Novel atropisomeric aminophosphine ligands with a bridge across the 5,5′-position of biphenyl for Rh(I)-catalyzed asymmetric hydrogenation

Article abstract of DOI:10.1016/j.tetlet.2008.04.140

A new type of atropisomeric bisaminophosphine ligands 2 with a bridge across the 5,5′-position of biphenyl has been developed. The axial chirality of this type of ligands can be retained by macro-ring strain produced from 5,5′-linkage of biphenyl even wit

Full text of DOI:10.1016/j.tetlet.2008.04.140

Tetrahedron Letters 49 (2008) 4106–4109
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Novel atropisomeric aminophosphine ligands with a bridge across the
5,50-position of biphenyl for Rh(I)-catalyzed asymmetric hydrogenation
*
Hao Wei, Yong Jian Zhang, Yijun Dai, Jiaming Zhang, Wanbin Zhang
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
A new type of atropisomeric bisaminophosphine ligands 2 with a bridge across the 5,50-position of biphen-
yl has been developed. The axial chirality of this type of ligands can be retained by macro-ring strain
produced from 5,50-linkage of biphenyl even without 6,60-substituents on biphenyls. The ligand (R)-2a
showed high catalytic activities and enantioselectivities (up to 95.3% ee and quantitative yields) for
Rh(I)-catalyzed asymmetric hydrogenation of a variety of methyl (Z)-2-acetamido-3-arylacrylates.
Ó 2008 Elsevier Ltd. All rights reserved.
Article history:
Received 1 February 2008
Revised 23 April 2008
Accepted 25 April 2008
Available online 29 April 2008
The design and synthesis of chiral phosphine ligands have
played a significant role in the development of transition-metal
catalyzed asymmetric reactions, and have attracted a great deal
of attention from both academia and industry.1 Since Noyori and
co-workers developed the historical axially chiral bisphosphine
ligand BINAP in the early 1980s,2 many bisphosphine ligands sup-
ported by an atropisomeric scaffold have been developed and
applied successfully in various asymmetric catalytic reactions.3 It
was found that modulation of the steric and electronic properties
of atropisomeric scaffold of the ligand could remarkably influence
on their efficiency in asymmetric catalytic reactions. Recently, the
steric design of the biaryl core has been extensively explored such
as BIPHEMP,4 MeO-BIPHEP,4 TunePhos,5 SEGPHOS,6 SYNPHOS,7
and DIFLUORPHOS.8 It was demonstrated that the ligand display-
ing a narrow dihedral angle of biaryl backbone showed better
enantioselectivities in Ru(II)-catalyzed asymmetric hydrogenation
of b-keto esters.9 Quite recently, we developed novel atropisomeric
framework 1 (Fig. 1),10 in which the biphenyl has only two coordi-
nating groups next to the axis, and the axial chirality of the ligand
can be retained by steric hindrance of two bulky coordinating
groups and macro-ring strain produced from 5,50-linkage of biphe-
nyls even without 6,60-substituents. The modulation of the length
of the backbone carbon chain could control the conformational
flexibility of the metal chelate rings formed with atropisomeric
ligands to provide more suitable chiral environment for asymmet-
ric catalysis. In our initial study, we found that the bisoxazolines10a
and bismethylenephosphines10b with atropisomeric framework 1
are provided with stable axial chirality at appropriate temperature,
and they can be effectively applied in Pd(II)-catalyzed asymmetric
Wacker-type cyclization of 2-allylphenols and Rh(I)-catalyzed
asymmetric hydrogenation of dehydroamino acids, respectively.
O
O
H
H
L
L
L
L
H
H
(CH2)n
(CH2)n
O
O
1 (aR)
1 (aS)
L = Coordination Group
Figure 1. Atropisomeric ligands with a bridge across the 5,50-positions of biphenyl.
Over the last three decades, most effort on asymmetric hydro-
genation has been focused on the use of rhodium and ruthenium
catalysts containing chiral phosphine ligands.
Recently, it has been shown that some of the results obtained
with chiral phosphinite,11 phosphite,12 phosphonite13 or phospho-
ramidite14 ligands can match those obtained by using phosphine
ligands. The easily prepared bisaminophosphine ligands were
also found to be effective ligands for the rhodium-catalyzed
asymmetric hydrogenation.15 For example, bisaminophosphines
(BDPAB) (see Fig. 2) with binaphthyl have effectively been applied
in asymmetric hydrogenation of enamides15c and dehydroamino
O
H
H
NHPPh2
NHPPh2
NHPPh2
NHPPh2
(CH2)n
O
2
BDPAB
* Corresponding author. Tel./fax: +86 21 5474 3265.
E-mail address: wanbin@sjtu.edu.cn (W. Zhang).
Figure 2. Bisaminophosphine ligands with axial biaryl backbone.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.140

H. Wei et al. / Tetrahedron Letters 49 (2008) 4106–4109
4107
acid derivatives.15a However, only a limited number of bisamino-
phosphines for asymmetric hydrogenation have been investi-
gated.16 Herein we wish to report novel bisaminophosphine
ligands with our atropisomeric framework 1, 2,20-bis(diph-
enylphosphinoamino)-5,50-(polymethylenedioxy)-1,10-biphenyls 2
(see Fig. 2), and their application in Rh(I)-catalyzed asymmetric
hydrogenation of de- hydroamino acid derivatives.
To evaluate the effectiveness of our atropisomeric bisamino-
phophine ligands 2 in Rh(I)-catalyzed asymmetric hydrogenation,
typically, (Z)-a-acetamidocinnamic acid (8) was used as a model
substrate for the hydrogenation. The hydrogenation reaction was
catalyzed by 1 mol % of the Rh(I)-(R)-2a complex generated in situ
by mixing Rh(COD)2BF4 with bisaminophosphine (R)-2a in metha-
nol at room temperature. It was found that the catalytic efficiency
largely depended on the hydrogen pressure. As shown in Table 1,
the hydrogenation reaction showed higher catalytic activity with
enhancement of the hydrogen pressure (entries 1–4). The enanti-
oselectivity also enhanced with the hydrogen pressure increasing
from 1 to 3 atm (from 88.2% to 89.7% ee, entries 1 and 2). However,
further raising the hydrogen pressure, the enantioselectivity
remarkably decreased (entries 3 and 4). With (R)-2b as a ligand,
which had decamethylenedioxy bridge, the enantioselectivity
decreased to 84.1% ee under 3 atm of hydrogen pressure (entry
5). The reaction medium had also effects on the catalytic activity
and enantioselectivity in the hydrogenation reaction (entries
6–9). The highest enantioselectivity (89.8% ee) was obtained in
THF under 3 atm of hydrogen pressure (entry 7).
Methyl (Z)-a-acetamidocinnamate (10a) was also used as a sub-
strate for the hydrogenation by mixing Rh(COD)2BF4 with bisami-
nophosphine (R)-2a in methanol at room temperature. It was
found that the catalytic efficiency also largely depended on the
hydrogen pressure, the ligand structure and a reaction solvent.
As shown in Table 2, the reaction results have similar tendencies
with hydrogenation of (Z)-a-acetamidocinnamic acid (8). However,
the obviously higher enantioselectivities for the hydrogenations of
methyl (Z)-a-acetamidocinnamate (10a) were observed than those
for the hydrogenations of corresponding acid 8 in the same reac-
tion conditions. The highest enantioselectivity (94.4% ee) for the
hydrogenation of methyl (Z)-a-acetamidocinnamate (10a) was
The optically pure bisaminophosphine ligands 2 were readily
prepared from 3-bromo-4-nitroanisole 317 as illustrated in Scheme
1. Ullman coupling of 3 with activated copper powder furnished
the biphenyl skeleton 4 in 78% yield. Demethylation of 4 with alu-
minum chloride and 1-dodecanethiol afforded compound 5 in 79%
yield. Cyclization of 5 with 1,8-dibromooctane and 1,10-dibromo-
decane led to the key intermediates 6a and 6b in 58% and 55%
yields, respectively. Straightforward reduction of 6a with Pd/C
under 5 atm of hydrogen gave quantitatively biphenyldiamine
7a. Then, we tried optical resolution of biphenyldiamine 7a to
obtain optically pure diamine compound, however, all attempts
in resolution of 7a failed. The reaction of racemic biphenyldiamine
7a with chlorodiphenylphosphine in the presence of triethylamine
furnished racemic bisaminophosphine 2a in 81% yield. The optical
separation of racemic 2a by chiral preparative HPLC using a Daicel
Chiralcel AD-H column afforded, respectively, enantiomerically
pure (>99% ee) (+)-2a and (ꢀ)-2a in 42% yield.18 In the same
way, the optically pure aminophosphine ligand 2b was prepared
from 5,50-decamethylenedioxy-biphenyl-2,20-dinitrate (6b) in 31%
overall yield.19 The sense of axial chirality of 2a was determined
by the major Cotton effects (CE) in the CD spectra. The CD curve
of (–)-2a displayed negative CE at 237.0 nm. This signed feature
is the characteristic of (R)-configuration at chiral axis, which was
in contrast with the spectra in the literature of axially chiral
biphenyl compounds.20
MeO
MeO
HO
O
MeO
a
c
b
NO2
NO2
NO2
NO2
H
H
NO2
NO2
(CH2)n
NO2
Br
HO
O
O
3
4
5
6a, n = 8
6b, n = 10
O
O
d
resolution
H
H
NH2
NH2
H
H
NH2
NH2
H
H
NH2
NH2
(CH2)n
(CH2)n
(CH2)n
O
O
O
O
(
S
)-7a n = 8
)-7b n = 10
7a-b
(
(
R
R
)-7a n = 8
)-7b n = 10
(
S
e
O
O
f
H
H
NHPPh2
NHPPh2
H
H
NHPPh2
NHPPh2
H
H
NHPPh2
NHPPh2
(CH2)n
(CH2)n
(CH2)n
O
O
O
(
R
) 2a
n = 8
(
S
)-2a n = 8
R
-
2a-b
(
) 2b
n = 10
(S)-2b n = 10
-
Scheme 1. Reagents and conditions: (a) Cu, 78%; (b) AlCl3, 1-dodecanethiol, CH2Cl2, 79%; (c) Br(CH2)nBr, K2CO3, DMF; for 6a, 58%; for 6b, 55%; (d) H2, Pd/C, EtOAc; for 7a,
100%; for 7b, 99%; (e) ClPPh2, Et3N, CH2Cl2; for 2a, 81%; for 2b, 79%; (f) chiral preparative HPLC, a Daicel Chiralcel AD-H column, for 2a, 42%; for 2b, 39%.

4108
H. Wei et al. / Tetrahedron Letters 49 (2008) 4106–4109
Table 1
Table 3
Rh(I)-Catalyzed asymmetric hydrogenation of (Z)-a-acetamidocinnamic acid using
Rh(I)-Catalyzed asymmetric hydrogenation of methyl (Z)-a-acetamido-3-arylacry-
bisaminophosphines (R)-2 as ligandsa
lates using bisaminophosphine (R)-2a as a liganda
Entry
Ligand
H2 press (atm)
Solvent
Conv.b (%)
eec (%)
Entry
Substrate
Ar
Conv.b (%)
eec (%)
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2a
2a
2a
2a
1
3
5
10
3
3
3
3
3
MeOH
MeOH
MeOH
MeOH
MeOH
IPA
THF
CH2Cl2
Acetone
75
96
88.2
89.7
81.9
66.6
84.1
83.1
89.8
89.6
81.8
1
2
3
4
5
6
7
8
10a
10b
10c
10d
10e
10f
Ph
100
100
100
100
100
100
100
100
94.4
95.3
95.1
94.3
93.4
92.1
91.3
91.6
100
100
100
100
100
100
91
p-Me-Ph
p-OMe-Ph
p-Cl-Ph
p-F-Ph
p-Br-Ph
o-Cl-Ph
m-Cl-Ph
10g
10h
a
a
All reactions were carried out at room temperature for 20 h. The catalyst was
prepared in situ from Rh(COD)2BF4 and ligand (substrate:Rh:ligand = 100:1:1.1).
All reactions were carried out at room temperature in CH2Cl2 for 20 h. The
catalyst was prepared in situ from Rh(COD)2BF4 and ligand (substrate:Rh:ligand =
100:1:1.1).
b
Determined by 1H NMR analysis.
c
b
Determined by 1H NMR analysis.
The enantiomeric excesses were determined by chiral HPLC using a Daicel
c
Chiralcel OD-H column. The S absolute configurations were assigned by comparison
The enantiomeric excesses were determined by chiral HPLC using a Daicel
of optical rotations with the literature data.21
Chiralcel OD-H column. The S absolute configurations were assigned by comparison
of optical rotations with the literature data.21
type of ligands can be retained by macro-ring strain produced from
5,50-linkage of biphenyl. The Rh(I) complex of bisaminophosphine
(R)-2a as a catalyst showed high catalytic activities and enantio-
selectivities (up to 95.3% ee) for the asymmetric hydrogenation
of various methyl (Z)-2-acetamido-3-arylacrylates 10. Further
study of development and application of this type of atropisomeric
ligands are in progress.
Table 2
Rh(I)-Catalyzed asymmetric hydrogenation of methyl (Z)-a-acetamidocinnamate
using bisaminophosphines (R)-2 as ligandsa
Entry
Ligand
H2 press (atm)
Solvent
Conv.b (%)
eec (%)
Acknowledgements
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2a
2a
2a
2a
1
3
5
10
3
3
3
3
3
MeOH
MeOH
MeOH
MeOH
MeOH
IPA
THF
CH2Cl2
Acetone
85
99
91.4
92.1
83.8
84.3
87.1
92.4
94.1
94.4
84.0
This work was supported by the National Natural Science Foun-
dation of China (Grant Nos. 20572070 and 20502015), the Ministry
of Education of China (EYTP and SRFDP).
100
100
100
100
100
100
91.7
References and notes
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a
All reactions were carried out at room temperature for 20 h. The catalyst was
prepared in situ from Rh(COD)2BF4 and ligand (substrate:Rh:ligand = 100:1:1.1).
b
Determined by 1H NMR analysis.
c
The enantiomeric excesses were determined by chiral HPLC using a Daicel
Chiralcel OD-H column. The S absolute configurations were assigned by comparison
of optical rotations with the literature data.21
obtained using bisaminophosphine (R)-2a as a chiral ligand in
methylene chloride under 3 atm of hydrogen (entry 8). With opti-
mization reaction conditions in hand, we have performed asym-
metric hydrogenations of various methyl (Z)-2-acetamido-3-
arylacrylates 10 catalyzed by Rh(I)-(R)- 2a complex. A variety of
substrates 10 can be hydrogenated to produce the corresponding
a-amino ester derivatives 11 in quantitative conversions with high
enantioselectivities (Table 3). The electronic nature of the phenyl
ring of the substrate had a little influence on the enantioselectivity
for the reaction. Electron-donating groups, 4-methyl-substituted
and 4-methoxy-substituted substrates, 10b (entry 2, 95.3% ee)
and 10c (entry 3, 95.1% ee), gave better ee values than halogenated
substrates (entries 4–6), while substitution at the ortho or meta
position of the phenyl ring of a substrate led to a slightly lower
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M.; Sun, J.; Lou, R. L.; Deng, J. G. J. Am. Chem. Soc. 1997, 119, 9570.
In summary, we have developed a new type of atropisomeric
bisaminophosphine ligands 2 with a bridge across the 5,50-position
of biphenyl. It was demonstrated that the axial chirality of this

H. Wei et al. / Tetrahedron Letters 49 (2008) 4106–4109
4109
12. For recent examples, see: (a) Huang, H.; Liu, X.; Chen, H.; Zheng, Z. Tetrahedron:
Asymmetry 2005, 16, 693; (b) Hua, Z.; Vassar, V. C.; Ojima, I. Org. Lett. 2003, 5,
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Commun. 2007, 1319; (b) Hou, G.-H.; Xie, J.-H.; Wang, L.-X.; Zhou, Q.-L. J. Am.
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X.; Zhou, Q.-L. J. Org. Chem. 2004, 69, 8157; (d) Reetz, M. T.; Mehler, G.;
Meiswinkel, A. Tetrahedron: Asymmetry 2004, 15, 2165; (e) Zanotti-Gerosa, A.;
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Nawa, M.; Hashimoto, H. Chem. Lett. 1980, 729; (d) Fiorini, M.; Giongo, G. M. J.
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17. Younong, Y.; Sudhir, K. S.; Angela, L.; Tsai, K. L.; Leroy, F. L.; Edmond, J. L. Bioorg.
Med. Chem. 2003, 11, 1475.
18. 2,20-Bis(diphenylphosphinoamino)-5,50 -(octamethylenedioxy)-1,10-biphenyl (2a):
1H NMR (CDCl3, 400 MHz): d 7.23 (m, 22H), 6.86 (dd, J = 8.8 Hz, 2.8 Hz, 2H),
6.75 (d, J = 9.2 Hz, 2.8 Hz, 2H), 4.55 (d, J = 6 Hz, 2H), 4.20 (m, 2H), 4.05 (m, 2H),
1.89 (m, 2H), 1.62 (m, 2H), 1.48–1.29 (m, 8H); 13C NMR (100 MHz, CDCl3): d
152.33, 137.55, 137.40, 131.34, 131.13, 130.88, 130.67, 128.91, 128.89, 128.54,
128.49, 128.47, 128.43, 119.08, 118.74, 118.55, 116.15, 67.16, 28.32, 27.99,
24.95; 31P NMR (CDCl3, 161 MHz) d 34.27; HRMS (Micromass LCT) calcd
14. For review, see: (a) Minnaard, A. J.; Feringa, B. L.; Lefort, L.; de Vries, J. G. Acc.
Chem. Res. 2007, 40, 1267. For recent examples, see: (b) Huo, X.-H.; Xie, J.-H.;
Wang, Q.-S.; Zhou, Q.-L. Adv. Synth. Catal. 2007, 349, 2477; (c) Giacomina, F.;
Meetsma, A.; Panella, L.; Lefort, L.; de Vries, A. H. M.; de Vries, J. G. Angew.
Chem., Int. Ed. 2007, 46, 1497; (d) Liu, Y.; Ding, K. J. Am. Chem. Soc. 2005, 127,
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X.; Jia, X.; Lu, G.; Au-Yeung, T. T.-L.; Lam, K.-H.; Lo, T. W. H.; Chan, A. S. C.
Tetrahedron: Asymmetry 2003, 14, 2687; (g) Pena, D.; Minnaard, A. J.; de Vries, J.
G.; Feringa, B. L. J. Am. Chem. Soc. 2002, 124, 14552; (h) Hu, A.-G.; Fu, Y.; Xie, J.-
H.; Zhou, H.; Wang, L.-X.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2002, 41, 2348.
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J. Am. Chem. Soc. 1998, 120, 5808.
for C44H43N2O2P2: 693.2800; found: 693.2808. For (R)-2a: mp 138–
2
5
2
5
141°C; ½aꢁD ꢀ32:8 (c 1, CHCl3). For (S)-2a: mp 138–141 °C; ½aꢁD þ31:9 (c 1,
CHCl3).
19. 2,20-Bis(diphenylphosphinoamino)-5,50 -(decamethylenedioxy)-1,10-biphenyl (2b):
1H NMR (CDCl3, 400 MHz): d 7.25 (m, 22H), 6.87 (dd, J = 8.8 Hz, 2.8 Hz, 2H),
6.73 (d, J = 9.2 Hz, 2.8 Hz, 2H), 4.44 (d, JPꢀH = 6.4 Hz, 2H), 4.12 (m, 2H), 4.00 (m,
2H), 1.85 (m, 2H), 1.61 (m, 2H), 1.41–1.29 (m, 12H); 13C NMR (100 MHz,
CDCl3):
d 152.44, 137.98, 137.80, 131.14, 130.93, 130.91, 130.71, 128.93,
128.85, 128.52, 128.45, 118.36, 118.07, 117.87, 117.10 68.96, 28.71, 28.65,
28.02, 25.10; 31P NMR (CDCl3, 161 MHz) d 32.86; HRMS (Micromass LCT) calcd
for C46H47N2O2P2: 721.3113; found: 721.3152. For (R)-2b: mp 119–121 °C;
25
25
½aꢁD ꢀ37:5 (c 1, CHCl3). For (S)-2b: mp 119–121 °C; ½aꢁD +39.7 (c 1, CHCl3).
20. Mislow, B. K.; Bunnenberg, E.; Records, R. J. Am. Chem. Soc. 1963, 85, 1342.
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M.; Mori, A.; Hashimoto, H. . Bull. Chem. Soc. Jpn. 1984, 2171; (c) Miyano, S.;