On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex

Article abstract of DOI:10.1016/S0022-328X(00)00041-3

The mechanism of hydroesterification of styrene using in situ-formed Pd(OTs)₂(PPh₃)₂ from Pd(OAc)₂, PPh₃ and TsOH in methanol has been investigated by isolation and characterisation of catalytically active intermediates. From reaction mixtures, Pd-hydridocarbonyl and Pd-acyl complexes were isolated and characterised, based on which a Pd hydride mechanism has been proposed. Formation of palladium hydride species has also been confirmed by ³¹P-NMR experiments.

Full text of DOI:10.1016/S0022-328X(00)00041-3

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 601 (2000) 100–107
On the mechanism of hydroesterification of styrene using an in
situ-formed cationic palladium complex
A. Seayad a, S. Jayasree a, K. Damodaran a, L. Toniolo b, R.V. Chaudhari a,*
a Homogenous Catalysis Di6ision, National Chemical Laboratory, Pune 411008, India
b Departmento di Chimica, Uni6ersita di Venezia, Calle Large-S, Marta 2137-30123, Venice, Italy
Received 8 November 1999; accepted 5 January 2000
Abstract
The mechanism of hydroesterification of styrene using in situ-formed Pd(OTs)2(PPh3)2 from Pd(OAc)2, PPh3 and TsOH in
methanol has been investigated by isolation and characterisation of catalytically active intermediates. From reaction mixtures,
Pd–hydridocarbonyl and Pd–acyl complexes were isolated and characterised, based on which a Pd hydride mechanism has been
proposed. Formation of palladium hydride species has also been confirmed by 31P-NMR experiments. © 2000 Elsevier Science
S.A. All rights reserved.
Keywords: Hydroesterification; Carbonylation; Styrene; Palladium; Mechanism; Catalytic cycle
1. Introduction
a mixture of Pd(OAc)2, PPh3 and TsOH as the catalyst
precursor system [9]. Since this catalytic system is found
to be promising with respect to catalytic activity and
selectivity and eliminates the use of corrosive pro-
moters, a detailed investigation on the mechanistic as-
pects has been carried out and presented in this paper
based on the isolation and characterisation of active
catalytic intermediates.
Carbonylation of vinyl aromatics [1] is an environ-
mentally clean synthetic route [2] for the preparation of
arylpropionic acids, especially 2-arylpropionic acids [3]
such as Ibuprofen [4] and Naproxen [5], a class of
non-steroidal anti-inflammatory agents. Palladium
complexes have generally been used as catalysts along
with halide promoters [6]. Formation of both branched
as well as linear isomers was observed in these reactions
and the regioselectivity and reaction rates vary depend-
ing on the catalyst system and the reaction conditions
used [7]. The use of cationic palladium complexes pro-
vides a halogen-free catalyst system for such reactions
under mild reaction conditions [8]. Recently, we have
reported a detailed study on the hydroesterification of
styrene using Pd(OTs)2(PPh3)2 (I), formed in situ from
Two mechanistic pathways proposed for the hydroes-
terification of olefins catalysed by palladium complexes
are a (a) hydride mechanism [10], which is initiated by
the insertion of an olefin into a palladiumꢀhydride
bond and proceeds through the formation of an acyl
complex as shown in Eq. (1), and (b) alkoxy mechanism
[11], which is initiated by the insertion of an olefin into
a palladiumꢀcarboalkoxy bond as per Eq. (2).
PdꢀHC“ꢁCPdꢀCꢀCꢀHC“OPdꢀCOꢀCꢀCꢀH
ROH
“ HꢀCꢀCꢀCOOR+PdꢀH
(1)
PdꢀCOR“OHPdꢀCOORC“ꢁCPdꢀCꢀCꢀCOOR
+
H
“/COHꢀCꢀCꢀCOOR+PdꢀCO
(2)
Scheme 1. Hydroesterification of styrene.
In principle, both the mechanisms can be operative
for the hydroesterification of styrene (Scheme 1) using
the present catalyst system consisting of Pd(OAc)2,
PPh3 and TsOH in methanol as a solvent. However, the
* Corresponding author. Tel.: +91-20-5893163; fax: +91-20-
5893260.
E-mail address: rvc@ems.ncl.res.in (R.V. Chaudhari)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00041-3

A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
101
Fig. 1. 31P-NMR spectra of a mixture of Pd(OAc)2–4PPh3–10TsOH in methanol: (A) immediately recorded; and (B) recorded after 1–2 h.
Scheme 2.
beneficial effects of hydrogen, water and TsOH in
promoting the carbonylation rates as observed in our
previous work [9] indicate the possibility of a hydride
mechanism. It is also likely that both the pathways are
operating simultaneously, since formation of palla-
diumꢀmethoxy complexes is quite facile in methanol as
a solvent [12] and that of palladiumꢀhydride is fa-
voured by the presence of acidic promoters [13b].
Hence, a detailed investigation on the nature of palla-
dium complexes formed under reaction conditions was
carried out and the results are discussed below.
in polar solvents like methanol as evidenced from
31P-NMR analysis, which showed three signals at
37.06 (l1), 30.58 (l2) and 29.19 (l3) ppm. These
signals are assigned to the dicationic complexes
[Pd(PPh3)2]2+(TsO−)2,
[(H2O)2Pd(PPh3)2]2+(TsO−)2
and [Pd(PPh3)3]2+(TsO−)2, respectively, according to
the values reported in the literature for similar palla-
dium dicationic complexes [13]. This solution showed a
different spectrum after standing for 1–2 h. Two new
signals at 38.35 (l4) and 33.82 (l5) ppm developed at
the expense of l1, l2 and l3 as shown in Fig. 1. This
change in the spectrum is due to the degradation of the
above-mentioned Pd-dicationic species in the presence
of residual water present in the solvent (ca. 1000 ppm).
The signal at l4 corresponds to a dicationic species
[Pd(m-OH)(PPh3)2]22+(TsO−)2 [14], which can be
formed as shown in Scheme 2. Our earlier studies [9]
2. Results and discussion
The in situ-generated Pd(OTs)2(PPh3)2 (I), from
Pd(OAc)2–4PPh3–10TsOH exists as a cationic species

102
A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
also showed that the isolated Pd(OTs)2(PPh3)2 was
highly moisture sensitive and changed to a sticky mass
in the solid state when exposed to air for 10–15 min.
The signal at l5 is assigned to triphenylphosphine ox-
ide, OꢁPPh3, in comparison with a standard sample in
methanol1.
No Pd–methoxy complexes were isolated in the
above case in the presence of excess TsOH. But when
the same reaction was carried out in the absence of
TsOH, a yellow microcrystalline complex, which turned
to red, was obtained on cooling the reactor contents to
room temperature. IR spectra of this complex showed
two characteristic strong carbonyl absorptions at 1955
and 1860 cm−1 corresponding to Pd(CO)(PPh3)3 and
[Pd(CO)(PPh3)]n, respectively [22]. 31P-NMR spectra of
this mixture of complexes showed three signals at 27.43
(l6), 22.19 (l7) and 20.12 (l8) ppm in DMF as a
solvent. The 31P-NMR spectrum of the mother liquor
after isolating the above complex showed three weak
signals at 19.02 (l9), 19.62 (l10) and 19.92 (l11) ppm in
addition to a sharp signal at −4.13 ppm corresponding
to free PPh3. A pale yellow complex was isolated on
evaporation of the solvent, the IR spectrum of which
showed strong carbonyl absorptions at 1671 and 1654
cm−1 and at 1304 and 1060 cm−1 corresponding to
Pd(COOMe)(OAc)(PPh3)2 as well as very weak absorp-
tions at 1630 and 1010 cm−1 corresponding to Pd-
(COOMe)2(PPh3)2 [23].
When the same reaction was carried out with two
equivalents of TsOH, pale yellow shining flakes of a
complex were obtained, which showed a carbonyl ab-
sorption at 1955 cm−1 corresponding to Pd(CO)-
(PPh3)3. Evaporation of the mother liquor gave a yel-
low complex, which also showed strong absorption at
1955 cm−1 along with weak carbonyl absorptions at
1751 and 1625 cm−1. No carbonyl absorption corre-
sponding to the carbomethoxy complex was observed
in this case. This suggests that formation of palladium–
carbomethoxy complexes is quite unlikely in the pres-
ence of TsOH. The instability of carbomethoxy species
in acidic medium has also been reported earlier [24].
In order to further investigate the nature of the
species formed in solution in the presence of CO under
reaction conditions (75°C and 500 psig of CO), experi-
ments were performed in which intermediate liquid
samples were taken under CO atmosphere followed by
31P-NMR analysis. An intermediate sample from the
reaction of Pd(OAc)2–4PPh3 with excess of TsOH (ten
equivalents) under 500 psig of CO at 75°C showed a
signal at 30.52 ppm (l12) and a broad signal between 18
and 25 ppm (l13) along with a small signal at l5 (that
of OꢁPPh3). The broad signal can be assigned to Pd(0)
carbonyls in fast equilibrium with free PPh3 as per Eq.
(3).
When the catalyst precursor system (Pd(OAc)2–
4PPh3–10TsOH) was treated with CO (500 psig) at
75°C for 30 min, shining white flakes of a complex were
observed on cooling the reactor contents to room tem-
perature. The supernatant solution was light yellow in
colour, which changed to black slowly on exposure to
air showing the precipitation of palladium metal. The
white complex was found to be highly unstable and
changed immediately to a light yellow and then to a
1
brown complex and finally to Pd metal. IR and H-
NMR spectra of the white complex were taken immedi-
ately. IR spectra showed absorption at 2154 cm−1
corresponding to PdꢀH [15] in addition to a carbonyl
1
absorption at 1955.7 cm−1. H-NMR spectra showed a
weak PdꢀH signal at −5.9 ppm. This PdꢀH signal was
found to be different from that observed from a mix-
ture of Pd(PPh3)4 with excess TsOH in CDCl3, which
showed two PdꢀH signals at −6.7 and −7.3 ppm,
corresponding
to
[HPd(PPh3)2S]+(TsO−)
and
[HPd(PPh3)3]+(TsO−), respectively, where S is a solvent
molecule. The in situ formation of palladium hydride
species from a mixture of Pd(0) complexes and acidic
promoters has also been reported earlier [16]. The
yellow flakes that formed from the white complex
showed only the carbonyl absorption at 1955.5 cm−1
and no signal corresponding to PdꢀH was observed in
1
IR or in H-NMR spectra. Elemental analysis of this
complex showed the presence of phosphorus and sulfur.
Further characterisations were difficult due to its insta-
bility even under inert atmosphere and at lower temper-
ature. From the decomposed solution of the complex in
CDCl3, transparent faint yellow crystals were obtained
in addition to palladium metal. The crystal was iden-
tified as PdCl2(PPh3)2·CDCl3 (by crystallography) and
may form by the interaction of palladium hydride with
the residual HCl present in CDCl3. With these evi-
dences, it is proposed that the isolated palladium
hydride complex has
a
formula [HPd(CO)-
(PPh3)2]+(TsO−) (VI). Palladium hydrides are generally
quite unstable for isolation [17] and have been fully
characterised only in a few cases [18]. Even though the
bridged binuclear palladium hydridocarbonyl species
has been well characterised [19], no mononuclear ana-
logue has been isolated and characterised though it has
been proposed [20]. A platinum analogue of VI was
reported earlier [21].
Pd(CO)(PPh3)3PXPh Pd(CO)(PPh3)2PXPh [Pd(CO)(PPh3)]n
3
3
(3)
When the spectrum was recorded with styrene imme-
diately added to a fresh intermediate sample of the
above reaction, two new signals were found to develop
at 23.65 (l14) and 24.09 (l15) ppm at the expense of the
signal at l12. The broad signal at l13 also disappeared
1 26.5 ppm in DMF as the solvent, PPh3ꢁO can be formed by the
interaction of H2O with Pdꢀphosphine complexes as explained by
Amatore et al. [13a].

A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
103
Fig. 2. (A) 31P-NMR spectrum of the mother liquor of a reaction of Pd(OAc)2–4PPh3–10TsOH in methanol under 500 psig at 75°C after keeping
overnight under CO. (B) Spectrum recorded after reaction with styrene.
Scheme 3.
and two new sharp signals at 19.35 (l16) and 18.72 (l17)
ppm, which may correspond to Pd(CO)(PPh3)n where
n=2 or 3, along with a broad signal from 4 to −4
ppm (l18) were developed. The new broad signal (l18)
may be due to Pd(0)–phosphine complexes in equi-
librium [25] with free PPh3 as shown in Eq. (4).
reaction was carried out under similar conditions to
those in which VI was isolated. The supernatant liquid
after keeping overnight under CO showed a signal at
l12 along with signals at l5, l16 and l17. A new signal
at 23.34 (l19) was also observed in this case. Styrene
was added to this reaction mixture (containing the
white flakes and the mother liquor) and stirred under
CO atmosphere at 40°C for 10 min. The intermediate
sample of this reaction also showed the development of
signals l14 and l15 at the expense of the signals at l12
and l19, in addition to a broad signal at l4 (Fig. 2). This
further confirms that the signal at l12 corresponds to
the palladium hydride species VI. The signal at l19 can
be assigned to a bridged hydrido–carbonyl species
[Pd(m-H)(m-CO)(PPh3)2]+2 (TsO−) (in comparison with
the value reported in the literature for similar com-
plexes) [27], which may form an equilibrium as shown
in Scheme 3.
Pd(PPh3)4PXPh Pd(PPh3)3PXPh Pd(PPh3)2
(4)
3
3
The signals at l14 and l15 correspond to Pd–alkyl
complexes and can be assigned to the species [(n-
St)Pd(CO)(PPh3)2]+(TsO−) (V) and [(iso-St)Pd-
(CO)(PPh3)2]+(TsO−) (VI). These values compare well
with those reported for similar alkyl complexes [26].
Since these alkyl complexes were formed at the expense
of the signal at l12, it can be assigned to the palladium
hydride species VI. In order to confirm that the signal
at 30.52 (l12) ppm is that of the PdꢀH species VI, a

104
A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
The catalytic activity of the isolated palladium hy-
concluded that the major pathway for the formation of
palladium hydride is through a kind of water-gas shift
reaction as shown in Scheme 4. Species IV on coordina-
tion with CO forms the active catalytic intermediate VI,
which initiates the catalytic cycle. As explained before,
Pd(0) species were also found to form under the reac-
tion conditions. This suggests that the active PdꢀH
species is in equilibrium with Pd(0) species such as
Pd(PPh3)4 or/and Pd(CO)(PPh3)3, as shown in Schemes
3 and 4. This fact is also supported by the formation of
PdꢀH species, on interaction of Pd(PPh3)4 with TsOH
as evidenced by the 1H-NMR experiments discussed
dride species VI was checked for the hydroesterification
of styrene. The catalytic activity without added TsOH
was found to be very low (TOF 52 h−1)2. But with
addition of eight equivalents of TsOH, the hydroester-
ification reaction proceeded with a TOF of 221 h−1
,
which gives further support to the suggestion that VI is
an active catalytic species. Under the reaction condi-
tions, the palladium hydride is in equilibrium with
Pd(0) complexes as shown before (Scheme 3) and the
lower activity of VI in the absence of TsOH is probably
because of the shift in the equilibrium more towards the
Pd(0) species, which subsequently transform to palla-
dium–methoxy complexes according to Eq. (2).
The active palladium hydride species VI can form
from Pd(OTs)2(PPh3)2 (I) through several reaction
pathways. It is known that methanol reduces Pd(II)
complexes to Pd(0) complexes [28], through the forma-
tion of a hydride IV as an intermediate (Scheme 4).
Since no formaldehyde formation was observed, it can
be inferred that formation of IV through III may not be
important. The hydride complex, IV, can also form
from I via a kind of water-gas shift reaction. Complex
I on interaction with CO and H2O gives a PdꢀCOOH
complex II, which forms the Pd hydride IV on elimina-
tion of a CO2 molecule [29].
before. The slight rate enhancement (TOF, 230 h−1
)
for hydroesterification reaction with Pd(PPh3)4, also
indicates the efficient formation of the active species VI
from the Pd(0) species. The palladium hydride VI was
also isolated when the catalyst precursor system
Pd(PPh3)4–10TsOH was treated with CO under similar
conditions as described earlier.
Isolation of the PdꢀH species under reaction condi-
tions and its reactivity towards insertion of styrene
forming the Pdꢀalkyl species, as well as for the hydroes-
terification reaction, indicate the involvement of the
hydride mechanism rather than the methoxy mecha-
nism. Another key intermediate in the catalytic cycle,
which can prove the involvement of the hydride mecha-
nism without ambiguity, is the palladium–acyl com-
plex. Generally, nucleophilic attack of methanol to
Pd–acyl complexes is regarded as the rate-determining
step in the case of carbonylation of olefins proceeding
through the PdꢀH mechanism [30]. Hence to isolate the
palladiumꢀacyl species, the hydroesterification reaction
was carried out at lower temperature (55°C) where the
reaction rate was very low. From this reaction, an
air-sensitive yellow crystalline complex was obtained on
keeping the reaction mixture under CO pressure for 2
days. The IR spectrum of this complex showed a strong
absorption at 1687 cm−1, characteristic frequency of a
Pd–acyl complex, and is assigned to PdꢀCOꢀsty spe-
GC analysis of the gas phase after a hydroesterifica-
tion reaction showed traces of CO2; this was confirmed
by reaction with large excess of catalyst (260 mg of
Pd(OAc)2), which showed a significant amount (0.61%
in the gas phase) of CO2 formation [9]. Hence it was
1
cies. H-NMR spectroscopy showed that the isolated
complex is a mixture of iso- and n-palladium–acyl
complexes (XI and XII). The complex was not so stable
probably due to the presence of weakly coordinating
ligands and immediate decomposition was observed.
In order to obtain a more stable acyl complex, the
hydroesterification reaction was carried out with PdCl2
as the catalyst precursor instead of Pd(OAc)2 and the
acyl complex formed was found to be more stable than
the earlier one. This complex showed strong carbonyl
absorption at 1685 cm−1 characteristic of a Pdꢀacyl
group. The 13C-NMR signal for the acyl CO was
observed at 234 ppm, which is 61 ppm downfield
compared with that of the free ester, 2. 1H-NMR
showed that the styrene moiety is coordinated in a
linear fashion and no iso-coordination was detected.
The observation of only one 31P-NMR signal (19.99
Scheme 4. Formation of the active palladium hydride.
2 With Pd(OAc)2–4PPh3 as the catalyst precursor, no reaction was
observed without added TsOH [9].

A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
105
Scheme 5. Catalytic cycle.
plausible one is the direct nucleophilic attack of
methanol to the Pdꢀacyl bond.
ppm) indicates that the two phosphine ligands are in
the same environment and the complex has a structural
formula trans-PdCl(COCH2CH2Ph)(PPh3)2 (XIII). Sim-
ilar types of acyl complexes have been reported to be
isolated from the hydrocarbonylation of hexene [31]
and propene [32] using PdCl2(PPh3)2 as the catalyst
precursor. The activity of complex XIII was checked by
reacting with methanol in a Schlenk flask under argon
atmosphere. Formation of 2 was observed (by GC as
well as by 1H-NMR) and the complex changed to
reddish–brown colour and finally decomposed to palla-
dium metal. This also confirms that the acyl complex is
a linear acyl species similar to the species XII. A small
difference in elemental analysis is due to the presence of
impurities and the purification beyond this was difficult
because of its unstable nature.
The isolation of PdꢀH as well as Pd–acyl complexes
under reaction conditions confirms the involvement of
the hydride mechanism for the hydroesterification of
styrene using Pd(OAc)2–PPh3–TsOH as the catalyst
system. As shown in Scheme 5, the catalytic cycle starts
with the insertion of styrene into the PdꢀH bond [33] of
the active palladium hydrido carbonyl species VI form-
ing the Pd–alkyl complex (VIII or X), which on migra-
tory insertion of CO gives the Pd–acyl complex (XI or
XII). The formation of product esters from the palla-
dium–acyl complex is the final step in the catalytic
cycle, which generates the active palladium hydride for
further catalysis. Methanol can attack the palladium–
acyl complex in two ways, i.e. through coordination to
the acyl complex or by the direct nucelophilic attack as
explained by Lin and Yamamoto [34] for the hydroes-
terification of benzylic halides. Though both the modes
of attack may be operating in the present system, most
3. Conclusions
The mechanism of hydroesterification of styrene us-
ing in situ-formed Pd(OTs)2(PPh3)2 complex from
Pd(OAc)2–PPh3–TsOH as the catalyst precursor sys-
tem was investigated. The catalytic cycle was found to
proceed through a hydride mechanism rather than a
methoxy mechanism. The hydride mechanism was
confirmed by characterisation of isolated active palla-
dium hydride and palladium–acyl complexes, which are
the key intermediates in the catalytic cycle. 31P-NMR
experiments carried out to investigate the nature of
species formed in solution in the presence of CO under
reaction conditions also confirmed the formation of
PdꢀH as well as the Pdꢀalkyl species.
4. Experimental
Styrene, Pd(OAc)2, PdCl2, p-toluenesulphonic acid
monohydrate (Aldrich, USA), PPh3 (Loba Chemie, In-
dia), and CO (Matheson, USA) were used as received
without further purification. Methanol was distilled and
degassed with argon before use and the water content
was analysed to be ca. 1000 ppm. Other solvents were
also distilled and degassed before use. Gas chromato-
graphic analysis was done on a Hewlett Packard 5890
Series GC, controlled by the HP Chemstation software,
by using an FFAP megabore column (30 m×0.53
mm×0.1 mm film thickness, on a poly ethyleneglycol

106
A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
stationary phase). Elemental analysis of the complexes
was carried out on a CHNS-O EA1108 Elemental
Analyser of Carlo Erba Instruments, Italy, and the ion
chromatography on a Waters Ion Chromatograph,
Austria, having Waters 432 Conductivity Detector, Wa-
TsO−). The complex VI changed to a light yellow
complex within a few minutes under argon atmosphere
and the IR spectrum of the resulting complex showed
no PdꢀH signal at 2154 cm−1 and l −5.9 ppm (1H-
NMR), while the carbonyl peak at 1955.7 cm−1 in the
IR spectrum was retained.
1
ters 600S Controller and Waters 626 Pump. H-, 13C-
and 31P-NMR spectra were recorded by Bruker MSL-
300 and Bruker AC-200 spectrometers. The 1H- and
13C-NMR chemical shift values are given relative to
TMS and those of 31P-NMR (121.19 MHz) are given
relative to external 85% H3PO4. The IR spectra in DRS
mode were obtained from a Shimadzu Hyper IR by
mixing samples with KBr and in transition mode using
a Perkin–Elmer Spectrum-2000 IR spectrometer.
4.3. Isolation of palladium–acyl complexes (XI and
XII)
Hydroesterification of styrene (7.2 mmol) was carried
out using a mixture of Pd(OAc)2 (0.223 mmol), PPh3
(0.892 mmol) and TsOH (2.23 mmol) in methanol with
low water content (200–500 ppm) at 55°C and 500 psig
of CO for 1–2 h. The contents in the reactor were
cooled to r.t. and kept for 2 days under CO atmo-
sphere. The yellow needle-shaped air-sensitive crystals
obtained were transferred to a Schlenk flask under
argon atmosphere and isolated as per the procedure
given for the isolation of VI. IR and NMR spectra of
the complex were taken immediately. IR (DRS, KBr):
4.1. Hydroesterification experiments
The hydroesterification reactions were performed in a
50 ml Parr autoclave, the details of which were given in
Refs. [8b,9]. In a typical experiment, known quantities
of styrene, Pd(OAc)2, PPh3, TsOH and methanol were
charged into the autoclave, and the contents were
flushed with nitrogen and then with CO. The autoclave
was heated to 75°C, and after attaining the tempera-
ture, the system was pressurised with CO to 500 psig.
The reaction was initiated by agitation (900 rpm). In
order to maintain pressure in the reactor, CO was fed
through a constant pressure regulator from a reservoir
vessel. After the reaction, the liquid samples were
analysed by gas chromatography.
.
1687 (PdꢀCOꢀSt), 1463, 1377, cm−1 1H-NMR4
(CDCl3): l 1.05 (s, CH3), 1.60 (PdꢀCOꢀCH2ꢀCH2),
2.20 (PdꢀCOCH2), 2.30 (s, CH3 of TsO−), 3.60
(PdꢀCOꢀCH), 7.10–7.70 (m, Ph of PPh3 and TsO−),
7.70–8.10 (m, Ph of acyl).
The complex changed immediately to reddish brown
on exposure to air in solid state and also decomposed
within a few minutes to palladium metal. 1H-NMR
spectroscopy showed that the isolated complex was a
mixture of iso (XI) and linear (XII) coordinated palla-
dium acyl complex.
4.2. Isolation of [HPd(CO)(PPh3)2]+(TsO−) (VI)
4.4. Preparation of PdCl(PPh3)2(COCH2CH2Ph) (XIII)
A mixture of Pd(OAc)2 (0.223 mmol), PPh3 (0.892
mmol), and TsOH (2.23 mmol) was charged to the
autoclave, flushed with nitrogen and subsequently with
CO, heated to 75°C, charged with 500 psig of CO and
stirred for 0.5–1 h. The reaction mixture was then
cooled to room temperature (r.t.) and kept under CO
atmosphere for 2–6 h. After depressurising CO and
flushing with argon, the white flake-like complex ob-
tained was transferred to a Schlenk flask under argon
atmosphere and the complex was allowed to settle
down. The mother liquor was decanted3 and the com-
plex washed with degassed methanol and n-hexane
successively and dried under vacuum to obtain the
complex VI, which is highly air sensitive and becomes
reddish brown within a few minutes of exposure to air
and was only moderately stable under argon atmo-
sphere. IR and NMR spectra were taken immediately.
IR (DRS, KBr): 2154 (w, PdꢀH), 1955.7 (w, PdꢀCO)
Hydroesterification of styrene (7.2 mmol) was carried
out with PdCl2 (0.565 mmol), PPh3 (2.29 mmol), and
TsOH (5.65 mmol) in methanol under 75°C and 500
psig of CO for 2–3 h. The contents in the reactor were
cooled to r.t., CO depressurised and the reaction mix-
ture kept in a freezer below 0°C for 2 days. The white
crystals formed were transferred to a Schlenk flask
under argon atmosphere and isolated as per the proce-
dure given for the isolation of complex VI. IR (DRS,
KBr): 1685.7 cm−1 (PdꢀCOꢀSt). 1H-NMR (CDCl3):
l
1.60 (t, 2H, PdꢀCOꢀCH2ꢀCH2), 2.50 (t, 2H,
PdꢀCOꢀCH2), 7.40–7.70 (m, 30H, Ph of PPh3), 7.70–
7.90 (m, 5H, Ph of acyl ligand). 13C-NMR (CDCl3): l
234 (PdꢀCOꢀSt), 59.84 (PdꢀCOꢀCH2), 32.92
(PdꢀCOꢀCH2ꢀCH2), 125–141 (Ph of PPh3 and acyl
ligand). 31P-NMR (CDCl3): l 19.98 s. Anal. Calc.5 for
1
cm−1. H-NMR (CDCl3), l −5.90 (s, PdꢀH), 2.10 (s,
4 The NMR spectrum was poorly developed because of the decom-
position of the compound.
3H, CH3 of TsOH), 7.50–8.10 (m, 35H, Ph of PPh3 and
5 The difference in calculated and experimental values is due to the
presence of a small amount of the linear ester product adhering to the
complex.
3 Decantation was done because the complex decomposes to a
brownish mass on filtration.

A. Seayad et al. / Journal of Organometallic Chemistry 601 (2000) 100–107
107
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C45H39ClOP2Pd: C, 67.59; H, 4.91; Cl, 4.43; P, 7.74.
Found: C, 66.36; H, 5.07; Cl, 4.10; P, 6.23%.
4.5. 31P-NMR experiments
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Samples for 31P-NMR experiments were prepared by
mixing Pd(OAc)2 (0.022 mmol), PPh3 (0.088 mmol) and
TsOH (0.22 mmol) in 2.5 ml of degassed methanol in a
10 mm NMR tube flushed with argon. The spectra were
recorded at 121.1 MHz by using 85% H3PO4 as the
external standard and internal D2O (added in a capil-
lary tube) as the deuterium source. For preparation of
samples under CO pressure, Pd(OAc)2 (0.112 mmol),
PPh3 (0.448 mmol) and TsOH (1.12 mmol) in 15 ml of
degassed methanol were charged to the 50 ml auto-
clave, flushed with argon followed by CO, heated to
75°C, pressurised to 500 psig with CO and kept under
stirring for 30 min. Intermediate samples were directly
taken under CO atmosphere to the NMR tube with the
help of a sampling device and the spectra recorded as
stated above.
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