Synthesis of a rigid and optically active poly(BINAP) and its application in asymmetric catalysis

Article abstract of DOI:10.1016/S0040-4039(00)00041-1

The first rigid and sterically regular chiral poly[2,2'- bis(diphenylphosphino)-1,1'-binaphthyl][poly(BINAP)] is synthesized. The application of this polymer ligand in asymmetric hydrogenations is examined. High enantioselectivity for the asymmetric hydro

Full text of DOI:10.1016/S0040-4039(00)00041-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1681–1685
Synthesis of a rigid and optically active poly(BINAP) and its
application in asymmetric catalysis
∗
Hong-Bin Yu, Qiao-Sheng Hu and Lin Pu
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, USA
Received 23 November 1999; accepted 3 January 2000
Abstract
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The first rigid and sterically regular chiral poly[2,2 -bis(diphenylphosphino)-1,1 -binaphthyl] [poly(BINAP)] is
synthesized. The application of this polymer ligand in asymmetric hydrogenations is examined. High enantioselec-
tivity for the asymmetric hydrogenation of ketones is achieved. The catalytic properties of the poly(BINAP)-based
catalysts are very similar to those of the BINAP catalysts. This study further demonstrates that rigid and sterically
regular polymer structures are able to preserve the catalytic properties of monomer catalysts. © 2000 Elsevier
Science Ltd. All rights reserved.
Application of chiral phosphine ligands in transition metal-catalyzed asymmetric hydrogenations has
been studied extensively in the past two decades.1–4 This activity has led to the discovery of a number
of highly enantioselective chiral phosphine-based catalysts which have been used in the pharmaceutical
industry for the synthesis of chiral drug molecules. Among the successful asymmetric hydrogenation
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0
catalysts are those made of optically active 2,2 -bis(diphenylphosphino)-1,1 -binaphthyl (BINAP) (R)-
or (S)-1.
1
–3
The BINAP-based transition metal catalysts are found to catalyze the hydrogenation of
compounds such as enamides and ketones with good enantioselectivity.
In our laboratory, we are interested in developing polymer-based chiral ligands for asymmetric
catalysis because using polymers allows the easy recovery and reuse of the often quite expensive chiral
5
ligands. We have recently demonstrated that incorporation of highly enantioselective monomer ligands
into rigid and sterically regular polymer backbones can generate highly enantioselective polymeric
6
,7
catalysts. Both the steric and electronic environments of the monomer catalysts can be maintained
in the rigid and sterically regular polymers. For example, we have converted a monomeric binaphthyl
compound (R)-2 into a polymer (R)-3 and have found that both (R)-2 and (R)-3 are excellent catalysts
∗
Corresponding author. E-mail: lp6n@virginia.edu (L. Pu)
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040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00041-1
tetl 16321

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for the asymmetric reactions of organozincs with aldehydes. We have also applied the same strategy to
construct an enantioselective polymeric catalyst based on the BINAP ligand. Herein, our synthesis of the
novel rigid and optically active poly(BINAP) and its application in asymmetric catalysis are reported.
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We have carried out the Suzuki coupling of a chiral binaphthyl monomer (R)-4 with 5 followed by
reduction of the phosphinoxy groups with trichlorosilane11 to prepare poly(BINAP) (R)-6 (Scheme 1).
This polymer is a yellow solid and is soluble in common organic solvents such as methylene chloride,
THF, chloroform and toluene. Gel permeation chromatography analysis shows that its molecular weight
is Mw=5800 and Mn=4300 (PDI=1.35) relative to polystyrene standards. The specific optical rotation of
12
3
1
(
R)-6 is [α] =36.6 (c=0.12, THF). The P NMR spectrum of the polymer shows a predominate singlet
D
1
at −14.8 ppm, identical to the phosphine signal of (R)-1 (−14.8 ppm). The H NMR spectrum of this
polymer is also well resolved, indicating a well-defined structure.
Scheme 1. Synthesis of a rigid and optically active poly(BINAP)
We have used (R)-6 to prepare a polymeric rhodium complex for the asymmetric hydrogenation of
4
,13
dehydroamino acid derivatives (Scheme 2).
The catalyst was obtained in situ by treatment of (R)-6
with Rh(COD) BF (COD=1,5-cyclooctadiene) in a 1.1:1 ratio in THF. The resulting catalyst suspension
2
4
was tested for the hydrogenation of dehydroamino acid derivatives at room temperature. The reactions
were conducted under 30 psi of hydrogen and were stopped until there was no further hydrogen uptake.
The experimental results are summarized in Table 1. In the presence of this poly(BINAP) catalyst, up to
7
5% ee was observed for the hydrogenation of Z-methyl α-(benzamido) cinnamate (entry 1). This result
is almost the same as the use of the rhodium complex made from the monomeric BINAP ligand (R)-1
entry 2). This again demonstrates that the catalytic properties of the monomer ligand are preserved in
the rigid polymer. The polymeric complex can be easily recovered by simple filtration under nitrogen
(

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after the reaction. The recovered polymer catalyst exhibited the same enantioselectivity as the original
polymer catalyst (entry 3).
Scheme 2. Asymmetric hydrogenation of dehydroamino acid derivatives by the poly(BINAP)–Rh catalyst
Table 1
a
Hydrogenation of dehydroamino acid using poly(BINAP) (R)-6 as the chiral ligand
We have also used poly(BINAP) (R)-6 to prepare a chiral polymeric ruthenium complex for the
asymmetric hydrogenation of methyl aryl ketones (Scheme 3). Recently, Noyori and co-workers reported
an air-stable BINAP/diamine–Ru complex which showed high selectivity in the hydrogenation of
1
4
ketones. Following the same procedure, a polymeric phosphine–diamine–Ru complex was prepared
by treatment of (R)-6 with [RuCl (C H )] in DMF followed by addition of (R,R)-DPEN [(R,R)-
2
6
6 2
31
DPEN=(R,R)-1,2-diphenylethylenediamine]. The resulting catalyst showed a predominate P NMR
signal at 46.3 ppm, very close to the corresponding monomeric BINAP complex (47.4 ppm).1 This
poly(BINAP) complex is air-stable which allows both the reaction work-up and the catalyst recovery to
be conducted in air. As shown in Table 2, this polymeric catalyst exhibits very good enantioselectivity
for the hydrogenation of certain ketones. The hydrogen pressure did not influence the stereoselectivity
but higher hydrogen pressure accelerated the reaction. The catalyst was recovered by precipitation with
methanol after work-up. The recovered polymer catalyst showed similar reactivity and selectivity to the
original catalyst. The catalytic properties of this poly(BINAP)-based catalyst are also very similar to
those of the monomer catalyst made from (R)-1.
4b
Scheme 3. Asymmetric hydrogenation of ketones by a poly(BINAP)–Ru catalyst
In summary, a novel rigid and sterically regular chiral polymeric phosphine ligand has been synthesi-
zed. The catalysts based on this poly(BINAP) ligand have been used in asymmetric catalysis and have

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Table 2
Asymmetric hydrogenation of ketones catalyzed by the poly(BINAP)–Ru complexa
exhibited very good enantioselectivity.15 The study of these polymer catalysts versus the monomeric
BINAP catalysts demonstrates again that the catalytic properties of an enantioselective monomer catalyst
can be preserved in a rigid and sterically regular polymer backbone. The use of the polymer-based
materials allows the easy recovery and reuse of these catalysts.
Acknowledgements
This research was supported by the Department of Chemistry at the University of Virginia. We also
thank the partial support of this research by the US National Science Foundation (DMR-9529805).
References
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4
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J. Chem. Soc., Perkin Trans. 1989, 1571. (e) Noyori, R.; Ikeda, T.; Ohkuma, T.; Widhalm, M.; Kitamura, M.; Takaya, H.;
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19, 1799 and references cited therein.
5
. Itsuno, S. In Polymeric Materials Encyclopedia; Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press:
Boca Raton, FL, 1996; Vol. 10, p. 8078.
6
7
. Pu, L. Chem. Eur. J. 1999, 5, 2227.
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4, 7940.
8
9
. (a) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (b) Giroux, A.; Han, Y.; Prasit, P. Tetrahedron Lett. 1997, 38, 3841.
0
0
. Preparation and characterization of (R)-4: To a mixture of (R)-6,6 -trifluoromethylsulfonyloxy-2,2 -bis(diphenylphosphin-
0
oxy)-1,1 -binaphthyl (1.650 g, 1.740 mmol, preparation to be published) and bis(pinacolato)diboron (1.770 g, 6.960 mmol)
in dioxane (15 mL) was added Pd(dppf)Cl2·CH
2 2
Cl (0.071 g, 0.087 mmol), dppf (0.048 g, 0.087 mmol) and KOAc (1.360
g, 13.920 mmol). The mixture was stirred at 85°C for 40 h and then filtered at room temperature. The solid was washed
1
with H
CDCl
2
O (4×10 mL), acetone (2 mL) and EtOAc (2×5 mL) to give (R)-4 as a pale grey solid 1.200 g (76%). H NMR
(
3
, 300 MHz) δ 1.38 (s, 24H, 8 CH ), 6.74 (d, J=8.7 Hz, 2H, Ar-H), 7.14–7.45 (m, 20H, Ar-H), 7.66, 7.70 (dd, J=8.1
3

1
685
Hz, J=12.3 Hz, 4H, Ar-H), 7.89 (d, J=8.1 Hz, 2H, Ar-H), 8.34 (s, 2H, Ar-H). 13C NMR (CDCl
, 75 MHz) δ 24.94, 25.48,
4.07, 126.14, 127.97, 128.13, 128.30, 129.75, 130.73, 131.14, 131.37, 132.01, 132.13, 132.60, 132.73,. 133.44, 133.57,
3
8
1
31
34.85, 134.99, 136.15, 142.57. P NMR (CDCl
3
, 121 MHz, 85% phosphoric acid as the external standard) δ 28.9 (s). MS
: C, 74.19; H, 6.00%. Found: C, 74.18; H, 6.76%.
+
(CI) m/e 906 (M , 100). Anal. calcd for C56
H
54
O
6
B
2
P
2
1
1
1
0. Rehahn, M.; Schlüter, A.-D.; Feast, W. J. Synthesis 1988, 386.
1. Takaya, H.; Akutagawa, S.; Noyori, R. Org. Synth. 1988, 67, 20.
2. Preparation of polymer (R)-6: (1) Under nitrogen, to a solution of (R)-4 (0.920 g, 1.020 mmol) and 2,5-dihexyl-1,4-
dibromobenzene (0.404 g, 1.000 mmol) in 10 mL THF was added Pd(dppf)Cl2·CH
CO (8 mL). The mixture was refluxed for 51 h followed by adding of 4-butyl-bromobenzene (100 µL). The resulting
mixture was refluxed for another 18 h. The organic layer of the reaction mixture was separated and washed with 1N HCl
2×5 mL) and brine (2×5 mL). After concentration, methanol (20 mL) was added to precipitate out the polymer. This
precipitation procedure was repeated three times to give a dark brown solid in 80.5% yield (0.720 g). (2) To a mixture of
the above polymer (0.600 g, 0.670 mmol), Et N (7 mL) and o-xylene (50 mL) was added HSiCl (5 mL). After the mixture
was heated at reflux for 48 h under nitrogen, NaOH (30%, 30 mL) was added carefully at room temperature which was then
stirred for 10 min. The resulting mixture was extracted with CH Cl
2 2
Cl (24 mg, 0.030 mmol) and 2 M
K
2
3
(
3
3
2
2
(3×50 mL). After filtration, methanol was then added
under nitrogen to precipitate out polymer (R)-6 (repeated three times) as a yellow solid in 93% yield (0.540 g).
3. Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L. J. Am. Chem. Soc. 1993, 115, 10125.
4. (a) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 2675. (b) Doucet, H.;
Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem.,
Int. Ed. 1998, 37, 1703.
1
1
15. A chiral polymer-based BINAP catalyst was recently reported to show high enantioselectivity in asymmetric hydrogenation:
Fan, Q.; Ren, C.; Yeung, C.; Hu, W.; Chan, A. S. C. J. Am. Chem. Soc. 1999, 121, 7407.