3102
Organometallics 2006, 25, 3102-3104
Electronic Effect of Diphosphines on the Regioselectivity of the
Palladium-Catalyzed Hydroesterification of Styrene
Ester Guiu,† Maria Caporali,† Bianca Mun˜oz,‡ Christian Mu¨ller,† Martin Lutz,§
Anthony L. Spek,§ Carmen Claver,‡ and Piet W. N. M. van Leeuwen*,†
Department of Chemical Engineering and Chemistry, Schuit Institute of Catalysis, EindhoVen UniVersity of
Technology, Den Dolech 2, 5600 MB EindhoVen, The Netherlands, Departament de Qu´ımica F´ısica i
Inorga`nica, UniVersitat RoVira i Virgili, Marcel.li Domingo s/n 43007 Tarragona, Spain, and BijVoet
Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity, Padualaan 8,
3584 CH Utrecht, The Netherlands
ReceiVed February 7, 2006
Summary: The electronic character of the diphosphine ligands
controls the regioselectiVity of the Pd-catalyzed carbonylation
of styrene. In the presence of electron-poor phosphines branched
esters are produced and high actiVities of the catalytic systems
are obserVed.
(110°) (2) showed high regioselectivity toward the linear
product7 (Scheme 1). In contrast with this last statement, Tanaka
and co-workers8 recently reported an unexpected selectivity for
the formation of branched products using a palladium catalyst
bearing a bulky electron-donor diphosphine but no evidence of
the presumed coordination as a cis bidentate ligand was given
to correlate the geometry with the observed regioselectivity.
In addition to the bite angle effect, other parameters such as
electronic and steric effects may be taken into consideration,
since they could play an important role in the regioselectivity-
determining step.9 Unfortunately, bidentate ligands lead pref-
erentially to linear products and, thus, as yet the plethora of
chiral bidentates cannot be used to produce esters enantiose-
lectively. Our aim is to obtain branched esters utilizing bidentate
systems and to show how the electronic and steric parameters
of the ligand influence the catalytic activity and product
selectivity of this reaction. Highly selective palladium-catalyzed
formation of branched esters from styrene and carbon monoxide
using DPEphos-based diphosphines and a Brønsted acid will
be presented. These findings may have a great impact on the
design of chiral bidentate systems that perform the reaction in
an enantioselective way.
The DPEphos-type ligands 3 and 4 were synthesized having
markedly different electronic properties, while the ligand 510
causes considerable steric hindrance (Scheme 1). The complexes
PdCl2(1) (8), PdCl2(3) (9), PdCl2(4) (10), and PdCl2(5) (11) were
prepared by direct reaction of Pd(COD)Cl2 with the correspond-
ing phosphines. The preferential coordination mode of the
ligands toward the Pd center is cis, as determined by 13C{1H}
NMR spectroscopy at room temperature. The geometry assign-
ment is straightforward because of the well-known recognizable
AX spin system observed in 13C{1H} NMR attributed to the
ipso aromatic carbon of the cis isomer.11 Reaction of 3 with
Pd(COD)CH3Cl led to the quantitative formation of the corre-
sponding Pd complex Pd(CH3)Cl(3) (14). The 31P NMR
spectrum of 14 revealed that the ligand is coordinated in a cis
The metal-catalyzed carbonylation of olefins is an attractive
process for the production of valuable compounds with several
pharmaceutical and agrochemical applications.1 Vinyl aromatics
are of special interest as starting materials, as they lead to
important products such as ibuprofen. In this reaction both
branched and linear carboxylic acid esters are formed. Regio-
selectivities depend on the catalytic system employed as well
as on the reaction conditions used.2 It is well-known that the
alkoxycarbonylation reaction might proceed by two different
mechanisms, which differ in the involvement of Pd-H or Pd-
COOR intermediates.3 It has been reported that insertion of
styrene into a metal-acyl bond leads to branched products,
while insertion into hydrides leads to linear products.4 Further-
more, regiocontrol depends on whether monophosphines or
diphosphines are used as the ligands, even though both systems
involve Pd-H species. It was found that monophosphines favor
trans intermediate species, and it was speculated that these trans
complexes lead to the formation of branched esters after
replacement of one coordinated monophosphine by the olefin.5
On the other hand, high linearity is achieved by using chelating
systems, in which diphosphines promote the most favored cis-
coordinated complexes. Also, a correlation between the selectiv-
ity and the P-Pd-P bite angles was found. Thus, diphosphines
with wide bite angles provide high conversions and almost
exclusively linear product.6 For instance, the palladium systems
based on DPEphos (natural bite angle 102.7°) (1) and Xantphos
* To whom correspondence should be addressed. E-mail:
† Eindhoven University of Technology.
‡ Universitat Rovira i Virgili.
§ Utrecht University.
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10.1021/om060121t CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/18/2006