SUPRAPhos ligands for the regioselective rhodium catalyzed hydroformylation of styrene forming the linear aldehyde

Article abstract of DOI:10.1039/b818665k

We report SUPRAphos-based rhodium catalysts that are unusually regioselective in the hydroformylation of styrene at low temperatures (40 °C) producing mainly the linear aldehyde (72%). This ligand-specific phenomenon is observed for two rhodium catalysts with heterobidentate phosphine-phosphoramidite ligands. We propose that the high selectivity for the linear product is caused by enhanced β-hydride elimination, due to specific substrate-ligand interactions, most likely π-π interactions.

Full text of DOI:10.1039/b818665k

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SUPRAPhos ligands for the regioselective rhodium catalyzed
hydroformylation of styrene forming the linear aldehyde†
P. Elsbeth Goudriaan, Mark Kuil, Xiao-Bin Jiang, Piet W. N. M. van Leeuwen and Joost N. H. Reek*
Received 21st October 2008, Accepted 12th December 2008
First published as an Advance Article on the web 23rd January 2009
DOI: 10.1039/b818665k
We report SUPRAphos-based rhodium catalysts that are unusually regioselective in the
hydroformylation of styrene at low temperatures (40 ◦C) producing mainly the linear aldehyde (72%).
This ligand-specific phenomenon is observed for two rhodium catalysts with heterobidentate
phosphine–phosphoramidite ligands. We propose that the high selectivity for the linear product is
caused by enhanced b-hydride elimination, due to specific substrate–ligand interactions, most likely p–p
interactions.
development of highly selective catalysts for the preparation of
Introduction
the branched aldehyde. Indeed, both rhodium catalysts containing
The conversion of alkenes into aldehydes represents an important
phosphorus ligands and unmodified rhodium catalysts favour the
formation of the branched product.7 Large bite angle ligands such
as xantphos, xantphite and calix-diphosphites give the highest
selectivities for the linear product8,7 in the hydroformylation of
styrene and the selectivity is commonly higher at elevated reaction
temperature and lower pressure. For example, application of
Rh(xantphos)(CO)2H at 60 ◦C and 10 bar of syn-gas results in
reaction, especially because several tools are available to convert
the aldehyde functionality into a variety of important functional
groups (carboxylic acids, alcohols, amines, etc). Undoubtedly,
the most important and clean reaction to introduce aldehyde
moieties is the hydroformylation of alkenes.1 This method is
industrially applied for the large scale synthesis of butanal and
butanol, 2-ethylhexanol and detergent alcohols.2 In asymmetric
hydroformylation, the most intensively studied substrate is styrene
(Scheme 1).3–5 The optically active aldehydes obtained from the
hydroformylation of styrene and other vinyl aromatics can be oxi-
dized to yield 2-arylpropionic acids, which are important synthetic
building blocks for the preparation of various pharmaceuticals,
such as the anti-inflammatory agents Ibuprofen, Ketoprofen and
Naproxen.6
◦
the formation of 44% of the linear aldehyde, whereas at 120 C,
70% linear product is formed.9 The enhanced selectivity for the
linear product at higher temperatures is ascribed to the reversible
formation of branched alkyl rhodium species, which regenerates
the styrene-adduct via b-hydride elimination (IIa → I), whereas
almost no b-hydride elimination from the linear alkyl-metal takes
place (IIb → I; Scheme 2). Consequently, part of the branched
rhodium alkyl species isomerises to the linear rhodium alkyl
complex, causing reversal of the selectivity from the branched
to the linear product.10
We11,12 and others,13 have recently introduced a new class of
bidentate ligands that form via self-assembly of two monodentate
ligands (Scheme 3). Among other motifs we have used the
zinc(II)porphyrin–pyridine interaction as an assembly motif to
generate bidentate phosphorus-based ligands. The role of the
porphyrin in the structures displayed in Scheme 3 is to bring the
P-donor atom attached to the pyridyl moiety in close proximity
to that attached to the porphyrin building block, in order to form
supramolecular, bidentate ligands. With the use of a small number
of building blocks a large library of (hetero-)bidentate phosphorus
ligands11b,d was generated that was successfully used in asymmetric
hydrogenation11d and asymmetric allylic alkylation.11c,d In our
efforts to use combinatorial approaches to search for novel
catalysts for the asymmetric hydroformylation of styrene, we
discovered that some of our SUPRAphos-ligands gave an unusual
selectivity for the linear aldehyde. The formation of the two
products follow different kinetics,14 which makes a combinatorial
approach even more useful, as will be reported in this contribution.
Scheme 1 Hydroformylation of styrene leading to chiral branched
aldehyde and the linear aldehyde.
While most investigations have focused on the formation of
the chiral aldehyde product, catalysts that produce selectively the
(non-chiral) linear aldehyde have received little attention. Most
likely, insertion of styrene gives mostly the branched hydrocarbyl
3
species, stabilized as a benzylic group coordinating in an h -
fashion or stabilized by a mesomeric interaction, which results in a
strong preference for the formation for the branched aldehyde. The
development of catalysts that form the linear aldehyde selectively,
from vinyl aromatics is therefore more challenging than the
Homogeneous and Supramolecular Chemistry, Van‘t Hoff Institute for
Molecular Sciences, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The
Netherlands. E-mail: j.n.h.reek@uva.nl; Fax: +31 205255604; Tel: +31
205256437
† This contribution is dedicated to Prof. Roeland J. M. Nolte on the occa-
sion of his 65th birthday and to celebrate his outstanding contributions to
molecular science.
Results and discussion
From an initial screening of a part of the SUPRAphos library
in the hydroformylation in styrene, the heterobidentate
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Table 1 Results of the hydroformylation of styrene using a small
SUPRAPhos phosphite–phosphoramidite librarya
Entry
L
Conversionb (%)
b/lc
ee (%)
1d
2
3
4
5
6
7
8
9
1·F
2·A
2·B
2·C
2·D
2·E
3·A
3·B
3·C
3·D
3·E
<1
16
8.8
13
8.8
5.9
39
16
1.9
11
1.0
8.3
2.5
0.8
7.9
7.5
5.3
6.6
0.5
2.9
3.4
76 (R)
16 (R)
22 (R)
25 (R)
2.2 (R)
4.0 (R)
7.3 (S)
0
27 (R)
10 (R)
5.2 (R)
10
11
13
a Conditions: [Rh(acac)(CO)2]
=
1.00 mM, [porphyrin phospho-
ramidite] = 5.00 mM, [pyridyl phosphine] = 5.00 mM, [diisopropylethyl
amine] = 50.5 mM, substrate: Rh = 1000, toluene, pCO/H2 = 20 bar,
T = 40 ◦C, 20 h. b Percentage conversion of styrene to aldehydes. c b/l =
branched/linear. d Reaction time = 88 h.
activity at 80 ◦C; 83% of the substrate is converted in 3 h compared
to 2% at 40 ◦C in 20 h. At 120 ◦C, the linearity decreases slightly
to 65% (entry 6).
Generally, selectivity for the linear product in styrene hydro-
formylation can be enhanced by applying lower syn-gas pressure.14
This will result in slower CO-coordination at complex II (II → III;
see Scheme 2); the resulting lower concentration of III will slow
down the migratory insertion of CO (III → IV), while complex
IIa will re-form complex I via b-hydride elimination. Complex
IIb is less prone to b-hydride elimination than IIa and it shows
fast migratory insertion of CO, thereby reversing the selectivity
towards the linear product. When the pressure is reduced from
20 bar to 10 bar of syn-gas, 72% linear product is obtained, albeit in
lower yield (entry 5). This suggests that at 20 bar syn-gas pressure,
there is already a considerable amount of b-hydride elimination.
Importantly, rhodium catalysts based on either ligand 3 or
ligand C applied as monodentate ligands do not show the peculiar
selectivity for the linear product. At 80 ◦C, both catalysts provide
Scheme 2 General mechanism of the hydroformylation of styrene.
Enhanced selectivity for the linear aldehyde at higher temperature is due
to fast equilibrium between IIa and IIb.
phosphite–phosphoramidite ligand 1·F was identified as a lead
structure for the formation of enantioselective catalysts, because
it provided 2-phenylpropionaldehyde in 76% ee.11j Unfortunately,
the activity of this catalyst was very low. We decided to make a
small sub-library of related ligands in order to improve activity
and selectivity (Scheme 3). The in situ formed rhodium catalysts
of these ligands were explored in the hydroformylation of styrene
at 40 ◦C and 20 bar CO/H2 in toluene and the results are displayed
in Table 1.
The higher activity we aimed for using this sub-library was
indeed achieved, but as regards enantioselectivity the results of
the new library were disappointing. Unexpectedly, two catalysts
(2·C and 3·C, entries 4 and 9) showed unusual preference for the
formation of linear product, even at low temperatures (40 ◦C).
To obtain more information about this particular selectivity of
the SUPRAphos ligand systems in hydroformylation, we applied
the rhodium catalysts based on these assemblies in the hydro-
formylation of both styrene and 1-octene at various temperatures
(Table 2).
◦
only 12% of the linear product (entry 8 and 10). At 120 C, the
selectivity for the linear product increases to 31% for the catalyst
based on ligand 3, whereas the catalyst based on C yields 17%
of the linear product (entry 9, 11). It is interesting to note that
the rhodium catalyst based on 3·C still provides a much higher
selectivity at 120 ◦C than the catalysts based on the monodentate
analogues (HRh(3)2(CO)2 and HRh(C)2(CO)2). This shows that
the bidentate character of the supramolecular ligand is retained
even at this high temperature.
In the hydroformylation of 1-octene, the catalyst based on ligand
3·C displays a relatively low selectivity of the linear product, 60%
both at 80 and 120 ◦C. In the latter experiment a high isomerization
is also observed (17%, entries 12 and 13). The linearity obtained
by the catalyst based on 3·C is lower than those obtained for
the catalysts based on the monodentate analogues (at 80 ◦C 64%
with HRh(3)2(CO)2 and 70% with HRh(C)2(CO)2). Interestingly,
the trends observed for styrene are not translated to similar
selectivities in the hydroformylation of 1-octene.15
Application of the catalyst based on SUPRAphos ligand 2·C
◦
◦
at 80 C shows similar selectivity as at 40 C: 54 and 51% linear
aldehyde (entry 1 and 2). On the contrary, the catalyst based on
ligand 3·C shows enhanced selectivity for the linear product at
80 ◦C; 69% of the linear product is formed, compared to 63% at
40 ◦C (entry 3 and 4). In addition, there is a remarkable increase in
We propose that the high selectivity for the linear product in the
styrene hydroformylation with HRh(2·C)(CO)2 and Rh(3·C)(CO)2
catalysts is caused by enhanced b-hydride elimination of the
branched metal–alkyl intermediate IIa. The relatively high
1802 | Dalton Trans., 2009, 1801–1805
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Scheme 3 SUPRAphos ligands used in the hydroformylation of styrene.
Table 2 Hydroformylation of styrene and 1-octene at different temperatures with porphyrin phosphoramidite–pyridyl phosphine rhodium complexesa
Entry
L
T/◦C
t/h
Conversion (%)b
b/lc
% Lineard
% Hydrogenatione
Styrene
1
2
3
4
5¢
6
7
8
9
10
11
2·C
2·C
3·C
3·C
3·C
3·C
2
40
80
40
80
80
120
80
80
120
80
120
20
3
20
3
3
1
3
3
1
3
13
49
1.9
83
24
86
8.1
83
>99
>99
>99
0.8
1.0
0.5
0.4
0.4
0.5
4.1
4.2
2.3
7.4
4.8
54
51
63
69
72
65
19
12
31
12
17
0.5
0.4
4.1
0.2
0.7
0.2
1.8
0.4
0.9
0.4
1.1
f
3
3
C
C
1
Entry
L
T/◦C
t/h
Conversion (%)b
l/bc
% Lineard
% Isomerisatione
1-Octene
12
13
14
15
16
17
3·C
3·C
3
80
120
80
120
80
3
1
3
1
3
1
60
86
86
99
99
99
2.0
2.6
2.4
2.5
2.4
2.4
60
60
64
67
70
68
9.7
17.2
8.2
7.0
1.7
3
C
C
120
3.0
a Conditions: [Rh(acac)(CO)2] = 1.00 mM, [porphyrin phosphoramidite] = 5.00 mM, [pyridyl phosphine] = 5.00 mM, [diisopropylethyl amine] =
50.5 mM, substrate: Rh = 1000, toluene, pCO/H2 = 20 bar. b Percentage conversion of styrene. c b/l = branched/linear; l/b = linear/branched selectivity.
d Percentage linear aldehydes to all products obtained from reaction. e Percentage of hydrogenated styrene (in the case of styrene hydroformylation) or
isomerised 1-octene in the case of octene hydroformylation. f At 10 bar CO–H2 1 : 1.
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percentage isomerization in 1-octene hydroformylation also in-
dicates a significant contribution of b-hydride elimination. This
implies that in the current system, hydride migration is reversible
at temperatures as low as 40 ◦C. This also explains the small
temperature effect displayed by our catalysts; at 120 ◦C the
linearity is only slightly higher than that observed at 80 ◦C. Why
the current catalyst based on the phosphoramidite–phosphine
ligand 3·C results in such high selectivity for the linear product
is unclear. A high selectivity for linear aldehydes is not typi-
cal of phosphoramidite ligands. For instance, the phosphine–
phosphoramidite ligand reported by Zhang preferentially forms
the branched aldehyde (b/l of 7.3–10.1, depending on the condi-
tions used).16 Also, catalysts based on monodentate and bidentate
phosphoramidite ligands reported by our group showed high
selectivity for the branched aldehyde (varying from 74–93%).17
The fact that particularly these two phosphine–phosphoramidite
ligands result in typical selectivity suggest a very ligand-specific
selectivity. Since our SUPRAphos ligands also provide the most
selective rhodium catalysts for the asymmetric hydrogenation of
aromatic cyclic enamides,11e it is tempting to speculate that the
pocket defined by the aromatic porphyrin and the aromatic groups
of the bis-naphthol moiety plays an important role in directing the
substrate, for instance by sandwiching the styryl group between
the porphyrin ring and aryl moieties of the ligand. In the current
hydroformylation reaction this could lead to an energetically stable
intermediate complex, reducing the rate of migratory insertion of
carbon monoxide and facilitating the b-hydride elimination in the
branched intermediate.
run on a Shimadzu GC-17A apparatus (split/split less, equipped
with a F.I.D. detector and a BPX35 column (internal diameter
0.22 mm, film thickness 0.25 mm, carrier gas 70 kPa He)). Chiral
gas chromatographic analyses were performed on an Interscience
Trace GC Ultra (F.I.D. detector) with a ph Megadex column
(internal diameter 0.1 mm, 5 m column, film thickness 0.1 mm).
Materials
With exception of the compounds given below, all other reagents
were purchased from commercial suppliers and used without
further purification. The preparation of ligands D–F and 1–3 is
described in another article.11j Dr P. H. M. Budzelaar (Koninkli-
jke/Shell Laboratorium Amsterdam) is gratefully acknowledged
for the donation of the pyridyl phosphorus ligands A–C.
Procedure for the hydroformylation of styrene and 1-octene
The experiments were performed in a stainless steel autoclave
(volume 150 mL), charged with an inset suitable for 14 reaction
vials (including Teflon stirring bars) for conducting parallel
reactions. The vessels (GC-vials) were oven-dried, cooled under
vacuum and filled under nitrogen. The substrate styrene was
freshly filtered over neutral alumina before use to remove possible
peroxide impurities. In a typical experiment, the vials were
filled with 0.50 mmol [Rh(acac)(CO)2], 2.50 mmol acceptor-type
phosphorus ligand (porphyrin phosphite or porphyrin amidite),
2.50 mmol donor-type phosphorus ligand (e.g. pyridyl phosphine),
4.4 mL diisopropylethylamine, 57.3 mL styrene and 28.7 mL decane
(internal standard) in 0.50 mL toluene. The vials were closed
and a needle was inserted to ensure gas uptake. The autoclave
was closed and four times purged with 15 bar of syn-gas
(H2–CO 1 : 1), pressurised to 20 bar syn-gas and subsequently
the reactions were stirred (see main text in this article for details
about temperature and reaction time). After the required time,
the autoclave was cooled to 0 ◦C, after which the pressure was
reduced to 1 bar. Two drops of tri-n-butyl phosphite were added
to all the reaction vials to prevent any further reaction. From each
vial two samples were taken and diluted with dichloromethane for
GC and chiral GC analysis. The samples for the chiral GC were
filtered over a plug of silica, the samples for normal GC analysis
were not filtered, because retention of the aldehydes on silica cause
deviation in the conversion and b/l selectivity determination. The
enantiomeric purity was determined by chiral GC (ph Megadex
column, internal diameter 0.1 mm, 5 m column, film thickness
0.1 mm; initial temperature = 40 ◦C and DT = 25 ◦C min-1; tR (R) =
5.65 min and tR (S) = 5.73 min). All reactions were performed in
duplicate.
Conclusions
The hydroformylation of styrene into phenylpropionalde-
hyde proceeds generally with high selectivity for the 2-
phenylpropionaldehyde (the branched product). The linear 3-
phenylpropionaldehyde can be formed by applying high tempera-
tures and low syn-gas pressures, in order to increase the amount
of b-hydride elimination of the branched alkyl-complex. In this
article, we report two SUPRAphos phosphoramidite-phospine
ligands that show unprecedented regioselectivity for the linear
aldehyde over the branched aldehyde in the rhodium catalyzed
hydroformylation of styrene, resulting in up to 72% linear aldehyde
at 80 ◦C and 10 bar CO–H2 1 : 1. We attribute this high selectivity
to slow migratory insertion of CO and hence enhanced b-hydride
elimination from the branched alkyl–rhodium species, resulting
in the re-formation of the rhodium–alkene complex. The slow
migratory insertion might be caused by the formation of a stable
intermediate species, in which the styrene is sandwiched between
the porphyrin ring and an aryl group of the other ligand.
Notes and references
Experimental
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