New chiral monodentate phospholane ligands by highly stereoselective hydrophosphination

Article abstract of DOI:10.1016/j.tetasy.2006.06.047

New chiral phospholanes 6 were prepared in both enantiomeric forms starting from l- and d-tartaric acid. The key step in the synthetic sequence is the double hydrophosphination of unsaturated chiral bis(lactone) 9 by NaPhPH(BH₃). This method wa

Full text of DOI:10.1016/j.tetasy.2006.06.047

Tetrahedron: Asymmetry 17 (2006) 2082–2087
New chiral monodentate phospholane ligands by
highly stereoselective hydrophosphination
Vitaliy Bilenko,a,b Anke Spannenberg,a Wolfgang Baumann,a
Igor Komarovb and Armin Bo¨rnera,c,
*
aLeibniz-Institut fu¨r Katalyse an der Universita¨t Rostock e.V., A.-Einstein-Str. 29a, 18059 Rostock, Germany
bKyiv Taras Shevchenko University, Volodymyrska-Str. 64, 01033 Kyiv, Ukraine
cInstitut fu¨r Chemie der Universita¨t Rostock, A.-Einstein-Str. 3a, 18059 Rostock, Germany
Received 8 June 2006; accepted 12 June 2006
Abstract—New chiral phospholanes 6 were prepared in both enantiomeric forms starting from L- and D-tartaric acid. The key step in the
synthetic sequence is the double hydrophosphination of unsaturated chiral bis(lactone) 9 by NaPhPH(BH3). This method was used for
the first time in the formation of chiral phospholanes. The structure of phospholane 6a was confirmed by X-ray crystallography.
r-Donor properties of the phospholanes were estimated by measurement of 1J(31P–77Se) coupling constants in the corresponding phos-
phine selenides. The new phospholanes were tested as ligands in the Rh-catalyzed enantioselective hydrogenation of functionalized
standard olefins (65–92% ee).
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
are in general more easily available than the corresponding
bidentate phosphines and therefore, also chiral monoden-
tate phospholanes, such as 1,6 2,4c,7 3,8 4,9 and 5,10 have
also been prepared. Some of them form active and enantio-
selective Rh hydrogenation catalysts.11,12
Catalytic enantioselective hydrogenation is of great impor-
tance both for science and industry, as it opens up simple
venues to a variety of optically active chemical com-
O
O
Ph
P
Ph
Ph
P
P
P
P
H
5
OH
Ph
Ph
3
Ph
2
1
4
pounds.1 In particular, Rh-, Ru-, and Ir-complexes bearing
chiral trivalent phosphorus compounds as ancillary ligands
play a dominant role. One of the most developed
approaches employs heterocyclic phosphines, in particular
phospholanes.2 Prominent examples are commercially
available bis(phospholanes) such as DuPHOS,3 RoPHOS,4
and ligands of the catASium M series.5 Monophosphines
This prompted us to search for a new chiral phospholane
motif, involving the development of a simple approach to
both enantiomers of the ligands. Therefore, we decided to
use inexpensive tartaric acid derivatives available in both
enantiomeric forms as starting materials. A literature
search revealed that the presence of bulky substituents at
1-, 2-, and 5-position of the phospholane ring seems to
be desirable in order to achieve enhanced enantioselectivity
in the hydrogenation reaction. Following these criteria, the
enantiomeric phospholanes 6a and 6b were prepared.
*
Corresponding author. E-mail: armin.boerner@ifok-rostock.de
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.06.047

V. Bilenko et al. / Tetrahedron: Asymmetry 17 (2006) 2082–2087
2083
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P
P
P
P
(all-S)-6a
(all-R)-6b
(all-R)-6a
(all-S)-6b
2. Results and discussion
no trace of the other diastereomer (possessing a trans-fused
five-membered ring) being observed. Obviously, the forma-
tion of trans-fused five-membered rings is energetically less
favored. The geometry of the compounds was confirmed by
H,H-COSY and NOESY experiments with compound 10a
(Fig. 1), and inspection of the X-ray crystal structure ana-
lyses of 6a and 10b (Fig. 2). Compounds 10a and 10b are
stable solids, which can be stored on air for a long time.
Free phospholanes 6a and 6b were generated by the reac-
tion of phosphine–boranes 10a and 10b with 1,4-diazabi-
cyclo[2,2,2]octane (DABCO).
2.1. Synthesis
Our synthesis of 6a and 6b commenced from dimethyl 2,3-
O-isopropylidenetartrate 7, commercially available in both
enantiomeric forms (Scheme 1, only the syntheses of all-R
enantiomers are shown; the opposite enantiomers were pre-
pared following the same methodology). The key transfor-
mation is the double hydrophosphination reaction between
phenylphosphine–borane adduct and bis(lactone) 9. The
latter was prepared according to a protocol by Le Corre
et al.13 Diester 8 representing the direct precursor of 9
was obtained following a known procedure.14 The pub-
lished procedure for the formation of 9 implies the isola-
tion of the Z,Z-isomer of 8, but we found that the use of
the diastereomeric mixture of 8 for the lactonization also
led to pure bis(lactone) 9 in 26% overall yield from 7.
It was of interest to estimate the r-donor properties of the
novel phospholanes.17 For this purpose, measurement of
the 31P–77Se NMR coupling constants in corresponding
phosphine selenides was suggested.18 The selenides were
prepared in the NMR tube by treating phospholanes 6a
and 6b with an excess of elemental selenium in CDCl3 at
room temperature. In general, the higher the magnitude
of the coupling constant, the lower the basicity of the phos-
The hydrophosphination of a,b-unsaturated carboxylic
acid derivatives or chalcones by borane complexes of sec-
ondary phosphines is known.15 It has been used by Le
Corre et al. for the synthesis of a related bis(diphenylphos-
phine) substituted chelating ligand.13 However, the analo-
gous reaction of monophosphine–borane complexes is
less studied.16 In this case, hydrophosphination is mostly
accompanied by hydroboration and decomposition of the
rather unstable phosphine–boranes.16b,c Therefore, this
step required extensive testing in order to identify the opti-
mal conditions. The use of NaH for the generation of the
phosphide anion, with THF as solvent, while carrying
out the reaction at 0 °C gave highest yields of the phospho-
lane–borane complexes 10a and 10b. These compounds
were then purified by column chromatography. It is note-
worthy that only a single diastereomer was isolated, with
1
phine. The magnitude of J(31P–77Se) is dependent upon
the nature of the organic groups bound to phosphorus.
Electron-withdrawing groups increase the coupling con-
stant, whereas electron-donating groups decrease it. The
presence of bulky substituents at the phosphorus atom like-
wise results in a decrease in the s-character of the electron
lone pair as a result of the widening of the relevant bond
angles.18
For the selenides obtained from 6a and 6b, larger
1J(31P–77Se) were observed than for the selenides prepared
from phospholanes 1 and 2 (Table 1).19 It can be concluded
that an increase in the number of fused rings at the phos-
pholane core causes a decrease in the r-donor properties
of the phosphine. However, this might also vary due to
O
1. DIBAH
COOMe
MsOH
O
Ph P
2.
3
COOMe
COOMe
O
O
O
COOMe
DME/H2O
O
O
O
COOMe
O
7
8
9
O
O
O
O
O
O
O
DABCO
R-PH(BH3)Na
P
P
a: R=Ph
b: R=cyclohexyl
BH3
R
R
10a,b
6a,b
Scheme 1. Synthesis of lactone-fused phospholane–borane adducts and free phospholanes 6a and 6b.

2084
V. Bilenko et al. / Tetrahedron: Asymmetry 17 (2006) 2082–2087
H7a
H4a
O
O
O
O
P
H 3a
H 7b
BH3
ppm
4.5
ppm
4.5
5.5
5.0
4.0
3.5
5.5
5.0
4.0
3.5
7b
7a
7a
7b
3a
4a
3a
4a
3.5
4.0
4.5
5.0
5.5
3.5
4.0
4.5
5.0
5.5
1H NOESY
1H COSY
Figure 1. 2D NMR spectra of compound 10a (measured in CDCl3). Correlations between signals of 4a-H and 7a-H, 3a-H and 7b-H confirm the transoid
arrangement of the five-membered rings.
(a)
(b)
Figure 2. Molecular structures of 6a (a) and 10b (b), from X-ray crystallography with only one of the two molecules in the asymmetric unit being depicted.
The thermal ellipsoids correspond to 30% probability.
the nature of the substituent at the phosphorus atom (com-
pare Ph and Cy in ligands 6a and 6b). The obtained data
correlate well with the behavior of 10a and 10b during
the deprotection with DABCO. Under the same conditions
10a reacted within 30 min, whereas 10b required 3 h for
complete deboronation. This provides clear evidence that
6b has a higher basicity than 6a.
2.2. Asymmetric hydrogenation
Phospholanes 6a and 6b were tested as monodentate
ligands in the Rh-catalyzed asymmetric hydrogenation of
functionalized standard olefins. The required precatalysts
were prepared in situ from the corresponding ligands and
[Rh(COD)2]BF4 (2:1 molar ratio). The hydrogenations

V. Bilenko et al. / Tetrahedron: Asymmetry 17 (2006) 2082–2087
2085
Table 1. 31P–77Se coupling constants (Hz) for phospholane selenides
a LECO CHNS-932. Mass spectra were recorded on an
AMD 402 spectrometer. Below, the syntheses of only one
enantiomer from each enantiomeric pair are given.
Phospholane
1J(31P–77Se) (Hz)
1
2
720 (Ref. 19)
749 (Ref. 19)
783
For the synthesis of 8 we followed the method described by
Krief et al.,14 except that the product was used in the next
step without further purification.
6a
6b
760
3.2. (5S)-5-[(2S)-5-Oxo-2,5-dihydro-2-furanyl]-2,5-dihydro-
2-furanone 9
were carried out at 1 bar of hydrogen pressure at 25 °C.
Some typical results are summarized in Table 2.
The crude mixture of 8 (26 g), obtained by the reported
method of Krief et al.,14 was refluxed in DME (160 ml)
with H2O (16 ml) and methanesulfonic acid (0.15 ml).
The mixture was evaporated to half and kept in a refriger-
ator. Then it was filtered and the precipitate was washed
with hexane and ethanol to give 4.7 g of 9 (26% yield based
It can be seen from the data in Table 2 that the phospho-
lane ligands 6a and 6b form active and quite selective
hydrogenation catalysts. In general, the enantioselectivity
of the hydrogenation is comparable with those previously
reported with Rh catalysts derived from ligands 1,6 3,8
and 5.10 It is noteworthy that high enantioselectivity can
be found with benzyl Z-b-acetylamidobutenoate as the
substrate in CF3CH2OH as a solvent.
on dimethyl 3,4-O-isopropylidenetartrate). Mp = 184 °C,
25
Ref. 14. 183–184 °C. ½aꢀD ¼ ꢁ1:8 (c 1, ethyl acetate), Ref.
25
14. ½aꢀD ¼ ꢁ1:8 (c 1, ethyl acetate).
3. Experimental
3.1. General
3.3. BH3 complex of (3aR,4aR,7aR,7bR)-4-phenylper-
hydrofuro[20,30:4,5]phospholo[3,2-b]furan-2,6-dione 10a
To a solution of phenylphosphine–BH3 complex (1.5 g,
12 mmol) in THF (80 ml) was added NaH (580 mg, 60%
in oil, 14.5 mmol) at 0 °C. The mixture was stirred for
20 min at 0 °C and 3 h at room temperature. Then the solu-
tion was added to a solution of 9 (2 g, 12 mmol) in THF
(800 ml) at 0 °C. After 15 min the cooling bath was re-
moved and the mixture stirred for 1.5 h at room tempera-
ture, then HOAc (1.3 ml) was added. The solution was
evaporated under vacuum and the product purified by
chromatography to give 0.5 g (15% yield) of compound
All reagents unless otherwise mentioned were purchased
from commercial sources and used without additional puri-
fication. The solvents were dried and freshly distilled under
argon before use. All reactions involving phosphines were
performed under an argon atmosphere by using standard
Schlenk techniques. Thin-layer chromatography was per-
formed on precoated TLC plates (silica gel). Melting points
are corrected. The optical rotations were measured on a
1
‘Gyromat-HP’ instrument. H, 13C, and 31P NMR spectra
were recorded at the following frequencies: 250.13 MHz
(1H), 75.48 MHz (13C), 121.49 MHz (31P). 2D NMR experi-
ments were recorded at 400.13 MHz. Chemical shifts of 1H
and 13C NMR spectra are reported in parts per million
downfield from TMS as an internal standard. Chemical
shifts of 31P NMR spectra are referred to H3PO4 as an
external standard. Elemental analyses were performed with
10a. Anal. Calcd for C14H16BO4P: C, 57.97; H, 5.56.
25
Found: C, 56.24; H, 5.42. ½aꢀD ¼ þ74:2 (c 3.0, CHCl3).
MS: m/z = 290 (M+, C14H16BO4P), 276 (M+ of free phos-
phine), 235, 192, 164, 149, 133, 108, 91, 57. 1H NMR
(CDCl3) d: 0.80 (br, 3H, BH3), 2.04 (dd, 1H), 2.88 (ddd,
1H), 3.05 (ddd, 1H), 3.24 (ddd, 1H), 3.42 (m, 1H), 3.72
3
3
(m, 1H), 5.34 (dd, 1H, JH,P = 19.1 Hz, JH,H = 5.7 Hz),
Table 2. Rh-catalyzed enantioselective hydrogenation of selected olefins using phospholanes 6a,b as ligands
[Rh(COD)2]BF4
+2 eq. Ligand,
H2 (1 bar), rt.
R2
R2
R1
NHAc
R1
NHAc
Ligand
R1
R2
Molar ratio Rh/substrate
Solvent
Timea (min)
ee (%)
(all-S)-6a
(all-S)-6b
(all-S)-6a
(all-S)-6a
(all-S)-6b
(all-S)-6b
(all-S)-6b
Ph
Ph
H
H
COOMe
COOMe
COOMe
COOMe
COOMe
Me
1:50
1:50
1:100
1:50
1:50
1:50
1:50
MeOH
THF
MeOH
CF3CH2OH
THF
230
55
35
17
6
76b (S)
65b (S)
68c (S)
81c (S)
78c (S)
82d (S)
92e (S)
H
COOMe
COOBn
CF3CH2OH
CF3CH2OH
200
180
Me
a Determined after full conversion. Analysis of reaction products.
b Determined by GC, 25 m Chirasil-Val, Alltech, 110 °C.
c Determined by GC, 4 m, XE 60 L-Valin-(tert.-butylamide), 85 °C.
d Determined by GC, 50 m, Chiraldex b-PH.
e Determined by HPLC, Chiralcel OJ, Daicel, n-hexane–ethanol/95:5, 1.4 ml/min.

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V. Bilenko et al. / Tetrahedron: Asymmetry 17 (2006) 2082–2087
3
3
5.51 (dd, 1H, JH,P = 17.4 Hz, JH,H = 6.5 Hz), 7.57 (m,
2H), 7.67 (m, 3H). 13C NMR (CDCl3) d: 31.1 (d,
J = 6.5 Hz, CH2), 31.6 (d, J = 2.9 Hz, CH2), 37.3 (d,
J = 32.3 Hz, CH), 39.8 (d, J = 29.3 Hz, CH), 85.9
(d, J = 1.7 Hz, CH), 86.5 (CH), 123.1 (d, J = 45.8 Hz,
CP), 130.3 (d, J = 10.5 Hz, CH), 133.4 (d, J = 9.5 Hz,
CH), 133.9 (d, J = 2.9 Hz, CH), 173.6 (C@O), 173.7
(C@O). 31P NMR (CDCl3) d: 55.7.
26.8 (d, J = 9.6 Hz, CH2), 29.5 (d, J = 12.9 Hz, CH2),
31.9 (d, J = 19.8 Hz, CH2), 34.3 (d, J = 18.1 Hz, CH),
35.0 (d, J = 31 Hz, CH2), 35.7 (d, J = 24.3 Hz, CH), 37.4
(d, J = 14.7 Hz, CH), 88.3 (s, CH), 88.9 (s, CH), 174.0
(d, J = 1.7 Hz, C@O), 174.7 (s, C@O). 31P NMR (C6D6)
d: 13.2 (s). For the corresponding selenide 31P NMR
(CDCl3) d: 73.1.
3.7. X-ray crystallographic study of compounds 6a and 10b
3.4. (3aR,4aR,7aR,7bR)-4-Phenylperhydrofuro-
[20,30:4,5]phospholo[3,2-b]furan-2,6-dione 6a
Data were collected on a STOE-IPDS diffractometer using
graphite-monochromated Mo-Ka radiation. The structures
were solved by direct methods (SHELXS-97) [G. M. Sheld-
rick, SHELXS-97, University of Go¨ttingen, Germany, 1997]
and refined by full-matrix least-squares techniques against
F2 (SHELXL-97). [G. M. Sheldrick, SHELXL-97, University of
Go¨ttingen, Germany, 1997] XP (BRUKER AXS) was used
for graphical representation.
To a solution of 10a (0.5 g, 1.74 mmol) in THF (20 ml) was
added DABCO (0.2 g, 1.78 mmol) and the mixture stirred
at 55 °C for 30 min. The solvent was evaporated and the
phosphine purified on silica gel (CHCl3/MeOH, 20/1) to
give 0.38 g (78% yield) of phospholane 6a. Analytical sam-
ple and crystals suitable for X-ray analysis were obtained
1
by crystallization from benzene. Mp = 164 °C. H NMR
(C6D6) d: 1.42–1.54 (m, 1H), 1.66–1.82 (m, 1H), 2.13–2.4
(m, 3H), 2.57–2.67 (m, 1H), 4.46 (t, J = 6.9 Hz, 1H), 4.77
(t, J = 6.6 Hz, 1H), 7.02–7.08 (m, 5H). 13C NMR (C6D6)
d: 31.0 (CH2), 34.1 (d, J = 34.6 Hz), 37.2 (d, J = 24 Hz,
CH), 38.4 (d, J = 13.2 Hz, CH), 88.4 (d, J = 3.3 Hz, CH),
90.1 (d, J = 4.4 Hz, CH), 128.9 (d, J = 7.2 Hz, CH),
130.4, 131.3 (d, J = 23.2 Hz, CP), 134.4 (d, J = 21.4 Hz,
C@H), 173.3 (C@O), 173.5 (d, J = 4 Hz, C@O). 31P
NMR(C6D6) d: 20.8. For the corresponding selenide 31P
NMR (CDCl3) d: 69.4.
Compound 6a: space group P212121, orthorhombic, a =
˚
11.014(2),
b = 11.543(2),
c = 19.755(4) A,
V =
2511.7(9) A , Z = 8, qcalcd = 1.461 g cmꢁ3, 38,627 reflec-
tions measured, 5775 were independent of symmetry and
5360 were observed (I > 2r(I)), R1 = 0.024, wR2 (all data) =
0.057, 375 parameters.
3
˚
Compound 10b: space group P212121, orthorhombic,
˚
a = 6.943(1),
b = 15.812(3),
c = 31.454(6) A,
V = 3453.3(12) A , Z = 8, qcalcd = 1.323 g cmꢁ3, 43,526
reflections measured, 6072 were independent of symmetry
and 5096 were observed (I > 2r(I)), R1 = 0.036, wR2 (all
data) = 0.086, 431 parameters.
3
˚
3.5. BH3 complex of (3aS,4aS,7aS,7bS)-4-cyclohexylper-
hydrofuro[20,30:4,5]phospholo[3,2-b]furan-2,6-dione 10b
Compound 10b was obtained analogously to 10a in 11%
yield, but after chromatographic purification the product
Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication numbers CCDC 607514 (6a) and 607515
(10b). Copies of the data can be obtained, free of charge,
on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44 1223 336033 or e-mail: deposit@
ccdc.cam.ac.uk].
was stirred in CHCl3–cyclohexane (1:1) mixture until
21
crystallization started. Mp = 101–102 °C. ½aꢀD ¼ ꢁ33:5
1
(c 5, acetone) H NMR (acetone-d6) d: ꢁ0.32 to 1.14 (br
q, J = 95 Hz, 3H, BH3), 1.25–1.62 (m, 5H), 1.7–1.9 (m,
5H), 2.14–2.36 (m, 1H), 2.67–3.17 (m, 4H), 3.4–3.55 (m,
1H), 3.65–3.81 (m, 1H), 5.14–5.35 (m, 2H). 13C NMR (ace-
tone-d6) d: 26.8 (d, J = 15 Hz, CH2), 27.1 (d, J = 5 Hz,
CH2), 28.1 (s, CH2), 29.8 (d, J = 5.6 Hz, CH2), 31.3
(d, J = 3.4 Hz, CH2), 33.3 (d, J = 28.2 Hz, CH), 34.3
(d, J = 32.2 Hz, CH), 37.8 (d, J = 33.9 Hz, CH), 88.1 (d,
J = 6.8 Hz, CH), 88.6 (d, J = 2.8 Hz), 173.6 (d,
J = 2.3 Hz, C@O), 173.7 (d, J = 5.8 Hz, C@O). 31P NMR
(acetone-d6) d: 56.7 (br).
Acknowledgment
We are grateful for the financial support provided by the
Deutsche Forschungsgemeinschaft (Graduiertenkolleg
1213).
3.6. (3aS,4aS,7aS,7bS)-4-Cyclohexylperhydrofuro-
[20,30:4,5]phospholo[3,2-b]furan-2,6-dione 6b
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