Study on dry-media microwave azalactone synthesis on different supported KF catalysts: Influence of textural and acid-base properties of supports

Article abstract of DOI:10.1039/b109413k

Twenty-five inorganic solids studied as supports for KF were screened with respect to the synthesis of the azalactone obtained by dry-media condensation of p-hydroxybenzaldehyde with hippuric acid in acetic anhydride (1:1:4, molar ratio), in 3 g of 10 wt%

Full text of DOI:10.1039/b109413k

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Study on dry-media microwave azalactone synthesis on different
supported KF catalysts: influence of textural and acid–base
properties of supports
2
Felipa M. Bautista, Juan M. Campelo, Angel García, Diego Luna,* José M. Marinas and
Antonio A. Romero
Departamento de Química Orgánica, Facultad de Ciencias, Universidad de Córdoba, Campus
Universitario de Rabanales, Edificio C-3, E-14014 Córdoba, Spain. E-mail: qo1lumad@uco.es
Received (in Cambridge, UK) 16th October 2001, Accepted 28th November 2001
First published as an Advance Article on the web 10th January 2002
Twenty-five inorganic solids studied as supports for KF were screened with respect to the synthesis of the azalactone
obtained by dry-media condensation of p-hydroxybenzaldehyde with hippuric acid in acetic anhydride (1 : 1 : 4,
molar ratio), in 3 g of 10 wt% KF support catalysts. The results obtained indicate that there was a number of
supported KF catalysts with conversions higher than or comparable to that obtained by the classical Erlenmeyer
method of azalactone synthesis. Furthermore, not only were there very important differences in the catalytic behavior
of different KF support systems, but also these differences are closely related to the procedure used in the reaction
heating. In this respect, the well-documented KF–Al2O3 was the best catalyst when reactions were conducted under
conventional heating. However, when reactions were carried out in a domestic microwave oven, KF supported on
AlPO4, TiO2 or Zn3(PO4)2 were better catalysts than KF–Al2O3. The importance of the textural and acid–base
properties of solid inorganic supports was also demonstrated with respect to the catalytic behavior of supported KF
catalysts. Finally, results obtained in the synthesis of thirteen different azalactones, lead us strongly to recommend
the procedure using microwave irradiation and KF–AlPO4, over classical heating with KF–Al2O3. The advantages are
good yields, easier work-up, a significant decrease in reaction times, and easy re-use of catalysts when operated in
dry-media, especially by use of a domestic microwave oven.
number of gas-phase organocationic reactions,17 as metal
Introduction
supports in the liquid-phase hydrogenation of alkenes and
Azalactones, or 2,4-substituted oxazolin-5-ones, are important
intermediates in the preparation of several fine chemicals,
including amino acids,1 peptides,2 antimicrobial or antitumor
compounds,3 some heterocycle precursors4 as well as biosensors
or coupling and/or photosensitive composition devices for pro-
teins.5 They are usually obtained by condensation reactions of
different arylaldehydes with hippuric acid in acetic anhydride,
in the presence of anhydrous sodium acetate as a homogeneous
basic catalyst, according to the Erlenmeyer method.6 This
classical method has generally been shown to produce Z-stereo-
isomers, although several procedures have reportedly obtained
the corresponding E-stereoisomers.7 Furthermore, Pb(OAc)2 is
used when the reaction is extended to α-hydroxyketones, α,αЈ-
dihydroxyketones, α-diketones and methylcyclohexanones.3b,8
Recently, an improved procedure has been reported for the syn-
thesis of arylideneoxazolones from hippuric acid and aromatic
aldehydes in the presence of acetic anhydride and anhydrous
zinc chloride9 as well as by using microwave irradiation.10
The use of environmentally friendly inorganic solids, as
catalysts or reaction media, is rapidly increasing11 because
these reactions, compared to their homogeneous counterparts,
often involve milder experimental conditions, an easier
set-up and work-up, more rapid reaction rates, increased yield
and/or selectivity, easy removal, recycling and reuse of
catalysts, and minimal wastes. Thus, we have studied the use of
the amorphous solids AlPO4 and AlPO4–Al2O3 as catalysts for
Diels–Alder reactions,12 1,3-dioxolane synthesis by acetaliza-
tion of different carbonyl compounds,13 tetrahydropyranylation
of alcohols and phenols,14 the liquid-phase retro-aldolization of
diacetone alcohol15 as well as for Knoevenagel condensations
in dry media.16 These solids have also been used as catalysts in a
alkynes bearing a variety of organic functional groups,18 as a
KMnO4 support in the benzaldehyde oxidation reaction,19 and
as a KF support in the Michael addition of nitromethane to
but-3-en-2-one.20
The classical Erlenmeyer synthesis of azalactones consists
of a condensation reaction between a carbonyl group and the
acidic methylene of hippuric acid (or acetylglycine), with the
loss of a water molecule, which works well with aromatic
aldehydes, moderately well with ketones, but often fails with
hindered or less active ketones. Thus, in principle, it is a con-
densation reaction very similar to that of Knoevenagel,
Michael or Aldol condensations, which can be effected by many
reagents including KF–Al2O3 catalysts in dry-media under
conventional heating,11a,c,21 as well as under microwave irradi-
ation.22 In fact, any condensation reaction where water or
alcohol are eliminated were found to constitute good candi-
dates for microwave treatment, and in like manner, reactions
like these that produce water are very sensitive to the catalytic
action of KF–Al2O3 catalysts.23
Thus, KF–Al2O3 is among the best studied and most fre-
quently employed supported reagents although the reasons for
its high catalytic activity remain a point of contention.23 The
influence of different alkali-metal fluorides24 was investigated
very early on even though there is still not much information
about the effects of different supports at the present time.25
In this respect, we had previously found that KF–ZnO was
a better catalyst than KF–Al2O3 in a Michael condensation
reaction carried out in dry-media conditions.20
Thus, in the present study we have undertaken a screening of
different inorganic solids as supports for KF in azalactone syn-
thesis, through the dry-media condensation of arylaldehydes
DOI: 10.1039/b109413k
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234
This journal is © The Royal Society of Chemistry 2002
227

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with hippuric acid in acetic anhydride, under conventional
heating, and under microwave irradiation in a domestic micro-
wave oven, at atmospheric pressure through use of an open
reaction vessel. Furthermore, the influence of textural and
acid–base properties of supports on the catalytic behavior
of the supported KF catalyst was also investigated because,
under dry-media reaction conditions, the textural and acid–
base properties of solid surfaces must play a similar role to that
of the solvent in homogeneous processes.
cedure,6 as well as with the same amount of NaOAc supported
on Al2O3-150, Table 2. Taking into account that the classical
Erlenmeyer procedure gave 69.1% yield and that the yield
obtained with a NaOAc–Al2O3-150 catalyst was 55.5% under
conventional heating, and 21.3%, when the microwave oven
was used, we can conclude that KF–NaOAc is a better catalyst
than NaOAc alone, and than Al2O3 alone or with NaOAc,
for azalactone synthesis. In this respect, when we try to effect
azalactone synthesis under microwave irradiation in homo-
geneous conditions (with NaOAc and without support) only a
2.4% yield was obtained.
The apparent contradiction with the literature where high
yields are obtained,10 is solved by taking into account that such
reactions, as compared to the present experimental conditions,
were carried out with a much greater amount of Ac2O (four
times as much). Thus, Ac2O also plays the role of solvent, which
provides the driving force of the condensation reaction due to
its high boiling point (139 ЊC). In fact, when carried out in this
way, a large quantity of Ac2O evaporated, so it was necessary to
work with a large amount of “Ac2O solvent” to obtain the
yields indicated.10
Results and discussion
The screening of twenty-five different inorganic solids studied
as supports for KF was carried out with respect to the synthesis
of the azalactone obtained by the dry-media condensation of
p-hydroxybenzaldehyde with hippuric acid in acetic anhydride
(1 : 1 : 4, molar ratio), with 3 g of 10 wt% KF support catalysts.
As outlined in Scheme 1, and according to the literature,6c the
At the present time the catalytically active basic sites in KF–
Al2O3 are associated with several reaction products of KF and
some surface atoms on the Al2O3 support.23 Thus, instead of
the support being the true catalyst, the interaction of several
support surface species and KF can give rise to basic sites,
which differ not only in nature, but also in number and
strength, and these may in fact act as the true catalysts.
Consequently, in order to determine the role of the surface
properties of inorganic solids on the catalytic behavior of
supported KF solids, we measured their surface areas (BET
method)12–20 as well as their acid–base properties (determined
by a spectrophotometric method using pyridine and benzoic
acid as titrant agents),12–20 Table 3. Furthermore, we developed
a correlation matrix by using all the data in Table 1 and Table 3.
The values of the slopes and intercepts obtained in the regres-
sion analysis of well-correlated parameter pairs are collected in
Table 4, where the corresponding significance levels are also
indicated.
Results obtained indicate that textural and acid–base proper-
ties of the support play an important role in the catalytic
behavior of 10 wt% supported KF catalyst but, at the same
time, these effects are not always exactly the same depending on
the heating procedure. This is in contrast with the good correl-
ation between YMW and YCLAS (the azalactone yields obtained
by microwave irradiation and by classical heating), which could
be interpreted as the standard of the support effects for both
kinds of reaction. However, when we consider the good correl-
ations obtained between the azalactone yield by classical
heating, YCLAS, and the inverse of the number of acid sites
titrated with pyridine, 1/XmA, the negative influence of the
acidity of the inorganic solid used as the KF support is obvious.
At the same time, for reactions conducted under microwave
irradiation, the specific basicity of the support (or number of
basic sites titrated by benzoic acid per unit surface area) XmB/
SBET, is that which basically determines the YMW values.
In summary, we plainly see that the support should
preferably not have acid sites for conventional heating, while
good yields are obtained for reactions conducted under micro-
wave heating when the number of basic sites is as high as
possible for a given surface area, SBET. Both requirements are
usually related, but they are not exactly the same, as can be seen
in Table 3. These results could indicate that, depending on the
type of heating used (classical or microwave), different basic
surface-active sites take part in the condensation reaction. In
Scheme 1
hydroxy group is acetylated at the same time as the condensa-
tion reaction is carried out.
The results collected in Table 1 indicate that, not only is there
a very pronounced influence of the support on the catalytic
behavior of supported KF catalysts, but also that these dif-
ferences are closely related to the activation procedure. Thus,
when the reaction was conducted under conventional heating,
the best results were obtained according to the following
sequence
KF–Al2O3-150 > KF–Zn3(PO4)2 > KF–Al2O3-90 >
KF–AlPO4 > KF–Fe2O3 > KF–SiO2Al2O3 (1)
with yields ranging between 72.5% and 39.8%. However, when
reactions were carried out with a domestic microwave oven, the
sequence was as follows
KF–AlPO4 > KF–TiO2 > KF–Zn3(PO4)2 >
KF–Al2O3-150 > KF–Cu2P2O7 > KF–AlPO4-A (2)
now with yields ranging between 86.8% and 37.1%.
Furthermore, there was a number of supported KF catalysts
with conversions higher, or comparable, to that obtained by the
Erlenmeyer method. Thus, additional experiments were carried
out to gain further insight into the nature of the active basic
sites of KF–Al2O3-150, which is the best catalyst under con-
ventional heating. The same reaction was carried out with
NaOAc, under the conditions of the classical Erlenmeyer pro-
the literature,23a the existence of a heterogeneity of active sites
it is considered for KF–Al2O3, not only in nature (FϪ, AlF4
Ϫ
,
AlF63Ϫ, OHϪ, O2Ϫ and CO32Ϫ), but also in their strength, which
will depend on the relative concentration of the KF and on
some aspects related to the synthetic procedure, such as time
228
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234

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Table 1 Isolated weights and yields obtained with different 10 wt% supported KF catalysts in the synthesis of 4-(4Ј-acetoxybenzylidene)-2-
phenyloxazol-5(4H )-one, under classical heating as well as by microwave irradiation under standard conditions, 15 minutes at 450 W
Classical heating
Isolated weight/g
Microwave irradiation
Isolated weight/g
Support
Yield (%)
Yield (%)
Al2O3–90
Al2O3–150
ZnO
Fe2O3
Sepiolite
SiO2
3SiO2Al2O3
TiO2
0.711
1.113
0.543
0.679
0.361
0.326
0.611
0.547
0.399
0.000
0.693
0.177
0.104
0.230
0.912
0.396
0.000
0.246
0.224
0.000
0.151
0.208
0.061
0.473
0.000
46.3
72.5
35.4
44.2
23.5
21.2
39.8
33.8
26.0
0.0
45.2
11.5
6.8
15.0
59.4
25.0
0.0
0.474
0.983
0.493a
0.537b
0.183
0.019
0.396
0.942a
0.000b
0.000
1.332
0.570
0.031
0.059
0.849a
0.610
0.000
0.393
0.360
0.000
0.546
0.000
0.120
0.083
0.000
30.9
51.2
32.2a
35.0a
11.9
1.2
25.8
61.3a
0.0b
0.0
Carbon
CuCrO4ؒ2CuOؒ2H2O
AlPO4ؒH2O
AlPO4–A
86.8
37.1
2.0
Ca2(PO4)2
FePO4ؒnH2O
Zn3(PO4)2ؒ4H2O
Cu2P2O7ؒnH2O
Al2(SO4)3ؒ18H2O
CaSO4ؒ2H2O
Cr2(SO4)3ؒnH2O
MnSO4
Fe2(SO4)3ؒ9H2O
CoSO4ؒ7H2O
NiSO4ؒ6H2O
CuSO4
3.2
55.3a
39.4
0.0
16.0
14.6
0.0
9.8
13.6
4.0
25.6
23.4
0.0
35.6
0.0
7.8
5.4
30.8
0.0
ZnSO4ؒ7H2O
0.0
a It was irradiated for 15 minutes at only 150 W. b It burned before two minutes, at only 150 W.
Table 2 Isolated weights and yields obtained with NaOAc catalysts, unsupported under classical Erlenmeyer experimental conditions, and sup-
ported on Al2O3, in the synthesis of 4-(4Ј-acetoxybenzylidene)-2-phenyloxazol-5(4H )-one, with classical heating as well as by microwave irradiation
Classical heating
Isolated weight/g
Microwave irradiation
Isolated weight/g
Catalyst
Yield (%)
Yield (%)
NaOAc
NaOAc–Al2O3-150
1.060a
0.852
69.1a
55.5
0.037
0.327
2.4
21.3
a Erlenmeyer conditions, 4 h under reflux.
Table 3 Surface area (SBET) and acid–base properties of different inorganic solids used as 10 wt% KF catalyst support
Acidity vs. pyridine
(XmA)/µmol gϪ1
Basicity vs. benzoic acid
(XmB)/µmol gϪ1
Support
SBET/m2 gϪ1
Al2O3–90
Al2O3–150
ZnO
65.7
45.3
4.0
4.9
9.3
0.4
357.5
289.6
2.0
Fe2O3
Sepiolite
SiO2
3SiO2Al2O3
TiO2
2.5
0.4
3.7
96.9
344.2
7.4
31.0
206.0
2.1
174.0
164.0
271.5
5.3
8.0
0.9
Carbon
743.0
4.9
124.0
4.5
42.3
212.1
2.7
25.4
0.1
13.8
37.8
1.6
0.4
0.4
67.2
11.5
12.2
14.5
12.4
132.0
20.6
104.0
297.9
48.5
8.9
1.6
9.0
6.1
20.0
0.3
0.5
0.3
12.8
21.4
1.1
CuCrO4ؒ2CuOؒ2H2O
AlPO4ؒH2O
AlPO4–A
19.6
173.9
37.6
11.7
< 0.1
< 0.1
13.4
5.3
Ca2(PO4)2
FePO4ؒnH2O
Zn3(PO4)2ؒ4H2O
Cu2P2O7ؒnH2O
Al2(SO4)3ؒ18H2O
CaSO4ؒ2H2O
Cr2(SO4)3ؒnH2O
MnSO4
Fe2(SO4)3ؒ9H2O
CoSO4ؒ7H2O
NiSO4ؒ6H2O
CuSO4
0.2
< 0.1
< 0.1
< 0.1
9.4
< 0.1
1.6
ZnSO4ؒ7H2O
3.8
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234
229

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and calcination temperature. When different supports are used,
the heterogeneity of surface active sites must increase in new
basic surface species and also in different strengths. Microwave
irradiation would probably depend on the total number of basic
active sites, but classical heating could be more dependent on
basic surface active sites of a certain strength, best obtained in
KF–Al2O3 through a co-operative action of FϪ on the Al2O3
surface.23c
With respect to the favorable results obtained with AlPO4
as a KF support for reactions carried out under micro-
wave activation, the possible use of this material as a microwave
substrate was proposed some years ago26 because it is com-
pletely isostructural with silica, exhibiting the same poly-
morphic transformations. However, for the moment these are
the first results presented in this respect.
conventional heating as well as by microwave irradiation. Thus,
we have selected the two best catalysts, KF–Al2O3-150 for con-
ventional heating and KF–AlPO4 for microwave irradiation,
to carry out a study on their stability over repeated use, and in
the condensation of several different carbonyl compounds
and hippuric acid, in order to synthesize different azalactones
(Scheme 2). From the results shown in Fig. 1 it is easy to confirm
the high operational stability of the catalysts studied because
after ten consecutive reactions, in the synthesis of 4-(4Ј-acetoxy-
benzylidene)-2-phenyloxazol-5(4H )-one under standard con-
ditions, the catalytic activity of both supported KF catalysts is
clearly maintained.
The yields obtained in the synthesis of different azalactones
in dry-media are comparable to those obtained by classical
Erlenmeyer synthesis as well as to those in the literature.
Classical Erlenmeyer synthesis affords generally the Z-
derivatives.7b Here the presence of E isomers was excluded by
TLC : the reaction products were always exactly the same,
independent of the reaction procedure, classical or dry-media.
However, these results also indicate that thermal activation
is probably the driving force in the microwave experiments,
taking into account that after microwave irradiation the
reaction is accompanied by a significant increase in tempera-
ture. These temperatures cannot easily be determined in the
domestic microwave oven used, however, values higher than
130 ЊC were obtained when they were measured approximately.
In general, we can conclude from the results in Table 5 that
the yields obtained with different procedures are similar, so that
the most interesting differences are associated with the easier
work-up, and the important decrease in reaction time when
the reaction is carried out in dry-media, especially when using
a domestic microwave oven. The main advantage of hetero-
geneous catalysis, however, is associated with the possibilities of
recovering and reusing the catalysts.
Finally, in accordance with the results in Fig. 1 and Table 5,
Fig. 1 Reusability of KF–AlPO4 (᭺) under microwave irradiation and
KF–Al2O3 (ᮀ) under classical heating, in the synthesis of 4-(4Ј-
acetoxybenzylidene)-2-phenyloxazol-5(4H )-one
under
standard
conditions.
we can confirm the general application of supported KF as
a basic catalyst for azalactone synthesis in dry-media, under
Conclusions
Table 4 Results for the single regression models (y = ax ϩ b) between
some surface properties of 10 wt% supported KF catalysts, in Table 1,
and the isolated yields (in %) under classical heating (YCLAS) and
microwave irradiation (YMW), in Table 2, as well as the corresponding
significance levels
The experiments described in this research were aimed at
extending the use of environmentally friendly inorganic solids
to the important classical Erlenmeyer azalactone synthesis.
We investigated not only the usual KF–Al2O3 basic catalysts,
but also the possibilities of different KF support catalysts,
taking into account results previously obtained in the Michael
condensation reaction where KF–ZnO was the best catalyst.20
Furthermore, in the present case we decided also to explore
the potential advantages of “MORE” chemistry (microwave-
induced organic reaction enhancement),27 by running the
reactions in a domestic microwave oven. Some results obtained
Y
x
a
b
Significance (%)
XmA
XmA
YCLAS
YMW
YMW
XmB
SBET
1/XmA
YCLAS
XmB/SBET
0.26
0.24
2.47
0.64
4.25
17.73
19.65
20.07
10.27
17.30
95.0
99.8
99.5
96.8
91.9
Table 5 Isolated yields obtained in the synthesis of different 4-ylidene-2-phenyloxazol-5(4H )-ones, with 10 wt% KF–AlPO4 under microwave
irradiation, with 10 wt% KF–Al2O3 under classical heating, and with CH3CO2Na by the standard Erlenmeyer synthesis procedure. References in the
literature are also included
Isolated yields (%)
Azalactone
a
Entry
(4-ylidene-2-phenyloxazol-5(4H )-one)
KF–AlPO4
KF–Al2O3
NaOAc
References
1
2
4-Benzylidene-
74.1
86.8
45.0
73.2
48.4
53.0
55.1
76.6
70.0
64.6
44.0
33.2
14.5
81.2
72.5
57.0
75.1
70.3
72.6
66.2
74.4
81.4
72.8
65.0
38.8
20.5
84.0
69.1
47.8
78.6
58.2
55.2
62.7
68.6
72.6
73.8
49.8
40.1
19.0
70–906c–g,7a,b,10
80–856c,d,7b
186c,d,7b
4-(4Ј-Acetoxybenzylidene)-
4-(2Ј-Acetoxybenzylidene)-
4-(4Ј-Methylbenzylidene)-
4-(2Ј-Methylbenzylidene)-
4-(4Ј-Nitrobenzylidene)-
4-(4Ј-Chlorobenzylidene)-
4-(4Ј-Methoxybenzylidene)-
4-(3Ј,4Ј-Methylenedioxybenzylidene)-
4-(3Ј-Phenylpropenylidene)-
4-(1Ј-Methyl-3Ј-phenylpropenylidene)-
4-(But-2Ј-enylidene)-
3
4
70–806c,e,7b,10
406f
5
6
606c–e,7b
7
70–796c–g,7b,10
63–806c–g,7b,10
70–806c,g,7b
10–756c,d,7a
8
9
10
11
12
13
4-Cyclohexylidene-
19.01a
a Erlenmeyer synthesis conditions,6 4 h under reflux.
230
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234

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Scheme 2
after the study were expected, such as those which led us to
Experimental
conclude that there were no important differences in yields
between reactions carried out according to the classical
Erlenmeyer synthesis and under dry-media conditions, with
classical heating or microwave irradiation, when the best
supported KF catalysts were used. Thus, the easier work-up
and the significant decrease in reaction times when dry-media
conditions were used, especially with the use of a domestic
microwave oven, lead us strongly to recommend this procedure
for the synthesis of any type of azalactone.
However, an important and unexpected conclusion obtained
was that there were not only very important differences between
the KF support catalysts observed during the screening of
catalysts, but also that these differences are also closely related
to the procedure used in the reaction heating. Thus, after
screening we can conclude that, under the present standard
experimental conditions the well documented23–25 Al2O3 was
the best support for KF when reactions are conducted under
conventional heating. However, this is not true when reactions
are carried out in a domestic microwave oven, where AlPO4,
TiO2 and Zn3(PO4)2 are better supports than Al2O3-150. What
is more, Al2O3 is not the only reasonably good KF support
catalyst for condensation reactions. Some inorganic solids,
such as phosphates or oxides [such as Zn3(PO4)2, AlPO4 and
Fe2O3] although not usually employed as supports for reagents,
could also easily be used because they are as accessible and
inexpensive as Al2O3. The study of supports other than Al2O3 is
even more necessary when microwave oven irradiation is used,
because the results clearly indicate that there are some inorganic
solids more appropriate for KF supports than Al2O3.
Apparatus
Melting points were determined on a Gallenkamp Mod. MPD-
350 apparatus and are uncorrected. Infrared spectra were
obtained for KBr pellets on a Bomem, Mod. MB-100 spectro-
photometer. 1H and 13C NMR spectra were obtained using
TMS as an internal standard, on a Bruker AC 400 spectrometer
at 400 and 100 MHz, respectively. High-resolution mass
spectra were performed on a VG AUTOSPEC (Micromass
Instruments) where samples were introduced through a solid
probe. Reactions were monitored by thin-layer chromatography
(TLC) on Merck silica gel 60 F254 plates. A conventional
domestic microwave oven (Corbero, 700 W max power,
equipped with a turntable, timer and five output powers) was
used for the microwave-assisted heating reactions.
Materials
Unless stated otherwise, chemicals were obtained from different
commercial sources and used without further purification.
Thus, hippuric acid as well as aldehydes and ketones used in
the synthesis of different azalactones were purchased from
Aldrich Chemical Co. Inorganic solids used as supports for
KF were from the following sources: TiO2 pure anatase was
obtained from Aldrich; natural Sepiolite was from Tolsa SA;
chromatographic grade SiO2 (Kieselgel 60), acidic Al2O3-90 and
basic Al2O3-150 (type T) were from Merck; ZnO, MnSO4,
Cr2(SO4)3ؒnH2O, Ca3(PO4)2 Al2(SO4)3ؒ18H2O and active
carbon were from Panreac. All the other inorganic solids listed
in Tables 1 and 3 [AlPO4 H2O, Zn3(PO4)2ؒ4H2O, and so on],
were from Probus. An amorphous AlPO4 (AlPO4–A), obtained
according to a sol–gel method previously described,12–20 was
also used as a support. This support was prepared by precipi-
tation from aqueous solutions of AlCl3ؒ6H2O and H3PO4
(85 wt%) at pH = 6.1 at the “precipitation end point”, with
ammonium hydroxide solution. The solid obtained was then
washed with isopropyl alcohol and dried at 120 ЊC for 24 hours.
In this respect, the importance of textural and acid–base
properties of solid inorganic supports in the catalytic behavior
of supported KF catalysts was also demonstrated. Thus, acidity
of supports seems to strongly inhibit the ability of KF support
catalysts for reactions under conventional heating. In addition,
the specific basicity of supports (number of basic sites/SBET
)
seems to strongly promote the KF catalysts in reactions carried
out in a microwave oven. Despite the lack of a more detailed
theoretical explanation, these differences could be ascribed to
the fact that the participation of KF surface active sites, which
take part in the condensation reactions, is not always exactly the
same and depends on whether the type of activation used is that
of classical heating or microwave.
Finally, according to our results we may conclude that,
before directly using the well-documented KF–Al2O3 as the
basic catalyst for a condensation reaction under dry-media
conditions, it is worthwhile investigating other supported KF
systems which might be more active and/or selective, especially
if a domestic microwave oven is to be used. For this purpose
AlPO4 should not be overlooked.
Synthesis and characterization of supported KF catalysts
Different inorganic solids were slowly heated in a muffle
furnace to reach 400 ЊC over the course of one hour (100 ЊC
every 15 min). They were then calcined for three hours at this
temperature, to remove completely the hydration water before
KF impregnation. Surface area, SBET, and acid–base properties
of the supports summarized in Table 3 were also determined
before KF deposition. These were obtained by nitrogen
adsorption12–20 as well as by a spectrophotometric method
described elsewhere,12–20 which allows titration of the amount
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234
231

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of irreversibly adsorbed benzoic acid (BA, pKa = 4.19) or
pyridine (PY, pKa = 5.25) employed as titrant agents for
basic and acid sites, respectively. The monolayer coverage at
equilibrium at 25 ЊC, Xm (in µmol gϪ1), was accomplished by
applying the Langmuir adsorption isotherm, and is assumed
to be a measure of the acidic or basic sites corresponding to the
specific pKa of the base or acid used in the titration.
Different 10 wt% supported KF catalysts were prepared by
impregnation of supports to incipient wetness with (1 : 1)
methanol–aqueous solutions of potassium fluoride (Merck,
pa.).20 Thus, the slurry obtained by mixing the support, 9 g,
and KF solution, 4 ml, (KF, 1 g, MeOH, 2 ml and water, 2 ml)
was stirred for 2 hours and then the solvent evaporated off on
a rotary vacuum evaporator. Afterwards, the samples were
dried at 150 ЊC in a muffle furnace for 24 h, and then stored in
a desiccator. Similarly, for comparative purposes, NaOAc–
Al2O3-150 was prepared by impregnation of anhydrous sodium
acetate (0.432 g, 5 mmol) on 3 g of support, but using 4 ml
anhydrous MeOH as the solvent.
Heterogeneously catalyzed condensation reactions with
NaOAc–Al2O3-150 were carried out in exactly in the same way
as with KF support catalysts.
The reuse of selected catalysts KF–AlPO4 and KF–Al2O3,
was investigated in the same reaction used in the screening
of different supported KF catalysts. Thus, the synthesis of the
azalactone 4-(4Ј-acetoxybenzylidene)-2-phenyloxazol-5(4H )-
one was successively carried out by the dry-media condensation
(microwave and conventional heating, respectively) of p-
hydroxybenzaldehyde with hippuric acid in acetic anhydride
(1 : 1 : 4, molar ratio), by repeatedly using the same 3 g of
10 wt% KF support catalyst, after extraction of reaction
products with boiling acetone, (15 ml, 3 times) and washing
with dichloromethane (15 ml). The catalysts recovered by filtra-
tion can be directly used in a new reaction because no activation
treatment is necessary
Spectroscopic data for products
4-Benzylidene-2-phenyloxazol-5(4H)-one (1). Mp 166 ЊC;
νmax/cmϪ1 1794, 1652, 1555, 1450, 1165, 864, 768, 698; 1H NMR
(CDCl3) δ 7.28 (s, 1H), 7.45–7.75 (m, 6H), 8.15–8.35 (m, 4H);
13C NMR (CDCl3) δ 126.6 (C), 129.0 (CH), 129.8 (CH), 130.1
(CH), 131.7 (CH), 132.0 (C), 133.3 (CH), 134.3 (CH), 134.4
(CH), 134.7 (C), 164.6 (C), 167.7 (C); HRMS (CIϩ) exact mass
calcd. for C16H11NO2 24907898, found: 249.07906.
General procedure for the synthesis of 4-ylidene-2-phenyl-
oxazol(4H)-5-ones
According to the standard Erlenmeyer procedure,6 a mixture of
hippuric acid (0.896 g, 5 mmol), anhydrous sodium acetate
(0.432 g, 5 mmol), acetic anhydride (2 ml, 20 mmol) and the
corresponding carbonyl compound, ketone or aldehyde,
(5 mmol), was placed in a 25 ml conical flask and heated on an
electric hotplate with constant shaking to liquefy the mixture
completely. The flask was then heated in a water bath for
4 hours, after which it was cooled to room temperature. Cool
water (10 ml) was added, and the mixture was allowed to stand
overnight. Crude azalactones were obtained after filtration and
washing with aqueous sodium carbonate to remove benzoic
acid. The yields indicated in Table 5 correspond to those
obtained after recrystallization from acetone–water (2 : 1).
To obtain the heterogeneously catalyzed condensation
reactions, the same amounts of reactants were used, but instead
of anhydrous sodium acetate, 3 g of the 10 wt% supported
KF catalyst (also containing 5 mmol of KF) was used. The
mixture of solid catalyst and reactants was stirred in dichloro-
methane (3–5 ml) in a rotary evaporator for fifteen minutes to
homogenize it, and then the solvent was evaporated off under
vacuum to obtain a powdered dry solid, with the reactants
adsorbed onto the solid surface. This operation must be done
very carefully to avoid the evaporation of the acetic anhydride.
One set of reactions was then conducted under conventional
heating: the solid was placed in a round-bottomed flask and
heated at reflux in a water bath for 1 hour (this operation can be
carried out on the same rotary evaporator). Another set of
reactions was conducted by microwave-assisted heating (15
minutes at 450 W), in an open Erlenmeyer flask (25 ml Pyrex
glass) covered with a watch glass. In both cases, the reaction
product was easily obtained after separation from the solid
catalyst, by extraction with boiling acetone, (15 ml, 2–3 times);
the extraction progress was followed by TLC analysis and after
cooling and volume reduction by evaporation on a vacuum
rotary evaporator, the crude azalactone was easily obtained by
precipitation.
4-(4Ј-Acetoxybenzylidene)-2-phenyloxazol-5(4H)-one (2). Mp
148 ЊC; νmax/cmϪ1 1796, 1759, 1659,1593, 1550, 1236, 1165,964,
880, 698; 1H NMR (CD3COCD3) δ 2.32 (s, 3H), 7.18–7.24 (m,
2H), 7.25 (s, 1H), 7.47–7.64 (m, 3H), 8.14–8.26 (m, 4H);
13C NMR (CD3COCD3) δ 21.2 (CH3), 122.1 (CH), 125.8 (C),
128.5 (CH), 129.0 (CH), 130.5 (CH), 131.3 (C), 133.4 (CH),
133.7 (CH), 152.9 (C), 163.8 (C), 167.5 (C), 168.8 (C); HRMS
(CIϩ) exact mass callcd. for C18H13NO4 307.08446, found
307.08363.
4-(2Ј-Acetoxybenzylidene)-2-phenyloxazol-5(4H)-one (3). Mp
132 ЊC; νmax/cmϪ1 1794, 1763, 1709, 1667, 1598, 1536, 1165,
1362, 1162, 861, 756, 698; 1H NMR (CD3COCD3) δ 2.42
(s, 3H), 7.29 (s, 1H), 7.34–7.70 (m, 4H), 7.90–8.95 (m,
5H); 13C NMR (CD3COCD3) δ 20.9 (CH3), 117.0 (CH),
120.9 (C), 123.8 (CH), 125.5 (CH), 126.0 (CH), 126.6 (CH),
127.2 (C), 129.7 (CH), 130.1 (CH), 132.9 (CH), 134.6
(CH), 135.4 (CH) 151.9 (C), 165.4 (C), 166.4 (C), 169 (C);
HRMS (CIϩ) exact mass calcd. for C18H13NO4 307.08446,
found 307.08419.
4-(4Ј-Methylbenzylidene)-2-phenyloxazol-5(4H)-one (4). Mp
141 ЊC; νmax/cmϪ1 1794, 1652, 1605, 1493, 1162, 818, 695;
1H NMR (CD3COCD3) δ 2.40 (s, 3H) 7.23 (s, 1H), 7.34 (d, 2H,
J = 7.9 Hz), 7.61–7.72 (m, 3H); 8.15–8.23 (m, 4H) 13CNMR
(CD3COCD3) δ 21.7 (CH3), 126.6 (C), 128.9 (CH), 130.0 (CH),
130.5 (CH), 131.9 (C), 133.4 (CH), 134.2 (C), 142.8 (C), 164.0
(C), 167.8 (C); HRMS (CIϩ) exact mass calcd. for C17H13NO2
263.09463, found 263.09485.
4-(2Ј-Methylbenzylidene)-2-phenyloxazol-5(4H)-one (5). Mp
141 ЊC; νmax/cmϪ1 1794, 1648, 1598, 1551, 1291, 1160, 861, 768,
1
698; H NMR (CDCl3) δ 2.51 (s, 3H) 7.25 (s, 1H), 7.31–7.36
With respect to microwave irradiation, it is important to
indicate that a total reaction time of 15 min corresponds, in
fact, to 3 cycles of 5 min, with cooling periods (30 seconds)
between them, which are used to monitor the reaction progress
by TLC, so that the yields indicated in Table 1 were those
optimized. In Table 1 both kinds of results obtained in the dry
media condensation of p-hydroxybenzaldehyde with hippuric
acid in acetic anhydride are collected, having been carried
out using different 10 wt% supported KF catalysts. Table 5
shows the results obtained with different oxazolones by using
the selected catalysts KF–AlPO4 and KF–Al2O3, respectively.
(m, 2H), 7.47–7.62 (m, 4H), 8.16 (d, 2H, J = 7.4 Hz), 8.77–8.82
(m, 1H); 13C NMR (CDCl3) δ 20.0 (CH3), 125.6 (C), 126.6
(CH), 128.4 (CH), 128.5 (CH), 128.9 (CH) 130.7 (CH), 131.2
(CH), 132.1 (CH), 132.2 (C), 133.2 (CH), 133.3 (C), 139.9 (C),
163.7 (C), 167.8 (C); HRMS (CIϩ) exact mass calcd. for
C17H13NO2 263.09463, found 263.09516.
4-(4Ј-Nitrobenzylidene)-2-phenyloxazol-5(4H)-one (6). Mp
241 ЊC; νmax/cmϪ1 1796, 1655, 1596, 1559, 1520, 1343, 1165, 864,
1
667; H NMR (CD3COCD3) δ 7.4 (s, 1H), 7.66–7.80 (m, 3H),
232
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234

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8.26–8.39 (m, 4H), 8.40–8.64 (m, 2H) 13C NMR (CD3COCD3)
δ 124.1, 124.4, 127.9, 129.5, 130.7, 133.9, 135.2, 138.9, 148.1,
158.8, 165.9; HRMS (CIϩ) exact mass calcd. for C16H10N2O4
294.06406, found: 294.06151.
(CH), 129.2 (C), 132.3 (CH), 159.3 (C), 161.2 (C), 165.6 (C);
HRMS (Clϩ) exact mass calcd. for C15H15NO2 241.11028,
found 241.11014.
Acknowledgements
4-(4Ј-Chlorobenzylidene)-2-phenyloxazol-5(4H)-one (7). Mp
197 ЊC; νmax/cmϪ1 1796, 1655, 1590, 1555, 1490, 1162, 826, 695;
1H NMR (CD3COCD3) δ 7.17 (s, 1H), 7.44 (d, 2H, J = 8.3 Hz),
7.53–7.63 (m, 3H) 8.1–8.2 (m, 4H); 13C NMR (CD3COCD3)
δ 125.8 (C), 128.5 (CH), 129.0 (CH), 129.3 (CH), 129.9 (CH),
132.1 (C), 133.5 (CH), 133.7 (C), 137.3 (C), 164.1 (C), 167.2
(C); HRMS (CIϩ) exact mass calcd. for C16H10ClNO2
283.04001, found 283.03953.
The authors gratefully acknowledge the subsidies granted by
the Ministerio de Ciencia y Tecnologia, DGI (Projet BQU2001/
2605), and by the Consejería de Educación y Ciencia (Junta de
Andalucía), Spain. The valuable help of Prof. M. Sullivan in the
grammatical revision of the manuscript is also acknowledged.
References
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Mp 157 ЊC; νmax/cmϪ1 1790, 1652, 1598, 1513, 1266, 1162, 1030,
1
830, 695; H NMR (CDCl3) δ 3.81 (s, 3H) 7.06–7.12 (m, 2H),
7.24 (s, 1H), 7.58–7.69 (m, 3H), 8.03–8.36 (m, 4H); 13C NMR
(CDCl3) δ 55.9 (CH3), 115.4 (CH), 126.6 (C), 127.5 (C) 128.8
(CH), 129.4 (C), 130.0 (CH), 132.0 (CH), 134.1 (CH), 135.2
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4-(3Ј,4Ј-Methylenedioxybenzylidene)-2-phenyloxazol-5(4H)-
one (9). Mp 195 ЊC; νmax/cmϪ1 1779, 1648, 1486, 1451, 1266,
1165, 926, 864, 698; 1H NMR (CD3COCD3) δ 6.17 (s, 2H) 7.02
(d, 1H, J = 8.0 Hz), 7.23 (s, 1H), 7.63–7.72 (m, 4H), 8.17–8.20
(m, 3H); 13C NMR (CD3COCD3) δ 103.1 (CH2), 109.5 (CH),
111.5 (CH), 126.7 (C) 128.8 (CH), 129.2 (C), 130.1 (CH), 130.4
(CH), 131.9 (CH), 132.3 (C), 134.2 (CH), 137.6 (C), 143.9 (C),
151.6 (C), 163.6 (C); HRMS (CIϩ) exact mass calcd. for
C17H11NO4 293.06881, found 293.06999.
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4-(3Ј-Phenylpropenylidene)-2-phenyloxazol-5(4H)-one (10).
Mp 151 ЊC; νmax/cmϪ1 1787, 1640, 1598, 1451, 1331, 1162, 872,
1
695; H NMR (CD3COCD3) δ 7.18 (s, 1H) 7.21 (S, 1H), 7.31–
7.49 (m, 4H), 7.59–7.72 (m, 4H), 8.03–8.16 (m, 3H); 13C NMR
(CD3COCD3) δ 123.9 (CH), 124.1 (CH), 126.7 (CH), 127.9 (C)
128.6 (CH), 128.8 (CH), 129.8 (CH), 130.0 (CH), 130.8 (CH),
133.1 (CH), 133.9 (CH), 134.3 (CH), 135.1 (C), 137.1 (C), 138.1
(CH), 144.7 (CH), 145.7 (CH), 162.9 (C), 165.7 (C); HRMS
(CIϩ) exact mass calcd. for C17H13NO2 275.09463, found
275.09470.
4-(1Ј-Methyl-3Ј-Phenylpropenylidene)-2-phenyloxazol-
5(4H)-one (11). Mp 155 ЊC; νmax/cmϪ1 1779, 1632, 1559, 1447,
1
1173, 965, 880, 695; H NMR (CD3COCD3) δ 4.15 (d, 3H,
J = 6.3 Hz), 7.47–7.53 (m, 7H), 7.93–8.13 (m, 5H); 13C NMR
(CD3COCD3) δ 41.8 (CH3), 127.0 (C), 128.1 (CH), 128.4 (CH)
128.6 (CH), 129.0 (CH), 129.3 (CH), 129.9 (CH), 130.4 (C),
132.2 (CH), 132.5 (CH), 133.6 (C), 134.2 (C), 135.3 (CH), 139.2
(CH), 167.7 (C), 171.4 (C); HRMS (CIϩ) exact mass calcd. for
C19H15NO2 289.11028 found 289.10999.
4-(But-2Ј-enylidene)-2-phenyloxazol-5(4H)-one (12). Mp 154
ЊC; νmax/cmϪ1 1791, 1655, 1559, 1451, 1327, 1181, 860, 698;
1H NMR (CD3COCD3) δ 1.94 (m, 3H) 6.63 (m, 1H), 6.98 (m,
2H), 7.55–7.63 (m, 2H), 7.66–7.70 (m, 1H), 8.07–8.11 (m, 2H);
13C NMR (CD3COCD3) δ 19.5 (CH3), 126.8 (C), 127.9 (C),
128.0 (CH) 128.6 (CH), 130.0 (CH), 133.5 (CH), 133.9 (CH),
145.2 (CH), 162.6 (C), 167.2 (C); HRMS (CIϩ) exact mass
calcd. for C13H11NO2 213.07898, found 213.07921.
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4-Cyclohexylidene-2-phenyloxazol-5(4H)-one (13). Mp 128
ЊC; νmax/cmϪ 2938, 2863, 1779, 1659, 1450, 1293, 1170, 979,
885, 704, H NMR (CD3COCD3) δ 1.68–1.81 (m, 6H), 2.83
(t, 2H, J = 6.1 H), 3.02 (t, 2H, J = 5.9 Hz), 7.26–7.54 (m, 3H),
8.01–8.06 (m, 2H); 13C NMR (CD3COCD3) δ 28.2 (CH2), 28.3
(CH2, 29.1 (CH2), 32.1 (CH2), 126.4 (C), 127.7 (CH), 128.8
1
J. Chem. Soc., Perkin Trans. 2, 2002, 227–234
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