Fluorapatite: Efficient catalyst for the Michael addition

Article abstract of DOI:10.1016/S0040-4039(03)00323-X

A Michael addition assisted by fluorapatite in heterogeneous media is described. Reaction between mercaptans and chalcone derivatives was studied at room temperature in methanol as solvent. By-products of usual undesirable reactions in Michael addition su

Full text of DOI:10.1016/S0040-4039(03)00323-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2463–2465
Fluorapatite: efficient catalyst for the Michael addition
Mohamed Zahouily,a,* Younes Abrouki,a Ahmed Rayadh,a Sa¨ıd Sebti,b Hamid Dhimanec and
Marc Davidc
aLaboratoire de Synthe`se Organique et Traitement de l’Information, UFR de Chimie Applique´e, Universite´ Hassan II,
Faculte´ des Sciences et Techniques, BP 146, 20650 Mohammadia, Morocco
bLaboratoire de Chimie Organique Applique´e et Catalyse, Universite´ Hassan II, Faculte´ des Sciences Ben M’Sik, BP 7955,
20702 Casablanca, Morocco
cLaboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, UMR 8601, Universite´ Rene´ Descartes (Paris 5),
45 rue des Saints-Pe`res, 75270 Paris Cedex 06, France
Received 2 February 2003; accepted 3 February 2003
Abstract—A Michael addition assisted by fluorapatite in heterogeneous media is described. Reaction between mercaptans and
chalcone derivatives was studied at room temperature in methanol as solvent. By-products of usual undesirable reactions in
Michael addition such as 1,2-addition, bis-addition and polymerisation are not observed. The yields obtained are good to
excellent. © 2003 Elsevier Science Ltd. All rights reserved.
Surface-mediated solid-phase reactions are of growing
interest1 because of their ease of set-up and work-up,
mild reaction conditions, rate of the reaction, selectiv-
ity, high yields, lack of solvent in some cases and the
low cost of the reactions as compared with their homo-
geneous counterparts. The development of an efficient
and selective solid base catalyst for the construction of
a carbonꢀcarbon and sulfurꢀcarbon bond continues to
be a challenging exploration in organic synthesis. The
versatile Michael reaction has numerous applications in
synthesis of fine chemicals2 and is classically catalysed
by base3 under homogeneous conditions. In heteroge-
neous phase various solid catalysts have been found
useful, including natural phosphate alone and doped by
develop a heterogeneous catalysis, we describe in this
paper, the use of the fluorapatite (FAP)10 as a new solid
support in Michael addition between chalcone deriva-
tives 1 and thiols 2, in methanolic solution at room
temperature (Scheme 1).
The synthesis of FAP in powder state is carried out by
reaction between diammonium phosphate, calcium
nitrate and ammonium fluoride in the presence of
ammonia (Scheme 2).
FAP was obtained by co-precipitation: 250 mL of a
solution containing 7.92 g of diammonium hydrogen
phosphate and 1 g of ammonium fluoride, maintained
at pH greater than 12 by addition of ammonium
hydroxide (15–20 mL), were dropped under constant
stirring into 150 mL of a solution containing 23.6 g
calcium nitrate (Ca(NO3)2·4H2O). The suspension was
potassium
fluoride,4
synthetic
diphosphate
Na2CaP2O7,5 zeolite,6 NiBr2/montmorillonite,7 MgꢀAl
hydrotalcite8 and other catalysts with more or less
success.9 In continuation of our ongoing programme to
Scheme 1.
Keywords: Michael addition; fluorapatite; heterogeneous catalysis; recyclable catalyst.
* Corresponding author. Fax: +212-23-315353; e-mail: zahouily@voila.fr
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00323-X

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M. Zahouily et al. / Tetrahedron Letters 44 (2003) 2463–2465
refluxed for 4 h. Doubly distilled water (DDW) was used
to prepare the solutions. The FAP crystallites were
filtered, washed with DDW, dried overnight at 80°C and
calcined in air at 700°C for 30 min before use. The final
product is identified by X-ray diffraction (space group
hexagonal system; a=9.364 A and c=6.893 A), infrared
spectra IR and chemical analysis (Ca=38.29%, P=
17.78% and Ca/P=1.66). The BET specific surface area
was found to be S=15.4 m2/g. The total pore volume was
calculated by the BJH method at P/Po=0.98 (Vt=0.0576
cm3/g).
Except in one case, all the combinations lead selectively
to the corresponding expected 1,4-adducts. No by-prod-
ucts resulting from the undesirable 1,2-addition and/or
bis-addition side reactions (usually observed under clas-
sical conditions in some cases) were observed. However
reaction of 2-aminothiophenol with the Michael acceptor
bearing a nitro group (1 with X=NO2) gives an 85/15
mixture of 1,4- and 1,3-addition products (entry 2). The
formation of this a priori unexpected regioisomer
requires both a strong nucleophilic thiol and a strong
electron-acceptor X group on the aromatic moiety of
chalcone derivative.
,
,
The use of FAP thus prepared was examined as hetero-
geneous catalyst in the Michael addition of three thiols
2 to enones 1 derived from acetophenone and various
p-substituted benzaldehydes. With such combinations
the 1,4-adducts were isolated, as solids, with yields
ranging from 57 to 96%, depending on the electronic
demand of both thiol 2 and enone 1 (Table 1).11
Solid catalysts become particularly interesting when they
can be regenerated. Indeed, in our case, FAP was
recovered quantitatively by simple filtration and regener-
ated by calcination for 15 min at 700°C. The recovered
catalysts was reused several times without loss of activity,
even after the seventh cycle product 3g was obtained with
the same yield.
Results in Table 1 show, as expected, that the reaction
time is highly dependent on the nucleophilicity of the
mercaptan 2. Moreover, the presence of electron-with-
drawing groups X on the aromatic ring of the Michael
acceptor 1 decreases the reaction time proportionally to
the value of the Hammet constant. Meanwhile, the
presence of electron-donating groups X increases the
reaction time, demonstrating the participation of both
the enone and the thiol in the rate controlling step of the
reaction.
For the catalytic activity of FAP in this Michael addition
we speculate that the reaction occurs at the surface rather
than inside tunnels of the catalyst. The dimensions of the
tunnels in our catalyst are not large enough compared
to those of zeolites.12 Thus, we estimate that probably the
surface of FAP presents multicatalytic active sites. The
basic sites (CaF2 and oxygen of PO4 group) enhance the
thiol nucleophilicity and the acidic sites (Ca2+ and
phosphorus of PO4 group) probably increases the enone
moiety polarization. Consequently, the SꢀC bond forma-
tion is accelerated and the final product is obtained after
protonation of the resulting enolate.
In summary, a simple procedure and work-up, relatively
fast reaction rates, mild reaction condition, good yields,
and selectivity of the reaction make the fluorapatite an
attractive and useful catalyst for the Michael addition.
Scheme 2.
References
Table 1. Synthesis of products 3 by Michael addition
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M. Zahouily et al. / Tetrahedron Letters 44 (2003) 2463–2465
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