Furanoside diphosphines derived from D-(+)-xylose and D-(+)-glucose as ligands in rhodium-catalysed asymmetric hydroformylation reactions

Article abstract of DOI:10.1016/S0957-4166(01)00081-7

Chirality transfer by furanoside diphosphines 1-3 was investigated in the Rh-catalysed asymmetric hydroformylation of prochiral olefins. In general, they induced high regioselectivities with branched aldehydes and moderate enantioselectivities of up to 58%. Improved activities were seen when a methyl substituent was introduced at C-(5) of the sugar residue. Systematic variation of the configuration at C-(5) suggests that there is a cooperative effect between stereocentres, which results in a matched combination for ligand 3 with (R)-configuration at the C-(5) stereogenic centre. Introduction of a methyl substituent at C-(5) induces a strong coordination preference. The solution structures of the species formed under hydroformylation conditions were also investigated.

Full text of DOI:10.1016/S0957-4166(01)00081-7

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 651–656
Furanoside diphosphines derived from
D
-(+)-xylose and
D
-(+)-glucose as ligands in rhodium-catalysed asymmetric
hydroformylation reactions
Montserrat Di e´ guez,* Oscar P a` mies, Gemma Net, Aurora Ruiz* and Carmen Claver
Departament de Qu ´ı mica F ´ı sica i Inorg a` nica, Universitat Rovira i Virgili, Pl. Imperial T a` rraco 1, 43005 Tarragona, Spain
Received 23 January 2001; accepted 15 February 2001
Abstract—Chirality transfer by furanoside diphosphines 1–3 was investigated in the Rh-catalysed asymmetric hydroformylation of
prochiral olefins. In general, they induced high regioselectivities with branched aldehydes and moderate enantioselectivities of up
to 58%. Improved activities were seen when a methyl substituent was introduced at C-(5) of the sugar residue. Systematic variation
of the configuration at C-(5) suggests that there is a cooperative effect between stereocentres, which results in a matched
combination for ligand 3 with (R)-configuration at the C-(5) stereogenic centre. Introduction of a methyl substituent at C-(5)
induces a strong coordination preference. The solution structures of the species formed under hydroformylation conditions were
also investigated. © 2001 Elsevier Science Ltd. All rights reserved.
1
. Introduction
enough understood to predict the type of ligand
required to achieve high levels of enantioselectivity. In
this context, the search for new efficient ligands which
can be prepared from simple starting materials is still of
great importance in catalysis research. Chiral auxiliaries
from the chiral pool have attracted much attention. In
recent studies sugar-derived ligands showed good con-
versions and excellent enantioselectivities in different
Hydroformylation is one of the most widely studied
homogeneous catalytic processes. In recent years,
asymmetric hydroformylation has attracted much
attention as a potential tool for preparing enantiomeri-
cally pure aldehydes, which are important precursors
for many fine chemicals. Catalyst precursors based on
Rh(acac)(CO) ] modified with chiral phosphorus lig-
1
1
3,4c,7
[
types of catalytic reactions,
e.g. hydrogenation,
2
ands are particularly well suited to enantioselective
hydroformylation. Moderate to excellent enantioselec-
hydroformylation, hydrocyanation and allylic alkyla-
tion, which demonstrates their potential. One of the
most successful families of sugar derivatives in asym-
tivities have previously been obtained in the hydro-
2
formylation 3of
styrene 4using
diphosphines,
phosphine–
metric catalysis is the 1,2-protected furanoses derived
4c,7f,7g,8
diphosphinites,
diphosphite and
from
D
-(+)-xylose and
D
-(+)-glucose (Scheme 2).
5
phosphite ligands. Recent theoretical studies with C -
2
diphosphine
ligands
attributed
the
moderate
In this paper we describe the use of C -furanoside
1
enantioselectivities achieved to minor stereodifferentia-
diphosphines 1–3 (Fig. 1) in the asymmetric hydro-
formylation of prochiral olefins. A feature of these
ligands is that the two donor sites are different and can
therefore match the coordination modes better, influ-
encing their reactivity and achieving good enantioselec-
tion of the different coordination modes by the catalyst
6
(
Scheme 1).
In situ spectroscopic studies on the most successful
diphosphite and phosphine–phosphite ligands suggest
that the presence of a single catalytically active species
in solution is the key to controlling efficient chirality
H
H
OC
P
P
P
4
b,5
transfer.
However, the mechanistic aspects of the
Rh
CO
Rh
CO
asymmetric hydroformylation reaction are still not well
P
CO
ee
ea
*
0
Corresponding authors. Fax: (+34)-977559563; e-mail: dieguez@
quimica.urv.es; aruiz@quimic.urv.es
Scheme 1.
957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00081-7

6
52
M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 651–656
O
Ph P
P O
O O
2
Ph P
RS
2
O
PPh
HO O
2
O
O
O
O
O
O
Hydrogenation 98% e.e.8a
Hydrogenation >99% e.e.7f
1,4-addition 62% e.e.8b
O
O
O
O
O
P
O
P
O
P
O O
O
P
O O
O
Ph P
2
O
O
PPh
2
O
O
O
O
O
O
Hydroformylation 61% e.e.8c
Hydroformylation 91% e.e.4c Hydrogenation 98% e.e.7g
Scheme 2.
9
tivity. Ligands 2 and 3 differ from ligand 1 at C-(5),
where a new stereogenic centre was introduced allowing
the effect of structural alterations at C-(5). The configu-
ration at C-(5) was expected to be important to the
course of the catalytic reaction because of its proximity
other vinyl arenes under different reaction parameters
(Scheme 3).
Since other precursors were reported to give lower
5
enantioselectivities, the catalysts were prepared in situ
7
g
to the metal centre.
from [Rh(acac)(CO) ] and the desired amount of
2
diphosphine. In general, high ratios of branched alde-
hyde 5 to linear aldehyde 6 and moderate enantioselec-
tivities were achieved (Table 1).
Ph P
2
PPh
O
Ph P
2
2
O
PPh
O
PPh2
PPh2
2
The effect of temperature, the ligand-to-metal ratio and
O
O
O
O
O
O
the partial pressures of CO and H on the enantioselec-
2
tivity were investigated using ligand 1 (Table 1). Entries
1–3 indicate that an excess of diphosphine (2 equiv.
with respect to Rh) was needed to prevent the forma-
1
2
3
10
Figure 1.
tion of the active [HRh(CO) ] achiral species and the
4
modified active species containing the diphosphine as a
monodentate ligand, which are known to lower the
enantioselectivity of the reaction.
2
2
. Results and discussion
2.1. Hydroformylation of vinyl arenes
Hydroformylation experiments under different partial
pressures of CO and H revealed that higher partial
2
Diphosphine ligands 1–3 were tested in the rhodium-
catalysed asymmetric hydroformylation of styrene and
pressures of CO lead to lower initial turnover frequency
(entries 2, 4 and 5). A comparison of entries 2, 4 and 5
CHO
CHO
CO / H2
+
Rh cat. / 1-3
R
R
R
4
a
b
c
R = H
(S)-5a/(R)-5a
(S)-5b/(R)-5b
(S)-5c/(R)-5c
6a
6b
6c
R = OMe
R = F
Scheme 3. Hydroformylation of vinyl arenes 4a–4c.

M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 651–656
653
a
Table 1. Hydroformylation of 4a–4c with [Rh(acac)(CO) ]/P-P (P-P=1–3)
2
TOFb
% Convc
% 5
d
% E.e.e
Entry
P-P
S
P-P/Rh
PCO/PH2
T (°C)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
3
3
3
4a
4a
4a
4a
4a
4a
4a
4b
4c
4a
4a
4a
4a
4a
1
2
4
2
2
2
2
2
2
2
2
2
2
2
1
1
1
2
0.5
1
1
1
1
1
1
1
1
80
80
80
80
80
40
40
80
80
80
40
80
40
40
87
39
37
28
58
5
82
37
36
23
52
4
84
96
96
95
95
97
97
95
94
96
97
97
97
97
6 (S)
35 (S)
34 (S)
34 (S)
33 (S)
51 (S)
50 (S)
38 (−)
32 (+)
29 (S)
44 (S)
39 (S)
58 (S)
57 (S)
f
5
58
38
36
50
6
54
7
33
35
45
6
48
7
0
1
2
3
4
0.5
12
10
a
b
c
d
e
f
Reaction conditions: P=30 bar, styrene (6.5 mmol), [Rh(acac)(CO) ] (0.013 mmol), toluene (15 mL).
2
TOF in mol substrate×Rh−1×h determined after 1 h reaction time by GC.
−1
%
%
Conversion after 5 h.
Regioselectivity in 5.
The % e.e. of 5 was measured by GC.
Determined after 72 h.
further shows that regio- and enantioselectivity are
virtually unaffected by varying the partial pressure of
CO.
bination for ligand 3 and a mismatched combination
for 2.
In conclusion, catalytic precursors based on rhodium
Lowering the reaction temperature to 40°C increased
the enantioselectivity (entry 6, 51% e.e.). The regio- and
enantioselectivity of the reaction were also found to
remain constant throughout the reaction, which shows
that the catalyst was stable under the reaction condi-
tions (entry 7).
systems with C -furanoside diphosphines, which are
1
readily available from the chiral pool, provide hydro-
formylated products with regioselectivities of up to 97%
with e.e.s of up to 58%. These results are similar to
those obtained with the most efficient diphosphine
2
based catalytic systems previously reported.
Substituting different groups in the para-position of the
substrate had no effect on the hydroformylation reac-
tion with respect to the conversion or the regio- and
enantioselectivity of the products (entries 8, 9 vs 2).
2.2. Solution structure of the species formed under
hydroformylation conditions
To identify the intermediates formed during the hydro-
formylation process, we carried out a solution spectro-
scopic study of the catalytic species formed by adding
two equivalents of the corresponding ligand to the
The other ligands were compared under ‘standard’ con-
ditions, i.e. a ligand-to-rhodium ratio of 2 and a syn gas
pressure of 30 bar. Activities improved when ligands 2
and 3 were used (entries 10 and 12 vs 2) in comparison
to the use of ligand 1. At 40°C with ligand 2, having
catalyst precursor [Rh(acac)(CO) ] in deuterated tolu-
2
ene and placing the resultant solution under 30 bar
pressure with syn gas.
(
(
(
S)-configuration at C-(5), gave (S)-5a with 44% e.e.
entry 11), while diastereomer 3 gave (S)-5a in 58% e.e.
entry 13). These results and those obtained with ligand
Initially, displacement of two carbon monoxide
molecules by the ligand caused the formation of a
mononuclear [Rh(acac)(P-P)] 7a–7c species (Scheme 4,
1
(entry 6) suggest that there is a cooperative effect
11
between stereocentres, which results in a matched com-
Table 2). Under these hydroformylation conditions
H
O
O
O
O
OC
P
CO
CO
P
P
CO / H2
P-P
Rh
Rh
Rh
P
CO
P-P = 1
7a
b
8a
b
2
3
c
c
Scheme 4.

6
54
M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 651–656
31
1
1
a
Table 2. Selected P{ H} and H NMR data for species 7 and 8
Hydride
l(P)
J{P-Rh}
l(P)
J{P-Rh}
J{P-P}
l(H)
J{H-P}
J{H-P}
J{H-Rh}
7a
7b
7c
8a
8b
8c
35.04
37.32
39.31
8.02
21.31
22.43
183.6
179.8
183.2
120.4
122.0
119.3
44.52
43.45
48.67
18.75
27.43
28.56
188.9
189.3
187.6
107.7
100.6
102.4
73.3
69.9
71.2
44.4
51.9
54.8
−8.58
−8.36
−8.42
34.5
19.7
18.2
84.3
102.2
100.9
11.1
11.6
11.3
a
Chemical shifts (l) in ppm and coupling constants (J) in Hz.
O
mediate values for the phosphorus–hydride coupling
constants indicate that there is an equilibrium between
the two equatorial–axial diastereoisomers in fast
OC
O
C
P
P
C
P
P
Rh
Rh
Rh
Rh
P
P
C
O
P
P
C
O
10
CO
exchange in the NMR time-scale. Interestingly, taking
9
2
into reference the typical values of J
in equatorial and axial positions,
for phosphines
it seems clear that
P-H
1
2a,d
H
Rh
P
one diastereoisomer is much more stable under these
conditions. Moreover, as in related systems containing
chiral diphosphines such as Bdpp, there is an equi-
OC
P
CO
12a,d
librium between mono- and binuclear species.
8a
No binuclear species were detected by HPNMR for
Scheme 5.
solutions containing ligands 2 and 3. Therefore, at
31
1
room temperature, the P{ H} NMR spectrum of
these solutions showed two doublets of doublets. These
were attributed to complexes 8b and 8c, respectively
7
a–7c reacted rapidly to form the hydridorhodium
(
Table 2). The spectrum also contained a doublet of
dicarbonyl species 8a–8c, which are known to be
doublets of doublets in the hydride region (Table 2),
again indicating hydride coupling with rhodium and the
two non-equivalent phosphorus atoms. Moreover, the
1
responsible for the catalytic activity.
3
1
1
At 25°C, the P{ H} NMR spectrum of solution con-
2
JP-H values and the VT NMR indicate that there is no
taining ligand 1 showed two broad doublets at 3.61
equatorial–axial exchange of phosphorus. This con-
trasts with the equilibrium usually reported for equato-
1
1
(
JP-Rh=145 Hz) and 6.74 ppm ( JP-Rh=149 Hz)
attributed to inactive dimers 9 and 10 (Scheme 5),
which have a RhꢀRh bond and two CO groups that
bridge the metal atoms. Dimer 10 has an additional
terminal CO group on each rhodium centre. Similar
compounds have previously been reported for other
diphosphine systems, with very similar rhodium–phos-
phorus coupling constants (around 145 Hz). After
heating the solution to 80°C, the P{ H} NMR spec-
trum showed two new doublets of doublets at 8.02 and
12a,d
rial–axial diphosphine chelates.
In conclusion, ligands 1–3 take advantage of their
C -symmetry to reduce their coordination modes in the
1
[
HRh(PP)(CO) ] complexes. The introduction of a
2
methyl substituent at C-(5) induces a strong coordina-
tion preference, therefore ligands 2 and 3 have only one
diastereoisomer under hydroformylation conditions.
However, this strong coordination preference affects
the enantioselectivity minimally. Enantioselectivity is,
however, affected by the configuration of the stereocen-
tres in the ligand, which produce a matched combina-
tion for ligand 3.
12
3
1
1
1
8.75 ppm. These correspond to the mononuclear
hydride–rhodium complex 8a (Table 2, Scheme 5).
1
The H NMR spectrum of this solution in the hydride
region revealed a doublet of doublets of doublets
(
Table 2, Fig. 2), which is indicative of hydride cou-
pling with rhodium and the two non-equivalent phos-
phorus atoms. The phosphorus–hydride coupling
constants are characteristic of a hydrido-rhodium dicar-
bonyl species2a,dwith equatorial-axially coordinating
1
diphosphines.
Small cis-phosphorus–hydride cou-
pling constants of between 1 and 20 Hz are reported in
HRh(PP)(CO) complexes with bis-equatorially coordi-
2
nating diphosphine. In contrast, a coupling constant of
1
1
06 Hz in trigonal bipyramidal HRh(CO) (DPEphos)
Figure 2. H HPNMR of the solution of [Rh(acac)(CO)
]/1 in
2
2
3,14
1
has been found for an apical phosphine.
The inter-
the hydride region under hydroformylation conditions.

M. Di e´ guez et al. / Tetrahedron: Asymmetry 12 (2001) 651–656
655
3. Experimental
Acknowledgements
3
.1. General techniques
We thank the Spanish Ministerio de Educaci o´ n y Cul-
tura and the Generalitat de Catalunya (CIRIT) for
their financial support (PB97-0407-CO5-01) and the
Generalitat de Catalunya (CIRIT) for awarding a
research grant (to O.P.). We are very much indebted to
Professor P. W. N. M van Leeuwen of the University of
Amsterdam, for his comments and suggestions.
All syntheses were performed by standard Schlenk tech-
niques under nitrogen or argon atmosphere.
a
1
5
7g,8a
[
Rh(acac)(CO)2] and ligands 1–3
were prepared by
previously described methods. Solvents were purified by
standard procedures. All other reagents were commer-
1
31
1
cially available and used as supplied. H and P{ H}
NMR spectra were recorded on a Varian Gemini 300
MHz spectrometer. Chemical shifts were relative to
References
1
31
SiMe4 ( H) as internal standard or H PO ( P) as
3
4
external standard. Gas chromatographic analyses were
run on a Hewlett–Packard HP 5890A instrument (split/
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internal diameter 0.2 mm, film thickness 0.33 mm,
carrier gas: 150 kPa He, F.I.D. detector) equipped with
a Hewlett–Packard HP 3396 series II integrator.
Hydroformylation reactions were carried out in a
home-made 100 mL stainless steel autoclave. Enan-
tiomeric excesses of hydroformylation products were
measured using a Hewlett–Packard HP 5890A gas
chromatograph (split/splitless injector, J&W Scientific,
FS-Cyclodex b-I/P 50 m column, internal diameter 0.2
mm, film thickness 0.33 mm, carrier gas: 100 kPa He,
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.3. In situ HPNMR characterisation of
[
HRh(CO) (PP)]
2
In a typical experiment a sapphire tube (ƒ=10 mm)
was filled under argon with solution of
Rh(acac)(CO) ] (0.030 mmol) and ligand (molar ratio
a
[
2
P/Rh=2) in toluene-d (1.5 mL). The HPNMR tube
8
was purged twice with CO and pressurised to the
appropriate pressure of CO/H . After shaking the sam-
2
ple until the desired temperature was reached, the solu-
tion was analysed.

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