Rh-catalyzed linear hydroformylation of styrene

Article abstract of DOI:10.1039/c2dt31738a

Usually the Rh-catalyzed hydroformylation of styrene predominantly yields the branched, chiral aldehyde. An inversion of regioselectivity can be achieved using strong π-acceptor ligands. Binaphthol-based diphosphite and bis(dipyrrolyl-phosphorodiamidite) ligands were applied in the Rh-catalyzed hydroformylation of styrene. High selectivities up to 83% of 3-phenylpropanal were obtained with 1,1-bi-2-naphthol-based bis(dipyrrolyl-phosphorodiamidite) with virtually no hydrogenation to ethyl benzene. The coordination chemistry of those ligands towards Rh(i) was investigated spectroscopically and structurally. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31738a

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Rh-catalyzed linear hydroformylation of styrene†‡
Evert Boymans,a Michèle Janssen,a Christian Müller,a,b Martin Lutzc and Dieter Vogt*a
Cite this: Dalton Trans., 2013, 42, 137
Usually the Rh-catalyzed hydroformylation of styrene predominantly yields the branched, chiral aldehyde.
An inversion of regioselectivity can be achieved using strong π-acceptor ligands. Binaphthol-based
diphosphite and bis(dipyrrolyl-phosphorodiamidite) ligands were applied in the Rh-catalyzed hydro-
formylation of styrene. High selectivities up to 83% of 3-phenylpropanal were obtained with 1,1-bi-
2-naphthol-based bis(dipyrrolyl-phosphorodiamidite) with virtually no hydrogenation to ethyl benzene.
The coordination chemistry of those ligands towards Rh(I) was investigated spectroscopically and
structurally.
Received 31st July 2012,
Accepted 27th September 2012
DOI: 10.1039/c2dt31738a
www.rsc.org/dalton
accessible via selective hydrogenation of the C–C double bond
in cinnamaldehyde or via oxidation of the hydroxyl moiety in
Introduction
The hydroformylation reaction is an atom efficient route for 3-phenylpropanol with e.g. CrO3.
the functionalization of alkenes towards aldehydes.1 Aldehydes
A few examples concerning hydroformylation of styrene
are versatile intermediates for the synthesis of various fine towards the linear product exist in the literature. Sémeril et al.
chemicals.2 Styrene is an often applied model substrate in the showed l/b ratios of about 3 in the Rh-catalyzed hydroformyl-
hydroformylation reaction of vinylarenes and usually the ation of styrene with calixarene-based diphosphites.5 Reek and
branched product that contains a stereogenic center is formed co-workers used a small library of SUPRAPhos phosphine–
predominantly (Scheme 1). Especially, using rhodium catalysts phosphoroamidite ligands and obtained 72% of the linear
with sigma-donating ligands such as triphenylphosphine (or aldehyde (l/b = 2.6).6 The catalyst that shows the highest selec-
without phosphorus ligands) the branched product is predo- tivity towards 3-phenylpropanal was recently described by
minantly formed.3,4
However, especially in pharmaceutical chemistry, catalysts shows l/b ratios of up to 96% (l/b = 22) in this reaction.7a
that selectively produce the linear aldehyde gain more and Relatively few studies on the mechanism of the hydroformy-
Zhang et al. Their substituted tetraphosphorus/Rh catalyst
more interest, also because their synthesis via other routes is lation of styrene to the linear product have appeared in the
often cumbersome. The linear 3-phenylpropanal is also literature.7b The proposed catalytic cycle is depicted in Fig. 1.
Lazzaroni and co-workers have shown that at lower temperatures
Scheme 1 Hydroformylation of styrene.
aLaboratory of Homogeneous Catalysis, Schuit Institute of Catalysis, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
bFreie Universität Berlin, Institute of Chemistry and Biochemistry, 14195 Berlin,
Germany
cCrystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht
University, Padualaan 8, 3584 CH Utrecht, The Netherlands
†Dedicated to Professor David Cole-Hamilton on the occasion of his retirement
and for his outstanding contribution to transition metal catalysis.
‡Electronic supplementary information (ESI) available. CCDC 904120 and
904121. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt31738a
Fig. 1 Generally accepted mechanism for the hydroformylation of styrene.
Dalton Trans., 2013, 42, 137–142 | 137
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(room temperature) the kinetic, branched product is predomi- molecular structure of [Rh(acac)L4] in the crystal as well as
nantly formed and deuterium labeling studies prove that the selected bond lengths and angles. The coordination geometry
formation of both the linear and the branched alkyl species 3 around the Rh center is square planar (angle sum 360.04(9)°)
is not reversible at low temperatures.4 At higher temperatures, with both enantiomers of the complex present in the crystal,
the insertion into the Rh–H bond becomes reversible. The since racemic 1,1′-bi-2-naphthol was used in the synthesis.
branched alkyl species 3b can convert back to complex 2 under
Crystals of [Rh(acac)L6] were obtained from acetonitrile.
β-hydrogen elimination, whereas this happens to a far lesser The molecular structure in the crystal is depicted in Fig. 5,
extent for the linear alkyl species 3a. Complexes 3a and 3b are
coordinatively and electronically (16 VE) unsaturated. However,
complex 3b can also become saturated by forming the 18 VE
η3-complex 3c. This equilibrium slows down the formation of
complex 4b. On the other hand, complex 3a can only form an
18 electron complex via coordination of CO. Therefore, the
coordination of CO (3a to 4a) is very fast.8
Here we report a group of π-accepting bidentate phosphorus
ligands in the selective Rh-catalyzed styrene hydroformylation
reaction and the tunability of the reaction outcome either to
the branched or to the linear aldehyde.
Fig. 3 Phosphorodiamidite ligands L5 and L6.
Results and discussion
Synthesis
Binaphthol-based ligands have been applied in various homo-
geneously catalyzed reactions.9a,b Diphosphite ligands based
on binaphthol, for example, have been applied in the hydro-
cyanation of styrene and 1,3-cyclohexadiene.10,11 Bini et al.
used binaphthol-based diphosphites in the hydrocyanation of
styrene towards the linear product.12
Reaction of 2 equiv. of the appropriate substituted phenol
or pyrrole with PCl3 in the presence of Et3N and subsequent
reaction with half an equivalent of binaphthol resulted in the
ligands L1–L4 in reasonable to good yields (Fig. 2).10 Ligands
L57 and L613 are among the best performing ligands reported
in the literature for the hydroformylation of respectively
styrene and 1-octene and are used for comparison purposes
(Fig. 3).
In order to obtain structural information for the precata-
lysts, attempts to crystallize the selected complexes were made.
Crystals of [Rh(acac)L4] were obtained from a mixture of
toluene and methanol at room temperature. Fig. 4 shows the
Fig. 4 The molecular structure of [Rh(acac)L4] in the crystal. Displacement
ellipsoids are drawn at a 50% probability level. Only one of two independent
molecules is shown. Disordered toluene solvent molecules have been omitted
for clarity. Selected bond lengths (Å): Rh1–P11 = 2.1674(6), Rh1–P21 = 2.1490(6),
Rh1–O31 = 2.0478(16), Rh1–O41 = 2.0587(16). Bite angle (°): P11–Rh1–P21
= 99.00(2).
Fig. 5 The molecular structure of [Rh(acac)L6] in the crystal. Displacement
ellipsoids are drawn at a 50% probability level. Selected bond lengths (Å): Rh1–
P1 = 2.1489(4), Rh1–P2 = 2.1635(4), Rh1–O4 = 2.0434(10), Rh1–O5 = 2.0523(10).
Bite angle (°): P1–Rh1–P2 = 93.183(14).
Fig. 2 Binaphthol-based ligands L1–L4.
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along with selected bond lengths. The coordination geometry (2075–1970 cm−1) than the corresponding ea coordinated com-
around the central Rh atom is square planar (angle sum plexes (2030–1920 cm−1).15
359.91(6)°) and the bite angle is 93.183(14)°, which is relatively
small.
Fig. 8 shows the carbonyl region of the IR spectrum of the
resting states for the three ligands bearing pyrrole substitu-
ents. [RhHL4(CO)2] shows absorptions with maxima at 2080
and 2022 cm−1 and [RhHL5(CO)2] at 2081 and 2027 cm−1
,
Electronic properties
indicative of an ee coordination mode. [RhHL6(CO)2] shows
four absorption maxima at 2070, 2031, 2009, and 1992 cm−1
Ni carbonyl complexes are well known to be generated quanti-
tatively from diphosphines and CO as shown in Fig. 6 and
their IR spectra are a useful probe for the electronic properties.
For this reason, Ni carbonyl complexes were prepared in situ
with ligands L1–L6. Reaction of 1 equiv. of the bidentate
ligands depicted in Fig. 2 and Fig. 3 with [Ni(cod)2] in toluene
led to the corresponding [Ni(cod)L] complexes. Purging the
toluene solutions with carbon monoxide for 30 s, during
which the color of the solution turned from yellow to colorless,
afforded the corresponding [Ni(CO)2L] complexes (Fig. 6).14
The Ni(0) complexes were investigated by means of IR spec-
troscopy in the ATR mode. By evaluating the A1 and B1 fre-
quencies of the CO ligands, the electronic properties of the
phosphorus ligands can be compared.14 An overview of these
frequencies is given in Table 1.
.
This suggests that there is more than 1 species present, which
might be a mixture of ee and ea complexes. For comparison
reasons, the IR data of the bidentate phosphine ligand Xant-
phos [HRh(Xantphos)(CO)2] are 2039, 1996, 1974 and
1949 cm−1, evidence for the coexistence of both the ee and ea
isomers.16 Application of this ligand showed low linearity in
the hydroformylation of styrene with an l/b ratio of 0.88.17
NMR spectroscopy of the resting state complex can give
complementary evidence for the coordination mode of the
bidentate dipyrrolyl-phosphorodiamidite ligands. These Rh-
complexes turned out to be stable when the syngas pressure
was released. The hydride resonances observed for the com-
plexes [HRhL4(CO)2] and [HRhL5(CO)2] show a broad singlet
(Table 2, and ESI‡), whereas the 31P NMR spectra show a broad
Ligands L1–L6 show relatively high values for the A1 and B1
frequencies as compared to phosphine and also phosphite
ligands.14b This means that ligands L1–L6 have pronounced
π-acceptor character.
In situ HP-IR spectroscopy
The coordination mode of ligands to the metal center is of
high importance for the selectivity and activity of the corre-
sponding catalyst. The resting state of the rhodium hydro-
formylation catalyst, HRh(L)(CO)2, shows trigonal bipyramidal
geometry. A bidentate ligand can coordinate either in equator-
ial–equatorial (ee) or equatorial–axial mode (Fig. 7).
Fig. 7 Coordination mode of a bidentate ligand in the catalyst resting state.
The coordination mode of the ligand was studied by means
of in situ high pressure IR spectroscopy, which enables investi-
gation of the catalyst resting state under ‘real’ reaction con-
ditions (i.e. catalytic Rh concentration, T, p). Complexes in
which the bidentate ligand coordinates in an ee fashion show
CO stretch frequencies at higher wave numbers
Fig. 6 Synthesis of [Ni(CO)2L] complexes.
Table 1 ATR IR frequencies of the A1 and B1 vibrations of CO in [(L)Ni(CO)2]
Entry
Ligand
A1 (cm−1
)
B1 (cm−1
)
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
2041
2044
2049
2048
2050
2041
1987
1990
2001
1993
1998
1989
Fig. 8 HP-IR spectra of [HRhL(CO)2], the resting state of the catalyst in the
hydroformylation reaction, in 2-methyltetrahydrofuran.
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caused by the sterically demanding tert-butyl groups in the
ortho position of the phenoxy moiety of the ligand. Indeed,
31P NMR of the precatalyst did not show any coordination,
i.e. only the phosphorus signal of the free ligand was
observed (130.3 ppm). For L2, a remarkable increase in l/b
from 0.97 to 7.1 was observed when the reaction temperature
was increased to 140 °C. Although consequently, an increased
hydrogenation of styrene was observed, yielding the by-
product ethyl benzene. The pyrrole substituted ligand L4
clearly shows a significantly higher regioselectivity, with an
l/b ratio of 4.9 at 80 °C. However, the increased regioselectiv-
ity at increased temperature does not occur in the catalytic
systems with L4 and L5.7 This is mainly due to the increased
hydrogenation of styrene at elevated temperatures. For com-
parison, two other bis(phosphorodiamidite) ligands (L5 and
L6) were tested under the same conditions, with comparable
selectivities compared to the catalytic system with L4.
Remarkably, ligands with more pronounced π-acceptor prop-
erties (see Tables 1 and 2) lead to higher selectivities to the
linear product under the same conditions. Especially for bis
(phosphorodiamidites) L4–L6, the trend in π-acceptor proper-
ties (Fig. 7) correlates to the achieved regioselectivity. More-
over, the bidentate ee-coordination of L4 and L5 in trigonal
bipyramidal [HRhL(CO)2] seems to be the preferred isomer
in the catalytic cycle to achieve high 3-phenylpropanal
selectivity.
Table 2 Spectroscopic data of [HRhL(CO)2] complexes at room temperature
δ 1Ha
(ppm)
δ
31Pa
(ppm)
JHP JHRh JRhP
Complex
(Hz) (Hz) (Hz) νCOb (cm−1
)
HRhL4(CO)2 −10.18 (s) 140.8 (d) n.d. n.d. 216.3 2080, 2022
HRhL5(CO)2 −10.72 (s) 141.5 (d) n.d. n.d. 212.3 2081, 2027
HRhL6(CO)2 −9.48 (dt) 135.5 (dd) 33.0 4.8 203.7 2070, 2031,
2009, 1992
a Measured in toluene-d8. b Measured in 2-methyltetrahydrofuran, s =
singlet, d = doublet, dt = doublet of triplets.
Table 3 Results of the Rh-catalyzed hydroformylation of styrene at full
conversion
TOF
(h−1
Hydrogenation
(%)
Selectivity
l/b ratio to linear (%)
Entry Ligand
T
)
1
2
3
4
5
6
7
8
9
L1
L2
L2
L3
L4
L4
L4
L5
L6
80
80
140 13 874 14
325 <1
1010 <1
0.37
0.97
7.1
—
4.9
2.9
4.7
5.8
2.5
27
49
88
—
83
74
82
85
71
80
80
60
—
3900
653 <1
—
1
140 11 657 10
80
80
2300 <1
7730 <1
Conditions: Rh : L : S = 1 : 2 : 2000, Tpref = 80 °C, p = 10 bar syn gas (CO/
H2, 1 : 1), Rh precursor = Rh(acac)(CO)2, [Rh] = 0.9 mM, solvent =
toluene, l/b ratio is determined by GC after gas-uptake ceased,
turnover frequencies (TOF) were determined at 20% conversion and
Fig. 9a shows that styrene is consumed with pseudo-first
are given in (mol aldehyde)(mol Rh)-1h−1
.
order kinetics.
A steady increase of 3-phenylpropanal,
2-phenylpropanal and ethyl benzene is illustrated over the first
30 min. Fig. 9b shows that the product distribution remains
constant over time. This demonstrates that the hydrogenation
activity is not caused by a deactivated hydroformylation cata-
lyst and that the catalyst is stable during the course of the reac-
tion. As a result, the hydrogenation of the olefin is most likely
in direct relation with temperature.
The reaction mechanism of the Rh-catalyzed hydroformyla-
tion, discussed in the introduction, can explain the increased
regioselectivity for the strongly π-accepting ligands L4–L6.
When applying strong π-acceptor ligands the 16 VE σ-alkyl
complexes 3a and 3b (see Fig. 1) are destabilized and a higher
activity should be observed. For species 3b (the branched
1-phenyl ethyl species), the unsaturation can also be overcome
by forming η3-stabilized species 3c. The regioselectivity is now
determined by the rate of CO coordination. This causes the
doublet. Due to these broad signals we were unable to deter-
mine the JHP and JHRh coupling constants. JPP coupling con-
stants were also not observed because the phosphorus atoms
are equivalent at room temperature. Complex HRhL6(CO)2
shows a doublet of triplets in the hydride region of the
1H NMR spectrum, with JHP = 33 Hz and JHRh = 4.8 Hz. The
corresponding 31P NMR spectrum shows a double doublet,
because of phosphorus coupling with both Rh (JRhP = 203.7
Hz) and hydride (JPH = 33 Hz). From these J coupling constants
in combination with HP-IR data it can be concluded that L6
exists as both the ee and ea-isomers.
Catalysis: hydroformylation of styrene to the linear aldehyde
The binaphthol-based ligands, depicted in Fig. 2, were exceptionally high l/b ratios for this kind of ligand, and
applied in the Rh-catalyzed hydroformylation of styrene. Their explains why the l/b ratio increases when ligands with stronger
performance in terms of activity and regioselectivity towards π-acceptor character are applied.
the linear product was compared with ligands L5 and L6
(Fig. 3).
After initial screening of the diphosphite ligands L1–L3
and bis(phosphorodiamidite) ligands L4 at 80 °C and 10 bar
Conclusions
of syngas pressure, the difference in performance was rather The Rh-catalyzed hydroformylation of styrene usually yields
pronounced. Whereas catalysts based on ligands L1, L2 and predominantly the branched product. By careful choice of the
L4 showed good activity in the reaction (Table 3, entries 1, 2 ligand, this regioselectivity can be inversed. It was demon-
and 5), the catalyst based on ligand L3 did not show any con- strated that ligands with pronounced π-acceptor properties
version (entry 4). The poor activity of the latter might be show both enhanced activity and linearity in this reaction.
140 | Dalton Trans., 2013, 42, 137–142
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Ligand L2 showed an l/b increase from 0.97 to 7.1 when
increasing the reaction temperature from 80 °C to 140 °C.
Experimental section
General procedure for the hydroformylation reactions
[Rh(acac)(CO)2] (1 equiv., 14.4 μmol, 3.7 mg) and the ligand
(2 equiv., 28.8 μmol) were dissolved in 15 ml toluene for
the preformation of the catalyst (1 h, 10 bar syngas, 80 °C).
Then styrene (2000 equiv., 28.8 mmol) was diluted with
toluene to 5 ml and added to the catalyst solution. Catalytic
conversions were determined by gas chromatography (GC) on
an ULTRA 2 column (25 m × 0.20 mm) using decane as an
internal standard. Retention times were compared with auth-
entic samples.
Acknowledgements
This work was financially supported by The Netherlands
Research School Combination Catalysis (NRSCC). We are grate-
ful to Dr Jos Wilting for valuable discussions and to Ton
Staring for his technical assistance.
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