Triethylammonium acetate [TEAA]: An efficient catalyst for one pot synthesis of tetrahydro-4H-chromene derivatives

Article abstract of DOI:10.2174/157017811795371430

An efficient and convenient approach towards the synthesis of series of tetrahydro-4H-chromenes via one pot three component coupling reactions of aldehyde, dimedone and malononitrile carried out at room temperature in presence of triethylammonium acetate

Full text of DOI:10.2174/157017811795371430

282
Letters in Organic Chemistry, 2011, 8, 282-286
Triethylammonium Acetate [TEAA]: An Efficient Catalyst for One Pot
Synthesis of Tetrahydro-4H-chromene Derivatives
Ravi Balaskara, Sandip Gavadea, Madhav Manea, Pramod Pabrekarc, Murlidhar Shingareb,
Dhananjay Mane*,a
aOrganic Research Laboratory, S.C.S. College, Omerga-413 606,( MS), India
bDepartment of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad-431 004 (MS), India
cV.G. Vaze College, Mulund, Mumbai
Received July 02, 2010: Revised August 22, 2010: Accepted February 10, 2011
Abstract: An efficient and convenient approach towards the synthesis of series of tetrahydro-4H-chromenes via one pot
three component coupling reactions of aldehyde, dimedone and malononitrile carried out at room temperature in presence
of triethylammonium acetate [TEAA] ionic liquid as a catalyst. Reusability of the catalyst with optimized reaction
conditions has also been investigated. The present methodology was found to be economically feasible and
environmentally benign.
Keywords: Triethylammonium acetate (TEAA), tetrahydro-4H-chromenes, room temperature.
INTRODUCTION
Tetrahydrobenzo[b]pyrans are an important class of
heterocyclic scaffolds in the field of drugs and
pharmaceutical. These compounds are widely used as
anticoagulant, diuretic, spasmolytic, anticancer and
antianaphylactic agents [22,23]. Also, they can be used as
cognitive enhancers for the treatment of neurodegenerative
disease including Alzheimer’s disease, amyotprophic lateral
sclerosis, Huntington’s disease, Parkinson’s disease, AIDS
associated dementia and Down’s syndrome, as well as for
the treatment of schizophrenia and myoclonus [24]. Other
than their biological importance, some 2-amino-4H-pyrans
have been widely used as photoactive materials [25]. The
conventional reported methods for synthesis of 4H-
chromenes involve the use of organic solvents such as
dimethylsulfoxide[26] and dimethylformamide[27].
Room temperature ionic liquids (RTILs) emerging as
greener alternative reaction media as well as catalyst towards
conventional organic solvents which is an important part of
today’s drive towards sustainable chemistry [1]. Ionic liquids
(ILs) were introduced because of their unique chemical and
physical properties of non-volatility, non-flammability,
thermal stability, and ease of recyclability [2]. Recently,
wide range of organic transformations has been carried out
in ionic liquids such as Baylis-Hillman [3], Beckmann
rearrangement [4], Hantzsch Polyhydroquinoline [5], Prins
reaction [6], aldol reactions [7], Esterification and
Transesterification [8], Diels–Alder reactions [9], Heck
Reaction [10], Suzuki cross-couplings [11], Friedel–Crafts
acylation [12] etc.
Recently,
synthesis
of
tetrahydrobenzo[b]pyran
In past several years a great deal of attention has been
given towards imidazolium based ionic liquids viz [bmim]Br
[13], [bmim]BF4 [14], [bmim]NTf2 [15], [hmim]PF6 [16],
[mim-tOH][OMs] [17] [Hbim]BF4 [18]. Although as a
unique advantage of ionic liquids as reaction media and
catalyst they cannot be applied in industries probably due to
high cost of ionic liquids, and their difficulty in separation or
recycling. Therefore, cost-effective and environmentally
practicable ionic liquids are gaining significant importance.
derivatives has been reported by using various catalysts like
LiBr [28], MgO [29], PEG1000-DAIL [30], K3PO4 [31], D,L-
proline,[32] (S)-proline,[33] hexadecyltrimethyl ammonium
bromide (HTMAB),[34] 4-dodecylbenzenesulfonic acid
(DBSA),[35] and (NH4)2HPO4.[36], sodium selenate [37],
tetra-methyl ammonium hydroxide[38] and amberlite IRA-
400 (OH-) [39]. Each of the above method has its own merit
with at least one of the limitations of low yields, such as
commercial unavailability of catalyst, long reaction times,
harsh reaction conditions and tedious work-up procedures.
Hence, improved method for multicomponent synthesis of
tetrahydro-4H-chromenes using inexpensive and mild
reagents coupled with simple reaction conditions and easier
work-up procedures is required.
Application of ammonium ionic liquids have drawn
much attention in the recent time for their use in organic
transformations [19, 20, 21]. In this report we had made a
successive attempt toward greener protocol by the
implementation of triethylammonium acetate [TEAA] ionic
liquid as highly mild, efficient and reusable catalyst for the
synthesis
of
tetrahydrobenzo[b]pyran
derivatives.
RESULT AND DISCUSSIONS
As part of our ongoing research[40-44] work to reduce
toxic waste and byproducts arising from chemical processes
by using mild, cheap and environmentally compatible
*Address correspondence to this author at the Organic Research Laboratory,
S.C.S. College, Omerga-413 606, (MS), India; Tel: +91 2475 252880; Fax:
+91 240 2403134; E-mail: dr_dvmane@rediffmail.com
1570-1786/11 $58.00+.00
© 2011 Bentham Science Publishers Ltd.

Triethylammonium Acetate [TEAA]
Letters in Organic Chemistry, 2011, Vol. 8, No. 4
283
R1
O
O
O
CHO
CN
CN
[TEAA]
CN
R1
Stirring / r.t.
O
NH2
3
4(a-n)
1(a-n)
2
Scheme 1. Synthesis of Tetrahydro-4H-chromene derivatives (4a-4n).
materials in the design of new synthetic methods, we have
implemented triethylammonium acetate (TEAA) as catalyst
and greener reaction media for the one pot three-component
synthesis of tetrahydro-4H-chromene derivatives (Scheme
1).
amount of catalyst did not show any improvement in the
reaction rate and yield of the product. Finally, 40 mol% of
the catalyst showed the better optimistic condition to forward
the reaction.
In order to explore the scope of the methodology a wide
range of structurally diverse and functionalized aromatic and
heteroaromatic aldehydes were examined, which underwent
cyclocondensation with excellent yields (Table 2, entries 4a-
4n). Various aldehyde bearing electron-withdrawing groups
(-Cl, -Br, -NO2, -F) and electron-donating groups (-OCH3, -
CH3, -OH) were employed and it is indeed gratifying to note
Initially, choosing an appropriate solvent is of crucial
importance for the successful organic synthesis. Therefore,
screening of solvents in three component reaction of 4-Cl-
benzaldehyde (1 mmol), malononitrile (1 mmol), dimedone
(1 mmol) in the presence of triethylammonium acetate (40
mol%) as a model reaction was investigated for the synthesis
Table 1. Solvent Effect on the Synthesis of Tetrahydrobenzo[b]pyran Using [TEAA] as Catalysta
Entry
Solvent + Catalyst
Time
Yield (%)b
1
2
3
4
5
6
H2O
: [TEAA]
: [TEAA]
: [TEAA]
4 h
3 h
28
52
64
76
82
97
DCM
THF
3 h
CH3CN : [TEAA]
1 h
EtOH
: [TEAA]
1 h
[TEAA]
20 min
aReaction condition: 4-Cl-benzaldehyde (1mmol), malononitrile (1mmol), dimedone (1mmol), Solvent (5 mL) with triethylammonium acetate (40 mol%) was stirred at r.t., bYields
were isolated.
of 2-Amino-7,7-dimethyl-5-oxo-4-(4-chlorophenyl)-5,6,7,8-
tetrahydro-4H-chromene-3-carbonitrile (Table 1). In each
case, the substrate was mixed together with TEAA (40
mol%) agitated with 5 mL of solvent. When H2O, DCM and
THF was used as solvent there is a retard in the rate of
reaction which ultimately resulted into decrease in the
product yield (Table 1, entry 1, 2 and 3). The results
obtained for CH3CN and EtOH were also not satisfactory.
After complete optimization, TEAA was found to be
appropriate reaction media and catalyst for this reaction
(Table 1, entry 4 and 5). There is dramatic acceleration in the
rate of the reaction and the yields were also found to be
improved as we introduce triethylammonium acetate as
reaction media and catalyst (Table 1, entry 6). Therefore,
[TEAA] was found to be a better catalyst and a reaction
media for proceeding the reaction with improved yield.
that substituents in the aromatic ring of aldehydes did not
show any obvious effect on the rate of the reaction and yield
of the product. The heteroaryl aldehydes, such as 2-
thiophenealdehyde and 2-furaldehyde reacted very smoothly
to obtain the corresponding derivatives in good yields.
Optimization of catalyst
150
120
97%
97%
96%
100
50
0
87%
35
72%
45
67%
58
22
50
20
40
20
60
0%
0
10
20
30
Mol (%)
Time (min)
Yield (%)
Fig. (1). Optimization of quantity of catalyst considering (Table 2,
entry 4f) as model reaction.
After brief screening of solvent we also optimized the
quantity of catalyst required for reaction to proceed further.
From (Fig. (1)) it is noteworthy that without any solvent and
catalyst the reaction did not proceed further. As the amount
of [TEAA] was increased from 10 mol% to 40 mol% there
was dramatically continuous improvement in the reaction
with respect to time and yield of the product. Increasing the
The recovery and reuse of solvent and catalyst are highly
preferable in terms of green synthetic process. Therefore,
with the success of these above reactions, we continued our
research by studying reusability and recycling of TEAA.

284 Letters in Organic Chemistry, 2011, Vol. 8, No. 4
Balaskar et al.
Table 2. Characterization of Tetrahydrobenzo[b]pyran Derivatives†
Entry
R1
Time (min.)
Yielda(%)
M.P.b (°C)
ReportedLit
4a
4b
4c
4d
4e
4f
C6H5
4-Cl-C6H4
10
20
15
25
20
15
25
35
30
40
35
45
30
35
95
97
95
94
96
95
94
92
93
89
91
88
93
92
227-229
208-210
230-232
180-182
200-202
195-197
193-195
218-220
207-209
237-239
197-199
230-232
211-213
197-199
228-230 [45]
207-209 [45]
230-232 [45]
177-178 [45]
196-198 [45]
192-194 [46]
190-193 [47]
220-222 [45]
206-208 [45]
236-238 [46]
200-202 [46]
228-230 [45]
210-212 [48]
230-223 [47]
3-Cl-C6H4
4-NO2-C6H4
4-Br-C6H4
2,4-Cl2-C6H3
4-F-C6H4
4g
4h
4i
4-Me-C6H4
4-OH-C6H4
3-OH-C6H4
4-OMe-C6H4
3-OH-4-OMe-C6H3
2-Thionyl
4j
4k
4l
4m
4n
2-Furyl
†Reaction condition: aldehydes (1 mmol), malononitrile (1 mmol) and dimedone (5,5-dimethylcyclohexane-1,3-dione) (1 mmol) in the presence of (TEAA) triethylammonium acetate
(40 mol%) stirred at room temperature. aIsolated yields of product are obtained before crystallization and bMelting points were compared with authentic samples and literature data.
After completion of reaction, the aqueous layer consisting of
the IL was subjected to distillation at temperature of 400C
and Vacuum Pressure upto 72 mbar for 2h to remove water
and remaining IL was reused. The recovery yield was 96-
98% which does not show any significant decrease up to 8
runs. Purity was measured on the basis of recovery yield of
the IL and product yield. From results there was no any
major changes observed in the yield of the product even after
six runs (Fig. 2).
General Procedure
mixture of benzaldehyde (0.10 g,
A
1
mmol),
malononitrile (0.06 g, 1 mmol) and dimedone (1 mmol) in
the presence of triethylammonium acetate (40 mol%) was
stirred at room temperature for appropriate time (Table 2).
The course of the reaction was monitored by TLC. After
completion of reaction, mixture was poured on ice cold
water (15-20 ml) and the obtained solid product was
collected by filtration. Further purification was accomplished
by recrystallization from ethanol to afford final product
which was in full agreement with the spectral data.
Yield…
Reusability of catalyst
Spectral analysis
4a: IR (KBr):ꢀmax = 3392, 3323, 3027, 2961, 2195, 1672
cm-1. 1H NMR (400 MHz, CDCl3): ꢁ 1.06 (s, 3H, CH3), 1.13
(s, 3H, CH3), 2.15-2.27 (m, 2H, CH2), 2.45 (s, 2H, CH2),
4.37 (s, 1H, CH), 4.58 (s, 2H, NH2), 7.14-7.32 (m, 5H, Ar-
H). EI-MS (m/z): 294 [M+].
97
97
95
96
94
95
1
3
5
Fresh
4b: IR (KBr): ꢀmax = 3387, 3320, 2957, 2187, 1671 1212
cm-1. 1H NMR (400 MHz, CDCl3): ꢁ 1.05 (s, 3H, CH3), 1.15
(s, 3H, CH3), 2.11-2.27 (m, 2H, CH2), 2.47 (s, 2H, CH2),
4.35 (s, 1H, CH), 4.57 (s, 2H, NH2), 7.17-7.34 (m, 4H, Ar-
H). EI-MS (m/z): 328 [M+].
Fig. (2). Reusability of TEAA considering (Table 2, entry 4e) as
model reaction.
EXPERIMENTAL
4k: IR (KBr): ꢀmax = 3457, 3326, 2968, 2157, 1657, 1381,
1216 cm-1. 1H NMR (400 MHz, CDCl3): ꢁ 1.07 (s, 3H, CH3),
1.16 (s, 3H, CH3), 2.17-2.28 (m, 2H, CH2), 2.45 (s, 2H,
CH2), 3.75 (s, 3H, -OCH3), 4.37 (s, 1H, CH), 4.58 (s, 2H,
NH2), 6.84-7.08 (m, 4H, Ar-H). EI-MS (m/z): 324 [M+].
Liquid aldehydes were distilled before use. Melting
points were determined in open capillaries and are
uncorrected. Ethyl acetate and n-Hexane was the solvent
system and silica gel was used as a chromatographic support.
IR spectra were recorded on JASCO FT-IR 4100
spectrometer in KBr with absorption in cm-1. HNMR was
1
4m: IR (KBr): ꢀmax = 3401, 3303, 3041, 2992, 2242, 1671
cm-1. 1H NMR (400 MHz, CDCl3): ꢁ 1.03 (s, 3H, CH3), 1.11
(s, 3H, CH3), 2.14-2.26 (m, 2H, CH2), 2.43 (s, 2H, CH2),
measured on Bruker DPX 400-MHz spectrometer. Mass
spectra were obtained using a Water-Micro Mass Quattro-II.

Triethylammonium Acetate [TEAA]
Letters in Organic Chemistry, 2011, Vol. 8, No. 4
285
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It can be concluded that, introduction of
triethylammonium acetate [TEAA] can be considered as an
interesting new alternate route for the synthesis of
tetrahydro-4H-chromene derivatives and the merits of this
method are highly inexpensive, ease of handling, easily
recycling and reuse of the catalyst. Therefore with increasing
“green” concern, the conditions employed in the present
method will make it environmental friendly and useful for
[19]
industrial
applications.
Further
applications
of
[20]
[21]
triethylammonium acetate (TEAA) for other reaction
systems are under investigation.
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