Synthesis of heteroaryl ketones via tandem reaction of 1,1-dibromoethenes

Article abstract of DOI:10.1016/j.tet.2011.05.111

A novel method for the synthesis of heteroaryl ketones through one-pot tandem reaction of 1,1-dibromoethenes with 2-amino(thio)phenols promoted by TBAF·3H₂O and RuCl₃(5%)/air was developed. This novel method includes several reactions in one-pot and utilizes economical yet efficient reagents to generate synthetically and biologically interesting heteroaryl ketones under mild conditions with good efficiency.

Full text of DOI:10.1016/j.tet.2011.05.111

Tetrahedron 67 (2011) 6369e6374
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of heteroaryl ketones via tandem reaction of 1,1-dibromoethenes
*
Xuesen Fan , Yan He, Xinying Zhang, Shenghai Guo, Yangyang Wang
School of Chemistry and Environmental Science, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, Henan Key Laboratory for Environmental Pollution
Control, Xinxiang, Henan 453007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2011
Received in revised form 23 May 2011
Accepted 27 May 2011
Available online 17 June 2011
A novel method for the synthesis of heteroaryl ketones through one-pot tandem reaction of 1,1-
dibromoethenes with 2-amino(thio)phenols promoted by TBAF$3H2O and RuCl3(5%)/air was de-
veloped. This novel method includes several reactions in one-pot and utilizes economical yet efficient
reagents to generate synthetically and biologically interesting heteroaryl ketones under mild conditions
with good efficiency.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Heteroaryl ketone
1,1-Dibromoethene
Tandem reaction
TBAF$3H2O
1. Introduction
benzoxazole through coupling of 1,1-dibromoethene with 2-
aminophenol under the promotion of 1,4-diazabicyclo[2.2.2]-oc-
Heteroaryl ketones are versatile synthetic building blocks and
valuable structural scaffolds of many therapeutic agents.1e4
Therefore, their preparation is a frequently encountered mission
for both organic and medicinal chemists. For this purpose, various
methods, such as FriedeleCrafts acylation of heteroaromatic
rings,5,6 condensation of metalated heterocycles with nitriles,7,8
oxidation of 1-heteroaromatic-1-alkanols,9,10 condensation of acyl
chloride with organometallics,11 palladium-catalyzed coupling of
aryl halides with organometallic agents,12e15 CeH activation of
heteroarenes,16 and condensation of 2-aminothiophenol with 2-
oxo-2-phenyl acetaldehyde,17 have been developed. While the
aforesaid methods are generally efficient and reliable, they often
suffer from expensive catalysts or specially made starting materials.
In addition, the use of highly reactive organometallic precursors
usually precludes the presence of labile functional groups on the
substrates and entails delicate reaction conditions. In this report,
we present a novel method for the synthesis of heteroaryl ketones
through one-pot tandem reaction of the readily available 1,1-
dibromoethenes and 2-amino(thio)phenols under mild conditions.
Recently, 1,1-dibromoethenes are emerging as powerful and
versatile synthetic intermediates. In addition to being widely used
in various cross coupling reactions,18e21 they are also finding in-
creasing applications in the synthesis of heterocycles.22e31 In this
regard, Shen et al. recently reported a preparation of 2-benzyl
tane (DABCO).32 During the formation of 2-benzyl benzoxazole, 1-
(2-bromoethynyl) benzene is generated through DABCO promoted
dehydrobromination of 1-(2,2-dibromovinyl) benzene and acts as
a crucial intermediate (Scheme 1).
NH2
N
OH
Br
DABCO
NMP
Bn
Ph
Br
Ph
100 oC, 10 h
O
Br
62%
Scheme 1. Synthesis of benzoxazole from 1,1-dibromoethene with DABCO.
It is then noticed that the above process gave 2-benzyl ben-
zoxazole only in moderate yield over long reaction period. The low
efficiency is reasonably attributed to the relatively poor capability
of DABCO in promoting the dehydrobromination of 1,1-
dibromoethene. We reasoned that this situation should be im-
proved with a more powerful dehydrobromination agent. In this
aspect, Mori et al. recently disclosed that tetra-n-butylammonium
fluoride (TBAF$3H2O) was a mild yet efficient base for the elimi-
nation of dibromoalkenes and gave the corresponding alkynyl
bromides in excellent yields.33
2. Results and discussion
The above-mentioned results prompted us to check the possi-
bility of developing an improved synthesis of 2-benzyl benzoxazole
* Corresponding author. Tel.: þ86 373 3329261; fax: þ86 373 3326336; e-mail
address: xuesen.fan@henannu.edu.cn (X. Fan).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.111

6370
X. Fan et al. / Tetrahedron 67 (2011) 6369e6374
Table 2
(3) from 1,1-dibromoethenes (1) and 2-aminophenols (2) with the
promotion of TBAF$3H2O (Table 1). To our delight, in the presence
of TBAF$3H2O, the yield and reaction time for the synthesis of 3
were considerably improved as compared to that promoted by
DABCO (Table 1, entries 1e5). In addition to 2-aminophenols, this
reaction was also successfully extended for the first time to 2-
aminothiophenols (4) and afforded the corresponding 2-
benzylbenzothiazoles (5) in good to excellent yields (Table 1, en-
tries 6e10).
Optimization for the synthesis of 6aa
O
1) Base
2) RuCl3(cat.)/oxidant
3) Solvent
NH2
OH
Br
N
+
O
Br
Br
Br
6a
1a
2a
Entry
Base
Oxidant (equiv)
Solvent
T (ꢀC)
t (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Ac
A
A
TBHP (3)
TBHP (1)
TBHP (0.5)
Air
DMF
DMF
DMF
DMF
60
60
60
60
60
60
60
60
60
80
5
6
59
57
58
53
Trace
d
Table 1
10
10
10
10
10
15
20
10
10
12
Preparation of 2-benzyl benzoxazoles (3) or benzothiazoles (5) promoted by
TBAF$3H2Oa
A
A
N2
DMF
X
XH
Br
+
TBAF 3H2O
DMF, 60 oC, 3 h
A
Air
[bmim]PF6
[bmim]BF4
DMF
DMF
DMF
R1
R2
R1
N
A
Air
Air
Air
Air
d
Br
NH2
Bc
Cc
A
41
37
61
60
61
R2
2, X = O
3, X = O
5, X = S
1
9
10
11
12
4, X = S
A
Air
DMF
DMF
100
80
A
Air
Entry
R1 in 1
R2 in 2 or 4, X
Product
Yieldb (%)
The significance of italics of entry 10 of table 2 is that with these conditions opti-
mum yield of 6a was obtained.
1
2
3
4
5
6
7
8
9
4-Br
3-NO2
3-NO2
4-Br
3-Br
H
H
H
H
4-F
H, O
H, O
4-Me, O
4-Cl, O
H, O
3a
3b
3c
3d
3e
5a
5b
5c
5d
5e
80 (60)c
78 (53)c
75 (52)c
82 (58)c
77 (55)c
76
78
76
65
81
a
Compounds 1a and 2a (1 mmol), DMF (5 mL) or IL (2 mL), RuCl3 (0.05 mmol).
Isolated yields.
A for TBAF$3H2O, B for TBAF in THF, C for DABCO.
b
c
H, S
much longer reaction period to complete and the yields of 6a were
lower (Table 2, entries 8 and 9) than those with TBAF$3H2O. After
the temperature and reaction time were optimized, the yield of 6a
was improved to 61% (Table 2, entry 10).
Based on the above observations, a plausible pathway for the
formation of 6 was depicted in Scheme 2. While the procedure is
thought to follow the route as envisioned above, air’s role as
a stoichiometric oxidant to oxidize the in situ formed Ru(II) back to
Ru(III) is worth to be noted. The use of a cost-free oxidant (air)
makes this tandem procedure toward heteroaryl ketones re-
markably sustainable.
4-Cl, S
5-Me, S
3-Cl, S
H, S
10
a
Compounds 1 and 2 or 4 (1 mmol), TBAF$3H2O (4 mmol), DMF (5 mL), 60 ꢀC, 3 h.
Isolated yields.
b
c
Yields with literature procedure.31
Inspired by the above results and taking into account of the high
efficiency and excellent chemoselectivity exhibited by ruth-
enium(III) reagents in a plethora of oxidative transformations,34e37
we envisioned a novel route toward 2-benzoxazyl ketone from
dibromoethene (1) and 2-aminophenol (2) through a one-pot
tandem process including: (i) TBAF$3H2O promoted dehydro-
bromination of 1 to give alkynyl bromide, (ii) condensation and
cyclization of alkynyl bromide with 2 to give 2-benzyl benzoxazole
(3), and (iii) benzylic oxidation of the in situ formed 2-benzyl
benzoxazole (3) by Ru(III) to afford 2-benzoxazyl ketone (6).
To study the feasibility of our envisioned route, 1-bromo-4-(2,2-
dibromovinyl)benzene (1a) and 2-aminophenol (2a) were used as
model substrates and various solvents, bases, oxidants, and tem-
peratures were screened (shown in Table 2). Initially,1a was treated
with TBAF$3H2O in DMF. Upon complete consumption of 1a as
monitored by TLC (60 ꢀC, 1 h), 2a was added together with RuCl3
(5% mol) and 3 equiv of tert-butyl hydroperoxide (TBHP), which
was used as a stoichiometric oxidant. The subsequent reaction
underwent smoothly to give 2-(4-bromobenzoyl)benzoxazole (6a)
in a yield of 59% (Table 2, entry 1). In further screening, it was found
that similar yields of 6a could be obtained with fewer amounts of
TBHP, or even without TBHP (Table 2, entries 2e4). In the absence of
TBHP, air is believed to act as a stoichiometric oxidant. Air’s role was
further confirmed by an independent experiment run under ni-
trogen, from which only trace amount of 6a was formed as in-
dicated by TLC analysis (Table 2, entry 5).
NH2
O
OH
Br
2a
TBAF
Ar
Ar
air
Br
or
Br
O
Ar
N
H
Ar
1
Ru(II)
Ru(III)
O
H
N
N
N
Ar
Ar
Ar
O
O
6
3
Scheme 2. Tandem pathway for the formation of 6.
Next, the scope of this reaction with various dibromoethenes (1)
and 2-aminophenols (2) was evaluated under the optimized con-
ditions determined above (as shown in Table 3). In general, all the
reactions were efficient and 2-benzoxazyl aryl ketone derivatives
(6) were obtained in moderate to good yields. For 2-aminophenols
with either electron-withdrawing or electrondonating groups, the
tandem process proceeded efficiently. In addition, various func-
tional groups, such as nitro, cyano, and halides, on the aromatic ring
of dibromoethenes were tolerated under the reaction conditions.
The above process was then extended to the synthesis of 2-
benzothiazyl ketones (7) by using 2-aminothiophenols (4) to re-
act with dibromoethenes. With 2-aminothiophenol as a fixed
substrate, we carried out the reaction with various dibromoethenes
(Table 4, entries 1e4, 7, and 13). It turned out that all the aromatic
substituted dibromoethenes reacted with 2-aminothiophenol
smoothly and 2-benzothiazyl ketones (7) were isolated in good
yields. It was also noted that the electronic effect of the substituents
on the aromatic rings affected the yields of 7 and the presence of
electron-withdrawing group gave better yields than that of
Then, the effect of different solvents was studied. In this regard,
DMF was replaced by ionic liquids. It was expected that the basic
property of fluoride ion in TBAF$3H2O would increase in these
highly polar solvents and thus accelerate the dehydrobromination
process. Unfortunately, with [bmim]PF6 or [bmim]BF4 as the re-
action medium, the dehydrobromination process was actually re-
tarded and no alkynyl bromide was formed from 1a as indicated by
TLC (Table 2, entries 6 and 7). As for the bases, reactions with the
promotion of either TBAF in THF (1 M solution) or DABCO required

X. Fan et al. / Tetrahedron 67 (2011) 6369e6374
6371
acetaldehyde under the promotion of FeCl3$6H2O.38 To our
knowledge, phenyl acetaldehyde derivatives are scarcely commer-
cially available. They are usually obtained through the Darzens
Table 3
Scope of the reaction leading to 2-benzoxazyl ketones (6)a
O
1. TBAF 3H2O
2. RuCl3 (5%)/air
DMF, 80oC, 10h
NH2
OH
Br
N
condensation between benzaldehydes and
a
-halo ester to give
a,b-
R2
R1
R1
+
O
Br
epoxy ester and subsequent saponification of
a,b-epoxy ester fol-
R2
lowed by decarboxylation. This sequence of reactions is hard to deal
with and the yields of phenyl acetaldehydes are low. On the other
hand, 1,1-dibromoetheres used in this protocol can be easily pre-
pared from aldehydes and tetrabromomethane with excellent
yields. Therefore, it is reasonable to state that the present study has
not only developed new chemistry of 1,2-dibromoethene, but also
provided a more practical and sustainable synthetic pathway to-
ward heteroaryl ketones.
1
6
2
Entry
R1 in 1
R2 in 2
Product
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
4-Br
4-Br
4-Br
4-NO2
4-NO2
3-Br
H
6a
6b
6c
6d
6e
6f
61
58
54
63
62
61
58
57
53
65
4-Me
4-Cl
H
4-Me
H
3-Br
3-Br
3-Br
4-CN
4-Me
H
4-Cl
H
6g
6h
6i
3. Conclusion
6j
In summary, TBAF$3H2O and RuCl3(5%)/air promoted one-pot
tandem reaction of 1,1-dibromoethenes with 2-amino(thio)phe-
nols was found to be a promising approach toward the synthetically
and pharmaceutically interesting heteroaryl ketones. To the best of
our knowledge, this is the first report in which heteroaryl ketones
are synthesized directly from 1,1-dibromoethenes. This novel
method includes several reactions in one-pot and utilizes eco-
nomical yet efficient reagents to generate molecular complexity
under mild conditions. It is also worth to be noted that it is com-
patible with various functional groups, which are not tolerated
with literature methods utilizing highly reactive organometallic
reagents. With the above-mentioned merits, this novel protocol
should be valuable for the construction of these kinds of hetero-
cycles with biological and medicinal interests. Biological evaluation
of the heteroaryl ketones obtained in this work is currently
underway.
a
Compounds 1 and 2 (1 mmol), TBAF$3H2O (4 mmol), RuCl3 (0.05 mmol), DMF
(5 mL), 80 ꢀC, 10 h.
b
Isolated yields.
Table 4
Scope of the reaction leading to 2-benzothiazyl ketones (7)a
O
1. TBAF 3H2O
2. RuCl3 (5%)/air
DMF, 80oC, 10h
NH2
SH
Br
N
R2
R1
R1
+
S
Br
R2
7
1
4
Entry
R1 in 1
R2 in 4
Product
Yieldb (%)
1
2
3
4
5
6
7
8
4-Br
4-Me
4-OMe
4-NO2
H
4-Me
H
4-F
H
H
H
H
4-Cl
4-Cl
H
4-Cl
3-Cl
3-Cl
5-Me
5-Me
H
7a
7b
7c
7d
7e
7f
7g
7h
7i
72
65
56
75
63
64
65
68
57
56
58
53
59
4. Experimental section
4.1. General information
9
H
10
11
12
13
4-Me
4-Me
4-OMe
3-Me
7j
7k
7l
Dibromoethenes were prepared through reaction of aldehydes
with carbon tetrabromide based on literature procedure.39 Other
reagents and solvents were purchased from commercial suppliers
and used without further purification. The 1H and 13C NMR spectra
were recorded at 400 MHz and 100 MHz, respectively. Chemical
shifts were reported in parts per million from tetramethylsilane
(TMS) as internal standard in CDCl3 solutions. Multiplicity was in-
dicated as follows: s (singlet); d (doublet); t (triplet); m (multiplet);
dd (doublet of doublets); td (triplet of doublets); br s (broad sin-
glet), etc. and coupling constants are given in hertz. The conversion
of starting materials was monitored by thin layer chromatography
(TLC) using silica gel plates (silica gel 60 F254 0.25 mm) and com-
ponents were visualized by observation under UV light (254 and
365 nm).
7m
a
Compounds 1 and 4 (1 mmol), TBAF$3H2O (4 mmol), RuCl3 (0.05 mmol), DMF
(5 mL), 80 ꢀC, 10 h.
b
Isolated yields.
electrondonating group. Additionally, nitro, methyl, methoxy, and
halide groups are well compatible with the reaction conditions. On
the other hand, aliphatic group-substituted dibromoethenes gave
2-substituted benzothiazoles (8a and 8b) without benzylic oxida-
tion (Scheme 3), indicating that under these mild conditions the
benzylic oxidation was highly chemoselective and only occurred
with methylene groups immediately between two aromatic
moieties.
4.2. Typical procedure for the preparation of 2-(4-bromo
benzyl)benzoxazole (3a)
1. TBAF 3H2O
2. RuCl3 (5%)/air
DMF, 80oC, 10h
N
S
NH2
SH
R
Br
+
To
a
solution of 1-(2,2-dibromovinyl)-4-bromobenzene
R
Br
(1 mmol) in DMF (5 mL) was added TBAF$3H2O (4 mmol). The
mixture was stirred at 60 ꢀC for 1 h. 2-Aminophenol (1 mmol) was
added and the mixture was stirred at 60 ꢀC for 2 h. Upon comple-
tion, the reaction mixture was cooled to room temperature and
diluted with diethyl ether (60 mL). The organic phase was washed
with brine, dried, filtered, and concentrated under vacuum. The
residue was purified by column chromatography on silica gel with
ethyl acetate/hexane (3e5%) to give 3a. Other 2-benzyl benzox-
azoles (3bee) and 2-benzylbenzothiazoles (5aee) were prepared
in a similar manner.
8a: R = Me, 52%
8b: R = Ph, 61%
Scheme 3. Reaction of aliphatic dibromoethenes with 2-aminothiophenol.
It is noted herein that this is the first report that heteroaryl
ketones were prepared directly from tandem reaction of o-ami-
nothiophenol or o-aminophenol with 1,1-dibromoethenes. Re-
cently, we have reported that 2-benzoyl benzothiazoles could be
obtained from the condensation of o-aminothiophenol and phenyl

6372
X. Fan et al. / Tetrahedron 67 (2011) 6369e6374
4.2.1. 2-(4-Bromobenzyl)benzoxazole (3a)32. Syrup; 1H NMR
(400 MHz, CDCl3) : 4.22 (s, 2H), 7.24e7.27 (m, 2H), 7.29e7.31 (m,
2H), 7.45e7.48 (m, 3H), 7.67e7.69 (m, 1H). 13C NMR (100 MHz,
124.9, 126.0, 126.1, 130.6, 130.7, 135.5, 153.2, 170.8. MS: m/z 244
d
(MH)þ.
CDCl3)
d
: 34.6, 110.4, 119.8, 121.4, 124.3, 124.9, 130.7, 131.9, 133.7,
4.3. Typical procedure for the preparation of 2-(4-bromo
benzoyl) benzoxazole (6a)
141.2, 151.0, 164.5. MS: m/z 288 (MH)þ.
4.2.2. 2-(3-Nitrobenzyl)benzoxazole
(3b). Syrup;
1H
NMR
To
a
solution of 1-bromo-4-(2,2-dibromovinyl)benzene
(400 MHz, CDCl3)
d
: 4.38 (s, 2H), 7.31e7.33 (m, 2H), 7.47e7.49 (m,
(1 mmol) in DMF (5 mL) was added TBAF$3H2O (4 mmol). The
mixture was stirred at 80 ꢀC for 1 h. 2-Aminophenol (1 mmol) and
RuCl3 (0.05 mmol) were then added. The mixture was stirred at
80 ꢀC for 9 h. Upon completion, the reaction mixture was cooled to
room temperature and diluted with diethyl ether (60 mL). The or-
ganic phase was washed with brine, dried, filtered, and concen-
trated under vacuum. The residue was purified by column
chromatography on silica gel with ethyl acetate/hexane (3e5%) to
give 6a. Other 2-benzoylbenzoxazoles (6bej) and 2-benzoyl ben-
zothiazoles (7aem) were prepared in a similar manner.
1H), 7.53 (t, J¼8.0 Hz, 1H), 7.68e7.70 (m, 1H), 7.73 (d, J¼8.0 Hz, 1H),
8.16 (dd, J1¼8.0 Hz, J2¼1.2 Hz, 1H), 8.28 (s, 1H). 13C NMR (100 MHz,
CDCl3) d: 34.8, 110.5, 120.0, 122.5, 124.1, 124.5, 125.1, 129.8, 135.2,
136.6, 141.1, 148.4, 151.0, 163.6. MS: m/z 255 (MH)þ. HRMS (FAB)
calcd for C14H11N2O3: 255.077 [MþH], found: 255.0775.
4.2.3. 2-(3-Nitrobenzyl)-5-methylbenzoxazole (3c). Syrup; 1H NMR
(400 MHz, CDCl3)
d
: 2.45 (s, 3H), 4.36 (s, 2H), 7.12 (d, J¼8.0 Hz, 1H),
7.34 (d, J¼8.0 Hz, 1H), 7.47 (s, 1H), 7.52 (t, J¼8.0 Hz, 1H), 7.71 (d,
J¼8.0 Hz,1H), 8.15 (d, J¼8.0 Hz,1H), 8.27 (s,1H). 13C NMR (100 MHz,
CDCl3)
d
: 21.4, 34.8, 109.9, 119.9, 122.5, 124.1, 126.2, 129.7, 134.3,
4.3.1. 2-(4-Bromobenzoyl)benzoxazole (6a)16. Colorless solid, mp
135.2, 136.7, 141.3, 148.4, 149.2, 163.6. MS: m/z 269 (MH)þ. HRMS
(FAB) calcd for C15H13N2O3: 269.0927 [MþH], found: 269.0929.
140e142 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.36e7.38 (m, 2H),
7.57e7.59 (m, 1H), 7.67 (d, J¼8.8 Hz, 2H), 7.76e7.78 (m, 1H), 8.12 (d,
J¼8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d: 110.6, 120.1, 124.7,
125.4, 126.1, 126.2, 129.0, 132.2, 142.0, 150.7, 162.1, 177.5. MS: m/z
4.2.4. 2-(4-Bromobenzyl)-5-chlorobenzoxazole (3d). Pale yellow
solid, mp 76e77 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 4.21 (s, 2H),
302 (MH)þ.
7.23e7.29 (m, 3H), 7.38 (d, J¼8.8 Hz, 1H), 7.47e7.50 (m, 2H), 7.66 (d,
J¼2.0 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d: 34.6, 111.2, 119.9, 121.5,
4.3.2. 2-(4-Bromobenzoyl)-5-methylbenzoxazole (6b). Yellow solid,
125.2, 129.8, 130.7, 132.0, 133.2, 142.3, 149.6, 165.9. MS: m/z 322
(MH)þ. HRMS (FAB) calcd for C14H10BrClNO: 321.9635 [MþH],
found: 321.9625.
mp 143e145 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 2.49 (s, 3H), 7.17 (d,
J¼8.4 Hz, 1H), 7.44 (d, J¼8.4 Hz, 1H), 7.54 (s, 1H), 7.66 (d, J¼8.4 Hz,
2H), 8.10 (d, J¼8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d: 21.5, 110.0,
119.9, 126.0, 126.2, 126.5, 128.9, 132.2, 134.6, 142.1, 148.9, 162.2,
178.9. MS: m/z 316 (MH)þ. HRMS (FAB) calcd for C15H11BrNO2:
315.9974 [MþH], found: 315.9981.
4.2.5. 2-(3-Bromobenzyl)benzoxazole
(400 MHz, CDCl3) : 4.24 (s, 2H), 7.22e7.24 (m, 1H), 7.31e7.33 (m,
3H), 7.42 (d, J¼8.0 Hz, 1H), 7.47e7.49 (m, 1H), 7.54 (s, 1H), 7.69e7.71
(m, 1H). 13C NMR (100 MHz, CDCl3)
: 34.8, 110.5, 119.9, 122.8, 124.3,
(3e). Syrup;
1H
NMR
d
d
4.3.3. 2-(4-Bromobenzoyl)-5-chlorobenzoxazole (6c). Pale yellow
124.9, 127.7, 130.3, 130.5, 132.0, 136.9, 141.2, 151.0, 164.3. MS: m/z
288 (MH)þ. HRMS (FAB) calcd for C14H11BrNO: 288.0025 [MþH],
found: 288.0019.
crystals, mp 130e132 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.33e7.35
(m, 1H), 7.48e7.50 (m, 1H), 7.67 (d, J¼8.0 Hz, 2H), 7.74 (s, 1H), 8.09
(d, J¼8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d: 111.3, 119.8, 120.0,
125.6, 126.7, 129.1, 130.2, 132.3, 143.1, 149.3, 163.4, 182.0. MS: m/z
336 (MH)þ. HRMS (FAB) calcd for C14H8BrClNO2: 335.9428 [MþH],
found: 335.9426.
4.2.6. 2-Benzylbenzothiazole (5a)38. Syrup; 1H NMR (400 MHz,
CDCl3)
d
: 4.45 (s, 2H), 7.29e7.39 (m, 6H), 7.46 (td, J1¼7.6 Hz,
J2¼0.8 Hz, 1H), 7.79 (d, J¼8.0 Hz, 1H), 8.01 (d, J¼8.0 Hz, 1H). 13C
NMR (100 MHz, CDCl3)
d: 40.6, 121.5, 122.8, 124.8, 125.9, 127.3,
4.3.4. 2-(4-Nitrobenzoyl)benzoxazole (6d). Brown solid, mp
128.8, 129.1, 135.7, 137.2, 153.2, 171.2. MS: m/z 226 (MH)þ.
139e141 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.52 (t, J¼8.4 Hz,1H), 7.62
(t, J¼8.4 Hz, 1H), 7.75 (d, J¼8.4 Hz, 1H), 7.98 (d, J¼8.4 Hz, 1H), 8.42
4.2.7. 2-Benzyl-5-chlorobenzothiazole (5b)38. Pale yellow solid, mp
(d, J¼8.8 Hz, 2H), 8.78 (d, J¼8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3)
74e77 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 4.43 (s, 2H), 7.29e7.37 (m,
d: 112.0, 122.6, 123.6, 126.2, 129.2, 132.1, 139.5, 140.6, 150.5, 150.8,
6H), 7.67 (d, J¼8.4 Hz, 1H), 7.98 (d, J¼2.0 Hz, 1H). 13C NMR
156.4, 178.8. MS: m/z 269 (MH)þ. HRMS (FAB) calcd for C14H9N2O4:
269.0563 [MþH], found: 269.0568.
(100 MHz, CDCl3) d: 40.6, 122.2, 122.6, 125.3, 127.5, 128.9, 129.2,
131.9, 133.9, 136.8, 154.1, 173.3. MS: m/z 260 (MH)þ.
4.3.5. 5-Methyl-2-(4-nitrobenzoyl)benzoxazole (6e). Colorless solid,
4.2.8. 2-Benzyl-6-methylbenzothiazole (5c)38. Syrup; 1H NMR
mp 188e190 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 2.51 (s, 3H),
(400 MHz, CDCl3) d: 2.45 (s, 3H), 4.41 (s, 2H), 7.24e7.36 (m, 6H),
7.24e7.25 (m, 1H), 7.50 (d, J¼8.0 Hz, 1H), 7.60 (s, 1H), 8.36e8.43 (m,
7.57 (s,1H), 7.86 (d, J¼8.4 Hz,1H). 13C NMR (100 MHz, CDCl3)
d
: 21.4,
4H). 13C NMR (100 MHz, CDCl3)
d: 21.5, 110.3, 120.4, 124.2, 127.5,
40.5, 121.2, 122.2, 127.2, 127.5, 128.8, 129.1, 134.8, 135.8, 137.3, 151.3,
128.3, 131.0, 135.1, 142.1, 149.2, 160.7, 184.5. MS: m/z 283 (MH)þ.
HRMS (FAB) calcd for C15H11N2O4: 283.072 [MþH], found: 283.0711.
170.0. MS: m/z 240 (MH)þ.
4.2.9. 2-Benzyl-4-chlorobenzothiazole (5d)38. Syrup; 1H NMR
4.3.6. 2-(3-Nitrobenzoyl)benzoxazole
(6f). Pink
solid,
mp
(400 MHz, CDCl3)
d
: 4.50 (s, 2H), 7.24e7.37 (m, 6H), 7.47 (d,
180e182 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.41e7.44 (m, 2H),
J¼7.6 Hz, 1H), 7.66 (d, J¼8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d:
7.63e7.66 (m, 1H), 7.76 (t, J¼8.0 Hz, 1H), 7.81e7.83 (m, 1H),
8.38e8.41 (m, 1H), 8.58e8.61 (m, 1H), 9.11 (s, 1H). 13C NMR
40.8, 120.1, 125.3, 126.2, 127.4, 127.5, 128.9, 129.3, 136.9, 137.1, 150.2,
173.0. MS: m/z 260 (MH)þ.
(100 MHz, CDCl3) d: 107.0, 110.9, 120.5, 122.5, 125.1, 125.8, 126.1,
130.1, 133.0, 141.8, 150.8, 160.5, 184.7. MS: m/z 269 (MH)þ. HRMS
(FAB) calcd for C14H9N2O4: 269.0563 [MþH], found: 269.0561.
4.2.10. 2-(4-Fluorobenzyl)benzothiazole (5e)38. Syrup; 1H NMR
(400 MHz, CDCl3)
3H), 7.46 (t, J¼7.6 Hz, 1H), 7.80 (d, J¼7.6 Hz, 1H), 7.99 (d, J¼8.4 Hz,
1H). 13C NMR (100 MHz, CDCl3)
: 39.7, 115.6, 115.8, 121.5, 122.8,
d: 4.41 (s, 2H), 7.04 (t, J¼8.4 Hz,1H), 7.31e7.36 (m,
4.3.7. 5-Methyl-2-(3-nitrobenzoyl)benzoxazole (6g). Pink solid, mp
d
168e170 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 2.51 (s, 3H), 7.22 (d,

X. Fan et al. / Tetrahedron 67 (2011) 6369e6374
6373
J¼8.4 Hz,1H), 7.49 (d, J¼8.4 Hz,1H), 7.58 (s,1H), 7.72(t, J¼8.0 Hz,1H),
8.35e8.38 (m,1H), 8.55e8.57 (m,1H), 9.06 (t, J¼1.6 Hz,1H).13C NMR
(d, J¼8.4 Hz, 2H), 7.51 (dd, J1¼8.4 Hz, J2¼1.6 Hz, 1H), 7.93 (d,
J¼8.4 Hz, 1H), 8.22 (d, J¼1.6 Hz, 1H), 8.46 (d, J¼8.4 Hz, 2H). 13C NMR
(100 MHz, CDCl3)
d
: 21.5,110.2,120.3,122.4,125.6,127.2,129.1,130.1,
(100 MHz, CDCl3) d: 21.9, 122.9, 125.1, 128.1, 129.3, 131.4, 132.0,
132.9, 135.0, 141.9, 148.6, 149.1, 160.6, 180.6. MS: m/z 283 (MH)þ.
HRMS (FAB) calcd for C15H11N2O4: 283.072 [MþH], found: 283.0729.
132.9, 135.1, 145.3, 154.6, 169.2, 184.4. MS: m/z 288 (MH)þ.
4.3.17. 2-Benzoylbenzothiazole (7g)40. Pale yellow solid, mp
4.3.8. 2-(3-Bromobenzoyl)benzoxazole (6h). Yellow solid, mp
97e98 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.55e7.63 (m, 4H), 7.69 (t,
107e109 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.44e7.52 (m, 2H), 7.58 (t,
J¼7.2 Hz, 1H), 8.04 (d, J¼7.2 Hz, 1H), 8.27 (d, J¼7.2 Hz, 1H), 8.58 (d,
J¼8.0 Hz, 1H), 7.72 (d, J¼8.0 Hz, 1H), 7.81 (d, J¼8.0 Hz, 1H), 7.96 (d,
J¼7.2 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d: 122.2, 125.7, 126.9,
127.6, 128.5, 131.3, 133.9, 134.9, 137.0, 153.9, 167.1, 185.4. MS: m/z
J¼8.0 Hz,1H), 8.55 (d, J¼8.0 Hz,1H), 8.71 (s,1H). 13C NMR (100 MHz,
CDCl3)
d
: 111.9, 122.5, 122.8, 125.9, 128.8, 129.7, 130.2, 133.7, 136.6,
240 (MH)þ.
137.1, 140.6, 150.4, 156.6, 179.0. MS: m/z 302 (MH)þ. HRMS (FAB)
calcd for C14H9BrNO2: 301.9817 [MþH], found: 301.9811.
4.3.18. 5-Chloro-2-(4-fluorobenzoyl)benzothiazole
solid, mp 149e151 ꢀC; 1H NMR (400 MHz, CDCl3)
(7h)38. Pink
: 7.23e7.26 (m,
d
4.3.9. 2-(3-Bromobenzoyl)-5-chlorobenzoxazole (6i). Colorless crys-
2H), 7.53 (dd, J1¼8.8 Hz, J2¼2.0 Hz, 1H), 7.94 (d, J¼8.8 Hz, 1H), 8.23
tals, mp 134e136 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.46 (t, J¼8.0 Hz,
(d, J¼1.6 Hz, 1H), 8.64e8.68 (m, 2H). 13C NMR (100 MHz, CDCl3)
d:
1H), 7.54 (d, J¼8.0 Hz,1H), 7.65 (d, J¼8.4 Hz,1H), 7.82 (d, J¼8.0 Hz,1H),
115.7, 115.9, 123.0, 125.2, 128.3, 130.9, 133.1, 134.2, 134.3, 135.2,
7.95 (d, J¼2.0 Hz, 1H), 8.53 (d, J¼8.0 Hz, 1H), 8.68 (s, 1H). 13C NMR
154.5, 165.2, 168.8, 183.1. MS: m/z 292 (MH)þ.
(100 MHz, CDCl3) d: 112.7,122.2,122.9,129.2,129.6,130.2,131.5,133.7,
136.3, 137.3, 141.6, 148.9, 157.5, 178.6. MS: m/z 336 (MH)þ. HRMS (FAB)
calcd for C14H8BrClNO2: 335.9428 [MþH], found: 335.9421.
4.3.19. 2-Benzoyl-4-chlorobenzothiazole (7i)38. Pale yellow crystals,
mp 111e112 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.48 (t, J¼8.0 Hz,
1H), 7.57e7.71 (m, 4H), 7.92 (d, J¼8.0 Hz, 1H), 8.68 (d, J¼8.0 Hz,
4.3.10. 2-(4-Cyanobenzoyl)benzoxazole (6j). Colorless solid, mp
2H). 13C NMR (100 MHz, CDCl3)
d: 120.6, 127.0, 128.0, 128.5, 130.6,
184e186 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.40e7.43 (m, 2H),
131.5, 134.1, 134.6, 138.4, 151.0, 167.8, 184.5. MS: m/z 274 (MH)þ.
HRMS (FAB) calcd for C14H9ClNOS: 274.0094 [MþH], found:
274.0099.
7.61e7.64 (m,1H), 7.82e7.84 (m, 3H), 8.37 (d, J¼8.4 Hz, 2H).13C NMR
(100 MHz, CDCl3) d: 110.9,114.7,120.5,125.1,126.1,127.9,128.8,131.1,
132.7, 141.8, 150.9, 160.9, 179.6. MS: m/z 249 (MH)þ. HRMS (FAB)
calcd for C15H9N2O2: 249.0665 [MþH], found: 249.0664.
4.3.20. 4-Chloro-2-(4-methylbenzoyl)benzothiazole (7j)38. Colorless
solid, mp 119e121 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 2.48 (s, 3H),
4.3.11. 2-(4-Bromobenzoyl)benzothiazole (7a)17. Pale yellow solid,
7.38 (d, J¼8.0 Hz, 2H), 7.47 (t, J¼8.0 Hz, 1H), 7.61 (d, J¼8.0 Hz, 1H),
mp 121e123 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.56e7.62 (m, 2H),
7.91 (d, J¼8.0 Hz, 1H), 8.59 (d, J¼8.4 Hz, 2H). 13C NMR (100 MHz,
7.69e7.73 (m, 2H), 8.02e8.04 (m,1H), 8.23e8.25 (m,1H), 8.46e8.49
CDCl3) d: 21.9, 120.6, 127.0, 127.9, 129.4, 130.5, 131.7, 132.0, 138.4,
(m, 2H).13C NMR (100 MHz, CDCl3)
d
: 122.2,125.7,127.0,127.8,129.5,
145.3, 151.1, 167.5, 184.1. MS: m/z 288 (MH)þ.
131.8, 132.7, 133.6, 137.0, 153.8, 166.7, 184.3. MS: m/z 318 (MH)þ.
4.3.21. 6-Methyl-2-(4-methylbenzoyl)benzothiazole
(7k)38. Pale
: 2.47 (s,
4.3.12. 2-(4-Methylbenzoyl)benzothiazole (7b)17. Colorless crystals,
yellow solid, mp 105e107 ꢀC; 1H NMR (400 MHz, CDCl3)
d
mp 94e96 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 2.47 (s, 3H, CH3), 7.36
3H), 2.54 (s, 3H), 7.34e7.40 (m, 3H), 7.80 (s, 1H), 8.11 (d, J¼8.4 Hz,
(d, J¼8.0 Hz, 2H), 7.53e7.58 (m, 2H), 7.99 (t, J¼8.4 Hz, 1H), 8.24 (d,
1H), 8.46 (d, J¼8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d: 21.8, 121.7,
J¼8.0 Hz, 1H), 8.48 (d, J¼8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d
:
125.1, 128.7, 129.2, 131.3, 132.5, 137.2, 138.1, 144.8, 152.1, 166.4, 185.0.
21.9, 122.2, 125.7, 126.9, 127.4, 127.5, 129.3, 129.7, 131.4, 132.3, 136.9,
MS: m/z 268 (MH)þ.
145.0, 153.9, 167.4, 184.9. MS: m/z 254 (MH)þ.
4. 3. 22. 2-(4-Methoxybenzoyl)-6-methylbenzothiazole
4.3.13. 2-(4-Methoxylbenzoyl)benzothiazole (7c)40. Colorless solid,
(7l)38. Colorless solid, mp 137e139 ꢀC; 1H NMR (400 MHz, CDCl3)
d:
mp 120e122 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 3.93 (s, 3H), 7.05 (d,
2.54 (s, 3H), 3.92 (s, 3H), 7.04 (d, J¼8.8 Hz, 2H), 7.39 (d, J¼8.4 Hz,1H),
J¼8.8 Hz, 2H), 7.52e7.61 (m, 2H), 8.02 (d, J¼8.0 Hz, 1H), 8.24 (d,
7.79 (s, 1H), 8.10 (d, J¼8.4 Hz, 1H), 8.63 (d, J¼8.8 Hz, 2H). 13C NMR
J¼8.0 Hz, 1H), 8.65 (d, J¼8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d
:
(100 MHz, CDCl3) d: 21.8, 55.5, 113.8, 121.7, 125.0, 127.9, 128.6, 133.8,
55.6, 113.9, 122.1, 125.5, 126.8, 127.4, 127.7, 133.8, 136.8, 153.9, 164.4,
137.2, 138.0, 152.1, 164.2, 166.8, 183.5. MS: m/z 284 (MH)þ.
167.9, 183.5. MS: m/z 270 (MH)þ.
4.3.23. 2-(3-Methylbenzoyl)benzothiazole (7m)38. Syrup; 1H NMR
4.3.14. 2-(4-Nitrobenzoyl)benzothiazole (7d). Yellow solid, mp
(400 MHz, CDCl3)
2H), 8.03 (d, J¼8.0 Hz,1H), 8.25e8.28 (m, 2H), 8.38 (d, J¼7.6 Hz,1H).
13C NMR (100 MHz, CDCl3)
: 21.5, 122.2, 125.7, 126.9, 127.6, 128.4,
d: 2.48 (s, 3H), 7.44e7.50 (m, 2H), 7.54e7.62 (m,
171e173 ꢀC; 1H NMR (400 MHz, CDCl3)
d: 7.59e7.63 (m, 2H),
8.04e8.06 (m, 1H), 8.25e8.28 (m, 1H), 8.40 (d, J¼8.8 Hz, 2H), 8.74
d
(d, J¼8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d
: 122.3, 123.5, 124.3,
128.6, 131.4, 134.8, 134.9, 137.0, 138.3, 153.8, 167.2, 185.7. MS: m/z
125.9, 127.3, 128.3, 132.3, 137.2, 139.7, 153.8, 165.8, 183.9. MS: m/z
285 (MH)þ. HRMS (FAB) calcd for C14H9N2O3S: 285.0335 [MþH],
found: 285.0332.
254 (MH)þ.
4.3.24. 2-Propylbenzothiazole (8a). Syrup; 1H NMR (400 MHz,
CDCl3)
d
: 1.07 (t, J¼7.6 Hz, 3H), 1.90e1.96 (m, 2H), 3.11 (t, J¼7.6 Hz,
4.3.15. 2-Benzoyl-5-chlorobenzothiazole (7e)38. Colorless crystals,
2H), 7.35 (t, J¼7.6 Hz, 1H), 7.46 (t, J¼7.6 Hz, 1H), 7.85 (d, J¼8.0 Hz,
mp 133e136 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 7.52e7.59 (m, 3H),
1H), 7.98 (d, J¼8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d: 13.7, 23.1,
7.69 (t, J¼7.6 Hz, 1H), 7.95 (d, J¼7.6 Hz, 1H), 8.24 (d, J¼2.0 Hz, 1H),
36.2, 121.4, 122.5, 124.6, 125.8, 126.6, 126.8, 135.1, 153.2, 172.2. MS:
8.55 (d, J¼7.6 Hz, 2H). 13C NMR (100 MHz, CDCl3)
d
: 123.0, 125.2,
m/z 178 (MH)þ.
128.2, 128.6, 131.3, 133.0, 134.1, 134.6, 135.2, 154.6, 168.9, 185.0. MS:
m/z 274 (MH)þ.
4.3.25. 2-Phenethylbenzothiazole (8b)17. Colorless solid, mp
50e52 ꢀC; 1H NMR (400 MHz, CDCl3)
d
: 3.22 (t, J¼8.0 Hz, 2H), 3.44
4.3.16. 5-Chloro-2-(4-methylbenzoyl)benzothiazole
solid, mp 115e117 ꢀC; 1H NMR (400 MHz, CDCl3)
(7f)38. Yellow
: 2.47 (s, 3H), 7.36
(t, J¼8.0 Hz, 2H), 7.21e7.38 (m, 6H), 7.46 (t, J¼8.0 Hz, 1H), 7.83 (d,
d
J¼8.0 Hz, 1H), 7.99 (d, J¼8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3)
d:

6374
X. Fan et al. / Tetrahedron 67 (2011) 6369e6374
12. Sergeev, A.; Spannenberg, A.; Beller, M. J. Am. Chem. Soc. 2008, 130, 15549.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Nos. 20972042 and 20911140238) and Innovation Scientists
and Technicians Troop Construction Projects of Henan Province
(No. 104100510019) for financial support.
Chem.
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Supplementary data
Supplementary data associated with this article can be found in
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