Methoxycarbonylation of olefins catalyzed by palladium complexes bearing P,N-donor ligands

Article abstract of DOI:10.1039/b704615b

The methoxycarbonylation of alkenes catalyzed by palladium(ii) complexes with P,N-donor ligands, 2-(diphenylphosphinoamino)pyridine (Ph ₂PNHpy), 2-[(diphenylphosphino)methyl]pyridine (Ph ₂PCH₂py), and 2-(diphenylphosphino)quinoline (Ph ₂Pqn) has been investigated. The results show that the complex [PdCl(PPh₃)(Ph₂PNHpy)]Cl or an equimolar mixture of [PdCl₂(Ph₂PNHpy)] and PPh₃, in the presence of p-toluensulfonic acid (TsOH), is an efficient catalyst for this reaction. This catalytic system promotes the conversion of styrene into methyl 2-phenylpropanoate and methyl 3-phenylpropanoate with nearly complete chemoselectivity, 98% regioselectivity in the branched isomer, and high turnover frequency, even at alkene/Pd molar ratios of 1000. Best results were obtained in toluene-MeOH (3: 1) solvent. The Pd/Ph₂PNHpy catalyst is also efficient in the methoxycarbonylation of cyclohexene and 1-hexene, although with lower rates than with styrene. Related palladium complexes [PdCl(PPh ₃)L]Cl (L = Ph₂PCH₂py and Ph₂Pqn) show lower activity in the methoxycarbonylation of styrene than that of the 2-(diphenylphosphinoamino)pyridine ligand. Replacement of the last ligand by (diphenylphosphino)phenylamine (Ph₂PNHPh) or 2- (diphenylphosphinoaminomethyl)pyridine (Ph₂PNMepy) also reduces significantly the activity of the catalyst, indicating that both the presence of the pyridine fragment as well as the NH group, are required to achieve a high performing catalyst. Isotopic labeling experiments using MeOD are consistent with a hydride mechanism for the [PdCl(PPh₃)(Ph₂PNHpy)]Cl catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b704615b

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Methoxycarbonylation of olefins catalyzed by palladium complexes bearing
P,N-donor ligands†
Pedro A. Aguirre,*a Carolina A. Lagos,a Sergio A. Moya,b Ce´sar Zu´n˜iga,b Cristian Vera-Oyarce,b
Eduardo Sola,c Gabriel Perisd and J. Carles Bayo´n*e
Received 27th March 2007, Accepted 30th August 2007
First published as an Advance Article on the web 25th September 2007
DOI: 10.1039/b704615b
The methoxycarbonylation of alkenes catalyzed by palladium(II) complexes with P,N-donor ligands,
2-(diphenylphosphinoamino)pyridine (Ph2PNHpy), 2-[(diphenylphosphino)methyl]pyridine
(Ph2PCH2py), and 2-(diphenylphosphino)quinoline (Ph2Pqn) has been investigated. The results show
that the complex [PdCl(PPh3)(Ph2PNHpy)]Cl or an equimolar mixture of [PdCl2(Ph2PNHpy)] and
PPh3, in the presence of p-toluensulfonic acid (TsOH), is an efficient catalyst for this reaction. This
catalytic system promotes the conversion of styrene into methyl 2-phenylpropanoate and methyl
3-phenylpropanoate with nearly complete chemoselectivity, 98% regioselectivity in the branched
isomer, and high turnover frequency, even at alkene/Pd molar ratios of 1000. Best results were obtained
in toluene–MeOH (3 : 1) solvent. The Pd/Ph2PNHpy catalyst is also efficient in the
methoxycarbonylation of cyclohexene and 1-hexene, although with lower rates than with styrene.
Related palladium complexes [PdCl(PPh3)L]Cl (L = Ph2PCH2py and Ph2Pqn) show lower activity in
the methoxycarbonylation of styrene than that of the 2-(diphenylphosphinoamino)pyridine ligand.
Replacement of the last ligand by (diphenylphosphino)phenylamine (Ph2PNHPh) or
2-(diphenylphosphinoaminomethyl)pyridine (Ph2PNMepy) also reduces significantly the activity of the
catalyst, indicating that both the presence of the pyridine fragment as well as the NH group, are
required to achieve a high performing catalyst. Isotopic labeling experiments using MeOD are
consistent with a hydride mechanism for the [PdCl(PPh3)(Ph2PNHpy)]Cl catalyst.
required for this reaction has been also reported.6 Furthermore,
in addition to phosphines, other ligands, such as S- and N-donor
Introduction
types, have been also used in Pd catalysts for these reactions.7
Heterobidentate P,N-donor ligands represent an important
class of ligands that have been applied in various catalytic
transformations,8 but they have been scarcely used in metal
catalyzed carbonylation reactions. The use of these ligands in Rh
catalyzed hydroformylation is fuzzy because the N-donor frag-
ment can be replaced by CO under reaction conditions, thus pro-
ducing monodenate or hemilabile ligands.9 As expected, in Pd(II)
complexes the substitution of the N-donor fragment by CO is more
difficult. For this reason the P,N-bonded chelate is normally pre-
served in the carbonylation reactions catalyzed by this metal. For
instance, Pd complexes containing this type of ligand are reported
to be active in the reductive carbonylation of nitroaromatics.10 On
the other hand, although CO–alkene polymerization is normally
carried out with Pd species of P,P- or N,N-donor ligands,11 there
have been a number of reports dealing with the performance of
Pd(II) complexes containing heterobidentate P,N-donor ligands in
this reaction.12,13 The [PdCl2(Ph2PNHpy)] complex (Ph2PNHpy =
2-(diphenylphosphinoamino)pyridine) has been reported to be
active in the carbonylation of ethanol to ethyl propionate.14 Pal-
ladium complexes with 2-(diphenylphosphino)pyridine (PPh2py)
have been shown to be effective catalysts in the carbonylation of
alkynes to produce methyl methacrylate and related products.15
To the best of our knowledge, only one P,N-donor ligand
has been used in the palladium hydroesterification reaction,
producing mainly the branched ester derived from styrene.16
Here we report on the methoxycarbonylation of alkenes, namely
Transition metal catalyzed hydrocarboxylation and hydroesteri-
fication of alkenes are attractive reactions for the synthesis of
valuable products for fine chemistry.1 In particular, a series of
commercial non-steroidal anti-inflammatory agents are accessible
through the reaction of vinyl aromatics with CO and H2O or
alcohols in the presence of a Pd catalyst.2 For this process,
high regioselectivities in the branched or linear product can be
obtained, because the regioselectivity of the reaction is strongly
dependent on the catalytic system employed, as well as the reaction
conditions used.3 Palladium–phosphine catalysts favor the forma-
tion of branched esters, while diphosphine leads to linear esters,4
although both the bite angle of the chelate and electronic effects
can strongly affect the selectivity of the reaction.5 A significant
effect on the selectivity of the counterion of the acid co-catalyst
aUniversidad de Chile, Facultad de Ciencias Qu´ımicas y Farmace´uticas,
Departamento de Inorga´nica y Anal´ıtica, Casilla 233 Santiago 1, Chile.
E-mail: paguirre@ciq.uchi le.cl
bUniversidad de Santiago de Chile, Facultad de Qu´ımica y Biolog´ıa,
Departamento de Qu´ımica de los Materiales, Bernardo O’Higgins 3363,
Santiago, Chile
cDepartamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales
de Arago´n, Universidad de Zaragoza-CSIC, 50009, Zaragoza, Spain
dServei Central d’Instrumentacio´ Cient´ıfica, Universitat Jaume I, 12071,
Castello´ de la Plana, Spain
eDepartament de Qu´ımica, Universitat Auto`noma de Barcelona, 08193,
Bellaterra, Barcelona, Spain
† CCDC reference number 629175. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b704615b
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Table 1 Crystal data for [PdCl(PPh3)(Ph2PNHpy-j2P,N)]Cl·(CH2Cl2)2
styrene, cyclohexene and 1-hexene catalyzed by Pd complexes of
2-(diphenylphosphinoamino)pyridine (Ph2PNHpy), 2-[(diphe-
nylphosphino)methyl]pyridine (Ph2PCH2py), 2-(diphenylphos-
phino)quinoline (Ph2Pqn), (diphenylphosphino)phenylamine
(Ph2PNHPh) and 2-(diphenylphosphinoaminomethyl)pyridine
(Ph2PNMepy) (Fig. 1).
2a
Empirical formula
M
C37H34Cl6N2P2Pd
887.78
Triclinic
¯
P1
9.6006(7)
13.8728(10)
15.8526(11)
94.061(2)
95.885(2)
99.201(2)
2065.1(3)
293(2)
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
V/A
T/K
Z
3
˚
2
l/mm−1
0.942
Fig. 1 P,N-Donor ligands.
Reflections collected
Unique reflections, (Rint
Goodness-of-fit on F2
R1a, wR2b [I > 2r(I)]
R1a, wR2b (all data)
11899
7270, 0.0725
1.029
0.0860, 0.2285
0.1777, 0.2861
)
Results and discussion
Synthesis of ligands and metal complexes
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = |Fo| − |Fc|/ Fo. b wR2 = [ [w(Fo − Fc2)2]|/ [w(Fo2)2]]1/2
.
2
Ligands Ph2PNHpy,17 Ph2PCH2py,18 Ph2Pqn19 and Ph2PNHPh20
were prepared by reported procedures, modifications of them or
by alternative methods, as described in the experimental part.
The ligand Ph2PNMepy was not previously reported, and it was
prepared from the reaction of 2-methylaminopyridine with n-BuLi
and PPh2Cl.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 1a
Pd(1)–N(1)
Pd(1)–P(2)
Pd(1)–P(3)
2.107(9)
2.234(3)
2.287(3)
Pd(1)–Cl(2)
P(2)–N(2)
P(2)–C(1)
2.337(3)
1.676(9)
1.786(12)
The reaction of the four bidentate ligands with PdCl2, in re-
fluxing acetonitrile, afforded the neutral complexes [PdCl2L] (L =
Ph2PNHpy 1a,14,17 Ph2PCH2py 1b, Ph2Pqn 1c,21 and Ph2PNMepy
1d), Scheme 1, those of Ph2PCH2py and Ph2PNMepy being
the only ones not previously reported. When these complexes
are reacted with equimolar amounts of PPh3 in CH2Cl2, they
produce analytically pure cationic complexes [PdCl(PPh3)L]Cl
(L = Ph2PNHpy 2a, Ph2Pqn 2c, and Ph2PNMepy 2d). The reaction
proceeds readily with the complexes 1a and 1d. However, the
complex 1c forms only after 6 h refluxing in CH2Cl2. Moreover,
after exhaustive reflux in this solvent, the reaction of PPh3 with 1b
yields only a mixture of 65% of 2b, evidenced by the two signals at
53.1 and 28.1 ppm in the 31P NMR spectra, and the signal of the
unreacted neutral complex 1b at 39.8 ppm, Scheme 1.
N(1)–Pd(1)–P(2)
N(1)–Pd(1)–P(3)
P(2)–Pd(1)–P(3)
N(1)–Pd(1)–Cl(2)
82.4(3)
176.7(3)
99.59(11)
92.4(3)
P(2)–Pd(1)–Cl(2)
P(3)–Pd(1)–Cl(2)
N(2)–P(2)–C(1)
166.89(13)
86.19(12)
106.2(5)
The structure of complex 2a was determined by a X-ray
diffraction study (Fig. 2, Table 1). The bond lengths and angles
(Table 2) around the Pd centre are very similar to those of related
Pd and Pt complexes.17 The diffraction study confirmed the cis
phosphorus geometry. Significant distortions for ideal square
Scheme 1
The PPh3 substitution reaction is completely stereoselective,
since only the cis phosphorus complexes were formed, as shown by
the small JP–P (ca. 6–10 Hz) observed in the 31P NMR of the four
complexes. On the other hand, the N coordination was corrobo-
rated through the downfield chemical shift (>9.50 ppm) observed
for the ortho-H in the aromatic ring, which is characteristic of
coordinated pyridine ligands.
Fig. 2 Crystal structure of [PdCl(PPh3)(Ph2PNHpy-j2P,N)]Cl·(CH2Cl2)2
2a. Carbon atoms are drawn as spheres and solvent molecules are omitted
for clarity.
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planar geometry were evident both in N(1)–Pd(1)–P(3) 82.4(3)◦
and P(2)–Pd(1)–Cl(2) 99.59(11)◦, arising respectively from the
chelate ring restriction and the steric compression of the two
adjacent phosphino fragments. The crystal structure is dominated
by a pairing between the cation complex and the chloride anion
through a hydrogen bond between the H of the amine and the
the conversion (99%), as well as the selectivity of the reaction.
On the other hand, the addition of PPh3 to complexes 1b and
1c faintly increases the output of the reaction (entries 5 and 6),
although the presence of PPh3 significantly reduces the formation
Pd metal. In the case of complex 1a, when the PPh3/complex molar
ratio was raised, entries 7 and 8, the rate of the reaction did not
improve, while the selectivity decreases. As expected, the results
obtained with an equimolar amount of 1a and PPh3 as catalysts,
are very similar to those obtained with the pre-formed catalyst 2a,
entry 9, the differences being a shorter induction period, as well
as better chemoselectivity for the pre-formed catalyst 2a. Under
the same screening reaction conditions as used for these catalytic
systems, the classical catalyst PdCl2(PPh3)2 only produces 56%
conversion, entry 10, which indicates that the Ph2PNHpy ligand
does not behaves merely as a monodentate P-donor ligand. When
in the solvent mixture (1,2-dichloroethane : MeOH = 15 : 5) the
chlorinated solvent was replaced by toluene, the results with the
1a/PPh3 catalyst significantly improve, since total conversion was
also reached in 6 h, but with nearly complete chemoselectivity
in esters and 97% of selectivity in the branched ester 3, entry 11.
Under the same reaction conditions a mixture of Pd2(dba)3 (dba =
dibenzylideneacetone), Ph2PNHpy and PPh3 (Pd : Ph2PNHpy :
PPh3 = 1 : 1 : 1), entry 12, yields low conversion (32% in 24 h)
and poorer regioselectivity than the chloride precursor 1a. The
replacement of 1,2-dichloroethane by toluene did not improve
the catalytic properties of 1b or 1c, entries 13 and 14. Therefore,
the catalyst 2a or an equimolar mixture of 1a plus PPh3 is
a better performing catalyst (99% conversion in 6 h) than the
classical PdCl2(PPh3)2 catalytic complex (56% conversion in 24 h),
indicating a beneficial effect of the heterobidentate ligand for the
methoxycarbonylation process.
˚
chloride anion, with a H · · · Cl distance of 2.27(1) A. The distance
˚
between the amine N atom and the chloride is 3.08(1) A, with a
N–H · · · Cl angle close to 160◦. The stability of the cis phosphorus
species in these compounds is also consistent with the previously
reported X-ray structures of [PdCl(L)(Lꢀ)]Cl (L = Ph2PNHpy-
P,N; Lꢀ = Ph2PNHpy-P,17 and L = Ph2Pqn-P,N; Lꢀ = Ph2Pqn-
P22), which show cis phosphine geometry.
The reaction of [PdCl2(CH3CN)2] with Ph2PNHPh in CH2Cl2
produces the neutral complex [PdCl2(Ph2PNHPh)(CH3CN)] 1e.
NMR experiments reveal that 1e reacts with PPh3 to yield the cis
complex 2e, Scheme 2, as shown by the two slightly broad signals
at 38.2 ppm and 22.5 ppm in the 31P NMR.
Scheme 2
Catalytic results
Screening reactions were carried out in the methoxycarbonylation
of styrene, Scheme 3, at 50 bar of CO, 75 ◦C, [Pd] = 2.0 mM in 1,2-
dichloroethane, and [styrene] : TsOH : [Pd] = 400 : 10 : 1. Under
these conditions, the neutral complexes [PdCl2L] 1a–c yielded very
poor conversions and selectivities, with [PdCl2(Ph2PNHpy)] 1a
giving a slightly better performance than the other two complexes,
entries 1–3, Table 3. Furthermore, extensive decomposition of the
complex into Pd black was observed in all cases. However, the
addition of PPh3 to complex 1a, entry 4, dramatically improves
Scheme 3
Table 3 Ligands and solvent effects in the methoxycarbonylation of styrene with 1a–c catalysts.a
Entry
Catalyst
Solvent
PPh3/Pd
Conversionb (%)
Esterc (%)
3/4d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1a
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
—
—
—
1
1
1
2
3
—
—
1
14
5
5
99
28
9
97
97
4
—
—
93
84
47
87
95
98
96
99
98
94
—
95
91
75/25
—
—
1b
1c
1a
92/8
81/19
78/22
89/11
79/21
90/10
95/5
97/3
85/15
91/9
—
1b
1c
1a
1a
2a
98 (20 h)
[PdCl2(PPh3)2]
56
99 (6 h)
32
5
<5
35
1a
1/2 Pd2(dba)3+ PPh2NHPy
1
1
1
1
1b
1c
1d
1e
97/3
93/7
1
54
a Reaction conditions: 4.0 × 10−2 mmol Pd in 15 mL of solvent (DCE is 1,2-dichloroethane, PhMe is toluene) and 5 mL of methanol; styrene : TsOH :
Pd = 400 : 10 : 1; P(CO) = 50 bar; T = 75 ◦C. b % of styrene converted after 24 h reaction, except when indicated. c % of esters (3 and 4) of the converted
substrate; (1-methoxyethyl)benzene was the only by-product. d Molar ratio between branched and linear ester.
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Table 4 Temperature and substrate concentration effects in the methoxycarbonylation of styrene with 1a catalysta
T(◦C)
Conv.(%)b
tof (h−1
)
Ester(%)d
3/4e
c
Entry
[substrate]/1a
Solvent
17
18
4
19
20
11
21
22
400
400
400
600
800
400
1000
1000
DCE
DCE
DCE
DCE
DCE
PhMe
PhMe
PhMe
60
70
75
75
75
75
75
90
61
74
99
71
63
99 (6 h)
99
10
12
17
18
21
67
42
41
98
93
93
92
93
99
98
98
94/6
94/6
92/8
93/7
94/6
97/3
95/5
96/4
99
a Reaction conditions: 4.0 × 10−2 mmol of 1a and 4.0 × 10−2 mmol PPh3 in 15 mL of solvent (DCE is 1,2-dichloroethane, PhMe is toluene) and 5 mL
of methanol; TsOH : Pd = 10 : 1; P(CO) = 50 bar. b % of styrene converted after 24 h reaction, except when indicated. c average turnover frequency at
the indicated conversion. d % of esters (3 and 4) of the converted substrate; (1-methoxyethyl)-benzene was the only by-product. e Molar ratio between
branched an linear ester.
Table 4 shows the effect of the temperature and substrate
concentration on the performance of the Pd/Ph2PNHpy catalyst.
When 1,2-dichloroethane/MeOH (3/1) was used as solvent the
reaction rate increases with temperature until 75 ◦C, entries 17
and 18 and 4. At higher temperatures, catalyst decomposition was
observed and the conversion decreases. On the other hand, the
reaction rate increases more than three times when the chlorinated
solvent was replaced by toluene, entry 11. Consistently, in this
solvent, 99% conversion was achieved in 24 h at 75 ◦C and
styrene/Pd molar ratio equal to 1000, entry 21. In these conditions,
when the temperature was raised to 90 ◦C, similar conversion
was achieved, but a massive decomposition of the catalyst was
observed at the end of the reaction, entry 22.
The complex 1a plus PPh3 was also assayed as catalyst for the
methoxycarbonylation of cyclohexene and 1-hexene, Scheme 4.
The results are collected in Table 5. Cyclohexene is converted into
methyl cyclohexanecarboxylate 6 with complete chemoselectivity,
although with lower reaction rates than styrene, entries 23–
25. 1-Hexene is converted into methyl 2-methylhexanoate 8 and
methyl heptanoate 9, with complete chemoselectivity, but with
very poor regioselectivity, entries 26–28, but with somewhat better
conversions than cyclohexene. Also, with these substrates, severe
decomposition of the catalyst was observed when the reaction was
run at 90 ◦C.
Catalytic species
In order to investigate which is the crucial structural characteristic
that makes catalyst 1a remarkably better at performing than 1b
and 1c in the methoxycarbonylation of styrene, complexes 1d and
1e were prepared and assayed in this reaction. In complex 1d,
the hydrogen of the amino group of ligand 1a was replaced by
a methyl group. This produces a very significant decrease in the
rate of the reaction (35% conversion in 24 h, entry 15 in Table 3,
versus 99% in 6 h for 1a, entry 11). On the other hand, complex 1e,
containing the Ph2PNHPh ligand, very similar to Ph2PNHpy in
terms of P-donor properties, but without the pyridine fragment,
produces also a lower conversion (54% in 24 h, entry 16) than
1a. These results suggest that in the Ph2PNHpy, both the pyridine
fragments, as well as the NH group, are crucial for its performance
in the catalytic methoxycarbonylation process.
A study of the species formed by 1a in the presence of PPh3
under variable reaction conditions was undertaken, in order to
gain some insight about the role of the heterobidentate ligand in
the catalytic process. An equimolar mixture of complex 1a and
PPh3 in toluene–MeOH in the presence of an excess of TsOH
(TsOH : Pd = 10 : 1) forms 2a as single species, as evidenced by
the 31P NMR spectrum. This means that pyridine coordination
is robust enough to be preserved in this acidic media produced
by TsOH.◦The catalytic mixture was pressurized with 50 bar of
CO at 70 C for 4 h and then its 31P NMR was registered under
Scheme 4
Table 5 Methoxycarbonylation of cyclohexene 5 and 1-hexene 7 with 1a as catalysta
Entry
Substrate
P
CO/bar
T/◦C
Conversionb (%)
Esterc (%)
9/8d
23
24
25
26
27
28
5
5
5
7
7
7
50
50
70
50
50
70
75
90
75
75
90
75
19
25
21
43
85 (12 h)
57
>99
>99
>99
>99
>98
>98
55/45
57/43
56/44
a Reaction conditions: 4.0 × 10−2 mmol 1a and 4.0 × 10−2 mmol PPh3 in 15 mL of toluene and 5 mL of methanol; substrate : TsOH : Pd = 400 : 10 : 1.
b % of substrate converted after 24 h reaction. c % of esters (6 for substrate 5 and 9 for substrate 7) of the converted substrate. d Molar ratio between linear
and branched ester for the reaction with substrate 7.
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Scheme 5
1 bar CO atmosphere. The spectrum shows that complex 2a is
still the major component of the reaction mixture. In addition, a
new singlet shows at 21.8 ppm, that can be tentatively assigned
to an non-coordinated P atom of the Ph2PNHpy ligand, as well
as two very minor products that appear as two doublets (113.8
and 82.2 ppm, Jp–p = 25 Hz; 92.9 and 82.1 ppm, Jp–p = 19 Hz).
In summary, the catalytic results seem to indicate that the role of
the PPh3 is to stabilize the Pd(0) species, since in its absence the
formation of Pd black was observed, but it is not clear if it is this
ligand or the phosphorus of the bidentate ligand that produces the
second vacancy required for progression of the catalytic cycle.
Two possible pathways have been proposed for the methoxy-
carbonylation of alkenes catalyzed by Pd phosphine and diphos-
phine complexes, namely the hydride and the carbomethoxy
mechanisms. Isotopic labeling experiments using CH3OD were
undertaken in order to elucidate which proceeds with catalyst
2a.23 Analysis of the reaction products by mass spectrometry and
hydride mechanism with a reversible H migration, as shown in
Scheme 5. Furthermore, preliminary kinetic experiments show
that the reaction is first order in the substrate. The results suggest
that the rate determining step is after the formation of the alkyl
complex. This seems to discard an intramolecular reaction of the
Pd(0) complex with the NH group of the ligand to form the
active Pd(II) hydride species, that would explain the remarkable
catalytic activity of complex 2a when compared with the rest
of the catalysts here reported. A more plausible explanation for
the activity of 2a is that the NH group assists the nucleophilic
attack of the MeOH, similarly to the proposed mechanism for
methoxycarbonylation of alkynes catalyzed by Pd complexes of
2-pyridylphosphine.15b Since the basicity of the ligand Ph2PNHpy
is not expected to be too different from that of Ph2PNMepy, the
better performance of catalyst 2a with respect to 2d could be
associated with the formation of a hydrogen bond between the
NH group and methanol.
1
2H and H NMR show that PhCH(COOMe)CH2D is the major
isotopomer of the branched ester 3 and its proportions remains
constant (ca. 65%) for most of the reaction period. The presence
of this isotopomer is compatible with either the hydride or the
carbomethoxy mechanisms. However, both the proportion of the
d2 isotopomer of the ester PhCH(COOMe)CHD2, as well as d1
and d2 styrene labeled at the terminal carbon, increase along the
reaction, Table 6. The presence of labeled unreacted substrate
and the d2 ester arising from it are interpreted in terms of a
Experimental
All manipulations were carried out under nitrogen using stan-
dard Schlenk techniques. Organic solvents and substrates were
purified by distillation over standard agents and deoxygenated
prior to use. The ligands Ph2PNHpy,17 Ph2PCH2py,18 Ph2Pqn19
and Ph2PNHPh20 were synthesized by reported procedures or
modifications of them. All other chemicals were commercial
products and were used without further purification. NMR data
were obtained on Bruker 500, 360 or 250 MHz spectrometers.
GC analysis was performed on a Hewlett-Packard Agilent 5890 N
chromatograph with a flame ionization detector and equipped
with an HP-5 column and GC-MS were recorded on a HP-
G1800A instrument.
Table 6 Isotopomer distribution in methyl 2-phenylpropanoate and un-
reacted styrene at two different conversions in the methoxycarbonylation
of styrene with CO and CH3OD catalyzed by 2aa
37% conversion
61% conversion
Ester
Styrene
Ester
Styrene
Ph2PNHpy
d0
d1
d2
33
63
4
89
10
1
29
63
8
73
20
7
A mixture of 2-aminopyridine (1.00 g, 10.6 mmol) and triethy-
lamine (1.50 mL, 10.6 mmol) was dissolved in THF (20 mL). The
solution was cooled at 0 ◦C and Ph2PCl (2.0 mL, 10.6 mmol) was
slowly added. The reaction was allowed to reach room temperature
and further stirred for 6 h. After this time, the ammonium salt
a Reaction conditions: 4.0 × 10−2 mmol of 2a in 15 mL of toluene and
5 mL of CH3OD; styrene : TsOH : Pd = 1500 : 10 : 1; P(CO) = 50 bar;
T = 80 ◦C.
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was separated by filtration and the solvent was removed under
vacuum. The oily residue obtained was dissolved in CHCl3 and
precipitated with Et2O, producing 2.8 g of white powder (96%
yield). The spectroscopic data are consistent with those reported
in the literature.17
[PdCl2(Ph2PNHpy-j2P,N)] 1a
The product was prepared through a more direct route than
that described in the literature.14,17 A mixture of PdCl2 (200 mg,
1.12 mmol) and Ph2PNHpy (315 mg, 1.12 mmol) were dissolved in
acetonitrile (30 mL) and heated to reflux for 3 h, producing a green
precipitate. The suspension was cooled, and the solid was filtered
off, rinsed with acetonitrile and diethyl ether, and dried under
vacuum, producing 450 mg (87% yield) of a green complex. NMR
and analytical data were consistent with those already reported.17
Ph2PCH2py
This ligand was prepared following the procedure previously
reported.18 1H-NMR (CDCl3; d, ppm): 8.40 (H6 py, dd, 4.9 Hz,
0.7 Hz, 1H), 7.77 (o-H Ph and H3 py, m, 5H), 7.46 (m-H ph, p-H
Ph and H4 py, m, 7H), 7.08 (H5 py, t, 6.9 Hz, 1H), 3.96 (PCH2, d,
JP–H = 16 Hz, 2H). 31P-NMR (CDCl3; d, ppm): −12.7 (s, 1P)
[PdCl2(Ph2PCH2py-j2P,N)] 1b
This complex was synthesized from 100 mg of PdCl2, as described
for 1a, yielding 225 mg (89% yield) of a pale green solid. Found
(calc. for C18H16Cl2NPPd): C 47.55 (48.01), H 3.54(3.67), N 3.08
Ph2Pqn
1
(2.95) %. H-NMR (CD2Cl2; d, ppm): 9.77 (H6 py, dd, 5.89 Hz,
A solution of 8-chloroquinoline (0.88 g, 5 mmol) in THF (10 mL)
was added dropwise to a solution of PPh2Li (0.96, 5 mmol) in
0.9 Hz, 1H), 7.91(o-H Ph and H3 py, m, 5H), 7.59 ( p-H Ph, m-H
Ph and H4 py, m, 7H), 7.43 ( H5 py, t, 6.0 Hz, 1H), 4.17 (PCH2,
d, JP–H= 15 Hz). 31P-NMR (CD2Cl2; d, ppm): 39.8 (s).
◦
THF at −78 C. The mixture was warmed to room temperature
and stirred for 2 h. After this time, the mixture was refluxed for
12 h. The solvent was then removed and 25 mL of a water–diethyl
ether (5 : 1) mixture was added. The suspension was filtered and
rinsed with a few mL of a mixture of water–diethyl ether 5 : 1
and dried under vacuum producing 1.1 g (70% yield) of a white
powered solid. 1H-NMR (CDCl3; d, ppm): 8.89 (H2 qn, dd, 4.3 Hz,
1.8 Hz, 1H), 8.18 (H4 qn, dd, 8.1 Hz, 1.8 Hz, 1H), 7.83 (H5 qn, d,
7.1 Hz, 1H) ), 7.45 ( H3 qn, m, 1H), 7.40 (H6 qn, m, 1H), 7.33 (m,
o-H Ph, m-H Ph, 10H), 7,15( H7 qn, m, 1H). 31P-NMR (CDCl3;
d, ppm): −17.0 (s, 1P).
[PdCl2(Ph2Pqn-j2P,N)] 1c
This complex was synthesized as described for 1a, from 100 mg
of PdCl2, producing 205 mg of a green product (75% yield).
Found (calc. for C21H16Cl2NPPd): C 51.40 (50.95), H 3.28 (3.08),
1
N 2.85(2.57) %. H-NMR (CDCl3; d, ppm): 10.45 ( H2 qn, dd,
5.3 Hz, 1.25 Hz, 1H), 8.54 (H4 qn, dd, 8.05 Hz, 1.25 Hz, 1H) 8.20
( H5 qn, dd, 9.6 Hz, 1,25 Hz, 1H), 8,01 ( H6 qn, dd, 7.1 Hz, 1.25 Hz,
1H), 7.90 ( o-PPh3, H3 qn, m, 5H), 7.71 (H7 qn, dd, 8.1 Hz, JP–H
=
5.3 Hz, 1H ), 7.52 (p-H Ph, m-H Ph, m, 6H ). 31P-NMR (CDCl3;
d, ppm): 42.4 (s, 1P).
PPh2NMepy
A solution of 1.0 g of 2-(methylamino)pyridine (9.3 mmol) in
20 mL of diethyl ether was cooled to −70 ◦C, and then n-BuLi
(9.8 mmol, 7.0 mL of a solution 1.4 M in hexane) was slowly
added. The mixture was stirred for 1 h and then PPh2Cl (2.1 g,
1.7 mL) was added dropwise. The final solution was allowed to
reach room temperature and it was further stirred overnight. 3 mL
of water were added and then the organic layer was extracted with
diethyl ether (2 ×10 mL). The diethyl ether solution was dried
over Na2SO4 for 30 min. After this time the solid was removed
by filtration and the solvent was evaporated on a vacuum line to
produce 2.7 g (65% yield) of an orange oil that becomes a pale
[PdCl2(Ph2PNMepy-j2P,N)] 1d
This complex was synthesized as described for 1a, from 100 mg of
PdCl2, producing 222 mg of a green product (85% yield). Found
(calc. for C18H17Cl2N2PPd): C 46.03 (46.20), H 3.64 (3.71), N
5.96(5.90) %. 1H-NMR (CDCl3; d, ppm): 9.65 (H6 py, d, 6.1 Hz,
1H), 7.9 (o-H Ph, H4 py, 5H), 7.69 (m-H Ph, p-H Ph, m, 6H), 7.50
(H5 py, t, 6.0 Hz, 1H), 6.81 ( H3 py, d, 8.0 Hz,3H), 2.98 (CH3–, d,
5.9 Hz, 3H). 31P-NMR (CDCl3; d, ppm): 95.7 (s, 1P).
1
yellow crystalline material under vacuum. H-NMR (CDCl3; d,
[PdCl2(Ph2PNHPh)(CH3CN)] 1e
ppm): 8.29 (H6 py, dd, 5.5 Hz, 2.8 Hz, 1H), 7.47 (m, o-H Ph, m-H
Ph, H3 py and H5 py, 12H), 6.77 (H4 py, m, 1H), 2.97 (CH3–, d,
1.6 Hz, 3H ). 31P NMR (CDCl3; d, ppm): 48.7 (s, 1P).
To a solution of 200 mg of [PdCl2(CH3CN)2] (0.78 mmol) in
CH2Cl2 (20 mL), 108 mg (0.39 mmol) of PPh2PNHPh in CH2Cl2
(5 mL) were added. The mixture was stirred for 5 h at room
temperature. The resulting yellow solution was concentrated and
precipitated with diethyl ether. The precipitate was collected by
filtration and rinsed with diethyl ether, producing 278 mg of a
bright yellow solid (72% yield). Found (calc. for C20H19Cl2N2PPd):
C 48.46 (47.95), H 3.86 (3.79), N 5.65(5.60) %. 1H-NMR (CDCl3;
d, ppm): 7.92 (o-H Ph, m, 4H), 7.46 (m-H Ph, p-H Ph, m, 6H),
6.99 (H3 aniline and H5 aniline, t, 7.5 Hz, 2H), 6.92 (N–H, d,
7.9 Hz, 1H), 6.77 ( H4 aniline, t, 7.3 Hz, 1H), 6.59 (H2 aniline and
H6 aniline, d, 8.0 Hz, 2H), 2.31 (s, CH3CN). 31P-NMR (CDCl3; d,
ppm): 35.0 (s, 1P).
PhPNHPh
This ligand was prepared from 360 mg of aniline, following the
same procedure used for PPh2PNHPy. The oil obtained was
dissolved in CHCl3 and precipitated with n-hexane, producing
1.5 g of white solid (75% yield). 1H-NMR (CDCl3; d, ppm): 7.47
(o-H Ph, m, 4H), 7.40 (m-H Ph, p-H Ph, m, 6H ), 7.22 (H3 aniline
and H5 aniline, t, 7.3 Hz, 2H), 7.04 (H2 aniline and H6 aniline,
m, 2H), 6.84 ( t, H4 aniline, 7.3 Hz, 1H), 4.41 (N–H, d, 8.0 Hz,).
31P-NMR (CDCl3; d, ppm): 25.9 (s, 1P).
5424 | Dalton Trans., 2007, 5419–5426
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[PdCl(Ph2PNHpy-j2P,N)(PPh3)]Cl 2a
methods and refined by the full-matrix method on the basis of F2
using the SHELXTL software package.26 All non-hydrogen atoms
were refined anisotropically, whereas the positions of the hydrogen
atoms were generated geometrically. Chlorine atoms of each one
of the two solvating molecules of CH2Cl2 in the crystal were
disordered among three of the four ideal tetrahedral positions.
Therefore site occupancies of 2/3 were assigned to each one of
the chlorine crystallographic positions. Hydrogen atoms of the
CH2Cl2 molecules were not introduced in this disorder refinement.
A mixture of 1a (200 mg, 0.45 mmol) and PPh3 (115 mg, 045 mmol)
were stirred at room temperature for 1 h in dichloromethane
(30 mL). The solution obtained was concentrated and precipitated
with diethyl ether. The product was filtered off and rinsed with di-
ethyl ether (2 × 5 mL), and dried under vacuum, producing 315 mg
of white powder (98% yield). Found (calc. for C35H30Cl2N2P2Pd):
C 58.55 (58.28), H 4.21 (4.12), N 3.90 (3.81) %. 1H-NMR (CDCl3;
d, ppm): 11.95 ( N–H, s, 1H), 9.15 (H6 py, dd, 5.3 Hz, 4.0 Hz, 1H),
7.84 (H3 py, d, 5.2 Hz, 1H), 7.75–7.21( m-H Ph, o-H Ph, H4 py,
26H), 6.89 (H5 py, dd, 9.0 Hz, 6.5 Hz, 1H), 31P-NMR (CDCl3; d,
ppm): 85.1 (d, JP–P = 6 Hz ), 26.1 (d, JP–P = 6 Hz).
Catalytic carbonylation
The catalytic methoxycarbonylation reactions were carried out
in a glass-lined stainless steel autoclave fitted with temperature
control unit and a sampling valve. In a typical experiment, 1a
(18.2 mg, 4.0 × 10−2 mmol), PPh3 (10.5 mg, 4.0 × 10−2) and TsOH
(69 mg, 4.0 × 10−1 mmol ) were dissolved in a mixture of 1,2-
dichloroethane or toluene (15 mL) and methanol or d1-methanol
(5 mL). The solution was introduced to the reactor, which was
purged three times with CO, and then charged to the required
pressure and heated to the desired temperature. Samples of the
reaction mixture were periodically extracted to be analyzed by
GC and the pressure was adjusted if necessary.
[PdCl(Ph2PCH2py-j2P,N)(PPh3)]Cl 2b
A mixture of 1b (200 mg, 0.44 mmol) and PPh3 (115 mg,
0.44 mmol) was dissolved in dichloromethane (30 mL), and heated
to reflux for 6 h. After this time, a mixture of complexes 1b and
2b was obtained, as shown by NMR spectroscopy. The product
was purified by recrystallization in chloroform–diethyl ether,
producing 205 mg (65% yield) of a yellow product. Found (calc.
for C36H31Cl2NP2Pd): C 60.31(59.90), H 4.36(4.01), N 1.95(1.87)
%. 1H-NMR (CDCl3) (d (ppm)): 9.48 (H6 Py, dd, 5.70 Hz, 1.9 Hz,
1H), 8.34 (H3 py, d, 7.7 Hz, 1H), 8.10–7.15 ( o-H Ph, m-H Ph,
p-H Ph, H4 py and H5 py, 27H), 5.00 (PCH2, d, JP–H = 15 Hz).
31P-NMR (CDCl3; d, ppm): 50.8 (s br), 28.3 (s br).
The kinetic order of the substrate was determine by the
classical logarithmic plot of the initial rate versus the substrate
concentration.
Isotopic labeling experiments
[PdCl(Ph2Pqn-j2P,N)(PPh3)]Cl 2c
Samples of the reaction of styrene with CO and MeOD, catalyzed
by 2a and TsOH in toluene were analyzed by GC-MS. The profile
of the relevant peaks, M to M + 4 for methyl 2-phenylpropanoate
and M − 4 to M + 3 for styrene, were simulated considering
only the presence of d0, d1 and d2 isotopomers for each species,
since this model gives a good fit to the experimental data. For the
simulation, it was assumed that there were not isotope effects on
fragmentation. 2H NMR was used to locate the deuterium atoms
in the mixture of isotopomers, as well to corroborate the relative
deuterium content of methyl 2-phenylpropanoate and styrene.
The procedure described above was used to prepare 2c from 200 mg
of 1c, producing 300 mg (98% yield) of an orange solid. Found
(calc. for C39H31Cl2NP2Pd): C 62.21 (61.17), H 4.14 (3.97), N 1.86
1
(1.57) %. H-NMR: (CDCl3; d, ppm): 10.21 (H2 qn, dd, 5.3 Hz,
1.25 Hz, 1H), 9.02 (H4 qn, dd, 8.05 Hz, 1.25 Hz, 1H), 8.52 ( H5
qn, d, 9.6 Hz, 1H), 7.96–7.16 (o-H Ph, m-H Ph, p-H Ph H3 qn,
H6 qn and H7 qn, d, 28H). 31P-NMR (CDCl3; d, ppm): 47.3 (d,
JP–P = 9 Hz), 26.4 (d, JP–P = 9 Hz).
[PdCl2(Ph2PN(CH3)py-j2P,N)(PPh3)]Cl 2d
Conclusions
The same procedure described for 2a was used to prepare 2d. Thus,
from 200 mg of 1d, 150 mg (89% yield) of a yellow solid were
obtained. Found (calc. for C36H32Cl2N2P2Pd): C 59.08 (59.27), H
4.41 (4.51), N 3.83(3.91) %. 1H-NMR (CDCl3; d, ppm): 10.65 (H6
py, d, 5.7 Hz, 1H), 7.90–7.67 (o-H Ph, m-H Ph, p-H Ph and H4 py,
m, 26H), 7.60 (H5 py, t, 6.0 Hz, 1H), 6.91 (H3 py, d, 7.0 Hz,1H),
3.05 (CH3–, d, 6.1 Hz). 31P-NMR (CDCl3; d ppm) 101.7 (s, br),
26.9 (s, br).
The complex 1a, [PdCl2(Ph2PNHpy-j2P,N)] (Ph2PNHpy = 2-
aminophosphinopyridine), in the presence of a stochiometric
amount of PPh3, or the preformed catalyst 2a, [PdCl(Ph2PNHpy-
j2P,N)(PPh3)]Cl are reasonably active catalysts in the methoxy-
carbonylation of styrene, although are less active for 1-hexene
and cyclohexene; under identical conditions, related complexes
[PdCl2(L-j2P,N)], L = 2-[(diphenylphosphino)methyl]pyridine,
2-(diphenylphosphino)quinoline (Ph2Pqn), (diphenylphosphino)-
phenylamine (Ph2PNHPh) and 2-(diphenylphosphinoamino-
methyl)pyridine (Ph2PNMepy) show very poor conversions in this
reaction. The use of (diphenylphosphino)phenylamine instead of
the heterobidentate ligands renders also very low conversions,
indicating that, in addition to the P-donor fragment, both the
presence of the coordinated pyridine, as well as the N–H group
are required for the good performance of the catalyst. The role of
these two fragments of the ligand in the catalytic process is not fully
understood, but some plausible explanations are proposed, based
on a preliminary kinetic study and isotopic labelling experiments.
X-Ray data collection and structure refinement
Crystals of 2a suitable for X-ray studies were grown by slow
diffusion of diethyl ether into a CH2Cl2 solution. A colorless
prismatic crystal was mounted on the tip of a glass fiber with
the use of epoxy cement. The data collection was carried out at
room temperature with a Bruker SMART CCD diffractometer
˚
using Mo-Ka radiation (k = 0.71073 A), with a frame width of
0.3◦ in x, and a counting time of 20 s per frame. The diffraction
frames were integrated using the SAINT package and corrected for
absorption with SADABS.24,25 The structure was solved by direct
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Dalton Trans., 2007, 5419–5426 | 5425
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The last experiments are consistent with a hydride type mechanism
for the catalyst 2a. Further work is currently in progress in order
to clarify the mechanism of this new type of catalyst.
9 (a) A. Aghmiz, A. M. Masdeu-Bulto, C. Claver and D. J. Sinou, J. Mol.
Catal. A: Chem., 2002, 184, 111; (b) J. Andrieu, J. M. Camus, P. Richard,
R. Poli, L. Gonsalvi, F. Vizza and M. Peruzzini, Eur. J. Inorg. Chem.,
2006, 51, 61; (c) Review article: G. Chelucci, G. Orru and G. A. Pinna,
Tetrahedron, 2003, 59, 9471.
10 P. Wehman, H. M. A. van Donge, A. Hagos, P. C. J. Kamer and
P. W. N. M. van Leeuwen, J. Organomet. Chem., 1997, 535, 187.
11 E. Drent and P. H. M. Budzeelaar, Chem. Rev., 1996, 96, 663.
12 A. D. Burrows, M. F. Mahon and M. Varrone, Dalton Trans., 2003,
4718.
13 (a) G. P. C. M. Dekker, A. Buijs, C. J. Elsevier, K. Vrieze, P. W. N. M.
van Leeuwen, W. J. J. Smeets, A. L. Spek, Y. F. Wang and C. H. Stam,
Organometallics, 1992, 11, 1937; (b) K. R. Reddy, W. W. Tsai, K.
Surekha, G. H. Lee, S. M. Peng, J. T. Chen and S. T. Liu, J. Chem.
Soc., Dalton Trans., 2002, 1776; (c) K. R. Reddy, C. L. Chen, Y. H. Liu,
S. M. Peng, J. T. Chen and S. T. Liu, Organometallics, 1999, 18, 2574;
(d) K. R. Reddy, K. Surekha, G. H. Lee, S. M. Peng, J. T. Chen and S. T.
Liu, Organometallics, 2001, 20, 1292; (e) P. Braunstein, M. D. Fryzuk,
M. Le Dall, F. Naud, S. J. Rettig and F. Speiser, J. Chem. Soc., Dalton
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Acknowledgements
The authors thank Fondecyt-Chile (Project N◦ 1050168), Uni-
versidad de Chile (Project CSIC19/04) and Spanish DGI
(project CTQ2005-09187-C02-01 for their financial support,
Cepedeq/Universidad de Chile for analytical support, and the
Spanish DGI and CYTED (Project V9) for bursaries for PA and
CZ.
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5426 | Dalton Trans., 2007, 5419–5426
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The Royal Society of Chemistry 2007
©