Ytterbium(III) bis(perfluorooctanesulfonyl)imide catalyzed one-pot synthesis of tetrahydrobenzo[b]pyrans in fluorous biphase system

Article abstract of DOI:10.3184/030823410X12863845344182

Ytterbium(III) bis(perfluorooctanesulfonyl)imide [Yb(NPf₂) ₃] has been prepared, and used as an efficient catalyst for the synthesis of 4H-benzo[b]pyran derivatives by a one-pot three-component condensation of aldehydes, active methylene compounds, and dimedone in a fluorous biphase system (FBS). The reaction proceeds smoothly and affords the corresponding 4H-benzo[b]pyrans in moderate to excellent yields. The catalyst is selectively dissolved in the fluorous phase, can be recovered simply by phase separation and reused several times without any obvious decrease of activity.

Full text of DOI:10.3184/030823410X12863845344182

568 RESEARCH PAPER
OCTOBER, 568–570
JOURNAL OF CHEMICAL RESEARCH 2010
Ytterbium(III) bis(perfluorooctanesulfonyl)imide catalyzed one-pot
synthesis of tetrahydrobenzo[b]pyrans in fluorous biphase system
Mei Hong and Chun Cai*
School of Chemical Engineering, Nanjing University of Science &Technology, Nanjing 210094, P. R. China
Ytterbium(III) bis(perfluorooctanesulfonyl)imide [Yb(NPf2)3] has been prepared, and used as an efficient catalyst for
the synthesis of 4H-benzo[b]pyran derivatives by a one-pot three-component condensation of aldehydes, active
methylene compounds, and dimedone in a fluorous biphase system (FBS). The reaction proceeds smoothly and
affords the corresponding 4H-benzo[b]pyrans in moderate to excellent yields. The catalyst is selectively dissolved in
the fluorous phase, can be recovered simply by phase separation and reused several times without any obvious
decrease of activity.
Keywords: fluorous biphase system, Lewis acid catalyst, bis(perfluorooctanesulfonyl)imide, ytterbium, three component
reaction, 4H-benzo[b]pyrans
4H-Benzo[b]pyran and its derivatives are of considerable
interest because of their wide range of biological and
pharmacological activities,1 such as spasmolytic, diuretic, anti-
coagulant, anti-cancer and anti-anaphylactic activities.2–4 The
pharmacological activities exhibited by these compounds are
mainly due to the presence of the heterocyclic ring system.
They can be used as cognitive enhancers for the treatment of
neurodegenerative disease including Alzheimer’s disease,
AIDS-associated dementia and Down’s syndrome, as well as
for the treatment of schizophrenia and myoclonus.5 A number
of 2-amino-4H-pyran derivatives are useful as photoactive
materials.6 Furthermore, these compounds can be employed as
cosmetics, pigments7 and utilised as potential biodegradable
agrochemicals.8 In view of their wide utility, researchers
have synthesised the 4H-benzo[b]pyran unit using different
methods involving microwave,9 ultrasonic irradiation10 or the
use of hexadecyltrimethylammonium salts,11 KF-alumina,12
rare earth perfluorooctanoates,13 (S)-proline,14 Mg/La mixed
oxide,15 PEG1000-based dicationic acidic ionic liquid,16 basic
quaternary ammonium salts,17 PF6,18 [bmim+][BF4−]19 as catalysts
in one-pot reactions. Each of the above methods has its own
merits, while some are disadvantaged by limitations such as
harsh reaction conditions, low yields, tedious work-up proto-
cols, or poor catalyst recyclability. Consequently, the develop-
mentofacatalystfortheefficientsynthesisof4H-enzo[b]pyrans
in one step still remains attractive to researchers.
Table 1 Effect of different catalysts on the condensation of
benzaldehyde, dimedone, and malononitrilea
Entry
Catalyst
Amount of catalyst
/mol%
Yield /%b
1
2
Yb(OTf)3
Yb(OPf)3
Yb(NPf2)3
Yb(NPf2)3
Yb(NPf2)3c
Yb(NPf2)3
La(NPf2)3
Nd(NPf2)3
Sm(NPf2)3
Gd(NPf2)3
Y(NPf2)3
1
73
1
82
3
0.2
0.5
1
68
81
4
5
92, 92, 90, 88, 88
6
1.2
1
91
67
63
71
75
81
83
7
8
1
9
1
10
11
12
1
1
Er(NPf2)3
1
a Reaction conditions: benzaldehyde (1 mmol), dimedone
(1 mmol), malononitrile (1 mmol), ethanol (2 mL), C10F18 (2 mL),
80 °C, 4 h.
b Isolated yields.
c Catalyst was reused in five times.
During the last decade, fluorinated Lewis acid catalysts in a
variety of synthetic reactions have been recognised because of
their strong Lewis acidity and the characteristic that they can
be immobilised in a fluorous solvent. Recently, the value of the
fluorous phase-separation and fluorous catalyst recyclability
under fluorous biphase system (FBS) has been recognised.20
We have introduced Yb(NPf2)3 as an efficient Lewis acid
catalyst for the allylation of 1,3-dicarbonyl compounds21 in
FBS. A key factor to accomplish the catalytic processes was
was believed to be the use of a sufficiently long perfluorinated
ligand –N(SO2C8F17)2, whose structural characteristic can
coordinate with a variety of metal cations to obtain the desired
Lewis acid catalysts with appropriate catalytic activity, as well
as the selective immobilisation in the fluorous phase.22 This
report describes a very efficient one-pot three-component
synthesis of tetrahydrobenzo[b]pyrans catalysed by the Lewis
acid catalyst Yb(NPf2)3 in FBS.
Yb(NPf2)3 showed the best catalytic activity in this reaction.
We also found that an increased yield could be obtained when
the catalyst loading was increased from 0.2 to 1 mol%, but
further increasing the amount of catalyst (over 1 mol% ) did
not provide further improvements (Table 1, entries 3–6). When
the reaction was completed, the fluorous layer containing
the catalyst could be separated easily by decantation at
room temperature and used five times with yields of products
ranging from 88 to 92% (Table 1, entry 5).
In order to make the reaction more accessible, we explored
different co-solvent systems taking a general organic solvent.
In each case, the substrates were mixed together with 5 mol%
Yb(NPf2)3 agitated with 2 mL co-solvent and 2 mL C10F18. The
results are shown in Table 2. It is important to note that the use
of a polar co-solvent such as ethanol, methanol or acetonitrile
resulted in higher product yield, and ethanol as co-solvent was
the most efficient. We assumed that the difference in solubility
of the catalyst and the substrates in polar or non-polar solvents
would have an influence on their effective miscibility.
First of all, we began to examine the catalytic activities of
different rare earth complexes to optimise the conditions for
reaction of benzaldehyde, dimedone and malononitrile in a
biphase of co-solvent ethanol and perfluorodecalin (C10F18,
cis- and trans-mixture) (Table 1). Among these catalysts,
In order to generalise this method, aldehydes 1, 5,5-
dimethyl-1,3-cyclohexanedione (dimedone) 2, and malononi-
trile or ethyl cyanoacetate 3 (see Table 3) were subjected to the
* Correspondent. E-mail: c.cai@mail.njust.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2010 569
Table 2 Effect of co-solvents on the condensation of
benzaldehyde, dimedone, and malononitrile catalysed by
Yb(NPf2)3a
MB154S IR analyser. UV-Vis spectra were obtained on a UV-1601
apparatus. Mass spectra were obtained with a Hewlett Packard 5989A
spectrometer. Inductively coupled plasma (ICP) spectra were mea-
sured on Ultima2C apparatus. Elemental analyses were performed on
a Yanagimoto MT3CHN recorder. Commercially available reagents
were used without further purification.
Entry
Co-solvent
Temperature
/°C
Yield /% b
Bis(perfluorooctanesulfonyl)imide:23,24 Ammonia gas (300 mmol)
was added into perfluorooctanesulfonyl fluoride (50 g, 99.6 mmol) at
-20 °C. After stirring at –20 °C for about 1 h, stirring was continued at
room temperature for another 1 h. The solid product was acidified to
pH about 2 with 2M HCl followed by addition of Et2O (100 mL). The
organic layer was dried over anhydrous Na2SO4, the solvent removed
under reduced pressure, and the residue dried in vacuo at 80 °C for
16 h to give C8F17SO2NH2 (87% yield). Then the mixture of perfluo-
rooctanesulfonyl fluoride (45.7 g, 91 mmol), perfluorooctanesulfon-
amide (43.4 g, 87 mmol) and Et3N (76 mL) was stirred under reflux
for 23 h. The lower brown fluorous layer was washed with 4M HCl to
yield a brown solid which was dried in vacuo at 70 °C for 6 h to afford
(C8F17SO2)2N HNEt3. Free (C8F17SO2)2NH was obtained in 50% yield,
using acidic ion exchange resin column.
1
2
3
4
5
6
95% C2H5OH
C2H5OH
CH3OH
H2O
80
80
65
80
80
80
84
92
83
35
62
71
PhMe
CH3CN
aReaction conditions: benzaldehyde (1 mmol), dimedone
(1 mmol), malononitrile (1 mmol), Yb(NPf2)3 (0.01 mmol ), C10F18
(2 mL), 4 h.
bIsolated yields.
Table 3 Yb(NPf2)3 catalysed the preparation of tetrahydro-
benzo[b]pyransa
Yb(NPf2)3: To a stirried solution of bis(perfluorooctanesulfonyl)imi
de (0.883 g, 0.9 mmol) in 10 mL H2O, ytterbium(III) chloride
(0.083 g, 0.3 mmol) was added and the mixture was stirred for 1 h
under reflux. Then the water was removed under reduced pressure at
80 °C for 16 h to afford a white powder of ytterbium(III) bis(perfluor
ooctanesulfonyl)imidein98%yield.Anal.CalcdforC48O12N3F102S6Yb:
Yb, 5.56. C, 18.52; N, 1.35. Found: Yb: 5.60, C: 18.45; N, 1.41%.
19F NMR (300 MHz, α,α,α-trifluorotoluene): δ –81.8 (CF3), –115.2
(α–CF2), –121.2 (β-CF2), –122.6 (3×CF2), –123.4 (γ–CF2), –126.9
(θ-CF2). IR(cm−1): 3475, 1651, 1338, 1207, 1153, 1084, 987, 945,
806, 740, 628. UV-vis (EtOH) λ (nm): 210.
Entry R1
R2
Time /h Product Yield /%b
1
2
C6H5
CN
4
5
4a
4b
4c
4d
4e
4f
92
80
86
96
96
89
82
80
88
37
83
85
71
93
81
87
4-CH3C6H4
4-CH3OC6H4
4-BrC6H4
4-ClC6H4
CN
3
CN
5
4
CN
3
Synthesis of tetrahydrobenzo[b]pyrans; general procedure
5
CN
3
A mixture of aldehyde (1 mmol), malononitrile (or ethyl cyanoace-
tate) (1 mmol), 5,5-dimethyl-1,3-cyclohexanedione (0.140 g, 1 mmol)
and Yb(NPf2)3 (0.031g, 1 mol%) in a biphase of ethanol (2 mL) and
C10F18 (2 mL) was stirred at 80 °C for an appropriate time. When the
reaction was completed as indicated by TLC, the mixture was cooled
to room temperature, the lower fluorous phase containing the catalyst
was separated and reused in the subsequent recycling reaction. The
upper phase was cooled in a refrigerator overnight; the solid product
which had crystallised was filtered off, washed with H2O and purified
by recrystallisation from EtOH (95%).
6
2,4-Cl2C6H3
3-NO2C6H4
4-NO2C6H4
4-HOC6H4
CH3(CH2)5
2-Furyl
2-Thienyl
C6H5
4-ClC6H4
CN
3
7
CN
3
4g
4h
4i
8
CN
3
3
9
CN
10
11
12
13
14
15
16
CN
24
3
4j
CN
4k
4l
4m
4o
4p
4q
CN
3
CO2C2H5
CO2C2H5
CO2C2H5
CO2C2H5
6
5
4-NO2C6H4
2,4-Cl2C6H3
5
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-phenyl-
5
4H-benzo[b]pyran (4a): White solid. m.p: 226–228 °C (lit.13 227–
Reaction conditions: aAldehyde (1 mmol), dimedone (1 mmol),
malononitrile or ethyl cyanoacetate (1 mmol), Yb(NPf2)3
(0.01 mmol), ethanol (2 mL), C10F18 (2 mL), 80 °C.
bIsolated yields.
1
229 °C). IR (KBr) ν: 3396, 3325, 2199, 1664, 1214, 1036 cm−1. H
NMR (300 MHz, CDCl3): 1.04 (s, 3H), 1.11 (s, 3H), 2.19 (d, 1H,
J = 16.1 Hz), 2.24 (d, 1H, J = 16.1 Hz), 2.45 (s, 2H), 4.40 (s, 1H),
4.53 (s, 2H), 7.18–7.27 (m, 5H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
tolylphenyl)-4H-benzo[b]pyran (4b): White solid. m.p. 208–210 °C
(lit.12 208–210 °C). IR (KBr) ν: 3426, 3330, 2193, 1678, 1140, 1041
cm−1. 1H NMR (300 MHz, CDCl3): 1.07 (s, 3H), 1.11 (s, 3H), 2.25 (s,
5H), 2.43 (s, 2H), 4.36 (s, 1H), 4.53 (s, 2H), 7.15–7.30 (m, 4H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
methoxyphenyl)-4H-benzo[b]pyran (4c): White solid. m.p. 192–
193 °C (lit.12 198–200 °C). IR (KBr) ν: 3378, 3315, 2193, 1687, 1655,
1141, 1034 cm−1. 1H NMR (300 MHz, CDCl3): 1.02 (s, 3H), 1.11 (s,
3H), 2.25 (s, 2H), 2.45 (s, 2H), 3.81 (s, 3H), 4.37 (s, 1H), 4.51 (s, 2H),
7.13–7.28 (m, 4H).
biphase system of ethanol and C10F18 at 80 °C in the presence
of a catalytic amount of Yb(NPf2)3 (Table 3). Both aldehydes
having electron-withdrawing and electron-donating groups on
the aromatic ring can undergo the reaction to afford the corre-
sponding 4H-benzo[b]pyrans in moderate to excellent yields
(71–96%). The position of the substituents on the aromatic
ring also did not show obvious effects on the yields. Longer
reaction times were required for the condensation with ethyl
cyanoacetate.
In summary, rare earth bis(perfluorooctanesulfonyl)imide
complexes RE(NPf2)3, especiallyYb(NPf2)3 proved to be effec-
tive catalysts for the synthesis of 4H-benzo[b]pyrans by the
condensation of aldehydes, malononitrile or ethyl cyanoace-
tate, and dimedone in FBS. The current methodology has the
advantages of operational simplicity, mild reaction conditions,
moderate to excellent yields of products and reusability of the
catalyst.
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
bromophenyl)-4H-benzo[b]pyran (4d): White solid. m.p. 201–203 °C
(lit. [14]: 203–205 °C). IR (KBr) ν: 3392, 3328, 2192, 1682, 1214,
1
1039 cm−1. H NMR (300 MHz, CDCl3): 1.02 (s, 3H), 1.12 (s, 3H),
2.25 (s, 2H), 2.45 (s, 2H), 4.35 (s, 1H), 4.71 (s, 2H), 7.12–7.41 (m,
4H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
chlorophenyl)-4H-benzo[b]pyran (4e): White solid. m.p. 215–217 °C
(lit.12 207–209 °C). IR (KBr) ν: 3370, 3324, 2178, 1674, 1215, 1014
cm−1. 1H NMR (300 MHz, CDCl3): 1.02 (s, 3H), 1.13 (s, 3H), 2.25 (s,
2H), 2.47 (s, 2H), 4.40 (s, 1H), 4.62 (s, 2H), 7.17–7.35 (m, 4H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(2p,4p-
chlorophenyl)-4H-benzo[b]pyran (4f): White solid. m.p. 115–117 °C
(lit.13 115-117 °C). IR (KBr) ν: 3363, 3156, 2966, 2192, 1686, 1607,
1473, 1040 cm−1. 1H NMR (300 MHz, CDCl3): 1.06 (s, 3H), 1.10 (s,
Experimental
Melting points were determined in a capillary tube and are uncor-
rected. 1H NMR and 19F- NMR spectra were recorded using a Bruker
Advance RX300 spectrometer. IR spectra were recorded on a Bomem

570 JOURNAL OF CHEMICAL RESEARCH 2010
3H), 2.27 (s, 2H), 2.45 (s, 2H), 4.56 (s, 1H), 4.89 (s, 2H), 7.13–7.39
(m, 3H).
4.81 (s, 1H), 6.28 (s, 2H), 7.46 (d, 2H, J = 8.8 Hz ), 8.01 (d, 2H,
J = 8.8Hz).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(3p-
nitrophenyl)-4H-benzo[b]pyran (4g): White solid. m.p. 210–212 °C
(lit.13 204–205 °C). IR (KBr) ν: 3324, 3212, 2185, 1679, 1211, 1037
cm−1. 1H NMR (300 MHz, CDCl3): 1.01 (s, 3H), 1.12 (s, 3H), 2.26 (s,
2H), 2.51 (s, 2H), 4.51 (s, 1H), 4.68 (s, 2H), 7.15–7.39 (m, 4H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
nitrophenyl)-4H-benzo[b]pyran (4h): White solid. m.p. 177–178 °C
(lit.13 171–172 °C). IR (KBr) ν: 3408, 3321, 2187, 1673, 1216, 1039
cm−1. 1H NMR (300 MHz, CDCl3): 1.01 (s, 3H), 1.12 (s, 3H), 2.25 (s,
2H), 2.47 (s, 2H), 4.53 (s, 1H), 4.68 (s, 2H), 7.41–8.15 (m, 4H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(4p-
hydroxyphenyl)-4H-benzo[b]pyran (4i): White solid. m.p. 208–210 °C
(lit.13 208–210 °C). IR (KBr) ν: 3357, 3250, 3163, 2191, 1664, 1558
cm−1. 1H NMR (300 MHz, CDCl3): 1.05 (s, 3H), 1.10 (s, 3H), 2.23 (s,
2H), 2.45 (s, 2H), 4.31 (s, 1H), 4.75 (s, 1H), 5.34 (s, 2H), 6.75–7.13
(m, 4H).
2-Amino-3-ethylcarboxylate-5,6,7,8-tetrahydro-7,7-dimethyl-5-
oxo-4-(2p,4p-chlorophenyl)-4H- benzo[b]pyran (4q): White solid. m.p.
176–178 °C (lit.14 172–174 °C). IR (KBr) ν: 3426, 3313, 2959, 1696,
1
1650, 1614, 867, 847, 804, 758 cm−1. H NMR (300 MHz, CDCl3):
1.02 (s, 3H), 1.12 (s, 3H), 1.16 (t, 3H, J = 7.2 Hz), 2.15 (s, 2H), 2.47
(s, 2H), 4.06 (q, 2H, J = 7.2 Hz), 4.98 (s, 1H), 6.21 (s, 2H), 7.16 (d,
1H, J = 8.0 Hz), 7.21 (d, 1H, J = 8.0 Hz), 7.35 (s, 1H).
Received 27 June 2010; accepted 20 August 2010
Paper 1000227 doi: 10.3184/030823410X12863845344182
Published online: 22 October 2010
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2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-heptyl-
4H-benzo[b]pyran (4j): White solid. m.p. 187–189 °C(lit.18 185–
2
3
4
5
6
7
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1
187 °C). IR (KBr) ν: 3387, 3319, 2185, 1681, 1146, 1040 cm−1. H
NMR (300 MHz, CDCl3): 0.86 (s, 3H), 1.08 (s, 3H), 1.10 (s, 3H),
1.14–1.26 (m, 10H), 2.28 (s, 2H), 2.29–2.35 (m, 2H), 3.41 (s, 1H),
4.46 (s, 2H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(2p-
furyl)-4H-benzo[b]pyran (4k): White solid. m.p. 218–220 °C (lit.12
218–220 °C). IR (KBr) ν: 3405, 3295, 3051, 2993, 2978, 2243, 1680,
1
1600, 730 cm−1. H NMR (300 MHz, CDCl3): 1.01 (s, 3H), 1.10 (s,
3H), 2.28 (s, 2H), 2.51 (s, 2H), 4.44 (s, 1H), 4.75 (s, 2H), 6.18–7.37
(m, 3H).
2-Amino-3-cyano-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4-(2p-
thienyl)-4H-benzo[b]pyran (4l): White solid. m.p. 224–226 °C (lit.19
224–226 °C). IR (KBr) ν: 3322, 3208, 2964, 2199, 1679, 1603, 1541,
1466, 758, 700 cm−1. 1H NMR (300 MHz, CDCl3): 1.08 (s, 3H), 1.13
(s, 3H), 2.28 (s, 2H), 2.44 (s, 2H), 4.61 (s, 1H), 4.79 (s, 2H), 6.92–7.14
(m, 3H).
2-Amino-3-ethylcarboxylate-5,6,7,8-tetrahydro-7,7-dimethyl-5-
oxo-4-phenyl-4H-benzo[b]pyran (4m): White solid. m.p. 155–157 °C
(lit.14 155–157 °C). IR (KBr) ν: 3403, 3290, 2956, 1693, 1655, 1606,
1524, 835, 796, 769, 725, 697 cm−1. 1H NMR (300 MHz, CDCl3): 0.96
(s, 3H), 1.09 (s, 3H), 1.16 (t, 3H, J = 7.1 Hz), 2.18 (s, 2H), 2.41 (s,
2H), 4.02 (q, 2H, J = 7.2 Hz), 4.71 (s, 1H), 6.11 (s, 2H), 7.16–7.25 (s,
5H).
2-Amino-3-ethylcarboxylate-5,6,7,8-tetrahydro-7,7-dimethyl-5-
oxo-4-(4p-chlorophenyl)-4H- benzo[b]pyran (4o): White solid. m.p.
150–152 °C (lit.14 153–155 °C). IR (KBr) ν: 3480, 3327, 2976, 1684,
1660, 1525, 919, 857, 842 cm−1. 1H NMR (300 MHz, CDCl3): 0.98 (s,
3H), 1.08 (s, 3H), 1.14 (t, 3H, J = 7.1 Hz), 2.17 (s, 2H), 2.42 (s, 2H),
4.06 (q, 2H, J = 7.2 Hz), 4.69 (s, 1H), 6.21 (s, 2H), 7.13–7.27 (s,
4H).
2-Amino-3-ethylcarboxylate-5,6,7,8-tetrahydro-7,7-dimethyl-5-
oxo-4-(4p-nitrophenyl)4H- benzo[b]pyran (4p): White solid. m.p.
183–185 °C (lit.14 183–185 °C). IR (KBr) ν: 3476, 3338, 1687, 1659
cm−1. 1H NMR (300 MHz, CDCl3) 0.98 (s, 3H), 1.10 (s, 3H), 1.15 (t,
3H, J = 7.2 Hz), 2.19 (s, 2H), 2.47 (s, 2H), 4.05 (q, 2H, J = 7.2 Hz),
8
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