A study on the catalytic pathways of the hydroalkoxycarbonylation reaction of styrene

Article abstract of DOI:10.1016/S0022-328X(98)01217-0

The catalytic pathways of the Pd(PPh₃)₂Cl₂/SnCl₂-catalysed styrene hydroalkoxycarbonylation reaction have been elucidated. Using deuterium labelling, the different reaction products were detected by mass spectral analysis and the deuterium content and its distribution determined by ¹H-, ²H- and ¹³C-NMR methods. The great number of labelled species supports the assumption that the hydrido (Pd-H) route is the operating mechanism of this system. Alkyl-metal intermediates easily undergo β-hydride elimination. This process is reversible for both isomers even at low reaction temperature

Full text of DOI:10.1016/S0022-328X(98)01217-0

Journal of Organometallic Chemistry 579 (1999) 147–155
A study on the catalytic pathways of the hydroalkoxycarbonylation
reaction of styrene
a
b
c
d
d,
´
Csilla Benedek , Ga´bor Szalontai , Agnes Go¨mo¨ry , Szila´rd To˜ro¨s , Ba´lint Heil *
a Research Group for Petrochemistry of the Hungarian Academy of Sciences, H-8201 Veszpre´m, PO Box 158, Hungary
b Department of Silicate and Materials Engineering, Uni6ersity of Veszpre´m, H-8200 Veszpre´m, PO Box 158, Hungary
c Central Research Institute of Chemistry of the Hungarian Academy of Sciences, H-1525 Budapest, PO Box 17, Hungary
d Department of Organic Chemistry, Uni6ersity of Veszpre´m, H-8201 Veszpre´m, Hungary
Received 18 June 1998; received in revised form 3 December 1998
Abstract
The catalytic pathways of the Pd(PPh3)2Cl2/SnCl2-catalysed styrene hydroalkoxycarbonylation reaction have been elucidated.
Using deuterium labelling, the different reaction products were detected by mass spectral analysis and the deuterium content and
its distribution determined by 1H-, 2H- and 13C-NMR methods. The great number of labelled species supports the assumption that
the hydrido (Pd–H) route is the operating mechanism of this system. Alkyl–metal intermediates easily undergo b-hydride
elimination. This process is reversible for both isomers even at low reaction temperature © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Carbonylation; Catalysis; Mechanism; Palladium
conditions and substrates [5]. In order to differentiate
between the two mechanisms in Knifton’s [6]
1. Introduction
Pd(PPh3)2Cl2/SnCl2 catalytic system and to gain an
insight into the pathways governing the reaction, the
In the hydroalkoxycarbonylation of olefins [1] with
different Pd(II) catalysts generally two mechanisms [2]
deuterioalkoxycarbonylation of styrene has been car-
are accepted. The ‘hydrido-route’, operating in systems
ried out.
producing mainly linear esters [3], involves olefin inser-
In this paper we describe a detailed investigation of
tion into Pd–H followed by CO insertion into the
the reaction performed at various temperatures and
Pd–alkyl bond, yielding the final product upon nucle-
different conversions. Using the labelled species deter-
ophilic attack of the alkanol. Alternatively, according
2
mined by MS and NMR (1H, H, 13C) techniques, we
to the ‘carboalkoxy’ mechanism, which is often invoked
deduce the catalytic pathways leading to these
for reactions where the major product is the branched
compounds.
one [4], a Pd–acyl species is formed by the attack of the
alcohol on Pd–CO. Insertion of the olefin into the
Pd–COOR bond is followed by protonolysis of the
metal–alkyl s bond to give the corresponding ester and
regeneration of the active form. These two mechanisms
are claimed to be operative depending on the reaction
2. Results
2.1. Deuterioalkoxycarbonylation of styrene
Deuterioalkoxycarbonylation of styrene (Scheme 1)
has been carried out at different temperatures between
60 and 120°C and 40 atm of CO with Pd(PPh3)2Cl2/
* Corresponding author. Tel.: +36-88-422022; fax: +36-88-
427492.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01217-0

148
C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
conversion. The main groups of signals are those aris-
ing from deuterio-styrenes, deuterio-3-phenyl-
propanoate and deuterio-2-phenylpropanoate.
The three resonances at 5.28, 5.75 and 6.70 ppm are
due to deuterium nuclei on the vinyl moiety of styrene.
The signal at 6.70 comes from the isotopomeric species
Ph–CD– having deuterium in position 2 while the two
absorptions at 5.28 and 5.75 ppm from the two geomet-
rical isomers (E)-1-deuterio-vinylbenzene and (Z)-1-
deuterio-vinylbenzene arising from the presence of
deuterium in the position 1.
Scheme 1. Deuterioalkoxycarbonylation reaction of styrene.
SnCl2/PPh3 as the catalyst precursor in toluene (Table
1). At 100°C the reaction has been studied for various
degrees of substrate conversion. No high-boiling com-
pounds are formed at these temperatures. The ratio
between ethyl-3-phenylpropanoate (b-isomer) and
ethyl-2-phenylpropanoate (a-isomer) was determined
by GLC analysis. The b-isomer predominates over a,
the ratio of the branched product increasing slightly
with the reaction temperature (from a/b=20.3/79.7 at
60°C to a/b=26.7/73.3 at 120°C) and the conversion
degree (from a/b=22.3/77.7 at 30% conversion to a/
b=25.2/74.8 at 100% conversion). Pure samples of the
linear ester and samples enriched in the branched form
were obtained by column chromatography on silica gel.
The crude reaction mixture as well as the purified
samples of isomeric esters were analysed by mass spec-
In the case of the linear ester, two signals were
recognised: that at 2.50 ppm from the methylene group
situated a to the CO moiety and that at 2.90 ppm from
the methylene group situated
b
to the CO.
Analogously, for the branched ester two resonances
were found, one at 1.53 ppm due to the methyl group
on the tertiary carbon atom and another at 3.66 ppm
due to the deuterium from the methine position.
Other features revealed by the spectrum were: (1)
incorporation of deuterium is almost the same in the
three possible positions of unconverted styrene; (2) in
the linear ester there is a larger incorporation of deu-
terium on the methylene group adjacent to the phenyl
ring than on the methylene near the CO function; (3) in
the branched ester deuterium was predominantly incor-
porated in the methyl group at low conversions and,
only to a small extent, in the methine group; (4) neither
in unconverted styrene nor in the reaction products
isotope labelling of the phenyl ring occurs.
1
2
troscopy and by H-, H-, 13C-NMR spectroscopy in
order to determinate the extent and the position of
deuterium incorporation in the molecules. Inspection of
2
the H-NMR spectra allowed the rapid and complete
identification of all the deuterated species present in
2
solution [7]. Fig. 1 shows the H-NMR spectrum of the
reaction mixture in toluene (with residual EtOD re-
moved) formed in the reaction at 100°C with 30%
2.2. Deuterium content and distribution in uncon6erted
styrene
Deuterium content of the residual olefin was deter-
mined by GC–MS analyses from the crude reaction
mixture. As the mass spectrum of styrene at 70 eV
complicates the analysis of deuterated styrene [8], the
measurements were carried out at 10–12 eV where the
peak at m/z 103 (M−1) disappears. Data gathered
under these circumstances (Table 2) show that at lower
conversions and temperatures the non-deuterated and
monodeuterated species predominate over the di- or
trideuterated ones which were found in increasing
amounts at 100°C with 55% conversion. The average
deuterium incorporation shows an increasing tendency
with increasing temperature and conversion.
Table 1
Degree of regioselectivity in the palladium-catalysed deuterioalkoxy-
carbonylation of styrene at various reaction temperatures and sub-
strate conversionsa
Reaction
temperature (°C) (h)
Reaction time Conversion (%)b a/bc
60
60
80
100
100
100
100
120
150
250
20
40
99
99
20.4/79.6
20.3/79.7
21.8/78.2
21.7/78.3
22.3/77.7
25.0/75.0
25.2/74.8
26.7/73.3
0.5
30
1
55
4.5
99
Deuterium distribution in unconverted styrene was
35
4
100
99
2
easily determined by H-NMR of the reaction mixture
after elimination of solvent and residual ethanol-d on
a Toluene solution (10 ml) containing 3.2 mmol of styrene, 28.9
mmol of d-ethanol, 75 mg of Pd(PPh3)2Cl2, 55 mg of SnCl2 and 50.1
mg of PPh3; CO, 40 atm; volume of reaction vessel is 50 ml.
b Refers to styrene; determined by GLC with toluene as internal
standard
the base of the incorporation calculated previously for
1
the products by means of H-NMR. The composition
of the mixture was determined by GLC. The global
incorporation values determined by NMR analyses are
in agreement with those found from MS data.
c Estimated accuracy 91%.

C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
149
Fig. 1. 2H-NMR spectrum (46 MHz, 20°C) of the reaction mixture in toluene (with EtOD removed) obtained by deuterioalkoxycarbonylation of
styrene at 100°C with 30% conversion.
2.3. Deuterium content of the carbonylated products
shows three singlets at l=30.94, 30.87, and 30.80 ppm
corresponding to –CH2– adjacent to a –CH2–,
–CHD– and a –CD2– group, respectively, and three
2.3.1. Mass spectral analysis
triplets at l=30.61 (1JCD=19.9 Hz), 30.54 (1JCD
=
The mass spectra of the products were determined
directly from the crude reaction mixture at low ionisa-
tion energy. The results obtained (Table 3) served only
for the qualitative analysis of low amounts of poly-
deuterated species which cannot be observed by NMR.
Thus the presence of 0, 1, 2, 3 or 4 deuterium atoms
can be noticed. While at lower temperatures and con-
20.3 Hz) and 30.47 ppm (1JCD=19.4 Hz) correspond-
ing to –CHD– groups adjacent to a –CH2–, a
–CHD– and a –CD2– group. As far as the methyl
group of the branched isomer (Fig. 3(a)) is concerned,
there is an in-phase singlet at l=18.58 ppm corre-
sponding to a CH3– adjacent to a –CH–, two in-phase
triplets at l=18.30 (1JCD=19.4 Hz) and 18.19 ppm
(1JCD=19.7 Hz) corresponding to CH2D groups adja-
cent to a –CH– and a –CD–, respectively and two
quintets at l=18.05 (1JCD=19.7 Hz) and 17.91 ppm
(1JCD=19.7 Hz) corresponding to CHD2– adjacent to
a –CH– and a –CD– group. The same spectrum
shows four singlets for the methine group (Fig. 3(b)) at
l=45.49, 45.43, 45.35 and 45.29 ppm, which can be
assigned to a –CH– adjacent to a CH3–, a CH2D–, a
CHD2– and a CD3–.
2
versions species containing less than two H atoms per
molecule predominate, in more severe conditions the
poly-deuterated ones are favoured.
2.3.2. Nuclear magnetic resonance analysis
The chromatographically purified reaction products
1
2
were analysed by H-, H- and13C-NMR. It was shown
that 13C-NMR spectra of the products permit the iden-
tification of some of the species present after the iso-
tope labelling in accordance with the original 13C
2
Deuterium content in different positions was calcu-
assignments and the H isotope shifts [9]. Thus, in the
1
lated from H spectra by integration of the signals of
case of the linear ester (Fig. 2), in the DEPT spectrum
[10] the methylene group a to the CO moiety (Fig. 2(a))
shows three anti-phase singlets at l=35.88, 35.81 and
35.74 ppm corresponding to –CH2– adjacent to a
–CH2–, –CHD– and a –CD2– group, respectively,
the groups where deuterium labelling occurs using the
resonance of the methylene group from the ester func-
tion as an internal standard. The relative values of
incorporations can be followed on the 2H-NMR spectra
as well. The results found (Table 4) point out that
deuterium is present to a larger extent in the linear
isomer than in the branched one but its amount
changes more rapidly in the case of the latter when
reaction conditions become more severe. As far as the
and three in-phase triplets at l=35.58 ppm (1JCD
=
19.9 Hz), 35.51 (1JCD=19.6 Hz) and 35.44 ppm
(1JCD=19.7 Hz) corresponding to –CHD– groups ad-
jacent to a –CH2–, a –CHD– and a –CD2– group.
Similarly, the methylene group b to the CO (Fig. 2(b))

150
C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
linear ester is concerned, at 99% conversion the amount
of deuterium atoms on both carbon atoms increases
with increasing reaction temperature. Similarly, at con-
stant temperature (experiments at 100°C) D incorpora-
tion becomes greater as the reaction progresses, these
effects being more pronounced at Ca atom. In the case
of the branched ester, at 99% conversion the deuterium
incorporated on the atoms Ca and Cb shows a rapid
increase, especially on the methyl group (from 0.60 at
60°C to 1.84 at 120°C), analogously, at 100°C further
progress of the reaction produces a significant growth
in the amount of D incorporated.
terioalkoxycarbonylation of styrene is carried out at
various temperatures and conversions.
The deuterated species observed in solution are par-
tially deuterated ethanol-d, mono-, di- and trideu-
teriostyrene labelled at the vinyl group, and mono-, di-,
tri- and tetradeuterated isomeric esters (a and b) being
labelled at the side-chain (EtOX, PhCXꢀCX2,
PhCX(CX3)COOEt and PhCX2CX2COOEt, X=H,
D). The formation of these species can be easily accom-
modated in terms of the reaction pathways described.
Thus, according to Scheme 2, the formation of the two
palladium–alkyl intermediates n% and b% is reversible
1
1
It should be noted that independent control experi-
ments, carried out under reaction conditions, in the
presence of the catalyst, proved that H/D exchange
between ethanol-d and both isomeric products is
negligible.
both at lower and higher temperatures; they undergo
b-hydride elimination to give the palladium–hydride
p-complexes 1 and 2. In complex 1, the insertion of the
olefin into the Pd–H bond could regenerate the starting
linear palladium–alkyl n1%, leading to the corresponding
linear 3-deuterated ester (3-d1-b), or produce the iso-
2.4. Deuterium content of residual ethanol-d
meric branched palladium–alkyl intermediate b%, from
2
which the 2-deuterated branched ester (2-d1-a) can be
The deuterium content of the residual ethanol-d
(Table 5) was determined via H-NMR analysis of the
formed. By the same process, complex 2 gives rise to
2
the linear palladium–alkyl intermediate n% and so to
2
crude reaction mixture using CDCl3 as an external
standard.
the linear 2-deuterated ester 2-d1-b or regenerates the
branched intermediate b%, producing the branched 3-
1
It must be noted that the mass spectrum of toluene
recovered from the reaction shows the normal isotopic
cluster expected for unlabelled toluene, however at
severe reaction conditions slight deuterium incorpora-
tion can be observed on the methyl group of the solvent
by NMR.
deuterated ester 3-d1-a. The overall process is, there-
fore, the isomerisation of the starting branched
metal–alkyl intermediate b1% to the linear isomeric inter-
mediate n2% and, conversely, the isomerisation of n1% to
b2%. On the other hand the p-complexes 1 and 2 can
undergo an intermolecular exchange process with unla-
belled styrene, producing respectively monodeu-
teriostyrenes 2-d1-sty and 1-d1-sty and unlabelled
p-complex 3. The latter can generate the linear (n) or
branched (b) unlabelled alkyl and hence the linear and
branched esters (b and a).
3. Discussion
The results of this investigation clearly show that
variable deuterium incorporation occurs in the sub-
strate and in the reaction products when deu-
Once labelled, styrene species may undergo further
reaction, resulting in di- and oligodeuterated derivatives
Table 2
Deuterium content via MS and NMR analyses of styrene recovered from the Pd-catalysed deuterioalkoxycarbonylation of styrene, after partial
conversions
Reaction temperature (°C)
Conversion (%)
Distribution of deuterium content Deuterium incorporation at each
(%)a
positionb
0D
1D
2D
3D
NDMc
D(HA) D(HB) D(HC) NDMc
60
100
100
40
30
55
43.0
40.8
25.2
46.3
46.2
38.8
10.5
12.2
27.5
0.2
0.8
3.7
0.68
0.73
1.05
0.19
0.24
0.25
0.22
0.24
0.34
0.32
0.30
0.43
0.73
0.78
1.02
a Via MS analysis.
b Via 1H and 2H-NMR.
c Average number of deuterium atoms per molecule.

C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
151
Table 3
Deuterium content of the linear and branched esters determined via MS analysis
Temperature (°C)
Conversion (%)
Deuterium incorporation of the b ester (%)
Deuterium incorporation of the a ester (%)
0D
1D
2D
3D
4D
0D
1D
2D
3D
4D
60
80
99
99
30
55
99
12.2
9.7
9.2
8.3
9.0
3.8
3.2
40.0
32.0
56.7
35.9
28.6
20.7
18.0
30.7
30.0
28.8
32.2
29.8
29.2
26.4
13.7
21.0
4.3
17.6
23.3
29.7
33.7
3.5
7.4
1.0
5.5
9.3
12.9
9.5
14.7
8.1
8.5
3.9
41.7
30.7
52.9
35
27.9
21.9
17.3
30.0
29.2
25.5
31.6
29.3
28.5
29.5
12.4
22.4
5.9
18.6
24.4
28.6
31.5
3.0
8.2
0.9
100
100
100
100
120
6.4
10.1
17.0
19.6
100
99
16.6
19.7
3.1
(Scheme 3). Thus, when 1-d1-sty gives the p-complex 4
formed in a less selective way. Independent control
experiments did show that H/D exchange between
EtOD and the ester products is negligible. The presence
of the deuterium in unconverted olefin confirms the
reversibility of the reaction [3] and suggests that dissoci-
ation of the alkyl–palladium intermediate takes place.
with a Pd–D species, dideuterated palladium–alkyls n%%
2
and b%% can be formed, producing dideuterated isomeric
1
esters (2,3-d2-b and 3,3-d2-a). A b-hydride elimination
process from b%%1 palladium–alkyl can create the p-com-
plex 5 from which 2,2-dideuterio-3-phenylpropanoate
(2,2-d2-b) is formed. In a similar manner, p-complex 6
leads to the dideuterated branched ester 2,3-d2-a. After
an intermolecular exchange process with styrene, com-
plexes 5 and 6 give rise to labelled dideuteriostyrenes
1,1-d2-sty and 1,2-d2-sty, respectively.
With 1,1-d2-sty as the starting molecule (Scheme 4)
metal–alkyls n%%% and b%%% are generated, from which
1
1
formation of 2,2,3-d3-b and 3,3,3-d3-a is possible. The
p-complex 8 can be formed from n%%% leading to the
1
isomeric ester 2,3,3-d3-a and to trideuterated styrene
1,1,2-d3-sty by elimination and exchange processes
which can not occur in the case of palladium–alkyl b%%%.
1
As far as 1,2-d2-sty is concerned, the two metal–alkyls
n%%% and b%%% are formed, the first giving rise directly to
2
2
the trideuterated 2,3,3-d3-b ester, no b-elimination be-
ing possible, while the latter leads to the 2,3,3-d3-a ester
and complex 8 from which the 2,2,3-d3-b linear ester
and 1,1,2-d3-sty can be derived.
Finally, at higher temperatures, trideuterated styrene
leads to the formation of the tetradeuterated esters
2,2,3,3-d4-a and 2,3,3,3-d4-b.
4. Conclusions
Significant deuteration of the unconverted styrene
and the ester isomers formed in hydroalkoxycarbonyla-
tion of styrene with EtOD using Pd(PPh3)2Cl2/SnCl2/
PPh3 as a catalyst was detected at low reaction
temperature (60°C). The composition of the complex
reaction mixture was successfully elucidated by mass
spectral analysis and NMR methods. The application
of 2H-NMR enabled the determination of the deu-
terium content of styrene directly from the reaction
mixture (after the removal of EtOD). Working under
severe reaction conditions, oligodeuterated species were
Fig. 2. 13C-NMR DEPT spectrum (75.4 MHz, CDCl3, 20°C) of the
methylene groups situated (a) a; (b) b to the CO group of the linear
ester obtained from the deuterioalkoxycarbonylation of styrene at
100°C with 99% conversion.

152
C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
Table 5
Deuterium content of the OD group of the residual ethanol-d
Reaction
temperature (°C)
Conversion (%) Deuterium content of OD
group (%)a
60
60
80
100
100
120
40
99
99
55
99
99
90.1
88.3
85.9
87.1
82.4
77.9
a Data calculated from 2H-NMR spectra using CDCl3 as external
standard.
labelled derivatives and strongly support the ‘hydrido’
route operating in the catalytic system employed. Re-
versible olefin insertion/b-hydride elimination prior to
carbonylation is responsible for the incorporation of
multiple deuterium atoms [5].
5. Experimental
Toluene was distilled from sodium under argon at-
mosphere. Ethanol-d was purchased from Fluka.
Pd(PPh3)2Cl2 [12] was prepared as reported in the liter-
ature. Anhydrous SnCl2 was obtained by reacting
SnCl2.2H2O with acetic anhydride and washing with
ether. GLC analyses were performed on a HP 5830 gas
chromatograph equipped with SPB-1 column (30 m×1
mm film depth) using flame ionisation as the detection
method and helium as the carrier gas. Column chro-
matography was carried out on a silica gel column
using hexane, benzene, chloroform, dichloromethane
and acetone as eluents.
Fig. 3. 13C-NMR DEPT spectrum (75.4 MHz, CDCl3, 20°C) of (a)
the methyl group and (b) the methine group of the branched ester
obtained from the deuterioalkoxycarbonylation of styrene at 100°C
with 99% conversion.
The kinetic deuterium isotope effect which controls
b-elimination [11] could account for the observed accu-
mulation of deuterated species. The reaction pathways
described in our paper explain the formation of the
Table 4
Deuterium content of the linear and branched esters via NMR analysis
Reaction temperature (°C)
Conversion (%)
D content at the C atoms of the linear
ester
D content at the C atoms of the
branched ester
D(Ca)
D(Cb)
NDMa
D(Ca)
D(Cb)
NDM
60
80
100
100
100
120
99
99
30
55
99
99
0.53
0.63
0.28
0.67
0.90
1.22
1.09
1.29
0.93
1.21
1.22
1.32
1.62
1.92
1.21
1.88
2.12
2.54
0.12
0.33
0.01
0.26
0.43
0.43
0.60
1.61
0.62
0.95
1.61
1.84
0.72
1.94
0.63
1.21
2.04
2.27
a Average number of deuterium atoms per molecule.

C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
153
Scheme 2. Pathways for monodeuteration of styrene and the derived monodeuterated isomeric esters.
Scheme 3. Pathways for dideuteration of styrene and the derived dideuterated isomeric esters.

154
C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
Scheme 4. Pathways for oligodeuteration of styrene and the derived oligodeuterated isomeric esters.
The mass spectra were measured using a VG ZAB-
5.1. Deuterioalkoxycarbonylation of styrene: general
procedure
SEQ instrument at 10–12 eV ionisation connected to a
HP-5890A gas chromatograph with a CP-Sil-8 CB
column (25 m×0.25 mm film depth) using helium as
the carrier gas.
The catalyst (75 mg, 0.11 mmol Pd(PPh3)2Cl2; 55 mg,
0.29 mmol anhydrous SnCl2; 50,1 mg, 0.19 mmol PPh3)
was introduced into a 50-ml stainless steel autoclave.
After sealing, a solution of styrene (0.37 ml, 3.2 mmol),
ethanol-d (1,7 ml, 28.9 mmol) in toluene (10 ml) was
introduced by suction. Then, carbon monoxide was
charged (40 atm pressure at room temperature) and the
1
2
The H-, H-, 13C-NMR spectra were recorded on a
Varian Unity 300 spectrometer, operating at 300, 46
and 75.4 MHz, respectively. The chemical shifts of the
2H-NMR measurements were determined by reference
to CDCl3 as an external standard.
.

C. Benedek et al. / Journal of Organometallic Chemistry 579 (1999) 147–155
155
mixture was magnetically stirred at the reaction temper-
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2
The residual mixture for partial conversions was
1
analysed by H-NMR also. Isomeric esters were sepa-
rated by column chromatography and studied further.
Acknowledgements
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The authors thank Zs. Cso´k (University of
Veszpre´m) for the NMR measurements and L. Kolla´r
for helpful discussions. This work was supported by the
Hungarian National Science Foundation (OTKA T-
016329 and T-016260).
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