F. Rajabi et al.
attention in recent years due to their wide range of appli-
cations. Coumarins are compounds exhibiting various bio-
logical activities including antibacterial, anticoagulant,
hypothermal, anti-helminthic [2–4] as well as different ap-
plications as food additives, perfumes and cosmetics [5].
Importantly, a large number of natural products with sig-
nificant biological activities and unique physical properties
also contain coumarin moieties in their chemical structure
[2]. Coumarin compounds have extensively been synthe-
sized using a number of known organic reactions including
Wittig [6], Perkin [7], Knoevenagel [8] and Pechmann [9]
type of chemistries. Among them, Pechmann condensation
processes can offer a significant advantage for coumarin
synthesis due to the simplicity of the protocol, the utilization
of widely available and relatively inexpensive reagents and
well known reaction mechanism via acid-catalysis [9].
Pechmann condensations generally involve the reaction
of phenolic compounds with a carboxylic acid or an ester
containing a b-carbonyl group to generate an intermediate
that undergoes dehydration to the final coumarin product.
Several protocols have extensively covered the synthesis of
coumarin derivatives via Pechmann condensation, mostly
catalysed by homogeneous acidic catalysts including
H2SO4, P2O5 [10], trifluoroacetic acid [11] as well as dif-
ferent metal salts such as FeCl3, TiCl4, InCl3, ZrCl4 [12–
15], Bi(NO3)3Á5(H2O) [16] and Cu(CH3CN)4PF6 [17]. The
use of homogeneous acid catalysts possesses inherent
corrosion, separation, product isolation and purification and
catalyst recycling issues, highly detrimental from the
practical point of view. Comparatively, the development of
efficient and environmentally benign solid acid catalysts
could provide advantages in the proposed coumarin syn-
thesis in terms of separation, product purification and cat-
alyst recycling. Alumina supported MoO3 [18], Keggin-
type heteropoly compounds [19] and silica sulfuric acid
[20] have therefore been explored as solid acid catalysts for
a greener synthesis of coumarins. In spite of clear advan-
tages of the reported heterogeneous solid acids utilized for
coumarin synthesis, most of the reported protocols still
entail long reaction times, high catalyst loadings, high re-
action temperatures and relatively poor reusable catalysts
[18]. A major challenge in the field deals with the design of
highly active, stable and reusable environmentally friendly
catalysts able to work under mild reaction conditions.
In continuation with research efforts from our groups
directed towards the design and development of advanced
nanocatalytic materials for fine chemicals production [21–
23], this contribution discloses an efficient, greener and
mild catalytic protocol for the synthesis of coumarin
derivatives using a previously reported and extensively
investigated Co/SBA-15 solid acid nanocatalyst. Excellent
yields to coumarins could be obtained using the Co/SBA-
15 system under solvent-free conditions.
2 Experimental
2.1 Catalyst Synthesis
Co/SBA-15 was synthesized as previously reported [21].
Briefly, salicylaldehyde (2 mmol, 0.244 g) was added to an
excess of absolute MeOH, to which 3-amino-
propyl(trimethoxy)silane (2 mmol, 0.352 g) was subse-
quently added. The solution instantly became yellow due to
imine formation. After 3 h, cobalt(II) acetate, Co(OAc)2Á
2H2O (1 mmol, 0.248 g) was added to the solution, and the
mixture further stirred for 3 h to allow the new ligands to
complex the cobalt. A color change from pink to olive
green is observed. Mesoporous silica (average pore di-
˚
ameter 60 A, 3 g) was activated by refluxing in concen-
trated hydrochloric acid (6 M) and then washed thoroughly
with the deionized water and dried before undergoing
chemical surface modification. This activation treatment
readily hydrolyses the siloxane Si–O–Si bonds to Si–OH
species which will be key to anchor the cobalt complex.
The complex and the activated silica were then mixed and
the mixture stirred overnight. The solvent is removed using
a rotary evaporator, and the resulting olive green solid
dried at 80 °C overnight. The final product was washed
with MeOH and water (to remove all physisorbed metal
species) until the washings were colourless. Further drying
of the solid product was carried out in an oven at 80 °C for
8 h.
2.2 Pechmann Reactions
To a mixture of phenol (1 mmol) and b-ketoester
(1 mmol), Co/SBA-15 (0.001 mmol) was added at 100 °C.
The mixture was stirred for 3 h (Table 2). The progress of
the reaction was monitored by thin layer chromatography
(TLC). After the complete conversion of the starting ma-
terials, as indicated by TLC, ethanol (10 mL) was added to
the reaction mixture was filtered and the solid residual
catalyst was washed with ethyl acetate (2 9 10 mL) and
then dried at 50 °C. After this, it could be used for the next
run. The filtrate was recrystallised from ethanol to get the
pure product. The products were characterized according to
1
their H and 13CNMR, IR, and melting point data.
3 Results and Discussion
Full characterization details of Co/SBA-15 have been
previously reported [21]. Typically, cobalt loading was
around 0.3 mmol g-1 and surface and textural properties of
Co/SBA-15 provided a surface area of 450 m2 g-1, with a
pore size of 3.6 nm and 0.77 cm3 g-1 mesoporous pore
volume, with an excellent homogeneous dispersion of
123
Rajabi, Fatemeh
