Catalytic application of two novel sandwich-type polyoxometalates in synthesis of 14-substituted-14H-dibenzo[a, j]xanthenes

Article abstract of DOI:10.1007/s12039-013-0384-z

Two sandwich-type polyoxometalates K₁₂[As₂W ₁₈Cu₃O₆₈]·30H₂O and K ₁₂[As₂W₁₈U₃O₇₄] ·21H₂O were found to be as novel efficien

Full text of DOI:10.1007/s12039-013-0384-z

c
J. Chem. Sci. Vol. 125, No. 2, March 2013, pp. 345–351. ꢀ Indian Academy of Sciences.
Catalytic application of two novel sandwich-type polyoxometalates in
synthesis of 14-substituted-14H-dibenzo[a, j]xanthenes
SHABNAM SHESHMANI
Department of Chemistry, Shahr-e Rey Branch, Islamic Azad University, P.O. Box 18155-144, Tehran, Iran
e-mail: shabnam_sheshmani@yahoo.com
MS received 4 August 2012; revised 12 November 2012; accepted 19 December 2012
Abstract. Two sandwich-type polyoxometalates K12[As2W18Cu3O68]·30H2O and K12[As2W18U3O74]·21H2O
were found to be as novel efficient catalysts for one-pot synthesis of various 14-aryl or alkyl-14H-
dibenzo[a, j]xanthenes. Three-component condensation reactions of β-naphthol with aromatic or aliphatic
aldehydes in the presence of these catalysts were investigated. These reactions were studied under different con-
ditions such as solvent-free media using both conventional heating and microwave irradiation and also several
solvents. Results showed that the optimum reaction time and the yield were obtained when reactions were car-
ried out under solvent-free conditions. Furthermore, the catalysts could be recovered conveniently and reused
efficiently.
Keywords. 14-Substituted-14H-dibenzo[a, j]xanthenes; sandwich polyoxometalates; catalyst; solvent-free.
1. Introduction
They are also candidates in photodynamic therapy
(PDT).20,21 Furthermore, benzoxanthenes are used as
Polyoxometalates (POMs) are well-known inorganic dyes,22 in laser technologies23 and in fluorescent
metal oxygen cluster species have been attracting more materials.24
interest in diverse fields such as catalysis, medicine,
The synthesis of 14H-dibenzo[a, j]xanthenes is
analytical chemistry and magnetism.1–6 The evolution generally achieved by dehydration of bis(2-hydroxy-1-
25
of POMs chemistry is dependent on the preparation naphthyl) methane using POCl3 or by boiling acetic
of novel polyoxoanions possessing unique structures acid diester of bis(2-hydroxy-1-naphthyl) methane,26
and properties. POMs are formed by the early transi- the reaction β-naphthol with formamide and carbon
tion metals of groups V and VI in their highest oxida- monoxide.27,28 However, those methods suffer from
tion states by condensation reactions in aqueous, acidic significant limitations, such as low yields of prod-
medium. The commonly used POMs building blocks ucts, the need for a prolonged reaction time (from
still largely focus on the well-known Keggin,7 Wells– 16 h to 5 days), large quantities of solvent and catalyst
Dawson,8 Anderson,9 Silverton10 and Lindquist-type11 involved, and harsh reaction conditions. Recently, ben-
anions.
zoxanthenes was obtained by the condensation reaction
The sandwich-type polyoxometalates obtained by between β-naphthol with aliphatic or aromatic aldehydes
reaction of the lacunary polyoxoanions with the in the presence of hydrochloric acid or phos-
transition-metal cations, are the excellent build- phoric acid,29 sulphuric acid,30 p-toluenesulphonic
ing blocks.12 Up to now, numerous sandwich-type acid,31 sulphamic acid,32 Amberlyst-15,33 HClO4-
polyoxometalates have been prepared and mostly SiO2,34 Dowex-50W,35 Yb(OTf)3,36 alum,37 Montmo-
belong to the well-known Weakly ([M4(X2W15O56)2]n− rillonite K-10,38 heteropoly acid,39 P2O5 or InCl3 40 and
and [M4(H2O)2(XW9O34)2]n−), Herve [M3(H2O)3(α- indion-13041 as catalyst.
XW9O33)2]n− (M = transition metal, X = AsIII, SbIII,
It is to be noted that no published reports are available
SeIV, BiIII) and Knoth-type [M3(A-XW9O34)2]n− (X = P on open-access literatures on the use of the sandwich-
and Si) sandwich-type structures.13–15 This type of com- type polyoxometalates as catalyst in the synthesis of
pounds has attracted special interest on their potential 14-aryl or alkyl-14H-dibenzo[a, j]xanthenes. In this
application as catalyst.16,17
study, an attempt was made to explain a facile and
Xanthenes especially benzoxanthenes, are an impor- efficient synthetic strategy for synthesis of 14-aryl or
tant class of biologically active heterocycles with poten- alkyl-14H-dibenzo[a, j]xanthenes using one-pot con-
tial application as antibacterial,18 antiviral agents.19 densation of β-naphthol with wide variety of aldehydes
345

346
Shabnam Sheshmani
in the presence of catalyst. Two sandwich-type poly- 2H2O (0.045 mol) and 1.58 g Na2HAsO4·7H2O
oxomatalates such as K12[As2W18Cu3O68]·30H2O and (0.005 mol) in water (50 mL) was added in a small por-
K12[As2W18U3O74]·21H2O as novel recyclable catalysts tion to a solution containing 5.27 g UO2(NO3)2·6H2O
were used.
(0.010 mol) and HNO3 6.4 M (8.600 mL) in water
(55 mL) and the resulting solution was refluxed for
2 h. Then, the solution was filtered off and 20 g KNO3
(0.200 mol) was added to the filtrate. The resulting
precipitate was filtered off, redissolved in hot water
and reprecipitated with the addition of 10 g KNO3
(0.100 mol). Consequently, the resulting solid was
recrystallized from hot water.42
2. Experimental
2.1 Materials and apparatus
All chemicals were obtained from Merck and Romil
was used without further purification. All yields were
referred to isolated products after purification. Pro-
ducts were characterized by comparison with authen-
tic samples by TLC, spectral and physical data. Melting
points were determined on Electrothermal 9100 appara-
tus and were uncorrected. IR spectra were recorded on a
Bruker Tensor 27 spectrometer with KBr plates. 1H and
13C NMR spectra were obtained on a Bruker DRX-500
AVANCE spectrometer at 500 and 125 MHz, respec-
tively in CDCl3 solvent. Mass spectra were recorded on
an Agilent technologies 5973 network mass selective
detector (MSD) operating at an ionization potential of
70 eV. Elemental analysis (C, H and N) were performed
on a Perkin-Elmer 2400 CHN elemental analyzer. The
reaction was irradiated using microwave oven with con-
trolled timer. The purity of the substances and the
progress of the reactions were monitored by analyti-
cal thin-layer chromatography (TLC) on silica gel 60F
containing the fluorescent indicator UV254.
2.3 Preparation of K12[As2W18Cu3O68]·30H2O
catalyst
The experimental procedure for preparation
of K12[As2W18Cu3O68]·30H2O was similar to
K12[As2W18U3O74]·21H2O with a replacement of
UO2(NO3)2·6H2O by Cu(NO3)2·3H2O.
2.4 Synthesis of 14-aryl or alkyl-14H-dibenzo[a, j]
xanthenes in presence of catalysts under
solvent condition
A mixture of an aldehyde (1 mmol), 0.28 g β-naphthol
(2 mmol) and 0.05 g K12[As2W18Cu3O68]·30H2O
(0.013 mmol, 1.3 mol%) or 0.05 g K12[As2W18U3O74]·
21H2O (0.008 mmol, 0.8 mol%) in solvent (10 mL)
was stirred at reflux for appropriate time (table 1). The
progress of the reaction was monitored by TLC. After
completion of the reaction, the reaction mixture was
filtered and the organic solvent was concentrated in
vacuum to afford crude product. The crystalline pure
product was obtained by further recrystalization from
ethanol.
2.2 Preparation of K12[As2W18U3O74]·21H2O catalyst
The detailed procedure of preparation of
K12[As2W18U3O74]·21H2O catalyst was reported else-
where.42 In brief, the solution of 14.85 g Na2WO4·
Table 1. The reaction between 4-fluorobenzaldehyde and β-naphthol using K12[As2W18Cu3O68]·30H2O (I) and
K12[As2W18U3O74]·21H2O (II) catalysts under different conditions.
I
II
Entry
Solvent
Methanol
ClCH2CH2Cl
H2O
Conditions
Time (h)/Yield (%)a
Time (h)/Yield (%)a
1
2
3
4
5
6
7
8
9
10
Reflux
Reflux
Reflux
Reflux
60◦C
10.00/trace
10.00/trace
10.00/32
10.00/trace
10.00/trace
8.00/25
5.30/53
2.30/95
10.00/trace
15.30 min/90
10.00/trace
10.00/trace
10.00/40
10.00/trace
10.00/20
6.30/36
4.00/60
2.00/94
5.00/trace
8.00 min/93
DMF
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free/MW
80◦C
100◦C
125◦C
140◦C
900 W
aThe yields refer to the isolated pure products.

Catalytic application of two novel sandwich-type polyoxometalates
347
2.5 Synthesis of 14-aryl or alkyl-14H-dibenzo[a, j] ν = 3068, 2926, 1623, 1591, 1513, 1457, 1399, 1340,
xanthenes in presence of catalysts under solvent-free
condition by conventional heating
1240, 1140, 1106, 1014, 959, 851, 827, 807, 744,
1
695 cm−1; HNMR (500 MHz, CDCl3): δ = 8.29 (2H,
d, J = 8.4 Hz), 7.95 (2H, d, J = 8.6 Hz), 7.88 (2H,
d, J = 4.2 Hz), 7.83 (2H, d, J = 5.4 Hz), 7.67 (2H,
d, J = 8.8 Hz), 7.61 (2H, t, J = 5.6 Hz), 7.50 (2H, d,
J = 8.9 Hz), 7.44 (2H, t, J = 7.9 Hz), 6.65 (1H, s);
13C NMR (125 MHz, CDCl3): δ = 152.6, 148.1, 145.9,
135.1, 130.8, 130.6, 129.6, 128.7, 127.2, 124.7, 123.8,
123.3, 117.8, 116.5, 36.8 ppm; MS (70 eV): m/z =
403 (M+, 6.5), 281 (100), 252 (17.4), 69 (53.5). Anal.
Calcd. for C27H17NO3: C, 80.39; H, 4.21; N, 3.47,
Found: C, 80.40; H, 4.47; N, 3.20.
The molar composition of aldehyde, β-naphthol and
catalysts were similar to solvent condition procedure.
This mixture was heated under solvent-free at 125◦C
with stirring. After completion of reaction (monitored
by TLC) the mixture was cooled and extracted with
ethyl acetate. The organic layer was oven-dried over
Na2SO4 and evaporated to give the crude product,
which was recrystallized from ethanol to afford pure
products.
2.6 Synthesis of 14-aryl or alkyl-14H-dibenzo[a, j]
xanthenes in presence of catalysts under solvent-free
condition by microwave irradiation
2.7c 14-(4-Fluorophenyl)-14H-dibenzo[a, j]xanthene
(3c): White solid. m.p.: 239–240◦C, IR (KBr):
ν = 3068, 2927, 1623, 1593, 1501, 1457, 1432, 1397,
1245, 1212, 1157, 1081, 958, 831 cm−1; 1HNMR
(500 MHz, CDCl3): δ = 8.38 (2H, d, J = 8.5 Hz), 7.88
(2H, d, J = 8.05 Hz), 7.84 (2H, d, J = 8.9 Hz), 7.64
(2H, t, J = 7.55), 7.53 (2H, d, J = 8.75 Hz), 7.51–
7.46 (4H, m), 6.87 (2H, d, J = 8.7 Hz), 6.52 (1H, s);
13CNMR (125 MHz, CDCl3): δ = 148.7, 131.3, 131,
129.6, 129.5, 129, 128.9, 126.8, 124.3, 122.4, 118,
117.1, 115.4, 115.2, 37.1 ppm; MS (70 eV): m/z = 376
(M+, 72.2), 281 (100), 252 (22.5), 178 (6.7), 75(3.5);
Anal. Calcd. for C27H17FO: C, 86.17; H, 4.52, Found:
C, 86.57; H, 4.59.
This reaction was carried out similar to solvent condi-
tion procedure in terms of molar composition. The mix-
ture was taken in a glass vial and irradiated (900 W) for
appropriate time. After completion of reaction (moni-
tored by TLC), the products were purified similar to
method conventional heating to give pure products.
2.7 Characterization of 14-aryl or alkyl-14H-dibenzo
[a, j]xanthenes
All the products obtained were characterized by spec-
troscopic methods such as IR, NMR (1H and 13C),
mass and elemental analysis. The spectral and analyti-
cal data of some representative aryl and alkyl 14H-
dibenzo[a, j]xanthenes are presented below.
2.7d 14-(4-Methylphenyl)-14H-dibenzo[a, j]xanthene
(3f): White solid. m.p.: 227–229◦C, IR (KBr):
ν = 3070, 2919, 1620, 1590, 1511, 1457, 1429, 1398,
1245, 1139, 1080, 960, 836, 811, 778, 740 cm−1;
1HNMR (500 MHz, CDCl3): δ = 8.44 (2H, d, J =
8.55 Hz), 7.86 (2H, d, J = 8.05 Hz), 7.82 (2H, d, J =
8.85 Hz), 7.62 (2H, t, J = 8.2 Hz), 7.53 (2H, d, J =
8.85 Hz), 7.47–7.43 (4H, m), 7.00 (2H, d, J= 7.9 Hz),
6.5 (1H, s), 7 (3H, s); 13CNMR (125 MHz, CDCl3): δ =
149.1, 142.5, 136.2, 131.8, 131.5, 129.5, 129.2, 129.1,
128.5, 127.1, 124.6, 123.1, 118.4, 117.8, 38, 21.2 ppm;
MS (70 eV): m/z = 372 (M+, 12.5), 281 (100), 252
(15), 69 (37.5); Anal. Calcd. for C28H20O: C, 90.32; H,
5.37, Found: C, 90.53; H, 5.45.
2.7a 14-Phenyl-14H-dibenzo[a, j]xanthenes (3a):
Colourless solid. m.p.: 183–185◦C, IR (KBr):
ν = 3074, 2886, 1621, 1592, 1514, 1490, 1456, 1402,
1
1252, 1079, 1027, 963, 827, 744, 700 cm−1; HNMR
(500 MHz, CDCl3): δ = 8.44 (2H, d, J = 8.45 Hz),
7.87 (2H, d, J = 7.9 Hz), 7.83 (2H, d, J = 8.85 Hz),
7.63 (2H, t, J = 7.7 Hz), 7.58 (2H, d, J = 8.3 Hz),
7.53(2H, d, J = 8.85 Hz), 7.45 (2H, t, J = 7.9 Hz),
7.19 (2H, t, J = 7.55 Hz), 7.03 (1H, t, J = 7.4 Hz),
6.5 (1H, s); 13CNMR (125 MHz, CDCl3): δ = 149.1,
145.4, 131.8, 131.4, 129.3, 129.2, 128.8, 128.6, 127.1,
126.7, 124.6, 123.1, 118.4, 117.7, 38.4 ppm; MS
(70 eV): m/z = 358 (M+, 22.5), 281 (100), 252 (13.3),
77 (4.4), Anal. Calcd. for C27H18O: C, 90.50; H, 5.02,
Found: C, 90.44; H, 5.08.
2.7e 14-(4-Methoxyphenyl)-14H-dibenzo[a.j]xanthenes
(3g): White solid. m.p.: 202–204◦C, IR (KBr):
ν = 3071, 2923, 1735, 1591, 1509, 1458, 1430,
1
1398, 1248, 1030, 960, 830, 808, 742 cm−1; HNMR
(500 MHz, CDCl3): δ = 8.42 (2H, d, J = 8.45 Hz),
7.87 (2H, d, J = 7.95), 7.82 (2H, d, J = 8.9), 7.62
(2H, t, J = 8.2 Hz), 7.52 (2H, d, J = 8.8 Hz), 7.40
2.7b 14-(4-Nitrophenyl)-14H-dibenzo[a, j]xanthene
(3b): Yellow solid. m.p.: 311–312◦C, IR (KBr):

348
Shabnam Sheshmani
(4H, m), 6.71 (2H, d, J = 8.8 Hz), 6.5 (1H, s), 3.65 3. Results and discussion
(3H, s); 13CNMR (125 MHz, CDCl3): δ = 158.2,
In order to optimize the reaction conditions and evalu-
149.1, 137.7, 131.8, 131.5, 129.5, 129.2, 129.1, 127.1,
124.6, 123.1, 118.4, 117.9, 114.2, 55.4, 37.5 ppm; MS
(70 eV): m/z = 388 (M+, 17.6), 281 (100), 252 (14.1),
57(6); Anal. Calcd. for C28H20O2: C, 86.59; H, 5.15,
Found: C, 86.43; H, 5.24.
ate the catalytic efficiency of two novel sandwich-
type polyoxometalate catalysts, initially a model
was chosen on condensation reaction between 4-
fluorobenzaldehyde (1 mmol) and β-naphthol (2 mmol)
in the presence of different amount of catalysts
K12[As2W18Cu3O68]·30H2O and K12[As2W18U3O74]·
21H2O separately under solvent and solvent-free con-
ditions using both conventional heating and microwave
irradiation. Results showed that among the tested sol-
vents such as methanol, 1,2-dichloroethane, water,
dimethyl formamide and solvent-free system, con-
densation of 4-fluorobenzaldehyde and β-naphthol
was more facile and proceeded to gave highest yield
when 1.3 mol% of K12[As2W18Cu3O68]·30H2O and
0.8 mol% of K12[As2W18U3O74]·21H2O under solvent-
free conditions are used (table 1). In the absence of
catalysts, this reaction could be carried out but the
product was obtained in very low yield even after pro-
longed time. In solvent-free media using microwave
2.7f 14-Ethyl-14H-dibenzo[a, j]xanthene (3j): White
solid, m.p.: 151–152◦C, IR (KBr): ν = 3064, 2927,
2873, 1620, 1590, 1514, 1456, 1433, 1399, 1256, 1139,
1029, 958, 829, 750 cm−1; 1HNMR (500 MHz, CDCl3):
δ = 8.27 (2H, d, J = 8.5 Hz), 7.89 (2H, d, J = 7.7 Hz),
7.79 (2H, d, J = 8.8 Hz), 7.66–7.45 (4H, m), 7.40 (2H,
d, J = 8.8 Hz), 5.58 (1H, t, J = 4.6 Hz), 2.15–2.03
(2H, m), 0.98 (3H, t, J = 7.4 Hz); 13CNMR (125 MHz,
CDCl3): δ = 150.1, 131.4, 131, 128.8, 128.2, 126.5,
124, 122.4, 117.5, 115.9, 31.6, 28.3, 9.01 ppm; MS
(70 eV): m/z = 310 (M+, 5), 281 (100), 252 (30), 140
(25), 57 (65); Anal. Calcd. for C23H18O: C, 89.03; H,
5.80, Found: C, 89.59; H, 5.97.
Scheme 1. Synthesis of 14-aryl or alkyl-14H-dibenzo[a, j]xanthenes in the presence of sandwich-type polyoxometalates.
Table 2. Synthesis of 14-aryl or alkyl-14H-dibenzo[a, j]xanthenes using K12[As2W18Cu3O68]·30H2O (I) and
K12[As2W18U3O74]·21H2O (II) catalysts under solvent-free conditions.
I Method A
I Method B
II Method A
II Method B
Time (h)/
Time (min)/
Yield (%)a
Time (h)/
Time (min)/
Yield (%)a
m.p. (◦C)
Ref.
Entry
R
Product
Yield (%)a
Yield (%)a
1
2
3
4
5
6
7
8
9
10
C6H5
4-NO2C6H5
4-FC6H5
4-ClC6H5
4-BrC6H5
4-CH3C6H5
4-OCH3C6H5
4-OHC6H5
CH3
3a
3b
3c
3d
3e
3f
3g
3h
3i
3.30/89
2.00/93
2.30/95
2.30/91
2.30/91
4.00/90
4.00/88
4.30/81
4.00/76
4.00/78
18.00/89
16.00/91
15.30/90
18.00/87
18.00/89
21.30/91
21.00/85
23.00 /81
25.00/74
24.00/79
2.30/88
2.00/91
2.00/94
2.00/92
2.30/90
3.00/92
3.30/89
4.00/82
3.30/75
4.00/80
10.00/85
10.00/95
8.00/93
8.00/90
8.30/92
9.00/93
12.00/85
12.00/79
12.00/72
12.00/77
183–18543
311–31244
239–24044
289–29044
297–29944
227–22944
202–20444
14045
149
CH3CH2
3j
151–15244
Method A: reaction carried out under conventional heating in solvent-free conditions
Method B: reaction carried out under microwave irradiation in solvent-free conditions
aYields refer to pure products and all products were characterized from their spectral data and were compared with authentic
samples

Catalytic application of two novel sandwich-type polyoxometalates
349
Scheme 2. Proposed mechanism for the synthesis of 14-substituted-14H-dibenzo[a, j]xanthenes.
irradiation K12[As2W18U3O74]·21H2O catalyst faci- requires long reaction time. It was found that under
litated the formation of dibenzo[a, j]xanthene in solvent-free conditions, the catalytic activities of
good yield, but K12[As2W18Cu3O68]·30H2O catalyst K12[As2W18U3O74]·21H2O and K12[As2W18Cu3O68]·
Table 3. Synthesis of 14-(4-fluorophenyl)-14H-dibenzo[a, j]xanthenes using different catalysts.
Catalyst
Conditions
Yield (%)
Time (h)
Ref.
K12[As2W18Cu3O68]·30H2O
K12[As2W18Cu3O68]·30H2O
K12[As2W18U3O74]·21H2O
K12[As2W18U3O74]·21H2O
H6[P2W18O62]·24H2O
H3[PW12O40]·3H2O
K5[CoW12O40]·3H2O
Indion-130
Solvent-free/125◦C
Microwave irradiation
Solvent-free/125◦C
Microwave irradiation
Solvent-free/100◦C
Solvent-free/100◦C
Solvent-free/125◦C
Solvent-free/110◦C
1,2 – Dichloroethane (reflux)
Water (reflux)
95
90
94
93
90
85
94
92
94
91
91
82
93
2.30
15.30 min
2.00
8.00 min
1.00
1.00
2.00
18.00 min
9.00
This work
This work
This work
This work
39
39
43
41
34
37
40
40
36
HClO4-SiO2
KAl (SO4)2·12H2O
4.00
P2O5
Solvent-free/80◦C
Solvent-free/80◦C
Solvent-free/90◦C
50.00 min
65.00 min
1.00
InCl3
Yb (OTf)3

350
Shabnam Sheshmani
30H2O were increased. In all the cases,
K12[As2W18U3O74]·21H2O was found to be a better 30H2O and K12[As2W18U3O74]·21H2O (time, yield,
reaction conditions) with efficiency of other cata-
Table 3 compares efficiency of K12[As2W18Cu3O68]·
catalyst than K12[As2W18Cu3O68]·30H2O (table 1).
After optimization of the reaction conditions, the lysts in synthesis of 14-(4-fluorophenyl)-14H-
generality of these conditions to other substrates was dibenzo[a, j]xanthene obtained by other groups. As it
studied. Various substituted aromatic and aliphatic can be seen, the used method is simpler, more efficient,
aldehyde were selected to undergo the reaction with β- and without toxic solvents.
naphthol in the presence of K12[As2W18Cu3O68]·30H2O
and K12[As2W18U3O74]·21H2O catalysts to pro-
4. Conclusions
duce the corresponding 14-aryl or alkyl-14H-
dibenzo[a, j]xanthenes under solvent-free conditions
(scheme 1). The results are summarized in table 2.
It was indicated that, electronic effects and the nature
of the substituted functional groups on the aldehyde
were affected on the reaction time. In comparison with
electron with-drawing groups on the aryl aldehydes
(table 2, entries 2–5), it was found that the presence of
electron donating groups on the aromatic ring (table 2,
entries 6–8) and also aliphatic aldehydes decreased
reaction rate (table 2, entries 9 and 10) and needed a
longer reaction time to give satisfactory yield.
In this work, we have developed a facile and efficient
method using sandwich-type polyoxometalates,
K12[As2W18U3O74]·21H2O and K12[As2W18Cu3O68]·
30H2O, as catalysts for one-pot synthesis of 14-
substituted-14H-dibenzo[a, j]xanthenes from β-
naphthol and various aldehydes. The catalytic activity
of these catalysts is remarkable and these are environ-
mentally benign. The advantages such as solvent-free
reaction conditions, shorter reaction time, simplicity
of the reaction, high product yields and the recycling
of the catalyst make K12[As2W18U3O74]·21H2O and
K12[As2W18Cu3O68]·30H2O as an efficient catalysts.
In a typical experimental procedure, when reaction
was complete, the catalyst was filtered, washed with
ethyl acetate, dried at 70◦C for 1 h, and finally reload-
ing the same catalyst for the new run without any
appreciable loss of activity.
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