Reductive dehalogenation of aryl halides over palladium catalysts deposited on SBA-15 type molecular sieve modified with amine donor groups

Article abstract of DOI:10.1016/j.molcata.2011.04.001

Supported palladium catalysts prepared from palladium(II) acetate and SBA-15 type molecular sieve modified with N-[2-(diethylamino)ethyl]-3- aminopropyl donor groups efficiently catalyse reductive dehalogenation of various bromo- and iodobenzenes with in situ formed triethylammonium formate as the hydrogen source. Since the corresponding chlorobenzenes react at considerably lower rates while fluorobenzenes remain unaffected, the catalytic system can be utilised for selective dehalogenations of multiply halogenated substrates. Compared with Pd/C tested earlier [N.A. Cortese, R.F. Heck, J. Org. Chem. 42 (1977) 3491], the catalysts deposited over modified SBA-15 are more reactive. Besides, they exert rather minor leaching of the metal component (8.4% during five consecutive runs with 0.5 mol.% Pd with respect to the substrate) and maintain their high activity during repeated use.

Full text of DOI:10.1016/j.molcata.2011.04.001

Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
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journal homepage: www.elsevier.com/locate/molcata
Reductive dehalogenation of aryl halides over palladium catalysts deposited on
SBA-15 type molecular sieve modified with amine donor groups
Petr St eˇ pni cˇ ka , Miloslav Semlera,b, Jan Demela,b,1, Arno sˇ t Zukal , Ji rˇ í Cejka
a,∗
b
b
ˇ
ˇ
a
Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, 128 40 Prague, Czech Republic
J. Heyrovsk y´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolej sˇ kova 3, 182 23 Prague, Czech Republic
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Supported palladium catalysts prepared from palladium(II) acetate and SBA-15 type molecular sieve
modified with N-[2-(diethylamino)ethyl]-3-aminopropyl donor groups efficiently catalyse reductive
dehalogenation of various bromo- and iodobenzenes with in situ formed triethylammonium formate
as the hydrogen source. Since the corresponding chlorobenzenes react at considerably lower rates while
fluorobenzenes remain unaffected, the catalytic system can be utilised for selective dehalogenations
of multiply halogenated substrates. Compared with Pd/C tested earlier [N.A. Cortese, R.F. Heck, J. Org.
Chem. 42 (1977) 3491], the catalysts deposited over modified SBA-15 are more reactive. Besides, they
exert rather minor leaching of the metal component (8.4% during five consecutive runs with 0.5 mol.% Pd
with respect to the substrate) and maintain their high activity during repeated use.
Received 14 February 2011
Received in revised form 1 April 2011
Accepted 4 April 2011
Available online 13 April 2011
Keywords:
Heterogeneous catalysts
Supported catalysts
Palladium
Dehalogenation
Hydrogen transfer
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Halogenated aromatic hydrocarbons are valuable synthetic
Typically, these reducing systems involve Ni or Pd as the metal
component and various hydrogen sources. The systems based on
Al–Ni nanoparticles/NaH/t-BuOH [7], silica-supported Ni/H2 [8],
Ni over C with various H-sources [9] or D2 [10] and, mainly,
various forms of supported Pd catalysts (e.g., Pd over MCM-41
[11], Pd over PM2 with formate or 2-propanol [12], Pd over C
with H2 [13], D2 [10], N H ·HCl [14], NEt /MeOH [15], or 2-
intermediates finding manifold use in common laboratory praxis
and in chemical industry. Their massive use and improper disposal
led to global contamination of the environment and, conse-
quently, stimulated interest in processes leading to degradation
of these chemically persistent pollutants. The methods utilised to
decompose haloaromatic compounds are often based on reductive
cleavage of the halogen–carbon bonds. Apart from their environ-
mental significance, these methods hold considerable synthetic
potential, particularly when allowing for selective modifications
of multiply functionalised aromatic molecules.
The dehalogenation reactions are typically performed with
reagents combining low-valent transition metal compounds –
either defined or generated in situ from suitable precursors (typ-
ically carbene [1] or phosphine [2] complexes) – and reducing
agents such as hydrides [3], alkali metals [4], zinc metal [5], or
Grignard reagents [6]. As practically more convenient alterna-
tives, however, appear to be systems based on a combination of
finely dispersed catalytically active metal and reducing agents.
2
4
3
propanol/alkali metal hydroxide [16] as the hydrogen source, Pd
on polystyrene–poly(ethylene glycol) resin/formate [17]) can serve
as typical examples. Systems exploiting unsupported [18] or sup-
ported rhodium [16,19] particles, deposited platinum/H2 [20], or a
mixed metal Pd–Rh/H2 system [21] have been reported, too.
Recently, we have prepared a series of palladium catalysts sup-
ported by siliceous materials modified with various aminoalkyl
groups at the surface and demonstrated the superiority of the
N-[2-(diethylamino)ethyl]-3-aminopropyl donor group [22]. The
generally good catalytic performance of these catalysts in C–C bond
forming reactions (Suzuki-Miyaura, Heck, and Sonogashira [22,23])
led us to explore their catalytic potential also in other Pd-catalysed
reactions. Considering the applications of deposited palladium
catalysts mentioned above, we decided to test our catalysts in
reductive dehalogenation of aryl halides. In this contribution, we
describe reductive dehalogenation of several model substrates
with triethylammonium formate (HCO H/NEt ) [13b] in the pres-
2
3
∗
Corresponding author. Fax: +420 221 951 253.
E-mail address: stepnic@natur.cuni.cz (P. Sˇ t eˇ pni cˇ ka).
ence of Pd catalysts supported by SBA-15 type molecular sieve
functionalised with the N-[2-(diethylamino)ethyl]-3-aminopropyl
groups.
1
Present address: Institute of Inorganic Chemistry, Academy of Sciences of the
Czech Republic, v.v.i., Rˇ e zˇ , Czech Republic.
1
381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.04.001

9
8
P. Sˇ t eˇ pni cˇ ka et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
O
O
O
SBA-15
Si
Cl
1
1
. H N(CH ) NEt
2
2 2
2
2
. NH
3
O
O
O
SBA-15
Si
NH
Et N
2
2a, 2b
Fig. 1. Changes in the amount of incorporated 3-chloropropyl groups (expressed as
wCl; open circles) and BET surface areas (filled circles) of material 1 with the reaction
time.
Pd(OAc)
2
the structure of pristine SBA-15 [25] remained largely unaffected
upon the incorporation of the modifying groups.
In the next step, support 1 was reacted with an excess of 2-
N,N-diethylamino)ethylamine in dry toluene (at 100 C for 24 h)
O
O
O
◦
(
SBA-15
Si
and the resulting material was converted to a free amine form by
treatment with ammonia in a water–ethanol mixture [22]. Com-
parison of the analytical data obtained for supports resulting from
two independent runs revealed the amination step to be rather
sensitive to the reaction conditions. Based on the residual chlorine
content, 67% of the 3-chloropropyl groups reacted in material 2a
NH
Pd(OAc) @ Et N
2
2
3a, 3b
−
1
Scheme 1. Preparation of materials 1–3.
(nitrogen content: 1.11 mmol g ), while the data for material 2b
−
1
suggested ca. 81% conversion (nitrogen content: 1.36 mmol g ).
As the last step, the amine-modified sieves were metalated with
palladium(II) acetate. Support 2a was used for the preparation of
a metal-saturated catalyst 3a via treatment with an excess of pal-
ladium(II) acetate (1 mmol Pd per 1 g of the support, reaction in
dichloromethane) and subsequent removal of some loosely bound
metal precursor by Soxhlet extraction. Material 2b was treated with
2
. Results and discussion
2
.1. Preparation of the catalysts
only 0.1 mmol Pd(OAc) per 1 g of the solid support to give catalyst
The support required for the preparation of the catalysts
2
3
b. According to elemental analysis, only 56% of the starting palla-
was obtained in two steps (Scheme 1). At first, all-siliceous
SBA-15 type mesoporous molecular sieve modified with 3-
chloropropyl groups in the walls (material 1) was prepared
by acid-induced co-condensation of sodium silicate and (3-
chloropropyl)triethoxysilane (ca. 8 mol.% of total Si in the reaction
mixture) in the presence of a non-ionic polar, triblock alkene-
oxide copolymer Pluronic P123 as the structure directing agent
−
1
dium(II) acetate was deposited in material 3a (Pd: 0.56 mmol g
;
molar Pd:N ratio = 1:1.8) while, in the case of 3b, the metal compo-
−
1
nent was deposited quantitatively (Pd: 0.11 mmol g ; molar Pd:N
ratio = 1:12).
Powder X-ray diffraction patterns confirmed that the meso-
porous structure of the supports remains virtually unaffected
[
24]. While trying to optimise the synthesis of material 1, we
found that the key properties of this support, namely the amount
of incorporated modifying groups (chlorine content) and textu-
ral properties, change greatly with the duration of hydrothermal
treatment (Fig. 1).
◦
As the optimal reaction time we chose seven days (at 90 C
under autogenous pressure). During this time the chlorine content,
reflecting the amount of anchored 3-chloropropyl groups, modestly
increased while prolonged reaction times led to its sharp drop. A
similar though less pronounced trend was noted also for BET sur-
face areas. The surface areas of material 1 increased during the first
ca. 5 days, then levelled-off (till ca. 15 days) and, finally, decreased.
It appears likely that extended reaction times lead to a (partial)
deterioration of the support, which is reflected by a decrease in the
amount of chlorine and the BET surface areas. The powder X-ray
diffraction patterns of material 1 prepared over different periods of
the hydrothermal treatment (Fig. 2) all showed a strong diffraction
◦
peak above 2ꢀ = 1 and a pair of relatively weaker peaks in the range
◦
of 2ꢀ = 1.3–1.8 . The patterns observed for individual samples did
Fig. 2. X-ray diffraction patterns of material 1 obtained after one (dotted line), three
not differ much from each other and, above all, they confirmed that
(dashed line) or seven days (solid line) of the solvothermal treatment.

P. Sˇ t eˇ pni cˇ ka et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
99
X
H
HCOOH/NEt
3
R
R
[Pd]
Scheme 2. General scheme of the reductive dehalogenation reaction (X = halide).
Fig. 3. Comparison of X-ray diffractions patterns of material 1 (dotted line), material
a (dashed line), and catalyst 3a (solid line).
2
Fig. 5. Kinetic profiles for dehalogenation of 4-bromo- (filled symbols) and 4-
iodotoluene (empty symbols) over 3a (circles) and 3b (triangles). Conditions:
◦
0
.5 mol.% Pd, 5 equiv. HCO2H, 80 C.
3
−1
(
0.7–0.8 cm g ) and pore diameters (around 7 nm). Finally, it
should be noted, that the data available do not offer any evidence
for a particular mode of interaction between the anchored palla-
dium and the surface, and for the role (coordination) of the nitrogen
groups.
2.2. Catalytic tests
For an initial catalytic testing in reductive dehalogenation
Fig. 4. Nitrogen adsorption–desorption isotherms for materials 1 (᭹), 2a (ꢀ), and
(
Scheme 2) we chose a complete series of 4-halotoluenes as simple
model substrates. Initial attempts to combine our catalysts with
-propanol/NaOH as a conventional hydrogen source were unsuc-
a (ꢁ). The isotherms of 2a and 3a are onset by 300 and 600 cm3
g
−1
, respectively,
3
to avoid overlaps (filled symbols = adsorption, empty symbols = desorption).
2
cessful. The dehalogenation reaction did not proceed even with the
more reactive halides (bromide and iodide) while the siliceous sup-
port was completely destroyed. Stimulated by the work of Cortese
and Heck [13b], we then decided to test our catalysts in combina-
tion with triethylammonium formate generated in situ from formic
acid and an excess of triethylamine.
during the amination and metalation steps (Fig. 3). The nitro-
gen adsorption isotherms for materials 1–3 revealed favourable
textural characteristics (Table 1) as it is typical for well-ordered
mesoporous structure of this kind. The isotherms (Fig. 4) showed
an increase in the adsorbed amount of nitrogen in the p/p0 range
ca. 0.65–0.75, being characteristic for mesoporous molecular sieves
with narrow pore size distributions. The calculated textural param-
eters (Table 1) indicate a considerable decrease in the BET surface
areas upon amination. Although the subsequent metalation had a
less pronounced effect, the BET surface areas determined for cat-
alyst 3a and 3b still differed considerably reflecting the amount
Activity of the catalysts was thus studied in dehalogenation of 4-
halotoluenes in a mixture of triethylamine and formic acid (5 equiv.
◦
HCO H with respect to the substrate) with 0.5 mol.% Pd at 80 C.
2
The reactions with 4-fluoro- and 4-chlorotoluene did not proceed
appreciably within 24 h. On the other hand, those employing the
more reactive 4-bromo- and 4-iodotoluene showed good conver-
sions with 4-iodotoluene being expectedly faster dehalogenated
than the bromo derivative with both catalysts (Fig. 5). Notably, the
catalysts showed significant differences in their relative activities:
2
−1
2
−1
of deposited palladium (cf. 491 m g
for 3b and 397 m g
for the metal-saturated material 3a; relative difference ca. 20%).
On the other hand, all materials showed similar void volumes
Table 1
a
Analytical data and textural parameters for materials and catalysts 1–3.
Material/catalyst
Pd content
Cl content
(mmol g
N content
(mmol g
BET surface
BJH void volume
(cm
BJH pore
diameter (nm)
−
1
−1
−1
2
−1
3
−1
g )
(mmol g
)
)
)
area (m
g
)
1
n.a.
n.a.
n.a.
0.56
0.11
0.72
0.24
0.14
0.22
0.17
n.a.
668
443
477
397
491
0.80
0.84
0.79
0.71
0.79
6.4
7.3
6.9
7.3
6.9
2
2
3
3
a
b
a
b
1.11
1.36
0.98
1.36
a
n.a. = not applicable.

1
00
P. Sˇ t eˇ pni cˇ ka et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
Table 2
Table 4
a
Optimisation of the reactions conditions in the dehalogenation of 4-bromotoluene
as the model substrate.
Recyclation tests with catalyst 3a.
a
Catalytic run
Conversion [%]
Pd in the liquid
phase [ppm]
Fraction of
total Pd [%]
◦
Entry
Pd loading
mol.%]
Amount of HCOOH
[equiv.]
T [ C]
Conversion [%]
[
1
2
3
4
5
69
82
86
100
100
4.1
1.1
1.4
3.8
1.1
3.0
0.8
1.0
2.8
0.8
1
2
3
4
5
6
7
8
9
0.2
0.5
1.0
2.0
5.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
5
5
5
5
5
1
2
5
10
20
5
5
5
80
80
80
80
80
80
80
80
80
80
60
65
70
75
80
28
46
81
94
90
10
25
42
33
36
4
a
Conditions: dehalogenation of 4-bromotoluene (1.0 mmol), 0.5 mol.% Pd (cata-
◦
lyst 3a), 5 equiv. HCO2H, 5 mL triethylamine, 80 C/12 h.
10
11
12
13
14
15
5 molar equivalents of the acid with respect to the substrate (i.e.,
5
mmol per 1 mmol of substrate in 5 mL of triethylamine as the
11
19
32
47
solvent). With higher amounts of the acid the reaction proceeded
still quite well. In contrast, the use of 2 or only 1 equiv. of HCO H
5
5
2
resulted in considerably lower conversions. Finally, changing the
reaction temperature (Table 2, entries 11–15) showed a very strong
influence (approximately exponential) of this reaction parameter
on the conversion. For instance, increasing the temperature from 60
a
The reaction time was 12 h in all experiments.
◦
to 80 C resulted in a more than ten-fold increase in the conversion
(4 → 47%).
Applying the optimised reaction conditions (catalyst 3a,
◦
0
.5 mol.% Pd, 5 equiv. HCO H, 80 C), dehalogenation of several
2
halide substrates was tested. The results collected in Table 3
show that extended reaction times (12 or 24 h) are required for
achieving satisfactory conversions with bromobenzenes bearing
electron-donating substituents (2-, 3- and 4-Me, 4-MeO; deacti-
vated substrates). In contrast, electron-withdrawing substituents
facilitate the halogen removal, which is reflected by very good
conversions at considerably shorter reaction times (cf. the data
for 4-bromoacetophenone and 4-bromobenzonitrile) [26]. How-
ever, this activation is not sufficient to allow for an efficient
reductive dehalogenation of aryl chlorides (Table 3). Apart from
the mentioned substrates, two asymmetric 1,4-dihalobenzenes
were included in testing to examine the possibility of selective
dehalogenation of multiply halogenated arenes. Both 1-bromo-4-
chlorobenzene and 1-iodo-4-chlorobenzene reacted selectively at
the site bearing the more reactive (heavier) halogen. For the for-
mer compound, a 69% conversion to chlorobenzene was achieved
within 24 h. The latter, more reactive substrate was deiodinated
almost completely within 2 h. Products resulting from chlorine loss
(i.e., bromo- or iodobenzene) were not detected by GC analysis.
In order to assess the possibility of catalyst recycling, another
series of experiments was carried out using catalyst 3a and
4-bromotoluene as the substrate. The data presented in Table 4
clearly demonstrate favourable properties of this catalyst as it
not only remained active (the conversion substantially increased
upon repeated use!) but also did not suffer from any massive
leaching of the deposited metal (max. 3% of total palladium per
one catalytic run, cumulative loss ca. 8.4% in five runs). Notably,
the amount of leached-out metal did not increase much when
the reaction time was prolonged. For instance, the reaction with
catalyst 3a and 4-bromo- and 4-iodotoluene proceeded with 80%
Fig. 6. Influence of the Pd loading on the conversion and turnover numbers.
whereas the metal saturated catalyst 3a performed rather similarly
with both substrates, particularly at the later stages of the reaction,
its less metalated counterpart 3b showed considerably different
reactivity towards bromo and iodotoluene (I ꢀ Br).
In the next step, the reaction conditions were optimised with
catalyst 3a by changing the amounts of the metal catalyst and the
reducing agent (triethylammonium formate, which is generated
in situ from HCO H and NEt ), and the reaction temperature. The
2
3
results summarised in Table 2 expectedly revealed the conversion
to increase with the metal loading up to 90% for 5 mol.% of the metal
catalyst (Table 2, entries 1–5) while the overall turnover numbers
decreased (TONs; Fig. 6). As a compromise, we chose 0.5 mol.%
metal loading (i.e., 0.5 mol.% of Pd with respect to substrate) for
all other experiments. Changing the amount of the reducing agent
(
Table 2, entries 6–10) revealed a clear optimum corresponding to
Table 3
a
Reactivity of catalyst 3a towards different haloaromatic compounds.
Substrate
Conversion [%]b
Substrate
Conversion [%]b
2
3
1
4
4
-Bromotoluene
-Bromotoluene
-Bromo-4-chlorobenzene
-Bromoacetophenone
-Bromobenzonitrile
6 (27/62)
5 (39/91)
16 (33/69)
78 (86/n.d.)
97 (n.d.)
4-Bromotoluene
4-Bromoanisole
1-Iodo-4-chlorobenzene
4-Chloroacetophenone
4-Chlorobenzonitrile
12 (28/80c)
4 (21/55)
87 (95/n.d.)
6 (28/53)
18 (n.d.)
a
b
c
◦
Conditions: 1 mmol of substrate in 5 mL of triethylamine, 5 equiv. HCO2H, 0.5 mol.% Pd catalyst, 80 C.
Conversion after 2 h (conversions achieved after 6/12 hours are given in parentheses if not specified otherwise). n.d. = not determined.
Conversion after 24 h.

P. Sˇ t eˇ pni cˇ ka et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
101
mesitylene (Fluka), formic acid (Lachema), sodium hydroxide
Lach-Ner) were used as received. Toluene (Lach-Ner) was dried
by standing over sodium metal and distilled. Dichloromethane
(
(
Lach-Ner) was dried over anhydrous K CO3 and distilled.
2
X-ray powder diffractograms were recorded with a Bruker D8
X-ray instrument using graphite-monochromatized Cu K␣ radia-
tion (ꢁ = 1.5412 A˚ ) and a position-sensitive detector Våntec-1 in
Bragg–Brentano arrangement. Adsorption isotherms of N2 were
◦
determined with a Micromeritics 2020 instrument at −196 C. Prior
to the measurement, the samples were degassed by heating to
◦
1
30 C and then evacuated till a pressure of 1 mPa was attained
(at least for 24 h).
The content of palladium was determined by optical emission
spectroscopy with inductively coupled plasma (ICP-OES) on an IRIS
Interpid II instrument (Thermo Electron) using axial plasma and
an ultrasonic nebuliser CETAC U-5000AT+ (plasma power: 1150 W,
Fig. 7. Powder X-ray diffraction patterns in the region of Pd (1 1 1) diffraction as
recorded for fresh catalyst 3a (A) and the catalyst recovered from dehalogenation
experiments with 4-bromotoluene (B: five runs of 12 h each, C: one run for 24 h)
−1
pressure in the nebuliser: 25.0 psi, auxiliary gas flow: 1 mL min
sample uptake: 2.40 mL min , wavelength used for spectropho-
tometric analysis: 324.270 nm). The samples were dissolved in a
,
−
1
and 4-iodotoluene (D: one run for 24 h). Conditions: 1 mmol substrate, 0.5 mol.% Pd,
◦
5
equiv. HCO2H, 5 mL triethylamine, 80 C. Note: The diffractograms are onset along
mixture of concentrated HNO and HF (2:3, both Suprapure, Merck;
3
the y-axis to avoid overlaps.
◦
5
0 C/15 min) and the solutions were diluted with redistilled water.
C, H, N, and Cl were determined by a conventional combustion
analysis.
and 84% conversions, respectively, with only 4.3% and 3.2% of total
palladium in the liquid phase after 24 h. The data obtained during
the recyclation experiments further indicate that the catalyst
develops during the reaction and repeated use. It appears likely
that palladium(II) initially deposited is reduced (with formate) to
form active Pd(0) species.
4.2. Preparation of the catalysts
Material 1. Commercial copolymer P123 (10.0 g) and
Na SiO ·9H O (27.35 g, 96.23 mmol) were dissolved in dem-
2
3
2
ineralised water (256 mL) by stirring in a polyethylene flask at
room temperature overnight. Concentrated aqueous HCl was
added (65 mL 35%, ca. 740 mmol) and the resulting mixture was
X-ray powder diffraction measurements (Fig. 7) revealed no
◦
differences in the region of low diffraction angles (2ꢀ < 5 ), thus sug-
gesting that the structure of the support remains unaffected during
the dehalogenation reaction. However, they clearly implicated a
formation of palladium metal particles. All materials recovered
after dehalogenation experiments exerted broad diffraction line
◦
heated in a bath kept at 45 C and stirred at 400 rpm. After 30 min,
neat (3-chloropropyl)triethoxysilane (1.96 g, 8.14 mmol; 7.6% of
◦
total Si) was introduced and stirring was continued at 45 C for 1 h.
◦
Then, the reaction mixture was transferred to a pre-heated oven
around 2ꢀ = 40 corresponding to (1 1 1) diffraction of palladium
◦
and allowed to react at 90 C for seven days. The separated product
metal [27], while fresh catalyst 3a (measured prior to catalytic use)
did not show any detectable peak in this region.
was filtered off, washed thoroughly with distilled water (until the
washings were neutral) and ethanol, and dried in the air. The crude
product was Soxhlet extracted with ethanol over 24 h and dried in
the air to afford 1 as a fine white powder. Powder X-ray diffraction
3
. Conclusions
◦ −1
2ꢀ/ ): 1.04 (s), 1.64 (w), 1.81 (w). Elemental analysis (mmol g ):
(
Deposited palladium catalysts prepared from palladium(II)
C 7.37, H 26.0, Cl 0.72.
acetate and SBA-15 type molecular sieve bearing N-[2-
diethylamino)ethyl]-3-aminopropyl donor groups at the surface
Material 2a. An oven-dried reaction flask was charged with
material 1 (6.5 g) and a stirring bar, and flushed with argon. Dry
toluene (140 mL) and 2-(N,N-diethylamino)ethylamine (2.4 mL,
(
as the support efficiently promote reductive dehalogenation of
bromo- and iodobenzenes with triethylammonium formate. The
corresponding chlorobenzenes react at considerably slower rates
while fluorobenzenes remain unaffected. Such different reaction
rates allow for selective dehalogenations of multiply halogenated
substrates. Compared to the commonly used Pd/C, the tested cata-
lysts are more active. In addition, they exert practically negligible
leaching of the metal component (8.4% during five consecutive
runs at 0.5 mol.% Pd loading) and remain active during repeated
use (at least for five consecutive runs).
1
7 mmol) were introduced successively and the resulting mixture
◦
was stirred at 100 C for 24 h. The solid was collected on a glass frit,
washed with ethanol and dried in the air. The intermediate was
suspended in a mixture of ethanol (100 mL) and aqueous ammonia
(25 mL 25%) and the suspension was stirred at room temperature
for 2.5 h. The solid was filtered off, washed with water until the
washings were neutral and then with ethanol and, finally, dried in
the air to give material 2a as a white powdery solid. Powder X-ray
◦
diffraction (2ꢀ/ ): 0.94 (s), 1.52 (m), 1.71 (m). Elemental analysis
−
1
(
mmol g ): C 7.09, H 20.0, N 1.11, Cl 0.24. Material 2b was pre-
−
1
4
. Experimental
pared in exactly the same manner. Elemental analysis (mmol g ):
C 9.43, H 27.5, N 1.36, Cl 0.14.
4.1. Materials and methods
Catalyst 3a. An oven-dried flask was charged with material 2a
(4.9 g), flushed with argon and sealed with a septum. Palladium(II)
Tri-block
copolymer
poly(ethylene
oxide)20(propylene
acetate (1.101 g, 4.9 mmol; 1.0 mmol per 1 g of the support) was dis-
solved in dry dichloromethane (10 mL) and the solution was filtered
through a 0.45 ␮m PTFE syringe filter directly to the solid support.
The resulting slurry was diluted with dichloromethane (40 mL) and
stirred under argon for 2 h. The solid was filtered off, extracted with
dichloromethane in a Soxhlet apparatus for 24 h to remove loosely
bonded palladium(II) acetate and, finally, dried in the air, afford-
oxide)70(ethylene oxide)20 (Pluronic P123, Aldrich), Na SiO ·9H O
2
3
2
(
(
Aldrich), HCl (35%, Lach-Ner), (3-chloropropyl)triethoxysilane
Aldrich), 2-(N,N-diethylamino)ethylamine (Aldrich), palladium
acetate (Aldrich), p-fluorotoluene (Aldrich), p-chlorotoluene
Fluka), p-bromotoluene (Fluka), p-iodotoluene (Aldrich), 2-
propanol (Penta), triethylamine (Acros), dioxane (Riedel-de Haën),
(

1
02
P. Sˇ t eˇ pni cˇ ka et al. / Journal of Molecular Catalysis A: Chemical 341 (2011) 97–102
ing catalyst 3a as a grey-orange powder. Powder X-ray diffraction
(c) J. V cˇ elák, J. Hetflej sˇ , Collect. Czech. Chem. Commun. 59 (1994) 1645;
(d) L. Lassová, H.K. Lee, T.S.A. Hor, J. Org. Chem. 63 (1998) 3538;
◦
−
1
(
2ꢀ/ ): 0.88 (s), 1.49 (w), 1.70 (w). Elemental analysis (mmol g ):
(
(
e) M. Czakóová, J. Hetflej sˇ , J. V cˇ elák, React. Kinet. Catal. Lett. 72 (2001) 277;
C 8.03, H 20.6, N 0.98, Cl 0.22, Pd 0.56.
f) J.M. Khurana, S. Kumar, B. Nand, Can. J. Chem. 86 (2008) 1052;
Catalyst 3b was obtained similarly starting from material 2b
4.8 g) and palladium(II) acetate (108 mg, 3 mmol; 0.10 mmol per
g of the support) in dichloromethane (10 + 40 mL). The product
(g) A.E.D. Fletcher, J.D. Hyatt, K.M. Ok, D. O’Hare, Green Chem. 11 (2009) 1343.
4] (a) F. Alonso, G. Radivoy, M. Yus, Tetrahedron 55 (1999) 4441;
[
(
1
(
1
b) F. Alonso, Y. Moglie, G. Radivoy, C. Vitale, M. Yus, Appl. Catal. A 271 (2004)
71;
was filtered off, washed with dichloromethane (250 mL) and dried
(c) Y. Moglie, F. Alonso, C. Vitale, M. Yus, G. Radivoy, Appl. Catal. A 313 (2006)
94.
in the air to give 3b as a yellowish powder. Powder X-ray diffraction
◦
−1
[5] J. Yamashita, Y. Inoue, T. Kondo, H. Hashimoto, Bull. Chem. Soc. Jpn. 58 (1985)
709.
[6] R. Hara, K. Sato, W.-H. Sun, T. Takahashi, Chem. Commun. (1999) 845.
7] (a) F. Massicot, R. Schneider, Y. Fort, S. Illy-Cherrey, O. Tillement, Tetrahedron
6 (2000) 4765;
(
2ꢀ/ ): 0.88 (s), 1.46 (w), 1.66 (w). Elemental analysis (mmol g ):
2
C 9.01, H 25.8, N 1.36, Cl 0.17, Pd 0.11.
[
5
4
.3. Catalytic tests
(
(
b) X. Jurvilliers, R. Schneider, Y. Fort, J. Ghanbaja, Appl. Organomet. Chem. 15
2001) 744.
Catalytic tests were performed with a Heidolph Synthesis I
device allowing for 16 parallel reactions. The reaction vessel
was charged with aryl halide (1.0 mmol), triethylamine (solvent;
[8] (a) C. Park, C. Menini, J.L. Valverde, M.A. Keane, J. Catal. 211 (2002) 451;
b) K.V. Murthy, P.M. Patterson, M.A. Keane, J. Mol. Catal. A: Chem. 225 (2005)
49.
9] B.H. Lipshutz, Adv. Synth. Catal. 343 (2001) 313.
[10] V.A. Yakovlev, V.V. Terskikh, V.I. Simagina, V.A. Likholobov, J. Mol. Catal. A 153
2000) 231, and references therein.
11] (a) P. Selvam, S.K. Mohapatra, S.U. Sonavane, R.V. Jayaram, Appl. Catal. B 49
2004) 251;
(b) P. Selvam, S.U. Sonavane, S.K. Mohapatra, R.V. Jayaram, Tetrahedron Lett.
5 (2004) 3071.
(
1
[
4
.0 mL), formic acid (5.0 mmol), mesitylene (1.0 mmol; internal
(
standard) and an appropriate amount of catalyst. For tests with
alcoholate reducing system, the reaction mixture consisted of aryl
halide (1.0 mmol), NaOH (2.0 mmol), 2-propanol (5.0 mL), mesity-
lene (1.0 mmol; internal standard) and catalyst 3a (0.5 mol.% Pd).
The reactions were monitored by withdrawing small aliquots of the
liquid phase, which were centrifuged at 4000 rpm for 10 min and
then analysed with Agilent 6850 gas chromatograph equipped with
a DB-5 column (0.18 mm diameter × 20 m, 0.18 ␮m film thickness)
and a FID detector.
[
[
(
4
12] M.A. Aramendía, V. Borau, I.M. García, C. Jiménez, A. Marinas, J.M. Marinas, F.J.
Urbano, C. R. Acad. Sci. Paris, Ser. IIc Chim. 3 (2000) 465.
[13] (a) R. Baltzly, A.P. Phillips, J. Am. Chem. Soc. 68 (1946) 261;
(b) N.A. Cortese, R.F. Heck, J. Org. Chem. 42 (1977) 3491.
[
14] P.P. Cellier, J.-F. Spindler, M. Taillefer, H.-J. Cristau, Tetrahedron Lett. 44 (2003)
191.
[15] H. Sajiki, A. Kume, K. Hattori, K. Hirota, Tetrahedron Lett. 43 (2002) 7247.
7
[
[
[
16] Y. Ukisu, T. Miyadera, J. Mol. Catal. A: Chem. 125 (1997) 135.
17] R. Nakao, H. Rhee, Y. Uozumi, Org. Lett. 7 (2005) 163.
18] (a) K. Fujita, M. Owaki, R. Yamaguchi, Chem. Commun. (2002) 2964;
Acknowledgements
(
b) T. Okamoto, S. Oka, Bull. Chem. Soc. Jpn. 54 (1981) 1265.
[19] Y. Ukisu, S. Kameoka, T. Miyadera, Appl. Catal. B 18 (1998) 273.
This work was financially supported by the Czech Science
Foundation (project no. 104/09/0561) and by the Ministry of
Education, Youth and Sports of the Czech Republic (project no.
MSM0021620857).
[
20] E.J. Creyghton, M.H.W. Burgers, J.C. Jansen, H. van Bekkum, Appl. Catal. A: Gen.
28 (1995) 275.
21] A. Ghattas, R. Abu-Reziq, D. Avnir, J. Blum, Green Chem. 5 (2003) 40.
1
[
[22] (a) J. Demel, Sujandi, S.-E. Park, J.
Cˇ ejka, P. Sˇ t eˇ pni cˇ ka, J. Mol. Catal. A: Chem. 302
(
(
(
2009) 28;
b) J. Demel, M. Lama cˇ , J. Cˇ ejka, P. Sˇ t eˇ pni cˇ ka, ChemSusChem 2 (2009) 442;
References
ˇ
ˇ
c) J. Demel, J. Cejka, P. St eˇ pni cˇ ka, J. Mol. Catal. A: Chem. 329 (2010) 13.
[
23] For a related catalyst, see: P.-H. Li, L. Wang, Adv. Synth. Catal. 348 (2005)
681.
[
1] (a) M.S. Viciu, G.A. Grasa, S.P. Nolan, Organometallics 20 (2001) 3607;
(
(
b) C. Desmarets, S. Kuhl, R. Schneider, Y. Fort, Organometallics 21 (2002) 1554;
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[
[
2] M.E. Cucullu, S.P. Nolan, T.R. Belderrain, R.H. Grubbs, Organometallics 18 (1999)
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1
[26] One referee suggested that the relatively higher conversion may be in this case
also due to a stronger interaction of these substrates with Pd.
5
(
[27] PDF card 5-0681: Palladium, cubic, space group Fm-3m (no. 225), a = 3.8898 A˚
◦
˚
◦
at 26 C; h k l 1 1 1, d = 2.246 A, relative intensity = 100, 2ꢀ ≈ 40 at Cu K␣.