Low symmetry metal complexes: Chloro cysteine ethyl ester-N,S triphenylphosphine palladium(II), a new regioselective hydrocarboxylation catalyst for 2-phenylpropanoic acid, and its crystal structure

Article abstract of DOI:10.1039/a809130g

The hydrochlorides of cysteamine, cysteine ethyl ester and penicillamine methyl ester, react with Pd(PPh₃)₄ to yield low symmetry complexes of palladium(II) that incorporate the N,S-aminothiolato ligand and triphenylphosphine; the complex with cysteine ethyl ester, structurally characterized by X-ray diffraction, proved to be an effective catalyst for the hydrocarboxylation of styrene with high selectivity (98%) towards 2-phenylpropanoic acid.

Full text of DOI:10.1039/a809130g

Low symmetry metal complexes: chloro cysteine ethyl ester-N,S
triphenylphosphine palladium(ii), a new regioselective hydrocarboxylation
catalyst for 2-phenylpropanoic acid, and its crystal structure
Julio Real,*a Montserrat Pagès,a Alfons Polo,b J. Francesc Piniellac and Ángel Álvarez-Larenac
a Departament de Química, Universitat Autònoma de Barcelona, E-08193, Bellaterra, Barcelona, Spain.
E-mail: real@cc.uab.es
b Departament de Química, Facultat de Ciències, Universitat de Girona, E-17071 Girona, Spain.
E-mail: dqapo@xamba.udg.es
c
Servei de Difracció de Raigs-X and Unitat de Cristallografia i Mineralogia, Universitat Autònoma de Barcelona,
E-08193, Bellaterra, Barcelona, Spain
Received (in Basel, Switzerland) 23rd November 1998, Accepted 28th December 1998
The hydrochlorides of cysteamine, cysteine ethyl ester and
penicillamine methyl ester, react with Pd(PPh3)4 to yield low
symmetry complexes of palladium(ii) that incorporate the
N,S-aminothiolato ligand and triphenylphosphine; the com-
plex with cysteine ethyl ester, structurally characterized by
X-ray diffraction, proved to be an effective catalyst for the
hydrocarboxylation of styrene with high selectivity (98%)
towards 2-phenylpropanoic acid.
Low symmetry complexes of the transition metals in general,
and the platinum group metals in particular, are of interest
because of their potential as selective catalysts in homogeneous
reactions. Geometric and electronic asymmetry is associated
with increased stability of one of the possible key intermediates,
which should give rise to improved selectivity. The preferred
ligands in many selective catalytic reactions are sophisticated
diphosphines. However, these are often even more expensive
than the precious metal itself. This report is part of our efforts to
evaluate the use of inexpensive, low symmetry ligands from the
chiral pool, in structurally well characterised complexes, to
promote selective homogeneous catalytic reactions.1
Scheme 1
Cysteine ligand type containing complexes are not very
common,2 but we have shown that chelate assisted oxidative
addition to Pt(0) is a reaction that can introduce acidic hydrogen
containing cysteamine and cysteine ethyl ester ligands in the co-
ordination sphere of the metal.3 However, in the case of
palladium the expected metal complexes would contain the
often-unstable Pd–H bond. In a new approach, we have added
an acid to quench the possible palladium hydride in order to
obtain stable palladium(ii) complexes in one step. The stoichio-
metric addition of acid has been achieved by the use of the
hydrochlorides of the ligands.
The hydrochlorides of cysteamine (HSCH2CH2NH2·HCl),
cysteine ethyl ester [HSCH2C*H(CO2Et)NH2·HCl] and also
penicillamine methyl ester [HSCMe2C*H(CO2Me)NH2·HCl]
reacted with Pd(PPh3)4 to yield the corresponding N,S-
aminothiolato chloro triphenylphosphine complexes in good
yield, with displacement of three PPh3 ligands, oxidation of the
metal and evolution of hydrogen (Scheme 1). In this way,
complexes 1–3 were obtained as stable crystalline solids.† The
cis-P,S coordination was established by single crystal X-ray
analysis. Complex 2 exhibits a very short Pd–S distance of
2.253(2) Å that is effectively equal to the Pd–P distance of
2.254(2) Å, owing to the non-bridging coordination of the sulfur
and the low trans influence of the chloro ligand.‡4 Complexes
1–3 are stable in solution and do not form sulfur-bridged dimers
or polymers, even when exposed to air. In these complexes four
very different atoms coordinate the metal, but only when the
asymmetric carbon containing cysteine and penicillamine are
used, is the symmetry plane of the square planar complex
broken and the compounds become optically active. Fig. 1
shows that in the solid state,5 the ethyl carboxylate substituent
in the chelate ring of 2 adopts an axial orientation and the
oxygen of the carbonyl group approaches the coordinated
nitrogen.§
Catalytic hydrocarboxylation of olefins is an interesting
reaction for the synthesis of the pharmacologically useful
2-arylpropionic acids, provided that a selective catalyst towards
the branched acid is used.6 Complex 2 has been subjected to a
preliminary evaluation in the hydrocarboxylation of styrene as
a model aryl olefin, with oxalic acid under CO pressure and no
added co-catalysts. Complex 2 has been found to be active, and
its selectivity in 2-phenylpropanoic acid (2-PP) was very high
(Table 1). Hydrogenation was the main side reaction, and it was
always under 0.2%. In low conversion conditions (i.e., when the
reaction was stopped after 2 h), yields increased steadily with
temperature. After 24 h, 75% conversions were reached and
selectivities as high as 98% in branched acid (2-PP) were
maintained. However, at 100 and 120 °C, lower than expected
conversions were observed. This result, together with the fact
that after depressurisation of the reactor, palladium black is
observed,¶ lead us to believe that the catalytic species suffers
degradation under such reaction conditions. The catalytic
reaction is faster at higher temperatures, but so is the
decomposition of the catalyst.
The 2-phenylpropanoic acid product is racemic in all cases.
Considering the fact that the reaction medium is strongly acidic,
and that as proposed7 it is reasonable to think that two free co-
ordination sites are needed for this reaction, we are suggesting
at this stage structure A for the active species. The relatively low
bulk of A is consistent with branched selectivity. Also, the lack
Chem. Commun., 1999, 277–278
277

of palladium, seem to indicate that sulfur ligands promote high
branched selectivity.
From a purely practical, synthetic point of view complex 2,
which is readily prepared from affordable, easily accessible
reagents, catalyses the conversion of styrene to 2-phenyl-
propanoic acid (98%) by reaction with oxalic acid under CO (30
bar), with a 75% yield at 80 °C in 24 h. Further work on the
synthesis, structure and evaluation of this novel class of
compounds in catalysis is under way and will be reported in due
course.
This work was financially supported by the DGICYT, project
PB91-0663-CO3; and the CICYT and DGR, project QFN95-
4725-CO3. We thank C. Claver for supplying a pre-print of ref.
8. M. P. thanks the UAB for a temporary teaching appointment
as her only support towards her doctorate.
Notes and references
† Synthesis of 1: A solution of HSCH2CH2NH2·HCl (0.52 mmol) in
methanol (5 ml) was slowly added to Pd(PPh3)4 (0.52 mmol) in toluene (20
ml) and stirred under nitrogen at 60 °C for 3 h to afford an orange solution.
Concentration of the solution caused precipitation of the product as an
orange solid, which was purified by recrystallization in acetone–diethyl
ether to yield [PdCl(SCH2CH2NH2)(PPh3)] 1 as dark orange crystals (65%
yield). The same procedure can be used for compounds 2 and 3. Compounds
1–3 gave satisfactory spectral (1H, 31P and 13C NMR and IR) and analytical
data.
‡ Crystal data for 2: C23H25ClNO2PPdS; M = 552.32; orthorhombic, a =
7.929(5), b = 10.330(2), c = 28.769(6) Å, V = 2356(2) Å3; T = 293 K;
space group P212121, Z = 4; m = 10.8 cm21; 2069 independent reflections
for 277 parameters; R(F) = 0.023 for 1910 reflections with I > 2s(I) and
Rw(F2) = 0.063 for all 2069 reflections; S = 1.129; Flack parameter
0.01(4). CCDC 182/1134. See http://www.rsc.org/suppdata/cc/1999/277/
for crystallographic files in .cif format.
§ The IR stretching frequencies for the CO and NH2 groups, and the distance
between the oxygen and hydrogen atoms O1…H–N of 2.18(4) Å, do not
support the existence of hydrogen bonding. The eclipsed conformation
could be favoured by an electrostatic interaction between the negatively
charged oxygen and a positively charged quaternary nitrogen.
¶ Powder X-ray diffraction showed it to be finely divided metal, and not
palladium sulfide of any crystallinity. This palladium black does not
catalyse the reaction, even when triphenylphosphine is added.
Fig. 1 The above projection of complex 2 includes all non-hydrogen atoms
(ORTEP-III, 50% probability). In the nearly eclipsed projection below, the
aryl rings have been omitted and the hydrogen atoms have been included, to
fully appreciate the axial position of the ethoxycarbonyl group. The C1–C2
bond forms an angle of 84.5(3)° with the coordination plane of the metal and
the O1–C1–C2–N torsion angle is 12.1(8)°. Selected distances (Å) and
angles (°): Pd–S 2.253(2), Pd–P 2.254(2), Pd–N 2.097(5), Pd–Cl 2.349(2),
S–C3 1.826(6), N–C2 1.483(7), C1–C2 1.509(8), C2–C3 1.482(8), C1–O1
1.192(7), C1–O2 1.323(8); S–Pd–P 92.92(6), S–Pd–N 85.7(1), Cl–Pd–P
93.63(6), Cl–Pd–N 87.7(1).
Table 1 Complex 2 as a catalyst in the hydrocarboxylation of styrene.
Conversion and selectivity towards 2-phenylpropanoic acid (2-PP) in the
reaction of styrene with oxalic acid under CO, at different temperatures over
2 and 24 ha
∑ Working in the same conditions as used for 2, we have found the
selectivity of PdCl2 or PdCl2(MeCN)2 plus 1 equiv. of PPh3 to be 85–88%
in 2-PP, although the second precursor is faster.
2 h
24 h
1 H.-U. Blaser, Chem.Rev., 1992, 92, 935; W. A. Herrmann and B. Cornils,
Angew. Chem. Int. Ed. Engl., 1997, 36, 1048.
Conversion Selectivity
Conversion Selectivity
2 H. S. Laurie in Comprehensive Coordination Chemistry, ed. G.
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T/°C
(%)
(%)
(%)
(%)
60
80
100
120
0.6
7
12
16
—
98
97
97
30
75
68
67
98
98
97
96
a
Conditions: 2 mmol styrene + 2.5 mmol H2ox·2H2O in 10 mL of DME
solvent; [styrene]/[2] = 50; CO pressure: 30 bar. No oxygen or co-catalysts
present.
3 J. Real, A. Polo and J. Duran, Inorg. Chem. Commun., 1998, 1, 457.
4 SHELX-97: G. M. Sheldrck, University of Göttingen, Germany, 1997.
5 M. N. Burnett and C. J. Johnson, ORTEP-III: Oak Ridge Thermal
Ellipsoid Plot Program for Crystal Structure Illustrations, Oak Ridge
National Laboratory Report ORNL-6895, 1996. PC version used: L. J.
Farrugia, J. Appl. Crystallogr., 1997, 565.
6 I. Tkachenko, in Comprehensive Organometallic Chemistry, ed. G.
Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 1987, vol.
8, pp. 101–223; H. Alper, J. B. Woell, B. Despeyroux and D. J. H. Smith,
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J. Am. Chem. Soc., 1990, 112, 2803; B. El Ali and H. Alper, J. Org.
Chem., 1993, 58, 3595; J. Mol. Catal., 1992, 77, 7; J.-Y. Yoon, E. J. Jang,
K. H. Lee and J. S. Lee, J. Mol. Catal. A, 1997, 118, 181.
of chiral induction would be consistent with structure A, in
which the asymmetric centre is displaced away from the metal.
At this point, we rule out complete detachment of the sulfur
ligand in the active species, because the activities and
selectivities observed with 2 were not those of a simple Pd(ii) +
PPh3 system with a phosphine/palladium ratio of unity, which is
the ratio in complex 2.∑ Furthermore, relevant results by Claver,
van Koten and coworkers8 with aryl aminothiolato complexes
7 B. El Ali and H. Alper, J. Mol. Catal., 1993, 80, 377; 1992, 77, 7.
8 D. Kruis, N. Ruiz, M. D. Janssen, J. Boersma, C. Claver and G. van
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Communication 8/09130G
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Chem. Commun., 1999, 277–278