A Tf2O-Promoted Synthesis of Functionalized Quinolines from Ketoximes and Alkynes

Article abstract of DOI:10.1002/adsc.201801724

A new general synthesis of quinolines was developed from ketoximes and alkynes in the presence of Tf₂O. It offered the first direct synthesis of quinolines by using the nitrilium salts generated in situ from a Tf₂O-promoted Beckmann rearrangement of ketoximes under very easy conditions. (Figure presented.).

Full text of DOI:10.1002/adsc.201801724

Title: A Tf2O-Promoted Synthesis of Functionalized Quinolines from
Ketoximes and Alkynes
Authors: Weiping Zheng, Weiguang Yang, Dongping Luo, Lin Min,
Xinyan Wang, and Yuefei Hu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801724
Link to VoR: http://dx.doi.org/10.1002/adsc.201801724

10.1002/adsc.201801724
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
A Tf2O-Promoted Synthesis of Functionalized Quinolines from
Ketoximes and Alkynes
Weiping Zheng,a Weiguang Yang,a Dongping Luo,a Lin Min,a Xinyan Wang,a* Yuefei
Hua*
a Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of
Chemistry, Tsinghua University, Beijing 100084, P. R. China.
Fax: 86-6277-1149; Tel: 86-6279-5380; E-mail: yfh@mail.tsinghua.edu.cn and wangxinyan@mail.tsinghua.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Methods a-d: The existing methods
Abstract. A new general synthesis of quinolines was
H
N
developed from ketoximes and alkynes in the presence of
Tf2O. It offered the first direct synthesis of quinolines by
using the nitrilium salts generated in situ from a Tf2O-
promoted Beckmann rearrangement of ketoximes under
very easy conditions.
R1
R1
R2
N
R2
COCl2, POCl3 or SOCl2
Tf2O, additive
O
Cl
b.
a.
SnCl4
R2
R2
C
R4
R1
R1
R1
C
R3
R2
N
N
R4
R3
Keywords: Beckmann rearrangement; ketoximes;
alkynes; quinolines; nitrogen heterocycles; triflic
anhydride.
N
N-aryl nitrilium salts
R2C
Cu(OTf)2
N
c.
heating
d.
R1
R1
R1
N2BF4
I
Due to the unique structure of quinoline,
functionalized quinolines have been widely used as
starting materials, intermediates and target products
in organic synthesis.[1,2] They have also found
significant applications in biology[3] and material
sciences.[4] Therefore, many strategies for the
synthesis of quinolines have been developed.
Method e. This work
OH
N
R4
N
R4
R3/(H)
R2
Tf2O, DCE, 80 oC, 12 h
R2
R1
R1
+
R3/(H)
1
2/4
3/5
Among the strategies for the synthesis of
quinolines, the cyclization of N-aryl nitrilium salts
with alkynes was reported as early as 1964[5] (Scheme
1a). Unfortunately, this strategy was rarely used for
its first forty years[6] because the difficult generation
of N-aryl nitrilium salts was incompatible with
alkynes. In recent years, rapid progress has been
made to achieve the generation of N-aryl nitrilium
salt and its cyclization with alkyne in one-pot. As we
know,[7] the nitrilium salts can be generated mainly
from four types of substrates: amides, nitriles,
ketoximes and isocyanides. The first two of them thus
far have been successfully used for the direct
synthesis of quinolines. The desired N-aryl nitrilium
salts could be generated in situ by Tf2O-promoted
dehydration of N-aryl amides[8] (Scheme 1b) or by
nucleophilic N-arylations of nitriles using aryliodo-
nium salts[9] or aryl diazo salts[10] (Scheme 1c and 1d).
Scheme 1. The existing methods and this method.
additives.[8a] For method-c and method-d, the
preparations of aryliodonium salts or aryl diazo salts
usually are inconvenient or expensive. Therefore,
development of methods for more efficient synthesis
of quinolines from different substrates is still highly
desirable. Herein, we report a direct method for the
synthesis of quinolines 3/5 from aryl ketoximes 1 and
alkynes 2/4 in the presence of Tf2O (method-e). In
this method, the N-aryl nitrilium salts were generated
in situ by a Tf2O-promoted Beckmann rearrangement
of ketoximes under simple conditions. The method is
characterized by using readily available substrates
under additive-free conditions.
It is well known that nitrilium salts are the key
intermediates in Beckmann rearrangement and their
reactivity varies with different reagents.[11] But thus
far, only one method has been reported to convert
ketoximes to quinoline,[6, 12] where N-aryl nitrilium
hexachloroantimonates needed to be premade from a
Although these pioneering works have their own
advantages, they also suffered from some
unavoidable drawbacks. For example, low efficiency
usually was observed when using terminal alkynes as
substrates in method-b, which caused by pyridine
1
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10.1002/adsc.201801724
Advanced Synthesis & Catalysis
Beckmann rearrangement using toxic reagents and
multi-step syntheses were used (Scheme 2).
entry
additive
---
base in work-up (time)b)
3a (%)c)
1
2
Et3N (< 3 min)
Et3N (< 3 min)
50
18
24
trace
9
OH
R2
O
R
2,6-Cl2-Py
2,6-F2-Py
2-Cl-Py
2-F-Py
---
N
N
O
3
Et3N (< 3 min)
R2
COCl2 or (COCl)2
R = Cl or COCl
R1
R1
4
Et3N (< 3 min)
1
5
Et3N (< 3 min)
SbCl5
N
R2
R4
N
6
Et3N (15 min)
71
80
79
76
71
R3
R4
R3
R2
(2)
R1
R1
7
---
Et3N (0.5 h)
SbCl6
8
---
10% aq. NaHCO3 (0.5 h)
10% aq. Na2CO3 (0.5 h)
10% aq. NaOH (0.5 h)
N-aryl nitrilium
hexachloroantimonates
3
9
---
Scheme 2. Early synthesis of quinolines from ketoximes.
10
---
a) After the mixture of 1a (0.5 mmol), 2a (0.5 mmol), Tf2O
(0.75 mmol) and an additive (0.75 mmol) in DCE (2 mL)
was stirred at 90 oC for 14 h, the reaction was quenched by
We assumed that these phenomena may be resulted
from unsuitable reactivity and compatibility between
the reagents in Beckmann rearrangement and alkynes.
Therefore, solving these problems may achieve a
novel and general method for the synthesis of
functionalized quinolines from ketoximes. Thus,
acetophenone oxime (1a) and 1,2-diphenyl ethyne
(2a) were used as model substrates to be tested with
different reagents of Beckmann rearrangement, such
as Ga(OTf)3,[13a] [Rh]/TfOH/PPh3,[13b] tBuCOCl/DMF,
b)
Et3N (0.5 mL). The time for the treatment of reaction
with a base. c) Isolated yields.
Further conditional tests (Table 2) indicated that
the reaction in fact could be finished within 12 h at 80
oC (entries 1-3). The yield of 3a was not improved by
increasing the amounts of 2a (entries 4), but it was
increased by increasing the amounts of 1a (entries 5-
6). The best results were obtained when two
equivalents of Tf2O were used (entries 7-8). Finally,
the entry 7 was assigned as our standard conditions.
[14a]
TsCl,[14b] TfCl/py,[14c] BAC/HFIP,[15a] TFA,[15b]
ClSO3H,[15c] H2NSO3H,[15d] PhI(OAc)2/BF3·OEt2,[16a]
NCS/PPh3,[16b] TCT/ZnCl2,[17a] TCT/DMF,[17b] and
Tf2O.[18] To our delight, the desired 2-methyl-3,4-
diphenylquinoline (3a) was obtained as a single
product in 50% yield by using Tf2O[18] (entry 1, Table
1), even though the complicated mixtures were
obtained by using all other reagents. To the best of
our knowledge, no such transformation has been
reported in literature to date.
Table 2. The effects of reaction conditions.a)
OH
N
Ph
N
Ph
Ph
Tf2O
DCE, temp, time
Ph
Me
+
Me
1a
2a
3a
Encouraged by our initial findings, we decided to
optimize further this transformation. As shown in
Table 1, the different additives were tested initially.
Surprisingly, the product 3a was obtained in only
18% yield when one equivalent of 2,6-Cl2-pyridine
was used as an additive[8] (entry 2). Similar low
yields were obtained when other pyridine derivatives
were used as additives (entries 3-5). These results
suggested that both Tf2O and the in situ formed TfOH
may play important roles in the Beckmann
rearrangement of 1a.[19] These results also reminded
us that the product 3a may form a salt with TfOH at
the end of the reaction, which may cause the low
yield of 3a. Therefore, the work-up step was tested by
using different bases for different times. As was
expected, 3a was obtained in 71% yield when the
reaction mixture was treated by Et3N for 15 minutes
(entry 6). The highest yield of 3a was obtained when
the treatment time was prolonged to 30 minutes
(entry 7). Although similar results were obtained by
using inorganic bases (entries 8-10), the conditions in
entry 7 were chosen for further tests.
entry
temp
(oC)
time
(h)
yield of 3a
1a/2a
(mmol)
(%)b)
1
2
80
80
70
80
80
80
80
80
14
12
12
12
12
12
12
12
0.5/0.5
0.5/0.5
0.5/0.5
0.5/0.6
0.55/0.5
0.6/0.5
0.6/0.5
0.6/0.5
84
84
74
83
87
90
93
93
3
4
5
6
7c)
8d)
a)
After the mixture of 1a, 2a and Tf2O (0.75 mmol) in
DCE (2 mL) was stirred at the given temperature and time,
the reaction was quenched by Et3N. b) Isolated yields.
1
c)
mmol of Tf2O were used. d) 1.25 mmol of Tf2O were used.
To generalize this method, the scopes of the
substrates and products were tested under standard
conditions. As shown in Scheme 3, by fixing 1,2-
diphenyl ethyne (2a), different 3,4-diphenyl
substituted quinolines 3a-3r were synthesized from
aryl alkyl ketoximes (1a-1m) or diaryl ketoximes
(1n-1r) in moderate to excellent yields. The products
3m and 3p were obtained in relatively low yields
because they were synthesized from two structurally
Table 1. The effects of additives and the bases.a)
OH
N
Ph
Tf2O, additive
DCE, 90 oC, 14 h
Ph
Ph
Ph
Me
+
N
Me
1a
2a
3a
2
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10.1002/adsc.201801724
Advanced Synthesis & Catalysis
special oximes.[20] The synthesis of 3s-3v indicated
that this method has a wide product diversity and 3-
iodoquinolin 3u provided an important example.
alkynyl imines were formed completely. For similar
reasons to 3p,[20] the product 5l was obtained in
relatively low yield. Under standard conditions, the
products 3a and 5a were prepared on 2-gram scales in
90% and 92% yields, respectively.
OH
R4
N
R4
R3
R2
Tf2O, DCE, 80 oC, 12 h
37-97%
R2
R1
R1
+
Based on the experimental results, a possible
pathway for the synthesis of 3a was proposed as
shown in Scheme 5. Initially, the oxime 1a carried
out a Tf2O-promoted Beckmann rearrangement to
generate the reactive intermediate nitrilium salt 8.[18]
When the salt 8 was captured by alkyne 2a, phenyl-
substituted alkenyl cation intermediate 9 formed.[8-11]
Finally, product 3a was obtained by an
intramolecular electrophilic substitution of 9. Based
on the proposed alkenyl cation intermediate 9, the
regioselectivity of products can be well explained.
Thus, the stronger electron-donating group in the
alkynes bearing two different groups regioselectively
locates at 4-position on quinoline ring in the products
3t-3v and 5a-5r, because such group can stabilize the
corresponding alkenyl cation intermediates (the
analogues of 9).
R3
2a-2e
N
1a-1r
3a-3v
3a, R1 = H, 93%
3f, R1 = 6-Cl, 80%[a]
3g, R1 = 6-Br, 82%[a]
3h, R1 = 8-Br, 67%
3i, R1 = 6-NO2, 71%
Ph
N
3b, R1 = 6-Me, 74%
3c, R1 = 8-Me, 88%
3d, R1 = 6-F, 80%a
3e, R1 = 8-F, 50%
5
Ph
6
R1
7
Me
8
Ph
Me Ph
Ph
Ph
Ph
Ph
Me
N
Me
N
Me
N
Me
3j
1:1 (77%)
3j'
3k (77%)
Ph
3o, R1 = Me, 91%
3p, R1 = OMe, 46%
3q, R1 = F, 83%
3r, R1 = Cl, 93%
Ph
Ph
R2
R1
Ph
N
3l, R2 = Et, 87%
3m, R2 = Bn, 37%
3n, R2 = Ph, 97%
N
R1
OH
Me
O
TfO
N
N
Tf
Tf
O
Tf
Ph
Ph
N
Me
N
Me
3s, R2 = Me, R3 = 4-Br-C6H4, R4 = 4-Br-C6H4, 74%
3t, R2 = Me, R3 = CO2Et, R4 = Ph, 52%
3u, R2 = Me, R3 = I, R4 = Ph, 67%
Ph
Ph
Me
R4
TfOH
8
7
1a
R3
R2
(2a)
Ph
Ph
3v, R2 = Ph, R3 = Me, R4 = Ph, 81%
Ph
N
Ph
N
Ph
TfO
N
H
Ph
Ph
Ph
Me
[a] 1.5 Equivalents of oxime 1 were used for higher yield of product.
TfO
Me
Me
N
TfOH
3a
10
9
Scheme 3. The synthesis of products 3a-3v.
Scheme 5. Proposed pathway for the synthesis of 3a.
As shown in Scheme 4, the desired products 5a-5r
were obtained smoothly from the corresponding
terminal alkynes 4a-4n under the standard conditions.
Since no pyridine additive was used, no byproduct
In summary, a new synthesis of quinolines was
developed from ketoximes and alkynes in the
presence of Tf2O. This method offers the first direct
synthesis of quinolines by using the nitrilium salts
generated in situ from a Tf2O-promoted Beckmann
rearrangement of ketoximes. Since the method was
operated under simple conditions and the ketoximes
are a huge family of substrates, we may expect that
this method will have widespread applications in
organic synthesis.
OH
N
R4
R4
Tf2O, DCE, 80 oC, 12 h
32-96%
R2
R1
R1
+
H
N
R2
5a-5r
1a,1l,1n,1o,1s
4a-4n
[b]
5a, R = H, 96%
5g, R = 3-Cl, 81%
5h, R = 4-F, 75%
5i, R = 2-F, 74%
R
5b, R = 4-Me, 84%
5c, R = 3-Me, 80%
5d, R = 4-Et, 75%
[b]
5j, R = 4-NO2, 85%
5k, R = 4-Ph, 75%
Experimental Section
[a]
5e, R = 4-Br, 70%
[a]
N
N
Ph
5f, R = 4-Cl, 80%
5l, R = 4-OMe, 32%
A typical procedure for synthesis of 2-methyl-3,4-
diphenylquinoline (3a). To a solution of (E)-aceto-
phenone oxime (1a, 81 mg, 0.6 mmol) and 1,2-diphenyl-
ethyne (2a, 89 mg, 0.5 mmol) in dry DCE (2 mL) was
Bun
Ph
o
added Tf2O (282 mg, 168 μL, 1.0 mmol) at 0 C. The
mixture was stirred at 0 oC for 10 min and then at 80 oC for
12 h. After the mixture was cooled to room temperature,
Et3N (0.5 mL) was added to quench the reaction and the
mixture was stirred at room temperature for another 30 min.
The reaction mixture was purified by a flash chromato-
graphy [silica gel, 10% EtOAc in petroleum ether (60–90
oC)] to give 137 mg (93%) of product 3a as a yellowish
Ph
N
Ph
Br
N
Me
[b]
[b]
5m (55%)
5n (84%)
5o (83%)
Ph
Ph
Ph
Me
Et
N
N
C6H4-4-Me
N
C6H4-4-Br
[b]
[b]
o
o
1
solid, mp 160−161 C (lit.[8b] 171–172 C). H NMR (400
MHz, CDCl3) δ 8.12 (d, J = 8.4 Hz, 1H), 7.67 (t, J = 8.4
Hz, 1H), 7.50 (d, J = 8.8 Hz, 1H), 7.37 (t, J = 8.0 Hz, 1H),
7.26–7.13 (m, 6H), 7.13–6.99 (m, 4H), 2.54 (s, 3H); 13C
NMR (100 MHz, CDCl3) δ 157.8, 147.0, 146.5, 138.6,
136.7, 134.0, 130.01 (2C), 129.96 (2C), 129.0, 128.6,
5p (96%)
5q (60%)
5r (70%)
[a] 1.5 Equivalents of oxime 1 were employed.
[b] 2.0 Equivalents of alkyne 4 were employed.
Scheme 4. The synthesis of products 5a-5r.
3
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10.1002/adsc.201801724
Advanced Synthesis & Catalysis
127.8 (2C), 127.6 (2C), 127.1, 126.7, 126.5, 126.2, 125.8,
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Peng, M. Li, Angew. Chem. Int. Ed. 2013, 52, 5323–
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The products 3b-3v and 5a-5r were prepared by the
similar procedure.
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This work was supported by National Natural Scientific
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[19] No product 3a was obtained from the same reaction
by complete replacement of Tf2O with TfOH.
[20] The lower yields of 3m and 3p may be caused by the
facts that their nitrilium salts 8m and 8p are partially
converted into the isomer 8m-A and the resonance
contributor 8p-A, respectively, by which the positive
charges on 8m and 8p are dispersed.
Ph
Ph
H
Ph
Ph
H
.
N
N
H
TfOH
OMe
TfO
8m-A
8m
Ph
8p
N
.
Ph
N
8p-A
OMe
4
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10.1002/adsc.201801724
Advanced Synthesis & Catalysis
COMMUNICATION
A Tf2O-Promoted Synthesis of Functionalized
Quinolines from Ketoximes and Alkynes
OH
R2
N
R4
R4
R3
R3
R2
Tf2O, DCE, 80 oC, 12 h
Adv. Synth. Catal. Year, Volume, Page – Page
R1
+
R1
Additive free
N
Easy procedure
High efficiency
Broad scope
forty products
in 32-97% yields
Weiping Zheng, Weiguang Yang, Dongping Luo,
Lin Min, Xinyan Wang,* Yuefei Hu*
5
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