P2O5/SiO2 as a new, efficient, and reusable catalyst for preparation of β-enaminones under solvent-free conditions

Article abstract of DOI:10.1080/00397910802513052

P2O5/SiO2 (30% w/w) was applied as an efficient, heterogeneous, and reusable catalyst for the synthesis of β-enaminones. The reactions were rapidly completed at 80°C under solvent-free conditions and products were obtained in good to excellent yields. Copyright Taylor & Francis Group, LLC.

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P2O5/SiO2 as a New, Efficient,
and Reusable Catalyst for
Preparation of β-Enaminones
Under Solvent-Free Conditions
Mohammad R. Mohammadizadeh a , A. Hasaninejad a
, M. Bahramzadeh a & Z. Sardari Khanjarlou a
a Department of Chemistry, Faculty of Sciences,
Persian Gulf University, Bushehr, Iran
Version of record first published: 04 Mar 2009.
To cite this article: Mohammad R. Mohammadizadeh, A. Hasaninejad, M.
Bahramzadeh & Z. Sardari Khanjarlou (2009): P2O5/SiO2 as a New, Efficient, and
Reusable Catalyst for Preparation of β-Enaminones Under Solvent-Free Conditions,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:7, 1152-1165
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Synthetic Communications1, 39: 1152–1165, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802513052
P2O5/SiO2 as a New, Efficient, and Reusable
Catalyst for Preparation of b-Enaminones Under
Solvent-Free Conditions
Mohammad R. Mohammadizadeh, A. Hasaninejad,
M. Bahramzadeh, and Z. Sardari Khanjarlou
Department of Chemistry, Faculty of Sciences,
Persian Gulf University, Bushehr, Iran
Abstract: P2O5=SiO2 (30% w=w) was applied as an efficient, heterogeneous, and
reusable catalyst for the synthesis of b-enaminones. The reactions were rapidly
completed at 80 ꢀC under solvent-free conditions and products were obtained in
good to excellent yields.
Keywords: Amines, b-dicarbonyl compounds, b-enaminone, P2O5=SiO2,
solvent-free
INTRODUCTION
Enaminones are enamines of b-dicarbonyl compounds, and their chemistry
has been widely reviewed.[1,2] They are valuable precursors in organic
synthesis,[1–7] in pharmaceutical development,[2–7] and for synthesis of
heterocyclic compounds.[4,5] Because of their wide range of activity and
importance, a variety of methods for the synthesis of b-enaminones have
been developed.[2–7] The most well-known and standard route for synthesis
of b-enaminones involves the direct condensation of b-dicarbonyl com-
pounds with amines in refluxing aromatic hydrocarbons with azeotropic
removal of water.[8] Several other improved methods for the synthesis of
Received August 19, 2008.
Address correspondence to Mohammad R. Mohammadizadeh, Department
of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran.
E-mail: mrmohamadizadeh@pgu.ac.ir
1152

P2O5/SiO2 as a New, Efficient, and Reusable Catalyst
1153
Scheme 1.
b-enaminones have been reported: silica gel under microwave–assisted
solvent-free conditions,[9] montmorilonite K–10,[10] Bi(OTf)3,[11]
NaAuCl4,[12] Zn(ClO4)2 ꢁ 6H2O,[13] Zn(OAc)2 ꢁ 2H2O,[14] InBr3,[15] CoCl2,[16]
CeCl3 ꢁ 7H2O,[17] I2,[18] HClO4 ꢁ SiO2,[19] ZrOCl2 ꢁ 8H2O,[20] Yb(OTF)3,[21]
Sc(OTf)3,[22] and ionic liquids.[23]
A detailed survey on the methods for the synthesis as well as the
chemistry of title compounds has been recently reported.[23,24] Many of
these methods suffer from drawbacks such as long reaction times, unsa-
tisfactory yields, low selectivity, use of hazardous solvents, lack of general
applicability, higher temperatures, applications of nonavailable and
costly reagents, and tedious experimental procedures. So, although differ-
ent methods are available for the synthesis of b-enaminones, development
of other facile, high-yielding, nonpolluting procedures is still of great
importance.
Heterogeneous organic reactions have been recently performed with
immobilized reagents on solid supports. These procedures have advan-
tages such as cleaner reactions, easier workup, reduced reaction times,
and ecofriendliness.[25] P2O5=SiO2 is an inexpensive, heterogeneous,
stable, and commercially available catalytic system with sometimes much
more reactivity than unsupported P2O5. This catalyst has been used for
the Ritter reaction,[26] sulfonylation and nitration of aromatic com-
pounds,[27,28] protection and deprotection of aldehydes[29] or alcohols,[30]
Beckmann or Schmidt rearrangements,[31] preparation of bisindolyl-
methanes,[32] and solvent-free synthesis of N–sulfonyl imines.[33]
In continuation of our previous studies[34] on the synthetic applica-
tions of heterogeneous catalysts, we have recently observed that silica-
supported phosphorous pentoxide (P2O5=SiO2) is a highly efficient
catalyst for the preparation of b-enaminones and b-enamino esters from
b-dicarbonyl compounds by treatment with amines (Scheme 1).
RESULTS AND DISCUSSION
To achieve the best results, we examined the efficiency of different
reaction media and catalyst amounts for the condensation reactions of
dimedone and aniline as a model reaction (Table 1). For each reaction

1154
M. R. Mohammadizadeh et al.
Table 1. Comparison of the reaction of dimedone with aniline under various
conditions
Catalyst
amount (g)
Temperature
(ꢀC)
Yield
(%)b
Entry
Solventa
Time
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Toluene
CHCl3
THF
EtOH
CH3CN
EtOAc
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.7
0.5
0.3
0.0
80
80
Reflux
80
Reflux
Reflux
Rt
50
80
80
80
80
80
5 h
5 h
5 h
20
10
10
25
20
30
45
68
96
98
96
10
28
75
10 h
10 h
10 h
2.5 h
1.5 h
3 min
4 min
11 min
5 h
SiO2 (0.5)
P2O5 (0.3)
2 h
4 min
80
aAll reactions were run using dimedone (1 mmol) and aniline (1.1 mmol) in
4 mL solvent.
bYields referred to GC yields.
condition, conversion of dimedone (1 mmol) and aniline (1 mmol) to 5,
5-dimethyl-3-(phenylamino)-2-cyclohexenone was measured by gas–
liquid chromatography using a 0.01 M solution of product in toluene
as standard. As can be see from Table 1, significant rate enhancement
and improved yields were observed using about 0.5 g of P2O5=SiO2 under
solvent-free conditions. It is noticeable that P2O5 has good efficiency:
however, it is not reusable, and its applications suffer from difficulties
because of its troublesome handling. SiO2 has not shown valuable effi-
ciency for this reaction under same conditions.
The experimental procedure for this reaction is remarkably simple
and does not require the use of organic solvents or inert atmospheres.
About 0.5 g of P2O5=SiO2 and the required amine were added to 1 mmol
of the 1,3-dicarbonyl compound, and the mixture was stirred at 80 ꢀC.
The reactions took place within only a few minutes to afford the product
in excellent yields.

P2O5/SiO2 as a New, Efficient, and Reusable Catalyst
1155
The generality of the method has been shown by the preparation of a
series of b-enaminones and b-enamino esters using various b-dicarbonyls
and amines (Table 2). The b-dicarbonyl compounds included here are both
b-diketones (linear and cyclic) and b-ketoesters. They reacted equally with
aromatic as well as aliphatic amines. In the case of 1,2-diaminoethane, 2
equiv of b-dicarbonyl compounds were used giving products with two
enaminone groups (entries c and f). In all these cases, the chemoselectivity
was very good; that is, only Z-b-enaminones and b-enamino esters were
1
formed predominantly as confirmed by H NMR spectra of the crude
products. The (Z)-selectivity in the products derived from acyclic diketones
and b-ketoesters was secured by intramolecular hydrogen bonding. In the
1H NMR spectra the proton of the –NH– group appeared in the region
of d 8.1–12.5. However, the b-enaminones derived from cyclic diketone,
1
5,5-dimethyl-1,3-cyclohexadione, displayed the H NMR spectra, having
the signals for the nonhydrogen bonded proton of the –NH– group in the
region of d 4.5–6.5 and thus indicating the (E)-configuration.
In addition to aniline and weakly activated anilines, this method was suc-
cessfully applied to aniline with strongly electron-withdrawing groups. For
example, 4-nitroaniline reacted with acetylacetone in the presence of
P2O5=SiO2 to form the corresponding b-enaminone in 80% yields (entry b).
Previously, a similar reaction carried out using other catalytic systems yielded
at best less than 30% of product.[17,19] In the cases of b-diketones with two dif-
ferent substituents, such as the commercially available benzoylacetone, we
obtained selectively the adduct deriving from a nucleophilic attack of the
amine to the carbonyl bearing a methyl group (entries d, e, and f).
The use of P2O5=SiO2 as a heterogeneous catalyst showed rate
enhancements, high yields, and short reaction times. However, one of
the important points regarding the heterogeneous catalysts is their deac-
tivation and reusability. From every reaction, the P2O5=SiO2 catalytic
system has been recovered by filtration of the reaction mixture and wash-
ing of the filtrate with acetone. Table 3 shows the results of five consecu-
tive runs of the reaction of dimedone with aniline and shows that the
catalyst could be reused several times without noticeable loss of activity.
This reusability demonstrates the high stability and turnover of the
P2O5=SiO2 catalytic system under operating conditions.
In conclusion, we have developed a simple, fast, and efficient proce-
dure for synthesis of b-enaminones and b-enamino esters catalyzed by
P2O5=SiO2 under solvent-free conditions. The mild conditions, simple
workup, high yields, and recyclability of the catalyst make our metho-
dology a valid contribution to the existing processes in the field of
b-enaminone synthesis. Furthermore, the method was successfully
applied for anilines bearing powerful electron-withdrawing substituents,
and their corresponding enaminones were obtained in good yields.

1156

1157

1158

1159

1160

P2O5/SiO2 as a New, Efficient, and Reusable Catalyst
1161
Table 3. Reusability of the catalyst for a series of five consecu-
tive runs of the reaction of dimedone with aniline
Time
(min)
Conversion
(%)
Yield
(%)
Entry
1
2
3
4
5
4
4
4
4
4
100
100
100
100
100
96
95
96
94
95
EXPERIMENTAL
Preparation of P2O5/SiO2 Catalyst
A mixture of SiO2 (5 g) and P2O5 (1.5 g) at room temperature was ground
vigorously for 30 min to give a P2O5=SiO2 catalytic system as a white
powder.
General Procedure
To a mixture of a dicarbonyl compound (1 mmol) and an amine
(1.1 mmol), P2O5=SiO2 (0.5 g) was added. The mixture was stirred at room
temperature. The reaction was monitored by thin-layer chromatography
(TLC). After completion of the reaction, mixture was diluted with EtOAc
(5 ml) and filtered. For the entries a–e and h–j, the filtrate was concen-
trated and the gummy mass was subjected to column chromatography
over silica gel using n-hexane=ethyl acetate (5:1) as eluent to obtain pure
b-enaminone. For the entries f, g, and k–x, solvent was evaporated and
the pure products were obtained by recrystallization of residue from
appropriated solvent. The catalyst was recovered from the residue.
Spectral (1H and 13C NMR) and Analytical Data of New
Compounds 3s–x
Compound 3s
1
Colorless solid; mp 225–227 ꢀC; H NMR (CDCl3, 250 MHz) d 1.13 (s,
6H), 2.10 (s, 2H), 2.24 (s, 2H), 2.76 (s, 3H), 5.73 (s, 1H), 7.02 (d,
J ¼ 7.1 Hz, 2H), 7.12 (s, 1H), 7.84 (d, J ¼ 7.1 Hz, 2H); 13C NMR (CDCl3,

1162
M. R. Mohammadizadeh et al.
63 MHz) d 26.4, 28.1, 32.8, 43.6, 50.3, 100.6, 115.8, 121.2, 121.6, 129.8,
133.0, 140.3, 143.2, 158.8, 196.8, 198.4. Anal. calcd. for C16H19NO3: C,
70.31; H, 7.01; N, 5.12. Found: C, 70.22; H, 7.01; N, 5.00.
Compound 3t
1
Colorless solid; mp 213–215 ꢀC; H NMR (CDCl3, 250 MHz) d 1.16 (s,
6H), 2.16 (s, 2H), 2.42 (s, 2H), 5.60 (s, 1H), 7.22 (d, J ¼ 10 Hz, 2H),
6.66 (broad pick, 1H), 7.89 (d, J ¼ 10 Hz, 2H); 13C NMR (CDCl3,
63 MHz) d 26.7, 32.8, 43.4, 50.4, 100.8, 106.7, 118.6, 122.1, 133.4,
143.2, 165.1, 198.7. Anal. calcd. for C15H16N2O: C, 74.97; H, 6.71; N,
11.66. Found: 75.03; H, 6.64; N, 11.76.
Compound 3u
1
Colorless solid; mp 219–220 ꢀC; H NMR (CDCl3, 250 MHz) d 1.02 (s,
6H), 2.14 (s, 2H), 2.36 (s, 2H), 5.13 (s, 1H), 6.66 (broad pick, 1H), 6.96
(d, J ¼ 8.6 Hz, 2H), 7.39 (d, J ¼ 8.6 Hz, 2H); 13C NMR (CDCl3,
63 MHz) d 26.8, 28.5, 31.8, 44.1, 50.4, 100.2, 121.6, 129.8, 132.7, 140.3,
143.2, 198.7. Anal. calcd. for C14H16BrNO: C, 57.16; H, 5.48; N, 4.76.
Found: C, 57.10; H, 5.55; N, 4.88.
Compound 3v
1
Colorless solid; mp 162–163 ꢀC; H NMR (CDCl3, 250 MHz) d 1.14 (s,
6H), 1.92 (s, 3H), 2.19 (s, 3H), 4.35 (s, 2H), 5.20 (s, 1H), 6.17 (broad pick,
1H), 7.22 (m, 2H), 7.68 (m, 1H), 8.55 (m, 1H); 13C NMR (CDCl3,
63 MHz) d 28.3, 32.9, 43.4, 46.8, 50.3, 96.3, 122.0, 122.7, 136.9, 148.8,
154.6, 162.2, 196.9. Anal. calcd. for C14H18N2O: C, 73.01; H, 7.88; N,
12.16. Found: 73.23; H, 7.96; N, 12.00.
Compound 3w
1
Colorless solid; mp 149–150 ꢀC; H NMR (CDCl3, 250 MHz) d 2.49 (s,
3H), 5.28 (s, 1H), 6.77 (d, J ¼ 7.5 Hz, 2H), 7.39–7.51 (m, 7H), 7.97 (d,
J ¼ 7.5 HZ, 2H), 8.10 (d, J ¼ 7.0 Hz, 2H), 12.88 (s, 1H); 13C NMR
(CDCl3, 63 MHz) d 26.1, 99.0, 112.3, 113.8, 121.6, 127.4, 128.2, 128.5,
128.9, 129.3, 130.1, 130.8, 131.8, 132.2, 135.5, 139.4, 144.1, 160.0,
163.4, 190.3, 196.8, Anal. calcd. for C23H19NO3: C, 77.29; H, 5.36; N,
3.92. Found: C, 77.08; H, 5.20; N, 4.06.

P2O5/SiO2 as a New, Efficient, and Reusable Catalyst
1163
Compound 3x
1
Colorless solid; mp 162–163 ꢀC, H NMR (CDCl3, 250 MHz) d 0.64 (d,
J ¼ 7.5 Hz, 6H), 1.01(s, 6H), 1.97 (m, 1H), 2.99 (s, 2H), 3.01 (s, 2H),
4.25 (d, J ¼ 7.5 Hz, 2H), 5.96 (s, 1H), 8.10 (s, 1H); 13C NMR (CDCl3,
63 MHz) d 20.3, 27.7, 27.9, 32.8, 43.1, 47.2, 51.1, 93.7, 128.0, 192.6, Anal.
calcd. for C12H21NO: C, 73.80; H, 10.84; N, 7.17. Found: C, 73.69; H,
10.93; N, 7.02.
ACKNOWLEDGMENTS
The authors thank the Research Committee of Persian Gulf University
for financial support of this work.
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