Direct C-2 acylation of thiazoles with aldehydes via metal- and solvent-free c-h activation in the presence of tert-butyl hydroperoxide

Article abstract of DOI:10.1055/s-0033-1340068

A novel and efficient methodology for the synthesis of heteroaryl ketones by C-H activation of aldehydes and thiazoles is developed. The reaction occurs smoothly, under metal-, acid- and solvent-free conditions using tert-butyl hydroperoxide as the oxidant under an air atmosphere, to afford a wide range of heteroaryl ketones in moderate to good yields. The sp² C-H bonds in the aldehyde and thiazole undergo direct oxidative cross-coupling, resulting in C-2 acylation of the azole. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0033-1340068

110▌
LETTER
lDettierrect C-2 Acylation of Thiazoles with Aldehydes via Metal- and Solvent-Free
C–H Activation in the Presence of tert-Butyl Hydroperoxide
Direct Oxidative Acylation Reactions
Ashok B. Khemnar, Bhalchandra M. Bhanage*
Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai-400 019, India
Fax +91(22)33611020; E-mail: bm.bhanage@gmail.com; E-mail: bm.bhanage@ictmumbai.edu.in
Received: 31.08.2013; Accepted after revision: 01.10.2013
tert-butyl hydroperoxide (TBHP) as an oxidant under an
Abstract: A novel and efficient methodology for the synthesis of
air atmosphere (Scheme 1).
heteroaryl ketones by C–H activation of aldehydes and thiazoles is
developed. The reaction occurs smoothly, under metal-, acid- and
solvent-free conditions using tert-butyl hydroperoxide as the oxi-
dant under an air atmosphere, to afford a wide range of heteroaryl
ketones in moderate to good yields. The sp2 C–H bonds in the alde-
hyde and thiazole undergo direct oxidative cross-coupling, resulting
in C-2 acylation of the azole.
O
O
R1
R2
N
metal-free
TBHP
N
S
R1
H
H +
S
neat, air
100 °C, 16 h
R3
R3
R2
Key words: acylation, C–H activation, oxidative cross-coupling,
metal-free, solvent-free
Scheme 1 Oxidative acylation of thiazoles with aldehydes
Initially, the reaction of 4,5-dimethylthiazole (1a) with
benzaldehyde (2a) was chosen as a model reaction, and
the effects of various parameters such as the catalyst, sol-
vent, oxidant, temperature and reaction time were studied.
At the outset, we screened various transition-metal salts
[Pd(OAc)2, Co(OAc)2, Fe(OAc)2 and FeSO4] for the oxi-
dative cross-coupling of 1a with 2a using tert-butyl hy-
droperoxide as the oxidant (Table 1, entries 1–4). It was
found that palladium acetate [Pd(OAc)2] and iron(II) sul-
fate (FeSO4) provided the desired product 3a in 10% and
38% yield respectively (Table 1, entries 1 and 4). Further-
more, when the reaction was carried out in the absence of
a metal salt, an improvement in the yield (45%) was ob-
served (Table 1, entry 5). Next, we investigated the effect
of different solvents on the reaction (Table 1, entries 6 and
7). However, when the reaction was carried out in the ab-
sence of a catalyst and solvent, the yield of the desired
product 3a increased to 73% (Table 1, entry 8).
Azole derivatives are important building blocks in various
natural products, agrochemicals and biologically active
molecules.1 In this regard, significant progress has been
made in developing efficient methodologies for the syn-
thesis of azoles substituted at C-2. Thus, C–C and C–N
bond formation between azoles and various moieties such
as alkynyl halides,2 amines,3 esters,4 alkyl halides,5 mesyl-
ates,6 nitriles,7 benzyl halides,8 and alkenyl halides9 have
been achieved with considerable efficiency by cross-
coupling at the C-2 position of azoles.
Acylation of azoles is an important approach to the syn-
thesis of various heteroaryl ketones. Anderson and co-
workers have developed C-2 acylations of azoles with
acyl chlorides.10 Benzaldehyde as an acylating reagent has
also been explored by various groups,11 the process re-
quiring lithium metal along with inorganic oxidants.
Beller and co-workers have reported carbonylative C–H
activation of azoles using aryl halides and carbon monox-
ide gas.12 Furthermore, the acylations of 2-phenylpyri-
dine,13 acetanilide14 and other moieties,15 via C–H bond
activation with different acyl moieties using noble metal
catalysts, have been reported.
We next screened various organic and inorganic oxidants
under metal- and solvent-free conditions (Table 1, entries
9–16), and observed that hydrogen peroxide and m-chlo-
roperoxybenzoic acid (MCPBA) were ineffective (Table
1, entries 9 and 10), whilst tert-butyl perbenzoate (TBPB),
cumene hydroperoxide (CHP) and aqueous tert-butyl hy-
droperoxide afforded the expected product in poor yields
(Table 1, entries 11–13). Inorganic oxidants did not work
under the same reaction conditions (Table 1, entries 14–
16). When the reaction was carried out using tert-butyl
hydroperoxide as the oxidant without an air atmosphere
the yield of the desired product decreased (Table 1, entry
17). Performing the reaction under an oxygen atmosphere
also led to a lower yield of the acylation product (Table 1,
entry 18). Furthermore, the effect of increasing the num-
ber of equivalents of benzaldehyde (2a) showed that the
yield of 3a was improved when the 1a:2a ratio was 1:4
(Table 1, entries 8 and 19). We also studied the effect of
the tert-butyl hydroperoxide loading and found that four
equivalents of the oxidant were necessary to obtain ac-
Metal-free C–C bond formation by C–H activation using
milder reaction conditions is a challenging prospect. In
this regard, Wang and co-workers reported metal-free
amidation16 and alkylation17 through oxidative cross-cou-
pling by C–H activation. In addition, the metal-free acyl-
ation of heteroaryl moieties has been explored.18 Thus, we
aimed to develop an efficient and mild protocol for the
acylation of thiazoles with aldehydes via sp2 C–H bond
activation at C-2. The present system works efficiently
under metal-, acid-, and solvent-free conditions using
SYNLETT 2014, 25, 0110–0114
Advanced online publication: 05.11.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1340068; Art ID: ST-2013-D0840-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Direct Oxidative Acylation Reactions
111
ceptable yields (Table 1, entries 8 and 20). Subsequently, present protocol is that acylation of thiazole 1a with
the effect of the reaction time and temperature were exam- ortho-, meta-, and para-substituted aldehyde derivatives
ined, and it was found that the highest yield of the product proceeded well, and the corresponding 2-acylated thiazole
3a was obtained at 100 °C within 16 hours.
products were obtained in good yields (Table 2, entries 1–
11), indicating that the steric and electronic effects of the
substituents on the aromatic rings of the aldehydes were
negligible. The acylation of thiazole 1a with aldehydes
bearing electron-donating substituents proceeded smooth-
ly to afford good yields of the corresponding products
(Table 2, entries 2–6).
Table 1 Optimization of the Reaction Conditionsa
Entry
Catalyst
Oxidant
Solvent
Yield (%)b
Effect of the catalyst
1
2
3
4
5
Pd(OAc)2
Co(OAc)2
Fe(OAc)2
FeSO4
TBHP
TBHP
TBHP
TBHP
TBHP
toluene
toluene
toluene
toluene
toluene
10
–
Aldehydes bearing halide substituents also furnished
good yields of the corresponding products (Table 2, en-
tries 7–11). The heteroaromatic aldehyde, thiophene-3-
carboxaldehyde (2l) smoothly underwent oxidative cross-
coupling and provided a moderate yield of the respective
product 3l (Table 2, entry 12). Moreover, an aliphatic al-
dehyde also reacted under the optimized reaction condi-
tions to give a moderate 47% yield of the coupled product
3m (Table 2, entry 13).19 When the reaction was carried
out using 4-methylthiazole (1b), satisfactory yields of the
corresponding products were obtained (Table 2, entries
14–16).20
–
38
45
–
Effect of the solvent
6
7
8
–
–
–
TBHP
TBHP
TBHP
PhCl
MeCN
–
5
10
73
A tentative mechanism to rationalize this transformation
is presented in Scheme 2. The reaction may proceed
through the generation of free radicals, the first step in-
volving homolysis of tert-butyl hydroperoxide to generate
a hydroxyl radical and an alkoxyl radical. In the next step,
these radicals abstract hydrogens from the sp2 C–H (C-2)
of 4,5-dimethylthiazole and the aldehydic C–H of benzal-
dehyde, generating the corresponding free radicals. Final-
ly, the free radicals of 4,5-dimethylthiazole and
benzaldehyde react with each other to form a new carbon–
carbon bond and generate the acylated derivative (3a) as
the major product, along with the minor homo-coupled
side product through termination of two radicals of 4,5-di-
methylthiazole. It should be noted that the reaction was
suppressed by a radical scavenger, such as 2,2,6,6-tetra-
methylpiperidine 1-oxyl (TEMPO).
Effect of the oxidant
9
10
11
12
13
14
15
16
17c
18d
19e
20f
–
–
–
–
–
–
–
–
–
–
–
–
H2O2
–
–
–
–
–
–
–
–
–
–
–
–
–
MCPBA
TBPB
CHP
–
20
16
25
–
TBHP (aq)
I2
K2S2O8
Ag2CO3
TBHP
TBHP
TBHP
TBHP
15
–
58
13
29
61
OH
O
+
O
OH
N
S
N
S
a Reaction conditions: 4,5-dimethylthiazole (1a) (1 mmol), benzalde-
hyde (2a) (4 mmol), oxidant (4 mmol), catalyst (10 mol%) if neces-
sary, solvent (3 ml) if necessary, 100 °C, air atmosphere, 16 h.
b Yield determined by GC.
5%
t-BuOH
N
S
N
S
O
c Without an air atmosphere.
H
d Under an oxygen atmosphere.
O
e Ratio of 1a/2a = 1:1.
OH
O
f TBHP (3 equiv, 5–6 M in decane).
.
O
H2O
H
N
With optimized reaction conditions in hand, the scope of
the protocol was extended for the synthesis of various ac-
ylated thiazole derivatives. As shown in Table 2, the reac-
tion worked efficiently with different aldehydes to
provide a wide range of acylated thiazole derivatives in
S
3a
70%
Scheme 2 A plausible reaction mechanism for the acylation of thia-
moderate to good yields. The important feature of the zoles with aldehydes
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 110–114

112
A. B. Khemnar, B. M. Bhanage
LETTER
Yield (%)b
70
Table 2 Oxidative Acylation of Thiazoles with Various Aldehydesa
Entry
1
Thiazole
Aldehyde
Product
O
O
O
O
O
H
N
N
H
H
H
H
H
H
H
S
S
S
S
S
1a
2a
O
3a
N
N
H
2
3
4
5
6
7
72
78
73
71
80
72
S
1a
2b
O
3b
N
N
H
S
1a
2c
O
3c
N
N
H
S
OMe
OMe
1a
2d
3d
O
O
O
N
OMe
OMe
N
OMe
OMe
H
S
S
S
1a
2e
3e
OMe
O
OMe
N
N
H
S
MeO
MeO
1a
2f
3f
O
O
N
F
N
F
H
S
S
OMe
OMe
1a
2g
3g
O
OMe
Br
O
OMe
Br
N
N
H
H
H
S
8
9
76
65
S
1a
2h
3h
O
O
N
N
H
S
S
F
F
1a
2i
3i
Synlett 2014, 25, 110–114
© Georg Thieme Verlag Stuttgart · New York

LETTER
Direct Oxidative Acylation Reactions
113
Table 2 Oxidative Acylation of Thiazoles with Various Aldehydesa (continued)
Entry
10
Thiazole
Aldehyde
Product
Yield (%)b
O
O
H
N
N
H
H
H
H
72
S
S
Cl
Cl
1a
2j
3j
O
Cl
O
Cl
N
N
N
N
H
11
12
13
77
56
47
S
S
S
S
Cl
Cl
1a
2k
3k
3l
O
O
O
N
H
S
S
S
1a
2l
N
O
O
H
S
2m
1a
3m
3n
3o
O
O
O
N
N
N
N
H
H
H
14
15
60
63
S
S
S
S
1b
2a
O
O
N
H
S
1b
2b
N
H
H
16
64
S
OMe
OMe
1b
2d
3p
a Reaction conditions: thiazole (1 mmol), aldehyde (4 mmol), TBHP (4 mmol, 5–6 M in decane), 100 °C, 16 h, air atmosphere, neat.
b Yield of isolated product.
The authors thank Dr. C. V. Rode from the National Chemical La-
boratory, Pune for providing HRMS analysis of products.
In summary, we have developed an efficient protocol for
the direct C-2 acylation of thiazoles with a range of alde-
hydes, via C–H activation under metal-, acid-, and sol-
vent-free conditions, to give the corresponding acylated
thiazole derivatives.21 Further applications of the present
protocol for acylation of various moieties are under prog-
ress.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 110–114

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A. B. Khemnar, B. M. Bhanage
LETTER
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An oven-dried 15 mL glass vial containing a magnetic stir
bar was charged with 4,5-dimethylthiazole (1a) (1 mmol)
and benzaldehyde (2a) (4 mmol). The vial was then flushed
with air and sealed with a cap. Next, TBHP (4 mmol, 5–6 M
in decane) was added dropwise with stirring and the mixture
was further stirred at 100 °C for 16 h under an air atm. After
cooling the mixture to r.t., it was washed with sat. NaHCO3
solution (1 × 30 mL). The product was extracted with
EtOAc (3 × 10 mL) and dried over Na2SO4. The solvent was
removed under vacuum and the crude residue was purified
by column chromatography (silica gel, 60–100 mesh; PE–
EtOAc) to afford pure coupled product 3a. 1H NMR (300
MHz, CDCl3): δ = 8.44–8.41 (m, 2 H), 7.56–7.51 (m, 3 H),
2.47 (s, 3 H), 2.45 (s, 3 H). 13C NMR (75 MHz, CDCl3):
δ = 184.08, 162.58, 151.77, 135.92, 133.25, 131.10, 128.68,
124.02, 15.14, 12.02. GC–MS (EI, 70 eV): m/z (%) = 217
(20) [M]+, 188 (52), 105 (100), 85 (37), 77 (87), 53 (10), 51
(31). HRMS (ESI): m/z [M + H]+ calcd for C12H12NOS:
218.0640; found: 218.0634.
(4,5-Dimethylthiazol-2-yl)(p-tolyl)methanone (3b)
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7.29 (d, J = 8.05 Hz, 2 H), 2.46 (s, 3 H), 2.44 (s, 3 H), 2.42
(s, 3 H). 13C NMR (75 MHz, CDCl3): δ = 183.69, 162.89,
151.60, 144.15, 135.57, 133.01, 131.23, 129.07, 21.80,
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202 (55), 119 (100), 91 (55), 89 (13), 86 (27), 65 (29), 45
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C13H14NOS: 232.0796; found: 232.0791.
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(2,6-Dimethylphenyl)(4,5-dimethylthiazol-2-
yl)methanone (3c)
1H NMR (300 MHz, CDCl3): δ = 7.52 (t, J = 7.6 Hz, 1 H),
7.05 (d, J = 7.6 Hz, 2 H), 2.45 (s, 3 H), 2.35 (s, 3 H), 2.19 (s,
6 H). 13C NMR (75 MHz, CDCl3): δ = 191.94, 161.71,
152.86, 138.78, 130.63, 129.32, 127.62, 119.38, 19.70,
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© Georg Thieme Verlag Stuttgart · New York