Efficient hydroxycarbonylation of aryl iodides using recoverable and reusable carbon aerogels doped with palladium nanoparticles as catalyst

Article abstract of DOI:10.1016/j.tet.2006.12.024

Electron-rich and electron-poor aryl iodides are converted, in high to excellent yields, into the corresponding carboxylic acids through a hydroxycarboxylation reaction catalyzed by a recoverable and reusable phosphine free palladium-carbon aerogel catalyst using lithium formate and acetic anhydride as an internal condensed source of carbon monoxide. The catalyst system can be reused several times without any appreciable loss of activity.

Full text of DOI:10.1016/j.tet.2006.12.024

Tetrahedron 63 (2007) 2519–2523
Efficient hydroxycarbonylation of aryl iodides using recoverable
and reusable carbon aerogels doped with palladium
nanoparticles as catalyst
c
a
b
a
Sandro Cacchi,a, Cosmin L. Cotet, Giancarlo Fabrizi, Giovanni Forte, Antonella Goggiamani,
*
Laura Martın, Sandra Martınez, Elies Molins, Marcial Moreno-Manas,d,! Francesco Petrucci,b
c
d
c
ꢀ
ꢀ
˜
Anna Roigc and Adelina Vallriberad,
*
aDipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Universitaꢁ degli Studi ‘La Sapienza’,
P. le A. Moro 5, 00185 Rome, Italy
bIstituto Superiore di Sanitaꢁ, Viale Regina Elena 299, 00161 Rome, Italy
cInstitut de Cieꢁncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Cerdanyola 08193, Spain
dDepartment of Chemistry, Universitat Autoꢁnoma de Barcelona, Cerdanyola 08193, Spain
Received 4 November 2006; revised 5 December 2006; accepted 11 December 2006
Available online 18 January 2007
Abstract—Electron-rich and electron-poor aryl iodides are converted, in high to excellent yields, into the corresponding carboxylic acids
through a hydroxycarboxylation reaction catalyzed by a recoverable and reusable phosphine free palladium–carbon aerogel catalyst using
lithium formate and acetic anhydride as an internal condensed source of carbon monoxide. The catalyst system can be reused several times
without any appreciable loss of activity.
Ó 2006 Published by Elsevier Ltd.
1. Introduction
is the most commonly used insoluble support for palladium.
Because of its stability and relatively low cost, palladium on
charcoal is widely used in hydrogenation processes and even
in carbon–carbon bond forming reactions. Silica has also
been used as an inert surface to adsorb palladium. A different
approach to support palladium catalysts involves the coordi-
nation of palladium to a ligand covalently bound to a polymer
backbone. However, in most of these cases, recovery and
reuse of the immobilized catalysts have been proven unsat-
isfactory. More recently, it has been found that microencap-
sulation of palladium in polymeric coating is an efficient and
cost-effective technique to ligate and retain palladium spe-
cies4 and, in the same direction, aerogels,5 a new class of
porous solids obtained via sol-gel processes coupled with
supercritical drying of wet gels, has also been shown to ex-
hibit a great potential for the preparation of heterogeneous
catalysts.6
Palladium catalysis is widely used in organic synthesis. In
the last decades, its utilization in a variety of transformations
has known a continuous impressive growth for achieving an
important place in the arsenal of the practicing organic
chemist.1 Palladium catalysts, however, are expensive and
this may limit their utilization in some cases. Furthermore,
the use of palladium catalysts might result in palladium con-
tamination of the desired isolated product, a significant prob-
lem for the pharmaceutical industry, which has to meet strict
specifications to limit the presence of heavy metal impurities
in active substances.2 A possible way to overcome these
problems, which are of great importance to transfer labora-
tory-scale methods to large scale cost-effective processes,
can be the utilization of immobilized palladium catalysts3
so as to permit their easy and economical removal from
the reaction mixture and their reutilization. Indeed, discov-
ery of new synthetic applications of this precious transition
metal has been paralleled by a number of studies devoted
to support palladium on inert materials. Activated carbon
As part of a program devoted to the development of new cat-
alyst systems, some of us found that carbon aerogels doped
with palladium nanoparticles are good catalysts for the
Mizoroki–Heck reaction.7 Since the palladium-catalyzed
hydroxycarbonylation of organic halides is a key step in
many synthetic processes, particularly in the preparation of
fine chemicals and complex molecules exhibiting biological
activities, we decided to explore the utilization of Pd–carbon
Keywords: Palladium; Aerogels; Nanoparticles; Carboxylic acids; Recycle.
* Corresponding authors. Tel.: +34 935813045; fax: +34 935811265;
e-mail addresses: adelina.vallribera@uab.es; sandro.cacchi@uniromal.it
!
Deceased on 20th February 2006.
0040–4020/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.tet.2006.12.024

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S. Cacchi et al. / Tetrahedron 63 (2007) 2519–2523
aerogel catalysts in the conversion of aryl iodides into the
corresponding carboxylic acids. As the development of tech-
niques where carbon monoxide is gradually generated in
situ, omitting the utilization of pressured carbon monoxide,
is a target of great current interest,8 we were particularly at-
tracted by the idea of combining the hydroxycarbonylation
protocol based on the in situ generation of carbon monoxide
from lithium formate and acetic anhydride9 with the utiliza-
tion of nanosized palladium particles embedded in carbon
aerogels as catalyst.
Herein, we report that phosphine free Pd–carbon aerogels
can effectively catalyze the hydroxycarbonylation of aryl
iodides using lithium formate and acetic anhydride as an
internal condensed source of carbon monoxide (Scheme 1).
Pd-carbon aerogel (5 mol%)
Figure 1. TEM image of Pd–carbon aerogel (35.3% of palladium).
LiCl, EtN(i-Pr)2
Ac2O
ArI
HCOOLi
ArCOOH
DMF, 100 °C
case) under the conditions that gave the best results with ho-
mogeneous conditions [2 equiv of acetic anhydride, 3 equiv
of HCOOLi, 2 equiv of EtN(i-Pr)2, 3 equiv of LiCl in 1.5 mL
of DMF at 80 ꢀC].9 The TEM image of Pd–carbon aerogel
(Fig. 1) showed nanoparticles with a mean particle size of
19ꢁ4 nm and the corresponding X-ray diffraction pattern
was characteristic of fcc-Pd. Other features of this particular
sample were a BET surface area of 422 m2 gꢂ1, a total pore
1
2
Scheme 1.
2. Results and discussion
Carbon aerogels doped with metallic palladium nanopar-
ticles were synthesized as described previously.7 Summariz-
ing, high content Pd–carbon aerogels have been prepared by
sol-gel polymerization of formaldehyde with potassium salt
of 2,4-dihydroxybenzoic acid, followed by the K+-exchange
with Pd2+ ions from 0.1 M Pd(OAc)2 acetone solution and
subsequent supercritical drying with CO2. Carbonization at
1050 ꢀC, under an inert atmosphere, transformed the metal
ion doped organic aerogels into metal nanoparticles-doped
carbon aerogels. Before its utilization in hydroxycarbonyl-
ation reactions, the catalyst was washed with DMF, the
solvent, which gave the best results with homogeneous catal-
ysis. A sample of Pd–carbon aerogel containing 46% of pal-
ladium in 8 mL of DMF was heated for three days at 100 ꢀC
and then recovered by decanting the solvent. The procedure
was repeated three times. Sector field inductively coupled
plasma mass spectrometry (SF-ICP-MS) analysis indicated
the level of palladium in DMF, after decanting, to decrease
from 16 to 8 ppm approximately (Table 1, entry 1), corre-
sponding to an original metal loss of 0.9% and 0.4%. The
resistance of Pd–carbon aerogel to leaching in other solvents
was also briefly investigated (Table 1).
volume of 0.9 m3 gꢂ1, and a bulk density of 0.85 g cmꢂ3
.
p-Toluic acid was obtained in very low yield showing that,
in comparison with palladium on charcoal, Pd–carbon aero-
gel is less reactive. Indeed, under the same conditions,
p-toluic acid was isolated in 89% yield after 22 h in the pres-
ence of 0.05 equiv of 5% palladium on charcoal. Pleasingly,
increasing the reaction temperature to 100 ꢀC led to the iso-
lation of p-toluic acid in 88% yield (24 h) in the presence of
Pd–carbon aerogel. The reaction most probably involves the
formation of acetic formic anhydride (generated in situ by
the reaction of formate anion with acetic anhydride), which,
as a result of its thermal instability,10 acts as an internal con-
densed source of carbon monoxide. The general applicability
of these conditions was investigated with a series of electron-
rich and electron-poor aryl iodides. Our results are summa-
rized in Table 2. Benzoic acids were usually isolated in
high to excellent yields. Minor amounts of aldehydes, very
likely formed via reduction of s-acylpalladium intermedi-
ates by formate anions,11 were isolated in some cases.
Table 2. The Hydroxycarbonylation of aryl iodides catalyzed by Pd–carbon
aerogela
Initial attempts to prepare carboxylic acids from aryl iodides
were carried out with p-iodotoluene as the model system and
0.05 equiv of Pd–carbon aerogel (a sample of Pd–carbon
aerogel containing 35.3% of palladium was used in this
Entry
Aryl iodide 1
Reaction time (h)
Yield (%)b of 2
1
2
3
4
5
6
7
p-Me–C6H4–I
24
8
10
10
24
4
88
90
2a
2b
2c
2d
2e
2f
p-EtOOC–C6H4–I
1-Iodonaphthalene
p-MeOC–C6H4–I
p-Cl–C6H4–I
p-NO2–C6H4–I
m-MeO–C6H4–I
89
77c
85d
89
Table 1. Leaching of palladium from Pd–carbon aerogel (ppm)a
24
94
2g
Entry Solvent
First washing Second washing Third washing
a
All reactions were carried out on a 0.463 mmol scale in 1.5 mL of DMF at
100 ꢀC using 1 equiv of aryl iodide, 3 equiv of HCOOLi, 2 equiv of Ac2O,
2 equiv of EtN(i-Pr)2, 3 equiv of LiCl, and 0.05 equiv of Pd–carbon
aerogel.
Yields are given for isolated products.
The corresponding aldehyde was isolated in 10% yield.
The corresponding aldehyde was isolated in 8% yield.
1
2
3
4
5
DMF
MeCN
Dioxane <1
THF
Toluene
16
2
8
2
<1
<1
—
8
<1
—
b
c
d
a
a
a
Below the detectability limit (2 ng/mL).

S. Cacchi et al. / Tetrahedron 63 (2007) 2519–2523
2521
We next explored the reutilization of the catalyst, one of the
main aims of this research effort. Using p-iodotoluene and
ethyl p-iodobenzoate as models of electron-rich and elec-
tron-poor aryl iodides, we investigated the number of times
that we could reuse the catalyst system. With p-iodotoluene
the Pd–carbon aerogel was reused 12 times without any
obvious loss in activity (Table 3). Its limit of recyclability
remains to be tested. The recovery of the catalyst is particu-
larly convenient since it involves only decanting the solution
and separating mechanically the Pd–carbon aerogel from the
salts in the presence of air, without any particular precaution.
SF-ICP-MS analysis of the solutions in the 12 runs showed
the level of palladium to range from 3 to 6 ppm approxi-
mately, corresponding to an original palladium loss of
0.1% and 0.2%. A comparison between the BET parameters
of the fresh aerogel catalyst and the used one reveals that
both the surface area and total pore volume have decreased,
from 422 to 171 m2 gꢂ1 and from 0.9 to 0.6 cm3 gꢂ1. We
attribute the decrease of the total pore volume and, conse-
quently the decrease of surface area, to the collapse of the
micropores during the many ambient dryings undergone
by the material after each use as catalyst, which is on the
other hand supported by the results of the BET analysis.
This indicates that the porous aerogel microstructure went
through a slow deterioration although it maintained a large
accessibility of the reactants to the palladium nanoparticles.
Neither a loss of activity (Table 3) nor an appreciable leach-
ing of palladium after 12 cycles was observed. The total
amount of palladium remained essentially the same (within
the experimental error; 35.3% in the initial sample and 36%
in the sample recovered after 12 cycles) and still in the form
of Pd(0) species, as shown by an X-ray diffraction (XRD)
analysis (Fig. 2).
Figure 2. XRD patterns of: (a) original Pd–carbon aerogel and (b) recovered
Pd–carbon aerogel after 12 reaction cycles.
conditions, its recovery is straightforward and it can be
reused several times without any appreciable loss of activity.
3. Experimental
3.1. General
€
Melting points were determined with a Buchi B-545 appara-
tus and are uncorrected. HCOOLi$H2O was dried in an oven
at 70 ꢀC for at least 24 h before use. Pd–carbon aerogels
were prepared as described previously.7 All other reagents
and solvents are commercially available and were used as
purchased without further purification. Reaction products
were purified by flash chromatography eluting with n-hex-
Similar results were obtained with ethyl p-iodobenzoate,
which was successfully reused 10 times (Table 3).
1
ane/AcOEt/AcOH mixture. H NMR (200 MHz) and 13C
To sum up, we have shown that phosphine free Pd–carbon
aerogel is an effective catalyst for the hydroxycarbonylation
of aryl iodides in the presence of lithium formate and acetic
anhydride as an internal condensed source of carbon mon-
oxide. Apparently it is air-stable under typical storage
NMR spectra (50.3 MHz) were recorded with a Bruker
Avance 200 spectrometer using DMSO-d6 as solvent and
TMS as a shift reference. IR spectra were recorded with
a Jasco FT/IR 430 spectrometer.
3.2. Sample treatment
Table 3. Recycling studies for the hydroxycarbonylation of p-iodotoluene
and ethyl p-iodobenzoate in the presence of Pd–carbon aerogel as catalysta
The content of Pd in five different organic solvents, namely
acetonitrile, dimethylformamide (DMF), dioxane, tetrahy-
drofuran (THF), and toluene, has been quantified. Three sam-
ples for each solvent were analyzed for a total of 15 samples.
Number of
cycles
Yield (%)b (with
p-iodotoluene)
Yield (%)b (with
ethyl p-iodobenzoate)
1
2
3
4
5
6
7
8
88
93
93
95
95
95
91
93
96
95
96
95
90
93
93
93
94
91
94
97
97
93
Samples in acetonitrile, dioxane, DMF, and THF were sim-
ply diluted with high-purity deionized water (EASY pure
UV, BarnsteadyThermolyne, Dubuque, USA) and directly
analyzed. In particular, samples in DMF were diluted in
the 1:400 ratio and the others in the 1:20 ratio.
9
As regards toluene, in consideration of its immiscibility with
water, samples were evaporated to dryness and subsequently
a 0.6 M HCl solution was addedto obtain a final1:20 dilution.
10
11
12
a
All reactions were carried out on a 0.463 mmol scale in 1.5 mL of DMF
at 100 ꢀC (p-iodotoluene, 24 h; ethyl p-iodobenzoate, 8 h) using 1 equiv
of aryl iodide, 3 equiv of HCOOLi, 2 equiv of Ac2O, 2 equiv of
EtN(i-Pr)2, 3 equiv of LiCl, and 0.05 equiv of Pd–carbon aerogel.
Yields are given for isolated products.
3.3. Instrumentation, reagents, and analysis
Determination of Pd at mass 105 was performed by means of
a sector field inductively coupled plasma mass spectrometry
b

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S. Cacchi et al. / Tetrahedron 63 (2007) 2519–2523
technique (SF-ICP-MS) in medium resolution (m/Dm¼
3000). The necessity of the medium resolution was manda-
tory to avoid those signals coming from molecular or iso-
baric species, such as 40Ar65Cu, could overlap the signal
of Pd at the chosen mass and overestimate the actual Pd
values. Quantification of Pd was carried out through the
standard addition calibration method in the case of dioxane,
DMF, and THF, whereas the external calibration curve was
adopted in the case of acetonitrile and toluene. Rhodium
(Rh), selected at mass 103, was used as an internal standard
to keep under control the instrumental drift. Single-element
calibrant and internal standard were prepared from 1000 mg/
mL stock solutions of Pd in 10% HNO3 and Rh in 10% HCl
by dilution with high-purity deionized water.
3.3.1.4. Compound 2e. Mp: 239–240 ꢀC; lit.8 mp 239–
1
240 ꢀC; H NMR d 13.16 (br s, 1H), 7.93 (d, J¼8.4 Hz,
2H), 7.54 (d, J¼8.5 Hz, 2H); 13C NMR d 166.9, 138.2,
131.6, 130.1, 129.1.
3.3.1.5. Compound 2f. Mp: 237–238 ꢀC; lit.8 mp 237–
1
238 ꢀC; H NMR d 13.6 (br s, 1H), 8.29 (dd, J1¼8.0 Hz,
J2¼1.9 Hz, 2H), 8.13 (dd, J1¼8.0 Hz, J2¼1.9 Hz, 2H); 13
NMR d 165.9, 150.0, 136.4, 130.7, 123.7.
C
3.3.1.6. Compound 2g. Mp: 105–107 ꢀC; lit.8 mp 105–
1
107 ꢀC; H NMR d 13.0 (br s, 1H), 7.57–7.38 (m, 3H),
7.20–7.16 (m, 1H), 3.79 (s, 3H); 13C NMR d 167.7, 159.8,
132.7, 130.2, 122.1, 119.4, 114.4, 55.8.
The Pd-doped aerogels were characterized by X-ray diffrac-
tion (XRD) with a D5000 Siemens X-ray powder diffrac-
tometer using Cu Ka incident radiation. Transmission
electron microscopy (TEM) observations were performed
using a JEOL-JEM-2010 microscope operating at 200 keV.
Surface area determinations were carried out following the
BET (Brunauer–Emmett–Teller) method with a ASAP-
2000 surface area analyzer (Micromeritics Instruments
Corp).
Acknowledgements
Work carried out in the framework of the National Project
‘Stereoselezione in Sintesi Organica. Metodologie ed Appli-
cazioni’ supported by the Ministero dell’Universita e della
Ricerca Scientifica e Tecnologica and by the University
‘La Sapienza’. Financial support from ‘Ministerio de Educa-
cion y Ciencia’ (Projects CTQ2005-04968-C02-01 and
ꢁ
ꢀ
MAT2003-01052) and ‘DURSI-Generalitat de Catalunya’
(Projects SGR 2005-00452 and SGR 2005-00305) is grate-
fully acknowledged.
3.3.1. Typical procedure for the preparation of carboxy-
lic acids from aryl iodides. A solution of HCOOLi$H2O
(97.2 mg, 1.389 mmol), EtN(i-Pr)2 (161 mL, 0.926 mmol),
and acetic anhydride (127 mL, 0.926 mmol) in anhydrous
DMF (0.5 mL) was stirred at room temperature for 1 h.
Then, p-iodotoluene (101.0 mg, 0.463 mmol), 35.3% Pd–
carbon aerogel (7.0 mg, 0.023 mmol), and LiCl (54.4 mg,
1.389 mmol) in anhydrous DMF (1 mL) were added. The re-
action mixture was stirred at 100 ꢀC for 24 h. After cooling,
the Pd–carbon aerogel was recovered by decanting the solu-
tion and separating the catalyst system mechanically (it was
simply picked up with a spatula and immersed in DMF to
maintain it wet till the subsequent utilization) from the salts
in the presence of air. Then, the reaction mixture was diluted
with ethyl acetate, washed with 2 N HCl, dried over Na2SO4,
and concentrated under reduced pressure. The residue was
purified by flash chromatography (silica gel, 30 g; n-hex-
ane/ethyl acetate/acetic acid 85/14/1 v/v) to give 55.5 mg
(88% yield) of 2a: mp: 179–180 ꢀC; lit.8 mp 179–180 ꢀC;
1H NMR d 12.78 (br s, 1H), 7.83 (d, J¼8.1 Hz, 2H), 7.28
(d, J¼8.1 Hz, 2H), 2.34 (s, 3H); 13C NMR d 167.9, 143.6,
129.9, 129.7, 128.6, 21.7.
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1
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