Palladium on calcium carbonate combined to 2-hydroxypropyl-α/β- cyclodextrins: A selective catalytic system for aqueous heck coupling and hydroarylation

Article abstract of DOI:10.1002/adsc.200800472

An efficient, selective and recoverable catalytic system for ligand-free aqueous Heck reactions using hydroxypropylated cyclodextrins (HPCDs) and palladium on calcium carbonate (Pd/CaCO₃) is highlighted. Remarkably, stereo- and chemoselectivities could be tuned by the cavity size of cyclodextrins, exploiting the relevance of host-guest interactions. UVVis experiments have led to strong evidence concerning an interplay between Pd(II) and α-HPCD, possibly ascribed to a reduction/stabilization effect of CDs. Unexpectedly, hydroarylation was the favored pathway with acrylonitrile which provided access to 3-phenylpropionitrile derivatives without usual hydride donors. Finally, determination of soluble Pd(0/II) via AAS enabled the definition of a predominant homogeneous mechanism in which TONs over 5000 were observed.

Full text of DOI:10.1002/adsc.200800472

FULL PAPERS
DOI: 10.1002/adsc.200800472
Palladium on Calcium Carbonate Combined to 2-Hydroxypropyl-
a/b-cyclodextrins: A Selective Catalytic System for Aqueous
Heck Coupling and Hydroarylation
Jaqueline D. Senra,a Luiz Fernando B. Malta,b Andrꢀa Luzia F. Souza,b
Lfflcia C. S. Aguiar,a,b and O. A. C. Antunesa,b,*
a
Nfflcleo de Pesquisas de Produtos Naturais, Universidade Federal do Rio de Janeiro, CCS Bloco H, Cidade Universitµria,
Rio de Janeiro RJ 21941-614, Brasil
Instituto de Química, Universidade Federal do Rio de Janeiro, CT Bloco A 641, Cidade Universitµria, Rio de Janeiro RJ
b
21941-909, Brasil
Fax : (+55)-21-2562-7559; e-mail: octavio@iq.ufrj.br
Received: July 28, 2008; Published online: November 5, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800472.
Abstract: An efficient, selective and recoverable cat- of CDs. Unexpectedly, hydroarylation was the fa-
alytic system for ligand-free aqueous Heck reactions vored pathway with acrylonitrile which provided
using hydroxypropylated cyclodextrins (HPCDs) and access to 3-phenylpropionitrile derivatives without
palladium on calcium carbonate (Pd/CaCO3) is high- usual hydride donors. Finally, determination of solu-
lighted. Remarkably, stereo- and chemoselectivities ble PdACHTNUTRGNE(NUG 0/II) via AAS enabled the definition of a pre-
could be tuned by the cavity size of cyclodextrins, ex- dominant homogeneous mechanism in which TONs
ploiting the relevance of host-guest interactions. UV- over 5000 were observed.
Vis experiments have led to strong evidence con-
cerning an interplay between Pd(II) and a-HPCD, Keywords: aqueous-phase catalysis; cyclodextrins;
possibly ascribed to a reduction/stabilization effect Heck reaction; host-guest systems; hydroarylation
Introduction
from the catalyst, along covalently modified metal
nanoparticles with supramolecular receptors[7] that
Palladium-catalyzed cross-coupling reactions are one can improve solubility and provide a range of inter-
of the most straightforward methods for the forma- esting properties.
[1]
À
tion of C C bonds in organic synthesis. In recent
Catalytic systems on the basis of water-soluble
years, the Heck reaction, commonly the coupling be- supramolecular receptors, such as cyclodextrins, calix-
tween aryl halides and olefins, has found widespread arenes and resorcinarenes, have drawn the attention
use in many industrial processes, especially in the syn- of synthetic chemists due to their singular activities,
thesis of fine chemicals and pharmaceutical inter- especially in terms of substrate selectivity.[8] The
mediates.[2] However, efficient Heck reactions are design of such systems can combine properties of a
usually carried out in organic solvents without catalyst metal core beyond a mass transfer effect, broadening
recycle, which jeopardize their major industrial appli- the pallet of valuable and elegant approaches in the
cation in terms of costs and effluent treatment. In search for effective catalysts.[8c]
view of this, considerable efforts have been directed
Cyclodextrins (CDs), cyclic oligosaccharides con-
to the development of benign processes. With this em- sisting of 6 (in a), 7 (in b) and 8 (in g) d-glucopyra-
phasis, common strategies include the use of alterna- nose units attached by a-1,4-linkages, are one of the
tive ligands to phosphines,[3] non-classical solvents,[4] most soluble supramolecular receptors and have
and phase-transfer conditions.[5] In particular, the use gained increasing attention in contemporary chemis-
of aqueous medium continues to attract attention, in try.[9] Their hydrophobic internal cavity can include
terms of cost, security and process conditions. Emerg- lipophilic molecules, causing significant increase of
ing conditions have involved biphasic catalytic sys- their solubility in polar solvents, e.g., water, a phe-
tems,[6] in which the product can be cleanly separated nomenon well understood in the light of the host-
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Jaqueline D. Senra et al.
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Table 1. Heck reactions catalyzed by the HPCD/Pd/CaCO3 system in aqueous medium.[a]
Entry
Ar-X
R1
R2
Cyclodextrin (mol%)
Yield [%][b]; (E/Z ratio)[b]
Product
1
2
3
4
5
6
7
8
9
10
11
12
13
C6H5I
C6H5I
4-MeOC6H4I
4-O2NC6H4I
4-O2NC6H4I
C6H5Br
4-MeCOC6H4Br
4-MeCOC6H4Br
C6H5I
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
H
H
H
CH3
CH3
CH3
H
a-HPCD (1)
No
80; (89/11)
61; (71/29)
97; (100/0)
trace[c]
1a
1a
1b
–
1c
1a
1d
–
1e
1e
–
1f
1f
a-HPCD (1)
a-HPCD (1)/No
b-HPCD (1)
a-HPCD (1)
a-HPCD (1)
No
a-HPCD (1)
b-HPCD (1)
No
54; (62/38)[d]
67; (100/0)
44; (100/0)[e]
Ph
CO2Me
CO2Me
CO2Bu
CO2Bu
CO2Bu
CO2Me
CO2Me
[f]
–
79; (94/16)
60; (68/32)
–
C6H5I
C6H5I
C6H5I
C6H5I
a-HPCD (100)
a-HPCD (1)
41; (100/0)[g]
52; (100/0)[g]
H
[a]
Reaction conditions: aryl halide (1.0 mmol), alkene (1.2 mmol), K2CO3 (2.0 mmol), Pd/CaCO3 (1 mol% based on Pd),
H2O/DMF (2:1, 15 mL), 1108C.
Determined by GC-MS.
Biphenyl and nitrobenzene were detected in 15 and 40% yields.
Nitrobenzene was obtained in 20% yield.
4,4’-Diacetyl-1,1’-biphenyl was also observed in 49%.
4,4’-Diacetyl-1,1’-biphenyl (82%) and acetophenone (17%) were the reaction products.
Using neat water.
[b]
[c]
[d]
[e]
[f]
[g]
guest approach. Indeed, cyclodextrin itself can be in the yield.[10b] Also, total regio- and stereoselectivi-
used to discriminate between substrates, which pro- ties were achieved, showing the beneficial effect of
vides a way for imparting selectivity[9d] to intrinsically CDs in this process.
non-selective reactions.
A more extensive survey of Heck couplings under
Because of our ongoing interest in the use of cyclo- these conditions is presented in Table 1 and Table 2.
dextrin derivatives in the study of inclusion phenom- For styrene, the reaction with iodobenzene in the
ena and in catalysis,[10a–c] we have been attracted to presence of a-HPCD, using K2CO3 as base, provided
explore aqueous cross-coupling reactions under supra- stilbene in a good yield (80%), although a slight mix-
molecular organometallic catalysis. Recently, we re- ture of Z/E diastereomers was observed (Table 1,
ported our first results on the use of hydroxypropylat- entry 1). As a reference, the reaction conducted in the
ed cyclodextrins (HPCDs) in aqueous Heck reactions absence of a-HPCD furnished a lower yield and de-
with aryl halides and methyl acrylate.[10b] This unpre- creased diastereoselectivity (Table 1, entry 2), suggest-
cedented catalytic system proved to be very effective ing the influence of the CD cavity. Application of the
using sub-stoichiometric amounts of a-HPCD in con- same conditions to activated and deactivated aryl io-
[10d]
junction with Pd/CaCO3
as palladium source. In dides led to different reactivity patterns (Table 1, en-
this work, we have enlarged our studies by examining tries 3–8). In particular, the reaction between 4-iodo-
HPCDs/Pd/CaCO3 systems in Heck reactions with a anisole and styrene, in the presence of a-HPCD, gave
range of aryl halides and olefins along with the effect the corresponding product with complete conversion
AHCTUNGTRENNUNG
in chemo- and stereoselectivities.
(Table 1, entry 3) but, surprisingly, the reaction failed
when using the most active 4-iodonitrobenzene
(Table 1, entry 4). In this case, the main product was
nitrobenzene deriving from hydrodehalogenation of
the substrate. However, under similar conditions a
Results and Discussion
In our previous study, the use of 1 mol% of both a- moderate yield of 4-nitrostilbene was observed in a b-
HPCD and Pd/CaCO3 in an aqueous mixture consist- HPCD medium (Table 1, entry 5).
ing of H2O/DMF (2/1) contributed to a huge increase
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Palladium on Calcium Carbonate Combined to 2-Hydroxypropyl-a/b-cyclodextrins
Table 2. Heck coplings of iodoarenes with acrylonitrile.a]
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Entry
Ar
Cyclodextrin (mol%)
Yield [%];[b] (E/Z ratio);[b] (Product) (Product)
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
a-HPCD (1)
No
b-HPCD (1)
b-CD (1)
a-HPCD (1)
No
a-HPCD (1)/No
b-HPCD (1)
19; (59/41); (2a)
0[c]
72 (3a)
0
35 (3a)
6 (3a)
57 (3b)
0
51; (52/48); (2a)
29; (51/49); (2a)
11; (67/33)[d]
0[e]
C6H5
4-MeO-C6H4
4-MeO-C6H4
4-O2N-C6H4
4-O2N-C6H4
0[f]
0
42; (55/45); (2c)[g]
31 (3c)
[a]
Reaction conditions: aryl iodide (1.0 mmol), acrylonitrile (1.2 mmol), K2CO3 (2.0 mmol), Pd/CaCO3 (1 mol%), CD (1
mol%), H2O/DMF (1:2, 15 mL), 1108C.
Determined by GC-MS.
10% biphenyl was detected.
Not isolated.
15% 4,4’-dimethoxy-1,1’-biphenyl was detected.
Nitrobenzene was the only product (30%).
[b]
[c]
[d]
[e]
[f]
[g]
1
The crude material (obtained as a 2c/3c mixture) was directly analyzed by H NMR.
Based on our previous result on the arylation of tions as above described, a-HPCD led to butyl a-
methyl acrylate, we also attempted to couple aryl bro- methylcinnamate in good yield and predominant E-
mides using this aqueous protocol. Under the same diastereoselectivity (Table 1, entry 9). In comparison,
conditions, the arylation of styrene with bromoben- a-HPCD-free medium was also applied but no signifi-
zene yielded the expected product in moderate yield cant yield was observed. Because of the more sterical-
(Table 1, entry 6). By contrast, when the reaction be- ly demanding character of this olefin, we sought to
tween the more reactive 4-bromoacetophenone and evaluate the cavity effect on the stereoselectivity em-
methyl acrylate was carried out, a substantial amount ploying b-HPCD at similar conditions. Whereas a sig-
of the Ullmann product was observed (Table 1, en- nificant E/Z mixture was formed (Table 1, entry 10),
tries 7 and 8), although a moderate yield of methyl 4- this result clearly indicates that the stereochemistry of
acetylcinnamate could be obtained in the presence of the product is induced by the cavity diameter, which
a-HPCD.
represents a good evidence that b-H elimination is as-
Since an analogous reactivity profile was displayed sisted by CD. This behavior has already been ob-
with 4-iodonitrobenzene (Table 1, entry 4), and from served by Cassez and co-workers in a different CD
the standpoint of supramolecular catalysis, it is con- system.[12]
ceivable that molecular recognition processes consti-
Acrylonitrile was quite unreactive at this
tute a decisive factor for the increase in (chemo)selec- H2O:DMF ratio, yielding cinnamonitriles only upon
tivity. In fact, these results are in line with the lower using a higher DMF ratio (1:2). In general, the pres-
association constants a-CD/substrate involving acyl- ence of a-HPCD led to the desired cross-coupling de-
and nitro-substituted arenes, when compared with b- rivative with an effective chemoselectivity (Table 2,
CD.[11] Considering that a and b-HPCD are mixtures entries 1 and 5) whereas biphenyl (Ullmann coupling)
of closely related derivatives containing different de- was the only product detected in its absence (Table 2,
grees of substitutions and isomeric forms, the respec- entries 2 and 6). Once again, Heck reactions with 4-
tive association constants would reflect an average iodonitrobenzene did not take place and hydrodeha-
effect which could explain the results in entries 4 and logenation products were predominant in the pres-
5. Therefore, alternative pathways seem to be favored ence/absence of a-HPCD (Table 2, entry 7). Most in-
in a predominant aqueous medium. Indeed, this be- terestingly, hydrocinamonitriles were the major prod-
havior suggests that the cavity is involved in the cata- ucts in an a-HPCD medium using iodobenzene or 4-
lytic cycle.
iodoanisole as the coupling partner (Table 2, entries 1
Butyl methacrylate, a substituted a,b-unsaturated and 5). To the best of our knowledge, this result has
carbonyl compound, was also employed for the prepa- not previously been observed in the context of Heck
ration of trisubstituted olefins. Under the same condi- coupling. Further reactions employing b-HPCD and
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native b-CD (Table 2, entries 3, 4 and 8) afforded the termination of soluble PdACTHNUTRGNE(UNG 0/II) species by using
Heck product in higher selectivity. So it could be pre- atomic absorption spectroscopy (AAS) was carried
sumed that the cavity diameter combined with the out. To address this issue, the Heck arylation of sty-
steric bulk/nature of the O-bound 2-hydroxypropyl rene was undertaken under the same general condi-
groups play a significant role towards the hydroaryla- tions.
tion pathway.
Beyond the experiments based on an aqueous-or- we obtained an estimation of the active palladium
ganic medium, we also have assayed the Heck reac- species from the Pd/CaCO3 support. Pd(0/II) found in
Analyzing the a-HPCD aqueous phase after 24 h,
AHCTUNGTRENNUNG
tion between methyl acrylate and iodobenzene apply- the solution at the end of the reaction corresponded
ing pure water. In neat water, formation of inclusion to 1.4% of the total palladium. So, this aqueous phase
complexes between iodobenzene-HPCD seems to be was tested in three consecutive runs, after product ex-
favored due to hydrophobic forces. This phenomenon traction and, as expected, no appreciable loss of activ-
was found to be critical, especially under stoichiomet- ity was observed. Subsequent analyses of the resultant
ric amounts of CD (Table 1, entries 12 and 13). Com- filtrate by AAS pointed to 1.3–1.0% of the initial Pd
bination with DMF provides significant solubility in- load (1”10À2 mmol) which supports the idea that
crease of aryl halides, which results in better diffusion active Pd particles remain in solution, presumably
and mass transfer of organic species. Assuming fur- due to a stabilization effect from CD units, as previ-
ther, it is possible that the use of an organic modifier ously supposed by us.[10b] Therefore, significant TON
lowers the medium polarity, which partially disables values over 5000 could be obtained from the catalytic
the hydrophobic association between cyclodextrin and reactions (Figure 1).
aryl halides, allowing the latter to interact with active
palladium species.
For comparison, in a closely related study by
Cassez et al.,[12] the increase in activity of Heck reac-
To highlight the potential and simplicity of our cat- tions catalyzed by Pd/C/randomly methylated b-CD
alytic system based on sub-stoichiometric HPCDs, we was proposed to be related to the adsorption of cyclo-
also compared it with the traditional micellar ap- dextrin on the catalytic surface, in which the reaction
proach using classical ionic and non-ionic amphiphiles takes place. Nevertheless, this idea contrasts with that
such as TBAB, Aliquat 336 and Triton X-100. As indi- prevailing here. Even though the reaction proceeding
cated in Table 3, only low to moderate yields were ob- in the heterogeneous medium cannot be ruled out,
served under the same conditions (1 mol% additive) our findings confirm the heterogeneous palladium is
by using the Heck reaction between iodobenzene and primarily the source of homogeneously active palladi-
styrene as a model. Effective couplings required at um species that actually perform the catalytic reac-
least 15–20 mol%, suggesting the reaction rate was tions.
dependent on self-assembly into micelles.[13] Indeed,
Aiming at investigating the interaction between Pd
the lower E/Z ratios, when compared with a-HPCD and a-HPCD, we obtained UV-Vis absorption spectra
(Table 1, entry 1), exploit the size and shape of the of aqueous palladium chloride solutions in the pres-
nanospace within which the reactions occur are deci- ence of a-HPCD. As observed in Figure 2, the ligand-
sive for boosting stereoselectivity.
to-metal charge transfer band at 235 nm suffered a
As an effort to evaluate the actual turnover num- great increase in its absorbance after 24 h. This is
bers of our CD system and to confirm our suspicions probably addressed to the interaction between OH
about a predominant homogeneous mechanism, de- groups of cyclodextrin and Pd(II), similar to that de-
scribed for a former Pd-dendrimer (G6-OH) interac-
tion.[14] Also the pronounced absorption in the 310–
375 nm range for the 24 h sample evidences the con-
Table 3. Heck reactions of iodobenzene with styrene using
stitution of Pd(0) clusters,[15] which is reasonable by
virtue of the microenvironment of CDs. Therefore,
we presume that cyclodextrins present in excess with
respect to Pd could act as capping agents for the
Pd(0) species during and after catalysis and recycling,
avoiding loss of activity through hydrophobic-hydro-
phobic stabilization.[16] On the basis of this reasoning,
the inclusion phenomena of substrates inside CDs
would allow a better interaction with Pd, resulting in
a favorable effect on catalytic performance.
different amphiphiles[a]
Entry
Amphiphile
mol (%)
Yield [%]b
1
2
3
4
5
6
TBAB
1
1
1
15
15
20
41 (62/38)
58 (74/26)
34 (68/32)
74 (79/21)
82 (80/20)
80 (77/23)
Aliquat 336
Triton X-100
TBAB
Aliquat 336
Triton X-100
[a]
Reaction conditions: phenyl iodide (1.0 mmol), styrene
(1.2 mmol), K2CO3 (2.0 mmol), Pd/CaCO3 (1 mol%), am-
phiphile, H2O/DMF (2:1, 15 mL), 1108C, 24 h.
Determined by GC-MS. The values in parentheses refer
to E/Z ratio.
These results further contain interesting informa-
tion about the role of HPCDs in the present system.
Although our investigation did not focus on mecha-
nistic aspects, some points regarding these findings
[b]
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Palladium on Calcium Carbonate Combined to 2-Hydroxypropyl-a/b-cyclodextrins
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Figure 1. TON values obtained from the recycling experiments with a-HPCD. Reaction conditions (2nd–4th runs): phenyl
iodide (1.0 mmol), styrene (1.3 mmol), K2CO3 (2.0 mmol), recovered aqueous phase, reflux, 24 h.
Figure 2. Absorption spectral of aqueous Pd(II) solutions observed upon UV-Vis light irradiation.
are worth noticing. First, the poorer chemoselectivity Indeed, this tendency suggests a slower rate preceding
obtained when using electron-withdrawing groups the migratory insertion step, in which fast reductive
substituted on aryl halides has already been observed coupling and parallel reduction reactions could be fa-
in aqueous Heck reactions. Similarly to Perosa and vored. Upon CD complexation, however, these path-
co-workers,[17] hydrodehalogenation and homocou- ways have a considerable smaller preference and,
pling pathways were particularly pronounced in the thus, the reaction outcome is dictated by the Heck
presence of these reactive halides, which provide evi- cycle.
dence that the respective intermediate from oxidative
Second, the catalytic hydroarylation of simple ole-
addition is the common one to competitive processes fins is exceedingly rare. Related profiles are common
taking place in poorly dispersed aqueous systems. in systems which are unable to reach the conforma-
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Jaqueline D. Senra et al.
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Figure 3. Proposed catalytic cycle in the presence of HPCDs.
tion required for syn-b-elimination of HPdX and, amount of a-HPCD, which also functions as a stabiliz-
thus, the reaction with a hydride donor can occur.[18] er, provides a means for reactivation and recycling.
Mainly due to the work of Wu,[19a] Hierso[19b] and Bar-
A proposal in line with our results is depicted in
ros[19c] with alkyne protocols, it is known that alcohol Figure 3.
functionalities provide an alternative route for hydro-
genolysis of organopalladium intermediates through
b-hydride elimination of palladium alcoholates. Acryl- Conclusions
onitrile, a less activated Michael acceptor, reacted
only in a CD medium to furnish a mixture of cinna- In conclusion, we have shown that the combination of
monitriles and their reduced product, whose ratio Pd/CaCO3 and a-HPCD, in sub-stoichiometric
could be tuned to a far greater degree depending on amounts, results in an efficient catalytic system for
the spatial restriction (cavity effect) and substituted the arylation of activated olefins with aryl halides in
groups on the primary macrocycle. Concerning the aqueous medium. Even though using a heterogeneous
later factor, it is possible that Pd hydride species catalyst, AAS measurements provided an insight into
could mostly be generated by interactions between mechanistic aspects and evidence that the support
hydroxypropyl groups from CD and the s-alkyl-Pd in- acts as a reservoir of catalytically active Pd species.
termediate from acrylonitrile insertion, leading to Indeed, the presence of 2-hydroxypropyl-a/b-cyclo-
Pd(0) species after one turnover. However, the reason dextrins always contributed to enhancements in the
for the outstanding selectivity towards acrylonitrile is yield along with a better diastereoselectivity profile
not yet clear .
(for a-HPCD), presumably induced by the cavity size.
Third, as suggested by UV-Vis spectra and consid- Central to supramolecular systems, molecular recogni-
ering the significant number of hydroxy groups[21] tion processes seem to operate as an important effect
present in the unique chemical environment of CDs, for boosting selectivity.
the reduction of Pd(II) species from the commercial
Interestingly, hydroarylation was the favored path-
Pd/CaCO3 catalyst[20] could also be an additional role way in the coupling with acrylonitrile, possibly a
played by HPCDs even held down by a low kinetics. result of the CD cavity size along with interactions
In this regard, subsequent reduction by the excess between 2-hydroxypropyl group and organopalladium
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Palladium on Calcium Carbonate Combined to 2-Hydroxypropyl-a/b-cyclodextrins
FULL PAPERS
The isolated products were collected, purified by flash
column chromatography and characterized by 1H and
13C NMR.
intermediate. So far, hydroarylation has not been ob-
served in common Heck systems which prompted us
to investigate the operative mechanism and also pro-
vide an attractive protocol to access 3-phenylpropio-
nitrile derivatives in the same pot without an addi-
tional hydrogen donor.
Recycling Experiments
The first run was carried out as described above. In a 50-mL
flask containing the recovered aqueous phase were poured
Furthermore, this catalytic system takes the advant-
age of using sub-stoichiometric amounts of the more phenyl iodide (0.11 mL, 1.0 mmol), styrene (0.14 mL,
1.3 mmol), potassium carbonate (276.4 mg, 2.0 mmol) and
5 mL N,N-dimethylformamide. The resulting mixture was
stirred and heated for 24 h at 1108C.
soluble and cheaper HPCDs – compared with the mi-
cellar protocol – to achieve good activities in aqueous
medium. Studies aimed at drawing on experimental
data to currently understand the role of CD along
with focusing on its application to the development of
new methodologies are underway in our laboratory.
Acknowledgements
The authors acknowledge CAPES, CNPq and FAPERJ, Bra-
zilian Governmental Agencies, for financial support. We also
are grateful to Professors Dr. D. S. Vaitsman and Dr. L. M.
Gomes for AAS measurements and Professor Dr. P. M. Es-
teves for GC/MS support.
Experimental Section
General Methods
All reactions were carried out with previously distilled
water using magnetic stirring. Reagents were obtained from
commercial sources and used without previous purification.
The a/b-HPCD were native a/b-CD partially O-2-hydroxy-
propylated with approximately 0.6 OH groups modified per
glucopyranose unit. Purification of reaction products was
carried out by flash chromatography using Merck silica gel
60 (230–400 mesh). Analytical thin layer chromatography
was performed on Merck silica gel 60-F plates (0.25 mm).
Visualization was accomplished with UV light. GC/MS anal-
yses were carried out on a Shimadzu GC-2010 coupled to a
quadrupole Shimadzu QP20105 network operating in a elec-
tron ionization mode at 70 eV. A DB-5 capillary column
(30 m”0.25 mm”0.25 mm) was used for the separations.
AAS measurements were performed on a Varian SpectrAA
220 FS employing a Varian Pd hollow cathode lamp. UV-Vis
spectra were recorded in a double beam Varian model Cary
1E using an aqueous PdCl2 solution (11.2 mM) or by the
treatment of this solution with 2-hydroxypropyl-a-cyclodex-
trin at a ratio of 1:1 at 258C. Before UV measurements,
samples were filtered through an Iso-Disc N-34 nylon mem-
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5 mL
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aryl
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