Palladium/magnesium-lanthanum mixed oxide catalyst in the heck reaction

Article abstract of DOI:10.1002/adsc.200505225

A new, reusable palladium/magnesium-lanthanum mixed oxide catalyst has been found to exhibit unprecedented high catalytic activity and selectivity for the heterogeneous Heck cross-coupling reaction of aryl halides with alkenes. The nature of the aryl halides has a determinate effect on the reaction yield. The corresponding reactions of a wide range of activated and non-activated aryl bromides and chlorides furnished the arylated product with high E/Z isomer ratios even already at 80°C. The stereoselectivity depended on the nature of the olefins.

Full text of DOI:10.1002/adsc.200505225

FULL PAPERS
DOI: 10.1002/adsc.200505225
Palladium/Magnesium-Lanthanum Mixed Oxide Catalyst in the
HeckReaction
a
a,
b
Agnieszka Cwik, Zolt a´ n Hell, * FranÅois Figueras
a
Department of Organic Chemical Technology, Budapest University of Technology and Economics, 1521 Budapest,
Hungary
Fax: (þ36)-1-463-3648, e-mail: zhell@mail.bme.hu
b
Institut de Recherches sur la Catalyse, 2, Avenue Albert Einstein, 69626 Villeurbanne C e´ dex, France
Received: May 31, 2005; Revised: November 29, 2005; Accepted: December 16, 2005
Abstract: A new, reusable palladium/magnesium-lan- vated and non-activated aryl bromides and chlorides
thanum mixed oxide catalyst has been found to exhib- furnished the arylated product with high E/Z isomer
it unprecedented high catalytic activity and selectivity ratios even already at 808C. The stereoselectivity de-
for the heterogeneous Heck cross-coupling reaction pended on the nature of the olefins.
of aryl halides with alkenes. The nature of the aryl
halides has a determinate effect on the reaction yield. Keywords: catalyst recycling; Heck reaction; magnesi-
The corresponding reactions of a wide range of acti- um-lanthanum mixed oxide; palladium
Introduction
Heterogeneous catalysis could solve some of these
problems. The catalysts could be recovered from the re-
The palladium-catalyzed coupling of terminal olefins action mixture by simple filtration, and then reused. In
with aryl and vinyl halides (the Heck reaction) is an im- recent years a number of reports have appeared about
portant reaction thanks to its high tolerance of function- the heterogeneous Heck reaction catalyzed by support-
[12]
[
1–3]
al groups and its general applicability.
The arylal- ed palladium catalysts such as palladium on charcoal,
[
13]
kenes obtained are used as intermediates for the syn- palladium on magnesium oxide, palladium on simple
[
4,5]
[14]
[14,15]
thesis of biologically active compounds, natural prod- oxides such as silica, alumina or titania,
palladi-
[6]
[7]
[14]
[16]
ucts, or as precursors of conjugated polymers. This um/Nb-MCM-41,
reaction has also been used in the pharmaceutical indus- palladium/zeolites.
palladium/polymeric materials,
These supports have a common
[
17–21]
try for the production of compounds such as naproxen or feature: they are neutral or solid acids. A few reports re-
[
8]
[22]
octyl methoxycinnamate.
The Heck reaction commonly proceeds in the pres- zeolites
ence of homogeneous palladium catalysts, generated weak bases.
from either palladium(0) compounds such as, e.g., From earlier works the support is known to affect the
late to the use of basic supports such as LDH, basic
[23] [24]
or alkali-exchanged sepiolite, which are
Pd(PPh ) and Pd (dba) , or palladium(II) salts, such electronic density at the surface of palladium and plati-
3
4
2
3
[
9]
as acetate and chloride. Several ligands such as phos- num atoms, their solubility in aqueous nitric acid, and
phines, phosphites, carbenes, or thioethers have been their catalytic properties for dechlorination. Whereas
[
1–3,10,11]
successfully employed for this reaction.
Howev- acid supports behave as electron acceptors, basic sup-
er, the homogeneous catalysis creates problems for re- ports have been demonstrated to behave as electron do-
[
25]
[26]
covery and reuse and might result in palladium contam- nors relative to palladium or platinum. The elec-
ination of the product. On the other hand, the use of tron donation was measured by the shift of the infrared
phosphines, which are commonly used as electron-do- band of CO at low coverage. The CO band shifted to-
nating ligands in the catalysts, is undesirable because ward higher frequencies on the supports which pos-
they are toxic and air-, as well as moisture-sensitive. sessed acid sites and towards lower frequencies on basic
Moreover, under Heck reaction conditions both the supports such as magnesium oxide or basic zeolites. The
phosphines and their palladium complexes are prone shift of CO between an acid and a weakly basic support
À1
À1
to decomposition so that excesses of phosphine and pal- reaches 100 cm for palladium and 50 cm for plati-
ladium are required. The excess of phosphine reduces num. Due to this electron donation the support can
the reaction rate while the higher amount of palladium play the role of phosphines in homogeneous complexes.
[
10]
[27]
increases the cost of the process.
Indeed, Choudary et al. have already reported the im-
Adv. Synth. Catal. 2006, 348, 523 – 530
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523

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Agnieszka Cwik et al.
mobilization of nanocrystallites of palladium on layered Results and Discussion
double hydroxide, which was highly active in non-aque-
ous ionic liquid media and microwave conditions for the
Preparation of Palladium/Magnesium-Lanthanum
Mixed Oxide
Heck reaction. Palladium on basic zeolites has recently
been reported to exhibit, for Heck coupling, an increase
of activity compared to acid zeolites and is related to the
[
23]
basicity of the support.
The magnesium-lanthanum mixed oxide was prepared
[31]
The metal-support interaction has also been observed by the co-precipitation method described earlier. It
to play a role on the rate of dissolution or sulfation of contained 39.8% La, 5.4% K and 37% H O. The powder
2
platinum on different supports. The rate of platinum dis- XRD pattern of the uncalcined mixed oxide showed the
[
28]
solution increased on acid supports, and the negative- diffraction lines of a hydrated lanthanum carbonate and
ly charged platinum, formed on hydrotalcite as support of magnesium and lanthanum hydroxides with param-
was less resistant to sulfur than neutral platinum parti- eters corresponding to those reported in the litera-
[26]
[31]
cles. The catalytic properties of palladium for the ture. This triphasic solid probably consisted of a layer
dechlorination of CF Cl have been found to be very of lanthanum oxide deposited on magnesia. This mixed
2
2
sensitive to the choice of the support: acid supports favor oxide is macroporous and shows a surface area of
2
dechlorination which was inhibited on basic oxides. The 37.6 m /g after calcination at 923 K.
kinetic study suggested that the selectivity was control-
The palladium(II)/magnesium-lanthanum mixed ox-
led by the bond strength between a carbene-like species, ide was prepared by ion exchange, using an aqueous sol-
CF , and the surface. The strength of this interaction was ution of sodium tetrachloropalladate(II), followed by
2
supposed to vary with the electron availability at the pal- washing. Using this procedure, highly dispersed chloro-
ladium surface, in good agreement with the shift of the palladate anions are fixed at the surface. Among the
IR CO band for palladium/Al O and palladium/ preparations of palladium nanoparticles, the chemical
2
3
[29]
AlF3. Both dechlorination of CF Cl and Heck reac- reduction of a palladium salt in an aqueous or organic
tions involve the oxidative addition of a chlorinated solution with reducing agents such as hydrazine, hy-
compound at the palladium atom, and strongly basic drogen
2
2
[
22]
[
17,33]
[34]
and sodium borohydride
has been de-
supports are expected to behave differently from zeo- scribed. We obtained the palladium(0)/magnesium-
lites in different aspects. In a recent work, Srivastava lanthanum mixed oxide by reduction of the palla-
[30]
et al.
into the solution during the reaction. After a significant drazine hydrate (Scheme 1).
initial leaching, palladium was trapped onto the SAPO- The resulting samples were studied by ICP-OES in or-
1 support at the end of the reaction. SAPO-31, similar der to determine the amount of palladium dispersed the
measured the amount of palladium leached dium(II)/magnesium-lanthanum mixed oxide with hy-
3
to zeolites or carbon, is an acid support and a cation ex- solid. The absolute metal content of the catalyst was 0.84
changer, and this experiment suggests that palladium and 0.73 mmol/g in palladium(II) and in palladium(0)
leaches as the cation. Basic supports are anion exchang- species, respectively.
ers, so that this type of redeposition is excluded, and it is
therefore interesting to check recycling in such a case.
Magnesium-lanthanum mixed oxide (MgLaO) ap- The Catalytic Activity of Palladium/Magnesium-
pears to be a stronger base than hydrotalcite or fluo- Lanthanum Mixed Oxide
[31]
rides showing for the Michael addition the same cata-
lytic properties as the proazaphosphatranes described The activity of palladium immobilized on magnesium-
[32]
by Kisanga et al. In this report, we present the prepa- lanthanum mixed oxide was examined in the Heck cou-
ration and characterization of a new palladium/magne- pling reaction. Some reaction parameters (solvent, reac-
sium-lanthanum mixed oxide catalyst (Pd/MgLaO) tion time and base concentration) were investigated to
and the examination of the catalytic properties, in the determine the optimal conditions. Iodobenzene and
Heck reaction, of this type of catalyst that does not re- styrene were chosen as model compounds for the initial
quire an inert atmosphere, is thermally stable, can be study (Scheme 2). The reactions were performed with a
easily handled and reused.
10% molar excess of styrene at 808C under ambient at-
mosphere in the presence of triethylamine.
When DMF was used as solvent, the coupling reaction
provided an excellent yield after 10 h even when the
Scheme 1. Synthesis of the catalyst.
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Palladium/Magnesium-Lanthanum Mixed Oxide Catalyst in the Heck Reaction
FULL PAPERS
mixed oxide in DMF in the presence of triethylamine
as base. In this case only 44% yield was obtained (Ta-
ble 1, entry 7) which is in agreement with the considera-
[
17]
tions described by Djakovitch and Koehler that palla-
dium(0) was the active species in heterogeneous sys-
tems.
Since aryl iodides are expensive substrates on a large
scale, after the initial optimization studies with iodoben-
zene and styrene, we reacted a wide variety of aryl hal-
ides with styrene (Scheme 3) in the presence of the
new catalyst system. In all cases E isomers were ob-
tained exclusively. The conversion in the reactions de-
pended on the nature of the halides and their substitu-
ents.
The best result was obtained in the reaction with the
highly reactive iodobenzene. With bromobenzene a
slower reaction was observed, but increasing the reac-
tion time from 10 h to 20 h led to higher reaction yield
(
Table 2, entry 3). With chlorobenzene the coupling re-
action afforded only 17% yield even after a longer reac-
tion time (25 h), at a much lower temperature than usu-
ally used for activating aryl chlorides. The yields de-
Scheme 2.
amount of the catalyst was reduced to 0.02 g (1.5 mol %
of palladium) (Table 1, entry 4). Only the trans-aryla-
tion product was obtained selectively. Neither a biaryl
derivative, resulting from a competing homocoupling
reaction, nor branched derivatives were detected as
by-products (Scheme 2). The solvent had a determinant
effect on the yield of the product. On changing to the less
polar toluene the yield decreased dramatically to 22%
(
Table 1, entry 5). In the reaction without triethylamine
only a 5% yield of desired stilbene was obtained (Ta-
ble 1, entry 6). This is in agreement with the currently
accepted mechanism for the Heck reaction that requires
the stoichiometric neutralization of hydrohalogenic acid
by an appropriate base.
To compare palladium(II) with palladium(0), the re-
action of iodobenzene and styrene was performed using
1.7 mol % of palladium(II)/magnesium-lanthanum Scheme 3.
[a]
Table 1. Effect of the solvent and the amount of catalyst on the yield.
[
b]
Entry
Solvent
Reaction time [h]
Catalyst [g] (mol % Pd)
Yield [%]
1
2
3
4
5
6
7
DMF
DMF
DMF
DMF
Toluene
DMF
DMF
20
10
10
10
10
10
10
0.1 (7.3)
0.1 (7.3)
0.05 (3.7)
0.02 (1.5)
0.05 (3.7)
0.02 (1.5)
0.02 (1.7)
99
95
95
95
22[
c]
5
[
d]
44
[a]
[b]
[c]
[d]
Reaction conditions: 1 mmol aryl halide, 1.2 mmol olefin, 1.2 mmol triethylamine, 808C.
Yield of isolated product.
Without triethylamine.
Palladium(II)/magnesium-lanthanum mixed oxide catalyst.
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Agnieszka Cwik et al.
Table 2. Heck reaction between aryl halides and styrene in the presence of palladium/magnesium-lanthanum mixed oxide cat-
[a]
alyst.
[
b]
Entry
Product
X
R
R’
R’’
Reaction time [h]
Yield [%]
1
2
3
4
5
6
7
8
9
3a
3b
3b
3c
3d
3e
3f
3g
3h
3i
I
H
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
10
10
20
20
20
20
20
25
20
20
20
20
95
11
67
90
95
73
66
17
72
70
80
78
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
COCH3
H
CH3
H
COCH3
H
OH
H
H
COCH3
H
CHO
H
H
1
1
1
0
1
2
H
H
H
COCH3
H
CHO
3j
3k
[
[
a]
Reaction conditions: 1 mmol aryl halide, 1.2 mmol styrene, 1.2 mmol triethylamine, 1.5 mol % palladium, 5 mL DMF, 808C.
Yield of isolated product.
b]
creased in the expected order I>Br>Cl. As expected,
better results were obtained in the presence of activated
aryl bromides and chlorides such as 4-bromoacetophe-
none, 4-chlorocetophenone, or 4-chlorobenzaldehyde
(
Table 2, entries 4, 9, and 11). When a bromobenzene
derivative with an electron-donating substituent was
used as reactant (p-bromotoluene), the yield of the
product decreased. Interestingly, the sterically hindered
3-methyl-2-bromophenol (entry 7) could also be cou-
pled with styrene in 66% yield under the above-men-
tioned reaction conditions. There was no example found
in the literature where aryl chlorides would react at such
a low temperature with good yield.
The steric effects of the acetyl group in the ortho posi-
tion (2-bromoacetophenone, 2-chloroacetophenone
and 2-chlorobenzaldehyde; Table 2, entries 5, 10 and
Scheme 4.
12) did not influence the yield significantly. These results to the exclusive formation of the E isomer, in these ex-
suggest that the electronic factors are determining for periments in some cases mixtures of the linear isomers
the rate of this reaction with palladium/magnesium-lan- E and Z were obtained (Scheme 4, entries 2, 3). The
thanum mixed oxide.
Next, we examined the efficiency of the palladium/
branched product was not observed in these cases either.
All the reactions were performed in the presence of
magnesium-lanthanum catalyst in the arylation of other palladium/magnesium-lanthanum catalyst with 1.2 mol
unsaturated compounds, acrylates, butyl vinyl ether and equivs. of triethylamine in DMF at 808C for 10 h. In all
1-hexene. In spite of the stereoselectivity observed in cases the corresponding coupling products were ob-
the reaction between iodobenzene and styrene leading tained in good to excellent yields (Table 3). The E/Z
[a]
Table 3. Palladium/magnesium-lanthanum mixed oxide-catalyzed Heck reaction with iodobenzene.
[
b]
Entry
Product
R
Reaction time [h]
Yield [%]
E:Z ratio
1
2
3
4
5
5a
5b
5c
5d
5e
COOEt
CN
OBu
10
10
10
10
10
75
80
96[
100: 0
81: 19
50: 50
100: 0
100: 0
c]
n-Bu
75
C(CH ) OH
80
3
2
[
a]
Reaction conditions: 1 mmol iodobenzene, 1.2 mmol alkene, 1,2 mmol triethylamine, 1.5 mol % palladium, 5 mL DMF,
08C.
Calculated by H NMR.
8
[
[
b]
c]
1
Reaction temperature: 608C.
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Palladium/Magnesium-Lanthanum Mixed Oxide Catalyst in the Heck Reaction
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stereoselectivity was determined from the coupling con- in the air, it could be stored under air without any loss
1
stants of the products in the H NMR spectra.
of activity.
An important conclusion of this work is the much
The decrease of activity suggests some small deactiva-
higher activity of the palladium/magnesium-lanthanum tion or leaching of palladium. It is still disputed whether
mixed oxide compared to palladium supported on acid the reaction takes place on the solid palladium sur-
[
14,39]
supports such as carbon, zeolites, MCM-41-type materi- face
als or oxides. All these solids catalyze the reaction of lyst.
or with palladium dissolved from the cata-
The test used to discriminate between the
[
40–42]
bromobenzene and styrene at 1408C for palladium on two possibilities is to make a “hot filtration” and run
charcoal or on zeolites, 1608C for palladium on ox- the reaction with the metal remaining in the solution.
[
35]
[36,37]
ides, and 1708C for palladium on MCM
or on If palladium has been leached the reaction could pro-
[38]
SBA. By contrast, hydrotalcite modified by exchange ceed by homogeneous catalysis. Hot filtration is re-
onto the surface of triphenylphosphine sulfonate works quired to exclude the possibility of precipitating the dis-
at 1208C.
solved palladium onto a support during cooling to room
The mechanism of the reaction is supposed to involve temperature, so that the catalyst could maintain its cat-
as first step the oxidative addition of the halocarbon on a alytic activity. This behavior has been observed, e.g., in
[
2,3]
zerovalent palladium atom. This step is the one also the palladium on charcoal catalytic systems and has
[
43,44]
involved in the hydrogenolysis of CÀCl bonds, therefore been described as the “boomerang effect”.
Indeed
[
45]
it is not surprising that a strong effect of the support is Alonso et al. have reported that palladium contents
observed in both cases. Both reactions should be affect- as low as 10 mol % catalyze the Heck reaction of bro-
À8
ed indirectly by the support through the change of the mobenzene on methylacrylate at 1608C, so that it is dif-
availability of electrons at the palladium surface.
ficult to exclude the catalysis by leached palladium spe-
cies. It has to be emphasized, however, that in this case
the reaction was run a temperature higher by 1008C
than that used here. The palladium content of the fil-
tered solid was determined by ICP-OES. The difference
Recycling of the Catalyst
The lifetime is an important point concerning the use of between the fresh and used catalyst was ca. 4%. If we as-
the catalyst both for industrial and pharmaceutical ap- sume that this mass is exclusively leached palladium, this
À3
plications. As the solid catalyst can be recovered easily wt i oo nu .l d correspond to 6Â10 mol % palladium in solu-
from the reaction mixture, we examined the reuse of
the catalyst in the reaction of iodobenzene and styrene.
The reaction of iodobenzene on styrene was used as
After the first run the catalyst was separated by filtra- model: iodoaryl derivatives are very reactive and it is
tion, washed with hexane to remove adsorbed organic then expected that if palladium was leached, the reac-
substrates and then allowed to dry at room temperature. tion could proceed in solution. The reaction mixture
The
recycled
palladium/magnesium-lanthanum was stirred for 2 h, then the solid catalyst was filtered
mixed oxide was then reused in the presence of triethyl- off at the reaction temperature. The 46% conversion
1
amine in DMF without any regeneration at 808C. Fig- at this point was determined by H NMR spectroscopy
ure 1 presents the results of recycling of the catalytic sys- from a sample taken from the filtrate. Then the superna-
tem. It was found that four catalyst runs were performed tant was allowed to react further in the absence of the
without any significant loss of catalytic activity (the use solid catalyst under identical reaction conditions. Fig-
of 0.02 g of catalyst gave 95%, 95%, 93%, 90% isolated ure 2 shows that, after removal of the solid, the reaction
st
nd
rd
th
yield of E-stilbene for the 1 , 2 , 3 , 4 cycles, respec- stopped. If the reaction does proceed with trace
tively). The catalyst was not sensitive to the humidity amounts of palladium in solution, these traces are trap-
Figure 2. Evolution of the yield as a function before and after
catalyst filtration.
Figure 1. Yields obtained with recycled catalyst.
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FULL PAPERS
Agnieszka Cwik et al.
ped by the solid in the filtration. However, the observa- nificantly higher than that of potassium fluoride supported on
g- or a-alumina, for which enthalpies of adsorption below
tion of a strong support effect on the activity of palladi-
um suggests that the reaction occurs at the surface.
1
20 kJ/mol have been determined. Calcined, then hydrated hy-
drotalcite (HDT-OH) adsorbed about 0.25 mmol/g with a con-
stant heat of adsorption of 100 kJ/mol, so magnesium-lantha-
num mixed oxide appears to be a stronger base with a smaller
number of sites.
Conclusion
In summary, we have developed a new heterogeneous
palladium/magnesium-lanthanum mixed oxide catalyst,
which catalyzed efficiently the coupling reaction of aryl
halides with avarietyof olefins. The conversion in the re-
actions depends on the nature of the aryl halide. The re-
action is regioselective and stereospecific. The reactions
can be performed with 1.5 mol % palladium at 808C.
The catalyst can be recovered by simple filtration and
reused without significant loss of activity under ambient
atmosphere. The catalyst is air stable, can be stored and
handled also under an atmosphere of air.
Preparation of Palladium(II)/Magnesium-Lanthanum
Mixed Oxide
Magnesium-lanthanum mixed oxide (1.5 g) was suspended in
1
1
50 mL of aqueous sodium tetrachloropalladate(II) (0.441 g,
.5 mmol) solution and stirred at 258C for 12 h under a nitro-
gen atmosphere. The solid catalyst was filtered, washed thor-
oughly with 500 mL of water, and vacuum-dried to obtain the
brown-colored palladium(II)/magnesium-lanthanum mixed
oxide (0.84 mmol of Pd/g).
Experimental Section
Commercially available starting materials were used without Preparation of Palladium(0)/Magnesium-Lanthanum
1
13
further purification. H NMR and C NMR spectra were re- Mixed Oxide
corded on a Bruker Avance 300 (300 MHz) spectrometer using
The catalyst (1 g) was reduced with hydrazine hydrate (1 g,
0 mmol) in ethanol (10 mL) for 3 h at room temperature, fil-
tered, and washed with ethanol to give an air-stable black pow-
der (0.73 mmol of Pd/g).
TMS as internal standard. Melting points were determined on
Gallenkamp apparatus and are uncorrected. TLC was per-
formed on Merk Kieselgel plates (60 F254) with hexane:acetone
2
(
4:1) as eluent. The surface areas were determined from the
isotherms of adsorption of N at 77 K using the BETequation.
2
X-ray powder diffraction (XRD) patterns were recorded on a
Philips instrument using CuKa radiation (l¼1.54184 Š). The
1
palladium content was determined by ICP-OES. The spectral
and physical data of the known compounds were identical
with those reported in the literature.
General Procedure for the HeckReaction
In a typical reaction, palladium/magnesium-lanthanum mixed
oxide catalyst (0.02 g, 1.5 mol % Pd) was added to a mixture of
aryl halide (1 mmol), olefin (1.2 mmol), and triethylamine
(
1.2 mmol) in DMF (5 mL). The mixture was stirred at 808C
Preparation of Magnesium-Lanthanum Mixed Oxide
for 10–25 h. Then the catalyst was filtered out, and washed
with hexane. The reaction mixture was diluted with water
and extracted with hexane (2Â10 mL). The combined organic
extracts were washed with water and brine, dried over magne-
sium sulfate and evaporated to give the product. The residue, if
necessary, was purified by column chromatography or recrys-
tallized to give the corresponding olefin. The known products
Magnesium and lanthanum nitrates (0.386 mol and 0.129 mol
in 0.5 L water for an atomic ratio Mg/La¼3) were coprecipitat-
ed at a constant pH¼10 using a mixture of potassium hydrox-
ide (1 mol) and potassium carbonate(0.26 mol) in 0.52 L of dis-
tilled water. The samples (about 0.15 g) were activated at
923 K. The chemical analyses of the solid catalyst gave the
1
composition: La: 39.8%, K: 5.4%, H O: 37%. The Mg/La ratio
were characterized by comparing the H NMR, and melting
point data with those reported in the literature.
2
in the solid is then 4.26, compared to 3 in the solution, therefore
the precipitation of lanthanum is not complete at pH 10. The
powder XRD pattern of the uncalcined magnesium-lantha-
num mixed oxide contains diffraction lines of a hydrated lan-
thanum carbonate as well as magnesium and lanthanum hy-
droxides. This triphasic solid probably consists of a layer of lan-
thanum oxide deposited on magnesia. The surface area of this
Selected Data
(E)-Stilbene (3a): white solid, mp 122–1238C (methanol) (lit.
2
[47]
1
mixed oxide is 37.6 m /g after calcination at 923 K, and sup-
123–1248C ); H NMR (300 MHz, CDCl ): d¼7.11 (s, 2H),
3
porting lanthanum on magnesia thus renders the surface area
7.23–7.28 (m, 2H), 7.33–7.38 (m, 4H), 7.48–7.50 (m, 4H);
1
3
stable. The enthalpy of adsorption of CO as measured by mi-
C NMR (300 MHz, CDCl ): d¼137.4, 128.5, 127.5, 126.6.
2
3
crocalorimetry allows us to evaluate the basic strength of the
solid, and the number of basic sites is then measured by the
amount adsorbed. Magnesium-lanthanum mixed oxide decar-
bonated at 673 K is a strong base, adsorbing about 10 mmol/g
(E)-Cinnamonitrile and (Z)-cinnamonitrile were isolated as
1
an E:Z mixture (5b). (E)-5b: H NMR (300 MHz, CDCl ): d¼
3
5.87 (d, 1H, J¼16.6 Hz), 7.39 (d, 1H, J¼16.6 Hz), 7.38–7.50
1
(m, 5H). (Z)-5b: H NMR (300 MHz, CDCl ): d¼5.44 (d,
3
of CO with an enthalpy >140 kJ/mol. This base strength is sig-
1H, J¼12.3 Hz), 7.13 (d, 1H, J¼12.3 Hz), 7.38–7.50 (m, 5H).
2
528
asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 523 – 530

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