Bis-alkoxycarbonylation of styrene by pyridinimine palladium catalysts

Article abstract of DOI:10.1039/b108804c

Pyridinimine-modified Pd(II) complexes of general formulae (N-N′)Pd(Y)₂ catalyze the methoxycarbonylation of styrene to give dimethyl phenylsuccinate as the largely major product [N-N′ = py-2-C(R) = N(2,6-R′C₆H₃), R = H, Me; R′ = Me, i-Pr; 6-Mepy-2-C(H)=N[2,6-(i-Pr)₂C₆H₃]; py-2-C(H)=N(C₆H₅); Y = acetate, trifluoroacetate]. The influence of various catalytic parameters on the overall conversion of styrene to carbonylated products and on the product selectivity has been studied by systematically varying the type of palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. By an appropriate choice of the structure of the pyridinimine ligand and of the reaction parameters, turn-over numbers as high as 96 and selectivities in dimethyl phenylsuccinate as high as 98% were obtained. In particular, the overall conversion of styrene is controlled by the steric properties of the alkyl substituents on the imine aryl group, while the nature of the substituent (H or Me) on the imine carbon influences the selectivity. The addition of 2 equivalents of TsOH to the catalytic mixtures generally increased the styrene conversion but lowered the selectivity in dimethyl phenylsuccinate due to greater production of methyl 3,6-diphenyl-4-oxohexanoate. Further additions of TsOH (up to 6 equivalents) resulted in better selectivities and lower conversions for all precursors.

Full text of DOI:10.1039/b108804c

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Bis-alkoxycarbonylation of styrene by pyridinimine palladium
catalysts
Claudio Bianchini,* Hon Man Lee, Giuseppe Mantovani, Andrea Meli*
and Werner Oberhauser
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di
Coordinazione (ISSECC) - CNR, Via J. Nardi 39, I-50132 Firenze, Italy.
E-mail: bianchin@fi.cnr.it; Fax: þ39 055247 8366
Received (in Strasbourg, France) 26th September 2001, Accepted 23rd November 2001
First published as an Advance Article on the web 8th March 2002
Pyridinimine-modified Pd(II) complexes of general formulae (N-N0)Pd(Y)2 catalyze the methoxycarbonylation
of styrene to give dimethyl phenylsuccinate as the largely major product [N-N0 ¼ py-2-C(R)=N(2,6-R0C6H3),
R ¼ H, Me; R0 ¼ Me, i-Pr; 6-Mepy-2-C(H)=N[2,6-(i-Pr)2C6H3]; py-2-C(H)=N(C6H5); Y ¼ acetate,
trifluoroacetate]. The influence of various catalytic parameters on the overall conversion of styrene to
carbonylated products and on the product selectivity has been studied by systematically varying the type of
palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid
(p-toluenesulfonic acid), and the CO pressure. By an appropriate choice of the structure of the pyridinimine
ligand and of the reaction parameters, turn-over numbers as high as 96 and selectivities in dimethyl
phenylsuccinate as high as 98% were obtained. In particular, the overall conversion of styrene is controlled by
the steric properties of the alkyl substituents on the imine aryl group, while the nature of the substituent
(H or Me) on the imine carbon influences the selectivity. The addition of 2 equivalents of TsOH to the catalytic
mixtures generally increased the styrene conversion but lowered the selectivity in dimethyl phenylsuccinate due
to greater production of methyl 3,6-diphenyl-4-oxohexanoate. Further additions of TsOH (up to 6 equivalents)
resulted in better selectivities and lower conversions for all precursors.
The alkoxycarbonylation of olefins is a metal-catalyzed
reaction that transforms relatively cheap feedstocks (CO, ole-
fins, alcohols) into esters, diesters and=or polyketones.1,2
Effective and selective catalyst precursors for this industrially
relevant reaction generally require the use of palladium(II) salts
in conjunction with either tertiary phosphorus or nitrogen
ligands.
In the case of styrene, the carbonylation in methanol in the
presence of palladium initiators and of an organic oxidant,
required to convert Pd0 to PdII, may yield different types of
products spanning from alternating polyketones, methyl cin-
namate, phenylpropanoates, dimethyl phenylsuccinate, to
dimethyl 2-oxo-phenylglutarates (Scheme 1).2–5 The chemo-
selectivity is essentially driven by the nature of the supporting
ligand.2–5 Dinitrogen ligands, particularly those with rigid
carbon backbones, form selective catalysts for the alternating
copolymerization leading to polyketones,2 while diphosphines
generate effective systems for the single, double or triple car-
bonylation of styrene yielding esters, diesters or oxo-gluta-
rates.3,4 Miscellaneous catalytic systems containing (chiral)
ligands with oxygen, sulfur, phosphorus and nitrogen donor
atoms may be appropriately tuned so as to give a wealth of
products with low to high chemo-, regio- and stereo-
selectivities.2,4,6
To the best of our knowledge, no palladium catalyst stabi-
lized by a chelating dinitrogen ligand has ever been reported to
catalyze the methoxycarbonylation of styrene, yielding phenyl
cinnamate, dimethyl phenylsuccinate or other diesters. There-
fore, we were truly surprised to discover that palladium(II)
precursors with certain pyridinimine ligands (Scheme 2) were
Scheme 1 Products obtainable from the methoxycarbonylation of styrene by palladium catalysis.
DOI: 10.1039/b108804c
New J. Chem., 2002, 26, 387–397
387
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performed on a Shimadzu QP 5000 apparatus equipped with a
column identical to that used for GC analysis.
Syntheses of ligands
The pyridinimine ligands were synthesized following pre-
viously reported procedures that have been modified when
necessary to improve the yield.12–16
py-2-C(H)=N(2,6-Me2C6H3), 1.13 A mixture of 2-pyridine-
carboxaldeyde (1.7 g, 15.9 mmol) and 2,6-dimethylaniline (1.9
g, 15.9 mmol) in 20 ml of MeOH was heated at reflux tem-
perature. After 4 h the solvent was removed under vacuum to
give an yellow oil that was used without further purification.
Yield: 60%. Anal. calcd for C14H14N2 : C, 79.97; H, 6.71; N,
13.32; found: C, 79.85; H, 6.75; N, 13.25%. IR (neat, cmꢀ1):
Scheme 2 Synthesis, structure and numbering scheme of the ligands
employed in this work.
1
able to catalyze the selective formation of dimethyl phenyl-
succinate upon methoxycarbonylation of styrene. We expected
to obtain alternating polyketones, in fact.2,7
n(C=N) 1645 (s). H NMR (CDCl3): d 2.18 (s, 6H, Me2C6H3),
3
6.95–7.13 (m, 3H, m- and p-Ar), 7.42 [ddd, J(HH) ¼ 7.7, 4.9,
4J(HH) ¼ 1.4 Hz, 1H, H5-py], 7.85 [td, 3J(HH) ¼ 7.7,
4J(HH) ¼ 1.8 Hz, 1H, H4-py], 8.30 [ddd, 3J(HH) ¼ 7.7,
4J(HH) ¼ 1.4, 5J(HH) ¼ 1.0 Hz, 1H, H3-py], 8.36 [s, 1H,
C(H)=N], 8.73 [ddd, 3J(HH) ¼ 4.9, 4J(HH) ¼ 1.8, 5J(HH) ¼ 1.0
Hz, 1H, H6-py].
Pyridinimines8–21 constitute a numerous family of chelating
dinitrogen ligands whose coordination chemistry closely
resembles that of a-diimine ligands8,9,21 and, like the latter,
have been used in palladium and nickel catalysis, particularly
in olefin oligomerization and polymerization,9,15,16,21 co-
oligomerization of ethylene with alkyl acrylates,9,15,21 and
CO=styrene copolymerization.7 Due to the formation of stable
metal complexes and the ease of functionalization, pyr-
idinimine ligands have been also employed to study funda-
mental processes such as migratory insertions in Pd(alkyl)(CO)
and Pd(acyl)(olefin) complexes.18,19
py-2-C(Me)=N(2,6-Me2C6H3), 2. A mixture of 2.0 g of 2-
acetylpyridine (16.5 mmol) and 8.0 g of 2,6-dimethylaniline
(66.0 mmol) was heated at 100 ꢁC. After 4 days, the resulting
residue was eluted on a neutral alumina column with ethyl
acetate–n-hexane (5 : 95) as eluent. Evaporation of the solvents
under reduced pressure gave a yellow oil. Yield: 42%. Anal.
calcd for C15H16N2: C, 80.32; H, 7.19; N, 12.49; found: C,
80.43; H, 7.20; N, 12.41%. IR (neat, cmꢀ1): n(C=N) 1644 (s).
1H NMR (CDCl3): d 2.05 (s, 6H, Me2C6H3), 2.20 [s, 3H,
C(Me)=N], 6.91–7.10 (m, 3H, m- and p-Ar), 7.40 [ddd,
3J(HH) ¼ 7.7, 4.9, 4J(HH) ¼ 1.3 Hz, 1H, H5-py], 7.83 [td,
3J(HH) ¼ 7.7, 4J(HH) ¼ 1.8 Hz, 1H, H4-py], 8.30 [ddd,
3J(HH) ¼ 7.7, 4J(HH) ¼ 1.3, 5J(HH) ¼ 0.9 Hz, 1H, H3-py],
In this work, we describe the synthesis of new pyridinimines
(N-N0) and of several palladium(II) complexes of general for-
mulae (N-N0)Pd(Y)2 (Y ¼ acetate, trifluoroacetate), which
have been employed as catalyst precursors to bring about the
oxidative carbonylation of styrene in MeOH.
Experimental
3
4
5
8.69 [ddd, J(HH) ¼ 4.9, J(HH) ¼ 1.8, J(HH) ¼ 0.9 Hz, 1H,
H6-py].
General procedure
py-2-C(H)=N[2,6-(i-Pr)2C6H3], 3.16 2-Pyridinecarboxalde-
yde (0.26 g, 2.48 mmol) and 2,6-diisopropylaniline (4.4 g, 2.48
mmol) were dissolved in 10 mL of ethanol and the resulting
mixture was heated at reflux temperature for 20 min. The
solvent was removed under reduced pressure and the residue
was chromatographed on basic alumina with n-pentane–ethyl
acetate (3 : 1) as eluent. Recrystallization from n-pentane
yielded pale yellow crystals, which were filtered off and washed
with cold n-pentane. Yield: 62%. Anal. calcd for C18H22N2: C,
81.16; H, 8.32; N, 10.52; found: C, 81.08; H, 8.36; N, 10.42%.
All manipulations were carried out under a nitrogen atmo-
sphere using Schlenk-type techniques. All the solid compounds
were collected on sintered-glass frits and washed with appro-
priate solvents before being dried in a stream of nitrogen. The
solvents were distilled under nitrogen over LiAlH4 (n-pentane,
n-hexane, THF), CaH2 (CH2Cl2) or Na (diethyl ether). MeOH
employed in carbonylation runs was heated at reflux tem-
perature over Mg shavings under an atmosphere of dry
nitrogen and distilled just prior to use. Styrene was freshly
distilled from LiAlH4 . All the other reagents and solvents were
used as purchased from commercial suppliers. The palladium
complex (COD)PdCl2 (COD ¼ 1,5-cyclooctadiene) was pre-
pared as described in the literature.22 Carbonylation reactions
were performed with a 250 ml stainless steel autoclave, con-
structed at the ISSECC-CNR (Firenze, Italy), equipped with a
magnetic drive stirrer, a Parr 4842 temperature and pressure
controller. Elemental analyses were performed using a Carlo
Erba Model 1106 elemental analyzer. Infrared spectra were
recorded on a Perkin–Elmer 1600 Series FT-IR spectro-
photometer. Deuterated solvents for NMR measurements
1
IR (Nujol mull, cmꢀ1): n(C=N) 1649 (s). H NMR (CDCl3): d
1.16 [d, 3J(HH) ¼ 7.0 Hz, 12H, CHMe2], 2.97 [sept.,
3J(HH) ¼ 7.0 Hz, 2H, CHMe2], 7.05–7.23 (m, 3H, m- and p-
3
Ar), 7.42 [ddd, J(HH) ¼ 7.7, 5.0, 4J(HH) ¼ 1.3 Hz, 1H, H5-
py], 7.86 [td, 3J(HH) ¼ 7.7, 4J(HH) ¼ 1.8 Hz, 1H, H4-py], 8.27
3
4
5
[ddd, J(HH) ¼ 7.7, J(HH) ¼ 1.3, J(HH) ¼ 0.9 Hz, 1H, H3-
py], 8.31 [s, 1H, C(H)=N], 8.73 [ddd, 3J(HH) ¼ 5.0,
5
4J(HH) ¼ 1.8, J(HH) ¼ 0.9 Hz, 1H, H6-py].
py-2-C(Me)=N[2,6-(i-Pr)2C6H3], 4.15 2-Acetylpyridine (1.5 g,
1.24 mmol) and 2,6-diisopropylaniline (2.2 g, 1.24 mmol) were
dissolved in 15 mL of MeOH containing a few drops of formic
acid and the resulting mixture was heated at reflux temperature
for 5 days. Partial evaporation of the solvent under reduced
pressure led to the precipitation of yellow crystals, which were
filtered off and washed with a small amount of cold MeOH.
Yield: 62%. M.p.: 79–80 ꢁC. Anal. calcd for C19H24N2: C,
81.38; H, 8.63; N, 9.99; found: C, 81.40; H, 8.60; N, 10.01%.
1
were dried over molecular sieves. H NMR spectra were col-
lected on a Bruker ACP-200 (81.01 MHz). Chemical shifts are
reported in ppm relative to TMS, referenced to the chemical
shifts of residual solvent resonances. GC analyses were per-
formed 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
mm film thickness) SPB-1 Supelco fused silica capillary col-
umn. The product composition was determined by using
acetophenone as the internal standard. GC=MS analyses were
1
IR (Nujol mull, cmꢀ1): n(C=N) 1649 (s). H NMR (CDCl3): d
388
New J. Chem., 2002, 26, 387–397

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1.16 [d, 3J(HH) ¼ 7.0 Hz, 12H, CHMe2], 2.23 [s, 3H,
analysis were obtained by slow evaporation of the solvent
from a diluted CH2Cl2 solution of 2a.
3
C(Me)=N), 2.76 [sept., J(HH) ¼ 7.0 Hz, 2H, CHMe2], 7.01–
7.21 (m, 3H, m- and p-Ar), 7.41 [ddd, 3J(HH) ¼ 7.7, 5.0,
4J(HH) ¼ 1.3 Hz, 1H, H5-py], 7.83 [td, 3J(HH) ¼ 7.7,
4J(HH) ¼ 1.8 Hz, 1H, H4-py], 8.37 [ddd, 3J(HH) ¼ 7.7,
4J(HH) ¼ 1.3, 5J(HH) ¼ 0.9 Hz, 1H, H3-py], 8.70 [ddd,
{py-2-C(H)=N[2,6-(i-Pr)2C6H3]}Pd(TFA)2 , 3a. Yellow micro-
crystals, yield 88%. Anal. calcd for C22H22F6N2O4Pd: C, 44.12;
1
H, 3.71; N, 4.68; found: C, 44.15; H, 3.70; N, 4.71%. H NMR
4
5
3J(HH) ¼ 5.0, J(HH) ¼ 1.8, J(HH) ¼ 0.9 Hz, 1H, H6-py].
3
(CD2Cl2): d 1.15 [d, J(HH) ¼ 6.8 Hz, 6H, CHMeMe0], 1.42 [d,
3J(HH) ¼ 6.8 Hz, 6H, CHMeMe0], 3.43 [sept., 3J(HH) ¼ 6.8 Hz,
2H, CHMeMe0], 7.28–7.44 (m, 3H, m- and p-Ar), 7.84 [ddd,
3J(HH) ¼ 7.9, 5.5, 4J(HH) ¼ 1.5 Hz, 1H, H5-py], 7.97 [s, 1H,
6-Mepy-2-C(H)=N[2,6-(i-Pr)2C6H3], 5. A mixture of 0.36 g
of 6-methyl-2-pyridinecarboxadehyde (2.97 mmol) and 0.55 g
of 2,6-diisopropylaniline (3.10 mmol) was heated at 100 ꢁC for
1 h. The resulting residue was allowed to stand overnight at
ambient temperature. The formed yellow solid was crystallized
from MeOH and water (a few drops). Yield: 70%. M.p.: 73–
76 ꢁC. Anal. calcd for C19H24N2: C, 81.38; H, 8.63; N, 9.99;
found: C, 81.40; H, 8.60; N, 10.01%. IR (Nujol mull, cmꢀ1):
3
4
5
C(H)=N], 7.99 [ddd, J(HH) ¼ 7.9, J(HH) ¼ 1.5, J(HH) ¼ 0.7
Hz, 1H, H3-py], 8.28 [td, 3J(HH) ¼ 7.9, 4J(HH) ¼ 1.5 Hz, 1H, H4-
py], 8.34 [ddd, 3J(HH) ¼ 5.5, 4J(HH) ¼ 1.5, 5J(HH) ¼ 0.7 Hz, 1H,
py H-6]. Crystals suitable for an X-ray analysis were obtained
by slow evaporation of the solvent from a diluted CH2Cl2
solution of 3a.
1
3
n(C=N) 1640 (s). H NMR (CDCl3): d 1.18 [d, J(HH) ¼ 6.9
Hz, 12H, CHMe2], 2.66 (s, 3H, Me-py), 2.98 [sept.,
3J(HH) ¼ 6.9 Hz, 2H, CHMe2], 7.08–7.21 (m, 3H, m- and
p-Ar), 7.29 [d, 3J(HH) ¼ 8.2 Hz, 1H, H5-py], 7.76 [t,
{py-2-C(Me)=N[2,6-(i-Pr)2C6H3]}Pd(TFA)2 , 4a. Yellow micro-
crystals, yield 90%. Anal. calcd for C23H24F6N2O4Pd: C, 45.07; H,
3.95; N, 4.57; found: C, 45.10; H, 3.90; N, 4.60%. 1H NMR
3
3J(HH) ¼ 8.2 Hz, 1H, H4-py], 8.11 [d, J(HH) ¼ 8.2 Hz, 1H,
3
(CDCl3): d 1.14 [d, J(HH) ¼ 6.9 Hz, 6H, CHMeMe0], 1.48 [d,
3J(HH) ¼ 6.6 Hz, 6H, CHMeMe0], 2.34 [s, 3H, C(Me)=N], 3.26
[sept., 3J(HH) ¼ 6.8 Hz, 2H, CHMeMe0], 7.15–7.43 (m, 3H, m- and
p-Ar), 7.85 [ddd, 3J(HH) ¼ 7.7, 5.6, 4J(HH) ¼ 1.5 Hz, 1H, H5-py],
H3-py], 8.30 [s, 1H, C(H)=N].
py-2-C(H)=N(C6H5), 6.12 This ligand was prepared as an
oily material following a procedure analogous to that descri-
bed for 1 starting from 2-pyridinecarboxaldyde (1.5 g, 14.0
mmol) and aniline (1.3 g, 14.0 mmol). Yield: 78%. Anal. calcd
for C12H10N2 : C, 79.10; H, 5.53; N, 15.37; found: C, 79.00; H,
4
7.92 [dd, 3J(HH) ¼ 7.7, J(HH) ¼ 1.5 Hz, 1H, H3-py], 8.32 [td,
3J(HH) ¼ 7.7, 4J(HH) ¼ 1.5 Hz, 1H, H4-py], 8.44 [dd,
3J(HH) ¼ 5.6, 4J(HH) ¼ 1.5 Hz, 1H, H6-py].
1
5.48; N, 15.21%. IR (Nujol mull, cmꢀ1): n(C=N) 1623 (s). H
{6-Mepy-2-C(H)=N[2,6-(i-Pr)2C6H3]}Pd(TFA)2 , 5a. Yellow
microcrystals, yield 93%. Anal. calcd for C23H24F6N2O4Pd: C,
45.07; H, 3.95; N, 4.57; found: C, 45.10; H, 3.90; N, 4.60%. 1H
NMR (CDCl3): d 7.24–7.46 (m, 5H, Ph), 7.4 (masked by Ph,
3
4
1H, H5-py), 7.81 [td, J(HH) ¼ 7.9, J(HH) ¼ 1.6 Hz, 1H, H4-
3
3
4
5
NMR (CD2Cl2): d 1.14 [d, J(HH) ¼ 6.8 Hz, 6H, CHMeMe0],
py], 8.21 [ddd, J(HH) ¼ 7.9, J(HH) ¼ 1.4, J(HH) ¼ 0.8 Hz,
1.42 [d, 3J(HH) ¼ 6.8 Hz, 6H, CHMeMe0], 2.71 (s, 3H, Me-py),
1H, H3-py], 8.62 [s, 1H, C(H)=N], 8.72 [ddd, 3J(HH) ¼ 4.9,
3
4J(HH) ¼ 1.6, J(HH) ¼ 0.8 Hz, 1H, H6-py].
5
3.50 [sept., J(HH) ¼ 6.8 Hz, 2H, CHMeMe0], 7.16–7.43 (m,
3H, m- and p-Ar), 7.61 [dd, 3J(HH) ¼ 7.8, 4J(HH) ¼ 1.4 Hz,
1H, H5-py], 7.76 [dd, 3J(HH) ¼ 7.8, 4J(HH) ¼ 1.4 Hz, 1H, H3-
py], 7.90 [s, 1H, C(H)=N], 8.09 [t, 3J(HH) ¼ 7.8 Hz, 1H, H4-py].
Synthesis of the palladium complexes
Bis-trifluoroacetate palladium complexes 1a–6a. In a typical
reaction, the appropriate diimine ligand (1.1 mmol) dissolved
in CH2Cl2 (10 ml) was added to a MeOH (10 ml) solution of
Pd(TFA)2 (1 mmol, TFA ¼ trifluoroacetate). The mixture was
allowed to stir for 10 min and then the volume was reduced
under a steady stream of nitrogen until the product began to
precipitate. Portionwise addition of diethyl ether (20 ml)
completed the precipitation of the product, which was col-
lected by filtration and washed with petroleum ether.
[py-2-C(H)=N(C6H5)]Pd(TFA)2 , 6a. Yellow brown micro-
crystals, yield 86%. Anal. calcd for C16H10F6N2O4Pd: C,
37.33; H, 1.96; N, 5.44; found: C, 37.50; H, 2.00; N, 5.40%.
1H NMR (CDCl3): d 7.39–7.51 (m, 5 H, Ph), 7.75 [ddd,
3J(HH) ¼ 7.8, 5.7, 4J(HH) ¼ 1.4 Hz, 1H, H5-py], 8.00 [dd,
3J(HH) ¼ 7.7, 4J(HH) ¼ 1.4 Hz, 1H, H3-py], 8.20 [s, 1H,
C(H)=N], 8.23 (m, 2H, H4-py and H6-py).
[py-2-C(H)=N(2,6-Me2C6H3)]Pd(OAc)2 , 1b. To Pd(OAc)2
(0.29 g, 1.30 mmol, OAc ¼ acetate) dissolved in MeOH (10 ml)
was added a solution of 1 (0.27 g, 1.30 mmol) in CH2Cl2 (10
ml). The mixture was stirred for half an hour and then con-
centrated to 5 ml under a steady stream of nitrogen. Portion-
wise addition of an 1 : 1 mixture of diethyl ether and n-pentane
(20 ml) led to the precipitation of a yellow-orange product,
which was filtered off and washed several times with n-pentane.
Yield: 65%. Anal. calcd for C18H20N2O4Pd: C, 49.74; H, 4.60;
N, 6.44; found: C, 49.20; H, 4.55; N, 6.40%. 1H NMR
(CDCl3): d 1.32 (s, 3H, CH3CO2), 2.05 (s, 3H, CH3CO2), 2.44
(s, 6H, Me2C6H3), 7.00–7.20 (m, 3H, m- and p-Ar), 7.72 [ddd,
3J(HH) ¼ 7.8, 5.6, 4J(HH) ¼ 1.1 Hz, 1H, H5-py], 7.99 [dd,
3J(HH) ¼ 7.8, 4J(HH) ¼ 1.1 Hz, 1H, H3-py], 8.11 [s, 1H,
C(H)=N], 8.19 [td, 3J(HH) ¼ 7.8, 4J(HH) ¼ 1.3 Hz, 1H, H4-py],
[py-2-C(H)=N(2,6-Me2C6H3)]Pd(TFA)2 , 1a. Yellow micro-
crystals, yield 76%. Anal. calcd for C18H14F6N2O4Pd: C,
39.83; H, 2.61; N, 5.16; found: C, 39.80; H, 2.50; N, 5.20%. 1H
NMR (CDCl3): d 2.43 (s, 6H, Me2C6H3), 7.04–7.23 (m, 3H, m-
and p-Ar), 7.79 [ddd, 3J(HH) ¼ 7.8, 5.6, 4J(HH) ¼ 1.1 Hz, 1H,
3
4
H5-py], 7.98 [dd, J(HH) ¼ 7.8, J(HH) ¼ 1.1 Hz, 1H, H3-py],
8.08 [s, 1H, C(H)=N], 8.23 [td, 3J(HH) ¼ 7.8, 4J(HH) ¼ 1.3 Hz,
1H, H4-py], 8.38 [dd, 3J(HH) ¼ 5.6, 4J(HH) ¼ 1.3 Hz, 1H, H6-
py]. Crystals suitable for an X-ray analysis were obtained by
slow evaporation of the solvent from a diluted CH2Cl2 solu-
tion of 1a.
[py-2-C(Me)=N(2,6-Me2C6H3)]Pd(TFA)2 , 2a. Yellow micro-
crystals yield 74%. Anal. calcd for C19H16F6N2O4Pd: C, 40.98;
H, 2.90; N, 5.03; found: C, 40.85; H, 2.91; N, 5.00%. 1H NMR
(CDCl3): d 2.31 [s, 3H, C(Me)=N], 2.41 (s, 6H, Me2C6H3), 7.07–
7.25 (m, 3H, m- and p-Ar), 7.92 [ddd, 3J(HH) ¼ 7.7, 5.6,
4J(HH)¼ 1.5 Hz, 1H, H5-py], 8.02 [dd, 3J(HH) ¼ 7.7
4J(HH)¼ 1.5 Hz, 1H, H3-py], 8.23 [td, 3J(HH) ¼ 7.7,
4J(HH)¼ 1.5 Hz, 1H, H4-py], 8.40 [dd, 3J(HH) ¼ 5.6,
4J(HH)¼ 1.5 Hz, 1H, H6-py]. Crystals suitable for an X-ray
3
4
8.44 [dd, J(HH) ¼ 5.6, J(HH) ¼ 1.3 Hz, 1H, H6-py].
[{py-2-C(H)=N(2,6-Me2C6H3)}Pd(THF)2](PF6)2 , 1c. A solid
sample of (COD)PdCl2 (0.18 g, 0.63 mmol) was added to a
stirred solution of 1 (0.13 g, 0.63 mmol) in THF (10 ml) at
ambient temperature. After 20 min, solid AgPF6 (0.35 g, 1.38
mmol) was added and the resulting slurry was stirred for 1 h.
New J. Chem., 2002, 26, 387–397
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Table 1 Selected 1H NMR data (CDCl3) for the pyridinimine ligands
and the corresponding palladium complexes
The mixture was filtered on celite to remove AgCl and the
yellow filtrate was concentrated to 5 ml. Addition of n-pentane
(20 ml) led to the precipitation of a yellow microcrystalline
solid, which was filtered off and washed with n-pentane. Yield:
68%. Anal. calcd for C22H29F12N2O2P2Pd: C, 35.25; H, 3.87;
N, 3.73; found: C, 35.21; H, 3.85; N, 3.70%. 1H NMR
(CDCl3): d 1.82 (m, 8H, THF, –CH2CH2–), 2.42 (s, 6H,
Me2C6H3), 3.65 (m, 8H, THF –CH2OCH2–), 7.09–7.34 (m,
3H, m- and p-Ar), 8.02 [ddd, 3J(HH) ¼ 5.5, 4J(HH) ¼ 1.5,
5J(HH) ¼ 0.5 Hz, 1H, H6-py], 8.10 [ddd, 3J(HH) ¼ 7.7, 5.5,
4J(HH) ¼ 1.5 Hz, 1H, H5-py], 8.12 [s, 1H, C(H)=N], 8.25 [ddd,
3J(HH) ¼ 7.7, 4J(HH) ¼ 1.5, 5J(HH) ¼ 0.5 Hz, 1H, H3-py],
Py-C(R)=
N-Ar
H5-py
H3-py
H4-py
H6-py
1
7.42 ddd
7.79 ddd
7.72 ddd
8.10 ddd
7.40 ddd
7.92 ddd
7.42 m
7.84 ddd
7.41 ddd
7.85 ddd
7.29 d
8.30 ddd
7.98 dd
7.99 dd
8.25 ddd
8.30 ddd
8.02 dd
8.27 d
7.99 ddd
8.37 ddd
7.92 dd
8.11 d
7.85 td
8.23 td
8.19 td
8.40 td
7.83 td
8.23 td
7.86 t
8.28 ddd
7.83 td
8.32 td
7.76 t
8.73 ddd
8.38 dd
8.44 dd
8.02 ddd
8.69 ddd
8.40 dd
8.73 d
8.34 ddd
8.70 ddd
8.44 dd
—
8.36 s
8.08 s
8.11 s
8.12 s
2.20 s
2.31 s
8.31 s
7.97 s
2.23 s
2.34 s
8.30 s
7.90 s
8.62 s
8.20 s
1a
1b
1c
2
2a
3
3a
4
4a
5
3
4
8.40 [td, J(HH) ¼ 7.7, J(HH) ¼ 1.5 Hz, 1H, H4-py].
Catalytic carbonylation of styrene
5a
6
6a
7.61 dd
7.4a
7.75 ddd
7.76 dd
8.21 ddd
8.00 dd
8.09 t
7.81 td
—
8.72 ddd
8.23 mb
8.23 mb
Typically, a MeOH solution (20 ml) of styrene (2 mmol) was
introduced by suction into a 250 ml autoclave, previously
evacuated by a vacuum pump, containing 0.01 mmol of cata-
lyst precursor along with the desired amounts of 1,4-benzo-
quinone (BQ) and p-toluenesulfonic acid (TsOH). The
autoclave was then pressurized with CO (generally 800 psi) at
room temperature and heated to 80 ꢁC. As soon as the reaction
mixture in the autoclave reached the desired temperature,
stirring (700 rpm) was applied for the desired time (generally 3
h). The reaction was stopped by cooling the autoclave to room
temperature by means of an ice-water bath. The pressure was
then released. The product composition was determined by
GC using acetophenone as the internal standard.
a
b
Masked by aryl hydrogen resonances. Signals overlapped.
Reaction of the ligands 1–6 with Pd(TFA)2 in MeOH–
CH2Cl2 at room temperature, followed by addition of diethyl
ether, yielded the corresponding neutral bis(trifluoroacetate)
complexes (N-N0)Pd(TFA)2 (1a–6a) in good yields (Scheme 3).
The bis(acetate) derivative [py-2-C(H)=N(2,6-Me2C6H3)]-
Pd(OAc)2 (1b) was obtained from 1 by substituting Pd(TFA)2
for Pd(OAc)2 , while the cationic complex [{py-2-C(H)=N(2,6-
Me2C6H3)}Pd(THF)2](PF6)2 (1c) was prepared by a more
complex procedure involving the substitution of the diene
ligand in (COD)PdCl2 by the pyridinimine 1 in THF, followed
by reaction of the dichloride intermediate with silver hexa-
fluorophosphate. All the complexes are quite air stable in the
solid state while they are stable in solutions of organic solvents
such as CH2Cl2 , acetone and MeOH only under a nitrogen
atmosphere. The complexes have been characterized by ele-
X-Ray structure determinations
Suitable crystals were sealed in a glass capillary and trans-
ferred to a Enraf–Nonius CAD4 diffractometer. Data were
collected at room temperature using Mo-Ka radiation
+
(l ¼ 0.71069 A). A set of 25 carefully centred reflections having
6.5 4 y 4 10.0ꢁ were used to determine the lattice constants.
The intensities of three standard reflections were measured
every 2 h for orientation and intensity control. This procedure
revealed no decay of intensity. The data were corrected for
Lorentz and polarization effects. Atomic scattering factors
with anomalous dispersion correction were taken from the
literature.23 Absorption corrections were applied via c scans.
The computational work was performed with a DIGITAL
DEC 5000=200 workstation using the program SHELX-93.24
The structures were solved by direct methods using the SIR92
program25 and all of the non-hydrogen atoms were found
through a series of Fo Fourier maps. Refinement was done by
full-matrix least-squares calculations, initially with isotropic
thermal parameters and finally with anisotropic thermal
parameters for all the atoms but the hydrogens. The phenyl
rings were treated as rigid bodies with D6h symmetry and
hydrogen atoms were introduced at calculated positions.
CCDC reference numbers 178390–178392. See http:==www.
rsc.org.=suppdata=nj=b1=b108804c= for crystallographic data
in CIF or other electronic format.
1
mental analysis and H NMR spectroscopy (selected data are
given in Table 1). Moreover, single-crystal X-ray structure
determinations were performed for the TFA complexes 1a–3a.
The 1H NMR characteristics of the complexes are in line
with those reported for related palladium(II) complexes con-
taining bidentate pyridinimine ligands and therefore do not
deserve any detailed analysis.15,16,18,20 It may be worthwhile to
stress, however, that the resonances of the protons in the 3
position are high-field shifted as compared to the free ligands,
which has been attributed to the difference between free
(s-trans) and coordinated (s-cis) conformations of the formal
1,4-diazabutadiene moiety (Schemes 2 and 3).18a In the free
ligand, the electron density on the imino nitrogen largely
deshields the vicinal H3 atom, in fact. Moreover, upon com-
plexation to palladium, the isopropyl groups in 3a, 4a, and 5a
became magnetically inequivalent (doublets at ca. d 1.15 and
1.45 vs. doublet at ca. d 1.17 in the free ligands), which can be
attributed to hindered rotation of the aryl rings.15,16 This
magnetic inequivalence was observed up to 80 ꢁC in MeOH-d4 .
Results and discussion
Syntheses and properties of the pyridinimine ligands and
of the corresponding palladium complexes
The pyridinimine ligands were conveniently prepared by con-
densation of either 2-pyridinecarboxaldeyde or 2-acetylpyr-
idine with equimolecular amounts of the appropriate anilines
(Scheme 2).12–16 All the ligands were authenticated by ele-
1
mental analysis and H NMR and IR spectroscopies. Selected
1H NMR data for all the ligands are presented in Table 1.
Scheme 3 Structure and numbering scheme of the bis-tri-
fluoroacetate complexes.
390
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At higher temperature, rapid decomposition of the complexes
occurred. In 1b, the methyl resonance of the acetate coligand
that is spatially closer to the imino aryl group is shifted upfield
(d 1.32 vs. 2.05) due to shielding by the aromatic p system.15,16
Crystallographic studies
Crystals suited for an X-ray analysis were obtained for 1a, 2a,
and 3a by slow evaporation of the solvent from diluted CH2Cl2
solutions. ORTEP drawings are shown in Fig. 1, while details
of data collection=refinement and selected bond distances and
angles are reported in Tables 2 and 3, respectively.
The overall molecular structure of the compounds investi-
gated can be described as a square plane with the palladium
centre coordinated by a chelating pyridinimine ligand and by
two cis TFA groups. The palladium atom in 3a, which features
a ligand bearing ortho i-propyl substituents, lies in the coor-
+
dination N2O2 plane [only 0.020(4) A above], the atoms of
+
which are coplanar within 0.096 A. In contrast, the metal
centre in the complexes containing 2,6-methyl substituents on
the imino aryl group (1a and 2a) is significantly displaced from
+
the coordination plane [below by 0.553(4) and 0.41(3) A,
0.566 A for 1a
+
respectively], with average deviations of
and 0.567 A for 2a.
+
The values of the chelating angles N1–Pd–N2 in 1a–3a are
comparable within the three compounds [from 79.7(3) to
81.1(3)ꢁ] and reflect the small bite sizes of the ligands.15,16,18
A feature common to all complexes is the almost perpen-
dicular orientation of the mean plane defined by the imino
aromatic ring with respect to the mean plane defined by the
atoms N(1), N(2), and Pd. As a consequence of this orienta-
tion, the free space at palladium in the coordination plane is
partly controlled by the imino aryl substituents.
The aromatic ring in 2a [78.1(4)ꢁ] is more tilted towards the
diazadiene backbone than in 1a and 3a [81.1(2) and 84.5(3)ꢁ,
respectively],15,16 which contrasts recent DFT studies on sub-
stituent effects in palladium-catalyzed olefin polymerization.26
Indeed, one would expect a more vertical orientation of the
aryl ring in 2a as compared to 1a due to the greater repulsion
between the methyl substituent at the aryl group and the
methyl substituent at the imino carbon atom.26 This apparent
contrast means that the aryl orientation in the solid state
is determined also by the TFA co-ligands. Consistently, the
Pd–O bonds are slightly longer and the N1–Pd–O1 angle is
slightly larger [96.4(3)ꢁ] in 2a than in 1a [94.3(2)ꢁ] and 3a
[93.8(3)ꢁ].
Fig. 1 ORTEP drawings of the pyridinimine palladium complexes
1a (a), 2a (b) and 3a (c).
Table 2 Crystal data and structure refinement details for 1a, 2a, and 3a
1a
2a
3a
Empirical formula
Formula weight
Crystal system
C
542.71
18H14F6N2O4Pd
C19H16F6N2O4Pd
556.74
Monoclinic
P21=n
8.481(4)
20.641(4)
12.241(3)
90.00
98.80(2)
90.00
2117.7(12)
4
0.957
3709
3709
0.0200
0.0667
0.2114
C22H22F6N2O4Pd
598.84
Monoclinic
P21=n
12.370(4)
17.071(2)
12.663(5)
90.00
105.55(4)
90.00
2576.1(13)
4
0.781
4725
4509
0.0185
0.0752
0.2123
Triclinic
P1
-
Space group
+
a=A
+
9.383(3)
9.774(2)
11.585(2)
85.90(2)
89.74(2)
70.79(2)
100.5(9)
2
0.994
3700
3508
0.0179
0.0636
0.1755
b=A
+
c=A
a=ꢁ
b=ꢁ
g=ꢁ
+
U=A3
Z
m=mmꢀ1
Reflections collected
Independent reflections
Rint
R1 [I>2s(I)]
wR2 (all data)
New J. Chem., 2002, 26, 387–397
391

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+
Table 3 Selected bond distances (A) and angles (ꢁ) for 1a, 2a, and 3a
PhC(OH)(OMe)Me in almost equimolar concentrations. The
formation of acetophenone may be explained by taking into
account a sequence of reactions analogous to that leading to
the production of acetaldehyde in the Wacker process, which
involves an intramolecular nucleophilic attack by H2O=OHꢀ
at the double bond of coordinated ethylene.27 The hemiketal is
apparently formed by addition of alcohol to the acetophenone
carbonyl group.28,29
1a 2a 3a
Pd–N(1)
Pd–N(2)
Pd–O(1)
Pd–O(2)
1.989(5)
1.989(5)
2.000(5)
2.000(5)
1.989(7)
1.995(9)
2.014(7)
2.039(6)
1.987(8)
1.984(8)
2.006(6)
2.026(6)
N(1)–Pd–N(2)
N(2)–Pd–O(2)
O(2)–Pd–O(1)
N(1)–Pd–O(1)
N(1)–Pd–O(2)
N(2)–Pd–O(1)
81.0(2)
96.0(2)
88.9(2)
94.3(2)
176.4(2)
173.7(2)
79.7(3)
97.9(3)
86.4(3)
96.4(3)
174.7(3)
173.7(3)
81.1(3)
98.2(3)
87.5(3)
93.8(3)
175.6(3)
171.4(3)
The precursor degradation in the presence of styrene was
studied by 1H NMR spectroscopy in commercial MeOH-d4 . It
was thus discovered that the decomposition of the palla-
dium(II) complexes is quite rapid (t1=2 ca. 5 min) and is
accompanied by the formation of both palladium metal and
cationic bis-chelate complexes of the general formula
[(N-N0)2Pd]2þ 15,30,31
The production of partially deuterated
.
Catalytic carbonylation of styrene
acetophenone and its hemiketal was observed by GC=MS
analysis. The formation of the bis-chelate complexes can be
accounted for by the reaction of water with styrene leading to
acetophenone and (N-N0)Pd(0) species.27 These latter are
known to decompose rapidly, in fact, yielding palladium metal
that precipitates as a black powder and free ligand that reacts
with the parent monochelate palladium(II) moiety to give the
corresponding bis-chelate complex. An identical mechanism
has been recently reported to explain the formation of bis-
chelates [(P-P)2Pd]2þ in the course of styrene carbonylation
reactions catalyzed by diphosphine-modified palladium(II)
catalysts in MeOH.4
Adopting the precautions previously described decreased
substantially the degradation of the precursors by a Wacker-
type process as neither acetophenone nor its hemiketal was
produced in appreciable amounts, as determined by GC. The
results obtained by applying different reaction times were
consistent with a good catalyst stability, at least within 3 h of
reaction. However, some catalyst degradation invariably
occurred, although in trace amounts, as black palladium metal
was observed in all reactions, particularly in those carried out
with a six-fold excess of acid.
The catalytic performance of the present pyridinimine palla-
dium complexes in the carbonylation of styrene in MeOH was
investigated under experimental conditions comparable with
those previously employed for analogous reactions catalyzed
by chelating diphosphine precursors of the formula [(P-P)
PdL2]nþ (L ¼ MeCN, bipy, n ¼ 2; L ¼ OAc, n ¼ 0)4 (see
Experimental). In a second stage, various parameters such as
the concentration of the protic acid, the CO pressure and the
reaction time were varied in an attempt at optimizing the
process as well as acquiring mechanistic information.
Unlike the reactions catalyzed by phosphine-modified pal-
ladium complexes,4 extreme care was adopted to avoid the
degradation of the metal precursors during the preparation of
the catalytic systems. Therefore, MeOH was carefully dried
and styrene was introduced into the reactor only after adding
the organic oxidant (BQ). In fact, we noticed that the palla-
dium precursors started decomposing to palladium metal and
free ligand already at room temperature upon addition of
excess styrene in commercial MeOH. It may be interesting to
report that the decomposition of the precursors is accom-
panied by the formation of both PhC(O)Me and its hemiketal
Table 4 Carbonylation of styrene catalyzed by (N-N0)Pd(II) complexesa
Entry Precursor TsOH=equivTime =h PCO=psi Conv(%) A (%) Sel A (%) B (%) Sel B (%) C–E (%) Sel C–E (%) DMO=TONb
1
2
3
4
5
6
7
8
9
1a
0
0
2
4
6
6
6
0
2
0
2
1.5
3
3
3
3
3
3
3
3
800
800
800
800
400
800
1500
800
800
800
800
35.1
44.3
57.3
50.2
51.2
41.3
24.9
4.0
1.5
1.0
1.1
0.5
1.2
0.2
0.0
0.0
0.7
1.0
0.8
4
2
2
1
2
0
0
0
1
2
2
29.8
37.7
47.2
46.6
48.1
40.0
24.4
3.4
85
85
82
93
94
97
98
85
75
87
84
3.8
5.6
9.0
3.1
1.9
1.1
0.5
0.6
12.5
5.0
6.9
11
13
16
6
4
3
9
20
11
4
0
3
19
42
15
28
13
0
1b
1c
15
24
11
15
52.2
46.1
47.2
39.0
40.1
39.5
10
11
3
3
12
13
14
15
16
2a
0
0
2
4
6
1.5
3
3
3
3
800
800
800
800
800
21.5
42.9
53.4
42.4
37.6
0.4
0.4
0.7
0.3
0.2
2
1
1
1
1
15.7
31.9
37.3
36.1
35.8
73
74
70
85
95
5.4
10.6
15.4
6.0
25
25
29
14
4
6
16
9
4
2
1.6
17
18
19
20
3a
4a
0
0
2
6
1.5
3
3
800
800
800
800
16.1
26.2
38.9
25.0
0.3
0.1
0.3
0.2
2
0
1
1
14.5
23.4
32.3
24.3
90
89
83
97
1.3
2.7
6.3
0.5
8
10
16
2
9
23
16
2
3
21
22
23
24
0
0
2
6
1.5
3
3
800
800
800
800
13.9
23.0
36.1
19.0
0.2
0.6
0.2
0.2
1
3
1
1
10.5
16.9
22.8
17.8
76
73
63
94
3.2
5.5
13.1
1.0
23
24
36
5
8
19
12
2
3
25
26
5a
6a
2
2
3
3
800
800
0.0
1.9
0.0
0.0
—
0
0.0
1.6
—
84
0.0
0.3
—
16
22
14
a
b
Pd(II) precursor (0.01 mmol); styrene (2 mmol); BQ (2 mmol); =MeOH (20 mL); 80 ꢁC; 700 rpm.
catalyst.
Mol of dimethyl oxalate produced per mol of
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The formation of A, B, D and E upon carbonylation of
styrene in MeOH implies a series of reactions that reþquire the
participation of methoxy initiators [(N-N0)Pd(OMe)] as well
as the occurrence of termination steps leading to the generation
of hydride species [(N-N0)PdH]þ via either b-H elimination
(products A and D) or methanolysis (products B and E).3a,b,4
As for the formation of C,3b the catalytic cycle requires the
occurrence of
a
1,2þ-migratory insertion process3a,b,g in
[(N-N0)Pd(H)(styrene)] and termination via methanolysis
(Scheme 5b). Given the nature of the products þobtained, the
migratory insertion in [(N-N0)Pd(acyl)(styrene)] must occur
with 2,1-regiospecificity.2,3j,32–34 Secondary insertions of this
type have been observed for a variety of styrene carbonylation
reactions leading to alternating polyketones,2 ketones or
esters.3,4 Secondary insertions of styrene have been also
observed in homogeneous oligomerization35 and polymeriza-
tion reactions of olefins.36
Scheme 4
The catalytic results are summarized in Table 4. The reac-
tion products are shown in Scheme 4: methyl cinnamate (A),
dimethyl phenylsuccinate (B), methyl 2,6-diphenyl-4-oxohex-
anoate (C), methyl 3,6-diphenyl-4-oxohex-5-enoate (D), and
dimethyl 2,5-diphenyl-4-oxoheptanedioate (E). These products
were identified by comparing their GC retention times and
GC=MS spectra with those of authentic samples.3a,b,4
Before reporting and commenting the results obtained, it
may be worth giving a brief description of the commonly
accepted mechanisms of styrene carbonylation by palladiu-
m(II) catalysis,2 particularly as regards the formation of the
products A–E (Scheme 5).
From a perusal of the catalytic data reported in Table 4, it is
apparent that, in the absence of the Brønsted acid co-reagent,
the ligands 1–4 give rise to efficient catalysts for the synthesis
of the bis-alkoxycarbonylation product B with selectivities
spanning from 73 to 90%. In particular, among the pyr-
idinimines investigated, 1 showed itself to be appropriately
tailored to provide the highest productivity in B (about 75 mol
per mol of catalyst in 3 h, entry 2). The remaining products
were the cinnamate A (max. 1.5%) and the oligomers C–E
obtained by alternating insertion of styrene and carbon
monoxide. Among the oligomers only C was produced in
Scheme 5 Proposed reaction schemes for the methoxycarbonylation of styrene with chelating dinitrogen-Pd(II) precursors; [Pd] ¼ [(N-N0)Pd]2þ
,
N-N0 ¼ dinitrogen ligand. u represents either a co-ligand or a solvent molecule.
New J. Chem., 2002, 26, 387–397
393

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Scheme 6
appreciable amounts, D and E never being obtained in con-
centrations higher than 1%. For this reason, the quantitative
data relative to the oligomeric products C–E have been col-
lected together in Table 4.
The selectivities in A were quite low indeed, in particular
when compared to those found in analogous styrene carbo-
nylation reactions in MeOH catalyzed by diphosphine-mod-
ified palladium complexes.3a,b, f,4 The low selectivity in methyl
cinnamate suggests that b-elimination is disfavoured over CO
insertion in Pd–CH(Ph)CH2CO2Me intermediates stabilized
by chelating dinitrogen ligands (Scheme 5a). There are two
possible explanations for this eventuality. The first is that the
greater electrophilic character of palladium(II) in the dinitro-
gen complexes might disfavour the b-elimination due to the
formation of more stable b-chelate complexes (equilibrium a,
Scheme 6).2b,d b-Keto chelate palladium complexes with
diimine ligands have been isolated, in fact.32,33 In particular,
both b- and g-chelates have been synthesized starting from the
complex [(PriDAB)Pd(Me)(NCMe)]þ where PriDAB is 1,4-
diisopropyl-1,4-diazabuta-1,3-diene (Scheme 7).32 Indeed, the
electrophilic character of the palladium centre in the pyr-
idinimine complexes is clearly shown by the occurrence of
styrene oxidation by water yielding acetophenone (vide infra).
An alternative explanation for the low selectivity in cinnamate
is that the rate of migratory insertion in Pd(CO)(alkyl) is faster
in the complexes stabilized by dinitrogen ligands18,37 than in
diphosphine analogues (equilibrium b, Scheme 6),38 as a con-
sequence of the greater electrophilicity of CO trans to nitrogen
as compared to CO trans to phosphorus.37a As for the
observed low selectivities in D and E, it may be simply pointed
out that the chain growth in the pyridinimine complexes is
disfavoured over termination by methanolysis leading to B
(Scheme 5a).
From a perusal of Table 4, one may also realize that the
overall yield in carbonylation products is controlled by the
steric properties of the alkyl substituents on the imine aryl
group, while the nature of the substituent (H or Me) at the
imine carbon influences the selectivity of the carbonylation
reaction. Indeed, the catalyst precursors 1a and 2a, bearing
methyl groups at the 2,6 positions of the aryl group, gave
more B (37.7 and 31.9%, respectively) as compared to the i-
propyl-substituted precursors 3a and 4a (23.4 and 16.9%,
respectively). In turn, the precursors 1a and 3a, containing the
–C(H)=N– group, were appreciably more selective in B than
the precursors with the –C(Me)=N– group (87av vs. 74av%).
The greater activity of 1a and 2a as compared to 3a and 4a may
be related to a more favourable steric control by the methyl
substituents over the i-propyl ones on the insertion of CO and
styrene into palladium-carbon bonds as well as the termination
Scheme 7
process by methanolysis (Scheme 5). The presence of i-propyl
groups may stabilize certain intermediates, thus hindering the
access of styrene, CO or MeOH to the palladium centre39,40
and ultimately resulting in reduced rates of product formation.
The different selectivities observed in the reactions catalyzed
by the complexes modified with the ligands 1–4 deserve a
comment. In view of the results reported in Table 4 and of
what was previously said on the relative abundance of the
oligomeric products, the lower selectivity in B is essentially due
to the greater production of oligomer C. Since B and C are
formed in catalytic cycles that involve different initiators (Pd–
OMe for B and Pd–H for C), the selectivity in B is effectively
controlled by the ability of the catalysts to favour (or diþs-
favour) the conversion of [(N-N0)PdH]þ to [(N-N0)Pd(OMe)] .
Exhaustive information may be found in the literature on the
factors that promote the formation of either [(L-L)Pd–H]þ or
[(L-L)Pd–OMe]þ in MeOH (L-L ¼ chelating diphosphine or
dinitrogen ligand).2 It is generally agreed that the presence of
an oxidant such as BQ favours the formation of the methoxy
complexes. Furthermore, the reaction between [(L-L)PdH]þ
and CO=MeOH to give [(L-L)PdCO2Me]þ has been inter-
preted in terms of a series of equilibria and reactions that see
the initial formation of an unstable Pd(0) species, later oxidized
to Pd(II) by BQ (Scheme 8). It has been suggested that the
latter process occurs with the intermediacy of palladium(0)
complexes containing a molecule of BQ, either Z2 or Z4 bon-
ded to the metal.2b,30b,41
A further evidence of the role played by BQ in shifting the
equilibria of Scheme 8 to the right has been provided by the
results obtained with the catalysts containing the pyridinimine
ligands 1–6. Control experiments have shown in fact that the
use of 300 instead of 200 equivalents of BQ in an experiment
carried out in the experimental conditions of entry 13 (Table 4)
led to a drastic decrease (from 25 to 6%) in the selectivity in
oligomer C whose formation requires a Pd–H initiator.
Given the complexity of the reaction sequence illustrated in
Scheme 8, it is not easy to determine the factors accounting for
Scheme 8
394
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the decreased conversion of Pd–H into Pd–OMe with ligands 2
and 4 as compared to ligands 1 and 3. Intuitively, we suggest
that the methyl group on the imine carbon induces a sterically
less favourable environment to the formation of BQ-contain-
ing intermediates, and hence to palladium(0) oxidation.
In the course of the catalyst screening, we have been also
looking for yield and=or selectivity dependence on the nature
of the co-ligands in the palladium precursors. Previous studies
of styrene methoxycarbonylation by palladium-diphosphine
catalysis had shown in fact that the substitution of acetate for
weaker co-ligands such as MeCN can result in an inversion of
selectivity from dimethyl phenylsuccinate to methyl cinna-
mate.4 In contrast to palladium-diphosphine catalysts, the
substitution of TFA (entry 2) for either acetate (entry 8) or
THF (entry 10) in the pyridinimine-modified precursor 1a gave
no substantial selectivity change. It was found, however, that
1b was less efficient than 1a and 1c, which may be accounted
for by the greater nucleophilicity of acetate vs. trifluoroacetate
and THF.4,30a
Unlike the co-ligand nature, an important role in deter-
mining both the catalytic activity and the selectivity seems to
be played by the concentration of the protic acid co-catalyst.
The addition of two equivalents of TsOH to the catalytic
mixtures has been found to generally increase the styrene
conversions as well as slightly decrease the selectivity in B due
to increased production of C. Notwithstanding the decreased
selectivity, the production of succinate B was greater than that
obtained in the absence of added acid (e.g., entry 3 vs. entry 2).
It is also worth pointing out that precursors 1a and 2a were
more efficient even in acidic media (57.3 and 53.4% conver-
sion, respectively) than 3a and 4a (38.9 and 36.1% conversion,
respectively). The increased conversion observed for all cata-
lysts in the presence of TsOH might be simply due to a larger
number of catalytically active sites as a consequence of
the weaker coordinating properties of the tosylate anion as
compared to carboxylate. A greater catalyst durability, occa-
sioned by the higher oxidizing power of BQ in an acidic
environment, may also contribute to enhance the catalyst
productivity.4,41b
In order to gain further insight into the ligand requirements
controlling the conversion and=or selectivity of the methoxy-
carbonylation reaction, some experiments have been carried
out with pyridinimine 5, containing ortho-i-propyl substituents
on the imino phenyl ring and a methyl substituent at the 6
position of the pyridine ring, and with pyridinimine 6 with no
substituent on either ring. Catalytic reactions carried out with
these precursors in the presence of two equivalents of acid gave
very low (6a, entry 26) or even no conversion (5a, entry 25). In
particular, the latter catalyst completely decomposed in 3 h.
Taken together, the results obtained with the pyridinimine
ligands 1–6 in the presence of 2 equivalents of TsOH allowed
us to infer the following structure=activity relationship. The
ligands bearing o-methyl substituents on the imine aryl group
(e.g., 1 and 2) give rise to more active catalysts (entries 3 and
14) as compared to pyridinimine 6 that contains an unsub-
stituted phenyl ring (entry 26). On a qualitative basis, the
increase in productivity may be associated to an increase in the
steric hindrance provided by the imine phenyl. This hypothesis
is indirectly supported by previous evidence according to
which both the migratory insertion of Pd(Me)(CO)18a and the
coordination of the olefin42 in palladium(II) fragments stabi-
lized by pyridinimine ligands is favoured when the latter
contain alkyl substituents at the 2 or=and 6 position(s) of the
phenyl group. A large steric hindrance such as in pyridinimines
3 and 4 containing i-propyl groups, however, reduced the
catalytic activity, which may be related to hindered access to
palladium of styrene, CO or MeOH. Consistently, when the
pyridinimine ligand bears 2,6-i-propyl groups at the phenyl
and a methyl group in the 6 position of the pyridine ring as in
5, the production of styrene carbonylation products was
practically suppressed (entry 25). A similar structure=activity
relationship has been recently reported for the copolymeriza-
tion of styrene and CO by 1,10-phenanthroline modified pal-
ladium(II) catalysts.30a In fact, a dramatic decrease in catalytic
Scheme 9 Proposed mechanism for the bis-methoxycarbonylation of styrene with pyridinimine precursors. u represents either a co-ligand or a
solvent molecule.
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activity was observed when 2,9-dimethyl-1,10-phenanthroline
was substituted for 1,10-phenanthroline. It cannot be ruled
out, however, that the low activity of the pyridinimines 3 and 4
may be due to competitive formation of palladacycles via
intramolecular C–H bond activation from an i-propyl group.43
Increasing the concentration of protic acid (from 2 to 4 to 6
equiv.) resulted for all precursors in a better selectivity in B
accompanied by decreased conversions, however. In the pre-
sence of 6 equiv. of TsOH, 1a–4a gave selectivities in the range
94–97% but with productivities even lower than those
obtained in the absence of acid co-reagent. We do not have a
clear-cut explanation accounting for the decreased activity and
increased selectivity in B at high acid concentrations. The drop
in activity in the presence of a large excess of TsOH may be
actually related to an increased amount of water in the cata-
lytic mixture. In fact, the acid is commercially available as the
monohydrated salt and also promotes the esterification of the
trifluoroacetate co-ligands by MeOH, yielding TFAMe and
H2O. As for the selectivity, we can only suggest that a high
acid concentration, favouring the oxidizing properties of
BQ,4,41b might favour the conversion of Pd–H to Pd–CO2Me
(Scheme 8).
In an attempt at determining the influence of the CO pres-
sure on the rate and selectivity of the methoxycarbonylation
reaction catalyzed by the pyridinimine complexes, 1a was
employed as precursor together with 6 equiv. of TsOH in some
experiments in which the initial CO pressure was varied from
400 to 1500 psi. Under these conditions, the conversion
decreased from 51.2% to 24.4%, while the selectivity in B was
practically unaffected. In actuality, a high CO concentration
might disfavour the coordination of styrene to the fragment
[(N0-N)Pd–CO2Me]þ in the early stages of the catalytic cycle
(Scheme 5a), thus favouriþng the formation of the intermediate
[(N-N0)Pd(CO)(CO2Me)] that is a precursor to dimethyl
oxalate (entries 5–7) (see below).
To conclude, incorporation of all of these data leads us to
propose the mechanism shown in Scheme 9 for the bis-meth-
oxycarbonylation of styrene with pyridinimine palladium(II)
catalysts.
As for the coordination structure around palladium in the
species reported in Scheme 9, we cannot state univocally
whether the alkyl or acyl groups are cis or trans to either
nitrogen donor atom. Previous studies in both the solid state
and solution of the structure of [(N-N0)Pd(alkyl)(L)]þ and
[(N-N0)Pd(acyl)(L)]þ complexes clearly show that the coordi-
nation geometry is variously controlled by the electronic and
steric properties of both nitrogen donor atoms and the fourth
co-ligand at palladium.15,18
Finally, it is worth saying a few words on the formation of
small amounts of dimethyl oxalate (DMO) that almost
invariably was found to contaminate the methoxycarbonyla-
tion of styrene (Table 4). The production of alkyl oxalates
from alcohols is generally achieved by either homogeneous
oxidative carbonylation by palladium(II) catalysis44 or carbo-
nylation of alkyl nitrites with Pd(0) catalysts in the homo-
geneous or heterogeneous phase.45 In the former case,
the reaction proceeds via formation of intermediates
LnPd(COOR)2 that ultimately release the oxalate by reductive
coupling.44 In theþpresent case, species with the formula
[(N-N0)Pd–CO2Me] may actually form from the carbonyla-
tion of Pd–OMe, and therefore it may be possible that the
observed production of DMO occurs via the mechanism
shown in Scheme 10. Oxidation by BQ of the Pd(0) complex
that forms upon reductive elimination of the oxalate re-
generates the catalytically active species (see Scheme 9).
In favour of a homogeneous process such as that proposed
in Scheme 10 there is the evidence that the production of DMO
increases with the CO pressure (entries 5–7) and decreases with
the concentration of the protic acid (entries 2–5; 13–16, 18–20
and 22–24). Indeed, it is reasonable to think that the con-
Scheme 10 Reaction sequence for the production of dimethyl ox-
alate.
centration of intermediate [(N-N0)Pd(CO)(CO2Me)]þ (step a in
Scheme 10) increases with the CO pressure, while a high
concentration of acid would shift step b to the left.
Acknowledgements
Thanks are due to the European Commission for the contract
n. HPRN CT 2000-00010 and to COST Action D17 for
sponsoring the Working Group 0007=2000. G. M. thanks ENI
(Italy) for a doctorate grant.
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