Cationic phosphine ligands with phenylguanidinium modified xanthene moieties - A successful concept for highly regioselective, biphasic hydroformylation of oct-1-ene in hexafluorophosphate ionic liquids

Article abstract of DOI:10.1039/b009919h

New guanidinium-modified diphosphine ligands with a xanthene backbone show high overall activity and high regioselectivity in the biphasic hydroformylation of oct-1-ene using hexafluorophosphate ionic liquids.

Full text of DOI:10.1039/b009919h

Cationic phosphine ligands with phenylguanidinium modified xanthene
moieties—a successful concept for highly regioselective, biphasic
hydroformylation of oct-1-ene in hexafluorophosphate ionic liquids
Peter Wasserscheid,*a Horst Waffenschmidt,a Peter Machnitzki,a Konstantin W. Kottsieperb and
Othmar Stelzerb
a Institut für Technische Chemie und Makromolekulare Chemie, RWTH Aachen, Worringer Weg 1, D-52074 Aachen,
Germany. E-mail: Wasserscheidp@itc.rwth-aachen.de
b Anorganische Chemie, Bergische Universität—GH Wuppertal, Gaußstr. 20, D-42097 Wuppertal, Germany.
E-mail: Stelzer@uni.wuppertal.de
Received (in Liverpool, UK) 6th December 2000, Accepted 29th January 2001
First published as an Advance Article on the web 14th February 2001
New guanidinium-modified diphosphine ligands with a
xanthene backbone show high overall activity and high
regioselectivity in the biphasic hydroformylation of oct-
1-ene using hexafluorophosphate ionic liquids.
Biphasic catalysis is a well-established method for effective
catalyst separation and recycling. In the case of Rh-catalysed
hydroformylation reactions this principle is technically realised
in the Ruhrchemie–Rhône-Poulenc process where an aqueous
catalyst phase is used.1,2 Unfortunately, this process is limited
to C2–C5 olefins due to the low water solubility of higher
olefins. As an alternative polar medium for biphasic hydro-
formylation, Chauvin et al. suggested novel solvents known as
ionic liquids (for general reviews see refs. 3–5). These authors
described in detail the biphasic hydroformylation of pent-1-ene
with [Rh(CO)2acac]/triarylphosphine in e.g. 1-butyl-3-methyl-
imidazolium hexafluorophosphate ([BMIM]PF6).6,7 However,
none of the tested ligands (PPh3, sulfonated triaryl phosphines)
allowed a combination of high activity, complete retention of
the catalyst in the ionic liquid and high selectivity for the desired
linear hydroformylation product. Recently, some of us
described the successful application of cobaltocenium ligands
in the Rh-catalysed hydroformylation of oct-1-ene in hexa-
fluorophosphate ionic liquids, demonstrating that the reaction
benefits from the use of ligand systems that are specifically
designed for this application.8
Cationic phosphine ligands containing phenylguanidinium
moieties were originally developed to make use of their
pronounced solubility in water.9,10 They have been shown to
form active catalyst systems in Pd-mediated C–C coupling
reactions between aryl iodides and alkynes9,11 (Castro–
Stephens–Sonogashira reaction) and Rh-catalysed hydro-
formylation of n-hexene in aqueous two-phase systems.12
In the present article, we report a new and very general
approach to immobilise homogeneous Rh-catalysts in hexa-
fluorophosphate ionic liquids. We found that the modification
of neutral phophine ligands with cationic phenylguanidinium
groups represents a very powerful tool to immobilise Rh
complexes in these ionic liquids. In detail, we describe the
synthesis of the new ligand 1a and its catalytic performance in
the biphasic, Rh-catalysed hydroformylation of oct-1-ene using
the ionic liquid [BMIM]PF6 as catalyst solvent. This ionic
liquid has been prepared from [BMIM]Cl13 according to a
method described by Fuller and Carlin14 or has been purchased
from Solvent Innovation GmbH, Cologne.15 First experiments
aimed to prove the general concept by comparing the ligand
PPh3 with the related guanidinium-substituted ligand 2a (Table
1, entries 1–5). 2a was prepared from 2b by anion exchange
with NH4PF6 in aqueous solution. 2b was prepared as
previously described by Stelzer et al.10
takes place in the organic layer. After the first catalytic run, 53%
of the used Rh is found in the organic layer (according to ICP
analysis) (Table 1, entry 1). In contrast, with ligand 2a the
hydroformylation reaction takes place uniquely in the ionic
liquid layer (Table 1, entries 3, 4). In the first catalytic run the
hydroformylation activity was found to be lower than in the case
of PPh3 (probably due to some mass transfer limitation of oct-
1-ene into the ionic liquid). However, due to the excellent
immobilisation of the Rh-catalyst with 2a (leaching is < 0.07%
per run according to ICP analysis (detection limit)), the catalytic
activity of the ionic catalyst solution is even slightly higher in
the third recycling run compared to the first run (probably due
to some preformation time of the active catalyst). Since
NaTPPTS is the most widely used ligand for immobilisation of
Rh-catalysts in aqueous catalyst phases, this ligand was tested
as well (Table 1, entries 5, 6). The results reveal good
immobilisation of the Rh-complex in the ionic liquid but much
lower activity of the resulting catalyst in the hydroformylation
of oct-1-ene in comparison with 2a.
Encouraged by the good results with the guanidinium-
substituted ligand, we decided to adopt this new immobilisation
Table 1 Hydroformylation results with different ligands in [BMIM]PF6
Ligand
(cycle)
Conversion
(%)
Octane +
i-octenes (%) n+isob
No.
TOFa h21
1
2
3
4
5
6
7
8
9
PPh3 (1)
PPh3 (3)
2a (1)
69.1
10.2
32.1
35.3
7.8
680
100
276
330
80
78
15
30
52
0.4
0.1
2.5
3.5
4.9
4.9
1.5
2.4
3.4
3.9
2.8
2.7
2.0
1.7
2.6
2a (3)
NaTPPTS
NaTPPTS
1a (1)
1a (3)
1a (5)
7.7
2.6
10.6
21.9
38.3
44.3
19.1
20.9
21.3
18.0
10
1a (7)
58
a Turnover frequency (TOF) in mol of octene converted per mol of Rh per
h. b Ratio of linear to branched aldehyde products; general conditions:
CO+H2 = 1; p(CO/H2) = 30 bar; T = 100 °C; L+Rh = 2; Rh-precursor:
Rh(CO)2acac, preformation time 30 min; t = 1 h (runs 1–6), 8 h (runs
7–10).
In the reaction with PPh3, good catalytic activity is observed
but obviously a significant part of the hydroformylation reaction
DOI: 10.1039/b009919h
Chem. Commun., 2001, 451–452
This journal is © The Royal Society of Chemistry 2001
451

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concept to a ligand structure that promises better regioselectiv-
ity in the hydroformylation reaction. It is well-known that
diphosphine ligands with large natural P–metal–P bite angles
form catalysts for highly regioselective hydroformylation
reactions.16 Here, xanthene-type ligands (P–metal–P ~ 110°)
developed from van Leeuwen’s group proved to be especially
suitable showing, e.g. an overall selectivity of 98% towards the
desired linear aldeyde in oct-1-ene hydroformylataion.17–19
Recently, some work has been published where xanthene-based
ligands have been immobilised on silica support.20
We describe here the first use of xanthene-based ligands in
biphasic catalysis using ionic liquids as catalyst solvent. Since
xanthene ligands such as, e.g. 1b show highly preferential
solubility in the organic phase in a biphasic oct-1-ene/[BMIM]
PF6 mixture, even at rt we developed the guanidinium-modified
xanthene ligand 1a for this purpose. The cationic ligand 1a was
synthesised in four steps according to Scheme 1.†
complexes in hexafluorophosphate ionic liquids. Hereby, the
electronic properties of the phosphine is not changed sig-
nificantly. Our approach may therefore be of interest not only
for hydroformylations but also for many other catalytic
reactions in ionic liquids. Further work to develop methods for
the immobilisation of neutral catalyst complexes in ionic liquids
is in progress.
We wish to thank the Fonds der Chemischen Industrie and the
Deutsche Forschungsgemeinschaft for financial support. P. W.
and H. W. thank Professor W. Keim for his continuous interest
in this research and the European Community for founding
under the BRITE 96-3745 project.
Notes and references
† Preparation of 1a: to a solution of 0.75 g (2.74 mmol) of the diprimary
phosphine 1c (prepared according to van Leeuwen et al.22) and 3.17 g
(10.96 mmol) of 3-iodophenylguanidine 227 mg (2 mol%) of tris(benzyl-
idene acetone)dipalladium(0) were added and the reaction mixture was
heated to 80–100 °C for 24 h. 31P{1H} NMR spectroscopic control of the
reaction mixture indicated that all of the diprimary phosphine had been
consumed. On evaporation of the solvent in vacuo (80 °C, 0.01 mbar) 4.14
g of a yellow–brownish coloured powder were obtained. It contains small
amounts of 3-iodophenylguanidine as indicated by the 13C{1H}-NMR
spectrum.
Mass spectrum [SIMS(DTE/DTT/Sul)]: cation: m/z = 919; 31P{1H}-
NMR data (161.98 MHz, 298 K, d4-methanol, referenced to H3PO4) for 1a:
dP = 215.9. 13C{1H}-NMR data (100.63 MHz, 298 K, d4-methanol,
referenced to TMSint, values in parentheses N = JPC + JPAC) for 1a: 157.1,
153.3 (C–O, 19.3 Hz), 141.0, 139.9 (13.2 Hz), 139.1 (6.0 Hz), 133.1 (12.0
Hz), 132.9, 131.5, 131.1 (6.0 Hz), 130.6 (19.3 Hz), 128.6, 126.2, 125.2, 39.5
(NMe2), 35.5 (CMe2), 32.3 (CMe2).
Scheme 1 Synthesis of ligand 1a. a) diethylchlorophosphite; b) LiAlH4–
chlorotrimethylsilane; c) 4 eq. 3-iodophenylguanidine, 2 mol% Pd2dba3 *
CHCl3, 80 °C, 24 h in DMF.
1 E. G. Kuntz, Fr. Pat. 2314910, (to Rhone-Poulenc); E. G. Kuntz,
CHEMTECH, 1987, 570.
The structure of 1a was established by its mass spectrum and
its 31P{1H}NMR chemical shift (dP = 215.9) comparable to
that of 1b (dP = 217.9)21 and of the meta guanidinium
phosphines 2a (dP = 24.7).10 The signal at dC = 157.1 in the
13C{1H}NMR spectrum of 1a may be assigned to the
carbonium carbon atoms of the guanidinium moieties, the dC
value of which compares well with that in 2a10 (dC = 155.8).
Some of the 13C{1H}NMR resonances appear as higher order
line pattern (X-parts of AAAX spin systems, A, AA = 31P, X =
13C) in agreement with the diphosphine structure proposed for
1a.
2 W. A. Herrmann and C. W. Kohlpaintner, Angew. Chem., Int. Ed. Engl.,
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After each hydroformylation run, in which 1a was used as
crude product, the organic layer was decanted off (under normal
atmosphere) and the remaining ionic catalyst layer remained in
the autoclave for the next run. It is noteworthy that the catalytic
activity increases during the first runs to obtain a stable level
only after the forth recycling run (Table 1, entries 7–10). This
behaviour is attributed to a certain catalyst preforming time as
well as to impurities of 3-iodophenylguanidine in the used
ligand sample (about 5 mass%). The latter are slowly washed
out from the catalyst layer over the first catalytic runs. After ten
consecutive runs an overall turnover number of 3500 mol oct-
1-ene per mol Rh-catalyst could be obtained. In good agreement
with the recycling experiments, the Rh-leaching into the organic
layer was found to be very low. With AAS and ICP analysis no
rhodium could be detected in the organic layer indicating a
leaching of less than 0.07%. In all experiments with 1a very
good selectivities for the linear aldehyde were obtained thus
proving that the attachment of the guanidinium moiety to the
xanthene backbone does not influence its known positive effect
on the regioselectivity of the reaction. This is in line with
previous IR and NMR studies in our laboratories showing that
the steric and electronic properties of arylphosphines is not
–
significantly changed by introduction of polar groups like SO3 ,
2–
PO3 and guanidinium in meta- or para-position to phospho-
rus.10
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Lett., 1995, 36, 75.
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W. N. M. van Leeuwen, Angew. Chem., Int. Ed., 1998, 37, 3299.
In conclusion, we could show that the modification of known
phosphine ligands with guanidinium groups represents a simple
and very efficient method to fully immobilise transition metal
452
Chem. Commun., 2001, 451–452