Ruthenium(II) Catalysed Highly Regioselective C-3 Alkenylation of Indolizines and Pyrrolo[1,2-a]quinolines

Article abstract of DOI:10.1002/ejoc.201901471

Discovered the Ruthenium(II) catalysed highly stereo- and regioselective protocol for the oxidative C-3 alkenylation of indolizines and pyrrolo[1,2-a]quinolines. The methodology represents the first example for the directing group assisted C–C bond formation reaction of the indolizines. Under mild reaction conditions, this method provides an ample substrate scope to produce C-3 alkenyl indolizines in excellent to moderate yields. However, pyrrolo[1,2-a]quinolines underwent alkenynation at elevated temperature to furnish C-3 alkenyl derivatives. The functionalized indolizines were selectively reduced to obtain their saturated derivatives.

Full text of DOI:10.1002/ejoc.201901471

A Journal of
Accepted Article
Title: Ruthenium(II) Catalysed Highly Regioselective C-3 Alkenylation
of Indolizines and Pyrrolo[1,2-a]quinolines
Authors: Pankaj Pandit Jadhav, Nilesh Machhindra Kahar, and Sudam
Ganpat Dawande
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901471
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901471
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10.1002/ejoc.201901471
European Journal of Organic Chemistry
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Ruthenium(II) Catalysed Highly Regioselective C-3
Alkenylation of Indolizines and Pyrrolo[1,2-a]quinolines
Pankaj Pandit Jadhav,[a] Nilesh Machhindra Kahar[a] and Dr. Sudam Ganpat Dawande[a]*
Among azaheteroaromatics, the indoilizines, which are structural
isomers of the indoles as well as their benzo fused derivative
pyrrolo[1,2-a]quinoline are present as principal structural cores in
various natural products,[8] pharmaceutical agents[9] and
Abstract: Discovered the Ruthenium(II) catalysed highly stereo-
and regioselective protocol for the oxidative C-3 alkenylation of
indolizines and pyrrolo[1,2-a]quinolines. The methodology
represents the first example for the directing group assisted the
C-C bond formation reaction of the indolizines. Under mild
reaction conditions, this method provides an ample substrate
scope to produce C-3 alkenyl indolizines in excellent to moderate
functional materials.[10] For instance, curvulamine (a)
and
curindolizine (b) produced by marine fungus Curvularia sp. IFB-
Z10 shows high selectivity towards antibacterial and anti-
inflammatory activity respectively.[8a] The indoxam (c) acts as a
potent sPLA2 inhibitor.[9a, 9b] The derivatives of Seoul fluor (d)
show a highly tunable and predictable emission wavelength.[11]
Similarly, the pyrrolo[1,2-a]quinoline (e) possesses antibacterial
activity[9d] and their hydrogenated derivatives (f) are known for
their antileukemic activity[9e] (Figure 1).
yields.
However,
pyrrolo[1,2-a]quinolines
underwent
alkenynation at elevated temperature to furnish C-3 alkenyl
derivatives. The functionalized indolizines were selectively
reduced to obtain their saturated derivatives.
Consequently, several reports are available for the synthesis of
indolizines and their derivatives which mainly start from
derivatives of pyridine,[12] pyrrole[13] and quinolines.[14] Moreover,
the functionalization of these heteroaromatics has been achieved
through direct electrophilic substitution, organocatalysis,[15] Lewis
acid catalysis,[16] as well as metalation followed by addition to
electrophilic species.[17] In contrast, very few reports are known for
the C-H bond functionalization reactions of indolizines. Most of
these C-C bond formation reactions utilize palladium as a catalyst
Introduction
The directing group assisted transition metal-catalyzed activation
of inert sp2 C-H bonds of aromatic and heteroaromatic compounds
have emerged as a potential tool in syntheses of molecules of
interest.[1] In this, ruthenium(II) catalysed-directing group assisted
C-H bond activation and further functionalization received
enormous attention due to the low catalyst cost, greater chemo-,
and regioselectivity.[2,3] Particularly, in the case of heteroaromatics,
the directing group assisted C-H bond functionalization is
convenient than the direct C-H functionalization due to better site
selectivity as well as atom economy.[4] Thus, these methods have
been extensively utilized in the functionalization of regularly
appearing nitrogen-containing aromatics such as indoles/
pyrroles,[5] pyridine,[6] and quinolines.[7]
which
includes
C-3
arylation,[18a-d]
acylation,
and
carbonylation,[18e,f]
alkynylation,[18g]
annelation,[18h]
homocoupling.[18i] On the other hand, the palladium catalysed
.
Figure 1. Selected examples having indolizines structural cores
[a]
Department of Chemistry, Institute of Chemical Technology,
Nathalal Parekh Marg, Matunga(East) Mumbai-400019,
Maharashtra, India
Scheme 1. Metal catalysed C-H alkenylation of indolizines
E-mail: sg.dawande@ictmumbai.edu.in
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201901471
European Journal of Organic Chemistry
COMMUNICATION
that the N-methylindolizine-2-carboxamide 1a resulted in 56% of
alkenylation reactions are reported only on the indolizines
possessing electron-withdrawing substituent at the C-1 position.
In this the styrene and acrylates give C-3 substituted 1-
arylvinylindolizines and indolizine arylates respectively (Scheme
1a).[19] Recently, our group reported highly stereo-selective C-3
functionalization of indolizines through Rh(II)-aza-vinyl carbene
transfer for the synthesis of their N-sulfonylaminealkenyl
derivatives by denitrogenative decomposition of N-sulfonyl-1,2,3-
triazoles (Scheme 1b).[20] Being an important structural framework
alkenylated product
tert-butyl(E)-3-(2-(methyl carbamoyl)
indolizin-3-yl)acrylate 3a (Table 1, entry 1) whereas, the
remaining C-2 functionalized indolizines were not reactive
towards the alkenylation reaction. When the reaction was carried
out at room temperature for 48 hours the yields were improved to
62 % (Table 1, entry 2). Next, we examined the alkenylation
reaction in presence of methanol, 1,4-dioxane, toluene,
chloroform, tetrahydrofurans (THF) and dimethyl sulfoxide
(DMSO) as solvents at room temperature (Table 1, entries 3-8).
With this study, we found that the reaction proceeds efficiently in
THF giving 90% yield of product 3a. The other solvents like
methanol, 1,4-dioxane, toluene, chloroform, were partially
effective but DMSO was ineffective. Further, we tested the
reaction for the role of additives like NaOAc, AgBF4, KPF6(Table
1, entries 9-11) and oxidants such as AgOAc, Ag2CO3 (Table 1,
entries 12 and 13). The detailed optimization studies revealed that
the best reaction conditions obtained on using 5 mol% [RuCl2(p-
cymene)]2 / 20 mol% of AgSbF6 / Cu(OAc)2.H2O (2 eqiuv) in THF
as solvent at room temperature to give 90% of product 3a.
indolizine requires
more attention towards selective
functionalization. Therefore, we continued our efforts towards the
C-3 alkenylation of indolizines through Ru(II) catalysed directing
group assisted C-H activation.
Results and Discussion
In the initial stage of Ruthenium(II) catalysed alkenylation we
examined the reaction with various C-2 functionalised indolizine
derivatives (0.1 mmol) such as indolizine-2-carboxylic acids, 2-
carboxylates, 2-acyl, 2-carboxamides with acrylate 2a (0.15
mmol) by using 5 mol% [RuCl2(p-cymene)]2 as catalyst, 20 mol%
AgSbF6 as additive and Cu(OAc)2.H2O (2 equiv) as oxidant in 1,2-
dichloroethane (DCE) at 80 ˚C. Under this condition, we found
Table 1. Optimization of reaction conditions for C-3 alkenylation
entry additive
oxidant
solvent
DCE
temperature
time
Hrs.
4
ayield
(%)
56
C
80
1
2
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
DCE
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
48
6
62
65
62
58
90
68
nr
3
MeOH
Dioxane
Toluene
THF
4
12
12
6
5
6
AgSbF6 Cu(OAc)2.H2O
7
AgSbF6
AgSbF6
NaOAc
AgBF4
KPF6
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
AgOAc
Chloroform
DMSO
THF
12
24
6
8
9
60
45
25
32
21
10
11
12
13
THF
6
THF
6
AgSbF6
AgSbF6
THF
6
Ag2CO3
THF
6
aReaction conditions: 0.1 mmol of N-methylindolizine-2-carboxamide 1a, 0.15 mmol of tbutyl acrylate 2a, 5 mol % [RuCl2(p-cymene)]2, additive, 2 equivalent oxidant
solvent (1 mL), bIsolated yields.
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10.1002/ejoc.201901471
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3g with 75-82% yields. While moderate yields were obtained for
alkenylation of N, N-dimethylindolizine-2-carboxamide giving
products 3h-3i in 66-70%. The structure of 3h was confirmed by
X-ray crystallographic analysis (CCDC1889069). The treatment
of hindered carboxamides with sterically crowded acrylates also
delivered products in good yields to give 3j-3k in 80-84%. N-
(benzyloxy)indolizine-2-carboxamide resulted in alkenylation with
cleavage of N-O bond to afford product 3l in 68% yield. Further
reactions of various N-alkyl derivatives of 5-methylindolizine-2-
carboxamide and acrylates showed relatively retarded rates to
give C-3 alkenyl indolizine derivatives 3m-3o in 67-72% yields. In
With the optimal reaction conditions in hand, we examined the
competence and limitations of the alkenylation of indolizine-2-
carboxamides with various acrylates (Table 2). The reaction of N-
methylindolizine-2-carboxamide
with
different
acrylates
proceeded efficiently to give their 3-alkenyl derivatives 3a-3d in
Table 2. Substrate scope and limitations
contrast,
N-alkylated
derivatives
8-methylindolizine-2-
carboxamide reacted with ease to give products 3p-3s with a 76-
88% yield. Subsequently, the treatment of indolizine carboxamide
with methyl vinyl ketone and (vinyl sulfonyl)benzene under similar
reaction condition furnished products 3t and 3u in 68% and 72%
respectively. While reaction with acrylonitrile resulted in product
3v as an isomeric mixture.
After the successful development of the alkenylation of indolizines,
we extended our studies towards the alkenylation of pyrrolo[1,2-
a]quinolines. When reactions were carried out under similar
conditions, the pyrrolo[1,2-a]quinoline-2-carboxamide were found
to be reluctant towards alkenylation resulting in only trace amount
product. With having detailed screening the optimal reaction
conditions obtained involve treatment of pyrrolo[1,2-a]quinoline-
2-carboxamide 4 with acrylate 2 using 5 mol% [RuCl2(p-cymene)]2,
Table 3. Ru(II) catalysed C-3 alkenylation of pyrrolo[1,2-a]quinoline
aReactions were carried out at 120 ˚C on 0.2 mmol of Pyrrolo[1,2-a]quinoline-2-
caboxamide 4, 0.3 mmol of acrylate 2, [RuCl2(p-cymene)]2 (5 mol %), AgSbF6
(20 mol%), Cu(OAc)2.H2O (0.4 mmol) in 2 mL DCE, bIsolated yields.
AgSbF6 (20 mol%) and
Cu(OAc)2.H2O (2 equiv) in 1,2-
dichloroethane as solvent at 120 C to produce C-3 alkenylated
pyrrolo[1,2-a]quinolines (see supporting information). Using this
optimal reaction condition we investigated the versatility of
methodology for different N-alkylated pyrrolo[1,2-a]quinoline-2-
carboxamides and acrylates to obtain products 5a-5f in 60-68%
yield (Table 3). The structure of the product 5a was confirmed by
analysis of 2D NMR spectrum. The observed regioselectivity in
pyrrolo[1,2-a]quinoline is attributed to the electron density of the
pyrrole unit of pyrrolo[1,2-a]quinolines.
The synthetic utility of alkenylation was demonstrated by selective
reduction of product 3a. The reaction of 3a with CoCl2 (1.5 equiv)
and NaBH4 (2 equiv) in ethanol at room temperature produced
product 6 in 71% yield (Scheme 2, a). Similarly, the hydrogenation
of 3a under 1 atmospheric pressure in presence of 10% Pd/C
using methanol as solvent resulted in a partial reduction to give
aReaction conditions: 0.2 mmol of indolizine-2-carboxamide 1, 0.3 mmol of
acrylate 2, [RuCl2(p-cymene)]2 (0.01 mmol), AgSbF6 (0.04 mmol),
Cu(OAc)2.H2O (0.4 mmol), THF (2 mL), bIsolated yields.
80-90% yields. Similarly, N-propylindolizine-2-carboxamide
reacted smoothly at room temperature with acrylates to give 3e-
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10.1002/ejoc.201901471
European Journal of Organic Chemistry
COMMUNICATION
methyl
3-(2-(methylcarbamoyl)-5,6,7,8-tetrahydroindolizin-3-
with stir bar was charged with [Ru(p-cymene)Cl2]2 (0.01 mmol),
N-alkyl indolizine-2-carboxamide 1 (0.20 mmol), acrylate 2 (0.30
mmol), AgSbF6 (0.04 mmol) and Cu(OAc)2·H2O (0.40 mmol). The
reaction tube was evacuated and refilled with nitrogen followed by
the addition of 2 mL anhydrous THF. The reaction mixture allowed
to stir at room temperature for 6-8 hrs. Then diluted with ethyl
acetate and filtered through a celite pad. The filtrate was
concentrated under reduced pressure and the crude product was
purified by silica gel column chromatography to obtain 3a-3v as
white solids.
yl)propanoate 7 in 78% (Scheme 2b).
General procedure for C-3 alkenylation of pyrrolo[1,2-
a]quinoline-2-carboxamide: A oven-dried screw cap reaction
tube equipped with stir bar was charged with [Ru(p-cymene)Cl2]2
(0.01 mmol), N-alkyl pyrrolo[1,2-a]quinoline-2-carboxamide 4
(0.20 mmol), acrylate 2 (0.30 mmol), AgSbF6 (0.04 mmol) and
Cu(OAc)2·H2O (0.40 mmol). The tube was evacuated and refilled
with nitrogen followed by the addition of 2 mL dry DCE. The
reaction mixture stirred at 120 °C for 8-12 hrs. Then the reaction
mixture was cooled to room temperature, diluted with ethyl
acetate and filtered through a celite pad. The filtrate was
concentrated under reduced pressure and the crude product was
purified by silica gel column chromatography to obtain 5a-5f as
white solids.
Scheme 2. Synthetic applications of C-3 alkenyl indolizines
The possible reaction mechanism for highly stereoselective C-3
alkenylation of indolizine-2-carboxamides with acrylates involves
the initial displacement of Cl-ligand by AgSbF6 from Ru(II)
complex to form a cationic Ru (II) species. This coordinates with
amide 1 followed by C-3 metalation to give five-membered
Procedure for synthesis of compound 6: To a oven dried 25
mL round bottom flask containing a solution of NaBH4 (4.00
mmol) and CoCl2 .6H2O (3.0 mmol) in 10 mL ethanol was added
methyl (E)-3-(2-(methyl carbamoyl)indolizin-3-yl)acrylate 3a (2.0
mmol ) at 0 oC under argon. The mixture was allowed to warm to
room temperature and stirred for 3 h. To the reaction mixture, cold
water (2 mL) was added and washed with a saturated NaCl
solution. The collected organic layers were dried over anhydrous
Na2SO4 and the solvents were evaporated under reduced
pressure. The residue was purified by flash column
chromatography using ethyl acetate/petroleum ether as an eluent
obtain product 6 as a white solid in 71% yield.
Procedure for synthesis of compound 7: In an oven-dried 10
mL round bottom flask equipped with stir bar added methyl (E)-3-
(2-(methyl carbamoyl)indolizin-3-yl)acrylate 3a (0.40 mmol) and 3
mL anhydrous methanol followed by 10% Pd/C. The reaction
mixture was stirred at room temperature with an H2 balloon for 10
hours. Then the reaction mixture was diluted with ethyl acetate
and filtered through celite pad and the filtrate was concentrated.
The crude product was purified by silica gel flash column
chromatography using ethyl acetate/petroleum ether to give the
product 7 as a white solid in 78% yield.
Scheme 3. Plausible mechanism of C-3 alkenylation
ruthenacycle intermediate 8. The formation of intermediate 8 was
confirmed with LC-MS analysis (see supporting information). This
on insertion of acrylate leads to intermediate 9 which immediately
undergoes β-hydride elimination affording C-3 alkenylation to give
products 3 and oxidant regenerates the catalytic system for the
next alkenylation cycle (Scheme 3).
Acknowledgments
We are grateful for financial support from the Department of
Science and Technology (DST) for financial support
(EEQ/2016/000609). We also thank Dr. Ajay Verma (IISERB) for
a single crystal analysis.
Conclusions
In summary, we have reported the first method for highly stereo-
and regioselective alkenylation of indolizines and pyrrolo[1,2-
a]quinolines through directing group assisted ruthenium(II)
catalysis. 2-substituted carboxamide functionality served as an
effective directing groups in C-3 alkenylation of various indolizines
and pyrrolo[1,2-a] quinoline derivatives giving excellent to
moderate yields. The C-3 alkenyl indolizines were selectively
reduced to obtain their hydrogenated derivatives.
Keywords: C-H activation • indolizine • alkenylation • ruthenium
• directing group
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This article is protected by copyright. All rights reserved.

10.1002/ejoc.201901471
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
C-H activation
Pankaj P. Jadhav, Nilesh M. Kahar,
Dr.Sudam G. Dawande*
Page 1. – Page 6.
Ruthenium(II) Catalysed Highly
Regioselective C-3 Alkenylation of
Indolizines and Pyrrolo[1,2-
a]quinolines
Developed an efficient method for highly stereo- and regioselective C-3
alkenylation of indolizines and pyrrolo[1,2-a]quinolones through Ru(II)
catalysis. The alkenyl indolizines obtained in excellent to moderate yields. The
synthetic utility of the products demonstrated through selective hydrogenation
reactions.
This article is protected by copyright. All rights reserved.