Cobalt-Catalyzed Decarboxylative 2-Benzoylation of Oxazoles and Thiazoles with α-Oxocarboxylic Acids

Article abstract of DOI:10.1021/acs.joc.5b01450

Cobalt-catalyzed decarboxylative cross-coupling of oxazoles and thiazoles with α-oxocarboxylic acids was developed through an sp² C-H bond functionalization process. This work represents the first example of cobalt-catalyzed decarboxylative C-H

Full text of DOI:10.1021/acs.joc.5b01450

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Note
Cobalt-Catalyzed Decarboxylative 2-Benzoylation of
Oxazoles and Thiazoles with alpha-Oxocarboxylic Acids
Ke Yang, Xinyong Chen, Yuqi Wang, Wanqing Li, Adnan A. Kadi,
Hoong-Kun Fun, Hao Sun, Yan Zhang, Guigen Li, and Hongjian Lu
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b01450 • Publication Date (Web): 09 Oct 2015
Downloaded from http://pubs.acs.org on October 9, 2015
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CobaltꢀCatalyzed Decarboxylative 2ꢀBenzoylation of
Oxazoles and Thiazoles with αꢀOxocarboxylic Acids
Ke Yang,†,§ Xinyong Chen,†,§ Yuqi Wang,† Wanqing Li,† Adnan A. Kadi,‡ HoongꢀKun Fun,‡ Hao
Sun,†,* Yan Zhang,†,‡ Guigen Li,†,* and Hongjian Lu†,*
† Institute of Chemistry & BioMedical Sciences, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
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‡ Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, Riyadh 11451,
Saudi Arabia
Graphical Abstract
Abstract Cobaltꢀcatalyzed decarboxylative crossꢀcoupling of oxazoles and thiazoles with αꢀ
oxocarboxylic acids was developed through an sp2 C−H bond functionalization process. This work
represents the first example of cobaltꢀcatalyzed decarboxylative C−H bond functionalization, and
provides an efficient mean of building some important bioactive heteroaryl ketone derivatives.
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Transitionꢀmetal catalyzed C−H functionalization process allows simplification and abbreviation of
synthetic procedures, and is one of the most powerful and facile methods with which to construct
complex molecules in synthetic organic and medicinal chemistry.1 Recently, cobaltꢀcatalyzed C−H
functionalization has met with success due to its economy, low toxicity and interesting reaction
modes.2,3 Many C−C4 and C−heteroatom5 bondꢀforming reactions have been achieved by this method
since the first example of cobaltꢀcatalyzed chelationꢀassisted C−H functionalization process was
reported by Murahashi.4x Due to their value in synthetic chemistry, C−C bondꢀforming reactions have
attracted significant attention, and various reagents including Nꢀtosylhydrazones, Nꢀcyanosuccinimide,
alkyl and aryl halides, sulfamates, aziridines, alkenes and alkynes have been utilized as coupling
partners.2,4 In the meanwhile, transitionꢀmetal catalyzed decarboxylative crossꢀcoupling reactions of
(hetero)arenes with carboxylic acids through sp2 C−H bond functionalization have also received
attention because of the low cost, stability, and general availability of carboxylic acids, one of the
coupling partners.6 Since 2008,7m Pd complexes have been recognized as most efficient catalysts in
decarboxylative C−H functionalization for the construction of versatile C−C bonds.7ꢀ9 Other transitionꢀ
metal complexes, including Rh, Ag, Cu and Ni complexes have very recently emerged as efficient
catalysts,10 but no decarboxylative C−H functionalization processes through cobalt catalysis have been
reported to date.
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2ꢀSubstituted oxazoles and thiazoles, present in many medicinally active compounds with a diverse
array of biological activities, medicines and functional materials.11 Previous methods to direct synthesis
of 2ꢀsubstituted oxazoles and thiazoles relies primarily on transitionꢀmetal catalyzed direct Cꢀ2
functionalizations of oxazoles and thiazoles.4r,7h,10a,10b,12 Although many transitionꢀmetal catalyzed
direct Cꢀ2 functionalizations have been reported, the success of direct Cꢀ2 acylation is still rare.10a,12f,12m
To continue our efforts on cobaltꢀcatalyzed C−H functionalization5e,5j and metalꢀcatalyzed
decarboxylative acylation of oxazoles,10a we disclose the cobaltꢀcatalyzed 2ꢀbenzoylation of oxazoles
and thiazoles with αꢀoxocarboxylic acids through an sp2 C−H bond functionalization reaction. This
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process represents the first example of decarboxylative crossꢀcouplings via an sp2 C−H
functionalization process with cobalt catalysis.
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Initially, we examined the decarboxylative crossꢀcoupling of benzoxazole (1a) and 2ꢀoxoꢀ2ꢀ
phenylacetic acid (2a) with CoCl2 in the presence of Ag2CO3 at 170 °C. After extensive screening of
solvents, 3ꢀfluorobenzotrifluoride (3ꢀFꢀC6H4CF3) was found to be optimal, and with this solvent, the
desired product benzo[d]oxazolꢀ2ꢀyl(phenyl)ꢀmethanone (3a) was obtained in 72% yield (Table 1,
entries 1ꢀ6). Subsequently, various cobalt catalysts were examined and we found that the reaction could
be effectively catalyzed by a variety of cobalt(II) complexes, including Co(OAc)2·4H2O, CoSO4·7H2O,
Co(acac)2, CoCO3· H2O and Co(ClO4)2·6H2O (entries 7ꢀ12). Cobalt(III) species, such as Co(acac)3 and
Co(bpy)3(ClO4)3 were also found to catalyze the reaction, albeit in low yield (entries 13 and 14). These
investigations identified Co(ClO4)2·6H2O as the most efficient catalyst, which provided the desired
product 3a in 92% isolated yield. Additionally, we found that replacement of Ag2CO3 with other
oxidants, such as K2S2O8, AgOAc or Ag2O, produced low yield of the desired product 3a (entries 15ꢀ
17). Finally, in the absence of the Co catalyst, only a trace amount of product 3a was formed (entry 18).
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Table 1. Optimization of reaction conditions.a
Entry
1
Co source (mol%)
CoCl2
Additives
Ag2CO3
Solvent
C6H6
Yield (%)b
30
2
3
CoCl2
CoCl2
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
C6H5CH3
C6H5F
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29
< 5
60
72
70
45c
61
10
4
CoCl2
C6H5Cl
5
CoCl2
C6H5CF3
6
CoCl2
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
7
CoCl2 · 6H2O
Co(OAc)2 · 4H2O
CoCO3· H2O
CoSO4 · 7H2O
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Co(acac)2
Co(ClO4)2 · 6H2O
Co(acac)3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
K2S2O8
AgOAc
Ag2O
3ꢀFꢀC6H4CF3
18
95(92)d
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3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
3ꢀFꢀC6H4CF3
Co(bpy)3(ClO4)3
Co(ClO4)2 · 6H2O
Co(ClO4)2 · 6H2O
Co(ClO4)2 · 6H2O
ꢀ
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< 5
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Ag2CO3
< 5
a
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), Co source (10 mol%), Additives (0.6 mmol), 2
b
1
mL of solvent, 170 °C, 24 h. Yields are based on 1a, determined by crude H NMR using
dibromomethane as the internal standard. c 35% of 1a isolated. d Isolated yield.
Table 2. Scope of oxazole derivatives.a
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a Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol), Co(ClO4)2·6H2O (10 mol%), Ag2CO3 (3.0 eq), 3ꢀ
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fluorobenzotrifluoride (2 mL), 170 °C, 24 h. Isolated yields. b 36 h.
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Table 3. Scope of αꢀoxocarboxylic acids.a
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O
N
O
Co(ClO4)2 . 6H2O (10 mol%)
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R
O
N
O
H
+
R
COOH
Ag2CO3 (3.0 eq)
3-F-PhCF3, 170 °C
1a
2
3
O
O
O
N
O
N
O
N
O
Me
OMe
F
3m 63%
3l 62%
3n 77%
O
O
N
N
O
O
N
O
O
F
Cl
Br
3o 72%
3p 67%
3q 81%
O
O
O
N
O
N
O
N
O
Cl
Br
CF3
3r 90%
3s 80%
3t 76%
a
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), Co(ClO4)2·6H2O (10 mol%), Ag2CO3 (3.0 eq), 3ꢀ
fluorobenzotrifluoride (2 mL), 170 °C, 24 h. Isolated yields.
With the optimum conditions defined, we explored the scope of the oxazole substrates. The current
catalytic system proceeds smoothly to provide 2ꢀacylated substituted oxazoles in good yield (Table 2).
Both electronꢀdonating (Me) and electronꢀwithdrawing groups (Cl and Br) on the phenyl ring of
benzoxazole are tolerated (3bꢀg), and this provides more options for further potential transformations.
Furthermore, both monoꢀ and diꢀsubstituted oxazoles were effective coupling partners, and the desired
products were isolated in good yield (3hꢀk). Next, the substrate scope study of αꢀoxocarboxylic acids
was also examined. As shown in Table 3, paraꢀ or metaꢀsubstituted phenylglyoxylic acids, with either
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an electronꢀdonating or an electronꢀwithdrawing group on the phenyl ring were good substrates in the
coupling reaction (3l–3t). In addition, we found that there were no desired products formed when orthoꢀ
chloroꢀsubstituted phenylglyoxylic acid, 4ꢀmethylꢀ2ꢀoxopentanoic acid or 2ꢀcyclopropylꢀ2ꢀoxoacetic
acid were used as the coupling reagents.
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We also explored the substrate scope of thiazoles and αꢀoxocarboxylic acids in the presence of cobalt
catalysts. In the reaction of benzothiazole 4a with 2ꢀoxoꢀ2ꢀphenylacetic acid (2a) under standard
conditions, 31% yield of the desired product 5a was formed and 37% of 4a was recovered. Obvious
decomposition was observed, probably due to the oxidative property of Co(ClO4)2 · 6H2O. Rescreening
of the cobalt catalysts identified Co(OAc)2·4H2O as the optimal catalyst in the presence of Ag2CO3 and
3ꢀfluorobenzotrifluoride at 170 °C (Table 4). Various αꢀoxocarboxylic acids 2 and thiazoles 4 were well
tolerated under these reaction conditions, providing the desired products in moderate to good yield (5aꢀ
5i).
Table 4. Scope of thiazoles and αꢀoxocarboxylic acids.a
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a Reaction conditions: 4 (0.2 mmol), 2 (0.6 mmol), Co(OAc)2·4H2O (20 mol%), Ag2CO3 (3.0 eq), 3ꢀ
1
2
3
fluorobenzotrifluoride (2 mL), 170 °C, 24 h. Isolated yields.
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5
Scheme 1. Radical trapping experiments
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Scheme 2. Intermolecular competition experiment
To gain insight into the reaction mechanism, some preliminary experiments were performed as
depicted in Schemes 1 and 2. It was found that the addition of TEMPO resulted in the decreased yield of
this reaction (Scheme 1, entries 1ꢀ4 ) and the formation of 2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl benzoate 6
(7% isolated yield) when 3 equivalent TEMPO was added (entry 4), suggesting that a radical
mechanism may be implicated in the reaction. Intermolecular competition experiments between 1e and
1f revealed that the reaction can reach in proximity reaction rate with electronꢀdeficient or electronꢀrich
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arene as the substrate (Scheme 2). To gain further insight into the reaction mechanism, especially the
rateꢀdetermining step for the transformation, deuteriumꢀlabeling experiments were carried out (Schemes
S1 and S2 in SI). Significant primary kinetic isotope effects were observed in a parallel experiment
(Scheme S1). Meanwhile, the intermolecular H/D exchange between Dꢀbenzoxazole [D]ꢀ1a and 2ꢀoxoꢀ
2ꢀphenylacetic acid 2a was also observed (Scheme S2). These results suggest that the C−H bond
cleavage of benzoxazoles is rateꢀlimiting in the process. In addition, cobalt(III) compounds were found
to be able to catalyze the reaction under the oxidative reaction conditions (entries 13 and 14, Table 1),
which illustrates that Co(III) species may involve in the C−H bond cleavage step.
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Although the mechanism of the reaction remains unclear, the following Co(III/IV/II) catalytic cycle
(Scheme 3) has been proposed on the basis of the results above and previous reports.5b Ag2CO3 may
oxidize the CoII catalyst to the CoIII species, which reacts with oxazole (1) in the presence of Ag2CO3 to
give the CoIII intermediate A. Subsequent addition of the acyl radical species B, which could be formed
by the oxidation of α–oxocarboxylic acid (2) in the presence of Ag2CO3,13 may produce cobalt(IV)
species C. Reductive elimination of the species C leads to the desired product 3 and regenerating the
CoII catalyst thus completing the catalytic cycle. However, a catalytic cycle involving Co(II/III/I)
oxidation states can not be excluded at the present stage.4, 5, 13
Scheme 3. A possible catalytic cycle
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In summary, we have developed an efficient cobalt(II)ꢀcatalyzed direct decarboxylative 2ꢀ
benzoylation of oxazole and thiazole derivatives through an sp2 C−H bond functionalization process.
This transformation is the first example of a decarboxylative C−H functionalization reaction with cobalt
catalysis, and provides an efficient complementary approach to important bioactive heteroaryl ketone
derivatives.
EXPERIMENTAL SECTION
General. All the solvents and commercially available reagents were purchased from commercial
sources and used directly. Thin layer chromatography (TLC) was performed on plates (silica gel) and
visualized by fluorescence quenching under UV light. Column chromatography was performed on silica
gel (200–300 Mesh) using a forced flow of 0.5–1.0 bar. The 1H and 13C NMR spectra were obtained on
1
400 MHz and 100 MHz spectrometer. H NMR data were reported as: chemical shift (δ ppm),
multiplicity, coupling constant (Hz), and integration. 13C NMR data were reported in terms of chemical
shift (δ ppm), multiplicity, and coupling constant (Hz). Mass (HRMS) analysis was obtained using TOF
LC/MS system with Electrospray Ionization (ESI).
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Materials. Azoles (1a, 1b, 1e, 1f, 1j), αꢀoxocarboxylic acids (2a) and thiazoles (4) were purchased from
commercial sources and used directly (1c, 1d, 1g) were prepared from condensation of the
corresponding 2ꢀaminophenol and triethyl orthoformate according to the reported procedure.14
Azoles(1h, 1i) were prepared from pꢀtoluenesulfonylmethylisocyanide and the corresponding
benzaldehyde in methanol according to the reported procedure.15 Azoles(1k) were prepared from methyl
2ꢀisocyanoacetate and the corresponding anhydride in THF according to the reported procedure.16
Other αꢀoxocarboxylic acids were prepared from oxidation of corresponding methyl ketones with SeO2
according to the reported procedure.17
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General procedures for the synthesis of product 3. A 35 mL ovenꢀdried pressure tube was charged
with oxazoles 1 (0.2 mmol), αꢀoxocarboxylic acid 2 (0.6 mmol), Co(ClO4)2 · 6H2O (7.3 mg, 0.02 mmol),
Ag2CO3 (165.4 mg, 0.6 mmol) and 3ꢀFꢀPhCF3 (2.0 mL). The tube was then sealed and stirred
vigorously at 170 ˚C for 24 h. After cooling to room temperature, the reaction mixture was diluted with
DCM (20 mL), filtered through a pad of Celite, and the filtrate was then concentrated in vacuo. The
residue was purified by flash chromatography on silica gel (1% ethyl acetate in petroleum ether, v/v) to
yield the desired products 3.
Benzo[d]oxazolꢀ2ꢀyl(phenyl)methanone (3a) : white solid, 41.0 mg, yield: 92% (known compound7o,18).
1H NMR (400 MHz, CDCl3) δ 8.58 (d, J = 8.0 Hz, 2H), 7.99 (d, J = 8.0 Hz, 1H), 7.77 – 7.71 (m, 2H),
7.63 – 7.58 (m, 3H), 7.52 (t, J = 7.6 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 180.7, 157.1, 150.4, 140.8,
135.0, 134.4, 131.03, 128.7, 128.5, 125. 8, 122.4, 111.9.
(6ꢀMethylbenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3b) : white solid, 29.4 mg, yield: 62% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.56 (d, J = 8. 0 Hz, 2H), 7.84 (d, J = 8.0 Hz, 1H), 7.71 (t,
J = 7.2 Hz, 1H), 7.60 (t, J = 7.6 Hz, 2H), 7.53 (s, 1H), 7.32 (d, J = 8.4 Hz, 1H), 2.58 (s, 3H). 13C NMR
(100 MHz, CDCl3) δ 180.6, 156.8, 150.8, 139.6, 138.7, 135.1, 134.2, 131.0, 128.6, 127.4, 121.8, 111.8,
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22.2.
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(6ꢀChlorobenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3c) : white solid, 44.3 mg, yield: 86% (known
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compound7a). H NMR (400 MHz, CDCl3) δ 8.56 (d, J = 8.0 Hz, 2H), 7.90 (d, J = 8.8 Hz, 1H), 7.76 –
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7.71 (m, 2H), 7.61 (t, J = 7.6 Hz, 2H), 7.50 (d, J = 8.8 Hz, 1H). C NMR (100 MHz, CDCl3) δ 180.2,
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157.5, 150.6, 139.5, 134.8, 134.6, 134.4, 131.0, 128.7, 126.8, 123.0, 112.4.
(6ꢀBromobenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3d) : white solid, 49.0 mg, yield: 81% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.57 – 8.55 (m, 2H), 7.92 (d, J = 1.6 Hz, 1H), 7.84 (d, J =
13
8.8 Hz, 1H), 7.75 – 7.71 (m, 1H), 7.65 – 7.58 (m, 3H). C NMR (100 MHz, CDCl3) δ 180.1, 157.3,
150.8, 139.9, 134.8, 134.5, 131.0, 129.5, 128.7, 123.4, 121.9, 115.3.
(5ꢀMethylbenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3e) : white solid, 25.0 mg, yield: 53% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.56 (d, J = 8.0 Hz, 2H), 7.75 – 7.70 (m, 2H), 7.60 (t, J =
7.6 Hz, 3H), 7.39 (d, J = 8.4 Hz, 1H), 2.55 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 180.7, 157.3, 148.7,
141.0, 135.8, 135.1, 134.2, 131.0, 129.9, 128.6, 121.9, 111.2, 21.6.
(5ꢀChlorobenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3f) : white solid, 44.8 mg, yield: 87% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.57 – 8.55 (m, 2H), 7.96 (d, J = 2.0 Hz, 1H), 7.73 (t, J =
7.6 Hz, 1H), 7.68 (d, J = 8.8 Hz, 1H), 7.61 (t, J = 7.6 Hz, 2H), 7.55 (dd, J = 8.8, 2.0 Hz, 1H). 13C NMR
(100 MHz, CDCl3) δ 180.2, 158.1, 149.0, 141.7, 134.7, 134.6, 131.3, 131.0, 128.9, 128.7, 122.1, 112.7.
(5ꢀBromobenzo[d]oxazolꢀ2ꢀyl)(phenyl)methanone (3g) : white solid, 55.0 mg, yield: 91% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.56 (d, J = 7.6 Hz, 2H), 8.13 (s, 1H), 7.76 – 7.68 (m, 2H),
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7.65 – 7.59 (m, 3H). C NMR (100 MHz, CDCl3) δ 180.2, 157.8, 149.4, 142.2, 134.7, 134.6, 131.6,
131.0, 128.7, 125.2, 118.5, 113.2.
(5ꢀ(4ꢀMethoxyphenyl)oxazolꢀ2ꢀyl)(phenyl)methanone (3h) : white solid, 44.6 mg, yield: 80% (known
compound18). 1H NMR (400 MHz, CDCl3) δ 8.50 – 8.47 (m, 2H), 7.81 – 7.79 (m, 2H), 7.66 (d, J = 7.2
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Hz, 1H), 7.58 – 7.53(m, 3H), 7.02 (d, J = 8.8 Hz, 2H), 3.89 (s, 3H). 13C NMR (100 MHz, CDCl3) δ
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178.6, 161.0, 156.6, 154.4, 135.5, 133.7, 130.7, 128.4, 127.1, 122.6, 119.4, 114.6, 55.6.
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(5ꢀ(4ꢀBromophenyl)oxazolꢀ2ꢀyl)(phenyl)methanone (3i) : white solid, 53.8 mg, yield: 82% (known
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compound19). H NMR (400 MHz, CDCl3) δ 8.49 (d, J = 8.0 Hz, 2H), 7.74 (d, J = 8.4 Hz, 2H), 7.71 –
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7.64 (m, 4H), 7.57 (t, J = 7.6 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ 178.7, 157.1, 153.2, 135.2, 133.9,
132.4, 130.8, 128.5, 126.8, 125.6, 124.3, 124.3.
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Ethyl 2ꢀbenzoyloxazoleꢀ5ꢀcarboxylate (3j) : white solid, 30.0 mg, yield: 62% (known compound19). H
NMR (400 MHz, CDCl3) δ 8.48 – 8.45 (m, 2H), 7.99 (s, 1H), 7.72 – 7.68 (m, 1H), 7.59 – 7.55 (m, 2H),
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4.48 (q, J = 7.2 Hz, 2H), 1.45 (t, J = 7.2 Hz, 3H). C NMR (100 MHz, CDCl3) δ 178.6, 158.2, 157.2,
144.1, 134.5, 134.5, 134.4, 130.9, 128.7, 62.3, 14.2.
Methyl 2ꢀbenzoylꢀ5ꢀphenyloxazoleꢀ4ꢀcarboxylate (3k) : yellow solid, 34.4 mg, yield: 56% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.52 (d, J = 7.6 Hz, 2H), 8.24 (d, J = 3.6 Hz, 2H), 7.70 (t, J
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= 7.2 Hz, 1H), 7.61 – 7.55 (m, 5H), 4.02 (s, 3H). C NMR (100 MHz, CDCl3) δ 178.6, 162.1, 157.5,
154.8, 134.7, 134.3, 131.5, 131.0, 129.1, 128.7, 128.6, 128.0, 125.9, 52.7.
Benzo[d]oxazolꢀ2ꢀyl(pꢀtolyl)methanone (3l) : white solid, 29.4 mg, yield: 62% (known compound7o). 1H
NMR (400 MHz, CDCl3) δ 8.49 (d, J = 8.4 Hz, 2H), 7.98 (d, J =8.0 Hz, 1H), 7.74 (d, J = 8.0 Hz, 1H),
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7.60 – 7.56 (m, 1H), 7.52 – 7.48 (m, 1H), 7.40 (d, J = 8.0 Hz, 2H), 2.50 (s, 3H). C NMR (100 MHz,
CDCl3) δ 180.2, 157.3, 150.4, 145.6, 140.8, 132.5, 131.2, 129.4, 128.3, 125.7, 122.3, 111.9, 21.9.
Benzo[d]oxazolꢀ2ꢀyl(4ꢀmethoxyphenyl)methanone (3m) : white solid, 31.9 mg, yield: 63% (known
compound7o). 1H NMR (400 MHz, CDCl3) δ 8.64 (d, J = 8.8 Hz, 2H), 7.97 (d, J = 8.0 Hz, 1H), 7.74 (d,
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J = 8.0 Hz, 1H), 7.59 – 7.55 (m, 1H), 7.51 – 7.48 (m, 1H), 7.07 (d, J = 8.0 Hz, 2H), 3.95 (s, 3H). C
NMR (100 MHz, CDCl3) δ 178.8, 164.7, 157.4, 150.3, 140.8, 133.6, 128.2, 128.0, 125.6, 122.2, 114.0,
111.8, 55.6.
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Benzo[d]oxazolꢀ2ꢀyl(4ꢀfluorophenyl)methanone (3n) : white solid, 37.0 mg, yield: 77% (known
compound7o). 1H NMR (400 MHz, CDCl3) δ 8.72 – 8.67 (m, 2H), 7.98 (d, J = 8.0 Hz, 1H), 7.75 (d, J =
8.0 Hz, 1H), 7.62 – 7.50 (m, 2H), 7.31 – 7.25 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 178.8, 166.6 (d, J
= 256.0 Hz), 156.9, 150.4, 140.7, 134.0 (d, J = 9.6 Hz), 131.4 (d, J = 2.9 Hz), 128.6, 125.8, 122.4, 115.9
(d, J = 21.9 Hz), 111.9.
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Benzo[d]oxazolꢀ2ꢀyl(4ꢀchlorophenyl)methanone (3o) : white solid, 37.0 mg, yield: 72% (known
compound19). 1H NMR (400 MHz, CDCl3) δ 8.60 – 8.56 (m, 2H), 7.97 (d, J = 8.0 Hz, 1H), 7.74 (d, J =
13
8.0 Hz, 1H), 7.61 – 7.55 (m, 3H), 7.53 – 7.49 (m, 1H). C NMR (100 MHz, CDCl3) δ 179.1, 156.8,
150.4, 141.1, 140.7, 133.3, 132.5, 129.0, 128.7, 125.9, 122.4, 111.9.
Benzo[d]oxazolꢀ2ꢀyl(4ꢀbromophenyl)methanone (3p) : white solid, 40.5 mg, yield: 67% (known
compound7o). 1H NMR (400 MHz, CDCl3) δ 8.51 – 8.49 (m, 2H), 7.97 (d, J = 8.0 Hz, 1H), 7.74 (d, J =
8.4 Hz, 3H), 7.61 – 7.57 (m, 1H), 7.51 (t, J = 7.6 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ 179.4, 156.8,
150.4, 140.7, 133.7, 132.5, 132.0, 130.0, 128.7, 125.9, 122.5, 111.9.
Benzo[d]oxazolꢀ2ꢀyl(3ꢀfluorophenyl)methanone (3q) : white solid, 39.0 mg, yield: 81% (known
compound7a). 1H NMR (400 MHz, CDCl3) δ 8.45 – 8.42 (m, 1H), 8.36 – 8.32 (m, 1H), 7.99 (d, J = 8.0
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Hz, 1H), 7.75 (d, J = 8.0 Hz, 1H), 7.63 – 7.56 (m, 2H), 7.54 – 7.50 (m, 1H), 7.45 – 7.40 (m, 1H). C
NMR (100 MHz, CDCl3) δ 179.1 (d, J = 2.0 Hz), 162.6 (d, J = 246.0 Hz), 156.8, 150.5, 140.7, 136.8 (d,
J = 7.1 Hz), 130.3 (d, J = 7.7 Hz), 128.7, 126.9 (d, J = 3.1 Hz), 125.9, 122.5, 121.4 (d, J = 21.4 Hz),
117.8 (d, J = 23.5 Hz), 111.9.
Benzo[d]oxazolꢀ2ꢀyl(3ꢀchlorophenyl)methanone (3r) : white solid, 46.2 mg, yield: 90% (known
compound19). 1H NMR (400 MHz, CDCl3) δ 8.59 (t, J = 2.0 Hz, 1H), 8.54 – 8.51 (m, 1H), 8.00 (d, J =
8.0 Hz, 1H), 7.75 (d, J = 8.4 Hz, 1H), 7.70 – 7.67 (m, 1H), 7.63 – 7.57 (m, 1H), 7.55 – 7.50 (m, 2H). 13C
NMR (100 MHz, CDCl3) δ 179.1, 156.7, 150.5, 140.7, 136.4, 134.9, 134.2, 130.9, 123.0, 129.2, 128.8,
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125.9, 122.6, 111.9.
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Benzo[d]oxazolꢀ2ꢀyl(3ꢀbromophenyl)methanone (3s) : white solid, 40.0 mg, yield: 80% (known
compound18). 1H NMR (400 MHz, CDCl3) δ 8.73 (t, J = 2.0 Hz, 1H), 8.59 – 8.57 (m, 1H), 8.00 (d, J =
8.0 Hz, 1H), 7.85 – 7.82 (m, 1H), 7.75 (d, J = 8.4 Hz, 1H), 7.62 – 7.58 (m, 1H), 7.54 – 7.46 (m, 2H). 13C
NMR (100 MHz, CDCl3) δ 179.0, 156.6, 150.5, 140.7, 137.1, 136.6, 133.8, 130.2, 129.7, 128.8, 125.9,
122.8, 122.6, 111.9.
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Benzo[d]oxazolꢀ2ꢀyl(3ꢀ(trifluoromethyl)phenyl)methanone (3t) : white solid, 44.2 mg, yield: 76%
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(known compound7a). H NMR (400 MHz, CDCl3) δ 8.89 – 8.84 (m, 2H), 8.01 – 7.96 (m, 2H), 7.77 –
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7.74 (m, 2H), 7.64 – 7.60 (m, 1H), 7.55 – 7.51 (m, 1H). C NMR (100 MHz, CDCl3) δ 179.1, 156.6,
150.5, 140.7, 135.5, 134.3, 131.4 (q, J = 33.0 Hz), 130.6 (q, J = 3.5 Hz), 129.3, 128.9, 127.9 (q, J = 3.9
Hz), 126.0, 122.6, 123.7 (q, J = 274.0 Hz), 112.0.
General procedures for the synthesis of product 5. A 35 mL ovenꢀdried pressure tube was charged
with thiazoles 4 (0.2 mmol), αꢀoxocarboxylic acid 2 (0.6 mmol), Co(OAc)2 · 4H2O (7.6 mg, 0.04 mmol),
Ag2CO3 (165.4 mg, 0.6 mmol) and 3ꢀFꢀPhCF3 (2.0 mL). The tube was then sealed and stirred
vigorously at 170 ˚C for 24h. After cooling to room temperature, the reaction mixture was diluted with
DCM (20 mL), filtered through a pad of Celite, and the filtrate was then concentrated in vacuo. The
residue was purified by flash chromatography on silica gel (gradient eluent of 1% ethyl acetate in
petroleum ether, v/v) to yield the desired products 5.
Benzo[d]thiazolꢀ2ꢀyl(phenyl)methanone (5a) : yellow solid, 28.7 mg, yield: 60% (known compound19).
1H NMR (400 MHz, CDCl3) δ 8.59 – 8.57 (m, 2H), 8.29 – 8.27 (m, 1H), 8.07 – 8.04 (m, 1H), 7.72 –
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7.69 (m, 1H), 7.64 – 7.56 (m, 4H). C NMR (100 MHz, CDCl3) δ 185.4, 167.1, 153.9, 137.0, 135.0,
134.0, 131.3, 128.6, 127.7, 127.0, 125.8, 122.2.
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Benzo[d]thiazolꢀ2ꢀyl(pꢀtolyl)methanone (5b) : yellow solid, m.p.: 89ꢀ90 C, 21.8 mg, yield: 43% H
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NMR (400 MHz, CDCl3) δ 8.50 (d, J = 7.6 Hz, 2H), 8.27 (d, J = 8.0 Hz, 1H), 8.05 (d, J = 7.6 Hz, 1H),
7.62 – 7.57 (m, 2H), 7.39 (d, J = 7.2 Hz, 2H), 2.50 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 185.0, 167.5,
153.9, 145.0, 137.0, 132.4, 131.4, 129.3, 127.5, 126.9, 125.7, 122.2, 21.9. IR (neat) ν 3004, 2916, 1637,
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1566, 1318, 1273, 1183, 1116, 889, 833, 764, 705 cmꢀ1 HRMS (ESI, m/z): calcd. for C15H12NOS
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(M+H)+: 254.0634, found: 254.0628.
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Benzo[d]thiazolꢀ2ꢀyl(4ꢀchlorophenyl)methanone (5c) : yellow solid, m.p.: 91ꢀ92 C, 27.4 mg, yield:
50%. 1H NMR (400 MHz, CDCl3) δ 8.60 – 8.57 (m, 2H), 8.28 – 8.26 (m, 1H), 8.06 – 8.04 (m, 1H), 7.65
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– 7.56 (m, 4H). C NMR (100 MHz, CDCl3) δ 184.1, 166.8, 153.8, 140.6, 137.1, 133.3, 132.7, 128.9,
127.8, 127.1, 125.8, 122.2. IR (neat) ν 3005, 2316, 1638, 1585, 1320, 1275, 1175, 1117, 891, 835, 764,
708 cmꢀ1. HRMS (ESI, m/z): calcd. for C14H9ClNOS (M+H)+: 274.0088, found: 274.0077.
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Benzo[d]thiazolꢀ2ꢀyl(4ꢀbromophenyl)methanone (5d) : yellow solid, m.p.: 90ꢀ91 C, 26.0 mg, yield:
41%. 1H NMR (400 MHz, CDCl3) δ 8.49 (d, J = 8.4 Hz, 2H), 8.27 (d, J = 8.0 Hz, 1H), 8.05 (d, J = 8.0
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Hz, 1H), 7.74 (d, J = 8.4 Hz, 2H), 7.65 – 7.57 (m, 2H). C NMR (100 MHz, CDCl3) δ 184.3, 166.8,
153.8, 137.1, 133.7, 132.8, 131.9, 129.5, 127.8, 127.1, 125.8, 122.2. IR (neat) ν 3010, 2987, 1639, 1581,
1394, 1275, 1173, 1115, 891, 835, 749, 710 cmꢀ1.HRMS (ESI, m/z): calcd. for C14H9BrNOS (M+H)+:
317.9583, found: 317.9568.
Benzo[d]thiazolꢀ2ꢀyl(4ꢀ(trifluoromethyl)phenyl)methanone (5e) : yellow solid, m.p.: 95ꢀ96 oC, 42.3 mg,
yield: 69%. 1H NMR (400 MHz, CDCl3) δ 8.69 (d, J = 8.0 Hz, 2H), 8.28 (d, J = 7.6 Hz, 1H), 8.07 (d, J
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= 7.6 Hz, 1H), 7.86 (d, J = 8.4 Hz, 2H), 7.66 – 7.59 (m, 2H). C NMR (100 MHz, CDCl3) δ 184.6,
166.3, 153.8, 137.8, 137.1, 134.9 (q, J = 32.7 Hz), 131.6, 128.1, 127.2, 125.9, 125.56 (q, J = 3.7 Hz),
126.3 (q, J = 272.0 Hz), 122.3. IR (neat) ν 3061, 3004, 1655, 1486, 1320, 1275, 1170, 1154, 893, 852,
715, 706 cmꢀ1..HRMS (ESI, m/z): calcd. for C15H9F3NOS (M+H)+: 308.0351, found: 308.0337.
(5ꢀChlorobenzo[d]thiazolꢀ2ꢀyl)(4ꢀ(trifluoromethyl)phenyl)methanone (5f) : yellow solid, m.p.: 135ꢀ136
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oC, 38.4 mg, yield: 57%. H NMR (400 MHz, CDCl3) δ 8.66 (d, J = 8.0 Hz, 2H), 8.13 (d, J = 9.2 Hz,
1H), 7.84 (d, J = 8.4 Hz, 2H), 7.44 (d, J = 2.4 Hz, 1H), 7.23 (dd, J = 9.2, 2.4 Hz, 1H), 3.96 (s, 3H). 13C
NMR (100 MHz, CDCl3) δ 184.3, 163.7, 160.1, 148.5, 139.3, 138.1, 134.8 (q, J = 32.2 Hz), 131.4,
126.6, 125.4 (q, J = 3.7 Hz), 123.7 (q, J = 274.2 Hz), 118.0, 103.4, 55.9. IR (neat) ν 3005, 2989, 1646,
1538, 1327, 1275, 1169, 1118, 897, 858, 763, 705 cmꢀ1. HRMS (ESI, m/z): calcd. for C16H11F3NO2S
(M+H)+: 338.0457, found: 338.0443.
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(6ꢀBromobenzo[d]thiazolꢀ2ꢀyl)(4ꢀ(trifluoromethyl)phenyl)methanone (5g) : yellow solid, m.p.: 104ꢀ105
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oC, 41.0 mg, yield: 60%. H NMR (400 MHz, CDCl3) δ 8.68 (d, J = 8.0 Hz, 2H), 8.27 (d, J = 1.6 Hz,
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1H), 7.98 (d, J = 8.8 Hz, 1H), 7.86 (d, J = 8.4 Hz, 2H), 7.58 (dd, J = 8.4, 2.0 Hz, 1H). C NMR (100
MHz, CDCl3) δ 184.1, 168.0, 154.5, 137.5, 135.3, 135.1 (q, J = 32.8 Hz), 133.3, 131.6, 128.7, 125.5 (q,
J = 3.7 Hz), 125.4, 123.1, 123.6 (q, J = 271.0 Hz). IR (neat) ν = 3005, 2989, 1650, 1583, 1325, 1275,
1170, 1129, 892, 839, 765, 704 cmꢀ1. HRMS (ESI, m/z): calcd. for C15H8ClF3NOS (M+H)+: 341.9962,
found: 341.9948.
(6ꢀMethoxybenzo[d]thiazolꢀ2ꢀyl)(4ꢀ(trifluoromethyl)phenyl)methanone (5h) : yellow solid, m.p.: 97ꢀ99
oC, 54.8 mg, yield: 71%. 1H NMR (400 MHz, CDCl3) δ 8.68 (d, J = 8.0 Hz, 2H), 8.21 (s, 1H), 8.12 (d, J
= 8.8 Hz, 1H), 7.85 (d, J = 8.0 Hz, 2H), 7.73 (dd, J = 8.8, 1.2 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ
184. 1, 166.7, 152.6, 138.6, 137.5, 135.0 (q, J = 32.7 Hz), 131.5, 131.0, 126.9, 125.5 (q, J = 3.7 Hz),
123.6 (q, J = 272.3 Hz), 124.9, 122.5. IR (neat) ν 3005, 2989, 1643, 1495, 1325, 1275, 1174, 1110, 865,
830, 765, 704 cmꢀ1. HRMS (ESI, m/z): calcd. for C15H8BrF3NOS (M+H)+: 385.9457, found: 385.9440.
(4,5ꢀDimethylthiazolꢀ2ꢀyl)(4ꢀ(trifluoromethyl)phenyl)methanone (5i) : yellow oil, 33.6 mg, yield: 59%.
1H NMR (400 MHz, CDCl3) δ 8.55 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 8.4 Hz, 2H), 2.51 (s, 3H), 2.48 (s,
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3H). C NMR (100 MHz, CDCl3) δ 182.9, 161.7, 152.2, 138.4, 136.9, 134.3 (q, J = 32.7 Hz), 131.3,
125.2 (q, J = 3.7 Hz), 123.7 (q, J = 271.0 Hz), 15.1, 12.0. IR (neat) ν3002, 2913, 1623, 1564, 1324,
1260, 1171, 1115, 918, 806, 750, 702 cmꢀ1.HRMS (ESI, m/z): calcd. for C13H10F3NOS (M+H)+:
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286.0508, found: 286.0513.
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2,2,6,6ꢀTetramethylpiperidinꢀ1ꢀyl benzoate (6) : white soild, 10.9 mg, yield: 7 % (3eq.TEMPO for 1h).
1H NMR (400 MHz, CDCl3) δ 8.16 – 8.04 (m, 2H), 7.60 (t, J = 7.6 Hz, 1H), 7.49 (t, J = 7.6 Hz, 2H),
1.89 – 1.44 (m, 6H), 1.30 (s, 6H), 1.15 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 166.4, 132.8, 129.8,
129.6, 128.5, 60.4, 39.1, 32.0, 20.9, 17.0. HRMS (ESI, m/z): calcd. for C16H24NO2 (M+H)+: 262.1802,
found: 262.1788.
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ASSOCIATED CONTENT
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Supporting Information. H and 13C NMR spectra of the desired product. Deuterium labeling studies.
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail: haosun@nju.edu.cn,
*Eꢀmail: guigenli@nju.edu.cn,
*Eꢀmail: hongjianlu@nju.edu.cn
Author contributions
§ These individuals contributed equally to this research.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
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We gratefully acknowledge the financial support from the National Natural Science Foundation of
China (21332005, 21372115, 21472085), the Ph.D. Programs Foundation of the Ministry of Education
of China (Grant 20130091120046), the Natural Science Foundation of Jiangsu Province (BK2012012,
BK20131267), Qing Lan Project, and the Deanship of Scientific Research at King Saud University for
funding the work through the research group project No. RGP VPPꢀ207.
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