Methoxycarbonylation of styrene to methyl arylpropanoates catalyzed by palladium(II) precursors with 1,1′-bis(diphenylphosphino)metallocenes

Article abstract of DOI:10.1016/j.molcata.2004.06.029

Palladium(II) complexes with 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1′-bis(diphenylphosphino)octamethylferrocene (dppomf), 1,1′-bis(diphenylphosphino)ruthenocene (dppr) and 1,1′- bis(diphenylphosphino)osmocene (dppo) have been synthesized and used to catalyze the methoxycarbonylation of styrene. Irrespective of the precursor, all the reactions gave methyl phenylpropanoates with prevalence of the linear isomer methyl 3-phenylpropanoate (up to 85% regioselectivity). The highest turnover frequency was obtained with the dppr precursor in the presence of p-toluenesulphonic acid co-catalyst (334 mol of styrene converted (mol of catalyst h)^-1). A reaction mechanism accounting for both activity and selectivity has been proposed on the basis of operando high-pressure NMR experiments and reactions with model compounds. The selective methoxycarbonylation of styrene to methyl phenylpropanoates, with large prevalence of the linear regioisomer, can be achieved by 1,1′- bis(diphenylphosphino)metallocenyl palladium(II) catalysts. The presence of methyl substituents on the cyclopentadienyl ligands, and the type of metal in the metallocenyl unit, which may vary from iron to ruthenium to osmium, control the catalytic activity.

Full text of DOI:10.1016/j.molcata.2004.06.029

Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
Methoxycarbonylation of styrene to methyl arylpropanoates
catalyzed by palladium(II) precursors with
1,1ꢀ-bis(diphenylphosphino)metallocenesଝ
Claudio Bianchinia,∗, Andrea Melia, Werner Oberhausera, Sebastien Parisela,
Oleg V. Gusevb, Alexander M. Kal’sinb, Nikolai V. Vologdinb, Fedor M. Dolgushinb
a
Istituto di Chimica dei Composti OrganoMetallici (ICCOM-CNR), Area di Ricerca CNR di Firenze, Via Madonna del Piano,
50019 Sesto Fiorentino (Firenze), Italy
N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences, Vavilov St. 28, 119991 Moscow, Russian
b
Received 4 March 2004; received in revised form 15 June 2004; accepted 18 June 2004
Abstract
Palladium(II) complexes with 1,1ꢀ-bis(diphenylphosphino)ferrocene (dppf), 1,1ꢀ-bis(diphenylphosphino)octamethylferrocene (dppomf),
1,1ꢀ-bis(diphenylphosphino)ruthenocene (dppr) and 1,1ꢀ-bis(diphenylphosphino)osmocene (dppo) have been synthesized and used to catalyze
the methoxycarbonylation of styrene. Irrespective of the precursor, all the reactions gave methyl phenylpropanoates with prevalence of the
linear isomer methyl 3-phenylpropanoate (up to 85% regioselectivity). The highest turnover frequency was obtained with the dppr precursor
in the presence of p-toluenesulphonic acid co-catalyst (334 mol of styrene converted (mol of catalyst h)−1). A reaction mechanism accounting
for both activity and selectivity has been proposed on the basis of operando high-pressure NMR experiments and reactions with model
compounds.
© 2004 Elsevier B.V. All rights reserved.
Keywords: 1,1ꢀ-Bis(diphenylphosphino)metallocenes; Alkoxycarbonylation; Palladium; Alkyl arylpropanoates; Regioselectivity
1. Introduction
Just to consider the ligands with diphenylphosphino
groups, at least four different structural variations (Scheme 1)
have been reported as being able to generate catalytically ac-
tive palladium(II) complexes [1–10].
1,1ꢀ-Bis(phosphino)metallocenes constitute a family of
potentially chelating tri- and bidentate ligands with many
applications in homogeneous catalysis in conjunction with
transition metals [1]. The success of these ligands in catal-
ysis is largely due to their unique flexibility that allows one
to build up molecular structures with different sandwiched
metals and substituents either on the cyclopentadienyl lig-
ands (Cp) or on the phosphorus atoms.
Most of the research work in catalysis has involved the
prototypical ligand 1,1ꢀ-bis(diphenylphosphino)ferrocene
(dppf). Among the applications of PdII-dppf complexes in
catalysis, carbon–carbon and carbon–nitrogen bond forming
reactions are of particular importance. Representative exam-
ples include cross-coupling reactions [3], Heck reaction [4],
carbonylation of chloroarenes [5], aryl halide amination [6],
hydroamination of alkynes [7a], glyoxalate-ene reaction with
chiral controllers [8], alternating copolymerization of CO and
ethylene [9] and methoxycarbonylation of ethylene [9a,b].
Palladium(II) complexes with 1,1ꢀ-bis(diphenylphos-
phino)octamethylferrocene (dppomf) and 1,1ꢀ-bis(diphenyl-
phosphino)osmocene (dppo) have been employed in the
ଝ
CCDC-229562 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge, UK;
fax∗: +44 1223 336 033; or deposit@ccdc.cam.ac.uk).
Corresponding author. Tel.: +39 0555225280; fax: +39 0555225203.
E-mail address: claudio.bianchini@iccom.cnr.it (C. Bianchini).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.06.029

36
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
(1)
[9b],
[Pd(H2O)2(dppomf)](OTs)2
(2)
[9b],
[Pd(OTs)(dppo)]OTs (4) [10], [Pd(H2O)(OTs)(dppe)]OTs
(5) [12] and [Pd(H2O)(OTs)(dppp)]OTs (6) [13] were
synthesized according to published procedures. CH2Cl2
and diethyl ether were distilled from CaH2 and LiAlH4,
respectively. All the other reagents and solvents were used
as purchased from commercial suppliers. All the isolated
solid samples were collected on sintered-glass frits and
washed with appropriate solvents before being dried under
a stream of nitrogen. Microanalyses were performed in
the Laboratory of Microanalysis of the N. Nesmeyanov
Institute of Organoelement Compounds. Infrared spectra
were recorded on a Specord M-82 spectrophotometer.
Conductivities were measured with an Orion model 990101
conductance cell connected to a model 101 conductivity
meter. The conductivity data [14] were obtained at sample
concentrations of ca. 10−3 M in nitroethane (ε = 19.7)
solutions. Deuterated solvents for routine NMR measure-
Scheme 1.
1
ments were dried over molecular sieves. H, 13C{1H}, and
31P{1H} NMR spectra were obtained on a Bruker ACP 200
spectrometer (200.13, 50.32, and 81.01 MHz, respectively).
Chemical shifts are reported in ppm (δ) with reference
to either TMS as an internal standard (1H and 13C{1H}
NMR spectra) or 85% H3PO4 as an external standard
(31P{1H} NMR spectra). The 10 mm sapphire NMR tube
was purchased from Saphikon, Milford, NH, while the
titanium high-pressure charging head was constructed at the
ICCOM-CNR (Firenze, Italy) [15]. Caution: Since high gas
pressures are involved, safety precautions must be taken at
all stages of studies involving high-pressure NMR tubes.
Carbonylation reactions were performed with a 120 ml
stainless steel autoclave, constructed at the ICCOM-CNR
(Firenze, Italy), equipped with a magnetic drive stirrer, a
Parr 4842 temperature and pressure controller. GC analyses
were performed on a Shimadzu GC-14 A gas chromatograph
equipped with a flame ionization detector and a 30 m
(0.25 mm i.d., 0.25 ␮m film thickness) SPB-1 Supelco fused
silica capillary column. The GC/MS analyses were per-
formed on a Shimadzu QP 5000 apparatus equipped with a
column identical with that used for GC analysis. The product
composition of the catalytic reactions was determined by
GC/MS, while the quantification of the products were carried
out by GC by using 1,2-dimethoxyethane as the internal
standard.
Scheme 2.
methoxycarbonylation of ethylene where both precursors
proved efficient and selective to give methyl propanoate
[9a,b,10].
In this paper, we report a study of the catalytic perfor-
mance of some known and new palladium(II) complexes
with dppf, dppomf, dppr and dppo in the methoxycarbonyla-
tion of styrene. Irrespective of the precursor, all the reactions
gave methyl phenylpropanoates with prevalence of the linear
isomer methyl 3-phenylpropanoate (up to 85% regiose-
lectivity) and quite high turnover frequencies (Scheme 2).
However, a significant dependence of the catalytic activity on
the 1,1ꢀ-bis(diphenylphosphino)metallocene was observed.
Operando high-pressure NMR experiments (HPNMR),
catalytic experiments with classical chelating diphosphines
such as 1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-
bis(diphenylphosphino)propane (dppp) and model reactions
with isolated intermediates have contributed to elucidate this
metallocene-effect as well as gain insight into the overall
carbonylation mechanism of styrene.
Preliminary data on the methoxycarbonylation of styrene
by dppo-PdII catalysis have been previously communicated
[10].
2.2. Catalytic carbonylation of styrene in MeOH
A solution of styrene (4 mmol) in MeOH (20 ml) was
introduced by suction into a 120 ml autoclave, previously
evacuated by a vacuum pump, containing 0.01 mmol of
catalyst precursor along with the desired amounts of
TsOH (p-toluenesulphonic acid monohydrate) and/or BQ
(1,4-benzoquinone). The autoclave was charged with CO to
the desired pressure at room temperature and heated to the
chosen temperature. As soon as the autoclave reached the
desired temperature, the mechanical stirring of the autoclave
2. Experimental
2.1. General procedures
All reactions and manipulations were carried out under an
atmosphere of nitrogen using Schlenk-type techniques. The
palladium complexes PdCl(Me)(COD) (COD = cycloocta-
1,5-diene) [11], PdCl2(dppr) [2b], [Pd(H2O)2(dppf)](OTs)2

C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
37
was switched on (700 rpm). After the desired time, the
2.5. HPNMR study of the carbonylation of styrene
reaction was stopped by cooling the autoclave to room
temperature by means of an ice-water bath. As the internal
temperature of the autoclave reached 0 ◦C, the unreacted CO
was released and 100 ␮l of the GC internal standard was
added.
catalyzed by 1–4 in TsOH containing MeOH
The reaction was followed by variable-temperature
31P{1H} and 1H NMR spectroscopy. A 10 mm sapphire HP-
NMR tube was charged sequentially with solutions of TsOH
(19 mg, 0.10 mmol) in MeOH-d4 (1 ml) and 3 (10.7 mg,
0.01 mmol) (or 1, 2, 4) in MeOH-d4 (1 ml) under nitrogen
at room temperature. The tube was placed into a NMR probe
at room temperature (31P{1H} NMR singlet at δ: 44.4). Addi-
tion of a 100-fold excess of styrene (114 ␮l, 1 mmol) caused
no change in the 31P{1H} NMR spectrum. In contrast, a dra-
matic change occurred when the tube was pressurized with
14 bar of CO at room temperature. The results of this study
are reported in Section 3.3.
Selected NMR data for [Pd2(␮-H)(␮-CO)(dppo)2]OTs
(14). 31P{1H} NMR (20 ◦C), δ: 24.4 (s). 31P NMR (20 ◦C),
δ: 24.4 (d, J(PH) = 42 Hz). 1H NMR (20 ◦C), δ: −7.14 (quin-
tet, J(HP) = 42.3 Hz, ␮-H), 4.71 (br s, H␤-Cp), 5.06 (br s,
Ha-Cp).
2.3. Synthesis of [Pd(H2O)(OTs)dppr]OTs (3)
To a solution of PdCl2(dppr) (0.51 g, 0.66 mmol) in
CH2Cl2 (50 ml) was added silver tosylate (0.40 g, 1.43 mmol)
at room temperature. After stirring overnight, the mixture
was filtered through Celite and the filtrate was concen-
trated to 4 ml. Addition of diethyl ether (30 ml) led to the
precipitation of an orange crystalline product, which was
washed with diethyl ether and dried in a stream of nitrogen.
Yield 0.60 g (84%). Anal. Calcd. for C48H44O7P2PdRuS2:
C, 54.06; H, 4.16. Found: C, 53.66; H, 4.48. ΛM (ni-
troethane) = 25 ꢀ−1 cm2 mol−1; this value is consistent with
a 1:1 electrolyte. 1H NMR (CD2Cl2), δ: 2.32 (s, 6H,
Me-OTs), 4.99 (s, 4H, H␤-Cp), 5.06 (s, 4H, H␣-Cp),
6.09 (s, 2H, H2O), 6.89 (d, 4H, J(HH) = 8.0 Hz, Ar-
OTs), 7.31 (d, 4H, J(HH) = 8.0 Hz, Ar-OTs), 7.46 (t, 8H,
J(HH) = 7.4 Hz, m-PPh2), 7.61 (t, 4H, J(HH) = 7.4 Hz, p-
PPh2), 7.99 (dd, 8H, J(HP) = 12.8 Hz, J(HH) = 7.4 Hz, o-
PPh2). 13C{1H} NMR (CD2Cl2), δ: 21.7 (s, Me-OTs),
2.6. Synthesis of [Pd(dppr)]2(OTs)2 (8)
A solution of 3 (120 mg, 0.11 mmol) in MeOH (5 ml) was
allowed to stand under stirring for 10 h at room temperature.
During this time there was a color change from yellow to dark
red. The solution was concentrated to dryness and the residue
was dissolved in 4 ml of CH2Cl2. The formed Pd black was
separated by filtration through Celite and the resulting clear
filtrate was concentrated to half of its volume. Addition of
4 ml of a 1:1 mixture of diethyl ether and n-hexane led to the
formation of a dark red powder, which was filtered off and
washed with n-hexane. Yield: 54 mg (69%). Anal. Calcd. for
C82H70O6P4S2Ru2Pd2: C, 56.14; H, 4.02. Found: C, 56.30;
H, 4.10. 1H NMR (CD2Cl2), δ: 2.30 (s, 6H, Me-OTs), 4.27
(br s, 8H, H␣-Cp), 5.73 (br s, 8H, H␤-Cp), 6.7–7.8 (m, 48H,
Ph + Ar-SO3). 13C{1H} NMR (CD2Cl2), δ: 21.6 (s, Me-
OTs), 63.1 (m, Cipso-Cp), 75.6 (br s, CH␣-Cp), 82.7 (br s,
CH␤-Cp), 126.6 (s, CH-OTs), 123.4 (m, ipso-PPh2), 129.0
(s, CH-OTs), 129.6 (t, J(CP) = 6.1 Hz, m-PPh2), 132.7 (s, p-
PPh2), 134.2 (t, J(CP) = 6.3 Hz, o-PPh2), 140.2 (s, Cipso-
OTs), 142.8 (s, Cipso-OTs). 31P{1H} NMR (CD2Cl2), δ:
6.3 (s).
1
76.1 (m, | J(CP) + 2J(CP)| = 70.4 Hz, Cipso-Cp), 78.3 (t,
J(CP) = 4.0 Hz, CH␤-Cp), 80.5 (t, J(CP) = 6.2 Hz, CH␣-Cp),
1
126.1 (s, CH-OTs), 128.4 (m, | J(CP) + 2J(CP)| = 59.1 Hz,
ipso-PPh2), 129.1 (s, CH-OTs), 129.6 (t, J(CP) = 6.1 Hz, m-
PPh2), 133.1 (s, p-PPh2), 134.9 (t, J(CP) = 6.2 Hz, o-PPh2),
140.8 (s, Cipso-OTs), 142.2 (s, Cipso-OTs). 31P{1H} NMR
(CD2Cl2), δ: 42.5 (s).
Single crystals of 3 suitable for an X-ray analysis were
obtained by slow diffusion of diethyl ether into a CH2Cl2
solution of 3 at room temperature.
2.4. Stability study of the catalyst precursors 1–4 in
MeOH by NMR spectroscopy
Typically, a sample of the appropriate compound
(0.012 mmol) was dissolved in MeOH-d4 (1 ml) at room tem-
perature. The solution was transferred into a 5 mm NMR tube
thatwasthenplacedintotheNMRprobeatroomtemperature.
1
31P{1H} and H NMR spectra were acquired every 30 min
2.7. Reaction of 4 with water: synthesis of
[Pd(H2O)2(dppo)](OTs)2 (10)
for 6 h. The results of this study are reported in Section 3.2.
Selected 1H NMR data for the complexes seen by NMR
during these experiments. Compound 2, δ: 1.32 (s, 12H,
Me-Cp*), 1.49 (s, 12H, Me-Cp*); compound 3, δ: 4.85 (br
s, H␤-Cp), 5.09 (br s, H␣-Cp); compound 4, δ: 4.76 (br
s, ␣H-Cp), 5.84 (br s, H␤-Cp); [Pd(dppomf)]2(OTs)2 (7),
δ: 0.51 (br s, 24H, Me-Cp*), 1.86 (br s, 24H, Me-Cp*);
[Pd(dppr)]2(OTs)2 (8), δ: 4.30 (br s, H␣-Cp), 5.55 (br s, H␤-
Cp); [Pd(dppo)]2(OTs)2 (9), δ: 4.49 (br s, H␣-Cp), 5.58 (br
s, H␤-Cp); [Pd(H2O)2(dppo)](OTs)2 (10), δ: 4.98 (br s, H␤-
Cp), 5.38 (br s, H␣-Cp).
Water (500 ␮l) was pipetted into a stirred CH2Cl2 (5 ml)
solution of 4 (114 mg, 0.10 mmol). After 1 h, the solution was
concentratedundervacuumtogive10asasolidresidue. Anal.
Calcd. for C48H46O8OsP2PdS2: C, 49.12; H, 3.95. Found:
1
C, 49.01; H, 3.92. H NMR (CD2Cl2), δ: 2.31 (s, 6H, Me-
OTs), 4.32 (s, 4H, H2O), 5.18 (br s, 4H, H␤-Cp), 5.29 (br
s, 4H, H␣-Cp), 6.9–8.0 (m, 28H, Ph + Ar-SO3). ΛM (ni-
troethane) = 30 ꢀ−1 cm2 mol−1; this value is consistent with
a 1:1 electrolyte.

38
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
2.8. Synthesis of PdCl(Me)(dppr) (15)
2.12. Reaction of 17 with MeOH: synthesis of
[Pd2(␮-H)(␮-CO)(dppr)2]B(Arꢀ)4 (13)
A solid sample of PdCl(Me)(COD) (66 mg, 0.25 mmol)
was added to a stirred solution of dppr (150 mg, 0.25 mmol)
in CH2Cl2 (20 ml) at room temperature. After 20 min, the
solution was concentrated to half of its original volume. Ad-
dition of a 1:1 mixture of diethyl ether and n-pentane (20 ml)
led to the precipitation of 15 as a white microcrystalline
solid, which was filtered off and washed with n-pentane.
Yield 140 mg (74%). Anal. Calcd. for C35H31ClP2PdRu:
C, 55.57; H, 4.13. Found: C, 55.30; H, 3.99. 1H NMR
(CD2Cl2), δ: 0.61 (dd, 3H, J(HP) = 7.7 Hz, J(HP) = 5.0 Hz,
Pd-Me), 4.01 (s, 2H, H␤-Cp), 4.61 (s, 2H, H␤-Cp), 4.85
(s, 2H, H␣-Cp), 4.93 (s, 2H, H␣-Cp), 7.2–8.0 (m, 20H,
Ar). 31P{1H} NMR (CD2Cl2), δ: 14.0 (d, 2J(PP) = 28.9 Hz),
37.8 (d).
MeOH (200 ␮l) was syringed into a 10 mm NMR tube
containing a solution of 17 (20 mg, 0.013 mmol) in CD2Cl2
(1.5 ml) under a stream of CO at room temperature. Af-
1
ter 30 min, H and 31P{1H} NMR spectra showed the al-
most quantitative formation of the binuclear complex 13 and
1
1
methyl acetate. H NMR (CD2Cl2), δ: −7.01 (quintet, H,
J(HP) = 41.6 Hz, ␮-H)), 4.43(s, 8H, H␤-Cp), 4.72(s, 8H, H␣-
Cp), 7.1–7.8 (m, 52H, Ph + Ar). 31P{1H} NMR (CD2Cl2),
δ: 19.7 (s). IR (CH2Cl2, cm−1): 1851 (CO).
2.13. X-ray data collection and structure determination
of 3
A suitable single crystal of 3 was analyzed with a Bruker
Smart 1000 CCD diffractometer. The diffraction data were
collected at 120 K. Atomic scattering with anomalous disper-
sion corrections were taken from Ref. [16]. The structure was
solved applying direct methods and refined by full-matrix
least squares against F2 using the SHELXTL-97 package.
The data were corrected for Lorentz and polarization effects.
All of the non-hydrogen atoms were found through a se-
quence of F0 Fourier maps and refined initially with isotropic
thermal parameters and finally with anisotropic thermal pa-
rameters. All of the hydrogen atoms were introduced at cal-
culated positions and refined by applying the riding model.
2.9. Synthesis of [PdMe(dppr)]B(Arꢀ)4 (16)
To a stirred solution of the chloride derivative 15 (150 mg,
0.20 mmol) in CH2Cl2 (15 ml) was added NaB(Arꢀ)4
(Arꢀ = 3,5-(CF3)2C6H3, 177 mg, 0.20 mmol) at room temper-
ature. After 15 min, the precipitated NaCl was filtered off
and the resulting solution was concentrated to dryness. Ad-
dition of a 1:10 mixture of diethyl ether/n-pentane (15 ml)
precipitated pale-yellow crystals of 16, which were collected
by filtration and washed with n-pentane. Yield 210 g (66%).
Anal. Calcd. for C67H43BF24P2PdRu: C, 50.79; H, 2.74.
Found: C, 50.00; H, 2.66. 1H NMR (CD2Cl2), δ: 2.02 (t, 3H,
J(HP) = 4.7 Hz, Pd-Me), 3.96 (s, 4H, H␣-Cp), 5.31 (s, 4H,
H␤-Cp), 7.4–7.9 (m, 32H, Ar). 31P{1H} NMR (CD2Cl2), δ:
−12.4 (s).
3. Results and discussion
3.1. Synthesis of the catalyst precursors
2.10. Synthesis of [Pd(COMe)(dppr)]B(Arꢀ)4 (17)
The palladium(II) complexes [Pd(H2O)2(dppf)](OTs)2
(1)
[9b],
[Pd(H2O)2(dppomf)](OTs)2
(2)
[9b],
Carbon monoxide was bubbled through a solution of 16
(100 mg, 0.06 mmol) in CH2Cl2 (10 ml) for 5 min at room
temperature. During this time the color of the solution turned
from yellow to red. The solution was then cooled to −20 ◦C
and pumped to dryness to give 17 as a red powder. Yield
90 mg (89%). Anal. Calcd. for C68H43BF24OP2PdRu: C,
50.29; H, 2.71. Found: C, 50.10; H, 2.56. IR (nujol mull,
cm−1): 1707 (CO). 1H NMR (CD2Cl2), δ: 2.75 (t, 3H,
J(HP) = 1.7 Hz, COMe), 4.24 (s, 4H, H␣-Cp), 5.18 (s, 4H,
H␤-Cp), 7.4–7.8 (m, 32H, Ar). 31P{1H} NMR (CD2Cl2), δ:
−13.8 (s).
[Pd(H2O)(OTs)dppr)]OTs (3) and [Pd(OTs)(dppo)]OTs
(4) [10] were obtained by reaction of the correspond-
ing bis-chloride derivative (P P)PdCl2 with Ag(OTs) in
dichloromethane, followed by ethyl ether addition.
X-ray diffraction analyses reported elsewhere for the tri-
flate analogues of 1 and 2, 1ꢀ and 2ꢀ showed the cations
[Pd(H2O)2(P P)]2+ to contain the palladium centers coor-
dinated by two cis water molecules taken from adventitious
moisture [9b], which reflects the low coordinating ability of
sulphonate anions. However, the molar conductivity data of
1 and 2 as well as those of 1ꢀ and 2ꢀ in polar solvents such
as nitroethane (ε = 19.7) [9b] are consistent with the forma-
tion of monocationic species with one tosylate (or triflate) ion
and one water molecule coordinated trans to the phosphorus
atoms (Scheme 3).
A single-crystal X-ray analysis and conductivity measure-
ments show that the new ruthenocenyl complex 3 contains a
k2-P,P metallocenyl ligand, one tosylate anion and one water
molecule coordinated to the palladium in both the solid state
and solution (Scheme 3). An ORTEP drawing of the com-
2.11. Reaction of 17 with styrene
A solution of 17 (20 mg, 0.013 mmol) and a 20-fold ex-
cess of styrene (30 ␮l, 0.26 mmol) in CD2Cl2 (1 ml) were
transferred into a 5 mm NMR tube at room temperature.
31P{1H} and H NMR spectra were acquired at room tem-
1
perature every hour. No new product was detected in an arch
of 24 h.

C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
39
Table 1
Crystallographic data for 3
Molecular formula
C48H44O7P2PdRuS2
Formula weight (g mol−1
Crystal color, shape
Crystal dimensions (mm)
T (K)
)
1066.43
Orange, prism
0.30 × 0.20 × 0.20
120(1)
Crystal system
Space group
Orthorhombic
Pbca
˚
a (A)
20.504(3)
20.372(3)
20.904(3)
90.000
90.000
90.000
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
V (A )
8732(2)
Z
8
F(0 0 0)
ρ (calcd.) (g cm−3
4320
1.622
Scheme 3.
)
˚
␭ (Mo K␣) (A)
0.71072
0.979
µ (mm−1
)
plex cation in 3 [Pd(H2O)(OTs)(dppr)]+ is provided in Fig. 1,
while crystallographic data and selected metrical parameters
are listed in Tables 1 and 2, respectively. The palladium center
is square planarly coordinated with a P(1) Pd P(2) crystal-
lographic bite angle of 97.47(4)◦, which is slightly smaller
than that in the corresponding neutral dichloride precursor
θ range (◦)
2.0–30.0
−28 ≤ h ≤28
−25 ≤ k ≤ 21
−29 ≤ l ≤ 29
12189
Index ranges
Number of collected reflections
Number of independent reflections
Rint
7849
0.0542
˚
(100.02(17) A) [17]. It is worth noticing that the crystallo-
Number of data/restrains/parameters
Refinement method
7849/0/550
Full-matrix least-squares on F2
1.015
graphic P Pd P bite angles in the dppf and dppomf deriva-
tives 1ꢀ and 2ꢀ are 96.37(1)◦ and 101.3(1)◦, respectively [9b].
The distances of ruthenium to the centroids of the Cp
rings in 3 are practically identical to each other (1.806(5) and
1.807(5) A) and match well those in the free ligand (1.803(4)
and 1.808(4) A) [2a]. The Cp rings are planar and tilted to-
GOF on F2
Final R1, wR2 indices [I > 2σ(I)]
Final R1, wR2 indices (all data)
Peak, hole in final difference map (e A
0.0573, 0.1215
0.0887, 0.1304
2.03, −0.68
−3
˚
)
˚
˚
wards the palladium atom. The angle between the plane nor-
mals related to the Cp rings is 9.0◦, while the dihedral angle
Fig. 1. ORTEP drawing of 3. Thermal ellipsoids are drawn at the 50% probability level.

40
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
P(1) Cp(1) Cp(2) P(2) exhibits a value of 33.8◦, which is
Table 2
Selected geometrical parameters for 3
comparable to that found in the corresponding neutral dichlo-
ride structure 34.1◦ [17]. The coordinated water molecule
participatesinbothintra-andintermolecularhydrogenbonds,
˚
Bond lengths (A)
Ru(1) Cp(1)a
Ru(2) Cp(2)b
Ru(1) CCp
1.806(5)
1.807(5)
2.121(4)–2.223(4)
4.215(5)
2.289(1)
2.240(1)
2.102(3)
2.126(3)
˚
with O· · ·H distances of 1.90 and 1.78 A, respectively, with
the oxygen atoms of both coordinated and free tosylate anions
[13b].
Ru(1)· · ·Pd(1)
Pd(1) P(1)
Pd(1) P(2)
Pd(1) O(1)
Pd(1) O(2)
Overall, the coordination geometry of 3 is similar to that
of 1ꢀ. Unlike the latter complex, 3 exhibits two rather dif-
˚
ferent Pd P bond lengths, Pd P(1) 2.289(1) A and Pd P(2)
P
P
CCp
CPh
1.799(5)–1.804(4)
1.809(5)–1.819(5)
3.404(1)
˚
2.240(1) A, which may reflect the presence of two different
ligands trans to phosphorus as well as solid state effects due
to the H-bonding network mentioned above.
P1· · ·P2
Bond angles (◦)
O(1)Pd(1)O(2)
P(1)Pd(1)P(2)
P(1)Pd(1)O(1)
P(2)Pd(1)O(2)
P(1)Pd(1)O(2)
P(2)Pd(1)O(1)
The osmocenyl complex 4 contains, in both the solid state
and solution, a k3-P,P,Os osmocenyl ligand with a dative
Os Pd bond and one tosylate group trans to Os (Scheme 3)
[10]. The major propensity of dppo than dppf or dppr to act as
tridentate ligand has been related to the higher electron den-
sity of the osmium center, which allows it to compete with
both tosylate and water molecules for coordination to palla-
dium (the nucleophilicity of the metallocene metal decreases
in the order Os > Ru > (Cp*)Fe > (Cp)Fe in fact [9a,b,10]).
On the other hand, it is noteworthy that both strong anionic
nucleophiles [10] and even water molecules (vide infra), if
provided in large excess, can lead to the formation of k2-P,P
dppo palladium complexes. Indeed, we have found that the
addition of water to CH2Cl2 solutions of 4 yields the bis-
solvento complex [Pd(OH2)2(dppo)](OTs)2 (10). Dppf, dp-
pomf and dppr can bind palladium in the k3-P,P,M mode too,
yet preferentially in conjunction with a fourth ligand featured
by either a large size (e.g., PPh3: dppf [18], dppomf [9b], dppr
[9d], dppo [9d]) or an electron-withdrawing character (e.g.,
C(O)OR: dppomf [9a] and C(O)R: dppomf [9a], dppr [this
work], dppo [10]).
88.23(11)
97.47(4)
174.00(9)
174.62(10)
86.06(8)
88.36(8)
Dihedral and pseudotorsion angles (◦)
P(1)Cp(1)Cp(2)P(2)a,b
C(1)Cp(1)Cp(2)C(6)a,b
33.8
34.1
36.3
−9.0
0.048
0.194
RuP(1)P(2)/Pd(1)P(1)P(2)
c
αCp
ꢁP(1)d
ꢁP(2)d
◦
˚
Bond lengths and angles (intramolecular hydrogen bond) (A, )
H(1B) O(4)
H(1B) O(1)
O(1)· · ·O(4)
O(1)H(1B)O(4)
1.90
0.77
2.622
157
◦
˚
Bond lengths and angles (intermolecular hydrogen bond) (A, )
H(1A) O(6)
H(1A) O(1)
O(1)· · ·O(6)
O(1)H(1A)O(6)
a
1.78
0.82
2.594
172
Cp(1) denotes the center of the C(1) C(5) Cp ligand.
Cp(2) denotes the center of the C(6) C(10) Cp ligand.
Dihedral angle between the Cp planes. The negative value corresponds
In conclusion, the solution structures of 1–3 or 4 depend
on a complex web of factors, which include the electronic and
steric features of the complementary ligand(s), the nature of
the sandwiched metal, the solvent and the water contents
in the latter. Scheme 4 provides a picture of the possible
equilibria and coordination structures exhibited by 1–4 in
reagent grade MeOH.
b
c
to the inclination of the Cp rings toward the Pd atom.
d
Deviations of the P atom from the corresponding Cp ring. The positive
value corresponds to the deviation toward the Ru atom.
Scheme 4.

C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
41
3.2. Stability study of the catalyst precursors 1–4 in
MeOH by NMR spectroscopy
aquo complex [Pd(OH2)2(dppo)](OTs)2 (10) (see below for
its independent synthesis and characterization). With time,
the 4/10 ratio remained practically constant, while the con-
centration of the 4/9/10 system, as a whole, decreased by ca.
60% in 6 h. Complex 10 contains a k2-P,P dppo ligand as
shown by the small difference between the chemical shifts of
the ␣ and ␤ hydrogen atoms of the Cp rings (ꢁδ(MeOH-
d4) = 0.40) [9a,b]. In compounds 4 and 9, both featured
by a k3-P,P,Os dppo ligand, the difference is much higher
(ꢁδ(MeOH-d4) = 1.08 and 1.09, respectively) [9a,b,10].
In conclusion, unlike 1, the metallocene palladium(II) pre-
cursors 2–4 tend to decompose spontaneously in MeOH.
Roughly, the stability of the precursors decreases in the or-
der 1 > 3 > 4 ≥ 2. This degradation can be inhibited by adding
TsOH. Indeed, as shown by independent NMR experiments,
the addition of an excess of TsOH to the mixtures of products
obtained by dissolving 2, 3 or 4 in MeOH-d4 regenerated 2
and 3 and gave a mixture of 4 and 10. Consistently, no forma-
tion of palladium(I) binuclear species was observed by NMR
spectroscopy upon dissolution of the palladium(II) precur-
sors 2–4 in MeOH-d4 acidified with TsOH.
Typically, a sample of the appropriate compound was dis-
solved in MeOH-d4 at room temperature. The solution was
transferred into a 5 mm NMR tube that was then placed into
the NMR probe at room temperature. 31P{1H} and 1H NMR
spectra were acquired every 30 min for 6 h. The dppf deriva-
tive1(31P{1H}NMR, δ:47.9)exhibitednoappreciabletrans-
formation during the course of the experiment [9a]. In con-
trast, just the first 31P{1H} NMR spectrum showed the partial
conversion (14%) of the dppomf complex 2 (δ: 71.6) into the
palladium(I) binuclear complex [Pd(dppomf)]2(OTs)2 (7, δ:
−1.3), which contains a k3-P,P,Fe dppomf ligand [9a]. Al-
though the 2/7 ratio did not appreciably vary during the ex-
periment, the intensity of the 31P NMR signals of these two
compounds decreased at least by half. Palladium metal was
observed to settle on the bottom of the NMR tube, however
in trace amount, which does not justify the intensity decrease
of 2 and 7. It is therefore likely that other species are formed
from either 2 or 7, each in a too low concentration to be seen
by NMR spectroscopy.
The formation of 7, in analogy with that of structurally
similar binuclear PdI complexes with chelating diphosphines
(P P) [9a], has been proposed to involve the coupling of
(dppomf)PdII and (dppomf)Pd0 moieties, the latter being
generated by palladium-promoted oxidation of MeOH to
formaldehyde (Scheme 5, steps a–d) [19,20].
A similar behavior was displayed also by the ruthenium
analogue 3 (31P{1H} NMR, δ: 44.4). In fact, the binuclear
complex [Pd(dppr)]2(OTs)2 (8) (31P{1H} NMR, δ: 6.8, see
below for the independent synthesis and NMR characteriza-
tion) appeared just in the first spectrum (20%). The stability
of the 3/8 system seems to be a little higher than that of the
dppomf system as the intensity of the NMR signals of the two
dppr compounds decreased by about one-fourth only within
the same residence time.
3.3. HPNMR study of the carbonylation of styrene
catalyzed by 3 in TsOH containing MeOH
Some carbonylation reactions of styrene in the presence
of 1–4 were followed by variable-temperature 31P{1H} and
1H NMR spectroscopy using a 10 mm-OD sapphire HPNMR
tube. A sequence of selected 31P{1H} NMR spectra relative
to the reaction assisted by 3 is reported in Fig. 2. A tube
was charged sequentially with MeOH-d4 solutions of TsOH
In the case of 4, the NMR picture was more complex.
Indeed, the first 31P{1H} NMR spectrum showed, besides
the signals due to the precursor [10] (δ: −17.5, 92%) and to
the palladium(I) binuclear complex [Pd(dppo)]2(OTs)2 (9)
(δ: 2.3, 3%), a third signal at δ: 50.4 (5%) due to the bis-
Fig. 2. Variable-temperature 31P{1H} NMR study (sapphire tube, MeOH-
d4, 81.01 MHz) of the carbonylation reaction of styrene catalyzed by 3: (a)
3 in the presence of TsOH (10 equiv) and styrene (100 equiv) under nitrogen
at room temperature; (b) after the tube was pressurized with 14 bar of CO
at room temperature; (c) at 80 ◦C; (d) after the tube was cooled to room
temperature; (e) at 80 ◦C; (f) after the tube was cooled to room temperature.
Scheme 5.

42
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
(10 equiv) and 3 under nitrogen at room temperature. The
tube was placed into a NMR probe at room temperature (trace
a, 31P{1H} NMR singlet at δ: 44.4). Addition of a 100-fold
excess of styrene caused no change in the 31P{1H} NMR
spectrum. In contrast, when the tube was pressurized with
14 bar of CO at room temperature, a highly noisy baseline
with no discrete signal appeared immediately (trace b), which
suggests the formation of species in fast dynamic exchange.
Thetemperaturewasthenincreasedgraduallyto50 ◦Cand
then to 80 ◦C. At 50 ◦C a broad 31P{1H} NMR signal began to
appear at ca. δ: 28, which became more intense at 80 ◦C (trace
c). On cooling to room temperature, the signal transformed
into a new one (trace d) at δ: 20.6 due to the ␮-hydrido-
␮-carbonyl complex [Pd2(␮-D/H)(␮-CO)(dppr)2]OTs (13)
(see below for its independent synthesis and characteriza-
tion). The 1H NMR spectrum showed the formation of both
linear and branched esters. To verify the relationship, if any,
between 13 and the species responsible for the broad signal
at δ: 28, the tube was heated again to 80 ◦C: the broad signal
reformed (trace e). After 15 min, the probe-head was allowed
to cool again to room temperature: 13 became the only vis-
ible species (trace f). This behavior may be explained by
the formation of catalytically active species in fast dynamic
exchange with 13 at temperatures higher than 50 ◦C. Note-
worthy, isolated 13 proved to be stereochemically rigid in
MeOH-d4 up to 100 ◦C. Consistent with the formation of
catalytically active species above 50 ◦C, a comparison be-
values in the range from 77% (with precatalysts 1, 3, and 4)
to 84% (with precatalyst 2). The highest turnover frequency
(TOF) of 334 mol of styrene converted (mol of catalyst h)−1
was obtained with the dppr derived catalyst. As a whole,
these results are of some relevance as they lie in the upper
range of activity and regioselectivity for this kind of reactions
[22].
A perusal of the data reported in Table 3 shows that only
the dppf precursor 1 converts a significant amount of styrene
in the absence of acid co-reagents (29.3% in 1 h at 42 bar CO
and 100 ◦C, run 1). Under these conditions, all the other pre-
cursors were much less active than 1, with conversions below
5% (runs 8, 16, 27). For all catalysts, a remarkable increase
in productivity, accompanied by a less important increase in
regioselectivity (1–2%), was observed by adding an excess of
TsOH. Indeed, the addition of 40 equiv of TsOH made 1–4
capable of converting 81.1, 50.6, 85.8 and 65.4% styrene
into A + B, respectively (runs 3, 11, 19, 29). Higher con-
centrations of acid increased neither the conversion nor the
selectivity. Overall, these results reflect the chemical stability
of the four precursors in either neutral and acid-containing
media as determined by operando NMR spectroscopy (vide
infra). Under acidic conditions, the catalysts were still active
within 2 h with no apparent change in selectivity (runs 5, 13,
25, 31).
A secondary, undesired, effect caused by the use of TsOH
co-catalyst was the formation of the ether PhCH(Me)OMe
(C), in consequence of the acid-catalyzed addition of MeOH
to the olefin double bond of styrene. This drawback is not
very important as the yields in C never exceeded 3% even
with 40 equiv of TsOH at 100 ◦C. Independent experiments
of styrene carbonylation in the absence of palladium catalyst,
yet with added TsOH, showed the formation of C in 2–3%
yield, depending on both acid concentration and temperature.
Neither A nor B were produced by substituting C for styrene
in the carbonylation reactions, which rules out any role of C
as intermediate to A and B.
In an attempt of determining the best experimental condi-
tions for high activity and selectivity as well as rationalizing
the reaction mechanism of the carbonylation of styrene cat-
alyzed by 1–4, a number of reactions were carried out varying
some experimental parameters such as pressure of CO, tem-
perature, reaction time and concentration of added oxidant
(1,4-benzoquinone, BQ).
In the CO pressure range from 14 to 42 bar, the conversion
of styrene increased steadily with no change in selectivity
(runs 4, 3/12, 11/23, 19, 21/29, 30). Higher pressures (e.g.,
63 bar) did not affect the selectivity, yet a decrease productiv-
ity was observed, which may reflect competition of CO with
styrene for coordination to palladium.
1
tween the H NMR spectra recorded at room temperature
before and after the last heating to 80 ◦C showed an increase
in the formation of esters. Analysis of the solution by GC
confirmed the formation of deuterated samples of both linear
and branched esters.
Similar NMR pictures were obtained when the precursors
1, 2 and 4 were used in the place of 3: the ␮-hydrido-␮-
carbonyl complexes 11 (dppf) [9a], 12 (dppomf) [9a], and 14
(dppo) were the only well-defined species detected by NMR
spectroscopy at room temperature, while broad humps were
observed at high temperature.
3.4. Catalytic studies
3.4.1. Reactions catalyzed by 1–4
The carbonylation of vinyl aromatics catalyzed by
palladium(II) precursors in polar media provides an effective
method for both the laboratory and large-scale production
of either arylpropionic acids (hydroxycarbonylation) or
alkyl arylpropanoates (alkoxycarbonylation) [21]. The use
of 1–3 or 4 as catalyst precursors for the methoxycar-
bonylation of the prototypical vinyl aromatic, styrene led
to the formation of mixtures of the saturated monoesters
methyl 3-phenylpropanoate (linear ester, A) and methyl
2-phenylpropanoate (branched ester, B) (Scheme 2) [22].
Experimental details and catalytic data are provided in
Table 3. Irrespective of the catalyst precursors and of the
experimental conditions, the reactions were featured by the
prevalent formation of the linear ester with regioselectivity
The productivity was found to increase with the tempera-
tureforallcatalysts. Thiseffectwasparticularlyimportantfor
the dppr precursor 3 which, at 100 ◦C, gave A and B in more
than double yield than at 80 ◦C (runs 22, 23). Above 100 ◦C,
however, the ether C was formed in a too large amount to
make the process appealing (run 24); therefore a temperature

Table 3
Methoxycarbonylation of styrene catalyzed by PdII complexes with dppf, dppomf, dppr, and dppo ligandsa
Run Catalyst precursor
number
BQ
(equiv)
HOTs
(equiv)
CO
(psi)
◦C
h
Conversion
(%)
Ether
Ether
Selectivity Ether
Selectivity Ether
Selectivity
Ether
E (%) of E
Selectivity CO products Selectivity of TOF CO
C (%) B (%) of B
A (%) of A
D (%) of D
(%)
CO products
products
1
2
3
4
5
6
7b
8
1
1
1
1
1
1
1
2
12
2
2
2
2
12
2
3
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
0
400
0
0
0
20
0
0
0
400
0
0
0
0
20
0
400
0
0
0
0
0
0
0
0
20
0
400
0
0
0
0
0
42
42
42
14
14
14
14
42
42
42
42
14
14
14
14
42
42
42
42
42
63
14
14
14
14
14
42
42
42
14
14
14
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
80
100
120
100
100
100
100
100
100
100
100
1
1
1
1
2
1
1
1
1
1
1
1
2
1
1
1
1
1
1
1
1
1
1
1
2
1
1
1
1
1
2
1
29.3
22.3
81.1
46.8
72.3
36.9
27.3
4.9
0.0
0.0
1.6
2.9
3.8
2.7
0.8
0.0
0.0
0.0
3.1
3.2
3.9
3.4
2.6
0.0
0.0
1.3
2.2
3.1
1.4
0.5
2.6
12.7
3.5
2.8
0.0
0.0
1.9
2.8
5.0
2.6
7.2
0.8
18.7
10.5
15.9
8.2
5.0
0.8
0.9
0.1
7.3
6.8
10.7
5.3
6.5
24.6
3.6
22.1
2.3
75.4
10.3
76.5
76.1
76.8
76.0
81.1
83.7
83.9
29.2
84.6
84.3
84.6
84.7
84.2
75.0
18.8
77.0
77.4
76.9
76.0
74.8
77.7
79.7
78.2
76.6
75.0
30.0
77.2
77.6
78.2
77.1
0.0
17.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.5
0.0
0.0
0.0
0.0
0.0
0.0
1.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.5
0.0
0.0
0.0
0.0
0.0
78.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
62.5
0.0
0.0
0.0
0.0
0.0
0.0
68.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
50.0
0.0
0.0
0.0
0.0
0.0
1.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.0
0.0
0.0
0.0
0.0
8.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
4.2
0.0
0.0
0.0
0.0
0.0
0.0
6.3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
10.0
0.0
0.0
0.0
0.0
29.3
22.3
79.5
43.9
68.5
34.2
26.5
4.9
100.0
100.0
98.0
93.8
94.7
92.7
97.1
100.0
100.0
100.0
93.9
93.1
94.7
91.1
94.1
100.0
100.0
97.3
97.4
96.4
97.8
98.4
96.4
84.5
96.2
94.8
100.0
100.0
97.1
93.8
92.2
93.7
117
89
40
40
40
40
40
0
23.5
23.9
23.2
24.0
18.9
16.3
16.1
4.2
15.4
15.7
15.4
15.3
15.8
25.0
6.3
23.0
22.6
23.1
24.0
25.2
22.3
20.3
21.8
23.4
25.0
10.0
22.8
22.4
21.8
22.9
60.8
33.4
52.6
26.0
21.5
4.1
318
176
137
137
106
20
9
0
0
5.6
2.4
4.7
0.7
5.6
2.4
22
10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
40
40
40
40
40
0
50.6
46.5
73.3
38.1
43.8
2.8
40.2
36.5
58.7
29.4
34.7
2.1
47.5
43.3
69.4
34.7
41.2
2.8
190
173
139
139
165
11
0.7
0.1
0
1.6
0.3
1.6
6
20
40
60
40
40
40
40
40
40
0
48.2
85.8
85.0
63.1
30.6
73.0
82.1
93.3
53.7
0.8
10.8
18.9
18.9
14.8
7.6
15.7
14.1
19.6
11.9
0.2
0.1
14.5
9.5
12.9
8.9
36.1
64.7
63.0
46.9
22.5
54.7
55.3
70.2
39.0
0.6
46.9
83.6
81.9
61.7
30.1
70.4
69.4
89.8
50.9
0.8
188
334
328
247
120
282
278
180
204
3
0
1.0
0.3
1.0
4
40
40
40
40
65.4
45.2
64.2
41.4
49.0
32.9
46.3
29.9
63.5
42.4
59.2
38.8
254
170
118
155
20
a
Catalyst precursor 0.01 mmol; styrene 4 mmol; solvent (MeOH) 20 ml; 700 rpm.
Solvent (EtOH) 20 ml.
b

44
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
Scheme 6.
Scheme 9. Proposed reaction schemes for the methoxycarbonylation of
styrene with chelating diphosphine-Pd(II) precursors; [Pd] = [(P P)Pd]2+
.
,
Scheme 7.
P
P
P = dppe, dppp, k2-P,P metallocene ligands, [M Pd] = [(P P)Pd]2+
M
P = k3-P,P,M metallocene ligands. (ꢀ) Either a co-ligand or a solvent
of 100 ◦C represents a limit to conjugate good activity and
selectivity.
M
molecule.
Some catalytic experiments were carried out using the bin-
uclear dppomf complex 12 in the place of the mononuclear
precursor 2 (runs 9, 14). Consistent with the HPNMR exper-
iments, the use of 12 led to the conversion of 38.1% styrene
with 84.7% selectivity in A. Since 2 gave 46.5% conversion
and 84.3% selectivity in A under identical conditions, the
␮-H-␮-CO complexes are likely catalyst resting state that
require an induction period to generate catalytically active
species; in our case, mononuclear Pd H complexes. Indeed,
previous studies of alternating copolymerization reactions of
ethylene with CO in MeOH have proved that ␮-H-␮-CO pal-
ladium(II) complexes with chelating diphosphines, includ-
ing dppf, are catalytically active and can re-enter the cataly-
sis cycle as Pd H through the reaction with the protic acid
(Scheme 6) [9a,23].
The addition of BQ to the initial catalytic mixtures caused
no beneficial effect either on the yield or on the regioselectiv-
ity. On one hand, the contemporaneous presence of 40 equiv
of TsOH and 20–100 equiv of BQ gave less esters (10–30%)
without improving the selectivity in isomer A that was around
77% with 1, 3 and 4 and around 84% with 2 (runs 6, 15,
26, 32). On the other hand, the use of a large excess of BQ
(400 equiv), in the absence of added acid, made the chemos-
electivity worse (runs 2, 10, 17, 28). Indeed, besides the sat-
urated esters A and B, methyl cinnamate (D) and dimethyl
phenylsuccinate (E) were also obtained (Scheme 7). Only
the dppf complex showed a significant production (17.4%)
of the major product methyl cinnamate (selectivity 78%), in
the presence of 400 equiv of BQ (run 2).
MeOH (Scheme 8) [24]. Indeed, the formation of D and
E requires Pd OMe initiators [25,26a,b], while the satu-
rated monoesters A and B are obtained from Pd H initiators
[26a,b,27].
3.4.2. Reactions catalyzed by
[Pd(H2O)(OTs)(dppe)]OTs and
[Pd(H2O)(OTs)(dppp)]OTs
In an attempt of gaining further insight into the role
played by the 1,1ꢀ-bis(diphenylphosphino)metallocene lig-
ands in governing the methoxycarbonylation of styrene, some
reactions were performed with catalyst precursors containing
classical diphosphines such as [Pd(H2O)(OTs)(dppe)]OTs
(5) [12] and [Pd(H2O)(OTs)(dppp)]OTs (6) [13] (Scheme 9).
Similardppe-PdII complexesareknowntocatalyzetheoxida-
tive carbonylation of styrene in MeOH to methyl cinnamate
and/ordimethylsuccinate[25], while6andrelateddpppcom-
plexes have been employed to convert styrene into different
types of low molecular weight oxygenates (esters, ketones,
aldeheydes) depending on the reaction parameters [26].
Under the present experimental conditions, both 5 and
6 gave very low conversions of styrene in either neutral or
acidic media (Table 4). Previous studies, including in situ
and operando HPNMR experiments, indicated the great in-
stability of the [(dppe)PdH]+ and [(dppp)PdH]+ moieties as
the principal factor accounting for the low catalytic activ-
ity in MeOH [25,26,28]. Indeed, unlike 1–4, compounds 5
and 6 undergo rapid and irreversible transformation, under
styrene methoxycarbonylation conditions, to give the bis-
chelates [(P P)2Pd](OTs)2 via the disruptive reduction of
half equivalent of PdII precursor to Pd0 and free diphosphine
ligand [25,28a,29].
The decrease in selectivity caused by the presence of BQ
in the reaction mixture has precedent in other carbonyla-
tion reactions of alkenes and can be rationalized by con-
sidering that BQ is able to convert Pd H into Pd OMe in
Scheme 8.

C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
45
Only in the presence of a large excess of oxidant BQ
(400 equiv), which is necessary to oxidize inactive Pd0 to
active PdII, the dppe and dppp precursors were able to con-
vert styrene to methyl cinnamate in good yield (65–70% in
1 h) and excellent selectivity (>95%) (Table 4, runs 2, 7)
[25]. As said above, the formation of the cinnamate implies
Pd OMe initiators that react with CO to form Pd CO(O)Me
intermediates. These insert styrene exclusively in 2,1-fashion
[24b,e,25,26i,30,31]. Therefore, only the cinnamate D can
form by ␤-H elimination, provided this reaction is faster
than CO insertion. If not, the succinate E might be obtained
by CO insertion, followed by methanolysis of Pd-acyl (see
Scheme 11). Since D was selectively produced, the ␤-H trans-
fer path is apparently favored over CO insertion for the dppe
and dppp systems.
3.5. Study of the methoxycarbonylation of styrene
catalyzed by model dppr complexes
The methyl complex [PdMe(dppr)]B(Arꢀ)4 (Arꢀ = 3,5-
(CF3)2C6H3) (16) was straightforwardly prepared by the re-
action of PdCl(Me)(dppr) (15) with NaB(Arꢀ)4 in CH2Cl2
solution. Under a CO atmosphere (1 bar) at room tem-
perature, 16 converted immediately into the acyl complex
[Pd(COMe)(dppr)]B(Arꢀ)4 (17) (Scheme 10).
The different hapticity of dppr in 16 and 17 with respect
to 15 was unequivocally put in evidence by comparing the
corresponding 31P{1H} and 1H NMR spectra. Indeed, as pre-
viously observed for Pd(II) complexes with dppo-, dppf-, and
dppf-related ligands [9a,b,10,18,32], the intramolecular for-
mation of M → Pd bonds causes a remarkable high-field shift
of the phosphorus resonance as compared to k2-P,P deriva-
tives. As an example, the 31P{1H} NMR spectra of 16 and 17
show singlets at δ: −12.4 and −13.8, respectively, for the two
magnetically equivalent phosphorus nuclei that resonate at δ:
33.0 in the dichloride PdCl2(dppr) [2a,17] and at δ: 14.0 (d,
J(PP) = 28.9 Hz) and 37.8 (d) in 15. The presence of Ru-Pd
bonds in 16 and 17 can also be inferred by the large chemical
shift difference between the ␣- and ␤-hydrogen atoms (1.36
and 0.94 ppm in 16 and 17, respectively) of the cyclopenta-
dienyl rings as compared to k2-P,P complexes (0.2–0.5 ppm)
[9a,b,10,18,32].
The acyl complex 17 in CD2Cl2 underwent fast methanol-
ysis at room temperature under a CO stream yielding the
␮-hydride-␮-CO complex 13 and methyl acetate [9a]. In
contrast, no reaction occurred when complex 17 was allowed
to react with a 20-fold excess of styrene in CD2Cl2 at room
temperature. The lack of reactivity of the acyl 17 towards
styrene and its fast reaction with MeOH are fully consistent
with the absence of products containing two styrene units in
the final catalytic mixtures.
3.6. Proposed mechanism of the methoxycarbonylation
of styrene catalyzed by 1–4
Scheme 11 reports a sequence of reactions that are gener-
allyconsideredaskeystepsinthemechanismofthemethoxy-

46
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
Scheme 10.
carbonylation of styrene by palladium(II)-chelating diphos-
phine catalysis [22a,b,24b,26d]. The results obtained with the
precursors 1–4 are fully consistent with this mechanism.
In the dashed box is illustrated the mechanism by which
1–4 can catalyze the production of the esters A and B in the
absence of oxidant. Under methoxycarbonylation conditions,
with or without added protic acid, (P P)PdII complexes can
form Pd H moieties by different routes, which include ␤-H
elimination of Pd OMe yielding HC(O)H (Scheme 5, step
b), water-gas-shift reaction, and protonation of Pd0 species
generatedinsitu[24]. Theinsertionofstyreneisthenstraight-
forward to give linear and branched Pd-alkyls (steps a and
b) which may interconvert into each other via a ␲ benzylic
species [33]. Migratory insertion of Pd(alkyl)(CO) gener-
ates the corresponding acyl complexes that, in the case of
dppomf, dppr, and dppo, should contain k3-P,P,M ligands
(step c) (see also Scheme 10). Finally, the esters A and B
are formed by nucleophilic attack of MeOH at the acyl car-
bon atoms (methanolysis, step d) [22a,b,32c], which has been
proposed to be the rate determining step of alkoxycarbony-
lation reactions of alkenes assisted by (P P)Pd H initiators
[22a,b,34]. In particular, it has been proved that the catalytic
activity decreases with the length of the alkyl chain of the
alcohol. Since we have observed a decrease of the catalytic
activity by replacing MeOH with EtOH, at even catalyst pre-
cursor and conditions (Table 3, run 7), it is very likely that the
methanolysis of Pd-acyl constitutes the rate-determining step
also in the methoxycarbonylation reactions reported in this
paper. Whether the methanolysis occurs via intermolecular
attack of MeOH at the acyl carbon atom or via intramolecu-
lar attack by coordinated MeOH (most favored pathway with
chelating diphosphines [32c]) is not possible to assess in the
absence of a specific kinetic study. It may be also possible
that both paths occur depending on the bonding mode of
the 1,1ꢀ-bis(diphenylphosphino)metallocene ligand, with dp-
pomf, dppr and dppo favoring the intermolecular mechanism.
In any case, the methanolysis of Pd-acyl is faster than the in-
sertion of styrene as no carbonylation product containing two
styrene units was obtained.
The high selectivity in the linear ester A obtained with
1–4 has precedent in several studies of methoxycarbonylation
of vinyl aromatics by palladium(II) catalysts modified with
Scheme 11.

C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
47
diphosphines bearing bulky phosphorus-substituents [22,35].
The reasons for the linear regioselectivity are still a matter of
debate. Steric effects, related to the interaction of the phos-
phino phenyl groups and the phenyl in the branched alkyl,
have been claimed to be determinant in favoring the forma-
tion of the linear ester [22c,36]. On the other hand, isotope
labelingstudieswith␣-deuterostyreneinMeOH-d4 havesug-
gested a kinetic control of the regioselectivity caused by the
slower insertion of CO into the Pd Cbranched-alkyl bond as
compared to the Pd Clinear-alkyl bond [26d]. In the case of
1–4, no appreciable influence of the CO pressure on the regio-
chemistry was observed from 14 to 42 bar. Therefore, we are
inclined to assign a major importance to direct steric effects
than to migratory insertion kinetics. Consistent this hypoth-
esis is the fact that the highest regioselectivity in A (85%)
was obtained with the dppomf complex 2 which exhibits the
largest crystallographic P Pd P bite angle (101.3(1)◦ versus
96.37(1)◦ in 1 and 97.47(4)◦ in 3) and hence contains four
phenyl rings spatially closer to the alkyl ligand than in the
other precursors.
3%. In our opinion, this is the indirect consequence of
the different propensity of the four 1,1ꢀ-bis(diphenylphos-
phino)metallocene ligands to bind palladium in the k3-P,P,M
mode. Indeed, in conjunction with electron-withdrawing co-
ligands such as the carbomethoxy C(O)OMe group, dp-
pomf, dppr, anddppoformveryrobustk3-P,P,Mpalladium(II)
complexes which do not react with alkenes [9a]. It cannot
be excluded that also dppf forms a complex of the formula
[Pd(C(O)OMe)(k3-P,P,Fe)]+ since the overall conversion of
styrene under oxidative conditions is rather low (22.3%).
However, some phenyl cinnamate (17.4%) is obtained, which
may be explained by the formation of a rather weak Fe Pd
bond. Consistent with the hypothesis that the k3-P,P,M bond-
ing mode in carbomethoxy complexes disfavors the insertion
of styrene, the precursors 5 and 6, with diphosphines, that
coordinate metal centers exclusively in k2-P,P fashion, give
rise to selective catalysts for the oxidative carbonylation of
styrene to methyl cinnamate (Table 4).
In the presence of a large excess of BQ, without TsOH, 1–4
produced four products, A, B, D and E, among which D was
the major product. However, only the dppf precursor 1 gave a
significant conversion of styrene (overall 22.3%). Despite the
moderate to scarce overall productivity, the reactions carried
out in the presence of 400 equiv of BQ provide interesting in-
formation on the mechanism of the oxidative carbonylation
of styrene as well as on the factors that differentiate the ac-
tivity of 1–4. Therefore, it is worthwhile commenting briefly
these reactions.
It is agreed that the formation ofthe esters D and E requires
a (P P)PdII OMe initiator which can be readily generated
by interaction of the PdII precursor with MeOH as illustrated
in step a of Scheme 5. In the course of the catalysis, it is just
BQ to provide new Pd OMe moieties through the oxidation
process shown in Scheme 8. The sequence of reactions yield-
ing D and E are shown in the lower section of Scheme 11. The
branched alkyl (step g) is obtained from Pd OMe by reaction
with CO (step e), followed by 2,1-insertion of styrene (step
g) [24b,e,25,26i,30,31]. The methyl cinnamate D is then re-
leased by ␤-elimination (step h). In consequence of this event,
the Pd H moiety is formed and from this the Pd OMe one
(Scheme 8). The same branched alkyl can also produce the
minor diester product E through CO insertion (step i), fol-
lowed by methanolysis (step j). It is important to notice that,
even in the presence of a large excess of BQ, some satu-
rated esters A and B were obtained: this means that: (i) some
Pd H moieties generated in step h enter the catalysis cycle
shown in the upper part of Scheme 11; (ii) the protonolysis
by MeOH of the branched alkyl, produced in step g, does not
occur. Indeed, were the protonolysis possible, only the linear
ester A would form, which is not experimentally observed
in fact.
4. Conclusions
1,1ꢀ-Bis(diphenylphosphino)metallocene palladium(II)
complexes with water and/or tosylate co-ligands are effective
and selective catalyst precursors for the methoxycarbonyla-
tion of styrene to methyl phenylpropanoates in the presence
of TsOH, yielding prevalently the linear regioisomer methyl
3-phenylpropanoate. Varying the metallocene metal between
Fe, Ru and Os or substituting Cp* for Cp did not appreciably
change either the chemoselectivity or the regioselectiv-
ity. In contrast, the nature of the metal and the type of
cyclopentadienyl ligand have been found to influence the
activity by controlling the chemical stability of the catalytic
systems.
The activity exhibited by the 1,1ꢀ-bis(diphenylphos-
phino)metallocene catalysts contrasts that of similar palla-
dium(II) catalysts stabilized by the chelating diphosphine
ligands dppe and dppp that, under comparable experimental
conditions, were almost inactive due to the very low stability
of the Pd H initiators. The reverse was observed by perform-
ing the carbonylation reactions in the presence of a large ex-
cess of BQ where Pd C(O)OMe initiators are formed. In
this case, the 1,1ꢀ-bis(diphenylphosphino)metallocene cat-
alysts, with the partial exception of the dppf precursor,
were almost inactive, while the dppe- and dppp-modified
catalysts gave the unsaturated ester methyl cinnamate in
good activity and excellent selectivity. The different activ-
ity and chemoselectivity exhibited by the catalysts with 1,1ꢀ-
bis(diphenylphosphino)metallocenes or dppe/dppp has been
interpreted in terms of the unique ability of the former to
act as tridentate k3-P,P,M ligands capable of forming robust
intramolecular M Pd bonds in M Pd C(O)OMe intermedi-
ates that are not easily disrupted by alkenes. The intermediate
behavior of the dppf precursor may reflect its low propensity
to form k3-P,P,Fe palladium(II) complexes.
As said above, only the dppf precursor 1 gave a sig-
nificant conversion of styrene in the presence of 400 equiv
of BQ, while the other catalysts showed yields lower than

48
C. Bianchini et al. / Journal of Molecular Catalysis A: Chemical 224 (2004) 35–49
[15] C. Bianchini, A. Meli, A. Traversi, Italian Patent FI A000025
Acknowledgments
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Thanks are due to the European Community (PALLA-
DIUM project, contract no. HPRN-CT-2002-00196) and the
Russian Foundation for Basic Research (grant no. 03-03-
32471) for financial support.
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