Flash vacuum pyrolysis over solid catalysts. 2. Pyrazoles over hydrotalcites

Article abstract of DOI:10.1021/jo0260894

Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as interlayer anions were employed. Between 400 and 600 °C, compound 1 remained almost unchanged and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 °C. At a higher temperature (660 °C), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene (5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions. Results showed that the yield is strongly dependent on the nature of the interlayer anion in the hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for ring fragmentation reaction.

Full text of DOI:10.1021/jo0260894

F la sh Va cu u m P yr olysis over Solid Ca ta lysts. 2. P yr a zoles over
Hyd r ota lcites
Elizabeth L. Moyano,† Margarita del Arco,‡ Vicente Rives,‡ and Gloria I. Yranzo*,†
INFIQC, Departamento de Quı´mica Orga´nica, Facultad de Ciencias Qu´ımicas, Universidad Nacional de
Co´rdobasCiudad Universitaria, 5000 Co´rdoba, Argentina, and Departamento de Quı´mica Inorga´nica,
Universidad de Salamanca, 37008-Salamanca, Spain
yranzogi@dqo.fcq.unc.edu.ar
Received J une 21, 2002
Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were
investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts
with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as
interlayer anions were employed. Between 400 and 600 °C, compound 1 remained almost unchanged
and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded
benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 °C. At a higher
temperature (660 °C), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene
(5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions.
Results showed that the yield is strongly dependent on the nature of the interlayer anion in the
hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for
ring fragmentation reaction.
SCHEME 1
In tr od u ction
Flash vacuum pyrolysis (fvp) reactions of pyrazoles
have proved to be an interesting tool to study the reaction
mechanisms of their own thermal reactions. These het-
erocycles lead to different reactions depending on N1
substitution. Thus, in the case of NH-pyrazoles, the main
reaction is nitrogen extrusion, affording a vinylcarbene
intermediate1,2 that is formed in a stepwise process where
the elimination of nitrogen is the rate-determining step
(Scheme 1).1
The team’s previous studies showed that substituents
in the vinylcarbene induced different reactions. There-
fore, it is possible to find hydrogen or alkyl migration,
insertion into aryl C-H bonds, addition to a carbonyl
bond,2 and others. These studies provides interesting
results concerning vinylcarbene’s reactivity, and in some
cases, they appeared to be an alternative synthetic
methodology for some interesting compounds but, due to
their large activation energies (182-298 Kj/mol), it is
necessary to use high temperatures (580-800 °C).2
Since it may be a problem in organic synthesis to use
these reactions, to smooth reaction conditions a hetero-
geneous system has recently been developed using zeo-
lites as solid catalysts.3 At that time, fvp reactions of 1,
2, and 3,5-dimethylpyrazole were studied. In the case of
1, as in the homogeneous system, the same reaction was
found, which is nitrogen extrusion. On the other hand, 2
afforded ring fragmentation to nitriles and isomerization,
while 3,5-dimethylpyrazole afforded ring fragmentation
as well as nitrogen extrusion, all of these reactions at
lower temperatures than in the homogeneous system,
thus showing an effective catalysis. These results showed
that the nitrogen extrusion reaction is catalyzed by
zeolites, provided the transition state is formed inside
the zeolites’ cavity. It is interesting to note that these
nitriles, arising from fragmentation reactions, have been
obtained in thermal reactions of other five-membered
rings but never in fvp of NH-pyrazoles.4 The same
isomerization reaction of 2 had already been described
† Universidad Nacional de Co´rdoba.
‡ Universidad de Salamanca.
(1) (a) Pe´rez, J . D.; Yranzo, G. I.; Phagouape´, L. M. Bull. Soc. Chim.
Fr. 1987, 129. (b) Pe´rez, J . D.; Yranzo, G. I.; Ferraris, M. A.; Claramunt,
R. M.; Lo´pez, C.; Elguero, J . Tetrahedron 1988, 44, (20), 6429. (c) Pe´rez,
J . D.; Yranzo, G. I. J . Anal. Appl. Pyrol. 1989, 16, 165. (d) Ferraris,
M. A.; Yranzo, G. I.; Pe´rez, J . D.; Cabildo, P.; Sanz, D.; Claramunt, R.
M.; Elguero, J . Anales Quı´m.sInt. Ed. 1996, 92, 3. (e) Pe´rez, J . D.;
Yranzo, G. I. Bull. Soc. Chim. Fr. 1986, 473-476.
(2) (a) Moyano, E. L.; Yranzo, G. I.; Elguero, J . J . Org. Chem. 1998,
63, 8188 and references therein. (b) Yranzo, G. I.; Moyano, E. L.; Rozas,
I.; Dardonville, C.; Elguero, J . J . Chem. Soc., Perkin Trans. 2 1999,
211.
(3) Moyano, E.; Yranzo, G. J . Org. Chem. 2001, 66, 2943-2947.
(4) (a) Lifshitz, A.; Wohlfeiler, D. J . Phys. Chem. 1992, 96, 11, 4505.
(b) Mitchell, W. R.; Paton, R. M. J . Chem. Res. (S) 1984, 58.
10.1021/jo0260894 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/24/2002
J . Org. Chem. 2002, 67, 8147-8150
8147

Moyano et al.
TABLE 1. F la sh Va cu u m P yr olysis Rea ction s of 1
for N-aryl-substituted pyrazoles,1 but this case with
zeolites was the first one described in fvp of an unsub-
stituted pyrazole. On the other hand, it was observed that
pyrazole isomerization was present only in protonic
zeolites with strong acid sites (Bro¨nsted sites), while
extrusion and fragmentation reactions were catalyzed on
materials having Bro¨nsted and Lewis sites or Lewis sites
only.3
As an extension of these studies on fvp reactions over
solid catalysts, anionic clays such as hydrotalcites were
chosen as catalytic materials. It is known that hydrotal-
cites or calcined hydrotalcites have been used extensively
as catalysts in many organic processes due to their basic
properties and mainly for their ability to exchange
anions.5 Thus, it is possible to find these materials as
efficient basic catalysts for the epoxidation of various
olefins6 and also for aldol condensation reactions.7
T (°C)
catalyst
% 1
T (°C)
catalyst
% 1
400
none
HT-1
HT-2
HT-3
none
HT-1
100a
95
88
500
HT-2
HT-3
none
HT-1
HT-2
HT-3
90
88
600
100a
89
70
500
100a
91
87
83
a
From ref 2a.
The aim of this study is to research the effect of the
basic sites as well as the hydrotalcite structure on
thermal reactions of NH-pyrazoles and product selectivity
in the process. This study may be a significant contribu-
tion in the development of heterogeneous fvp systems,
since the existing literature does not provide any com-
parative studies between zeolites and hydrotalcites with
the same substrates.
Therefore, some hydrotalcites having different struc-
tures and interlayer spaces were chosen with carbonate
(HT-1), nitrate (HT-2), and silicate (HT-3). The substrates
employed are NH-pyrazole (1) and 3,5-diphenylpyrazole
(2), which afford nitrogen extrusion in the homogeneous
systems.1e,2a
It is important to point that the name anionic clay is
applied to natural or synthetic layered hydroxides con-
taining anionic species in the interlayer space. The
general formula describing the chemical composition of
III
anionic clays is [MII
M )x/n‚mH2O. When
x(OH)2](An-
1-X
MII is Mg, MIII is Al, and the interlaminar anion (A) is
carbonate, the compound is usually known as hydrotal-
cite [Mg6Al2(OH)16CO3‚4H2O]. The structure consists of
brucite layers, which are positively charged because of
partial substitution by trivalent cations, with the inter-
layer space filled with anions (to balance the positive
charge of the layers) and water molecules.
Resu lts a n d Discu ssion
F la sh Va cu u m P yr olysis of NH-P yr a zole (1). The
fvp experiments were carried out between 400 and 600
°C using hydrotalcites HT-1, HT-2, and HT-3 as solid
catalysts. The results here obtained, as well as the ones
previously reported in the homogeneous system,1,2 are
shown in Table 1. It can be seen that 1 has very low
reactivity under the experimental conditions, and high
amounts of unreacted material were recovered. Some
unidentified volatile products were formed, probably
arising from ring fragmentation or coke reactions, and
also starting material was adsorbed on the catalyst
surface. No propyne, was detectedsthe expected product
from a nitrogen extrusion reaction. This is an important
difference when comparing these reactions with the ones
over zeolites.3
In reactions where HT-2 was used, loss of the inter-
layer anion as gaseous NO2 was observed. This elimina-
tion is accompanied by an increase of the surface area,
which results in the collapse of the layered structure and,
after the thermolysis, the material became useless for
other experiments. It is worthwhile to mention here that
this loss was also detected when HT-2 was submitted to
experimental conditions without substrates, showing the
instability of this material.
As can be seen, these solids show anion-exchange
properties5,8 and, depending on the nature of the cations
and the calcination temperature, the recovery of the
structure is possible.9
Anionic clays may be synthesized by different methods
such as precipitation, hydrothermal synthesis, anion
exchange, and others. However, they are usually pre-
pared by coprecipitacion of metal hydroxides of divalent
and trivalent cations in a basic aqueous solution in the
presence of the anions that will be located in the
interlayer. Physicochemical and structural properties of
the resulting solids depend on the precipitation pH,
temperature, aging, washing, drying, and crystallization
conditions. When the clays are calcined at moderate
temperatures, new materials are formed, and a mixture
of the oxides of the starting cations is developed. These
new materials are used as catalysts in different relevant
processes.5,10 Previous studies on these materials showed
that these systems are usually more effective as catalyst
after calcinations where the layered structure is lost
starting from 350 °C and large specific areas are devel-
oped, the largest being with silicate.11
It was also seen that the decomposition of 1 was
greater when HT-3 was used. In this case, a small
amount (20%) of substrate was adsorbed by the catalyst.
This adsorption was low at 500 °C, due probably to a
partial saturation of the material. Since the catalysts
underwent continuous deactivation during the reaction,
it was impossible to recover the total amount of the
catalytic material employed as a result of a carbonaceous
deposit (coke), which restricted their lifetime.
(5) Roy, A.; Forano, C.; El Malki, K.; Besse, J . Expanded Clays and
Other Microporous Solids, Occelli, M. L., and Robson, V. N. R., Eds.;
New York, 1992; p 108.
(6) Ueno, S.; Yamaguchi, K.; Yoshida, K.; Ebitani, K.; Kaneda, K.
Chem. Commun. 1998, 3, 295.
(7) Rao, K. K.; Gravelle, M.; Valente, J . S.; Figueras, F. J . Catal.
1998, 173, 1, 115.
(8) (a) Bennani, M. N.; Tichit, D.; Figueras, F.; Abouarnadasse, S.
J . Chim. Phys. Phys.Chim. Biol. 1999, 96, 1498. (b) Cavani, F.; Trifiro,
F.; Vaccari, A. Catal. Today 1991, 11, 1.
(9) (a) Chibwe, K.; J ones, W. Chem. Mater. 1989, 1, 489. (b) Del Arco,
M.; Malet, P.; Trujillano, R.; Rives, V. J . Mater. Chem. 1999, 9, 1499.
(10) (a) Uzunova, E.; Klissurski, D.; Kassabov, J . J . Mater. Chem.
1994, 4, 153. (b) Uzunova, E.; Klissurski, D.; Mitov, I.; Stefanov, P.
Chem. Mater. 1993, 5, 576.
(11) Del Arco, M.; Gutierrez, S.; Martin, C.; Rives, V. Phys. Chem.
Chem. Phys. 2001, 3 (1), 119.
8148 J . Org. Chem., Vol. 67, No. 23, 2002

Flash Vacuum Pyrolysis over Solid Catalysts
SCHEME 2
SCHEME 3
and fvp of 3,6-diphenyl-1,2,4,5-dioxodiazine.13 It is pro-
posed that this fragmentation reaction go through a
mechanism that involves a three-membered ring inter-
mediate, as shown in Scheme 3.
In this case both nitriles may be formed by H and Ph
migrations. It is interesting to remark that this reaction
is catalyzed over acid (zeolites) as well as over basic solids
(hydrotalcites). While in the former fragmentation is a
competitive reaction, in the latter this is the exclusive
reaction, thus making this catalytic process more selec-
tive; in other words, fragmentations seem to need only
active sites while isomerizations need Bro¨nsted acid ones.
Some polycyclic aromatic hydrocarbons such as phenan-
threne (5) and anthracene (6) were also formed at high
temperatures from benzonitrile and phenylacetonitrile.
These transformations were also observed in fvp on
zeolites and proceeded through radical intermediates,
since dibenzyl was identified as reaction product when
toluene was used as carrier gas. The use of toluene as
carrier gas is a normal test reaction to detect radicals.
On the other hand, in reactions at higher tempera-
tures, phenylindenes 7 and 8 were also found in the
reaction products. These compounds arise from the
expected nitrogen extrusion through the intermediacy of
vinylcarbene 2a (Scheme 2). However, it should be
emphasized that this reaction is not catalyzed by hydro-
talcites at low temperatures.
A very significant conversion of substrate was observed
over HT-2. This behavior could be related to a high
surface area developed in the catalyst in the fvp system.
The values of specific surface areas are enhanced about
4 times when the material is calcined at 600 °C11 (at
normal pressures), this enlargement may be increased
under fvp conditions, even when the material was already
calcined. Thus, the formation of mixed oxides with high
catalytic properties can be accelerated and the fragmen-
tation process catalyzed.
TABLE 2. F la sh Va cu u m P yr olysis Rea ction s of 2
T (°C)
catalyst
% 2
T (°C)
catalyst
% 2
450
none
HT-1
HT-2
HT-3
none
HT-1
100a
73
5
560
HT-2
HT-3
none
HT-1
HT-2
HT-3
0
39
47a
34
0
660
45
560
100a
60
20
a
From ref 2a.
F la sh Va cu u m P yr olysis of 3,5-Dip h en ylp yr a zole
(2). This pyrazole was subjected to fvp between 450 and
660 °C using HT-1, HT-2, and HT-3 as catalytic materi-
als. In this case, two different reactions were present:
(a) ring fragmentation, starting from 450 °C, and (b)
nitrogen extrusion at the same temperatures of reactions
in the homogeneous systems (660 °C). The results of these
experiments, as well as the ones reported earlier for the
homogeneous system,2 are shown in Scheme 2 and Table
2.
When fvp reactions of 2 were studied over zeolites, it
was found that 2 afforded two different reactions, i.e.
fragmentation to benzonitrile (3) and phenylacetonitrile
(4) and isomerization to 2,5-diphenylpyrazole,3 however,
in reactions over hydrotalcites, the only catalyzed reac-
tion was fragmentation, affording nitriles 3 and 4. In both
cases (zeolites and hydrotalcites), 2 afforded phenylin-
denes 7 and 8 at the same temperatures of the homoge-
neous reactions, showing that at low temperatures the
catalysis is not effective. Fragmentation reactions cor-
respond to N-N and C3-C4 bond fissions that were
previously reported in shock tube reactions of isoxazole12
Besides, HT-1 and HT-3 showed lower activity than
HT-2. Under fvp conditions these HTs are more stable
and it may be that the increase of the surface area by
calcination is slower than for HT-2 under the experimen-
tal conditions. In the case of HT-3, a portion of substrate
was adsorbed by the catalyst, causing an earlier poison-
ing of the material. A possible explanation can be the
higher interlayer space (11.4 Å) in silicate hydrotalcite,
(12) Aldous, G. L.; Bowie, J . H.; Thompson, M. J . J . Chem. Soc.,
Perkin Trans I 1976, 1.
(13) Mitchell, W. R.; Paton, R. M. J . Chem. Res. (S) 1984, 58.
J . Org. Chem, Vol. 67, No. 23, 2002 8149

Moyano et al.
Al nitrates in NaOH-NaNO3 aqueous solution by the method
previously described.15 The silicate catalyst is obtained by
anionic exchange of the nitrate precursor (HT-2) using the
SiO2-NaOH-H2O system15 In all cases, the ratio of cations
Mg:Al was 2:1. Powder X-ray diffraction diagrams were
recorded for the three samples, the signals corresponded to a
hydrotalcite structure. Diffraction values were 7.4, 8.8, and
11.4 Å, very close to those reported for hydrotalcite-like
materials containing carbonate, nitrate, and silicate, respec-
tively.15 The specific surface areas, calculated following the
BET method,11 for the uncalcined and calcined HTs at 600 °C
were the following (m2 g-1): 86 and 120 (HT-1), 25 and 85 (HT-
2), 72 and 75 (HT-3). The catalysts were pressed, fractured,
sieved to the desired particle size fraction of 12-20 mesh, and
stored under ambient atmosphere. The materials were calcined
at 300 °C under nitrogen flow for 3 h when they were prepared.
F la sh Va cu u m P yr olysis of 1. Pyrazole 1 was com-
mercially available from SIGMA and purified by sublimation
in vacuo. The amount of unreacted starting material was
measured by 1H NMR using nitromethane as internal stan-
dard. After the fvp reactions, both starting material and a
mixture of unknown volatile products were obtained. A portion
of substrate was adsorbed on the catalyst’s surface. This was
detected by checking the mass of catalyst before and after each
reaction.
which favors the trapping of molecules inside the hydro-
talcite structure when the interlaminar anion is elimi-
nated, thus inactivating the catalyst.
Con clu sion s
NH-pyrazole did not react when HT-1, HT-2, and HT-3
were used as catalysts. Under the same conditions, 2
afforded nitriles by a fragmentation reaction at 450 °C.
In both cases (pyrazoles 1 or 2) no nitrogen extrusion
reaction was observed, unlike thermolysis in the presence
of zeolites. This fact may be an indication that the
nitrogen extrusion reaction needs acid catalysis.
The results here reported indicate that hydrotalcites
show low activity but selectivity in fvp reactions of NH-
pyrazoles, in comparison with zeolites, provided that the
pyrazole is ring-substituted. Among the different materi-
als used in this study, HT-2 containing nitrate as
interlayer anion showed better performance toward
fragmentation reaction.
Exp er im en ta l Section
F la sh Va cu u m P yr olysis of 2. This compound was
synthesized from dibenzoylmethane as described in the
literature.16After the reaction was finished, the products were
separated by column chromatography using benzene:ethyl
acetate (95:5). Compounds 3, 5, and 6 were unidentified by
comparison with authentic samples. Nitrile 4 was identified
by its NMR spectrum, and results of CG/MS analysis agreed
with previously reported ones.17
Gen er a l. Flash vacuum pyrolysis was carried out in a Vycor
glass reactor using a GAYNOR PRDH temperature controller
and a Thermolyne 21100 tube furnace. Oxygen-free dry
nitrogen or a mixture of nitrogen/toluene was used as carrier
gas. Samples to be pyrolyzed were 50-80 mg. Contact times
were around 10-2 s and pressures of 0.02 to 0.01 Torr were
used. In a typical run, 0.75-1.50 g of fractured catalyst was
placed along the reactor (30 cm length, 1 cm diameter) using
ceramic wool fiber as inert support. Products were trapped at
the liquid air temperature, extracted with solvent, and sub-
mitted to different analyses or separation techniques. Gas
chromatography/mass spectrometry (GC/MS) analyses were
performed with an SE-30 column, using helium as eluent at
a flow rate of 1 mL/min and a heating rate of 40 °C for 5 min
and 400 to 280 °C for 40 min. Chemical shifts are reported in
parts per million (ppm) downfield from TMS. Column and thin-
layer chromatography were performed on silica gel. Conversion
values in reactions of 1 and 3 were determined by quantifying
substrate by 1H NMR using nitromethane as internal stan-
dard.
Ack n ow led gm en t. This work was supported by
SECyT-UNC (Co´rdoba, Argentina), TWAS and INFICQ
(CONICET, Argentina), to whom the authors are in-
debted. Synthesis of hydrotalcites was performed by
E.L.M. in Spain, with an AL-E fellowship.
J O0260894
(14) Reichle, W. T. J . Catal. 1980, 63, 295.
(15) Del Arco, M.; Gutierrez, S.; Martin, C.; Rives, V.; Rocha, J . J .
Solid State Chem. 2000, 151, 272.
(16) Elguero, J .; J acquier, R. Bull. Soc. Chim. Fr. 1996, 90.
(17) Organic Syntheses; Wiley: New York, 1990; Collect. Vol. VII,
p 27.
Ca ta lysts. Catalyst HT-1 was prepared following the
methodology described by Reichle,14 and the sample having
nitrate (HT-2) was obtained by coprecipitation from Mg and
8150 J . Org. Chem., Vol. 67, No. 23, 2002