidazolium-based scaffolds have been
rapidly developed with advances in both N-heterocyclic
carbenes (NHCs)1,2 and anion recognition chemistry,3 as
well as with room temperature ionic liquids (RILs).4 N-
Heterocyclic carbenes have emerged as a relevant family
of ancillary ligands in organometallic chemistry due to
their practical applications in both homogeneous and het-
erogeneous systems. In homogeneous catalysis, the cata-
lytic potential of N-heterocyclic carbenes has been
expanded by the inclusion of an ensemble of bidentate
ligands. These incorporate an N-heterocyclic carbene
moiety and a chiral donor subunit (e.g., oxazolinyl-imida-
zolylidene), with the aim of enhancing their catalytic ac-
tivity in stereoselective transformations in organic
synthesis.2 Since the synthesis of the first type of oxazoli-
nyl-N-heterocyclic carbene A, reported by Herrmann et
al.,5a the new bidentate ligand prototypes B–F, among
others, have been utilized as efficient chiral catalysts
(Figure 1). Thus, Burgess and co-workers examined the
asymmetric hydrogenation of alkenes using iridium(I)
catalysts type B,5b Gade and co-workers reported type C
ligands,5c–e,6 Crudden and co-workers reported type D
ligands,5f and Nanchen and Pfaltz reported type A and E
ligands.5g Bolm et al. have described the type F ligands
with a chiral paracyclophane linker.5h
D
E
F
Figure 1 Different types of oxazolinyl-N-heterocyclic carbene bi-
dentate ligands
a convenient multistep route, for the synthesis of cations
1–5, which proceeds in three steps starting from interme-
diate 8. In contrast, bis(oxazolines) 6 and 7 were directly
obtained in two steps. To examine the catalytic potential
of palladium(II)–bidentate ligands generated in situ from
cations 1–3, 5, and bis(oxazoline) 6 and palladium(II)
acetate, the classical Suzuki–Miyaura reaction was used
to validate the catalytic efficiency. In this context, only
one report describes a palladium(II)–bis(oxazoline) cata-
lyst system generated in situ.7
Although different multistep routes could be applied for
the synthesis of oxazolinyl-imidazolium ligand precursors
1–3, (S)-4, and (S)-5, the coexistence of two different ter-
minal heterocyclic moieties, joined through the spacer,
implies the stepwise introduction of the two rings. A rea-
sonable pathway to the target ligand precursors appeared
to involve the incorporation of the imidazolium quaterna-
ry moiety in the last synthetic step. This route requires the
preparation of [(hydroxymethyl)aryl]acetonitrile 8, fol-
lowed by sequential conversion into the title ligand pre-
cursors (Scheme 1). In accordance with the standard
protocol, bis(oxazoline) ligands 6 and (S)-7 were prepared
from 2,4,6-trimethyl-1,3-phenylenediacetonitrile (9).
The present study focuses on the oxazolinyl-imidazolium
salts 1–5 and the corresponding bis(oxazoline) ligands 6
and 7 with a 2,4,6-trimethyl-1,3-phenylene interannular
spacer. The oxazoline units contain (or not) a stereocenter
at C4 and are combined with the N-methylimidazolium
ring for 1 and 4 or sterically hindered N-arylimidazolium
subunits for 2, 3, and 5 (Scheme 1). We have determined
The synthesis of the key intermediate [3-(hydroxymeth-
yl)-2,4,6-trimethylphenyl]acetonitrile (8) began with the
transformation of commercially available bis(chloro-
methyl) derivative 10 into the known compound 11,8
which led to the cyanomethyl compound 8 upon treatment
with calcium carbonate/water in dioxane (Scheme 2). Us-
ing calcium carbonate/water in dioxane,9a the hydrolysis
SYNTHESIS 2007, No. 6, pp 0865–0872
x
x
.
x
x
.2
0
0
7
Advanced online publication: 20.02.2007
DOI: 10.1055/s-2007-965938; Art ID: T15406SS
© Georg Thieme Verlag Stuttgart · New York

866
E. Alcalde et al.
PAPER
Me
Me
Me
Me
Me
Me
Me
Me
CN
CN
Me
Me
O
N
+
Me
Br–
OH
CN
O
Me
O
N
N
N
9
8
N
R1
R1
R2
R2
R3
R2
R1
6, 7
R1
1–5
R2
R1
Me
R2
R3
Me Me
H
6
Me Me
2,4,6-Me3C6H2 Me Me
2,6-i-Pr2C6H3 Me Me
1
2
3
(S,S)-7
i-Pr
(S)-4 Me
2,4,6-Me3C6H2
H
H
i-Pr
i-Pr
(S)-5
Scheme 1
of compound 11 gave alcohol 8 in quantitative yield. efficient in the presence of 50 mol% of cadmium acetate
However, an alternative and less efficient procedure has hydrate giving (oxazolinylmethyl)benzyl alcohol 15 in
been applied to the related (cyanomethyl)benzyl alco- 52% yield, which was transformed into the bromomethyl
hols.9b To shorten the sequential protocol leading to derivative 16 by bromination with phosphorus tribromide
alcohol 8, we attempted the conversion of [(chlorometh- in the absence of base. When the bromination was per-
yl)aryl]acetonitrile 11 into a 2-[3-(chloromethyl)-2,4,6- formed using phosphorus tribromide (1 equiv) and pyri-
trimethylbenzyl]oxazoline. As this proved to be ineffec- dine (either 1.5 or 1.0 equiv) and both the reaction
tive, resulting in the formation of compound 13, we em- temperature (0 °C to –20 °C) and time (12 h to 4 h) were
ployed the standard transformation of arylacetonitrile 8 varied, only decomposition products were observed. Fi-
into arylacetic acid 14, which, instead, produced polymer- nally, quaternization of N-alkylimidazoles 17–19 with
ization products.
[(bromomethyl)aryl]oxazoline 16 under neutral condi-
tions yielded the corresponding targeted cations 1–3. Fol-
lowing the same stepwise synthetic protocol, chiral
cationic ligand precursors (S)-4 and (S)-5 were obtained
starting from arylacetonitrile 8 and (+)-(S)-2-amino-3-
methylbutan-1-ol [(S)-20] (Scheme 3).
Oxazolinyl-imidazolium ligand precursors 1–3, (S)-4, and
(S)-5 were prepared following a three-step sequence
shown in Scheme 3, a synthetic strategy that relies on the
formation of the imidazolium ring from suitable function-
alized key intermediates 15 and (S)-21. Oxazoline ring
formation was examined based on the condensation of Bis(oxazoline) ligands 6 and (S)-7 were prepared from
arylacetonitrile 8 with amino alcohol 12 mediated by cad- commercially available 2,4-bis(chloromethyl)-1,3,5-tri-
mium acetate hydrate. While we initially utilized similar methylbenzene (10), which was transformed into give
conditions to those used in variety of oxazoline sys- 2,4,6-trimethyl-1,3-phenylenediacetonitrile (9). The reac-
tems,10,11 yields were low (£25%). After examining vari- tion of 9 with excess of 2-amino-2-methylpropan-1-ol
ous reaction conditions, the condensation proved most (12) (3.75 equiv) and a catalytic amount of cadmium(II)
Me
Me
Me
Cl
Me
Me
Cl
Me
Cl
CaCO3, H2O
NaCN
anhyd DMSO
34%
dioxane
99%
OH
CN
Me
CN
Me
Me
8
10
11
OH
Cd(OAc)2·2H2O
C6H5Cl
H2SO4, H2O
reflux, 6 h
Me
Me
NH2
12
reflux, 7 d
42%
Me
Me
Me
Me
Me
Me
CN
HO
CO2H
Me
CN
Me
Me
N
14
OH
13
Scheme 2
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

PAPER
Imidazolium Salt–Oxazoline/Palladium(II) Acetate In Situ Catalyst Systems
867
OH
N
Me
Me
17 R = Me
18 R = 2,4,6-Me3C6H2
19 R = 2,6-i-Pr2C6H3
Me
N
Me
Me
Br
Me
Me
Me
NH2
N
PBr3
12
8
R
Cd(OAc)2·2H2O
C6H5Cl
anhyd THF
99%
Me
O
anhyd DMF
O
Me
OH
Me
O
+
N
N
Br –
N
Me
52%
15
N
16
Me
Me
R
Me
Me
1
2
3
R = Me
R = 2,4,6-Me3C6H2 82%
R = 2,6-i-Pr2C6H3 71%
93%
OH
H
N
17 R = Me
18 R = 2,4,6-Me3C6H2
NH2
Me
Me
Me
Br
Me
Me
Me
N
PBr3
(S)-20
R
8
anhyd DMF
Cd(OAc)2·2H2O
C6H5Cl
anhyd THF
99%
O
OH
Me
N
Me
(S)-22
Me
O
O
+
N
N
N
Br –
53%
N
(S)-21
H
H
H
R
R = Me
68%
R = 2,4,6-Me3C6H2 75%
(S)-4
(S)-5
Scheme 3
acetate hydrate (0.05 equiv) gave bis(oxazoline) 6 in 79% explore the catalytic efficiency of the new ligand precur-
yield, whereas condensation of dinitrile 9 with (S)-20 led sors. To this end, the palladium(II) catalysts were gener-
to chiral bis(oxazoline) (S)-7 in 49% yield. Increasing the ated in situ, thereby simplifying the reaction. However,
amount of cadmium acetate hydrate to 0.5 equivalents im- the exact amount and chemical composition of the catalyst
proved the yield of (S)-7 to 59% (Scheme 4).
remain unknown.
The cross-coupling reaction between aryl halides and Palladium(II) catalyst systems were produced in situ from
arylboronic acids (Suzuki–Miyaura reaction) is the most palladium(II) acetate. Catalytic amounts of ligand precur-
versatile method for the synthesis of substituted biaryls. It sor 1–3, 5, or 6 and palladium(II) acetate were stirred for
is frequently utilized to show the efficiency of palladi- two hours in the reaction medium (Cs2CO3, dioxane) at
um(II) catalysts and efforts are currently directed towards 80 °C before the aryl halide and phenylboronic acid were
obtaining improved results with deactivated substrates added. All experiments were stopped after two hours, and
such as aryl chlorides at very low catalyst loadings.12 Ac- the reaction mixture was treated to isolate the coupling
cordingly, the Suzuki–Miyaura reaction was selected to product (Table 1).
Me
Cl
Me
Cl
Me
10
NaCN
anhyd DMSO
93%
OH
H
OH
Me
Me
Me
Me
NH2
Me
Me
Me
Me
NH2
12
(S)-20
Cd(OAc)2·2H2O
C6H5Cl
Cd(OAc)2·2H2O
C6H5Cl
Me
Me
O
O
CN
CN
Me
O
O
N
N
N
N
59%
79%
9
H
Me
Me
H
Me
Me
6
(S,S)-7
Scheme 4
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

868
E. Alcalde et al.
PAPER
Table 1 Suzuki–Miyaura Reactions between Aryl Halides and Phenylboronic Acid Using an In Situ Catalyst Systema
R
X
B(OH)2
L, Pd(OAc)2
Cs2CO3
+
dioxane
2 h, 80 °C
R
Entry
R
X
Ligand
Catalyst (mol%)
Yieldb,c (%)
1
2
OMe
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
1
2
3
5
6
1
2
3
5
1
0.5
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
0.5
91
93
84
36
65
12
23
25
23
93
3
4
5
6
7
8
9
10
COMe
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
3
5
6
1
2
3
5
6
1
2
3
5
6
1
2
3
5
2
3
5
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
0.1
2.5
2.5
2.5
2.5
2.5
1
94
99
49
90
92
97
95
19
66
11
83
71
13
19
6
COMe
1
73
74
17
77
71
8
1
1
Me
2.5
2.5
2.5
a Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.5 mmol), Cs2CO3 (2 mmol), catalyst [ligand precursor/Pd(OAc)2], dioxane
(3 mL), 80 °C, 2 h.
b Yield after chromatographic purification.
c Average of two runs.
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

PAPER
Imidazolium Salt–Oxazoline/Palladium(II) Acetate In Situ Catalyst Systems
869
Initially, we examined the coupling of 4-bromoanisole (a thereby developing their catalytic performance in asym-
deactivated bromoarene) and phenylboronic acid, using metric reactions.
0.5 mol% of oxazolinyl-imidazolium salts 1–3 and 5 or
bis(oxazoline) 6 and palladium(II) acetate (Table 1, en-
tries 1–5). The results showed excellent yields for imid-
Melting point: CTP-MP 300 hot-plate apparatus with ASTM 2C
thermometer. IR (NaCl or KBr disks): Nicolet 205 FT spectropho-
tometer. Optical rotations [a]D20: Perkin-Elmer 241 polarimeter us-
azolium salts 1–3 and were comparable to those reported
by Gade et al.,5c in which the reaction was carried out with
0.2 mol% isolated palladium–oxazolinyl-N-heterocyclic
carbene complex. Moderate yields were, however, ob-
tained for compounds 5 and 6. When the catalyst loading
was 0.1 mol%, the yield of 4-methoxybiphenyl decreased
(Table 1, entries 6–9).
1
ing a 589 nm Na light. H NMR: Varian Gemini 200 (200 MHz),
Varian Gemini 300 (300 MHz), and Mercury 400 (400 MHz) spec-
trometers at 298 K with chemical shifts were referenced relative to
TMS for CDCl3. 13C NMR: Varian Gemini 200 (50.3 MHz), Varian
Gemini 300 (75.4 MHz), and Mercury 400 (100.6 MHz) spectrom-
eters at 298 K. Chemical shifts were referenced relative to the cen-
tral peak CDCl3 (d = 77.0). MS were obtained using CI or EI at 70
eV in a Hewlett-Packard spectrometer (HP-5989A model). ESI+
MS was carried out using a Waters ZQ spectrometer (Micromass
Instruments). TLC: Merck precoated silica gel 60 F254 plates or
Merck neutral alumina 60 F254 plates using UV light (254 nm) as
a visualizing agent and/or 3% aq H2PtCl2–10% aq KI (1:1) or
KMnO4 in EtOH soln. Column chromatography: silica gel 60 ACC
(Merck) or neutral alumina 90 activity II–III (Merck). Microanaly-
ses were performed on a Carlo Erba 1106 analyzer.
Better results were obtained when the reaction was carried
out with an activated bromoarene like 4-bromoacetophe-
none. When using 0.5 mol% or 0.1 mol% of oxazolinyl-
imidazolium salt 1–3 and palladium(II) acetate, the cou-
pling reaction produced similarly satisfactory isolated
yields (>90%) (Table 1, entries 10–12, 15–17). This con-
firms the catalytic efficiency of the in situ palladium(II)/
oxazolinyl-N-heterocyclic carbene system even at low
concentrations. Compounds 5 and 6 demonstrated their
efficiency when using 0.5 mol% of the catalyst system.
However, yields decreased when catalyst system loading
was lowered to 0.1 mol% (Table 1, entries 13, 14 vs. 18,
19).
2,4-Bis(chloromethyl)-1,3,5-trimethylbenzene (10), 2-amino-2-
methylpropan-1-ol (12), 1-methyl-1H-imidazole (17), (S)-2-amino-
3-methylbutan-1-ol [(S)-20], 4-bromoanisole, 4-bromoacetophe-
none, 4-chloroacetophenone, 4-chlorotoluene, and phenylboronic
acid were purchased from commercial sources. 2,4,6-Trimethyl-
1,3-phenylenediacetonitrile (9),8 [3-(chloromethyl)-2,4,6-trimeth-
ylphenyl]acetonitrile (11),8 1-mesityl-1H-imidazole (18),13 and 1-
(2,6-diisopropylphenyl)-1H-imidazole (19),13 were prepared as pre-
viously described.
Bearing in mind that the Suzuki–Miyaura reaction involv-
ing chloroarenes requires higher catalyst loading (2.5–3
mol%),12 we examined cross-coupling with 4-chloroace-
tophenone using 2.5 mol% of either 1–3, 5, or 6 and
palladium(II) acetate. Among these, oxazolinyl-N-aryl-
imidazolium ligand precursors 2 and 3 provided the best
results; moreover, when the in situ catalyst system was de-
creased to 1 mol% the isolated yield was maintained
(Table 1, entries 21, 22 vs. 26, 27). Going further, this
coupling was carried out using 4-chlorotoluene in the
presence of 2.5 mol% of ligand precursor 2 or 3 and pal-
ladium(II) acetate and the diaryl product was formed in
71% or 77% isolated yield, respectively (Table 1, entries
29, 30). Notably, catalysis with oxazolinyl-N-heterocy-
clic carbene ligand precursor 2 showed that coupling with
either 4-chlorotoluene or the activated 4-chloroacetophe-
none resulted in similar isolated yields for the diaryl prod-
uct as shown in Table 1, when comparing yields given in
entries 29 to 21 and 29 to 26.
[3-(Hydroxymethyl)-2,4,6-trimethylphenyl]acetonitrile (8)
To a stirred soln of 11 (0.69 g, 3.30 mmol) in anhyd dioxane (20
mL) was added a suspension of CaCO3 (1.70 g, 17.01 mmol) in H2O
(20 mL); the mixture was heated to reflux temperature for 12 h. The
resulting white suspension was cooled to r.t., acidified with 5 M
HCl, and extracted with EtOAc (3 × 100 mL). The organic layer was
washed with sat. aq Na2CO3 (3 × 100 mL) and dried (anhyd
Na2SO4) and the solvent was evaporated to dryness to produce 8
(0.62 g, 99%) as a yellow solid; mp 120–122 °C.
IR (KBr): 3297, 2249, 995 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.34 (s, 3 H, CH3), 2.37 (s, 3 H,
CH3), 2.43 (s, 3 H, CH3), 3.63 (s, 2 H, CH2CN), 4.69 (s, 2 H,
CH2OH), 6.93 (s, 1 H).
13C NMR (50.3 MHz, CDCl3): d = 15.6 (CH3), 18.0 (CH2CN), 19.5
(CH3), 20.1 (CH3), 59.2 (CH2OH), 117.4 (CN), 125.7, 130.5 (CH),
135.2, 136.2, 136.3, 137.2.
MS (EI): m/z (%) = 189 [M+] (24), 171 [M+ – 18] (100).
Anal. Calcd for C12H15NO·0.1 EtOAc: C, 74.89; H, 8.06; N, 6.97.
Found: C, 74.90; H, 8.06; N, 6.86.
In summary, new simple imidazolium-based bidentate
ligand precursors 1–3, (S)-4, and (S)-5 and bis(oxazoline)
ligands 6 and (S,S)-7 were prepared using an efficient
multistep sequence. A Suzuki–Miyaura reaction was used
(3-{[(3-Cyanomethyl-2,4,6-trimethylbenzyl)(2-hydroxy-1,1-
dimethylethyl)amino]methyl}-2,4,6-trimethylphenyl)acetoni-
to illustrate the activity of the palladium(II) catalyst sys- trile (13)
A soln of Cd(OAc)2·2 H2O (0.08 g, 0.30 mmol), 11 (1.25 g, 6.02
tem produced in situ from oxazolinyl-imidazolium cat-
ions 1–3 and (S)-5 or bis(oxazoline) 6 and palladium(II)
acetate. Of these, sterically hindered N-arylimidazolium
cations 2 and 3 demonstrated their efficiency with both
aryl bromides and aryl chlorides using low catalyst system
loading. The synthetic protocol applied to these simple
cationic ligand precursors could be adapted to a variety of
oxazolinyl-N-heterocyclic carbene chiral frameworks,
mmol), and 2-amino-2-methylpropan-1-ol (12, 1.08 mL, 11.29
mmol) in chlorobenzene (25 mL) was stirred at reflux temperature
under an argon atmosphere for 7 d. The resulting soln was evaporat-
ed to dryness. The resulting orange oil was purified by column chro-
matography (silica gel, EtOAc–MeOH mixtures of increasing
polarity) to provide 13 (1.08 g, 42%) as a yellow oil.
IR (KBr): 3161, 2252 cm–1.
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

870
E. Alcalde et al.
PAPER
1H NMR (200 MHz, CDCl3): d = 1.22 [s, 6 H, C(CH3)2], 2.34 (s, 6
H, CH3), 2.36 (s, 6 H, CH3), 2.42 (s, 6 H, CH3), 3.39 (s, 2 H,
CH2OH), 3.64 (s, 4 H, NCH2), 3.77 (s, 4 H, CH2CN), 6.93 (s, 2 H).
matography (neutral alumina, hexanes–EtOAc mixtures of
increasing polarity) to give (S)-21 (0.77 g, 53%) as a white solid; mp
112–114 °C.
13C NMR (75.4 MHz, CDCl3): d = 15.7 (CH3), 18.2 (CH2CN), 19.6
(CH3), 20.1 (CH3), 23.4 (CH3), 40.3 (NCH2), 54.7, 68.7 (CH2OH),
117.4 (CN), 125.8, 130.7 (CH), 134.2, 135.6, 135.9, 137.1.
[a]D20 –36.0 (c 1.0, MeOH).
IR (KBr): 3191, 1651 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.83 (d, J = 6.8 Hz, 3 H, i-Pr),
0.91 (d, J = 6.8 Hz, 3 H, i-Pr), 1.67–1.79 (m, 1 H, i-Pr), 2.33 (s, 3
H, CH3), 2.36 (s, 3 H, CH3), 2.37 (s, 3 H, CH3), 3.66 (s, 2 H, CH2-
oxazoline), 3.81–3.92 (m, 2 H, OCH2), 4.10–4.19 (m, 1 H, NCH),
4.70 (s, 2 H, CH2OH), 6.89 (s, 1 H).
MS (CI): m/z (%) = 432 [M+ + 1] (100).
Attempted Preparation of [3-(Hydroxymethyl)-2,4,6-trimeth-
ylphenyl]acetic Acid (14)
A mixture of 8 (0.59 g, 3.12 mmol), H2SO4 (2.93 mL, 95–98%
w/w), and H2O (3.51 mL) was stirred and heated at reflux tempera-
ture for 6 h. The mixture was cooled to r.t. and poured into ice. The
resulting brownish solid was filtered, washed with cold H2O, and
dried under vacuum, generating an unidentified product.
13C NMR (75.4 MHz, CDCl3): d = 15.7 (CH3), 17.7 (CH3), 18.8
(CH3), 19.5 (CH3), 20.4 (CH3), 29.1 (CH2-oxazoline), 32.2 (CH),
59.6 (CH2OH), 66.7 (OCH2), 71.7 (NCH), 130.2 (CH), 130.7,
134.8, 135.8, 137.0, 137.1, 165.3.
MS (EI): m/z (%) = 275 [M+] (69), 256 [M+ – 19] (100).
{3-[(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)methyl]-2,4,6-tri-
methylphenyl}methanol (15)
Anal. Calcd for C17H25NO2: C, 74.12; H, 9.15; N, 5.09. Found: C,
74.19; H, 9.26; N, 5.24.
A soln of Cd(OAc)2·2 H2O (0.60 g, 2.25 mmol), 8 (0.85 g, 4.49
mmol), and 2-amino-2-methylpropan-1-ol (12, 0.81 mL, 8.53
mmol) in chlorobenzene (80 mL) was stirred at reflux temperature
under an argon atmosphere for 7 d. The resulting soln was evaporat-
ed to dryness. The resulting orange oil was purified by column chro-
matography (silica gel, hexanes–EtOAc mixtures of increasing
polarity) to provide 15 (0.61 g, 52%) as a yellow oil.
(4S)-2-[3-(Bromomethyl)-2,4,6-trimethylbenzyl]-4-isopropyl-
4,5-dihydrooxazole [(S)-22]
To a soln of (S)-21 (0.45 g, 1.62 mmol) in anhyd THF (25 mL) was
added PBr3 (0.15 mL, 1.62 mmol) at –20 °C under an argon atmo-
sphere and the mixture was stirred for 1 h. It was then was washed
with ice-cold sat. aq NaHCO3 (30 mL) and the aqueous phase was
extracted with CH2Cl2 (3 × 50 mL). The combined organic phases
were dried (anhyd Na2SO4), filtered, and evaporated under reduced
pressure to give (S)-22 (0.54 g, 99%) as a yellow oil, which was sub-
jected to the next step without further purification.
IR (CHCl3): 3345, 1658 cm–1.
1H NMR (200 MHz, CDCl3): d = 1.21 [s, 6 H, C(CH3)2], 2.31 (s, 3
H, CH3), 2.34 (s, 3 H, CH3), 2.39 (s, 3 H, CH3), 3.61 (s, 2 H, CH2-
oxazoline), 3.86 (s, 2 H, OCH2), 4.66 (s, 2 H, CH2OH), 6.87 (s, 1 H).
1H NMR (200 MHz, CDCl3): d = 0.84 (d, J = 6.8 Hz, 3 H, i-Pr),
0.92 (d, J = 6.8 Hz, 3 H, i-Pr), 1.67–1.84 (m, 1 H, i-Pr), 2.35 (s, 3
H, CH3), 2.37 (s, 3 H, CH3), 2.41 (s, 3 H, CH3), 3.67 (s, 2 H, CH2-
oxazoline), 3.86–3.95 (m, 2 H, OCH2), 4.10–4.22 (m, 1 H, NCH),
4.59 (s, 2 H, CH2Br), 6.89 (s, 1 H).
13C NMR (50.3 MHz, CDCl3): d = 15.6 (CH3), 19.6 (CH3), 20.4
(CH3), 28.2 (CH3), 29.2 (CH2-oxazoline), 59.3 (CH2OH), 66.7, 79.1
(OCH2), 130.1 (CH), 130.4, 135.7, 136.8, 136.9, 163.9.
MS (EI): m/z (%) = 261 [M+] (60), 242 [M+ – 19] (100).
Anal. Calcd for C16H23NO2·0.21 EtOAc: C, 72.29; H, 8.89; N, 5.01.
Found: C, 72.23; H, 8.98; N, 5.32.
Oxazolinyl-imidazolium Salts 1–5; General Procedure
[(Bromomethyl)aryl]oxazoline 16 or (S)-22 (1 equiv) and N-substi-
tuted imidazoles 17, 18, or 19 (1.1–1.5 equiv) were dissolved in an-
hyd DMF and heated to 80 °C under an argon atmosphere for 12 h,
and the solvent was then removed under vacuum. The white solids
obtained were washed several times with Et2O and used without fur-
ther purification. The yields were not optimized.
2-[3-(Bromomethyl)-2,4,6-trimethylbenzyl]-4,4-dimethyl-4,5-
dihydrooxazole (16)
To a soln of 15 (0.55 g, 1.94 mmol) in anhyd THF (30 mL) was add-
ed PBr3 (0.18 mL, 1.94 mmol) at –20 °C under an argon atmosphere
and the mixture was stirred for 1 h. It was then washed with an ice-
cold sat. aq NaHCO3 (30 mL) and the aqueous phase was extracted
with CH2Cl2 (3 × 50 mL). The combined organic phases were dried
(anhyd Na2SO4), filtered, and evaporated under reduced pressure to
yield 16 (0.62 g, 99%) as a yellow oil, which was subjected to the
next step without further purification.
1-{3-[(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)methyl]-2,4,6-tri-
methylbenzyl}-3-methylimidazolium Bromide (1)
The above procedure was followed using oxazoline 16 (0.50 g, 1.54
mmol), 1-methyl-1H-imidazole (17, 0.18 mL, 2.31 mmol), and an-
hyd DMF (5 mL). The product was obtained as a hygroscopic solid;
yield: 93%.
1H NMR (200 MHz, CDCl3): d = 1.25 [s, 6 H, C(CH3)2], 2.34 (s, 3
H, CH3), 2.36 (s, 3 H, CH3), 2.40 (s, 3 H, CH3), 3.64 (s, 2 H, CH2-
oxazoline), 3.87 (s, 2 H, OCH2), 4.59 (s, 2 H, CH2Br), 6.89 (s, 1 H).
IR (NaCl): 2967, 1733 cm–1.
13C NMR (50.3 MHz, CDCl3): d = 15.4 (CH3), 19.2 (CH3), 20.5
(CH3), 28.2 (CH3), 29.3 (CH2-oxazoline), 30.4 (CH2Br), 66.7, 79.1
(OCH2), 130.2 (CH), 130.6, 131.8, 135.9, 136.8, 137.8, 163.5.
1H NMR (300 MHz, CDCl3): d = 1.18 [s, 6 H, C(CH3)2], 2.24 (s, 6
H, CH3), 2.31 (s, 3 H, CH3), 3.58 (s, 2 H, CH2-oxazoline), 3.84 (s,
2 H, OCH2), 4.07 (s, 3 H, CH3-Im), 5.53 (s, 2 H, CH2-Im), 6.94 (s,
1 H), 6.98 (s, 1 H, Im), 7.54 (s, 1 H, Im), 10.05 (s, 1 H, Im).
MS (EI): m/z (%) = 324 [M+] (4), 245 [M+ – 79] (19), 244 [M+ – 80]
(100).
13C NMR (75.4 MHz, CDCl3): d = 16.2 (CH3), 20.0 (CH3), 20.4
(CH3), 28.2 (CH3), 29.2 (CH2-oxazoline), 36.9 (CH3-Im), 48.4
(CH2-Im), 66.9, 79.1 (OCH2), 120.8 (Im), 123.5 (Im), 126.0, 131.0
(CH), 131.7, 136.7, 136.8, 137.5 (Im), 139.4, 163.2.
(3-{[(4S)-4-Isopropyl-4,5-dihydrooxazol-2-yl]methyl}-2,4,6-tri-
methylphenyl)methanol [(S)-21]
A soln of Cd(OAc)2·2 H2O (0.70 g, 2.64 mmol), 8 (1 g, 5.28 mmol),
and (+)-(S)-2-amino-3-methylbutan-1-ol [(S)-20, 1.04 g, 10.04
mmol) in chlorobenzene (25 mL) was stirred at reflux temperature
under an argon atmosphere for 7 d. The resulting soln was evaporat-
ed to dryness. The resulting orange oil was purified by column chro-
MS (ESI+): m/z (%) = 326 [M]+ (100), 731 [2 M + Br]+ (2).
Anal. Calcd for C20H28BrN3O·1.5 H2O: C, 55.43; H, 7.21; N, 9.70.
Found: C, 55.83; H, 7.18; N, 9.30.
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

PAPER
Imidazolium Salt–Oxazoline/Palladium(II) Acetate In Situ Catalyst Systems
871
1-{3-[(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)methyl]-2,4,6-tri-
methylbenzyl}-3-3-(2,4,6-trimethylphenyl)imidazolium Bro-
mide (2)
1-(3-{[(4S)-4-Isopropyl-4,5-dihydrooxazol-2-yl]methyl}-2,4,6-
trimethylbenzyl)-3-(2,4,6-trimethylphenyl)imidazolium Bro-
mide [(S)-5]
The above procedure was followed using oxazoline 16 (0.73 g, 2.24
mmol), 1-mesityl-1H-imidazole (18, 0.46 g, 2.46 mmol), and anhyd
DMF (6 mL). The product was obtained as a white solid; yield:
82%; mp 148–150 °C.
The above procedure was followed using oxazoline (S)-22 (0.23 g,
0.69 mmol), 1-mesityl-1H-imidazole (18, 0.14 g, 0.75 mmol), and
anhyd DMF (2 mL). The product was obtained as a white solid;
yield: 75%; mp 130–132 °C.
IR (KBr): 2967, 1734 cm–1.
[a]D20 +4.7 (c 1.0, MeOH).
1H NMR (400 MHz, CDCl3): d = 1.21 [s, 6 H, C(CH3)2], 2.07 (s, 6
H, CH3), 2.34–2.37 (m, 12 H, CH3), 3.64 (s, 2 H, CH2-oxazoline),
3.88 (s, 2 H, OCH2), 6.02 (s, 2 H, CH2-Im), 6.97 (s, 1 H), 7.00 (s, 2
H), 7.12 (s, 1 H, Im), 7.34 (s, 1 H, Im), 10.38 (s, 1 H, Im).
IR (KBr): 2959, 1731 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.82 (d, J = 1.0 Hz, 3 H, i-Pr),
0.86 (d, J = 1.0 Hz, 3 H, i-Pr), 1.74–1.86 (m, 1 H, i-Pr), 2.04–2.09
(m, 6 H, CH3), 2.24–2.42 (m, 12 H, CH3), 3.47–3.94 (m, 4 H), 4.08–
4.28 (m, 1 H, NCH), 5.76–6.09 (m, 2 H, CH2-Im), 6.95–7.05 (m, 3
H), 7.09–7.20 (m, 1 H, Im), 7.31–7.39 (m, 1 H, Im), 9.91–10.54 (m,
1 H, Im).
13C NMR (100.6 MHz, CDCl3): d = 16.2 (CH3), 17.6 (CH3), 20.1
(CH3), 20.5 (CH3), 21.0 (CH3), 28.2 (CH3), 29.3 (CH2-oxazoline),
49.3 (CH2-Im), 66.9, 79.2 (OCH2), 121.9 (Im), 122.9 (Im), 126.8,
129.9 (CH), 130.6, 131.1 (CH), 131.9, 134.1, 136.8, 137.7 (Im),
137.7, 139.3, 141.4, 163.4.
13C NMR (100.6 MHz, CDCl3): d = 16.2 (CH3), 17.5 (CH3), 17.5
(CH3), 19.3 (CH3), 19.4 (CH3), 19.9 (CH3), 20.6 (CH3), 21.0 (CH3),
28.8 (CH), 37.7 (CH2-oxazoline), 48.8 (CH2-Im), 69.8 (OCH2),
71.6 (NCH), 122.3 (Im), 124.3 (Im), 126.9, 129.7 (CH), 129.8
(CH), 130.7, 131.2 (CH), 132.0, 133.9, 134.0, 136.8 (Im), 136.9,
137.8, 139.3, 141.0, 170.2.
MS (ESI+): m/z (%) = 430 [M]+ (100), 941 [2 M + Br]+ (1).
Anal. Calcd for C28H36BrN3O·1.7 H2O: C, 62.23; H, 7.33; N, 7.78.
Found: C, 62.27; H, 7.27; N, 7.42.
1-{3-[(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)methyl]-2,4,6-tri-
methylbenzyl}-3-(2,6-diisopropylphenyl)imidazolium Bromide
(3)
MS (ESI+): m/z (%) = 444 [M]+ (53), 462 [M + H2O]+ (100).
Anal. Calcd for C29H38BrN3O·1.5 H2O: C, 63.15; H, 7.49; N, 7.62.
Found: C, 63.08; H, 7.45; N, 7.22.
The above procedure was followed using oxazoline 16 (0.59 g, 1.81
mmol), 1-(2,6-diisopropylphenyl)-1H-imidazole (19, 0.46 g, 1.99
mmol), and anhyd DMF (5 mL). The product was obtained as a
white solid; yield: 71%; mp 132–134 °C.
Bis(oxazolines) 6 and (S,S)-7; General Procedure
A soln of Cd(OAc)2·2 H2O (0.05 or 0.5 equiv), dinitrile 20 (1
equiv), and amino alcohol 12 or (S)-20 (3.75 equiv) in chloroben-
zene was stirred at reflux temperature under an argon atmosphere
for 7 d and then the solvent was removed under vacuum. The oily
residue was purified by column chromatography (silica gel, hex-
anes–EtOAc–MeOH mixtures of increasing polarity).
IR (KBr): 2965, 1735 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.15 (d, J = 6.8 Hz, 6 H, i-Pr),
1.21 [s, 6 H, C(CH3)2], 1.23 (d, J = 6.8 Hz, 6 H, i-Pr), 2.20–2.40 (m,
11 H), 3.64 (s, 2 H, CH2-oxazoline), 3.87 (s, 2 H, OCH2), 6.10 (s, 2
H, CH2-Im), 7.00 (s, 1 H), 7.13 (s, 1 H), 7.29 (s, 1 H), 7.31 (s, 1 H),
7.51–7.57 (m, 2 H, Im), 10.33 (s, 1 H, Im).
1,3-Bis[(4,4-dimethyl-4,5-dihydrooxazol-2-yl)methyl]-2,4,6-tri-
methylbenzene (6)
13C NMR (100.6 MHz, CDCl3): d = 16.1 (CH3), 20.0 (CH3), 20.5
(CH3), 24.0 (CH3), 24.5 (CH3), 28.3 (CH3), 28.7 (CH), 29.3 (CH2-
oxazoline), 49.5 (CH2-Im), 66.9, 79.2 (OCH2), 122.3 (Im), 123.9
(Im), 124.7 (CH), 126.9, 130.1, 131.1 (CH), 131.9, 131.9 (CH),
136.8, 137.7 (Im), 137.9, 139.3, 145.3, 163.3.
The above procedure was followed using Cd(OAc)2·2 H2O (0.07 g,
0.25 mmol), dinitrile 9 (1 g, 5.05 mmol), 2-amino-2-methylpropan-
1-ol (12, 1.80 mL, 18.90 mmol), and chlorobenzene (25 mL). The
product was obtained as a yellow solid; yield: 79%; mp 100–102
°C.
MS (ESI+): m/z (%) = 472 [M]+ (100), 1025 [2 M + Br]+ (2).
IR (KBr): 1658 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.23 (s, 12 H), 2.31 (s, 6 H), 2.34
(s, 3 H), 3.64 (s, 4 H, CH2-oxazoline), 3.85 (s, 4 H, OCH2), 6.87 (s,
1 H).
Anal. Calcd for C31H42BrN3O·2 H2O: C, 63.26; H, 7.88; N, 7.14.
Found: C, 63.35; H, 7.86; N, 6.74.
1-(3-{[(4S)-4-Isopropyl-4,5-dihydrooxazol-2-yl]methyl}-2,4,6-
trimethylbenzyl)-3-methylimidazolium Bromide [(S)-4]
The above procedure was followed using oxazoline (S)-22 (0.52 g,
1.53 mmol), 1-methyl-1H-imidazole (17, 0.18 mL, 2.31 mmol), and
anhyd DMF (4 mL). The product was obtained as a hygroscopic sol-
id; yield: 68%; 1H NMR purity: 80%.
13C NMR (100.6 MHz, CDCl3): d = 16.3 (CH3), 20.4 (CH3), 24.1
(CH3), 28.3 (CH3), 29.6 (CH2-oxazoline), 66.8, 79.1 (OCH2), 130.1
(CH), 130.3, 135.9, 136.8, 164.0.
MS (EI): m/z (%) = 342 [M+] (100).
Anal. Calcd for C21H30N2O2: C, 73.65; H, 8.83; N, 8.18. Found: C,
73.68; H, 8.77; N, 8.18.
1H NMR (400 MHz, CDCl3): d = 0.84–1.00 (m, 6 H, CH3), 1.80–
1.90 (m, 1 H, i-Pr), 2.18–2.33 (m, 9 H, CH3), 3.61–3.78 (m, 4 H,
CH2-oxazoline), 3.94–4.02 (m, 6 H), 5.32–5.60 (s, 2 H, CH2-Im),
6.98 (s, 1 H, Im), 7.19 (s, 1 H), 7.42 (s, 1 H, Im), 10.26 (s, 1 H, Im).
1,3-Bis{[(4S)-4-isopropyl-4,5-dihydrooxazol-2-yl]methyl}-
2,4,6-trimethylbenzene [(S,S)-7]
The above procedure was followed using Cd(OAc)2·2 H2O (0.67 g,
2.52 mmol), dinitrile 9 (1 g, 5.05 mmol), (+)-(S)-2-amino-3-meth-
ylbutan-1-ol [(S)-20, 1.95 g, 18.90 mmol), and chlorobenzene (25
mL). The product was obtained as a yellow solid; yield: 59%; mp
80–82 °C.
13C NMR (100.6 MHz, CDCl3): d = 15.8 (CH3), 19.3 (CH3), 19.5
(CH3), 20.5 (CH3), 25.1 (CH3), 28.6 (CH), 36.1 (CH3-Im), 37.5
(CH2-oxazoline), 47.9 (CH2-Im), 56.5 (NCH), 61.4 (OCH2), 121.2
(Im), 123.3 (Im), 126.0, 130.5 (Im), 131.0 (CH), 132.1, 136.4,
137.6, 139.5, 170.0.
[a]D20 –0.87 (c 1.0, MeOH).
MS (ESI+): m/z (%) = 358 [M + H2O]+ (100), 762 [2 M + Br]+ (2).
IR (KBr): 1661 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.83 (d, J = 6.8 Hz, 6 H, i-Pr),
0.91 (d, J = 6.8 Hz, 6 H, i-Pr), 1.68–1.80 (m, 2 H, i-Pr), 2.32 (s, 6
Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York

872
E. Alcalde et al.
PAPER
H), 2.37 (s, 3 H), 3.66 (s, 4 H, CH2-oxazoline), 3.84–3.91 (m, 4 H,
OCH2), 4.11–4.21 (m, 2 H, NCH), 6.88 (s, 1 H).
(3) (a) Beer, P. D.; Gale, P. A. Angew. Chem. Int. Ed. 2001, 40,
486. (b) Gale, P. A. Annu. Rep. Chem., Sect. B 2005, 101,
148. (c) Matthews, S. E.; Beer, P. D. Supramol. Chem. 2005,
17, 411. (d) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion
Receptor Chemistry; Royal Society of Chemistry:
Cambridge, 2006. (e) Gale, P. A.; Quesada, R. Acc. Chem.
Res. 2006, 39, 465. (f) Beer, P. D.; Sambrook, M. R.; Curiel,
D. Chem. Commun. 2006, 2105.
(4) (a) Handy, S. T. Chem. Eur. J. 2003, 9, 2938. (b) Gozzo, F.
C.; Santos, L. S.; Augusti, R.; Consorti, C. S.; Dupont, J.;
Eberlin, M. N. Chem. Eur. J. 2004, 10, 6187. (c) Welton, T.
Coord. Chem. Rev. 2004, 248, 2459.
13C NMR (100.6 MHz, CDCl3): d = 16.4 (CH3), 17.8 (CH3), 18.8
(CH3), 20.4 (CH3), 29.4 (CH2-oxazoline), 32.3 (CH), 69.7 (OCH2),
71.8 (NCH), 130.1, 130.4 (CH), 135.9, 136.6, 165.4.
MS (EI): m/z (%) = 370 [M+] (100).
Anal. Calcd for C23H34N2O2·0.65 H2O: C, 72.27; H, 9.31; N, 7.33.
Found: C, 72.19; H, 9.16; N, 7.57.
Suzuki–Miyaura Reaction with Palladium(II); General Proce-
dure
(5) (a) Herrmann, W. A.; Goossen, L. J.; Spiegler, M.
Organometallics 1998, 17, 2162. (b) Perry, M. C.; Cui, X.;
Powell, M. T.; Hou, D.-R.; Reibenspies, H. J.; Burgess, K. J.
Am. Chem. Soc. 2003, 125, 113. (c) César, V.; Bellemin-
Laponnaz, S.; Gade, L. H. Organometallics 2002, 21, 5204.
(d) César, V.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade,
L. H. Chem. Eur. J. 2005, 11, 2862. (e) Poyatos, M.;
Maisse-François, A.; Bellemin-Laponnaz, S.; Peris, E.;
Gade, L. H. J. Organomet. Chem. 2006, 691, 2713. (f) Ren,
L.; Chen, A. C.; Decken, A.; Crudden, C. M. Can. J. Chem.
2004, 82, 1781. (g) Nanchen, S.; Pfaltz, A. Chem. Eur. J.
2006, 12, 4550. (h) Bolm, C.; Focken, T.; Raabe, G.
Tetrahedron: Asymmetry 2003, 14, 1733.
Under an argon atmosphere, a Schlenk tube was charged with 1,4-
dioxane (3 mL), Pd(OAc)2 (see Table 1), ligand (see Table 1), and
Cs2CO3 (2 mmol). The mixture was heated to 80 °C for 2 h and then
the mixture was cooled to r.t. and the aryl halide (1 mmol) and aryl-
boronic acid (1.5 mmol) were added in turn. The Schlenk tube was
heated to 80 °C and stirred for 2 h. The mixture was then allowed to
cool to r.t., and was purified by filtration through a pad of Celite. It
was subsequently concentrated and finally purified by flash chro-
matography.
Acknowledgment
This research was supported by the Vicerrectorat de Recerca
(2006), Universitat de Barcelona and the Dirección General de In-
vestigación (Ministerio de Educación y Ciencia) project CTQ2006-
1182/BQU. Thanks are also due to the agaur, 2005SGR00158
(Generalitat de Catalunya).
(6) Poyatos, M.; Maisse-François, A.; Bellemin-Laponnaz, S.;
Peris, E.; Gade, L. H. Organometallics 2006, 25, 2634; and
references cited therein.
(7) Tao, B.; Boykin, D. W. Tetrahedron Lett. 2002, 43, 4955.
(8) Wu, C.; Decker, E. R.; Blok, N.; Bui, H.; Chen, Q.; Raju, B.;
Bourgoyne, A. R.; Knowles, V.; Biediger, R. J.; Market, R.
V.; Lin, S.; Dupré, B.; Kogan, T. P.; Holland, G. W.; Brock,
T. A.; Dixon, R. A. F. J. Med. Chem. 1999, 42, 4485.
(9) (a) Moorthy, J. N.; Venkatakrishnan, P.; Mal, P.;
Venogopalan, P. J. Org. Chem. 2003, 68, 327. (b) Alcalde,
E.; Ayala, C.; Dinarès, I.; Mesquida, N. J. Org. Chem. 2001,
66, 2291.
(10) (a) Aggarwal, V. K.; Bell, L.; Coogan, M. P.; Jubault, P. J.
Chem. Soc., Perkin Trans. 1 1998, 2037; and references
cited therein. (b) Griffith, J. A.; Rowlands, G. J. Synthesis
2005, 3446.
(11) (a) Gerisch, M.; Krumper, J. R.; Bergman, R. C.; Tilley, D.
Organometallics 2003, 22, 47. (b) Kim, J.; Kim, S. G.;
Seong, H. R.; Ahn, K. H. J. Org. Chem. 2005, 70, 7227.
(12) (a) Kotha, S.; Lahiri, K.; Kashinath, D. Tetrahedron 2002,
58, 9633. (b) Littke, A. F.; Fu, G. C. Angew. Chem. Int. Ed.
2002, 41, 4176.
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Synthesis 2007, No. 6, 865–872 © Thieme Stuttgart · New York