Facile Synthesis of 2-Acylthieno[2,3- b]quinolines via Cu-TEMPO-Catalyzed Dehydrogenation, sp2-C-H Functionalization (Nucleophilic Thiolation by S8) of 2-Haloquinolinyl Ketones

Article abstract of DOI:10.1021/acs.orglett.9b04598

An efficient, solvent-free synthesis of 2-acylthieno[2,3-b]quinolines is reported from 2-halo-quinolinyl ketones through Cu-TEMPO catalyzed dehydrogenation, sp²-C-H functionalization using elemental sulfur as thiol surrogate (sulfur source) and tetrabutylammonium acetate as an ionic reaction medium. The optimized reaction conditions give excellent product yields under mild reaction conditions with chemoselectivity and broad functional group tolerance. The synthetic importance of the synthesized molecules is showcased further by Friedl?nder annulation, reduction, and alkene functionalization reactions.

Full text of DOI:10.1021/acs.orglett.9b04598

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Letter
Facile Synthesis of 2‑Acylthieno[2,3‑b]quinolines via Cu-TEMPO-
Catalyzed Dehydrogenation, sp2‑C−H Functionalization
(Nucleophilic Thiolation by S8) of 2‑Haloquinolinyl Ketones
Chitrala Teja and Fazlur Rahman Nawaz Khan*
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ABSTRACT: An efficient, solvent-free synthesis of 2-acylthieno[2,3-
b]quinolines is reported from 2-halo-quinolinyl ketones through Cu-
TEMPO catalyzed dehydrogenation, sp2-C−H functionalization using
elemental sulfur as thiol surrogate (sulfur source) and tetrabutylammonium
acetate as an ionic reaction medium. The optimized reaction conditions
give excellent product yields under mild reaction conditions with chemos-
electivity and broad functional group tolerance. The synthetic importance
̈
of the synthesized molecules is showcased further by Friedlander annula-
tion, reduction, and alkene functionalization reactions.
n the past few years, nitrogen-containing heterocycles have
(i).8 Xie and co-workers reported a domino reaction of o-
Ibeen considered important due to their engaging biological isothiocyanato-(E)-cinnamaldehyde with α-halocarbonyl com-
pounds, utilizing L-proline and DBU in methanol (ii).4
activities. Among them, quinoline subunits exhibit potential
pharmacological activities,1−3 which are also available in natu-
Koketsu and co-workers reported an iodine mediated synthesis
of thieno[2,3-b]quinoline derivatives via electrophilic cycliza-
tion of 3-alkynyl-2-(methylthio) quinolines. (iii).9 Doucet and
co-workers documented one-pot sequential imination and intra-
molecular arylation of thiophene-3-carbaldehyde with
2-bromoaniline. (iv)10 A few other reports have been
published.11,12
ral products and used as essential intermediates for many organic
transformations. In this regard, thienoquinolines are quinoline-
fused sulfur-containing heterocycles and are privileged scaffolds
available in natural products, pharmaceutical drugs, and also
organic light-emitting materials.4,5 The 2-acylthienoquinoline
scaffold is commonly found in various drug molecules and
shows its broad range of bioactivities (Figure 1). For example,
However, the above literature reports show several advan-
tages and limitations, including expensive metal catalysts and
external ligands, limited substrate scope, use of the conven-
tional organic solvents at high temperatures, harsh reaction
conditions, multistep starting materials, and long reaction
intervals to obtain products. To the best of our knowledge, the
synthesis of 2-acylthieno[2,3-b]quinolines from 2-halo-alky-
lated ketones is a challenging concern, including less reactive
C(sp2) and C(sp2) sites, elemental sulfur as a thiol source, and
Cu powder as a catalyst for cyclization. Here, we report the
synthesis of 2-acylthieno[2,3-b]quinolines via Cu-TEMPO
catalyzed dehydrogenation and sp2-C−H functionalization
(S8-thiolation) from 2-haloquinolinyl ketoalkanes (Scheme 1b).
Initially, we started our investigation using 3-(2-chloroqui-
nolin-3-yl)propan-1-one 1a (2-chloroquinolinyl ketoalkanes)
as a substrate for the synthesis of 2-acylthienoquinolines, using
Cu(OAc)2 as a catalyst, TEMPO as a novel oxidant, DBU as
Figure 1. Representative examples of biologically active 2-acylthieno-
[2,3-b]quinolines.
thienoquinolines are novel disruptors of the PKCε/RACK2
inhibitors.6 Protein kinase inhibitors in cancer cells show func-
tional interactions with protein kinase receptor against VEGFR1
and CHK2 receptors.7
Received: December 28, 2019
A very few literature reports are available for the synthesis of
thienoquinolines (Scheme 1a). Mahadevan and co-workers
reported a one-pot reaction of 3-formyl-2-mercaptoquinolines
with 1-chloroacetone using K2CO3 under microwave conditions,
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© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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ligand, and S8 (elemental sulfur) as a sulfur precursor in DMF
at 90 °C and have observed that the reaction yielded a trace
amount of the desired product (30%) 2a (Table 1, entry 1).
The exchange of ligand to DABCO, 2,2-bipyridyl, and 1,10-
phenanthroline was carried out (entries 2−4). Here, we have
observed that 2,2-bipyridyl ligand is effective in obtaining 2a in
45% yield, compared to other ligands, because it is a bidentate
chelating ligand that can easily form a complex with transition
metals.13,14 Then change of oxidant to TBHP is effected, and
no increment in product yield was observed (entry 5).
TEMPO is a well-known radical initiator and oxidant and is
used as a mediator for many controlled radical polymerization
reactions.15−18 Further, optimization of the reaction is done to
improve the efficiency of the thiol surrogate, explored with
Na2S, Na2S2O3, K2S5, and KEX (potassium ethyl xanthogen-
ate) as various sulfur precursors (entries 6−9). Though it
provided 2a in trace to moderate yields, S8 (sulfur) and KEX
provided 2a in 45, 40% of yields (entries 3 and 9). Conse-
quently, S8 (sulfur) was chosen as a sulfur surrogate because of
its cost-effectiveness compared to the expensive KEX. So far,
the reaction was attempted with various ligands and oxidants,
but the yields were not acceptable. Subsequently, the reaction
was screened with Cu salts CuCl2, CuBr2, CuI, CuSO4,
Cu(OTf)2, and Cu -powder (entries 10−15). The results show
that CuI, Cu(OTf)2, and only Cu provided 2a in 52%, 56%,
and 55% of yields, respectively. Interestingly, Cu itself shows
its catalytic activity in terms of product yield compared with
other copper catalysts. Cu was chosen as the metal catalyst for
further studies. The solvent effect was studied, using DMSO
and toluene under Cu catalysis, but there is no improvement in
the yield was observed (entries 16 and 17).
Scheme 1. (A) Previous Reports: Thieno-quinoline Core
Motifs. (B) This Report: Cu-Catalyzed Synthesis of
2-Acylthieno[2,3-b]quinolines from 2-Haloquinolinyl
Ketones through Dehydrogenation and sp2-C−H
Functionalization, Using S8 as Thiol Source
Surprisingly, when the reaction was screened using TBA
salts as a basic ionic medium in the presence of Cu, a better
yield was observed, and it eliminated the use of conventional
volatile organic solvent (VOCs). Several literature procedures
wherein tetrabutylammonium salts were utilized as ionic
a
Table 1. Optimization of Reaction Conditions
f
entry
catalyst
Cu(OAc)2
oxidant
ligand
solvent
S-surrogate
temp (°C)
time (h)
yield (%)
1
2
3
4
5
6
7
8
TEMPO
TEMPO
TEMPO
TEMPO
TBHP
DBU
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Toluene
TBAB
S8
S8
S8
S8
90
90
90
90
90
90
90
90
90
90
80
80
90
100
90
110
100
90
14
10
8
12
15
8
8
8
8
10
9
10
15
6
6
5
30
25
45
34
26
38
trace
26
40
32
41
52
18
56
55
51
46
81
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuCl2
CuBr2
Cu−I
CuSO4
Cu(OTf)2
Cu
DABCO
2,2′-bipyridyl
1,10-phenanthroline
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
S8
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
Na2S
Na2S2O3
K2S5
KEX
S8
S8
S8
S8
S8
S8
S8
S8
S8
9
10
11
12
13
14
15
16
17
18
Cu
Cu
Cu
5
4
B
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Table 1. continued
f
entry
catalyst
oxidant
ligand
solvent
S-surrogate
temp (°C)
time (h)
yield (%)
19
20
21
22
23
24
25
26
27
28
29
Cu
Cu
Cu
Cu
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
2,2′-bipyridyl
TMAB
TEAB
TBAI
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
S8
90
90
90
90
90
90
100
100
90
90
90
90
90
110
90
90
6
5
8
3
10
8
10
9
10
12
8
75
77
80
86
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
TBAA
PdCl2
Pd(OCOCH3)2
PdCl2(PPh3)2
Pd(PPh3)4
trace
trace
trace
30
25
71
87
86
55
88
Ru(p-cymene)Cl2]2
Zn(OTf)2
Yb(OTf)2
Cu
Cu
Cu
b
30
3
3
3
3
c
31
d
32
e
33
Cu
Cu
e
34
3
a
Standard reaction conditions: 2-halo-quinolinyl ketone, 1a (1.0 mmol), catalyst (20 mol %), TEMPO (10 mol %), ligand (20 mol %), Thiol-
b
c
surrogate (2 equiv) in 2 mL of solvent in closed reaction vial at 90 °C or 1 equiv of TBA-ILs. Reaction under nitrogen atmosphere. 30 mol % of
Cu catalyst. Temperature change. Change in the equiv of S8, Isolated yields.
d
e
f
a,b
liquids (ILs) as additive or solvent instead of VOCs in
reactions included Heck coupling, Suzuki, Stille, Sonogashira,
and other cyclization reactions.19−26 At first, TBAB (tetrabu-
tylammonium bromide) was tried using Cu as a catalyst
(20 mol %), TEMPO (10 mol %), 2,2-bipyridyl (20 mol %),
and S8 thiol-surrogate at 90 °C, the reaction yielded 81% of 2a
(entry 18). To our delight, the dehydrogenation followed by
thiolation under TBAB as a reaction medium gave an encour-
aging result, which was screened further with other TBA salts
including TMAB, TEAB, TBAI, and TBAA as ILs using the
same optimized conditions, which provided good amounts of
desired product 2a in 75%, 77%, 80%, and 86%, respectively
(entries 19−22).
Scheme 2. Synthesis of 2-Acylthieno[2,3-b]quinolines
The TBAA (tetrabutylammonium acetate) ionic medium gave
the expected product in good yield compared to all other ILs.
Further, the reaction under inert N2 atmosphere gave a lower yield
in contrast to the standard conditions (2a in 71% yield) (entry 30).
When 30 mol % of Cu catalyst was used, there was no improve-
ment in the yield (entry 31). Furthermore, there was no dif-
ference in the observed yield when the temperature was
increased to 110 °C (entry 32). Additionally, the S8 equivalent
was evaluated, utilizing 1 equiv of S8 as sulfur surrogate, and a
decrease in the product yield (55% of 2a) was observed, and
when 3 equiv of S8 was used an identical result was observed
(entries 33 and 34). Further, dehydrogenation and sp2-CH
functionalization (thiolation) were tried with palladium, ruthe-
nium, and metal triflates, but there is no fruitful result observed
(entries 23 and 29). As per known literature reports, copper
catalysis is an efficient tool for dehydrogenation and cyclization
strategies.27−30 Compared to other metal catalysts, it is also
cost effective. Accordingly, our optimization studies showed
that use of Cu (20 mol %) as an efficient catalyst, TEMPO
(10 mol %) as an oxidant, 2,2-bipyridyl (20 mol %) as a ligand,
S8 (2 equiv) elemental sulfur as a thiol-surrogate, and 1 equiv
of TBAA as an ionic medium or solvent for the reaction gives
2-acylthieno[2,3-b]quinoline 2a in excellent yield up to 86%
within 3 h.
a
Standard reaction conditions: 2-chloroquinolinyl ketone, 1a
(1.0 mmol), Cu (20 mol %), TEMPO-oxidant, (10 mol %), ligand
(20 mol %), thiol-surrogate (2 equiv) in TBAA reaction medium at
90 °C. Isolated yields.
b
Having optimized reaction conditions in hand (Table 1,
entry 22), the substrate scope was explored with a wide variety
of substrates. For instance, 3-(2-chloroquinolin-3-yl)-1-phenyl-
propan-1-one, 1, was prepared from acetophenone and
2-chloro-quinolinyl alcohols according to our previous literature
procedure.31 The electron-donating groups from acetophenone
C
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The above results indicate that the electronic and steric
effects on 2-chloroquinolinyl ketones do not affect the reac-
tion, and the corresponding products in excellent yields up to
86% were observed. In addition, the control experiments and
1H NMR studies were performed to identify the reaction
mechanism. (The results are discussed in Scheme S1).
Scheme 3. Synthesis of 4-Phenylquinolin-3-ylthieno[2,3-
b]quinolin-2-yl)methanones
a,b
On the basis of the above experimental studies and previous
reports,27,32−35 a two-step concerted plausible reaction
mechanism was proposed for the formation of 2-acylthieno-
[2,3-b]quinoline (Scheme-4). In step I, initially, Cu as a Lewis
acid reacts with ketone A to form metal−enolate complex B.
After homolysis of the Cu(II)−enolate bond which generates
Cu(I) species and formation of α-radical intermediate C will
then react with active TEMPO radical to result in formation of
α-TEMPO radical substituted ketone D. The α-TEMPO
radical undergoes rapid TEMPO−OH elimination to form
enone E, and TEMPO−OH elimination at the α-position occurs
via β-H abstraction assisted by another molecule of TEMPO
leading to formation of α,β-unsaturated carbonyl (chalcone)
via dehydrogenation F. Finally, the oxidation of Cu(I) species
by TEMPO or TEMPOH again generates active Cu(II) species
with tetramethylpiperidine and water.27,36 In step II, again the
oxidative addition of copper to chalcone intermediate takes
place to yield G and ligand exchange to leading to Cu−S inter-
mediate H, which further undergoes reductive elimination to
give sulfur-substituted α-C−H intermediate I. Finally, Cu catalyzed
nucleophilic thiolation at α-C−H takes place to give thieno-
quinoline product J. However, in-depth mechanistic studies
and applications of this newly developed strategy are underway.
The synthesized 2-acylthieno[2,3-b]quinoline 2 was further
a
Standard reaction conditions: 4-phenylquinolin-3-yl-3-(2-chloroqui-
nolin-3-yl)propan-1-one 3a (1.0 mmol), Cu catalyst (20 mol %),
TEMPO oxidant, (10 mol %), ligand (20 mol %), thiol surrogate
b
(2 equiv) in TBAA reaction medium at 90 °C. Isolated yields.
̈
functionalized at active o-NH2-ketone using Friedlander annu-
lation37 in acidic medium to obtain thieno[2,3-b]quinolin-2-
yl)quinolin-3-yl)ethan-1-ones 5 in up to 92% yield (a).
Likewise, reduction of compound 2 at 2-acyl (ketone) position
using NaBH4 in methanol38 led to thieno[2,3-b]quinolin-
2-yl)(phenyl)methanol 6 in 96% yield (b). Further, during
functionalization of compound 4, the methyl group at the
2-position was effective to obtain 2-styrylquinolin-3-ylthieno-
[2,3-b]quinolin-2-yl)methanones 7 in 61% yield using
aldehyde in the presence of acidic medium39 (c) (Scheme 5).
In summary, a solvent-free protocol is reported for the syn-
thesis of 2-acylthieno[2,3-b]quinoline from saturated ketones
via Cu-TEMPO-catalyzed dehydrogenation followed by
substitution (−Me, −OMe, and −NH2) facilitated smoothly to
give the corresponding 2-acylthienoquinolines 2f, 2g, 2h, and 2o
in excellent yields (Scheme 2). Likewise, electron-withdrawing
groups from acetophenone (−F and −Cl) facilitated the
reaction smoothly to provide 2-acylthieno quinolines 2i and 2k
in good yield. In the quinoline alcohol, methyl groups of
phenyl ring at the ortho, meta, and para positions did not affect
the product yields (steric effect). The quinoline-sub-
stituted ketones instead of acetophenones were also transformed
smoothly to product 4, as given in (Scheme 3). Compared to
product 2, product 4 was obtained in slightly lower yields
because of steric effects at the acyl substituent.
Scheme 4. Proposed Reaction Mechanism
D
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Scheme 5. Synthetic Application of 2-Acylthieno[2,3-
b]quinolines
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04598.
Copies of 1H, 13C NMR, Mass and IR spectra of the syn-
thesized products, control experiment, additional data,
figures and tables (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Fazlur Rahman Nawaz Khan − Organic and Medicinal
Chemistry Research Laboratory, School of Advanced Sciences,
Vellore Institute of Technology, Vellore 632014, Tamil Nadu,
India; orcid.org/0000-0001-7828-526X;
Email: nawaz_f@yahoo.co.in
Author
Chitrala Teja − Organic and Medicinal Chemistry Research
Laboratory, School of Advanced Sciences, Vellore Institute of
Technology, Vellore 632014, Tamil Nadu, India
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Complete contact information is available at:
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Notes
́
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ACKNOWLEDGMENTS
We are thankful to SIF-VIT for providing NMR, GC−MS, IR,
and CHNS instrumentation facilities.
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Balakrishna, M.; Kumar, C. G.; Hussaini, S. M. A.; Sridhar, B.;
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E
https://dx.doi.org/10.1021/acs.orglett.9b04598
Org. Lett. XXXX, XXX, XXX−XXX