The synthesis of novel C2-symmetric P,N-chelation ruthenocene ligands and their application in palladium-catalyzed asymmetric allylic substitution

Article abstract of DOI:10.1016/j.tetlet.2006.11.121

Novel air-stable C₂-symmetric tetrasubstituted ruthenocene-based ligands were readily synthesized and used for palladium-catalyzed asymmetric allylic substitution showing excellent enantioselectivity and high catalytic activity.

Full text of DOI:10.1016/j.tetlet.2006.11.121

Tetrahedron Letters 48 (2007) 585–588
The synthesis of novel C2-symmetric P,N-chelation
ruthenocene ligands and their application in
palladium-catalyzed asymmetric allylic substitution
Delong Liu, Fang Xie and Wanbin Zhang*
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
Received 16 September 2006; revised 13 November 2006; accepted 20 November 2006
Available online 12 December 2006
Abstract—Novel air-stable C2-symmetric tetrasubstituted ruthenocene-based ligands were readily synthesized and used for palla-
dium-catalyzed asymmetric allylic substitution showing excellent enantioselectivity and high catalytic activity.
Ó 2006 Published by Elsevier Ltd.
Ferrocene-based chiral ligands designed for asymmetric
synthesis have attracted tremendous scientific interest
over the past decades,1 with some having been success-
fully applied in industrial processes.2 The reason as to
why these ligands were a versatile class of compounds
is that structural modification can be readily made by
introduction of a desired functional group on the cyclo-
pentadienyl rings according to the demand of the reac-
tion type. Among the vast variety of ferrocene-based
ligands, the design and the preparation of C2-symmetry
planar chiral ferrocenes are of great importance in the
development of transition metal-catalyzed enantioselec-
tive reactions.3,4 Several years ago, the Ikeda group
reported the C2-symmetric ferrocene-based ligands
1,10-bis(oxazolinyl)-2,20-diphenylphosphino ferrocene 1
(Fig. 1). This type of ligands afford interesting complex-
ation behavior with palladium(II) and excellent enantio-
selectivity for palladium-catalyzed asymmetric allylic
substitution.3 Since then, many C2-symmetry tetrasub-
stituted chiral ferrocene ligands have been developed
showing excellent enantioselectivity for asymmetric
synthesis.4
Comparing with chiral ferrocene ligands, the chiral
ruthenocene ligands have received much less attention.
To the best of our knowledge, there are few reports on
chiral ruthenocene ligands.5 It is known that the dis-
tances between the two cyclopentadienyl rings in ferro-
˚
cene and ruthenocene are 3.32 and 3.68 A, respectively
(Fig. 2).6 The longer distance by about 10% in rutheno-
cene than their ferrocene analogs would be expected to
present different complexation behavior with transition
metals. These ligands will probably give rise to different
enantioselectivity and catalytic activity in catalyzed
asymmetric synthesis too. Considering these presump-
tions, we report herein the preparation of the novel
C2-symmetric tetrasubstituted ruthenocene compounds,
their complexation behavior with palladium(II) and
application in palladium-catalyzed asymmetric allylic
substitution.
O
O
R
R
N
N
PPh2
PPh2
Ru
Fe
PPh2
PPh2
N
N
R
R
O
O
Ph
Ph
Ph
Ph
1 (a: R=i-Pr, b: t-Bu)
P
2 (a: R=i-Pr, b: t-Bu)
P
3.32A
P
Fe
Ru 3.68A
P
Figure 1.
Ph
Ph
Ph
Ph
*
Corresponding author. Tel./fax: +86 21 54743265; e-mail: wanbin@
sjtu.edu.cn
Figure 2.
0040-4039/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2006.11.121

586
D. Liu et al. / Tetrahedron Letters 48 (2007) 585–588
COCl
aminoalcohols
complex (10)10 but not P,P-chelation complex (11)
COOH
COOH
(COCl)2
n
-BuLi
(Fig. 3), after an additional 1 equiv of palladium(II)
had been added. We also tried to investigate the com-
plexation of 2a with 1,3-diphenylallylpalladium(II)
chloride dimer, but the 1H NMR and 31P NMR
were complicated and difficult to conclusively analyze.
Ru
Ru
Ru
dry-ice
COCl
3
4
5
Y=96%
HO
O
O
N
With the novel C2-symmetric tetrasubstituted rutheno-
cene compounds in hand, we first applied them in the
palladium-catalyzed asymmetric allylic substitution
(Scheme 2).11 All of these ligands afforded high enantio-
selectivity for the palladium-catalyzed asymmetric
allylic substitution of rac-1,3-diphenyl-2-propenyl
acetate, similar to that of the corresponding ferrocene
compounds.3b In addition, higher catalytic activity was
observed since all reactions were completed within
30 min at room temperature in dichloromethane. How-
ever, it took 6 h for this reaction when 1 was used.3b
The results are shown in Table 1.
N
R
R
R
H
s
-BuLi
MsCl
Ru
Ru
H
N
N
O
Ph2PCl
R
O
HO
6a, 6b
7a, 7b
y=58-62% from 4
PPh2
PPh2
O
O
N
O
N
R
R
R
R
R
N
PPh2
+
Ru
Ru
Ru
+
PPh
2N
PPh
2N
N
O
R
O
O
PPh2
RP
not detected
(
SP
,
SP) 2a, 2b
(SP
,
RP) 8a, 8b
(
, RP) 9a, 9b
The influence of reaction conditions and structure of
ligands upon the allylic substitution were taken into
y=52-55%
y=7-12%
Scheme 1. a: R = i-Pr, b: R = t-Bu.
Cl
Cl
Pd
Compound 2 can be easily prepared from 3 via five steps
(Scheme 1). Thus, treatment of 3 with 4 equiv n-butylli-
thium and 2.6 equiv TMEDA in n-hexane for 19 h gave
lithiated compound, which followed by carboxylation
reaction and acidification afford 1,10-dicarboxylic ruthe-
nocene 4 in 96% yield. This acid then reacted with
8 equiv oxallyl chloride in dichloromethane to give the
1,10-dichlorocarbonylruthenocene 5. Without any purifi-
cation, 5 was directly reacted with 2.4 equiv chiral
aminoalcohol and then treated with 2.6 equiv methyl
sulfonyl chloride in ‘one-pot’ to afford the 1,10-bis[(S)-
4-substituted oxazolin-2-yl]-ruthenocene 7 in 58–62%
overall yields from 4. Compound 7 which was treated
with 2.6 equiv of sec-butyllithium in THF at ꢀ78°C
for a period of 2 h and then for 20 min at 0 °C. The
dilithiated species were then reacted with 2.6 equiv of
chlorodiphenylphosphine at 0 °C for 3 h. Thus, 27,8
was obtained as a major product with yields of 52–
55% and (SP,RP)-89 was also isolated as a minor product
with yields of 7–12%. (RP,RP)-9 could not be detected in
this procedure. The absolute configuration of the prod-
uct was deduced according to that of compound 1.3a
N
N
N
Ph2
P
PPh2
O
O
O
O
Cl
Cl
Ru
Ru
Pd
PPh2
Cl
P
Ph2
N
Pd
Cl
10
11
Figure 3.
H3COOC
Ph
COOCH3
Ph
η3
OAc
Ligand, [Pd( -C3H5)Cl]2
Base
+ H2C(CO2CH3)2
Ph
Ph
Scheme 2.
Table 1. Allylic substitution of 1,3-diphenyl-2-propenyl acetate with
dimethyl malonatea
Entry
Ligand
T (°C)
Time
eeb (%)
Enantiomerc
1d
2
2a
2a
2a
2a
2a
2a
2b
rt
rt
rt
0
ꢀ25
40
rt
3 h
82
92
82
92
92
92
98
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
To our delight, the obtained tetrasubstituted rutheno-
cene compound 2 is more air-stable than 1. In the case
of the preparation of 1, work up and purification with
column chromatography had to be done under argon
all the time.3a During this work, no oxidation of phos-
phorus atoms in 2 could be detected upon exposure to
air during synthesis. Furthermore, 2 can be stored in
air atmosphere for more than ten months without any
change.
0.5 h
0.5 h
20 h
20 h
10 min
0.5 h
3e
4
5
6
7
a Molecular
ratio:
[Pd(g3-C3H5)Cl]2/ligand/substrate/BSA/H2C-
(CO2Me)2 = 2.5/3.0/100/300/300. Reactions were conducted under
nitrogen in CH2Cl2 at room temperature and all reactions gave above
95% isolated chemical yields. The catalyst was prepared by treating
[Pd(g3-C3H5)Cl]2 with ligands in CH2Cl2 at 20 °C for 1 h before use.
b Determined by the HPLC using chiral OD-H column.
c Assigned through comparison of the sign of specific rotations with the
literature data.12
Next, the complexation of 2a with palladium(II) in
acetonitrile-d3 was examined. When 2a was mixed
with 1 equiv of dichlorobis(acetonitrile)palladium(II) in
acetonitrile-d3, three components including 2a were
observed by 1H NMR and 31P NMR. Finally, the
mixture gave a single C2-symmetric 1:2 P,N-chelation
d NaH was used as a base.
e THF was used as solvent.

D. Liu et al. / Tetrahedron Letters 48 (2007) 585–588
Table 2. The effect of planar chirality to the allylic substitutiona
Entry Ligand Base Solvent Time eeb Enantiomerc
587
account. First of all, the effect of the base on the
catalytic reaction was examined. Primarily, when
sodium hydride was used as the base, 82% ee was
obtained. The enantioselectivity was enhanced to 92%
ee if the base was changed to BSA (Table 1, entries 1
and 2). The solvent also affected the enantioselectivity.
When using THF instead of dichloromethane, the
enantioselectivity decreased to 82% ee (Table 1, entries
2 and 3). However, the temperature had no effect on
the enantiometric excess in this substitution (Table 1,
entries 2, 4–6).
(h)
(%)
1
2
3
2a
12
13
BSA CH2Cl2 0.5
92
87
59
(S)-(ꢀ)
(S)-(ꢀ)
(S)-(ꢀ)
BSA CH2Cl2
1
BSA CH2Cl2 1.5
a Molecular
ratio:
[Pd(g3-C3H5)Cl]2/ligand/substrate/BSA/H2C-
(CO2Me)2 = 2.5/6.0/100/300/300. Reactions were conducted under
nitrogen in CH2Cl2 at room temperature and all reactions gave above
95% isolated chemical yields. The catalyst was prepared by treating
[Pd(g3-C3H5)Cl]2 with ligands in CH2Cl2 at 20 °C for 1 h before use.
b Determined by the HPLC using chiral OD-H column.
c Assigned through comparison of the sign of specific rotations with the
literature data.12
It was shown that the substituent R on the oxazolinyl
ring had much effect on the enantioselectivity and a
bulkier group gave a better ee value. When 2b having
tert-butyl group was used as a chiral ligand, up to
98% ee was obtained in the allylic substitution (Table
1, entry 7).
and catalytic activity in this substitution (Table 2,
entries 2 and 3).
In order to examine whether the planar chirality had ef-
fect on the catalytic reaction, the 1,2-substituted ruthe-
nocene ligands 12 and 13 were also synthesized as
shown in Scheme 3. Compounds 1213 and 1314 were eas-
ily prepared from 1-carboxylic ruthenocene via steps 4–6
with overall yields of 30–61%, and then employed in the
palladium-catalyzed asymmetric allylic substitution
(Table 2).
In summary, we have prepared the novel C2-symmetric
tetrasubstituted ruthenocene ligand 2 and applied it in
the palladium-catalyzed asymmetric allylic substitution.
Comparing to the corresponding ferrocene ligand 1,
much higher catalytic activity and comparable excellent
enantioselectivities were observed. Furthermore, it was
found that these novel ligands were rather stable toward
air, which makes them suitable for commercial pro-
cesses. Studies of the tetrasubstituted ruthenocene
ligand 2 and the 1,2-substituted ruthenocene ligands 12
and 13 showed that the planar chirality has an influence
on the enantioselectivity and catalytic activity in this
substitution. Structural modifications of these ligands
are currently being conducted in our laboratories.
It was found that the C2-symmetric tetrasubstituted
ruthenocene ligand 2 had better enantioselectivity and
higher activity than both 12 and 13 in the palladium-
catalyzed asymmetric allylic substitution (Table 2),
although it was not clear whether the process was
affected by electric or/and steric effects, especially in
view of the small amount of P,P-chelation complex.
Meanwhile, it could be concluded that the planar
chirality had an influence on the enantioselectivity
Acknowledgments
This work was partly supported by the Excellent Young
Teachers Program of MOE, PRC and the National Nat-
ural Science Foundation of China (No. 20572070).
HO
O
COOH
COCl
N
H
i-Pr
(COCl)2
L-valinol
Ru
Ru
Ru
DIPEA
References and notes
O
PPh2
O
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i
-Pr
i
-Pr
N
1)
2)
s
-BuLi / Me3SiCl
N
MsCl
Ru
Ru SiMe3
52%
n
-BuLi / Ph2PCl
DIPEA
79% from 1-carboxylic
ruthenocene
1) s-BuLi / TMEDA
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TBAF
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N
PPh2
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12
13
77%
74%
Scheme 3.

588
D. Liu et al. / Tetrahedron Letters 48 (2007) 585–588
(d) Zhang, W.; Yoneda, Y.-I.; Kida, T.; Nakatsuji, Y.;
76.7, 77.3, 77.8, 78.6, 78.7, 78.8, 78.9, 128.1, 128.21,
128.25, 128.27, 128.32, 128.38, 128.42, 128.49, 128.5,
128.8, 128.9, 132.6, 132.8, 132.9, 133.0, 134.6, 134.8,
134.9, 135.1, 162.9, 163.0; 31P NMR (CDCl3, 162 Hz, 85%
H3PO4): d ꢀ16.77, ꢀ15.72; MS (MALDI): m/z 823
[M+1+] (100); HRMS calcd for C46H47N2O2P2Ru
823.2151, found 823.2159.
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hedron: Asymmetry 2006, 17, 116–123.
10. Data for 10: 1H NMR (CD3CN, 400 Hz): d 0.85 (d,
J = 6.8 Hz, 6H), 0.95 (d, J = 6.8 Hz, 6H), 2.77–2.86 (m,
2H), 3.84 (br s, 2H), 3.88 (br s, 2H), 4.38 (t, J = 9.2 Hz,
2H), 4.46–4.48 (m, 2H), 5.12–5.15 (m, 2H), 5.16 (br s, 2H),
7.25–7.96 (m, 20H); 31P NMR (CD3CN, 162 Hz, 85%
H3PO4): d 18.09.
11. Selected papers: (a) Trost, B. M.; Strege, P. E. J. Am.
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W.; Kida, T.; Nakatsuji, Y.; Ikeda, I. Synlett 1999, 1319–
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M.; Yamamoto, N.; Suenaga, T.; Sakamoto, M.; Fujita,
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4221–4226; (b) Abbenhuis, H. C. L.; Burckhardt, U.;
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logr. 1959, 12, 28.
27
7. Data for 2a: mp 178–180 °C; ½aꢁD ꢀ263.6 (c 0.61, CHCl3);
1H NMR (CDCl3, 400 Hz): d 0.60 (d, J = 6.8 Hz, 6H),
0.86 (d, J = 6.8 Hz, 6H), 1.65–1.71 (m, 2H), 3.67 (t,
J = 8 Hz, 2H), 3.82 (br s, 2H), 3.89–3.91 (m, 2H), 4.23–
4.27 (dd, J = 8, 9.6 Hz, 2H), 4.67 (br s, 2H), 5.41 (br s,
2H), 7.17–7.32 (m, 20H); 13C NMR (CDCl3, 100 Hz): d
17.4, 19.1, 32.1, 69.6, 72.2, 75.8, 77.4, 78.0, 78.1, 81.9, 82.1,
84.5, 84.8, 128.1, 128.2, 128.3, 128.4, 128.5, 128.9, 132.8,
133.0, 134.4, 134.6, 137.9, 138.0, 139.2, 139.3, 163.3, 163.4;
31P NMR (CDCl3, 162 Hz, 85% H3PO4): d ꢀ16.85; MS
(MALDI): m/z 823 [M+1+] (100); HRMS calcd for
C46H47N2O2P2Ru 823.2151, found 823.2154.
12. Wimmer, P.; Widhalm, M. Tetrahedron: Asymmetry 1995,
6, 657–660.
27
13. Data for 12: mp 136–138 °C; ½aꢁD ꢀ97.51 (c 0.46, CHCl3);
1H NMR (CDCl3, 400 Hz): d 0.67 (d, J = 6.8 Hz, 3H),
0.79 (d, J = 6.8 Hz, 3H), 1.64–1.68 (m, 1H), 3.67 (t,
J = 7.6 Hz, 1H), 3.79–3.84 (m, 1H), 3.94 (br s, 1H), 4.16–
4.20 (dd, J = 8, 10 Hz, 1H), 4.59 (s, 5H), 4.67 (br s, 1H),
5.31 (br s, 1H), 7.27–7.40 (m, 10H); 13C NMR (CDCl3,
100 Hz): d 17.7, 18.7, 32.2, 69.7, 72.1, 72.8, 74.3, 73.1 (5C),
76.5, 76.6, 79.5 (d, J = 18 Hz), 81.7 (d, J = 17.6 Hz),
128.0, 128.1, 128.2, 128.3, 128.4, 128.9, 132.6, 132.8, 134.6,
134.8, 138.3 (d, J = 13.6 Hz), 139.8 (d, J = 14.5 Hz); 31P
NMR (CDCl3, 162 Hz, 85% H3PO4): d ꢀ14.94; MS
(MALDI): m/z 528 [M+1+] (100); HRMS calcd for
C28H29NOPRu 528.1025, found 528.1041.
27
8. Data for 2b: mp 150–151 °C; ½aꢁD ꢀ282.9 (c 1.04, CHCl3);
1H NMR (CDCl3, 400 Hz): d 0.70 (s, 18H), 3.71–3.80 (m,
4H), 3.82 (br s, 2H), 4.15 (t, J = 8.4 Hz, 2H), 4.72 (br s,
2H), 5.36 (br s, 2H), 7.12–7.29 (m, 20H); 13C NMR
(CDCl3, 100 Hz): d 25.9, 33.7, 68.7, 76.2, 76.3, 76.4, 77.7,
78.2, 78.3, 82.0, 82.2, 84.0, 84.2, 128.1, 128.2, 128.3, 128.4,
128.5, 128.8, 132.7, 132.9, 134.4, 134.6, 137.9, 138.0,
139.4, 139.5, 162.8; 31P NMR (CDCl3, 162 Hz, 85%
H3PO4): d ꢀ16.93; MS (MALDI): m/z 851 [M+1+] (100);
HRMS calcd for C48H51N2O2P2Ru 851.2464, found
851.2473.
27
14. Data for 13: mp 169–171 °C; ½aꢁD +130.47 (c 0.46,
CHCl3); 1H NMR (CDCl3, 400 Hz):
d 0.62 (d,
J = 6.8 Hz, 3H), 0.64 (d, J = 6.8 Hz, 3H), 1.51–1.57 (m,
1H), 3.86–4.00 (m, 3H), 4.60 (s, 5H), 4.67 (br s, 1H), 5.30
(br s, 1H), 7.26–7.42 (m, 10H). 13C NMR (CDCl3,
100 Hz): d 18.0, 18.1, 32.6, 69.7, 72.1, 72.5, 73.1 (5C),
74.4, 76.8, 76.9, 79.2 (d, J = 18.3 Hz), 81.5 (d, J = 16 Hz),
128.0, 128.1, 128.2, 128.3, 128.4, 128.8, 132.6, 132.9, 134.8,
135.0, 138.4 (d, J = 12 Hz), 139.6 (d, J = 11.5 Hz); 31P
NMR (CDCl3, 162 Hz, 85% H3PO4): d ꢀ15.74: MS
(MALDI): m/z 528 [M+1+] (100); HRMS calcd for
C28H29NOPRu 528.1025, found 528.1036.
27
9. Data for 8a: mp 87–88 °C; ½aꢁD ꢀ62.9 (c 0.44, CHCl3); 1H
NMR (CDCl3, 400 Hz): d 0.49–0.54 (m, 9H), 0.71 (d,
J = 6.8 Hz, 3H), 1.36–1.43 (m, 1H), 1.48–1.56 (m, 1H),
3.38 (t, J = 8.4 Hz, 1H), 3.70–3.72 (m, 1H), 3.76–3.80 (m,
1H), 3.85–3.94 (m, 2H), 3.96–4.02 (m, 1H), 4.16 (br s, 2H),
4.78–4.80 (q, J = 2.8 Hz, 2H), 5.26 (br s, 1H), 5.29 (br s,
1H), 7.59–7.43 (m, 20H); 13C NMR (CDCl3, 100 Hz): d
17.4, 18.0, 18.1, 18.9, 31.9, 32.7, 69.5, 69.6, 72.1, 72.3, 76.4,