Coordination chemistry and asymmetric catalysis with a chiral diphosphonite

Article abstract of DOI:10.1002/ejic.200400132

The improved synthesis of the chiral diphosphonite, XantBino (1), based on a xanthene backbone and bearing chiral binaphthyl groups on both P-atoms is described together with its Pd^II and Rh¹ complexes. The ³¹P NMR spectra of both complexes point out that the two phosphorus atoms are chemically inequivalent. The complex cis-[PdCl₂(1)] (2) is structurally characterized by NMR spectroscopy and X-ray crystallography. The molecular structure reveals an unusually small bite angle for this member of the xantphos family of only 100°. The rhodium-catalyzed asymmetric hydroformylation of styrene and vinyl acetate as well as the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate, applying this chiral diphosphonite 1, are described. Low enantiomeric excesses are obtained in the asymmetric hydroformylation, while use of the catalyst precursor [Rh(cod)(1)]BF₄ (3) results in a promising enantiomeric excess in the Rh-catalyzed asymmetric hydrogenation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400132

FULL PAPER
Coordination Chemistry and Asymmetric Catalysis with a Chiral
Diphosphonite
[
a]
[a]
[a]
[a]
Jarl Ivar van der Vlugt, Jos M. J. Paulusse, Eric J. Zijp, Jason A. Tijmensen,
[b]
[b]
[c]
[a]
Allison M. Mills, Anthony L. Spek, Carmen Claver, and Dieter Vogt*
Keywords: Diphosphonite compounds
/ Asymmetric hydrogenation / Palladium / Rhodium / Asymmetric
hydroformulation / Enantioselectivity
The improved synthesis of the chiral diphosphonite,
XantBino (1), based on a xanthene backbone and bearing
chiral binaphthyl groups on both P-atoms is described to-
gether with its Pd and Rh complexes. The P NMR spectra
of both complexes point out that the two phosphorus atoms
droformylation of styrene and vinyl acetate as well as the
asymmetric hydrogenation of methyl (Z)-2-acetamidocinna-
mate, applying this chiral diphosphonite 1, are described.
Low enantiomeric excesses are obtained in the asymmetric
hydroformylation, while use of the catalyst precursor
II
I
31
are chemically inequivalent. The complex cis-[PdCl
2
(1)] (2)
4
[Rh(cod)(1)]BF (3) results in a promising enantiomeric ex-
is structurally characterized by NMR spectroscopy and X-ray
crystallography. The molecular structure reveals an un-
usually small bite angle for this member of the xantphos
family of only 100°. The rhodium-catalyzed asymmetric hy-
cess in the Rh-catalyzed asymmetric hydrogenation.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
hydrogenation of enamides,[
8c]
the copper-catalyzed ad-
[
10]
dition of diethylzinc to enones, and the hydroformylation
The rhodium-catalyzed asymmetric hydroformylation of
vinylarenes, with styrene as a model substrate, using rho-
dium-based catalysts, is a typical benchmark reaction for
the comparison of new catalyst systems.[1] A number of li-
gand classes have been applied in this reaction, aiming at a
thorough understanding of both the kinetics and the origin
[
11]
of octenes.
Little precedent exists on the application of
chiral) diphosphonites in the transition metal catalyzed
hydroformylation of alkenes or vinyl arenes. Dahlenburg
and coworkers reported on chiral dialkyl-derived diphos-
phonites L , their platinum complexes, and the Pt/Sn-cata-
(
2
[
2]
lyzed hydroformylation of styrene, but only very low enantio-
of enantioselectivity, for example, diphosphanes, phos-
[
12]
[
3]
[4]
selectivities were obtained.
The groups of Schmutzler
phane-phosphites, diphosphites, aminophosphane-pho-
[
5]
[6]
and Börner published a joint study on calix[4]arene-based
achiral diphosphonites L2 in the Rh-catalyzed hydrofor-
mylation of terminal alkenes. The regioselectivities re-
mained well below 2, indicating only monodentate behavior
sphinites, aminophosphanes, and other bifunctional sys-
[
7]
tems. However, only few catalytic systems show satisfac-
tory enantioselectivity for the desired branched aldehyde,
with BINAPHOS (L ) as the current benchmark ligand in
many respects (Figure 1). Phosphonites have been largely
1
[
13]
of these ligands systems.
overlooked in homogeneous catalysis to date. Recently,
We have previously reported on the use of chiral xan-
some reports have appeared on diphosphonites in asymmet- thene-based diphosphonites in the asymmetric hydrocyan-
[
8]
[9]
ric hydrogenation as well as in hydroboration, making ation of vinyl arenes that showed good enantioselectivities
[
14]
use of several backbones. Next to this, monophosphonites of up to 63% for 4-isobutylstyrene. We recently published
have attracted some attention, especially for the asymmetric on the synthesis and use of sterically constrained achiral
diphosphonites L4 in the rhodium-catalyzed hydrofor-
[
[
[
a]
b]
c]
Schuit Institute of Catalysis, Laboratory of Homogeneous
Catalysis, Eindhoven University of Technology,
mylation of alkenes and showed that these systems are well-
suited for the conversion of 2-butene into n-pentanal.[ To-
15]
P. O. Box 513, 5600 MB Eindhoven, The Netherlands
Laboratory for Crystal and Structural Chemistry, Department gether with our studies on the coordination chemistry of
of Chemistry, Utrecht University,
these xanthene-derived diphosphonites towards various
Padualaan 8, 3584 CH Utrecht, The Netherlands
[16]
Departament de Qu ´ı mica F ´ı sica i Inorg a` nica, Universitat transition metals, this prompted us to explore the poten-
Rovira i Virgili,
tial of chiral diphosphonite 1 in various asymmetric cata-
Pl. Imperial T a` rraco 1, 43005 Tarragona, Spain
lytic reactions. In this paper we report on the improved syn-
thesis of compound 1, its coordination with palladium and
Fax: (internat.) ϩ 31-40-2455054
E-mail: d.vogt@tue.nl
Eur. J. Inorg. Chem. 2004, 4193Ϫ4201
DOI: 10.1002/ejic.200400132
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4193

D. Vogt et al.
FULL PAPER
Figure 2. Chiral xanthene-based diphosphonite used in this study
Scheme 1. Synthetic route to compound 1: i) nBuLi, TMEDA,
Et
2
2 2
O, Ϫ40 °C, 16 h; ii) ClP(NEt ) , pentane, Ϫ60 °C, 16 h.; iii) 2,2Ј-
dihydroxy-1,1Ј-binaphthyl (2 equiv.), toluene, tetrazole, ∆T, 1 day
erate to good yield.[14] Subsequently, the chiral binaphthyl
groups were introduced to obtain the desired chiral diphos-
phonite. This reaction is catalyzed by tetrazole as pro-
tonation agent, which is a slight but important modification
of the literature procedure. Addition of tetrazole shortened
the reaction time considerably, analogous to the synthesis
Figure 1. Representation of several reported literature ligand sys-
1 2
tems: L ) Nozaki’s BINAPHOS; L
) cyclopentane-based diphos- of sterically constrained diphosphonites previously reported
phonites by Dahlenburg; L
3
) calix[4]arene-derived diphosphonite
[15]
by our group. The pure compound was obtained by lay-
of Schmutzler and Börner (the ellipsoid represents a p-tert-butylca-
ering a solution of the crude product in dichloromethane
with acetonitrile. This was omitted in the earlier method-
ology. In general, the second step led to an unwanted side-
4
lix[4]arene unit); L ) our own sterically constrained achiral diphos-
phonites
rhodium, and the use of this ligand in the rhodium-cata- product (Ͻ10%), most likely the monophosphonite, which
lyzed asymmetric hydroformylation of styrene and vinyl justified the crystallization step. The 31P NMR spectrum of
acetate and in the asymmetric hydrogenation of methyl (Z)- the pure compound showed a singlet at δ ϭ 178.0 ppm,
2
-acetamidocinnamate (Figure 2).
which seems remarkably low-field, even for a phosphonite
moiety. This value reflects the fact that both electronic fac-
tors as well as conformational constraints strongly affect
the chemical shift of the 31P nucleus.
Results and Discussion
Synthesis of Diphosphonite 1 and Its Dichloropalladium
Complex 2
We were interested in the detailed coordination behavior
of XantBino (1) towards palladium and rhodium. Pre-
[
14]
Diphosphonite 1 was prepared in a straightforward man- viously, only the bischelate Ni(1) was described.
From
2
ner, using a two-step procedure, as illustrated in Scheme 1. the reaction of PdCl (cod) with ligand (R,R)-1 at room tem-
2
The [bis(diethylamino)]diphosphonite was derived from the perature, a yellow compound was obtained, for which the
31
commercial starting material 9,9-dimethylxanthene in mod-
P NMR spectrum shows two doublets at δ ϭ 143.1 ppm
4194
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Eur. J. Inorg. Chem. 2004, 4193Ϫ4201

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite
FULL PAPER
[
20,22]
and 137.4 ppm. The observed coupling constant JP-P is served by X-ray crystallography.
4
In complex 2, the
1
41
2
54
6.5 Hz, apparently indicating that the phosphorus atoms angles PdϪP ϪC and PdϪP ϪC were 109.87(11)° and
are chemically inequivalent. This is attributed to the chiral, 108.57(12)°, respectively (Figure 4). The dihedral angle be-
1
1
2
bulky binaphthyl units. From the H NMR spectrum, it ap- tween the metal plane Cl ϪPdϪCl and the ligand plane
1
2
41
pears that the methyl groups in the backbone are also in- P ϪP ϪC is 115.7°, indicating that the palladium is actu-
equivalent, indicating that the aromatic skeleton of the xan- ally positioned outside the plane defined by the backbone
[
14,17]
thene is bent.
and the phosphorus atoms, as illustrated in This is quite
Single crystals suitable for X-ray analysis were grown by unusual for cis-Pd-diphosphorus complexes. A similar
layering a dichloromethane solution of complex 2 with structural disposition of the metal atom, together with a
acetonitrile. The molecular structure of cis-[PdCl (1)] is pre- small ligand bite angle, was found for an unexpected ortho-
2
sented in Figure 3, together with selected bond lengths and
angles. It is evident that ligand 1 acts as a bidentate ligand,
coordinating in a cis-fashion to the palladium atom.
The coordination around the central palladium atom is
1
2
distorted square-planar, with a P ϪPdϪP angle of
9
1
2
9.99(3)°, while the Cl ϪPdϪCl angle is 90.15(4)°. There
are few examples on palladium diphosphonite complexes
known in literature. However, the bond lengths for PdϪP
1
˚ ˚
2
[
PdϪP is 2.2449(9) A and PdϪP is 2.2532(9) A] and
1
˚
2
PdϪCl [PdϪCl is 2.3412(10) A and PdϪCl is 2.3442(9)
A] are normal, in agreement with those found by Agbos-
sou and Schmutzler. Compared with crystal structures
of Pd complexes known for xanthene diphosphane ligands,
the bite angle of 100° for ligand 1 is unusually small. For
instance, we previously reported on trans-coordinated di-
phosphonites and both van Leeuwen et al. and Buch-
wald et al.
Only in the case of Pd complexes or with cationic pal-
˚
[
18]
[19]
[
16]
[20]
[
21]
reported on trans-Pd(Xantphos) complexes.
Figure 4. Illustration of the geometry around the square-planar
0
palladium atom in complex 2, showing the dihedral angle between
ladium species has cis-coordination of Xantphos been ob- the metal plane and the ligand plane
Figure 3. ORTEP representation of complex 2, cis-[PdCl
2
(1)]; displacement ellipsoids are drawn at the 50% probability level; all hydrogen
1 2 1 2 1 1
atoms and solvent molecules are omitted for clarity; PdϪP 2.2449(9); PdϪP 2.2532(9); PdϪCl 2.3412(10); PdϪCl 2.3442(9); P ϪO
1
2
2
3
2
4
1
41
2
54
5
46
5
55
1
.613(2); P ϪO 1.601(2); P ϪO 1.800(3); P ϪO 1.604(3); P ϪC 1.800(3); P ϪC 1.815(3); O ϪC 1.380(4); O ϪC 1.388(4);
5
1
2
1
2
1
2
1
1
1
2
2
1
3
PdϪO 3.284(2); P ϪP 3.4455(12); Cl ϪPdϪCl 90.15(4); P ϪPdϪP 99.99(3); Cl ϪPdϪP 82.70(3); Cl ϪPdϪP 172.15(3); Cl ϪPdϪP
70.01(3); Cl ϪPdϪP 86.23(3); O ϪP ϪO 102.46(12); O ϪP ϪO 103.07(13); PdϪP ϪO 116.50(9); PdϪP ϪO 117.78(9); PdϪP ϪO
2 4 1 41 2 54 46 5 56 45 47 50
2
2
1
1
2
3
2
4
1
1
1
2
2
1
1
23.91(9); PdϪP ϪO 110.02(10); PdϪP ϪC 109.87(11); PdϪP ϪC 108.57(12); C ϪO ϪC 112.6(3); C ϪC ϪC 106.8(3);
1
1
41
2
1
41
3
2
54
4
2
54
O ϪP ϪC 106.07(13); O ϪP ϪC 102.70(13); O ϪP ϪC 102.50(14); O ϪP ϪC 107.67(15)
Eur. J. Inorg. Chem. 2004, 4193Ϫ4201
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D. Vogt et al.
FULL PAPER
metallated Pd-diphosphonite species described previously
[
16]
by our group.
The structure of complex 2 also bears close resemblance
to the molecular structures for the related platinum com-
[
23]
plex of ligand 1, which will be communicated elsewhere,
[24]
as well as for cis-[PtCl (Xantphos)].
This indicates that
2
such dispositioning of the metal atom is highly favored in
Scheme 2. General reaction scheme for the Rh-catalyzed hydrofor-
the crystal packing regardless of the metal atom involved. mylation of styrene
This phenomenon must therefore be based on steric crowd-
ing induced by the binaphthyl units. The dihedral angle of
the two aromatic rings in the xanthene backbone is found
Several hydroformylation experiments were performed at
to be 107.8°. The distance between the two phosphorus
different synthesis gas pressures (10 and 20 bar) and at dif-
ferent temperatures (40Ϫ100 °C). The amount of hydrogen-
ation was usually less than 1% and no other side products
were formed. The results obtained are summarized in
Table 1. At 40 °C, a pressure of 20 bar instead of 10 bar
led to an increase in the reaction rate of approximately 33%.
The enantiomeric excess was found to be lower (entry 2
vs. 1). The selectivity for the branched aldehyde remained
constant within experimental error. Turnover frequencies
were determined at conversion levels of around 20Ϫ40%
from the linear increase of conversion in time. Several
samples were taken throughout the reaction without press-
ure loss.
The effect of pressure on the reaction rate is more signifi-
cant at temperatures of 60 °C and higher. In all cases, the
initial activity (TOF) is lower at higher pressures. Also, the
enantioselectivity is lower at higher pressure. The regiose-
lectivity shows a reverse trend, viz. a positive effect of press-
ure on the regioselectivity to branched product and a nega-
tive influence of the reaction temperature. This is more pro-
˚
atoms is 3.4455(12) A, while the distance between the pal-
ladium atom and the oxygen atom in the backbone is
˚
.284(2) A, which is too long to expect any interaction.
[25]
3
When ligand (R,R)-1 was reacted with Rh(cod)(acac), the
P NMR spectrum reveals two doublets of doublets, as-
3
1
cribed to the cationic complex 3, [Rh(cod)(1)]BF . The av-
4
erage signals appear at δ ϭ 169.7 ppm and at δ ϭ
1
1
61.4 ppm, with coupling constants J
of 218 Hz and
Rh-P
2
JP,P of 24 Hz. Although no further characterization was
performed on this compound, the existence of this ABX
pattern implies that the two phosphorus atoms are chemi-
cally inequivalent in the rhodium complex, as in the pal-
ladium case. The RhϪP coupling constant is similar to val-
ues reported by Börner,[ Puddephatt, and Shum, al-
13]
[26]
[27]
though the latter report dealt with a Rh(acac)-diphosphon-
1
ite derivative, with a J
of 266 Hz.
Rh-P
Asymmetric Hydroformylation
In order to evaluate the catalytic properties of enantiom- nounced at a pressure of 10 bar, as the regioselectivity is
erically pure (R,R)-1, it was applied in the rhodium-cata- only 56% at 100 °C and increases to 81% at 20 bar. In all
lyzed asymmetric hydroformylation of styrene (Scheme 2). cases, the enantiomeric excess (ee) is poor, with a maximum
To the best of our knowledge, no reports have appeared on of 33% at 100 °C.
the use of chiral diphosphonite ligands in rhodium-cata-
No attempts were made to modify the ligand system in
lyzed asymmetric hydroformylation. As a rhodium precur- order to obtain higher enantioselectivity. Goertz et al. al-
sor, Rh(acac)(CO) was used. Before addition of the sub- ready showed that for the asymmetric hydrocyanation, the
2
strate, the catalytic system was activated for 1 h with 20 bar introduction of bulky substituents in the 3,3Ј-positions of
syngas at the reaction temperature to generate the catalytic the binaphthyl units can lead to higher ee’s.[14] Buisman et
resting state.
al. observed a similar effect in the asymmetric hydrofor-
Table 1. Rh-catalyzed asymmetric hydroformylation of styrene applying (R,R)-XantBino (1)
Entry[a]
T [°C]
p [bar]
Time [h]
Conversion [%][b]
Sel.branched [%][b]
ee [%][c]
TOF [h ]
Ϫ1 [d]
[
[
e]
e]
1
2
3
4
5
6
7
8
40
40
60
60
80
80
100
100
10
20
10
20
10
20
10
20
48
72
5
8
3
3
2
2
63
90
54
93
99
100
94
96
91
92
85
87
80
85
56
81
17 (R)
13 (R)
22 (R)
16 (R)
27 (R)
20 (R)
33 (R)
24 (R)
12
17
260
140
880
720
n.d.
n.d.
[
[
a]
b]
2
Reaction conditions: 1.55 mL styrene (13.5 mmol); 3.45 mL toluene; [Rh(acac)(CO) ] ϭ 0.9 m, L:Rh ϭ 1.2:1; styrene:Rh ϭ 1000:1.
[c]
[d]
Determined by GC analysis. Enantiomeric excess, determined by chiral GC analysis. Turnover frequency determined at 20Ϫ40%
Ϫ1 Ϫ1 [e]
conversion; defined as (mol substrate converted)·(mol rhodium) ·h
196
.
Preformation time 15 hours.
4
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Eur. J. Inorg. Chem. 2004, 4193Ϫ4201

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite
mylation of styrene but with diphosphites derived from less
FULL PAPER
[
4e]
rigid backbones.
Furthermore, steric crowding around
these positions led to a significant decrease in enantioselec-
tivity.
As with thermodynamic control of the stereoselection,
the ee decreases when the temperature is increased, and Scheme 3. General reaction scheme for the Rh-catalyzed hydrofor-
mylation of vinyl acetate
experiments
temperature.
are
usually
conducted
at
low
[
28Ϫ30]
There are nonetheless some studies on
the rhodium-catalyzed hydroformylation of styrene in
which the enantioselectivity increased with increasing
system would perform in this specific reaction (Scheme 3).
The substrate vinyl acetate differs from styrene in two im-
portant aspects, viz. a) vinyl acetate cannot form η com-
[
31Ϫ34]
temperature.
Hanson et al. studied the Pt/Sn-cata-
3
lyzed asymmetric hydroformylation with diamino-substi-
tuted versions of the known ligands CHIRAPHOS and
BDPP. A linear decrease in the ee with temperature was
observed using the CHIRAPHOS derivative, while with the
BDPP ligand the sign of enantioselection changed with
temperature. The (S)-enantiomer was formed in excess at
low temperature, while the (R)-enantiomer was the major
product at 60 °C and above. Furthermore, the enantioselec-
tivity of the (R)-enantiomer increased with reaction tem-
perature, contradictory to the ‘‘normal’ behavior seen with
CHIRAPHOS. This phenomenon, already discussed by
Scharf in a review on dramatic temperature effects on selec-
tivities in catalysis,[ was explained by the existence of
competing pathways of diastereomeric intermediates of a
single chelate conformation of the catalyst system, hence
indicating kinetic control.
plexes and b) the carbonyl oxygen can participate in bond-
ing to the metal.
In all catalytic runs the catalyst was preformed for 1 hour
at 60 °C and 10 bar. Reaction at 60 °C and 20 bar of H2
led to an enantiomeric excess of 51%, which is significantly
higher than for styrene. The regioselectivity was extremely
high with an b/l ratio exceeding 90. The reaction of vinyl
acetate showed the normal temperature dependence; the ee
dropped to 33% at a reaction temperature of 80 °C. This
3
supports the hypothesis that the η -benzyl intermediate
might induce kinetic control of the ee with styrene as a
substrate, leading to an unusual temperature effect. Since
this intermediate species is absent in the case of vinyl acet-
ate, no such dependence was found for this substrate.
35]
Asymmetric Hydrogenation
For the catalytic system with ligand 1 we also observed
a positive dependency of the enantiomeric excess on the
reaction temperature, as illustrated in Figure 5. The highest the Rh-catalyzed asymmetric hydrogenation of methyl (Z)-
ee of 33% is found at 100 °C and 10 bar. The ee is lower at 2-acetamidocinnamate (I) (Scheme 4).
higher pressure (for example, entries 3 and 4 vs. 5 and 6 in
As a third test reaction, diphosphonite 1 was applied in
[
37]
Table 1). A discussion on the proposed mechanism for this
particular system is discussed hereafter (vide infra).
Scheme 4. General reaction scheme for the Rh-catalyzed hydrogen-
ation of methyl (Z)-2-acetamidocinnamate
The catalysis was carried out at ambient temperature,
with a substrate to rhodium ratio (S/Rh) of 100:1. Before
addition of the substrate, a concentrated solution of
[
Rh(cod)(1)]BF , complex 3, in methanol was stirred for 3
4
hours under 1.2 bar of H in order to preform the catalyst.
2
During catalysis a static H pressure (1.2 or 5 bar) was ap-
2
plied. The reaction rate was constant up to 80% conversion
for the reaction that applied 1.2 bar of H , indicating that
2
the catalyst system remains stable and active. Also the ee
remained constant at 54% over the full conversion range.
Substrate depletion leads to the expected drop in reaction
Figure 5. Plot of the positive relationship between the ee and the rate at even higher conversion. Under these nonoptimized
reaction temperature, at two pressures
Ϫ1
conditions, the rates were low (about 40 h ) relative to
[
8a]
[8b]
rates reported by Reetz
and Claver
for the hydrogen-
It is known that the results obtained in the rhodium-cata- ation of different substrates that applied diphosphonites.
lyzed asymmetric hydroformylation of vinyl acetate can be For the paracyclophane-based diphosphonite system em-
[
8c]
Ϫ1
drastically different when applying the same catalytic sys- ployed by Zanotti-Gerosa,
tem. We were therefore interested to see how our Rh/(1) were reported.
TON’s of up to 2000 h
[
36]
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D. Vogt et al.
FULL PAPER
The enantioselectivity of 54% with the present Conclusions
[
Rh(cod)(1)]BF system 3 is promising, but still moderate
4
The synthesis of diphosphonite ligand 1 (XantBino) has
been improved. The coordination chemistry of this ligand
was explored for palladium and rhodium. The molecular
compared with the diphosphonite systems mentioned be-
fore (vide supra). The (S)-enantiomer is formed in excess
with the catalyst containing (R,R)-(1). Similar catalytic re-
sults were obtained when applying a pressure of 5 bar H2.
After 2 hours, the conversion reached 94%, with an ee of
structure for cis-[PdCl (1)], complex 2, has been described,
2
revealing an out-of-plane positioning of the metal atom at
a dihedral angle of 115°. The bite angle of 100° of the di-
phosphonite ligand is unusually small for a xanthene-de-
rived phosphorus ligand. The first application of a chiral
diphosphonite in the rhodium-catalyzed asymmetric hydro-
formylation of styrene is reported, leading to high activities
47%. The higher activity is expected since the concentration
of hydrogen is evidently increased.
Temperature Dependence in Asymmetric Hydroformylation
of Styrene
Ϫ1
with turnover frequencies of up to 1420 h . The maximum
enantioselectivity obtained was low; however an interesting
increase was observed at higher temperature. In the asym-
metric hydrogenation of methyl (Z)-2-acetamidocinnamate,
low activities were found, yet the enantioselectivity was pro-
mising, with an enantiomeric excess of 54%, which is en-
couraging, given the nonoptimized structure and con-
ditions.
In the case of transition metal catalyzed functionalization
3
of vinyl arenes, participation of η -benzyl intermediates
38Ϫ41]
(
Scheme 5) have been discussed.[
Depending on the li-
gand influence and on the temperature, pre-equilibria can
exist that involve coordination of the substrate, σ-alkyl and
η -benzyl intermediates. Obviously chirality is already in-
duced during substrate binding but the direction could be
3
influenced by a reversible insertion step. With (our) π-ac-
3
ceptor ligands the η -benzyl species could be favored, es-
Experimental Section
pecially at low temperature giving rise to thermodynamic
control over the enantioselection. At higher temperature, General: Chemicals were purchased from Aldrich, Acros and
the σ-alkyl species could become more important, possibly Merck, and used as received. Synthesis gas (CO/H , 1:1) was pur-
2
resulting in a kinetically controlled process. This is in line chased from Carburos Metalicos. All preparations were carried out
under argon using standard Schlenk techniques. Solvents were dis-
tilled from sodium/benzophenone (THF, ether, toluene, hexane and
ethanol) or calcium hydride (CH Cl and CDCl ) prior to use. All
2 2 3
with the pressure dependence of the ee and of the b/l ratio
observed (vide supra) as higher CO pressure will slow down
pre-equilibria and enhance product formation from inter-
mediates once they are formed. Definitely, this effect is very
interesting, and shedding more light on the underlying pro-
cesses will give new insight into the stereoselection mecha-
nism of the asymmetric hydroformylation. Unfortunately,
glassware was dried by heating under vacuum. The NMR spectra
1
were recorded on a Varian Mercury 400 spectrometer ( H,
13
1
31
1
C{ H}, P{ H}). Chemical shifts are given in ppm referenced
1
13
1
to solvent ( H, C{ H}) or an 85% aqueous solution of H
3
PO
4
31
1
(
P{ H}). GC spectra were recorded on a HewlettϪPackard
the available data do not allow any conclusions yet. De- 5890A Chromatograph equipped with a Ultra-2 column (styrene)
tailed studies on this phenomenon are presently underway. or a Shimadzu 17A with a PONA column (vinyl acetate and hydro-
Scheme 5. Proposed stereoselection mechanism in the rhodium-catalyzed asymmetric hydroformylation of styrene (and other vinyl arenes)
198 www.eurjic.org Eur. J. Inorg. Chem. 2004, 4193Ϫ4201
4
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite
FULL PAPER
1
1
2
genation). Enantiomeric excesses were determined on
a
(d, J ϭ 8.8 Hz, 1 H), 7.95 (dd, J ϭ 8.0, J ϭ 4.8 Hz, 2 H), 7.89
1
1
HewlettϪPackard 5890A with a Cyclodex β-I/P column (styrene)
a GC Vega 6000 equipped with a Cyclodex β-I/P column (vinyl
acetate) or a Shimadzu 2010 equipped with a Chirasil--Val column
(d, J ϭ 8.8 Hz, 1 H), 7.81 (m, 2 H), 7.70 (m, 3 H), 7.65 (dt, J ϭ
2
1
1
8.4, J ϭ 1.2 Hz, 1 H), 7.58 (t, J ϭ 8.8 Hz, 1 H), 7.56 (t, J ϭ
8.0 Hz, 2 H), 7.51 (dd, J ϭ 7.2, J ϭ 1.2 Hz, 1 H), 7.46 (m, 1 H),
7.46 (t, J ϭ 8.0 Hz, 1 H), 7.34 (m, 2 H), 7.23 (m, 2 H), 7.21 (t,
J ϭ 6.8 Hz, 1 H), 7.13 (m, 1 H), 7.09 (dd, J ϭ 6.4, J ϭ 2.0 Hz,
1
2
42]
[43]
1
(
(
hydrogenation). PdCl
2
(cod),[
[Rh(cod)Cl]
2
,
and [Rh(cod)-
[
44]
1
1
2
acac)] were prepared according to literature procedures.
1
1
1
H), 6.90 (d, J ϭ 8.8 Hz, 1 H), 6.09 (d, J ϭ 9.6 Hz, 1 H), 1.96
4
9
,5-Bis[bis(diethylamino)phosphonito]-9,9-dimethylxanthene:[14]
,9-dimethylxanthene (2.85 g, 13.56 mmol) and N,N,NЈ,NЈ-tetra-
31
1
(s, 3 H, CCH
3 3 3
), 1.67 (s, 3 H, CCH ) ppm. P{ H} NMR (CDCl ):
δ ϭ 143.1 (d, JP,P ϭ 46.5 Hz), 137.4 (JP,P ϭ 46.5 Hz) ppm.
C H36Cl O P Pd (1016.14 g mol ): calcd. C 65.01, H 3.57; found
55 2 5 2
methylethylenediamine (TMEDA) (3.32 g, 28.48 mmol) were dis-
solved in diethyl ether (50 mL). The solution was cooled to Ϫ40
°
Ϫ1
C 65.34, H 3.63%.
C. n-Butyllithium (11.4 mL, 2.5  in hexane) was added to the
mixture dropwise. The solution was allowed to warm up to room
(6.0 g, 18.99 mmol) eric literature procedure.[ [Rh(cod)(acac)] (0.047 g, 0.15 mmol)
2 2
temperature and stirred overnight. ClP(NEt )
4
[Rh(cod)((R,R)-1)]BF (complex 3): This is a modification of a gen-
45]
was dissolved in diethyl ether (40 mL) and added to the reaction
mixture at Ϫ80 °C. The solution was allowed to warm up to room
temperature and stirred overnight. The solvent was removed by
evaporation, and the resulting solid was dissolved in pentane
4
was dissolved in THF (2 mL). HBF (54% solution in diethyl ether)
(0.053 g, 0.60 mmol) was added. During addition, the solution col-
ored darker. (R,R)-1 (0.126 g, 0.15 mmol) was dissolved in THF
(2 mL) and added to the solution, yielding an orange suspension.
Diethyl ether (approximately 20 mL) was added, and the mixture
was stirred vigorously for 5 minutes. The yellow precipitate formed
was isolated and washed with diethyl ether (10 mL). Volatiles were
removed in vacuo to leave a yellow/orange solid. Yield 68% (0.14 g,
(40 mL) and filtered to remove solids. Excess TMEDA was re-
moved by azeotropic evaporation with toluene (3 ϫ 20 mL). Pen-
tane (10 mL) was added to the solid, and the mixture was stored
overnight at Ϫ25 °C. The crystals that had formed were separated
from the solution by filtration. The crystals were dried in vacuo,
3
1
1
0.10 mmol). P{ H} NMR (CDCl
3
): δ ϭ 169.3 (dd, JRh-P ϭ 219,
1
yielding a pure dark yellow solid. Yield (4.64 g, 61.3%). H NMR
JP-P ϭ 24 Hz), 161.1 (dd, JRh-P ϭ 216, JP-P ϭ 24 Hz.) ppm.
(
1
(
CDCl
.60 (6 H, CCH
CDCl ): δ ϭ 91.5 (s) ppm.
3
): δ ϭ 7.38 (m, 4 H), 7.02 (d, 2 H), 3.05 (m, 16 H, NCH
2
),
Hydroformylation of Styrene: Reactions were performed using a
stainless steel autoclave (75 mL) equipped with an inner glass be-
aker. Styrene was filtered through neutral alumina before use. Gen-
) ppm. 31P{ H} NMR
1
), 1.00 (m, 24 H, NCH
3 3
3
(
2
R,R)-4,5-Bis(dinaphtho[d,f][1,3,2]dioxaphosphepin-4-yl)-9,9-di- erally, Rh(acac)(CO) (3.5 mg, 13.5 µmol) and 1 (1.2 equiv.) were
methylxanthene (1): This is a modification of a literature pro-
cedure.
both dissolved in toluene (5 mL), and the combined solution was
brought into the preheated autoclave, which was then pressurized
[
14]
4,5-Bis[bis(diethylamino)phosphonito]-9,9-dimethylxan-
thene (2.00 g, 3.58 mmol) and (R)-(ϩ)-2,2Ј-dihydroxy-1,1Ј-binaph- to 20 bar of synthesis gas. After the appropriate preformation time,
thyl (2.05 g, 7.16 mmol) were dissolved in toluene (50 mL). A cata-
lytic quantity of tetrazole (about 10 mg, 0.14 mmol) was added as
the autoclave was depressurized, and the substrate solution, con-
taining styrene (1.55 mL, 13.5 mmol) in toluene (3.45 mL) was ad-
a protonation agent. The solution was heated to 90 °C for 16 hours. ded, and 20 bar of synthesis gas was applied. During catalysis,
Diethylamine formed during the reaction was removed twice under
vacuum. After reaction, the solvent was removed in vacuo, and the
remaining crude product was dissolved in CH Cl (5 mL) and lay-
2 2
samples were withdrawn without pressure loss for GC analysis.
After reaction, the autoclave was cooled and depressurized, and
the reaction mixture taken out. Oxidation of the aldehydes to the
carboxylic acids was done directly afterwards, to prevent racemiz-
ation and to allow for analysis by chiral GC.
ered with acetonitrile (20 mL). The precipitated product was iso-
lated by filtration and washed with acetonitrile (10 mL). After dry-
ing, a yellow powder was obtained as a pure product (0.70 g, 31%).
Hydroformylation of Vinyl Acetate: Reactions were performed
using a stainless steel autoclave (75 mL) equipped with an inner
1
The other enantiomer can be obtained by the same procedure. H
1
NMR (CDCl
3
): δ ϭ 7.94 (d, J ϭ 8.8 Hz, 2 H, binolH), 7.90 (d,
2
glass beaker. Generally, Rh(acac)(CO) (3.0 mg, 11.6 µmol) and 1
1
1
J ϭ 8.0 Hz, 2 H, binolH), 7.84 (d, J ϭ 8.0 Hz, 2 H, binolH), 7.63
d, J ϭ 8.8 Hz, 2 H, binolH), 7.58 (d, J ϭ 8.8 Hz, 2 H, binolH),
.55 (dd, J ϭ 8.0, J ϭ 0.8 Hz, 2 H, binolH), 7.42 (t, J ϭ 8.8 Hz,
H, ArH), 7.32 (d, J ϭ 8.4 Hz, 2 H, binolH), 7.28 (dquin, 4 H,
(
2 equiv.) were both dissolved in benzene (10 mL), and the com-
1
1
(
bined solution was charged into the autoclave, which was then
pressurized to 20 bar of synthesis gas and heated to reaction tem-
perature. After preformation, vinyl acetate (1.5 mL, 16.3 mmol) in
benzene (5 mL) was added. After reaction, the reaction mixture
was quantitatively withdrawn, and a weighed amount of ethyl pro-
pionate (0.5 mL, 4.4 mmol) was added. After a quantitative distil-
lation, an aliquot was withdrawn for GC analysis (both for conver-
sion and enantiomeric excess).
1
2
1
7
1
6
1
2
1
J ϭ 8.0, J ϭ 1.2 Hz, binolH) 7.18 (d, J ϭ 8.8 Hz, 2 H, binolH),
1
6
.92 (d, 1J ϭ 8.8 Hz, 2 H, binolH), 6.90 (d, J ϭ 7.2 Hz, 4 H,
binolH), 1.79 [s, 6 H, C(CH ): δ ϭ
] ppm. 13C NMR (CDCl
52.6, 149.9, 149.1, 133.0, 132.5, 131.5, 130.9, 130.4, 129.9, 129.0
d, JP,C ϭ 16.8 Hz), 128.5, 128.2 (d, JP,C ϭ 6.8 Hz), 126.9 (d, JP,C
3
)
2
3
1
(
ϭ
3
.8 Hz), 125.8 (d, JP,C ϭ 19.0 Hz), 124.6 (d, JP,C ϭ 20.6 Hz), 123.3,
21.9 (d, JP,C ϭ 5.3 Hz), 34.1 [C(CH ], 31.9 [C(CH ] ppm.
): δ ϭ 178.0 (s) ppm. C55
mol ): calcd. C 78.75, H 4.33; found C 78.52, H 4.18%.
1
3
)
2
3
)
2
Hydrogenation of Methyl (Z)-2-Acetamidocinnamate (I): Reactions
(838.82 g were carried out in a stainless steel autoclave (75 mL). 3 (11.4 mg,
3
1
!
P{ H} NMR (CDCl
3
H
36
O
5
P
2
Ϫ1
0.01 mmol) was dissolved in methanol (1 mL), and the solution was
2
activated under 1 bar of H for 2 hours. Methyl (Z)-2-acetamido-
cinnamate (0.22 g, 1.00 mmol) was dissolved in methanol (9 mL),
and the substrate solution was charged into the catalyst solution,
and the appropriate H pressure was applied. During reaction,
2
samples were withdrawn for GC analysis.
cis-[PdCl ((S,S)-1)] (complex 2): PdCl (cod) (13.2 mg, 46.2 µmol)
2
2
and (S,S)-1 (38.7 mg, 46.2 µmol) were dissolved in CH Cl (5 mL)
2
2
and stirred for 1 hour at room temperature. The solvent was re-
moved in vacuo to leave a light-yellow solid. Yellow block-shaped
crystals of complex 2 were grown by slow diffusion of acetonitrile
1
into a dichloromethane solution. H NMR (CDCl
3
): δ ϭ 8.37 (d,
Crystal Structure Determination of 2: Intensity data were collected
1
1
1
J ϭ 8.8 Hz, 1 H), 8.20 (d, J ϭ 8.8 Hz, 1 H), 8.16 (d, J ϭ 8.8 Hz, using graphite-monochromated Mo-K
α
radiation, on a Nonius
1
1
1
H), 8.08 (d, J ϭ 8.8 Hz, 1 H), 8.03 (t, J ϭ 8.8 Hz, 1 H), 8.00
KappaCCD diffractometer. A correction for absorption was con-
Eur. J. Inorg. Chem. 2004, 4193Ϫ4201
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4199

D. Vogt et al.
FULL PAPER
sidered unnecessary. The structure was solved by automated direct
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[46]
2
methods using SHELXS-97, and refined on F using SHELXL-
[
47]
9
7. The absolute configuration of 2 was established by anomal-
ous dispersion effects to be (S,S)-[PdCl (1)], Flack parameter
.013(19). The crystal structure contains voids (1406.0 A /unit cell)
2c]
2
[
2d]
G. Scapacci, J. Organomet. Chem. 2001, 621, 26Ϫ33.
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0
Nettekoven, P. C. J. Kamer, M. Widhalm, P. W. N. M. van
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nitrile). Their contribution to the structure factors was ascertained
using PLATON/SQUEEZE (499 e/unit cell). All non-hydrogen
atoms were refined with anisotropic displacement parameters. All
hydrogen atoms were constrained to idealized geometries and al-
lowed to ride on their carrier atoms with an isotropic displacement
parameter related to the equivalent displacement parameter of
their carrier atoms. Structure validation and molecular graphics
preparation were performed with the PLATON package.[
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Table 2. Selected crystallographic data for complex 2, cis-
4f]
2
2
2
2
2
[
PdCl
2
(1)]; Rint ϭ Σ[|F
o
Ϫ F
o
(mean)|] / Σ[F
o
]; wR(F ) ϭ {Σ[w(F
||) / Σ|F
o
2
2
2 2 1/2
Ϫ F
c
) ] / Σ[w(F
o
) ]} ; R(F) ϭ Σ(||F
o
| Ϫ |F
c
o
|
[
[
5] [5a]
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000, 6, 1496Ϫ1504.
Formula
C
55
H
36Cl
2
O
5
P
2
Pd ϩ solvent
6] [6a]
M (g molϪ1)
[a]
For a recent review see: J. Ansell, M. Wills, Chem. Soc. Rev.
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6b]
2
002, 31, 259Ϫ268. [
E. J. Zijp, J. I. van der Vlugt, M.
Crystal size (mm)
Crystal system
Space group
0.06 ϫ 0.12 ϫ 0.30
orthorhombic
P2 2 2 (no. 19)
1 1 1
Fioroni, D. M. Tooke, A. L. Spek, C. Müller, D. Vogt, unpub-
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[
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11.3183(1)
17.4368(1)
27.4181(3)
5411.10(8)
4
7b]
˚
b (A)
˚
c (A)
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Z
d
calcd. (g cmϪ3
[a]
[a]
)
1.247
0.544
150
8c]
) (mmϪ1)
α
µ(Mo-K
T (K)
3
687Ϫ3690.
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Unique reflections (Rint
59674
12279 (0.064)
0.0411
[9]
)
4
R
wR
F
2
[10]
(F ) (all data)
0.0975
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Eur. J. Inorg. Chem. 2004, 4193Ϫ4201

Coordination Chemistry and Asymmetric Catalysis with a Chiral Diphosphonite
FULL PAPER
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Early View Article
Published Online September 7, 2004
Eur. J. Inorg. Chem. 2004, 4193Ϫ4201
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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