Natural phosphate doped with potassium fluoride and modified with sodium nitrate: Efficient catalysts for the Knoevenagel condensation

Article abstract of DOI:10.1016/S0040-4039(02)00092-8

A high yield synthesis of activated alkenes is described using the inexpensive natural phosphate alone, doped with potassium fluoride or modified by sodium nitrate. The Knoevenagel condensation was carried out in mild conditions at room temperature in met

Full text of DOI:10.1016/S0040-4039(02)00092-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1813–1815
Natural phosphate doped with potassium fluoride and modified
with sodium nitrate: efficient catalysts for the Knoevenagel
condensation
Sa¨ıd Sebti,* Abdellatif Smahi and Abderrahim Solhy
Laboratoire de Chimie Organique Applique´e et Catalyse, Universite´ Hassan II, Faculte´ des Sciences Ben M’Sik B.P. 7955,
Casablanca, Morocco
Received 25 December 2001; revised 9 January 2002; accepted 11 January 2002
Abstract—A high yield synthesis of activated alkenes is described using the inexpensive natural phosphate alone, doped with
potassium fluoride or modified by sodium nitrate. The Knoevenagel condensation was carried out in mild conditions at room
temperature in methanol. The catalytic activity of these phosphates are compared. © 2002 Elsevier Science Ltd. All rights
reserved.
Catalysis of organic reactions by natural inorganic
solids is an important new dimension in preparative
organic chemistry. The applications of inorganic solids
such as natural calcium phosphate (NP)1 as efficient
catalyst of inorganic transformations have been
exploited recently for several reactions.2,3 The NP has
several advantages, as it is inexpensive, environmentally
friendly, non-toxic, recoverable, reusable and also a
mild catalyst.
for the Knoevenagel condensation between an aldehyde
1 and an activated methylene 2 at room temperature in
methanol (Scheme 1).
Natural phosphate comes from an extracted ore in the
region of Khouribga (Morocco). The fraction of 100–
400 mm grain size has been washed with water, calcined
at 900°C for 2 h, washed again, calcined at 900°C for
0.5 h and ground (63–125 mm). The structure of NP is
similar to that of fluorapatite (Ca10(PO4)6F2), as shown
by X-ray diffraction pattern and chemical analysis.3f
The Knoevenagel reaction is one of the most important
CꢀC bond forming reactions available, because the
alkenes synthesized are very useful intermediates in
organic synthesis. In recent years, the Knoevenagel
reaction has been carried out in heterogeneous media in
the presence of several solids.4 Recently, we have shown
that NP and synthetic phosphates are excellent catalysts
of this reaction.2,5
The KF/NP was prepared by evaporating to dryness a
solution of potassium fluoride (1 g) in water in the
presence of NP (8 g). On the other hand, the impregna-
tion of the NP with a solution of sodium nitrate
(NaNO3/NP=1/2 w/w) followed by calcination gives
the modified natural phosphate NaNO3/NP. The gen-
eral procedure of preparation and characterization of
this catalyst were described in our previous work.6,7 In
a typical procedure, the aldehyde 1 (1.5 mmol) and the
active methylene compound 2 (1.5 mmol) in methanol
(1 mL) were stirred with phosphate catalyst (NP, KF/
NP or NaNO3/NP) (0.1 g) at room temperature for the
specified time (Table 1). Workup included extraction
Potassium fluoride supported on NP has been used to
promote several reactions.3 Recently, we have reported
that sodium nitrate modified NP can promote
efficiently the Claisen–Schmidt condensation6 and the
epoxidation of electron-deficient alkenes.7
We report here the utilization of NP alone, doped by
KF and modified by NaNO3 as heterogeneous catalysts
Keywords: natural phosphate; potassium fluoride; sodium nitrate;
heterogeneous catalysis; Knoevenagel condensation.
* Corresponding author. Tel.: +212 61 46 48 19; fax: +212 22 24 96
72; e-mail: saidsebti@yahoo.fr
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00092-8

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S. Sebti et al. / Tetrahedron Letters 43 (2002) 1813–1815
with CH2Cl2, filtration and evaporation. Products were
subsequently purified by distillation under vacuum or
recrystallization and identified by 1H NMR and IR
spectroscopy.
The results for a variety of alkenes prepared by the
Knoevenagel condensation catalyzed by NP, KF/NP
and NaNO3/NP are summarized in Table 1. It shown
that NP used in methanol can promote the Knoeve-
Table 1. Synthesis of alkenes 3 by the Knoevenagel reaction using NP, KF/NP and NaNO3/NP
Figure 1. Comparison of the catalytic activity of NP, KF/NP and NaNO3/NP in the synthesis of alkene 3g.

S. Sebti et al. / Tetrahedron Letters 43 (2002) 1813–1815
1815
nagel condensation with good yields. In the case of 3d
and 3g, the reaction rate is much slower and requires a
comparatively longer time for completion. However,
under the same conditions used for NP, the catalysts
KF/NP and NaNO3/NP are more reactive, the best
being NaNO3/NP (Fig. 1).
Commun. 1999, 29, 1057; (e) Alahiane, A.; Rochdi, A.;
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Rhihil, A.; Nazih, R.; Tahir, R. Appl. Catal. A 2001, 206,
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Borszeky, K.; Mallat, T.; Baiker, A. Tetrahedron: Asym-
metry 1997, 8, 3745; (h) De la Cruz, P.; Diez-Barra, E.;
Loupy, A.; Langa, F. Tetrahedron Lett. 1996, 37, 1113; (i)
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Jorda, J. L. Chem. Commun. 1999, 593; (k) Wiame, H. M.;
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V.; Villemin, D.; Jaffre´s, P. A. Green Chem. 2001, 3, 52.
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In summary, we have reported an efficient and conve-
nient route to the synthesis of the activated alkenes by
a Knoevenagel condensation. NP doped by potassium
fluoride or modified by sodium nitrate enhance the
yields obtained and reduce considerably the longer
reaction times. The yields are very high in a few
minutes.
Acknowledgements
Financial assistance of Ministry of Education, Govern-
ment of Morocco (PROTARS, P2T3/59) and CER-
PHOS, OCP group is gratefully acknowledged.
References
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