Palladium-catalyzed direct arylation of thiazoles with aryl bromides

Article abstract of DOI:10.1016/S0040-4020(03)00879-2

Thiazole, 2-phenyl or -alkyl substituted one and benzothiazole are efficiently arylated with aryl bromides at the 2- and/or 5-position(s) in the presence of Pd(OAc)₂ and a bulky phosphine ligand using Cs₂CO₃ as base. 2-Phenyl-5-thiazolecarboxanilide undergoes successive diarylation at the 4- and 5-positions accompanied by decarbamoylation.

Full text of DOI:10.1016/S0040-4020(03)00879-2

Tetrahedron 59 (2003) 5685–5689
Palladium-catalyzed direct arylation of thiazoles with
aryl bromides
*
Aya Yokooji, Toru Okazawa, Tetsuya Satoh, Masahiro Miura and Masakatsu Nomura
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 13 May 2003; revised 4 June 2003; accepted 4 June 2003
Abstract—Thiazole, 2-phenyl or -alkyl substituted one and benzothiazole are efficiently arylated with aryl bromides at the 2- and/or
5-position(s) in the presence of Pd(OAc)2 and a bulky phosphine ligand using Cs2CO3 as base. 2-Phenyl-5-thiazolecarboxanilide undergoes
successive diarylation at the 4- and 5-positions accompanied by decarbamoylation.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Various thiazole derivatives are known to exhibit pharma-
cological activities.1 Aryl-substituted thiazoles are of
importance not only as medicinal agents,1 but also as
organic functional materials such as fluorescent dyes2a,b and
liquid crystals.2b,c Among the most useful methods to
prepare such arylheterocycles is the palladium-catalyzed
cross-coupling of either heteroaryl halides with arylmetals
or aryl halides with heteroarylmetals.3 Thus, the reaction
has been extensively studied.
The arylation of unsubstituted thiazole (2) was first carried
out using various tertiary phosphines in order to examine the
effect of ligands on the reaction. As the arylating reagent,
4-bromoanisole (1a) (Table 1) was employed, since an
electron-donating substituent on aryl halides often causes a
significant problem, that is scrambling of the aryl moieties
with those of triarylphosphines.7 When 2 (1 mmol) was
treated with 1a (2.4 mmol) in the presence of Pd(OAc)2
(0.1 mmol) and PPh3 (0.2 mmol) using Cs2CO3 as base in
DMF at 1508C for 4 h, 2,5-di(4-anisyl)thiazole (3a) was
Meanwhile, it is known that aryl halides can couple directly
with a number of five-membered heteroaromatics including
thiazoles at their 2- and/or 5-position(s) in the presence of a
palladium catalyst.4 This method has a significant
advantage, not requiring stoichiometric metalation of the
heterocycles. We previously reported that azole compounds
and thiophenes were effectively arylated using aryl
bromides or iodides in the presence of Pd(OAc)2–PPh3
and K2CO3 or Cs2CO3, as catalyst and base, respectively.5a
For sulfur-containing substrates, i.e. thiazoles and thio-
phenes, addition of a copper(I) halide such as CuI as a
promoter or a cocatalyst was required to perform the
reaction effectively. After that, it was also found that by
using bulky phosphine ligands, which may yield coordina-
tively unsaturated active palladium species,6 the arylation of
2- or 3-substituted thiophenes with aryl bromides proceeded
efficiently without adding the copper species.5b Therefore, it
was anticipated that thiazoles could also be arylated
effectively under similar conditions. Consequently, the
arylation reaction of thiazoles has been investigated further.
The results are described herein.
Table 1. Reaction of thiazole (2) with 4-bromoanisole (1a)
Entry
Ligand
Ligand/Pd
% Yield 3ba
1b
2
3
4
5
6
7
8
9c
10d
PPh3
P(4-MeOC6H4)3
P(t-Bu)3
P(biphenyl-2-yl)(t-Bu)2
P(2-MeC6H4)3
P(cyclohexyl)3
P(t-Bu)3
P(t-Bu)3
P(t-Bu)3
P(t-Bu)3
2
2
2
2
2
2
1
3
2
2
6
28
76 (66)
78
3
8
39
42
53
16
Reaction conditions: [1a]/[2]/[Pd(OAc)2]/[Cs2CO3]¼2.4:1:0.1:2.4 (in
mmol), in DMF at 1508C for 4 h under N2.
Determined by GC. The value in parentheses is isolated yield.
a
b
Keywords: arylation; aryl halides; palladium and compounds; thiazoles.
*
Anisyl(phenyl)thiazoles (ca. 15%) were also formed.
K2CO3 was used in place of Cs2CO3.
o-Xylene was used in place of DMF.
c
Corresponding author. Tel.: þ81668797361; fax: þ81668797362;
d
e-mail: miura@chem.eng.osaka-u.ac.jp
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00879-2

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A. Yokooji et al. / Tetrahedron 59 (2003) 5685–5689
formed in only 6% yield together with trace amounts of
anisyl- and phenylthiazoles and ca. 15% combined yield
of two possible anisylphenylthiazoles, which was con-
firmed by GC and GC–MS (entry 1). The formation of
the phenyl-containing products suggests that the unde-
sired scrambling occurred. Using P(4-MeOC6H4)3 to
avoid the scrambling improved the product yield, but it
was still low (entry 2). In expectation, a bulky phosphine
P(t-Bu)3 remarkably promoted the reaction to give 3a in
76% (entry 3), no scrambling being observed. Almost the
same product yield was obtained using double amounts of
the substrates (Table 2, entry 1). Another bulky
reaction (entry 9). o-Xylene as solvent was not effective
for this reaction (entry 10).5b It is noted that in each
run, formation of the singly arylated products was not
detected or negligible even at the early stage of the
reaction, suggesting that the second arylation is signifi-
cantly faster than the first one. Thus, the precedence of 2-
and 5-arylations under the present conditions is not
definitive.9
Table 2 summarizes the results for the arylation of 2,
2-phenylthiazole (4), 2-isobutylthiazole (5) and benzo-
thiazole (6) using Pd(OAc)2 and P(t-Bu)3 or P(biphenyl-2-
yl)(t-Bu)2 in the presence of Cs2CO3 in DMF. The reactions
of 2 with bromobenzene (1b), 4-bromochlorobenzene (1c)
and 1-bromonaphthalene (1d) gave the corresponding 2,5-
diarylated thiazoles 3b–d in good yields, as did that with 1a
(entries 1–5). The 5-arylations of 4 with 1a and of 5 with 1b
proceeded smoothly (entries 6 and 8). The 2-arylation of
6 with 1a–c also took place to give 2-arylbenzothiazoles
8
phosphine P(biphenyl-2-yl)(t-Bu)2 showed similar per-
formance (entry 4), whereas P(2-MeC6H4)3 and
P(cyclohexyl)3 could not enhance the reaction (entries 5
and 6). The amount of ligand was also an important
factor determining the reaction efficiency (entries 3, 7 and
8). A maximum yield was obtained at a P/Pd ratio of 2.
Use of K2CO3 in place of Cs2CO3 as base retarded the
Table 2. Reaction of thiazoles 2 and 4–6 with aryl bromides 1
Entry
Bromide (mmol)
Thiazole (mmol)
Liganda
Time (h)
Product, % yieldb
1
2
3
4
1a: X¼OMe (4.8)
1b: X¼H (4.8)
1b: X¼H (4.8)
1c: X¼Cl (4.8)
2 (2)
2 (2)
2 (2)
2 (2)
L1
L1
L2
L1
8
8
3
10
3a: X¼OMe, 78 (69)
3b: X¼H, 90 (70)
3b: X¼H, 95 (76)
3c: X¼Cl, 76 (63)
5
1d (4.8)
2 (2)
L1
8
3d, 78 (55)
6
7c
1a (1.2)
1a (1.2)
4 (1)
4 (1)
L2
L2
5
18
7, 88 (72)
7, 72
8d
1b (1.2)
5 (1)
L1
1
8, 93 (71)
9e
1a (1)
1a (1)
1a (1)
1b (1)
1b (1)
1c (1)
6 (1.2)
6 (1.2)
6 (1.2)
6 (1.2)
6 (1.2)
6 (1.2)
L1
L1
L1
L1
L1
L1
1
4
1
1
1
1
9a: X¼OMe, 56
9a: X¼OMe, 75
9a: X¼OMe, 76 (71)
9b: X¼H, 65
10e
11e,f
12e
13e,f
14e,f
9b: X¼H, 76 (63)
9c: X¼Cl, 75 (45)
The reaction was carried out using Pd(OAc)2-ligand(0.05 and 0.1 equiv. to thiazole) and Cs2CO3 (equimolar amount to 1) in DMF at 1508C under N2 unless
otherwise noted.
a
L1¼P(t-Bu)3, L2¼P(biphenyl-2-yl)(t-Bu)2.
b
Determined by GC. The value in parentheses is isolated yield.
Reaction in o-xylene.
c
d
Pd(OAc)2-ligand (0.1 and 0.2 equiv to 5).
Pd(OAc)2-ligand (0.05 and 0.1 equiv to 1).
CuBr (0.2 mmol) was added.
e
f

A. Yokooji et al. / Tetrahedron 59 (2003) 5685–5689
5687
Table 3. Reaction of 2-phenyl-5-thiazolecarboxanilide (10) with aryl
bromides (1)
Scheme 1.
9a–c. It is noted that addition of a catalytic amount of CuBr
to the reaction of 6 somewhat enhanced the reaction (entries
9 and 12 vs 11 and 13).
Entry ArBr
Time (h)
Product, % yielda
In the previous study of thiophene arylation, it was also
found that 2-thiophenecarboxamides underwent an
unprecedented multiple arylation on treatment with
excess aryl bromides (Scheme 1).5b In this reaction, the
carbamoyl group acts as a directing group for the arylation
at the 3-position, after which it is cleaved and the 2-position
as well as the 5-position is arylated to give 2,3,5-
triarylthiophenes. The reaction seems to be useful as a
unique method for the successive arylation of the b- and
a-positions.
1
1a
24
Consequently, the arylation of thiazole nucleus was
examined in order to see applicability of the method. Due
to its ready availability, 2-phenyl-5-thiazolecarboxanilide
(10) was employed as substrate. The reaction of 10 with
1a in the presence of Pd(OAc)2, P(biphenyl-2-yl)(t-Bu)2
and Cs2CO3 in o-xylene could occur accompanied by
decarbamoylation to give the expected product, 4,5-di(4-
anisyl)-2-phenylthiazole (11a) in 67% yield (Table 3,
entry 1).10 Similarly, treatment of 10 with 1b, 1c and
4-bromobiphenyl (1e) in o-xylene afforded the
corresponding 4,5-diaryl-2-phenylthiazoles 11b, 11c and
11e.
2
1b
48
3
1c
24
It should be noted that the reaction of 10 with 1a did
not proceed in DMF. This may be due to the fact that
the first 4-arylation is difficult to occur in a polar
solvent, in which directing effect of the amide function
seems to be hampered.11 This contrasts with the fact that
the 5-arylation of 4 in DMF was faster than that in
o-xylene (see Table 2, entries 6 and 7). This may be
attributed to electrophilic character of the latter
reaction.5a,9b
As an additional experiment, 2-phenyl-5-oxazolecarboxa-
nilide (12) was treated with 1b under the same conditions
with those employed in entry 2 of Table 3, in order to
compare the reactivity of it with that of 10. While the
expected product, 2,4,5-triphenyloxazole (13) (33%) was
obtained (Scheme 2), a considerable amount of 2,4-
1e
4
48
Reaction conditions: [1]/[10]/[Pd(OAc)2]/[ligand]/[Cs2CO3]¼2.5:0.5:
0.05:0.1:2.5 (in mmol), in refluxing o-xylene under N2.
Determined by GC. The value in parentheses is isolated yield.
a
diphenyloxazole (ca. 20%) was accompanied. This result
suggests that the latter oxazole is less reactive than the
corresponding thiazole under the conditions employed.
In summary, we have described that the palladium-
catalyzed direct arylation of thiazole at the 2- and
5-positions with aryl bromides proceeds efficiently using a
bulky phosphine ligand in DMF. In contrast, use of a less
polar solvent such as o-xylene is essential for the successive
Scheme 2.

5688
A. Yokooji et al. / Tetrahedron 59 (2003) 5685–5689
4,5-diarylation of a thiazole substrate having 5-carboxa-
nilide function as sacrificial group. These methods seem to
be useful for preparing oligoaryl compounds having a
thiazole unit.
1319 are known. The characterization data of new
compounds are given below.
3.2.1. 2,5-Di(4-chrolophenyl)thiazole (3c). Mp 131–
1328C; 1H NMR d 7.39 (d, J¼8.3 Hz, 2H), 7.43 (d,
J¼8.7 Hz, 2H), 7.52 (d, J¼8.3 Hz, 2H), 7.89 (d,
J¼8.7 Hz, 2H), 7.98 (1H, s); 13C NMR d 127.55, 127.82,
129.25, 129.35, 129.70, 131.98, 134.31, 136.13, 138.36,
139.57, 166.06; MS m/z 305, 307, 309 (Mþ). Anal. calcd for
C15H9Cl2NS: C, 58.84; H, 2.96; N, 4.57. Found: C, 58.87;
H, 3.04; N, 4.51.
3. Experimental
3.1. General
1H and 13C NMR spectra were recorded at 400 and
100 MHz, respectively, for CDCl3 solutions unless other-
wise noted. MS analysis was made by EI. GC analysis was
carried out using a Silicone OV-17 glass column (i.d.
2.6 mm£1.5 m). 2-Phenylthiazole (4) was prepared accord-
ing to a published procedure.12 2-Phenyl-5-thiazolecarbox-
anilide (10) and 2-phenyl-5-oxazolecarboxanilide (12) were
prepared from 5-bromo-2-phenylthiazole12 and 5-bromo-2-
phenyloxazole13 by a carbonylation method reported
previously using aniline and PdCl2(PPh3)2 as nucleophile
and catalyst, respectively.14 Amide 10: Mp 219–2208C; 1H
NMR (DMSO-d6) d 7.14 (t, J¼7.3 Hz, 1H), 7.39 (t,
J¼8.0 Hz, 2H), 7.54–7.56 (m, 3H), 7.73 (d, J¼7.6 Hz,
2H), 8.02–8.45 (m, 2H), 8.69 (s, 1H), 10.47 (s, 1H); 13C
NMR (DMSO-d6) d 120.62, 124.30, 126.71, 128.94, 129.57,
131.37, 132.68, 135.83, 138.52, 144.96, 158.62, 170.93;
HRMS m/z (Mþ) calcd for C16H12N2OS 280.0687, found
280.0665. Amide 12: Mp 164.5–1658C; 1H NMR d 7.19 (t,
J¼7.3 Hz, 1H), 7.39 (t, J¼7.8 Hz, 2H), 7.49–7.55 (m, 3H),
7.65–7.68 (m, 2H), 7.91 (s, 1H), 8.00 (s, 1H), 8.11–8.14
(m, 2H); 13C NMR d 120.31, 125.11, 126.31, 127.03,
129.01, 129.19, 131.63, 133.22, 136.81, 144.82, 154.90,
162.58; HRMS m/z (Mþ) calcd for C16H12N2O2 264.0899,
found 264.0901. Other starting materials were commer-
cially available. The solvents employed were purified by
standard methods before use. The following experimental
procedure may be regarded as typical in methodology and
scale.
3.2.2. 2,5-Di(1-naphthyl)thiazole (3d). Mp 122–1238C; 1H
NMR d 7.53–7.68 (m, 7H), 7.92–7.98 (m, 5H), 8.11 (s,
1H), 8.27–8.30 (m, 1H), 8.98 (d, J¼8.3 Hz, 1H); 13C NMR
d 125.10, 125.32, 125.32, 125.94, 126.32, 126.41, 126.92,
127.40, 128.41, 128.53, 128.60, 128.64, 128.78, 129.28,
130.48, 130.57, 130.71, 131.91, 133.86, 134.12, 137.05,
142.47, 167.81; MS m/z 337 (Mþ). Anal. calcd for
C23H15NS: C, 81.87; H, 4.48; N, 4.15; S, 9.50. Found: C,
81.68; H, 4.67; N, 4.08; S, 9.39.
1
3.2.3. 2-(Isobutyl)-5-phenylthiazole (8). Oil; H NMR d
1.03 (d, J¼6.8 Hz, 6H), 2.14 (m, 1H), 2.88 (d, J¼7.3 Hz,
2H), 7.30 (t, J¼7.3 Hz, 1H), 7.38 (dd, J¼7.8, 7.3 Hz, 2H),
7.53 (d, J¼7.8 Hz, 2H), 7.83 (s, 1H); 13C NMR d 22.30,
29.82, 42.57, 126.59, 127.95, 129.00, 131.69, 137.61,
138.46, 169.68; MS m/z 217 (Mþ). Anal. calcd for
C13H15NS: C, 71.85; H, 6.96; N, 6.44. Found: C, 71.81;
H, 6.87; N, 6.17.
3.2.4. 4,5-Di(4-methoxyphenyl)-2-phenylthiazole (11a).
Mp 122–1238C; 1H NMR d 3.81 (s, 3H), 3.83 (s, 3H), 6.83–
6.89 (m, 4H), 7.32 (d, J¼8.8 Hz, 2H), 7.40–7.46 (m, 3H),
7.55 (d, J¼8.8 Hz, 2H), 7.98–8.00 (m, 2H); 13C NMR d
55.23, 55.29, 113.68, 114.18, 124.48, 126.33, 127.76,
128.84, 129.77, 130.27, 130.81, 131.74, 133.79, 150.05,
159.16, 159.50, 164.68; MS m/z 373 (Mþ). Anal. calcd for
C23H19NO2S: C, 73.97; H, 5.13; N, 3.73, S, 8.59. Found: C,
73.91; H, 5.12; N, 3.71; S, 8.43.
3.1.1. Reaction of thiazole (2) with 4-bromoanisole (1a).
In a 100 cm3 two-necked flask was placed Cs2CO3
(4.8 mmol, 1.56 g), which was then dried at 1508C in
vacuo for 2 h. Then, Pd(OAc)2 (0.1 mmol, 22.4 mg), P(t-
Bu)3 (0.2 mmol, 40.5 mg), 1a (4.8 mmol, 898 mg), 2
(2 mmol, 170 mg), 1-methylnaphthalene (ca. 100 mg) as
internal standard and DMF (5 cm3) were added. The
resulting mixture was stirred under N2 at 1508C for 8 h.
After cooling, the reaction mixture was extracted with ethyl
acetate and dried over sodium sulfate. Column chromato-
graphy on silica gel using hexane–ethyl acetate (95:5) as
eluent gave 2,5-di(4-anisyl)thiazole2b,c (3a) (410 mg, 69%):
Mp 174–1758C; 1H NMR d 3.84 (s, 3H), 3.86 (s, 3H), 6.93–
6.97 (m, 4H), 7.51 (d, J¼8.7 Hz, 2H), 7.85–7.90 (m, 3H);
13C NMR d 55.37, 55.40, 114.29, 114.49, 124.17, 126.73,
127.73, 127.84, 137.88, 138.22, 159.63, 161.03, 166.24; MS
m/z 297 (Mþ). Anal. calcd for C17H15NO2S: C, 68.66; H,
5.08; N, 4.71; S, 10.78. Found: C, 68.47; H, 5.10; N, 4.68; S,
10.41.
3.2.5. 4,5-Di(1,10-biphenyl-4-yl)-2-phenylthiazole (11e).
Mp 194–1958C; 1H NMR d 7.34–7.38 (m, 2H), 7.41–7.50
(m, 7H), 7.52 (d, J¼8.4 Hz, 2H), 7.57–7.64 (m, 8H), 7.74
(d, J¼8.0 Hz, 2H), 8.04–8.06 (m, 2H); 13C NMR d 126.46,
126.97, 127.00, 127.00, 127.35, 127.40, 127.62, 128.76,
128.87, 128.93, 129.52, 129.97, 130.03, 131.04, 132.83,
133.63, 134.00, 140.22, 140.50, 140.69, 140.92, 150.52,
165.56; MS m/z 465 (Mþ). Anal. calcd for C33H23NS: C,
85.13; H, 4.98; N, 3.01; S, 6.89. Found: C, 84.87; H, 5.05;
N, 2.96; S, 6.77.
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