Jafari et al.
industries [12] has been an area of research in the last decades.
consume the starting material as specified in Table 2. After the
completion of the reaction, diethyl ether (10 ml) was added to
the reaction mixture and the catalyst was recovered by a
simple filtration. Petroleum ether/diethyl ether was removed
by a simple distillation. The sole corresponding pyrroles were
obtained after the recrystallization in ethanol-water and
consequently simple filtration.
Method B: Amine (1 mmol), 2,5-dimethoxytetrahydro-
furan (1.2 mmol), and HPA/SiO2 10% (2.5 mol%) were mixed
carefully, put in the MW vessel and irradiated in the
microwave oven as specified in Table 2. After the completion
of the reaction, diethyl ether (10 ml) was added to the reaction
mixture to dissolve the product and the catalyst was recovered
by a simple filtration. Diethyl ether was removed by a simple
distillation. The sole corresponding pyrroles were obtained
after recrystallization in ethanol-water and consequently
simple filtration.
Heteropoly acids are more active catalysts than the
conventional inorganic and organic acids for various reactions
in solutions [13]. They are used as industrial catalysts for
several liquid-phase reactions [14-17]. Among heteropoly
acids, polytungstic acids are the most widely used catalysts
owing to their high acid strengths, thermal stability, and low
reducibility. Catalysts based on heteropoly acids such as
Bronsted acids have many advantages over liquid acid
catalysts. They are non-corrosive and environmentally benign,
presenting fewer disposal problems. Solid heteropoly acids
have attracted special attention in organic synthesis owing to
easy work-up procedures, easy filtration, and minimization of
cost and waste generation due to the reusability and
recyclability of the catalysts [18]. Supported heteropoly acid
on silica gel has been used as an effective catalyst for Diels
Alder [19a] and Fries rearrangement [19b], Friedel Crafts
reactions [19c,d], oxathioacetalization of carbonyl compounds
[20a], synthesis of β-acetamido ketones [20b], Isomerisation
of α-pinene oxide [20c], and photocatalytic oxidation of
benzylic alcohols to carbonyl compounds [20d].
In the last few years, we have concentrated on the
exploration of new catalytic activities of H3PW12O40 and its
salts [12d,12e,20a,20e], for basic chemical transformations. In
this article, we report that H3PW12O40 supported on silica gel
(H3PW12O40/SiO2) could be used as a highly efficient catalyst
for the condensation of amines with 2,5-dimethoxytetrahydro-
furan to produce N-substituted, 2- and 3-unsubstituted pyrroles
in solution or under solvent-free microwave irradiation
(Scheme 1).
Selected Analytical and Spectral Data for New
Compounds
1-(3-Pyrrole-1-phenyl)-ethanone (Table 2, entry 9).
M.p.: 55-56 °C; FT-IR: υmax(neat) = 3064, 1681, 1587, 1497,
1444, 1340, 1249, 1075, 723, 687 cm-1;ꢀ1H NMR (500 MHz,
CDCl3): δ = 2.64 (s, 3H), 6.29 (s, 2H), 7.46 (s, 2H), 7.59 (dd,
J = 8.0, 7.6 Hz, 1H), 7.80 (d, J = 7.6 Hz, 1H), 7.83 (d, J = 8
Hz, 1H), 8.03 (s, 1H) ppm; 13C NMR (125 MHz, CDCl3): δ =
27.3, 110.9, 111.3, 111.6, 119.1, 119.6, 124.2, 124.9, 125.3,
129.7, 130.5, 138.6, 140.6, 198.0 ppm.
1-(3-((1H-pyrrol-1-yl)methyl)benzyl)-1H-pyrrole
(Table 2, entry 10). M.p.: 92-93 °C; FT-IR: υmax(neat) =
3000, 2930, 1607, 1520, 1443, 1407, 1325, 1301, 974, 751
cm-1;ꢀ1H NMR (500 MHz, CDCl3): δ = 4.72 (s, 4H), 4.99 (s,
4H), 5.89 (s, 4H), 6.58 (s, 1H), 6.74 (d, J = 7.5 Hz, 2H), 7.24
(t, J = 7.5 Hz, 1H) ppm; 13C NMR (125 MHz, CDCl3): δ =
47.1, 105.9, 123.5, 124.8, 128.3, 129.7, 139.5 ppm.
EXPERIMENTAL
General Procedure for the Synthesis of Pyrroles
Method A: To a solution of amine (1 mmol), 2,5-
dimethoxytetrahydrofuran (1.2 mmol) in petroleum ether 40-
60 was added HPA/SiO2 10% (2.5 mol%) and stirred to
Bis(4-(1H-pyrrole-1-yl)phenyl)methan (Table 2, entry
11). M.p.: 94-95 °C; FT-IR: υmax(neat) = 2919, 1516, 1402,
Scheme 1
ꢀ
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