A novel dicationic phenoxaphosphino-modified Xantphos-type ligand - A unique ligand specifically designed for a high activity, selectivity and recyclability

Article abstract of DOI:10.1039/b209486j

A novel phenoxaphosphino-modified ligand has been prepared and successfully employed in the rhodium catalysed hydroformylation of 1-octene in ionic liquids showing unprecedented high selectivity and activity without detectable rhodium or phosphorus leaching during recycling experiments.

Full text of DOI:10.1039/b209486j

A novel dicationic phenoxaphosphino-modified Xantphos-type ligand—
a unique ligand specifically designed for a high activity, selectivity and
recyclability†
Raymond P. J. Bronger,a Silvana M. Silva,b Paul C. J. Kamera and Piet W. N. M. van Leeuwen*a
a Institute of Molecular Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV
Amsterdam, The Netherlands. E-mail: pwnm@science.uva.nl; Fax: +31(0)205256456
b Laboratory of Molecular Catalysis, Instituto de Quimica, UFRGS, Av. Bento Gonçalves, 9500, Porto
Alegre 91501-970 RS, Brazil
Received (in Cambridge, UK) 1st October 2002, Accepted 25th October 2002
First published as an Advance Article on the web 15th November 2002
A novel phenoxaphosphino-modified ligand has been pre-
pared and successfully employed in the rhodium catalysed
hydroformylation of 1-octene in ionic liquids showing
unprecedented high selectivity and activity without detect-
able rhodium or phosphorus leaching during recycling
experiments.
bromination on the 4,5 positions of the xanthene backbone
followed by dilithiation and reaction with 10-chloro-2,8-dime-
thylphenoxaphosphine yielded 2. Reaction of 2 with two
equivalents of 1-methylimidazole at 80 °C followed by a X2—
PF6 (X = halogen) exchange gives the desired ligand 1
(Scheme 1).
2
The catalyst was immobilised by mixing 5 mg of Rh(CO)2(a-
cac) and four equivalents of ligand in an acetonitrile/BMI·PF6
mixture. After stirring for 1 h the acetonitrile was removed by
evaporation under reduced pressure at 60 °C for 3 h affording a
red ionic liquid catalyst solution. The hydroformylation of
1-octene was carried out at 100 °C under 17 bar CO–H2 (1+1)
using 3 mL of ionic liquid and 3 mL of 1-octene. At
approximately 30% conversion the reaction was stopped by
venting the gases and cooling down the autoclave in an ice-bath.
Recycling experiments were performed by removal of the
octene–aldehyde layer followed by subsequent charging of
fresh 1-octene, repressurisation and heating the autoclave to the
desired temperature. The results of seven consecutive recycling
experiments are shown in Table 1.
The applied catalysis conditions were the same as the
optimised conditions described by Dupont et al. where
sulfonated Xantphos was used as modifying ligand.7 Already
from the first experiment it is clear that very high linear over
branched (l/b) ratios are obtained by employing this ligand,
albeit with moderate activity. The increase in catalytic activity
observed for recycle experiments 2 through to 4 possibly
indicate that incomplete conversion to the active rhodium–
hydride species has taken place. Others also reported this
effect.6 Only from recycle experiment 4 were stable catalysis
results obtained. The relatively low activity for this ligand
system and the red color of the rhodium complex in the ionic
liquid suggests the presence of a dimeric rhodium species.11 In
order to shift the equilibrium between the rhodium–hydride and
the rhodium-dimer towards the rhodium–hydride a higher
partial hydrogen pressure was applied (Scheme 2). Indeed, the
catalytic activity could be enhanced threefold, to TOFs > 300
h21, by applying higher pH2 while keeping the pCO constant
(Table 1, entries 6 and 7).
In recent years room temperature ionic liquids (RTIL) have
proven to be attractive mediums for many homogeneously
catalysed reactions. Much attention has been devoted to this
chemistry due to the prospects of ‘green’ catalysis, because easy
product and catalyst separation is possible. In many cases the
products are slightly soluble in the ionic phase and as a result the
catalyst can be separated by simple phase separation and
recycled.1 Compared to the aqueous two-phase catalysis
concept, the scope of two-phase catalysis can be extended to
substrates that are poorly soluble or insoluble in aqueous media.
In addition, the use of phosphites,2 phosphonites and phosphi-
nites as modifying ligands for two-phase catalysis in RTILs
comes within reach because degradation reactions, such as
hydrolysis,3 are less likely to occur.
For hydroformylation reactions 1-butyl-3-methylimidazo-
lium hexafluorophosphate (BMI·PF6) is often used as reaction
medium, usually showing good activity,2,4,5 selectivity2,5,6 or
complete retention of the catalyst.6 However, to the best of our
knowledge, no RTIL-system for hydroformylation is known
that combines all three aspects.
In order to obtain a catalytic system that combines a high
activity, high selectivity and a good retention, a novel
diphosphine ligand was designed. Recent studies have shown
that various xanthene based diphosphines show a relatively high
selectivity when modified for use in RTILs.6,7 For the
hydroformylation of 1-octene in conventional solvents like
toluene, ligands based on the xanthene(-type) backbone always
proved to be very active and selective.8 Especially when
phenoxaphosphino-modified ligands are used the catalytic
activity is enhanced considerably.9 Third, the effect of the
nature of the anions and cations of the ionic liquid and ligand
has a major influence on the recyclability of the catalyst.
Excellent retention of the catalyst was observed by adjusting the
ligand to the ions of the solvent.2 Therefore, we opted to
synthesize a phenoxaphosphino-modified xantphos-type ligand
that contains two 1-methyl-3-pentylimidazolium hexafluor-
ophosphate moieties (1) resembling closely the reaction
medium (Fig. 1).
The excellent recycling results were confirmed by ICP
analysis of the octene–aldehyde mixture after catalysis. No
phosphorus ( < 100 ppb) or rhodium ( < 5 ppb) leaching was
The ligand can easily be prepared via a six-step synthetic
route. First, a Friedel–Crafts acylation using 5-bromovaler-
icacid chloride and 9,9-dimethylxanthene was carried out. The
ketone functionalities were catalytically reduced using InCl3
10
and chlorodimethylsilane,‡
because standard methods for
reduction, like the Wolff-Kischner or Clemmensen reduction,
cannot be used due to the reactivity of the bromo-group. Next,
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b209486j/
Fig. 1 Novel dicationic ligand.
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CHEM. COMMUN., 2002, 3044–3045
This journal is © The Royal Society of Chemistry 2002

Scheme 1 Synthesis of ligand 1 (yields in parenthesis). Reagents and conditions: (a) 2.2 eq. AlCl3/2.2 eq. 5-bromovalericacid chloride (90%), (b) 1) InCl3/
chlorodimethylsilane (86%), 2) Br2 (90%), (c) 1) n-BuLi, 280 °C, 30 min., 2) 10-chloro-2,8-dimethylphenoxaphosphine (48%), (d) 1) 1-methylimidazole,
80 °C, 8 days in CH3CN–toluene (46%), 2) KPF6 (77%).
Table 1 Hydroformylation of 1-octenea
In conclusion, we have reported for the first time, the use of
a phenoxaphosphino-modified ligand in RTILs. This novel
Cycle % Isom.b TOFb,c/h21 l/bb
% Sel.b
catalytic system offers a unique combination of a high activity,
a high selectivity for the preferred linear aldehyde with
complete retention of the catalyst in the ionic phase (see Table
2 for a comparison with some other systems). Besides, the
catalyst proved to be stable for more than two weeks under
ambient conditions, which shows the high stability of the
system. Nevertheless, because of the aforementioned reasons,
storage under inert atmosphere is advisable. Further work to
improve activity, in order to enable selective hydroformylation
of internal olefins, is currently in progress.
1
2
3
4
5
6d
7d
11.8
8.3
7.8
7.7 112
9.6 107
13.3 318
17.7 305
65
88
93
44
49
44
44
38
49
55
86.2
89.8
90.1
90.3
88.1
85.0
80.8
a Conditions: T = 100 °C, p(CO–H2, 1+1) = 17 bar, ligand/Rh = 4,
substrate/Rh
= 988. In none of the experiments was hydrogenation
observed. b Linear over branched ratio, percent isomerisation to 2-octene,
percent linear aldehyde and turnover frequency were determined at ~ 30%
alkene conversion. c Turnover frequency = (mol aldehyde) (mol Rh)21
Financial support from Celanese GmbH is gratefully ac-
knowledged.
h21 d pH2 = 40 bar, pCO = 6 bar.
.
Notes and references
‡ By applying these conditions the bromo-groups were partially substituted
by chloro-groups. This substitution is clearly observed in both 1H NMR and
GC–MS spectra. This substitution did not appear to have a negative effect
on further reactions, although reaction of 1-methylimidazole with bro-
moalkyl groups proceeds much faster than reaction with chloroalkyl
groups.
Scheme 2 Equilibrium between Rh-dimer and Rh–hydride species.
1 Comprehensive information about this field can be found in the many
reviews that have appeared on this subject: D. Zhao, M. Wu, Y. Kou and
E. Min, Catal. Today, 2002, 74, 157; P. Wasserscheid and W. Keim,
Angew. Chem., Int. Ed., 2002, 39, 3772; H. Olivier-Bourbigou and L.
Magna, J. Mol. Catal. A: Chemical, 2002, 419; J. Dupont, C. S. Consorti
and J. Spencer, J. Braz. Chem. Soc., 2000, 11, 337; C. M. Gordon, Appl.
Catal. A, 2001, 222, 101; R. Sheldon, Chem. Commun., 2001, 2399.
2 F. Favre, H. Olivier-Bourbigou, D. Commereuc and L. Saussine, Chem.
Commun., 2001, 1360.
3 P. C. J. Kamer, J. N. H. Reek and P. W. N. M. van Leeuwen, in Rhodium
Catalyzed Hydroformylation, ed. P. W. N. M. van Leeuwen and C.
Claver, Kluwer Academic Publishers, Dordrecht, 2000, ch. 3.
4 D. J. Brauer, K. W. Kottsieper, C. Liek, O. Stelzer, H. Waffenschmidt
and P. Wasserscheid, J. Organomet. Chem., 2001, 630, 177.
5 C. C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt and P.
Wasserscheid, Organometallics, 2000, 19, 3818.
detected at all. Furthermore the catalyst proved to be extremely
stable even under atmospheric conditions without taking special
precautions. Hydroformylation of 1-octene after storing the
ionic liquid containing the catalyst at room temperature under
air for more than 14 days resulted in equal catalysis results as
the previous hydroformylation experiment, although a slightly
higher rate of isomerisation is observed. The increased
isomerisation might be due to formation of acidic impurities
caused by some anion hydrolysis with moisture from air.
Table 2 Comparison of different systemsa
Rh
P
leachingc leachingc
6 P. Wasserscheid, H. Waffenschmidt, P. Machnitzki, K. W. Kottsieper
and O. Stelzer, Chem. Commun., 2001, 451.
Ligand backbone Ref.
TOFb/h21
l/b
(%)
(%)
7 J. Dupont, S. M. Silva and R. F. de Souza, Catal. Lett., 2001, 77,
131.
Phenold
2e
4
5
6
7
240
552
810
52
13
1
16
21
13
2
nr
< 0.2
< 0.07j
nr
nrf
nr
nr
nr
nr
2-Imidazoliumg
Cobaltoceniumh
Xanthenei
8 M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M. van
Leeuwen, K. Goubitz and J. Fraanje, Organometallics, 1995, 14, 3081;
L. A. van der Veen, M. D. K. Boele, F. R. Bregman, P. C. J. Kamer, P.
W. N. M. van Leeuwen, K. Goubitz, J. Fraanje, H. Schenk and C. Bo,
J. Am. Chem. Soc., 1998, 120, 11616; L. A. van der Veen, P. H. Keeven,
G. C. Schoemaker, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van
Leeuwen, M. Lutz and A. L. Spek, Organometallics, 2000, 19, 872.
9 L. A. van der Veen, P. C. J. Kamer and P. W. N. M. van Leeuwen,
Organometallics, 1999, 18, 4765; L. A. van der Veen, P. C. J. Kamer
and P. W. N. M. van Leeuwen, Angew. Chem., Int. Ed., 1999, 38,
336.
Xanthenek
32
Our
system 317
Xanthene
49
< 0.07j
< 0.4j
a Conditions: reaction medium = BMI·PF6, substrate = 1-octene, T = 100
°C, p(CO–H2) = 10–46 bar. b Turnover frequency = (mol aldehyde) (mol
.
Rh)21 h21 c Percentage of leached rhodium/phosphorus of initial intake.
d Monophosphite ligand. e Substrate = 1-hexene, T = 80 °C. f nr = Not
reported. g Monophosphine ligand. h Diphosphine ligand. i Phenylguani-
dium modified diphosphine ligand. j Detection limit of ICP analysis.
k Sulfonated diphosphine ligand.
10 T. Miyai, M. Ueba and A. Baba, Synlett., 1999, 2, 182.
11 A. Castellanos-Páez, S. Castillon, C. Claver, P. W. N. M. van Leeuwen
and W. G. J. de Lange, Organometallics, 1998, 17, 2543.
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