Anion-cation co-operative catalysis by artificial sweetener saccharine-based ionic liquid for sustainable synthesis of 3,4-dihydropyrano[: C] chromenes, 4,5-dihydropyrano[4,3- b] pyran and tetrahydrobenzo [b] pyrans in aqueous medium

Article abstract of DOI:10.1039/c8ra06889e

In this study, a saccharine-based ionic liquid [Bmim]Sac has been found to be a sustainable catalyst for the synthesis of 3,4-dihydropyrano[c]chromenes, 4,5-dihydropyrano[4,3-b]pyran and tetrahydrobenzo[b]pyrans scaffolds through Domino Knoevenagel-Michae

Full text of DOI:10.1039/c8ra06889e

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Anion–cation co-operative catalysis by artificial
sweetener saccharine-based ionic liquid for
sustainable synthesis of 3,4-dihydropyrano[c]
chromenes, 4,5-dihydropyrano[4,3-b]pyran and
tetrahydrobenzo[b]pyrans in aqueous medium†
Cite this: RSC Adv., 2018, 8, 38974
*
Himani Sharma and Suman Srivastava
In this study, a saccharine-based ionic liquid [Bmim]Sac has been found to be a sustainable catalyst for the
synthesis of 3,4-dihydropyrano[c]chromenes, 4,5-dihydropyrano[4,3-b]pyran and tetrahydrobenzo[b]
pyrans scaffolds through Domino Knoevenagel–Michael reaction. The easy recovery of the catalyst and
high yield of the products make the protocol attractive, sustainable and economical. A mechanistic
hypothesis is discussed using the concept of cooperative catalysis based on the dual (electrophilic/
nucleophilic) activation of reactants by [Bmim]Sac. Furthermore, dual hydrogen bonding of saccharinate
anions plays an important role in the activation of nucleophiles.
Received 17th August 2018
Accepted 5th November 2018
DOI: 10.1039/c8ra06889e
rsc.li/rsc-advances
ammonium acetate,10e visible light,10f thiourea dioxide,10g silica-
graed ionic liquids,10h crown ether complex cation ionic
liquids (CECILs),10i SDS,10j [TETA]TFA,10k and starch solution10l
Introduction
Currently, ionic liquids have attracted signicant attention in
green organic synthesis owing to their unique properties such
as low vapor pressure, wide liquid range, good conductivity and
large electrochemical window.1 In addition to these, the gold
benchmark for green chemistry is functional ionic liquid-
mediated synthesis (FILMs).2 Nowadays, FILMs has become
a novel approach representing an attempt to describe “design
capacity of ionic liquids”, which makes them an accurate
working system rather than simply novel media, and their
properties can be altered to suit the requirement of a particular
process.3 This unique property of the materials obtained by
FILMs gives them ability to serve as catalysts. For example, ILs
with acidic groups have been used in Fischer esterication,
alcohol dehydrodimerization, pinacol rearrangement,4 and
Mannich reactions;5 with basic groups, they have been utilized
in Markovnikov addition,6 Michael addition,7 and absorption of
CO2 and SO2.8
The 3,4-dihydropyrano[c]chromene and tetrahydrobenzo[b]
pyran units are privileged, heterocyclic motifs that form the
core of a large family of natural products with strong bioactivity
proles.9 Multicomponent methods have been reported for the
synthesis of 3,4-dihydropyrano[c]chromenes employing
L-proline–melamine,10a magnetic nanoparticle-tagged ionic liq-
uid,10b SiO2/H3PW12O40 nanohybrid material,10c [DBU][Ac],10d
as catalysts. Some studies on the multicomponent entry to tet-
rahydrobenzo[b]pyran motifs have reported employing H2O/
PEG-400,11a sulfonic acid-functionalized magnetic Fe3ÀxTixO4
nanoparticles,11b
L-tyrosine,11c
Fe3O4@SiO2-imid-PMAn
magnetic nanocatalyst,11d inorganic–organic hybrid magnetic
nanocatalyst11e and magnetite-dihydrogen phosphate11f as
catalysts.
Despite the availability of these methods, ionic liquid-
mediated syntheses of 3,4-dihydropyrano[c]chromenes and
tetrahydrobenzo[b]pyrans are still less explored and there
remains enough scope for an efficient, high yielding, and mild
approach to achieve such systems. With increasing concerns
about environmental protection, synthesis of ILs from non-toxic
materials is desirable. As a part of our attempt to develop
synthesis of biologically important heterocycles12 via green
methodology,13 we herein report a saccharine-based ionic
liquid14-mediated protocol for the synthesis of 3,4-dihydropyr-
ano[c]chromene and tetrahydrobenzo[b]pyran (Fig. 1). The
saccharin group was chosen as it is less toxic than other ionic
liquids.16
Results and discussion
[Bmim]Sac was synthesized by the reported procedure, as
shown in Fig. 2.15 Initially, the reaction between benzaldehyde
(4a), malononitrile (5) and 4-hydroxycoumarin (6) was
employed as the model reaction to screen ILs in water, ethanol
Department of Applied Sciences, National Institute of Technology, Delhi, IAMR
Campus, Sec A-7, Narela, Delhi 110040, India. E-mail: sumanbhu08@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra06889e
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and other common solvents to develop appropriate reaction
conditions.
As evident from the results summarized in Table 1, the
[Bmim]-based ionic liquid with different anions could catalyse
the reaction. However, the reaction of [Bmim]Sac anions pro-
ceeded very well as compared to that with others in neat as well
as in water and afforded the product 7a with moderate to
excellent yield (82% and 95%), respectively. The results are
summarized in Table 1. The use of water as solvent improved
the yield of the desired product slightly and also reduced the
amount of catalyst from 20 mol% to 5 mol% effectively (Table 1,
entries 11 and 14). Higher amount of [Bmim]Sac was needed for
proper mixing of reactant only in the absence of water. In the
presence of imidazole, saccharine and sod saccharinate as
catalysts, no product was observed (Table 1, entries 7, 8, and 9).
The inuence of the reaction temperature and the amount of
the ionic liquid on the catalysis performance was also studied.
The reaction proceeded slowly at room temperature, and the
Fig.
1 3,4-Dihydropyrano[c]chromenes, 4,5-dihydropyrano[4,3-b]
pyran and tetrahydrobenzo[b]pyrans.
ꢀ
reaction yield increased with increasing temperature to 80 C.
To show the merit of our procedure, we have compared our
result for the synthesis of 3,4-dihydropyrano[c]chromenes using
[Bmim]Sac with the result of some other ionic liquids reported
in literature for the same transformation. The results are
summarized in Table 1 (entry 15–21). As can be clearly seen
from Table 1, the best result was obtained at 80 ꢀC in the
Fig. 2 Synthesis of [Bmim]Sac.
Table 1 Optimisation conditions for 3,4-dihydropyrano[c]chromenes
S. no.
Catalyst/ILa
Condition
Time (min)
% yieldc
Ref.
1
2
3
4
5
6
7
8
[Bmim]Br
[Bmim]Cl
80 ꢀC, water
80 ꢀC , water
80 ꢀC, water
80 ꢀC, water
80 ꢀC, water
80 ꢀC, water
80 ꢀC, water
80 ꢀC, water
80 ꢀC, water
75
80
90
56
59
65
70
75
62
NR
NR
NR
NR
82
75
50
95
94
86
95
90
94
93
85
[Bmim]OH
[Bmim]BF4
[Bmim] SO3H
[Bmim]PF6
Imidazole
Saccharine
Sod saccharinate
—
[BMim]Sac
[Bmim]Sac
[Bmim]Sac
[Bmim]Sacb
[Sipim]HSO4
[TETA]TFA
Starch solution
[18-C-6K][OAc]
NH4OAc
120
100
120
24 h
24 h
24 h
24 h
10
45
60
10
30
20
25
15
3
13
120
9
10
11
12
13
14
15
16
17
18
19
20
21
Water, 80 ꢀ
C
80 ꢀC
Ethanol, 80 ꢀ
C
Methanol, 65 ꢀ
C
Water, 80 ꢀC
100 ꢀC, 0.08 mmol
10h
10k
10l
10i
10e
10g
10j
Ethanol–water, reux (5 mol%)
50 ꢀC, 4 ml
EtOH, reux, (30 mol%)
EtOH, reux (15 mol%)
Water, 70 ꢀC (10 mol%)
Water, 60 ꢀC (20 mol%)
Thiourea dioxide
SDS
a
Reaction conditions: benzaldehyde (1 mmol), malononitrile (1 mmol), 4-hydroxy coumarin (1 mmol), water (2 ml), catalyst (20 mol%). b Catalyst
(5 mol%). c Isolated yield.
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presence of 5 mol% of catalyst. Similar optimizations were dicarbonyl compound for the synthesis of 4,5-dihydropyrano
performed for products 7b and 9a; in all cases, 80 ꢀC and [4,3-b]pyran derivatives. As expected, the reaction proceeded
5 mol% of catalyst were the optimum conditions.
smoothly, giving the corresponding products in good to excel-
Using these optimized conditions and to show the generality lent yields with aliphatic as well as aromatic aldehydes (Table 2 ,
and scope of this methodology, reactions were explored for the entry 11–14).
synthesis of a wide variety of 3,4-dihydropyrano[c]chromene
A mechanism for the probable sequence of events is given in
and tetrahydrobenzo[b]pyran derivatives using aldehydes, Fig. 3. The reaction proceeds via three steps: Knoevenagel
malononitriles and different 1,3-dicarbonyl compounds (4- condensation, Michael addition, and then intramolecular
hydroxy
coumarin,
5,5-dimethyl-1,3cyclohexanedione/1,3- cyclization, as presented in Fig. 3. The Knoevenagel adduct
cyclohexanedione and 1,3-cyclohexanedione, respectively) in formed from the ionic liquid-catalyzed condensation of alde-
the presence of [Bmim]Sac (5 mol%) in an aqueous medium hydes and malononitrile subsequently undergoes Michael
under reux conditions. The results have been summarized in reaction with carbonyl compounds possessing a reactive
Tables 2 and 3. Indeed, there is no difference in reactivities methylene group (4-hydroxycoumarin, 4-hydroxy-6-methyl-2-
among
cyclohexanedione,
5,5-dimethyl-1,3cyclohexanedione/1,3- pyrone, 5,5-dimethyl-1,3cyclohexanedione and 1,3-cyclo-
1,3-cyclohexanedione and 4-hydrox- hexanedione); aer cyclization, it affords pyran annulated
ycoumarin. The effect of electron-withdrawing substituents, heterocyclic systems.
electron-releasing substituents and halogens of the aromatic
Bmim cations of ionic liquids activate electrophiles by the
ring of aldehydes on the reaction results was investigated. The proton in the 2-position of the imidazolium ring through
reaction time of aromatic aldehydes having electron- hydrogen-bond interaction with the carbonyl and nitrile groups
withdrawing substituents and halogens produced higher yield of aldehyde and malononitrile. Simultaneously, anions of ionic
of products and faster reactions than that observed for their liquids activate nucleophiles by accepting the hydrogen bond.
electron-rich counterparts (Table 2, entry 2).
The dual activation of nucleophiles and electrophiles by the
The attempt to synthesise 3,4-dihydropyrano[c]chromene cations and anions of ionic liquids is crucial to promote the
and tetrahydrobenzo[b]pyran derivatives using aliphatic alde- reaction in high yields. As can be seen in Fig. 3, saccharinate
hyde (n-butyraldehyde) was successful, and the results are anions also play an important role in the dual activation of 1,3-
summarized in Table 2 (entry 10) and Table 3 (entry 24 and 25). dicarbonyl intermediate nucleophile. It is proposed that an
To expand the scope of the present catalytic system, we used “electrophile nucleophile dual activation” phenomenon of
substrate 4-hydroxy-6-methyl-2-pyrone as the cyclic 1,3- [Bmim]Sac through “dual hydrogen bond formation by
Table 2 Synthesis of 3,4-dihydropyrano[c]chromenes
S. no.
Ra
Product
Time (min)
Yieldb
Melting point
Literature melting point18
1
2
3
4
5
6
7
8
Ph
7a
7b
7c
7d
7e
7f
7g
7h
7i
10
50
35
75
45
45
30
40
70
50
45
40
15
45
95
85
90
81
85
84
93
89
70
85
87
90
93
88
262–263
252–253
264–265
266–267
265–267
257–258
255–256
257–258
226–230
195–200
239–242
218–220
236–238
222–224
260–261
250–251
261–263
267–269
266–268
258–259 (ref. 10d)
250–251
4-MeOC6H4
4-NO2C6H4
4-OHC6H4
4-ClC6H4
4-FC6H4
3-NO2C6H4
4-BrC6H4
2-C5H4OS
CH3(CH2)2
4-BrC6H4
CH3(CH2)2
Ph
255–258
9
228–230 (ref. 19)
193–195 (ref. 17)
240–242 (ref. 11d)
220–222 (ref. 10i)
10
11
12
13
14
7j
11a
11b
11c
11d
4-MeOC6H4
223–225 (ref. 10m)
a
Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), 4-hydroxy coumarin/4-hydroxy-6-methyl-2-pyrone (1 mmol), water (2 ml),
[Bmim]Sac (5 mol%). b Isolated yield.
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Table 3 Synthesis of tetrahydrobenzo[b]pyrans
S. no.
Ra
Product
Time (min)
Yieldb
Melting point
Literature melting point18
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Ph
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
9k
12a
12b
12c
12d
12e
12f
12g
10
25
20
75
45
25
15
35
60
45
60
40
15
40
30
30
25
30
96
82
91
80
88
92
94
90
88
89
80
90
92
89
86
90
92
93
238–240
201–203
179–181
269–270
212–213
195–197
209–211
200–201
226–228
193–194
200–202
223–225
219–221
196–200
189–191
228–230
200–205
198–201
227–228
194–196
178–180
265–266
207–209
191–193
208–211
196–198
230–231 (ref. 19)
192–193 (ref. 11h)
205–207 (ref. 11h)
224–226 (ref. 11g)
220–222 (ref. 10k)
—
186–189 (ref. 10k)
—
—
4-MeOC6H4
4-NO2C6H4
4-OHC6H4
4-ClC6H4
4-FC6H4
3-NO2C6H4
4-BrC6H4
2-C5H4OS
CH3(CH2)2
C6H5CH]CH
4-ClC6H4
Ph
4-BrC6H4
4-MeOC6H4
4-MeC6H4
CH3(CH2)2
4-FC6H4
—
a
Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), 5,5-dimethyl-1,3cyclohexanedione/1,3-cyclohexanedione (1 mmol), water (2 ml)
[Bmim]Sac (5 mol%). b Isolated yield.
saccharinate anions and charge–charge interactions” occurs
(Fig. 3).16
The reusability of ionic liquid [Bmim]Sac was also inves-
tigated using the reaction between benzaldehyde,
Fig. 4 Reusability of ionic liquid [Bmim]Sac.
malononitrile and 4-hydroxycoumarin as a model system.
Ionic liquid can be recovered from the reaction system and it
is interesting to note that the recovered IL was reused for ve
successive batches of reactions to afford pure products aer
crystallization (Fig. 4). Therefore, it can be concluded that
this catalytic system has great potential in industrial appli-
cations (Fig. 4).
Conclusions
We have introduced a green domino Knoevenagel–Michael
multicomponent reaction procedure for novel and highly effi-
Fig. 3 Proposed mechanism for dual activation of IL.
cient synthesis of 3,4-dihydropyrano[c]chromene and
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tetrahydrobenzo[b]pyran derivatives in the presence of [Bmim]
Sac as a non-toxic and green ionic liquid in aqueous media. This
procedure also offers other signicant advantages including
simple operation, excellent yield, short reaction time, atom
economy, scaling up to multigram quantities, and ease of
separation. Also, the catalyst can be easily recovered and reused
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
H. S. and S. S. are thankful to DST for the nancial assistance.
SAIF-CDRI and IIT Mandi were acknowledged for providing the
spectral analytical data.
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