7954
S. Sebti et al. / Tetrahedron Letters 42 (2001) 7953–7955
phate calcined at 900°C, showed the apparition of two
new phases which can be attributed to sodium phos-
phate and calcium oxide. This phenomenon can be due
to a possible reaction in the solid state between sodium
nitrate and the natural phosphate. On the contrary, the
structures of NP and NaNO3 are not modified by
calcination of NaNO3/NP at 300 or 500°C, in agree-
ment with their low catalytic activity. It is worth noting
that both sodium phosphate9–11 and calcium oxide15
have been used in heterogeneous catalysis. Thus, they
can contribute to an increase in the basic activity of
NP. However, the intensity of their peaks in X-ray
pattern of NaNO3/NP is very low.
range of 40–74% after 24 h, which can be increased to
70–93% with longer reaction times. It is important to
note that, under the same conditions, unmodified natu-
ral phosphate displayed almost no activity, p-methoxy-
acetophenone did not react and with acetophenone
yields did not exceed 10% after 24 h. It is even possible
to further reduce the amount of catalyst. Thus, 78%
yield of chalcone 3a was obtained, after 48 h of reac-
tion, with only 0.05 g of NaNO3/NP. It is then clear
that doping NP with sodium nitrate remarkably
increases its catalytic activity.
To sum up, the modified natural phosphate (NaNO3/
NP) is a new inorganic reagent, which can represent an
important breakthrough in the development of basic
solid catalysts. Several chalcones have been synthesised
with high yields using catalytic amounts of NaNO3/NP.
This phosphate is easily prepared from inexpensive
precursors.
The reactions were carried out between arylaldehydes 1
(2.5 mmol) and acetophenones 2 (2.5 mmol) at room
temperature in the presence of a catalytic amount of
NaNO3/NP (0.1 g) in methanol (1–3 mL) (Scheme 1).
First of all we tried to determine the best nitrate/NP
ratio (r=m0/m1). With this aim, we carried out the
synthesis of chalcone 3a with 0.1 g of NaNO3/NP using
different nitrate/NP ratios (r=1/2, 1/3, 1/5, 1/8, and
1/15 w/w). The yields obtained after 24 h of reaction
(98, 78, 67, 8 and 2%, respectively) show that sodium
nitrate/NP=1/2 w/w is the optimal composition.
Acknowledgements
Financial assistance of the Ministry of Education, Gov-
ernment of Morocco (PROTARS, P2T3/59) and
CICYT (Project MAT99-1176) is gratefully acknowl-
edged. We are also indebted to OCP for financial
support and technical assistance.
Thereafter, we carried out the synthesis of several chal-
cones 3 using 0.1 g of NaNO3/NP (1/2) (Table 1). All
1
products were isolated, purified, and analysed by H,
13C NMR, IR, and MS. Only the E-isomers were
obtained. The yields were, in general, very high and
exceeded 90% after 24 h of reaction, except for the
chalcones obtained by condensation of arylaldehydes
with p-methoxyacetophenone (3f, 3g and 3h). The low
reactivity of this acetophenone, due to the presence of
an electron-donor group, accounts for the yields in the
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