Ytterbium(III)-catalyzed three-component reactions: Synthesis of 4-organoselenium-quinolines

Article abstract of DOI:10.1039/c7nj01803g

4-Organoselenium-quinolines were synthesized via model multi-component Povarov (MCR) reactions between p-anisidine, ethyl glyoxylate and ethynyl(phenyl)selane, catalyzed by the Lewis acid Yb(OTf)₃, with yields ranging from moderate to good. This method is advantageous in that there is no requirement for an oxidant and that the reaction has a wide scope. These 4-organoselenium-quinolines allowed access to 4-alkynyl-quinolines, which were synthesized via copper free Sonogashira cross-coupling reactions catalyzed by a new palladium catalyst that employs an oxazoline derivative as a ligand. A plausible mechanism was proposed based on HRMS studies.

Full text of DOI:10.1039/c7nj01803g

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Ytterbium(III)-Catalyzed Three-Component Reactions: Synthesis of
4-Organoselenium-quinolines and Application in Copper-free
Sonogashira Cross-coupling Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Isadora M. de Oliveira,a Stanley S. N. Vasconcelos,b Cristiane S. Barbeiro,b Thiago C. Correra,a
Anwar Shamim,a Daniel C. Pimenta,c Ignez Caracelli,d Julio Zukerman-Schpector,e Hélio A.
Stefani,b* Flávia Manarinf*
DOI: 10.1039/x0xx00000x
www.rsc.org/
4-organoselenium-quinolines were synthesized via model multi-
component Povarov (MCR) reactions between p-anisidine, ethyl
glyoxylate and ethynyl(phenyl)selane, catalyzed by the Lewis acid
Yb(OTf)3, with yields ranging from moderate to good. This method
is advantageous in that there is no requirement for an oxidant and
that the reaction has a wide scope. These 4-organoselenium-
quinolines allowed access to 4-alkynyl-quinolines, which were
synthesized via copper free Sonogashira cross-coupling reactions
catalyzed by a new palladium catalyst that employs an oxazoline
derivative as a ligand. A plausible mechanism was proposed based
on HRMS studies.
component strategies for the synthesis of functionalized
quinolines.12 This protocol is an inverse electron-demand aza-
Diels-Alder reaction between aryl imines (formed in situ) and
terminal alkynes. The classical Povarov reaction employed
electron-rich olefins; however, alkynes can also be used as
dienophiles in the Povarov reaction.
There are several Povarov methodologies in the literature
based on Lewis acid catalysis,13 Bronsted acid3 and even
catalyst-free14 conditions to obtain 2,4-disubstituted quinoline
derivatives. However, similar approaches of multi-component
Povarov reactions involving chalcogen atoms as substituents in
the quinoline ring nucleus, which is a privileged structure, have
been unexplored.
The quinoline ring system is present in a wide array of natural
compounds,1 and materials2. These natural quinoline-bearing
compounds and materials have been reported to exhibit a
large range of biological properties such as neuroprotective3,
anti-tuberculosis4, anti-trypanosomal,5 anti-cancer6 and anti-
toxoplasmosis effects.7
Selenium-containing compounds have found great utility in
biology and chemistry due to their highly promising
properties.15 Unsaturated organoselenides can be used as
versatile synthetic intermediates as an alternative to
organohalides in cross-coupling reactions.16 In the
pharmacology field, a series of selenium-containing clioquinol
derivatives were evaluated as multi-functional anti-
Alzheimer´s disease (AD) agents. These compounds notably
exhibited inhibition of metal-induced Aβ aggregation.17
In this paper, we report a synthetic route to access novel 2,4-
dissubstituted selenium-quinoline derivatives via multi-
component (MCR) Povarov reactions and their subsequent use
as intermediates in copper-free Sonogashira cross-coupling
reactions.
Based on their versatility in multi-disciplinary fields, the
development of new synthetic strategies for the preparation
of quinoline derivatives has attracted great attention in the
field of organic synthesis.
Several methods have been reported for the synthesis of
quinoline rings; the classical methodologies that date from
1880 usually involve harsh reaction conditions and show
various limitations for diversity-oriented synthesis.11 However,
the Povarov reaction is one of the most straightforward multi-
The unique structural features and potential bioactivity of
quinolines prompted us to investigate this cycloaddition
reaction under various conditions. Initially, screening of
reaction parameters, including catalyst, solvent, temperature
and reaction time, was conducted using a model multi-
component (MCR) Povarov reaction between p-anisidine 1a
ethyl glyoxylate 2a and ethynyl(phenyl)selane 3a (Table 1).
,
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catalyst is readily recovered from the reaction mixtures and
can be reused with equivalent catalytic capacity.19
Table 1. Optimization of Multi-component (MCR) Povarov Reaction Conditions.
In relation to the optimization of the reaction conditions, we
found that prolonging the reaction time did not improve the
yields. The effect of a nitrogen atmosphere was evaluated,
however there were no major changes in yield (Table 1, entry
12).
Therefore, we supposed that the yield of this reaction might
be improved by using an additive, such as TEMPO, to act as an
Entry
Catalyst (mol %) Time(h)
Solvent
Yield (%)e
1
2
Yb(OTf)3 (10)
FeCl3 (10)
4
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
DCM
Toluene
69
40
60
47
53
45
35
42
40
65
60a
62b
75
50c
47d
oxidant
to
produce
quinolines
from
di-
and
18
20
19
19
20
3
tetrahydroquinolines. However, we did not observe any
change in the yield when we used TEMPO as an oxidant (Table
1, entry 11). However, increasing the catalyst loading to 20
mol%, increased the yield of 4a to 75% (Table 1, entry 13). We
3
Sc(OTf)3 (10)
Ag(OTf) (10)
In(OTf)3 (10)
SnCl2
4
5
6
next
investigated
different
solvents
(acetonitrile,
7
TfOH (10)
dichloromethane and toluene) for the Yb(OTf)3 catalyzed
reaction, however acetonitrile proved most suitable (Table 1,
entries 1, 14 and 15).
8
I2 (10)
22
18
22
10
18
2
9
AuCl3 (10)
10
11
12
13
14
15
Yb(OTf)3 (10)
Yb(OTf)3 (10)
Yb(OTf)3 (10)
Yb(OTf)3 (20)
Yb(OTf)3 (10)
Yb(OTf)3 (10)
Among the various conditions tried, the best result was
obtained with Yb(OTf)3 (10 mol%) as catalyst and acetonitrile
as solvent at 80 ºC, providing the cyclization product in 69%
(Table 1, entry 1). Confirmation of the NMR spectroscopic
results was accomplished by the X-ray crystal structure of
compound 4a (See SI).
20
17
We next explored the generality of the multi-component
(MCR) Povarov reaction for the synthesis of selenium-
quinoline derivatives, using a variety of substituted anilines
(Table 2). The results indicated that both electron-poor and
electron-rich aryl substituents were tolerated. For example, p-
Cl, p-Br and o-Cl susbtituted anilines afforded the
Conditions: p-anisidine (0.5 mmol, 1.0 eq.), ethylglyoxylate (0.5 mmol, 1.0 eq.),
catalyst (10 - 20 mol%), solvent (3.0 mL), ethynyl(phenyl)selane (0.6 mmol, 1.2
eq.), in N2 atmosphere. aTEMPO. bAmbient atmosphere. cTemperature 45 °C.
dTemperature 100 °C. eIsolated yields.
When substrates 1a, 2a, and 3a, in a ratio of 1:1:1.2, were
corresponding products 4c
30-40%. p-OMe, p-OH and p-Me substituted anilines provided
the corresponding products 4a 4e and 4g in moderate to good
, 4d and 4f in yields ranging from
reacted in the presence of Yb(OTf)3 (10 mol%) in acetonitrile
under reflux for 4 h, compound 4a was isolated in 69% yield
(Table 1, entry 1). Several other Lewis and Bronsted acids, such
as FeCl3, SnCl2, TfOH, I2 and AuCl3, were also screened as
catalyst, however the desired product 4a was achieved in poor
yields of (Table 1, entries 2, 6-9). Triflates have proved to be
efficient Lewis acid catalysts for the synthesis of several
important biologically active heterocyclic molecules due of
their distinct properties such as moisture insensitivity, stability,
reusability and high catalytic activity.18 They have also been
used efficiently for the synthesis of quinoline derivatives,7,13
therefore we decided to screen the effectiveness of Yb(OTf)3,
Sc(OTf)3, Ag(OTf) and In(OTf)3 (Table 1, entries 1, 3-5).
However, Yb(OTf)3 was found to be the most effective when
compared to all the catalysts studied.
,
yields of 40-69%. When p-NO2 substituted aniline was used,
the formation of the imine intermediate was not observed.
Substituent position on the anilines also exerted a noticable
effect, namely the yields of reactions with ortho-substituted
anilines were comparitively lower than the yields of reactions
with para-substituted anilines (4f and 4h).
Table 2. Scope of Anilines in the Multi-component (MCR) Povarov reactions.
Recently, Sakhujaet al. reported the importance of Yb(OTf)3 as
a useful Lewis acid that has attracted attention in organic
synthesis. It is probably one of the most effective of the
lanthanide triflates, owing to its position near the end of the
lanthanide series. With a larger number of protons in its
nucleus, Yb3+ is one of the smallest lanthanides and the
hardest acid of the series. Therefore, the high Lewis acidity of
Yb3+ may be attributed to its small ionic radius. The incomplete
number of electrons in the 4f orbital of Yb3+ results in a high
tendency for this orbital to become filled. Reaction conditions
using this catalyst are generally mild and often only room
temperature and/or low-boiling solvents are required. The
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Similarly, different aldehydes were applied to make a library of
selenium-substituted quinolines (Table 3). Good to moderate
yields were achieved with a variety of aromatic, alkyl and
heteroaromatic aldehydes.
Table 3. Scope of Aldehydes in the Multi-component (MCR) Povarov reactions.
The reaction was found to tolerate both electron-withdrawing
and electron-donating groups and the yields were the same for
both.
To demonstrate the synthetic utility of 4-organoselenium-
quinolines derivatives, compound 4a was subjected to
Sonogashira cross-coupling reactions. We tried the reactios
with
ethyl-6-methoxy-4-(phenylselene)
quinoline-2-
carboxylate 4a and 2-methylbut-3-yn-2-ol 7a, in the presence
of various palladium catalysts, copper salts, bases, and
solvents (Table 5).
We next examined different substituted aromatic selenium
acetylenes in the multi-component (MCR) Povarov reactions
(Table 4). The desired products were obtained in yields ranging
from 30-69%. Phenyl and benzyl alkynes were used in order to
determine the influence of phenyl and benzyl groups linked to
selenium. We observed that alkynes containing the phenyl
group led to product formation with comparatively higher
Table 5. Screening of the Sonogashira cross-coupling reaction.
yield examples 4a and 6b
.
Table 4. Scope of Alkynes in the Multi-component (MCR) Povarov reactions.
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Yield
(%)
shown (Table 5, entry 7). The structure of compound 8a was
confirmed unequivocally by X-ray crystallographic analysis (See
SI).
Entry
Catalyst (mol%)
Base
Solvent
1
2
Pd(PPh3)Cl2 (10)/Cu(OAc)2 (20)
Pd(Ph3)4 (10)/Cu(OAc)2 (20)
Pd(OAc)2 (10)/Cu(OAc)2 (20)
Pd(PPh3)Cl2 (10)/CuI (20)
Pd(PPh3)Cl2 (10)/Cu(OAc)2 (20)
Pd(PPh3)Cl2 (5)/Cu(OAc)2 (10)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 2 (20)
PdCl2(10)/ligand 3 (20)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
PdCl2
Et3N
Et3N
Et3N
Et3N
DIPA
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Cs2CO3
DIPA
Et3N
Et3N
Et3N
Et3N
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
ACN
65ª
50ª
15ª
50ª
57ª
76ª
75ª
10ª
30ª
10c
Using the optimized reaction conditions, the scope of this
coupling reaction was evaluated using different alkynes to
obtain a library of alkyne substituted quinoline derivatives, as
shown in Table 6.
3
4
Table 6. Sonogashira Cross-coupling Reactions.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
63a,g
20
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
PdCl2(10)/ligand 1 (20)
15ª
25ª
37ª
57d
MeOH
THF
27b
DMF
28a,e,f,g
Conditions: 4a(0.15 mmol, 1.0 eq.), 7a (0.3 mmol, 2.0 eq.), palladium catalyst (
- 10 mol%), copper catalyst (10 - 20 mol%), ligand (20 mol%), base (0.3 mmol, 2.0
eq.), solvent (1.0 mL), in N2.aTemperature 80 °C. bTemperature 60 °C. cRoom
temperature. dTemperature 50 °C. eMW. fTime 1h. gAmbient atmosphere.
The Sonogashira cross-coupling reactions employing the
oxazoline derivative as a ligand were promising, tolerating the
use of different aromatic and alkyl alkynes. Alkynes with alkyl
groups (8f) and highly electron-withdrawing groups (8h) gave
lower yields. Alkynes with electron-donating groups gave the
products 8a-e, and 8g in good yields.
Our preliminary studies focused on the reaction of ethyl-6-
methoxy-4-(phenylselene)quinoline-2-carboxylate 4a with 2-
methylbut-3-yn-2-ol 7a in the presence of 10 mol% of
Pd(PPh3)Cl2 and 20 mol% of Cu(OAc)2, with Et3N as base and
DMF as solvent at 80 ºC. Quinoline 8a was isolated in 65%
yield after 24 h (Table 5, entry 1). Subsequently, different
palladium species such as Pd(PPh3)4, Pd(OAc)2, PdCl2/L1,
PdCl2/L2 and PdCl2/L3 were also tested (Table 5, entries 2, 3,7-
9). PdCl2/L1 proved to be the more efficient catalyst in these
reactions. Among the catalysts used, those with oxazoline
ligands afford the product in higher yields with no need for the
use of copper as co-catalyst. PdCl2 without the oxazoline ligand
gave the product in lower yields.
A mechanism was proposed based on experimental data
obtained by using high resolution mass spectrometry (HRMS),
as shown in Scheme 1. Initially, complex
A forms in situ from
the oxazoline ligand and palladium chloride, as detected by
mass spectrometry [m/z 535.975]+. Then, the palladium-
chlorine bonds break forming complex
Oxidative addition20 of the 4-phenylselenium-quinoline 4a
B
[m/z 466.010]+.
leads to the complex
coordination with the
C
[m/z 853.064]+ (See SI) followed by
-electrons of the alkyne 7a. We believe
π
Other common bases that we used in this reaction, such as
Et3N, Cs2CO3 and DIPA, were effective too, although lower
yields were obtained (Table 5, entries 12-14). This indicated
that base played an important role in the reaction. The effect
of the solvent was also investigated. Changing the solvent to
ACN, MeOH and THF failed to improve the yield of the product
8a (Table 5, entries 7, 15-17). Further investigation revealed
that temperature was also a key factor in this reaction. When
the reaction was run at room temperature, the product 8a was
obtained in only 10% yield (Table 5, entry 10). The highest
yield of 75% was obtained under the optimized conditions as
that the coordination of the palladium with the alkyne
(complex ) acidifies the terminal hydrogen, facilitating its
removal after the addition of the base. Reductive elimination
of complex leads to the coupling product 8a and the
D
E
regeneration of catalyst
spectra see the SI).
B (for additional information and
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organoselenium-quinolines
derivatives.
Our
reactions
proceeded in the absence of oxidants and tolerated a wide
variety of anilines, alkynes and aldehydes. Furthermore, we
demonstrated the synthetic potential of 4-organoselenium-
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in copper-free Sonogashira cross-coupling reactions.
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Acknowledgements
The authors gratefully acknowledge financial support from the
São Paulo Research Foundation (FAPESP grant 2012/00424-2
to HAS, 2016/04289-3 to I.M.O. and 2013/17960-7 to S.N.S.V.),
São Paulo Research Foundation (FAPESP-grant 2014/15962-5,
FAPESP-grant 2015-08539-1 to T.C.C.) and Coordination of
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Improvement of Higher Level Personnel (CAPES) for
a
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fellowship (CAPES 23038.006960/2014-65 to T.C.C), and The
National Council for Scientific and Technological Development
(CNPq) for fellowships (306119–2014-5 to HAS, 308480/2016-3
to IC, 305626/2013–2 to JZ-S), The National Council for
Scientific and Technological Development (CNPq) for
a
fellowship (459816/2014-4 F.G.M.). We thank Professor Regina
H. A. Santos from IQSC-USP for the X-ray data collection.
Graphical Abstract
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