Lithium tetrafluoroborate catalyzed highly efficient inter- and intramolecular aza-Michael addition with aromatic amines

Article abstract of DOI:10.1016/j.crci.2011.09.006

Lithium tetrafluoroborate has been demonstrated for the first time to be an efficient catalyst in intermolecular aza-Michael addition aromatic amines to electron deficient alkenes. Suitability of the same catalyst in intramolecular aza-Michael addition leading 2-aryl-2,3-dihydroquinolin-4(1H) ones has also been described.

Full text of DOI:10.1016/j.crci.2011.09.006

C. R. Chimie 14 (2011) 1059–1064
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Preliminary communication/Communication
Lithium tetrafluoroborate catalyzed highly efficient inter- and
intramolecular aza-Michael addition with aromatic amines
b
U.P. Lad a, M.A. Kulkarni a, U.V. Desai a, , P.P. Wadgaonkar
*
a Department of Chemistry, Shivaji University, Kolhapur 416004, India
b Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Lithium tetrafluoroborate has been demonstrated for the first time to be an efficient
catalyst in intermolecular aza-Michael addition aromatic amines to electron deficient
alkenes. Suitability of the same catalyst in intramolecular aza-Michael addition leading 2-
aryl-2,3-dihydroquinolin-4(1H) ones has also been described.
Received 24 May 2011
Accepted after revision 15 September 2011
Available online 4 November 2011
ß 2011 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
Keywords:
Michael addition
Anilines
Lithium tetrafluoroborate
2-aryl-2,3-dihydroquinolin-(4H)-1- ones
1. Introduction
are well suited to primary aliphatic as well as secondary
amines; however, aromatic amines being weakly nucleo-
Michael addition of heteroatom nucleophiles to conju-
gated alkenes constitutes an important strategy in carbon-
heteroatom bond forming reactions [1]. It opens an easy
philic, fail to undergo the Michael addition reaction.
It is well known that, nucleophilicity of aromatic
amines is highly solvent dependent [6] and they are twice
more nucleophilic in aqueous medium [7]. Thus, Michael
addition of anilines was explored earlier in aqueous
medium [8a–g]. These water-mediated protocols often
require longer reaction time, thermal or MW activation
and furnish the desired products in poor to moderate
yields. To alleviate these problems, in recent years Michael
addition of anilines has also been investigated using YNO3,
SiO2.AlCl3, SiCl4, RuCl3 / PEG, I2, etc. as catalysts [9a–f]. It is
worth mentioning that, despite remarkable success of
these protocols, their scope has been limitedly explored for
intermolecular aza-Michael addition reaction. Conse-
quently, the development of a versatile and an efficient
protocol applicable to inter- as well as the intramolecular
Michael addition of anilines, leading preferably to biologi-
cally active compounds is highly desirable. In continuation
with our ongoing studies on the development of new
synthetic methodologies [10], herein we report our studies
on aza-Michael reaction with special reference to aromatic
amines as Michael donors (Scheme 1).
route for the pivotal synthetic intermediates like b-amino
carbonyls, esters, nitriles and amides which find applica-
tions in the synthesis of natural products, chiral auxiliaries,
bioactive compounds, pharmaceuticals, fine chemicals, etc.
[2]. Classical Mannich reaction of enolates with imines is a
useful alternative for the introduction of such functionali-
ty; however, its limited scope coupled with harsh reaction
conditions makes it a less attractive route in the synthesis
of complex molecules [3,4]. In contrast, the addition of
nitrogenous nucleophiles to electron deficient alkenes
(aza-Michael reaction) has attracted much attention from
organic chemists principally due to its operational
simplicity and high atom economy. Over the past few
years, a good number of protocols have been reported for
this important reaction [5a–h]. These efficient protocols
* Corresponding author.
E-mail address: uprabhu_desai@rediffmail.com (U.V. Desai).
1631-0748/$ – see front matter ß 2011 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
doi:10.1016/j.crci.2011.09.006

1060
U.P. Lad et al. / C. R. Chimie 14 (2011) 1059–1064
H
N
R2
R2
NH2
+
LiBF4
Neat
EWG
EWG
R1
R1
1
2
3
O
O
LiBF4
R3
reflux
CH3CN,
NH2
N
H
R3
4
5
R1= Me, OMe, Br, NO2
R2= H/Me
EWG = CN, COOMe, CONH2
R3= Ar/Sub. Ar
Scheme 1.
2. Results and discussion
electrophilicity to facilitate the addition of nitrogenous
nucleophiles [9a].
A systematic study was undertaken to develop the most
suitable conditions for the reaction between aniline and
methyl vinyl ketone as model substrates. Accordingly, an
equimolar mixture of aniline and methyl vinyl ketone was
stirred together at ambient temperature under solvent-
free condition. The reaction was exorgenic and furnished
Lithium tetrafluoroborate is a well-known mild Lewis
acid catalyst available commercially and unlike lithium
perchlorate it is non-explosive, non-oxidizing as well as a
non-nucleophilic agent. It acts as a slow release source of
boron trifluoride and provides a convenient route to effect
organic transformations under neutral conditions [11]. It
the desired
b
-amino ketone in excellent yield, in a short
has been used earlier by us in the synthesis of a-
time (30 min, 93%, GCMS).1 To demonstrate the generality
of the reaction conditions, various ring-substituted ani-
lines were initially reacted with methyl vinyl ketone and,
having obtained the satisfactory results, the studies were
extended to other conjugated ketones. It is worth
mentioning that the reactions of anilines with ethyl vinyl
ketone were very smooth however; under the established
reaction condition their reaction with cyclohexenone was
unprecedented2 while chalcone furnished corresponding
aminonitriles [10c] and by others in the synthesis of
homoallylic acetates [12a], in intramolecular Diels alder
reactions [12b], etc. A literature survey revealed that,
although a variety of Lewis acids have been used earlier in
Michael addition of aniline to conjugated alkenes, to the
best of our knowledge, there are no reports on the use of
lithium tetrafluoroborate for either the inter- or intramo-
lecular aza-Michael reaction. With reference to our earlier
experience [10c], we planned to explore the suitability of
LiBF4 in Michael addition of anilines to electron deficient
alkenes.
During the initial exploratory reactions, addition of
aniline to methyl acrylate was studied as a model reaction
using LiBF4 (10 mol %) as catalyst. The reaction was too
sluggish at room temperature but it went to completion in
acceptable time (30 min) on heating the reaction mixture
up to 70 8C under solvent-free condition. The studies on the
effect of catalyst loading as well as change in the reaction
temperature did not exhibit any appreciable change in the
yield of desired Michael addition product. Thus, under the
established reaction conditions for the aniline in hand,
various substituted anilines were screened for their
addition to methyl acrylate and the protocol was then
extended towards other Michael acceptors viz. methyl
methacrylate, acrylonitrile as well as acrylamide. In
extending the scope of the protocol, it was observed that
aniline can also undergo addition to poorly electrophilic
chalcone under the same reaction condition (entry 14,
Table 1) This initial success to intermolecular aza-Michael
addition reaction prompted us to explore the suitability of
LiBF4 in intramolecular aza-Michael addition using 2’-
aminochalcone as a model substrate. This is because, the
b-amino ketone in fair yield. In examining the scope of the
protocol, it was further noticed that anilines failed to
undergo addition to other Michael acceptors such as
methyl acrylate, acrylonitrile as well as acrylamide. These
observations clearly project the essentiality of an appro-
priate activator viz. energy source, solvent or catalyst for
successful aza-Michael reaction with wide scope.
From a mechanistic viewpoint, the success of the aza-
Michael reaction relies either on the increase in nucleo-
philicity of anilines or the electrophilicity of conjugated
alkenes, and the use of a Lewis acid catalyst is one of the
keys in performing the aza-Michael reaction efficiently. It
is well accepted that the metal ion from Lewis acid forms a
strong coordinate bond with the electron withdrawing
groups in conjugated alkenes and thereby increase their
1
The reaction between alkyl vinyl ketone with anilines is exothermic
and we suppose that the heat liberated during the reaction is one of the
driving forces of the reaction. In support of this supposition, computa-
tional studies on the calculation of Gibbs free energy as well as heat of
formation of the products is currently on and these results will be
communicated separately.
2
The resultant products decompose upon standing.

U.P. Lad et al. / C. R. Chimie 14 (2011) 1059–1064
1061
Table 1
Lithium tetrafluoroborate catalyzed Michael addition of anilines to conjugated alkenes.
Entry
M. Donor (1)
M. acceptor (2)
Catalyst (mol %)
–
Temp (8C)
Time (min)
Yield (%)a
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
RT
30
30
40
30
30
50
93
92
90
91
87
67
O
2a
7
1a
1b
1c
1d
–
RT
30
30
30
90
85
93
84
8
O
9
10
2b
11
12
13
1a
1b
1c
–
RT
20
25
25
85
81
82
O
2c
14
1a
10
10
RT
70
30
30
20
55
O
Ph
Ph
2d
15
16
17
18
19
1a
1b
1c
1d
1e
70
15
25
10
20
10
90
95
93
91
90
O
MeO
2e
20
1a
10
70
20
87
O
MeO
2f
21
22
23
1a
1c
1d
10
10
70
70
20
15
25
94
95
85
NC
2g
24
25
26
1b
1c
1e
30
30
10
94
95
85
O
H2N
2h
a
Isolated yields; all compounds gave satisfactory spectroscopic data; 1a: Aniline; 1b: 4-Methyl aniline; 1c: 4-Methoxy aniline; 1d: 4-Chloro aniline; 1e:
4-bromoaniline; 1f: 3-nitro aniline.
applicability of the resultant 2-aryl-2,3-dihydroquinoline-
4-(1H) one, as a valuable precursor in the synthesis of
medicinally important compounds is well documented
[13,14].
To test the feasibility of the protocol, to a stirred
solution of 2-amino chalcone, 4a, in acetonitrile was added
LiBF4 (10 mole %) and stirring was continued at room
temperature. Timely analysis of the reaction mixture
(comparison with authentic sample) revealed the forma-
tion of desired quinolones (20%, tlc, 4 h). However, the
reaction furnished the desired dihydroquinolone, 5a on
heating the reaction mixture (70 8C) for a short time
(20 min). Without modifying the reaction conditions, the
scope of the protocol was examined using a variety of

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U.P. Lad et al. / C. R. Chimie 14 (2011) 1059–1064
Table 2
Lithium tetrafluoroborate catalyzed synthesis of 2-aryl-1,2,3,4-tetrahydroquinolin-4-(1H)-ones
O
O
LiBF4
CH3CN, Reflux
NH2 R3
N
H
R3
No.
a
Product (5)
Time (min)
20
Yield (%)
98
References
[15c]
[15e]
O
N
H
b
10
10
95
95
[15c]
[15e]
O
N
H
OMe
c
[15c]
[15e]
O
N
H
Me
d
15
90
–
O
N
H
e
20
20
91
94
[15c]
[15e]
O
N
H
Cl
f
[15c]
O
OMe
OMe
N
H
g
h
i
20
10
20
86
95
93
[15c]
O
NO2
N
H
–
–
O
N
H
O
O
N
H
Yields refer to pure isolated products; b: all the compounds gave satisfactory spectral analysis.

U.P. Lad et al. / C. R. Chimie 14 (2011) 1059–1064
1063
2-aminochalcones. In all the studied cases, corresponding
4.2. Spectral data of representative compounds
2-aryl-2,3-dihydroquinoline-4(1H)-one was obtained in
excellent yield and purity (Table 2). There are only a few
reports on the synthesis of such quinolones [15]. However,
the protocol developed by us is operationally simple and
high yielding.
4.2.1. 2-[4-(Propan-2-yl phenyl)]-2,3-dihydroquinolin-
4(1H)-one, 5d
1H NMR (300 MHz, CDCl3,):
d 1.27 (d, 6H, J = 6.9 Hz, 2 x
CH3), 2.73–2.98 (m, 2H, CH2), 4.52 (brs, 1H, NH), 4.73 (dd,
1H, J = 13.8 & 3.9 Hz, CH), 6.68–7.40 (m, 7H, ArH), 7.88 (dd,
1H, J = 7.8 & 1.4 Hz, ArH); 13C NMR (75 MHz, CDCl3,):
d
3. Conclusion
23.97, 33.86, 46.40, 58.22, 115.87, 118.34, 118.99, 126.64,
127.01, 127.61, 135.37, 138.33, 149.30, 151.61,
193.52 ppm; ESIMS: m/z = 266 (M + H).
In conclusion, we have demonstrated for the first time
the use of LiBF4 as a highly efficient catalyst in promoting
inter- as well as intramolecular aza-Michael addition of
aromatic amines to a range of Michael acceptors.
4.2.2. 2-(2,6-Dimethylphenyl)-2,3-dihydroquinolin-4(1H)-
one,5h
1H NMR (300 MHz, CDCl3):
d 2.5–2.62 (m, 7H, 2 x CH3 &
1H from CH2), 3.19–3.30 (m, 1H, 1H from CH2), 4.52 (brs,
1H, NH), 4.73 (dd, 1H, J = 15.6 & 3.6 Hz, CH), 6.70–7.37 (m,
6H, ArH), 7.92 (dd, 1H, J = 7.8 & 1.4 Hz, ArH); 13C NMR
4. Experimental
(75 MHz, CDCl3):
118.65, 127.88,127.98, 129.72, 129.91, 135.28, 135.36,
137.02, 151.93, 193.84 ppm; ESIMS: m/z = 252 (M + H).
d 20.50, 21.42, 54.16, 115.95, 118.02,
Various aromatic amines, Michael acceptors and LiBF4
(Aldrich/Lancaster) were used as received while various 2’-
amino chalcones were prepared by potassium phosphate
catalyzed condensation of 2-amino acetophenone with a
range of aldehydes. IR spectra were recorded as neat or as
KBr disc on Perkin-Elmer [FT-IR-783] spectrometer. 1H
(300 MHz) and 13C (75.4 MHz) NMR spectra were recorded
on Bruker-AC-300 spectrometer as CDCl3 solutions of
4.2.3. 2-Furanyl-2,3-dihydroquinolin-4(1H)-one, 5i
1H NMR (300 MHz, CDCl3):
d 2.92 (dd, 1H, J = 16.1 &
5.4 Hz, CH), 3.00 (dd, 1H, J = 16.3 & 9.8 Hz, CH), 4.79–4.84
(m, 2H, CH & NH), 6.25 (d, 1H, J = 3.2 Hz, ArH), 6.32 (m, 2H,
ArH), 7.37–7.58 (brs, 3H, ArH), 7.84 (dd, 1H, J = 7.9 & 1.2 Hz,
ArH); 13C NMR (75 MHz, CDCl3):
samples using TMS as internal standard and
expressed in ppm.
d values are
d 41.9, 50.7, 106.8, 110.3,
116.0, 118.5, 119.1, 127.3, 135.4, 142.4, 150.4, 153.3,
192.6 ppm; ESIMS: m/z = 214 (M + H).
4.1 General experimental procedure
4.2.4. 2-(3,4-dimethoxyphenyl)-2,3-dihydroquinolin-4(1H)-
4.1.1. Intermolecular aza-Michael reaction
one,5f
To the stirred solution of aniline (2 mmol) and Michael
acceptor (except conjugated ketone) (2 mmol) was added
lithium tetrafluoroborate (10 mol %) and the mixture was
stirred at appropriate temperature (Table 1) till comple-
tion of the reaction (TLC). Water (10 mL) was added and
the product was extracted with ethyl acetate (3 Â 15 mL).
The combined organic extract was washed with water,
dried (anhy. Na2SO4) and solvent removed. Purification of
the resultant product by short column chromatography
over silica gel (60–120 mesh) furnished the desired
Michael addition product.
1H NMR (300 MHz, CDCl3):
d
2.87 (m, 2H, CH2), 3.86 (s,
6H, 2 x OCH3), 4.48 (brs, 1H, NH), 4.78 (dd, 1H, J = 13.2 &
4.0 Hz, CH), 6.70–7.33 (m, 6H, ArH), 7.85 (dd, 1H, ArH). 13
NMR (75 MHz, CDCl3): 46.7, 56.02, 58.4, 56.06, 109.5,
111.3, 116.0, 118.5, 119.02, 119.0, 127.6, 133.6, 149.1,
149.3, 151.7, 135.5, 193.5 ppm.
C
d
Acknowledgments
Authors (UVD and MAK) thank UGC, New Delhi for
financial assistance. We are also thankful to DST and UGC,
New Delhi for providing NMR and elemental analysis
facilities to the Chemistry Department of Shivaji Universi-
ty, Kolhapur under FIST and SAP program.
4.1.2. Intramolecular aza-Michael reaction
A mixture of 2’-amino chalcone (2 mmol) and lithium
tetrafluoroborate (10 mol %) was stirred at 70 8C until
completion of the reaction (TLC). Water (10 mL) was
added and the product was extracted with ethyl
acetate (3 Â 15 mL). The combined organic extract was
washed with water, dried (anhy. Na2SO4) and solvent
removed. Purification of the resultant product by short
column chromatography over silica gel (60–120 mesh)
furnished the desired 2-aryl-2,3-dihydroquinolin-4-
(1H)-one.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.crci.2011.09.006.
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