Synthesis of novel diphosphines from D-(+)-glucose. Use in asymmetric hydrogenation

Article abstract of DOI:10.1016/S0957-4166(00)00440-7

A new family of bidentate diphosphine ligands which contain a glucofuranoside as a simple but highly effective chiral backbone are reported. These ligands are prepared in a few steps from readily available D-(+)-glucose. These new ligands have been applie

Full text of DOI:10.1016/S0957-4166(00)00440-7

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4701–4708
Synthesis of novel diphosphines from
D-(+)-glucose.
Use in asymmetric hydrogenation
Montserrat Die´guez,a,* Oscar Pa`mies,a Aurora Ruiz,a Sergio Castillo´nb and Carmen Clavera
aDepartament de Qu´ımica F´ısica i Inorga`nica, Universitat Rovira i Virgili, Pl. Imperial Tarraco 1, 43005, Spain
bDepartament de Qu´ımica Anal´ıtica i Orga`nica, Universitat Rovira i Virgili, Pl. Imperial Tarraco 1, 43005, Spain
Received 16 October 2000; accepted 3 November 2000
Abstract
A new family of bidentate diphosphine ligands which contain a glucofuranoside as a simple but highly
effective chiral backbone are reported. These ligands are prepared in a few steps from readily available
D
-(+)-glucose. These new ligands have been applied to the rhodium-catalysed hydrogenation of a,b–unsat-
urated carboxylic acid derivatives under very mild reaction conditions, with enantiomeric excesses of up
to 98%. © 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
The enantioselective hydrogenation of prochiral substrates is one the most important applica-
tions of asymmetric catalysis.1 Although the use of numerous chiral diphosphines in asymmetric
hydrogenation has been successfully reported,2 the design and synthesis of new readily available,
highly active and enantioselective chiral ligands continues to be an active field of research.
One of the simplest methods of obtaining chiral ligands is the transformation of readily
available natural homochiral compounds (the so-called chiral pool). Despite the accessibility at
low cost of the carbohydrate synthons and the excellent enantioselectivities obtained in different
processes,3 the full potential of the carbohydrate chiral pool to provide ligands has not been
fully exploited.
In the context of our studies on furanoside ligands,4–6 we have described the synthesis of a
new diphosphine derived from xylose (Scheme 1, xylophos) and its successful use in the
Rh-asymmetric hydrogenation of alkenes with enantioselectivities of up to 98%.5 Moreover, we
have recently shown that the rate and enantioselectivity in the Rh-asymmetric hydroformylation
of styrene with the furanoside diphosphite 1 (Scheme 1, ee=61%)6a was notably improved by
* Corresponding author. Fax: +34 977559563; e-mail: dieguez@quimica.urv.es
0957-4166/00/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00440-7

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M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
introducing a new stereocentre at the C-5 position6 (Scheme 1, 2, ee=90%).6b These results
suggested that diphosphine ligands related to xylophos, but with a new stereocentre at C-5,
could also provide more active and enantioselective hydrogenation catalytic systems.
O
O
O
R H
O
P O
P
OMe
OMe
Ph2P
t-Bu
O
O
O
PPh2
O
O
=
O
O
O
O
t-Bu
xylophos
1 R= H
2 R= Me
Scheme 1.
In this context we now report the synthesis of the new diphosphines 3,5,6-trideoxy-3,5-bis-
diphenylphosphine-1,2-O-isopropylidene-a- -glucofuranose 3 and 3,5,6-trideoxy-3,5-bis-diphenyl-
phosphine-1,2-O-isopropylidene-a- -glucofuranose 4 (Scheme 2) and their use in the Rh-
D
D
catalysed enantioselective hydrogenation of prochiral a,b-unsaturated carboxylic acid deriva-
tives. With these two diastereomers we can explore the effects of the stereogenic C-5 atom,
whose configuration is expected to be important for the course of the catalytic reaction because
of its proximity to the metal. In addition, these ligands are particularly interesting from a
practical point of view since they can be prepared in very few steps from available -(+)-glucose.
D
H
H
PPh2
O
PPh2
Ph2P
O
PPh2
O
O
O
O
3
4
Scheme 2.
2. Results and discussion
2.1. Synthesis of diphosphines
The new C1-symmetrical homochiral diphosphines 3 and 4 were synthesised very efficiently in
two steps from the corresponding alcohols 6-deoxy-1,2-O-isopropylidene-a- -allofuranose 6 and
6-deoxy-1,2-O-isopropylidene-b- -talofuranose 7, as shown in Scheme 3. These diols were
readily prepared from natural
-(+)-glucose 5.7 The reaction of 6 and 7 with trifluoromethane-
D
L
D
sulfonic anhydride in the presence of pyridine afforded the ditriflate compounds 8 and 9,
respectively. Subsequent addition of a slight excess of potassium diphenylphosphide in THF
provided easy access to the desired diphosphines 3 and 4.

M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
4703
H
H
H
TfO
PPh2
O
PPh2
HO
O
O
ii)
iii)
O
O
O
O
O
OH
OTf O
i)
6
8
3
D-(+)-glucose
5
i)
H
H
H
OTf
O
OH
O
Ph2P
iii)
O
PPh2
ii)
O
O
O
O
O
OH
OTf
O
7
9
4
Scheme 3. Synthesis of diphosphine ligands 3 and 4. (i) Ref. 7. (ii) Tf2O, Py, CH2Cl2, −20 to 25°C. (iii) KPPh2, THF,
−78 to 25°C.
Diphosphines 3 and 4 were isolated in good yields by flash chromatography under argon
atmosphere as white crystalline solids that are stable at air in solid state. The elemental analysis
1
and mass spectrometry data were in agreement with the assigned structure. The 31P, 13C and H
NMR spectra provided further unequivocal evidence to identity the compounds (see Section 4).
The 31P NMR spectrum of 4 displays a large phosphorus–phosphorus coupling constant
(4JP–P) of 34.7 Hz. In the 13C NMR spectrum, double doublets due to the coupling with both
non-equivalent phosphorus atoms were observed for the furanoside carbon atoms C-3 and C-5
with JP–C between 7 and 22 Hz. Unlike the 31P NMR spectrum of 4, no phosphorus–phosphorus
coupling constant was observed for diphosphine 3. The inverse configuration at C-5 of the sugar
backbone gave rise to a solution structure without the correct conformation to appreciable
phosphorus–phosphorus coupling.8
2.2. Hydrogenation of prochiral olefins
Diphosphine ligands 3 and 4 were tested in the Rh-catalysed asymmetric hydrogenation of a
variety of prochiral a,b-unsaturated carboxylic acid derivatives 10–13 under mild reaction
conditions (1 bar of H2). The results are shown in Table 1.
Reaction of the prochiral olefin Z-a-acetamidocinnamic 10 using the catalyst precursor
containing ligand 3 afforded a quantitative yield of hydrogenation product with 52% ee (entry
1). The related ester 2-acetoamidomethacrylate 11 and substrate 12 gave similar results at room
temperature (entries 2–3). When itaconic acid 13 was used as a substrate the enantioselectivity
substantially decreased (23% ee, entry 4). This is in line with the lower substrate acceptability of
the six-membered ring Rh–diphosphine catalysts.1,9 Catalytic precursor containing ligand 4
hydrogenated substrates 10–12 with higher enantioselectivities (up to 93% ee, entries 5–7).
Decreasing the reaction temperature to 0°C enhanced the enantioselectivity (up to 98%, entries
9 and 10).
The data demonstrate that the enantiomeric excesses are strongly dependent on the absolute
configuration of the C-5 stereogenic centre of the carbohydrate backbone, while the activity and

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M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
Table 1
Asymmetric hydrogenation of a,b-unsaturated carboxylic derivatives 10–13a
Entry
Substrate
Ligand
t (h)
Conv. (%)
ee (%)
1
2
3
4
5
6
7
8
10c
11b
12b
13e
10c
11b
12b
13c
11b
12b
12b
3
3
3
3
4
4
4
4
4
4
2
2
1
2
2
2
1
2
6
5
1
100
100
75
100
100
100
80
100
95
100
50
52 (S)
53 (S)
54 (S)
23 (R)
92 (S)
91 (S)
93 (S)
59 (R)
98 (S)
98 (S)
92 (S)
9d
10d
11
e
–
a Cat./substrate=1/100, substrate=1 mmol, solvent=methanol (6 ml), T=20°C, P=1 atm.
b Conversions and enantiomeric excesses measured by GC.
1
c Conversions measured by H NMR and enantiomeric excesses measured by polarimetry.
d T=0°C.
e Hydrogenation of 12 using catalyst precursor [Rh(cod)(xylophos)]BF4 (14).
the sense of the asymmetric induction are not affected (entries 1–4 versus 5–8). Thus, the best
enantioselectivities are obtained by using the catalytic precursor containing the diphosphine
ligand with a (R)-configuration in the C-5 (up to 98%).
As with the related diphosphite ligands,6 introducing the methyl substituent in the carbohy-
drate backbone considerably enhanced the rate (entries 3 and 7 versus 11). The fact that the
conversions were better with the catalyst containing more hindered and slightly basic ligands 3
and 4 than with the Rh–xylophos catalyst precursor 14 could suggest that the rate-determining
step is the addition of H2.
The results obtained in the enantioselective Rh-catalysed hydrogenation compete favourably
with the most efficient diphosphine with five- to seven-membered chelate rings previously
reported in the literature.1 Moreover, the Rh-catalyst containing ligand 4 is one of the few
examples of six-membered chelate rings that have led to enantioselectivities of above 90%
(Skewphos,10 Josiphos,2g xylophos5).
3. Conclusion
A new family of C1 diphosphine ligands has been easily prepared in a few steps from the
readily available
D
-glucose. Initial studies directed towards their use as chiral ligands in the

M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
4705
asymmetric hydrogenation of a,b-unsaturated carboxylic acid derivatives revealed high enan-
tioselectivities (up to 98%) under very mild reaction conditions.
The configuration of carbon C-5 strongly influences the enantioselectivity. The best results are
obtained using ligand 4 with (R)-configuration in the C-5. The results also show that a methyl
substituent in the carbon C-5 significantly increases the activity.
A thorough investigation of the Rh-complexes by NMR spectroscopy and theoretical calcula-
tions is currently in progress in order to determine the structure of the catalyst precursors and
explain the different enantioselective efficiencies of the catalysts. The results will be reported in
due course.†
4. Experimental
4.1. General procedures
All syntheses were performed by standard Schlenk techniques under a nitrogen or argon
atmosphere. Compounds 6-deoxy-1,2-O-isopropylidene-a-
D
-allofuranose 6 and 6-deoxy-1,2-O-
isopropylidene-b-
L
-talofuranose 77 were prepared by previously described methods. Solvents
were purified by standard procedures. All other reagents were used as commercially available.
Elemental analyses were performed on a Carlo Erba EA-1108 instrument. H, 13C{1H} and
1
31P{1H} NMR spectra were recorded on a Variant Gemini 400 MHz or 500 MHz spectrometer.
Chemical shifts are relative to SiMe4 (1H and 13C) as internal standard or H3PO4 (31P) as
external standard. All assignments in NMR spectra were determined by means of COSY, 31P–1H
and 13C–1H correlation experiments. EI mass spectra were obtained on a HP 5989 A spectrom-
eter. Gas chromatographic analyses were run on a Hewlett–Packard HP 5890A instrument
(fused silica capillary column 25 m×0.25 mm Permabond
L
-Chirasil-Val) equipped with a
Hewlett–Packard HP 3396 series II integrator. Optical rotations were measured at 25°C on a
Perkin–Elmer 241 MC polarimeter. Hydrogenation reactions were performed in a previously
described hydrogen–vacuum line.11
4.2. 6-Deoxy-1,2-O-isopropylidene-3,5-di-O-trifluoromethanesulfonyl-h-
D
-allofuranose 8
Anhydrous pyridine was added (0.8 ml, 10 mmol) to a solution of 6-deoxy-1,2-O-isopropyli-
dene-a- -allofuranose 6 (0.75 g, 3.7 mmol) in dichloromethane (20 ml). After 10 minutes,
D
trifluoromethanesulfonic anhydride (1.5 ml, 8.9 mmol) was added dropwise at −20°C and the
mixture was allowed to react at room temperature for 20 minutes, after which the solvent was
evaporated. The residue was purified by flash chromatography on a small column of neutral
1
silica (hexane:ethyl acetate, 1:1) to produce the triflate 0.97 g (56%) as a colourless liquid. H
3
NMR: l: 1.21 (s, 3H, CH3), 1.40 (d, 3H, H-6, J6–5=7.1 Hz), 1.47 (s, 3H, CH3), 4.17 (dd, 1H,
3
3
3
3
H-4, J3–4=7.8 Hz, J4–5=1.8 Hz), 4.67 (dd, 1H, H-2, J2–1=3.6 Hz, J2–3=5.1 Hz), 4.82 (dd,
1H, H-3, J3–2=5.1 Hz, J3–4=7.8 Hz), 5.20 (m, 1H, H-5), 5.73 (d, 1H, H-1, J1–2=3.6 Hz). 13C
3
3
3
NMR: l: 16.0 (C-6), 26.6 (CH3), 77.2 (C-2), 78.5 (C-4), 79.9 (C-3), 82.6 (C-5), 103.8 (C-1), 114.6
1
1
(CMe2), 118.2 (q, CF3, JC–F=316.2 Hz), 118.6 (q, CF3, JC–F=314.4 Hz).
† Preliminary NMR results indicate that there is an equilibrium between two different conformers when ligand 3 is
coordinated to rhodium. For ligand 4 only one conformer is observed. Thus, the absolute configuration of C-5 seems
to influence the stability of the different possible Rh-conformers and therefore the enantioselectivity of the process.

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M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
4.3. 6-Deoxy-1,2-O-isopropylidene-3,5-di-O-trifluoromethanesulfonyl-i-
L
-talofuranose 9
Treatment of 6-deoxy-1,2-O-isopropylidene-b-
L
-talofuranose 7 (0.61 g, 3.0 mmol) with tri-
fluoromethanesulfonic anhydride as described for compound 8 afforded ditriflate 9, which was
purified by flash chromatography (eluent: hexane:ethyl acetate, 1:1). Yield: 0.77 g (55%) of a
1
3
colourless liquid. H NMR: l: 1.41 (s, 3H, CH3), 1.56 (s, 3H, CH3), 1.62 (d, 3H, H-6, J6–5=6.7
3
Hz), 4.11 (m, 1H, H-4), 4.81 (m, 2H, H-2, H-3), 5.12 (m, 1H, H-5), 5.89 (d, 1H, H-1, J1–2=3.6
Hz). 13C NMR: l: 17.1 (C-6), 26.4 (CH3), 26.7 (CH3), 77.1 (C-2), 78.3 (C-4), 80.5 (C-3), 82.3
1
1
(C-5), 103.9 (C-1), 114.7 (CMe2), 118.5 (q, CF3, JC–F=319.0 Hz), 118.7 (q, CF3, JC–F=321.1
Hz).
4.4. 3,5,6-Trideoxy-3,5-bis(diphenylphosphine)-1,2-O-isopropylidene-i- -idofuranose 3
L
Potassium diphenylphosphide (4.2 ml, 2.1 mmol) was slowly added at −78°C to a solution of
ditriflate 8 (0.94 g, 2 mmol) in THF (10 ml). The mixture was allowed to react at room
temperature for 30 minutes, after which the solvent was evaporated. The residue was purified by
column chromatography (solvent: toluene) under argon to give the diphosphine 0.74 g (69%) as
a white solid. 31P NMR, l: −25.7 (s, 1P), 2.0 (s, 1P). 1H NMR: l: 0.91 (ddd, 3H, H-6, 3J6–5=6.7
Hz, J6–P=9.3 Hz, J6–P=2.7 Hz), 1.14 (s, 3H, CH3), 1.38 (s, 3H, CH3), 2.89 (d, 1H, H-3,
3
3J3–4=4.4 Hz), 3.39 (m, 1H, H-5), 4.09 (m, 1H, H-4), 4.56 (dd, 1H, H-2, J2–1=3.6, J2–P=6.5
3
Hz), 4.65 (d, 1H, H-1, J1–2=3.6 Hz), 7.32 (m, 12H, CHꢀ), 7.61 (m, 8H, CHꢀ). 13C NMR: l:
12.9 (t, C-6, JC–P=4.9 Hz), 26.1 (CH3), 26.3 (CH3), 31.7 (t, C-5, JC–P=18.9 Hz), 44.6 (dd, C-3,
J
C–P=23.1 Hz, JC–P=6.1 Hz), 81.5 (t, C-4, JC–P=10.9), 83.4 (C-2), 104.3 (C-1), 110.3 (CMe2),
127.5, 127.9, 128.2, 128.4, 128.8, 129.2, 132.5, 132.8, 133.0, 134.2, 134.5, 134.9, 135.2 (CHꢀ). MS
+
’
(70 eV, EI): m/z: 541 [M ]; C33H34O3P2 (540.57) calcd: C, 73.32; H, 6.34; found: C, 73.42; H,
6.39.
4.5. 3,5,6-Trideoxy-3,5-bis(diphenylphosphine)-1,2-O-isopropylidene-h- -glucofuranose 4
D
Treatment of ditriflate 9 (0.7 g, 1.5 mmol) with potassium diphenylphosphide (3.3 ml, 1.65
mmol) as described for compound 3 afforded diphosphine 4, which was purified by flash
chromatography (eluent: toluene). Yield: 0.56 g (70%) of a white solid. 31P NMR, l: −24.3 (d,
1P, 4JP–P=34.7 Hz), −2.0 (d, 1P, 4JP–P=34.7 Hz). 1H NMR: l: 1.01 (t, 3H, H-6, J=7.4 Hz), 1.14
(s, 3H, CH3), 1.25 (s, 3H, CH3), 3.05 (m, 1H, H-3), 3.31 (m, 1H, H-5), 4.11 (m, 1H, H-4), 4.43
3
(dd, 1H, H-2, J2–1=3.0, J2–P=4.6 Hz), 4.53 (m, 1H, H-1), 7.2–7.6 (m, 20H, CHꢀ). 13C NMR:
l: 13.8 (C-6), 26.3 (CH3), 26.5 (CH3), 31.8 (dd, C-5, JC–P=15.1 Hz, JC–P=20.5 Hz), 45.1 (dd,
C-3, JC–P=22.2 Hz, JC–P=7.5 Hz), 81.4 (dd, C-4, JC–P=18.1, JC–P=10.9 Hz), 84.0 (C-2), 103.9
(C-1), 114.0 (CMe2), 127.9, 128.4, 128.8, 129.1, 129.4, 129.6, 132.4, 132.9, 133.4, 133.9, 134.2,
+
’
134.6, 135.0 (CHꢀ). MS (70 eV, EI): m/z: 541 [M ]; C33H34O3P2 (540.57) calcd: C, 73.32; H,
6.34; found: C, 73.39; H, 6.26.
4.6. Hydrogenation of prochiral olefins
In a typical run, a Schlenk was filled with a methanol solution (6 ml) of substrate (1 mmol)
and catalyst precursor (8.38 mg, 0.01 mmol). It was then purged three times with H2 and
vacuum. The reaction mixture was then shaken under H2 (1 atm) at 293 K. After the desired

M. Die´guez et al. / Tetrahedron: Asymmetry 11 (2000) 4701–4708
4707
reaction time, the solvent was removed. The following procedures were used to isolate the
hydrogenation products:
For 2-methylsuccinic acid and N-acetylphenylalanine, the residue was dissolved in 0.5 M
NaOH and separated from the insoluble catalyst by filtration. The filtrate was acidified with
diluted HCl, extracted with ether and washed with water. The ether phase was dried over
1
sodium sulfate and evaporated to dryness. The extent of the conversion was measured by H
NMR. The enantiomeric excesses were determined by polarimetry.12
For N-acetylalanine methyl ester and N-acetylphenylalanine methyl ester the residue was
dissolved in CH2Cl2 and filtered over a plug of silica to remove the catalyst. Conversion and
enantiomeric excesses were determined by gas chromatography.
Acknowledgements
We thank the Spanish Ministerio de Educacio´n y Cultura and the Generalitat de Catalunya
(CIRIT) for their financial support (PB97-0407-CO5-01).
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