Synthesis of chiral bis(oxazolinyl)bipyridine ligands and related helical metal complexes

Article abstract of DOI:10.1016/S0957-4166(02)00069-1

Enantiomerically pure tetradentate bis(oxazolinyl)bipyridine ligands 1 have been synthesized in high yields. X-Ray analysis showed that the copper(I) complexes of ligands 1 had a C₂-symmetric helical structure. Ruthenium complexes of ligands 1 prepared in situ were found to be efficient in the asymmetric cyclopropanation of styrene with ethyl diazoacetate.

Full text of DOI:10.1016/S0957-4166(02)00069-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 161–165
Synthesis of chiral bis(oxazolinyl)bipyridine ligands and related
helical metal complexes
Yi-Zhou Zhu,a Zhi-Peng Li,b Jun-An Ma,a Fang-Yi Tang,b Li Kang,b Qi-Lin Zhoua,* and
Albert S. C. Chanc
aState Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
bInstitute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China
cOpen Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hong Kong, China
Received 11 January 2002; accepted 8 February 2002
Abstract—Enantiomerically pure tetradentate bis(oxazolinyl)bipyridine ligands 1 have been synthesized in high yields. X-Ray
analysis showed that the copper(I) complexes of ligands 1 had a C2-symmetric helical structure. Ruthenium complexes of ligands
1 prepared in situ were found to be efficient in the asymmetric cyclopropanation of styrene with ethyl diazoacetate. © 2002
Elsevier Science Ltd. All rights reserved.
1. Introduction
which was prepared from the dibromobipyridine 3 as
shown in Scheme 1. The reaction of 6,6-dibromo-2,2-
bipyridine, prepared from the coupling of 2,6-dibro-
mopyridine,6 with the anion of ethyl isobutyrate gave
diester 4 in 98% yield. Ester exchange of 4 with amino
alcohols in xylene provided diamides 5 in 75–81% yield.
Conversion of hydroxyl groups of 5 to chlorides 6
followed by treatment with NaOH in methanol pro-
duced the desired tetradentate ligands 1 in 62–76%
overall yield from 5.
The design and synthesis of suitable chiral catalytic
ligands is a key goal in the field of asymmetric catalysis
and a number of very efficient chiral catalysts have
been developed.1 Among the tremendous number of
reported ligands for chiral catalysis, those possessing
central chirality, axial chirality and planar chirality
occupy a predominant position.2 Chiral helical ligands,3
catalysts4 and inducers5 have recently been used in
asymmetric catalysis, and some have provided encour-
aging enantioselectivities. Of these compounds, we are
particularly interested in ligands which, upon coordina-
tion with metal ions, form chiral helical metal com-
plexes for use as chiral catalysts in asymmetric catalysis.
The well-designed examples of this type of ligand
include binaphthol derivatives3d and bisamides.4c
Herein we describe the synthesis of a new class of
tetradentate chiral bipyridine ligands 1 and the related
helical metal complexes 2.
To investigate the coordination behavior of tetra-
dentate ligands 1 and the structures of their metal
complexes, the copper(I) complexes 2 were synthesized.
The bis(oxazolinyl)bipyridine ligands 1 were mixed with
Cu(CH3CN)4PF6 (1:1) in CH2Cl2 and stirred at room
temperature for 2 h. After evaporation of solvent, the
crude solid was recrystallized with anhydrous methanol
to give a reddish-brown crystalline 2 (Scheme 2).
The single-crystal structure of complex 2a was deter-
mined by X-ray analysis shown in Fig. 1.7 Selected
bond lengths and bond angles are listed in Table 1. As
expected, the ligand 1a assembled with Cu(I) through
the coordination of four nitrogen atoms to form a
helical complex 2a. The complex in the crystal is C2-
symmetric and the configuration of helical ring is P,
and the angles between the CuꢀN(oxazoline) bond and the
coordination plane (N1ꢀCuꢀN1A) are 36.3°. The dis-
2. Results and discussion
The bis(oxazolinyl)bipyridine ligands 1 were synthe-
sized in high yields from enantiomerically pure amino
alcohols and the diethyl bipyridinedicarboxylate 4
* Corresponding author. E-mail: qlzhou@public.tpt.tj.cn
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00069-1

162
Y.-Z. Zhu et al. / Tetrahedron: Asymmetry 13 (2002) 161–165
H2N
R
OH
CO2Et
N
N
N
O
N
N
N
CO2Et EtO2C
C
O C
Br
Br
NH OH
HO
HN
3
4
R
R
5
OH-, MeOH
(S,S)-1a, R = Bn
(S,S)-1b, R = i-Pr
(S,S)-1c. R = Ph
(R,R)-1d, R = Ph
R
R
SOCl2
N
O
N
N
O
N
N
N
C
O
C
NH Cl
O
Cl
HN
1
R
R
6
Scheme 1.
N
N
N
Cu(CH3CN)4PF6
CH2Cl2, RT
R
N
R
N
N
N
Cu
R
N
O
O
O
O
R
(S,S)-1
(P)-2
Scheme 2.
Figure 1. Crystal structure of complex 2a.
propane structures in organic synthesis.10 A few exam-
ples have demonstrated that the multinitrogen ligands
induce good enantioselectivity in copper- or ruthenium-
catalyzed cyclopropanation.11 To investigate the
efficiency of ligands 1 for enantiocontrol in the cyclo-
propanation reaction, the complexes 2 were first tested
for catalytic ability in the asymmetric cyclopropanation
of styrene with ethyl diazoacetate. No reaction was
observed in refluxing CHCl3; however, the ruthenium
complexes, prepared in situ from [Ru(p-cymene)Cl2]2
tances between the copper atom and pyridine nitrogen
,
are 2.096(3) A which lie within the range reported in
the copper(I) complexes of bipyridine.8 While the
,
lengths of CuꢀN(oxazoline) bonds [1.988(3) A] are slightly
longer than those in the copper complexes of
bisoxazoline.4c,9
Transition metal-catalyzed asymmetric cyclopropana-
tion of olefins with diazo acetates is one of the most
important methods for the construction of chiral cyclo-

Y.-Z. Zhu et al. / Tetrahedron: Asymmetry 13 (2002) 161–165
163
−10°C under nitrogen. The mixture was stirred at the
same temperature for 30 min and then cooled to −78°C.
Ethyl isobutyrate (2.78 g, 24 mmol) was added. At this
temperature the mixture was allowed to stand for a
further 45 min, and 6,6%-dibromo-2,2%-bipyridyl (3.14 g,
10 mmol) was added. Under the light of a 200 W
tungsten lamp, the mixture was kept at −78°C for 10
min and then allowed to stir overnight at rt. The
mixture was treated with saturated aqueous NH4Cl
solution, and extracted with diethyl ether. The organic
layer was washed with saturated NaCl and dried over
Na2SO4, filtered and concentrated under reduced pres-
sure. The crude product was purified by column chro-
matography on silica gel with PE/EtOAc (10:1) to give
4 as a white solid (3.76 g, 98%). Mp 57–59°C. IR:
2983m, 1734vs, 1567s, 1436s, 1254s, 1148s, 1124m,
1022m, 808m, 760m. 1H NMR: 8.33 (d, J=7.8 Hz, 2H),
7.75 (t, J=7.8 Hz, 2H), 7.28 (d, J=7.8 Hz, 2H), 4.16
(q, J=7.2 Hz, 4H), 1.66 (s, 12H), 1.88 (t, J=7.2 Hz,
6H). MS (m/z, %): 384 (45, M+), 355 (14), 311 (68), 237
(100). Anal. calcd for C22H28N2O4: C, 68.75; H, 7.29;
N, 7.29. Found: C, 68.71; H, 6.88; N, 7.51%.
,
Table 1. Selected bond lengths (A) and bond angles (°) in
the complex 2a
,
Bond lengths (A)
Bond angles (°)
N(1)ꢀCuꢀN(1A)
N(1)ꢀCuꢀN(2)
N(2)ꢀCuꢀN(2A)
N(1A)ꢀCuꢀN(2A)
N(2)ꢀCuꢀN(1)ꢀC(1)
N(1A)ꢀCuꢀN(1)ꢀC(1)
CuꢀN(1)
CuꢀN(2)
N(1)ꢀC(1)
N(1)ꢀC(5)
N(2)ꢀC(9)
N(2)ꢀC(11)
C(1)ꢀC(1A)
2.096(3)
1.988(3)
1.345(4)
1.353(4)
1.273(5)
1.490(5)
1.497(7)
79.8(2)
88.3(1)
120.7(2)
88.3(1)
136.12
−7.57
N(2A)ꢀCuꢀN(1)ꢀC(1) −81.48
and the ligands 1 were found to be effective in this
reaction. As an example, the cyclopropanation of sty-
rene with ethyl diazoacetate catalyzed by the Ru com-
plex of ligand 1a (1 mol%) provided cyclopropanation
products in 56% yield (cis/trans 34:66) with 24% e.e. for
the trans isomer.
3. Conclusion
4.3. Synthesis of 5
In summary, a series of new chiral tetradentate bis(oxa-
zolinyl)bipyridine ligands 1 have been synthesized in
high yields. These ligands, upon coordination with cop-
per(I), formed C2-symmetric helical complexes. The
ligands 1 were proven to be efficient in the ruthenium-
catalyzed asymmetric cyclopropanation of styrene.
Although the enantioselectivity of the reaction was not
high, it clearly shows the potential of these ligands and
their helical complexes for enantiocontrol in asymmet-
ric catalysis.
4.3.1. General procedure. Anhydrous xylene (24 mL),
amino alcohol (12 mmol), 4 (4 mmol) and NaCN (39
mg, 10 mol%) were added in sequence into a Schlenk
flask equipped with a N2 inlet. The mixture was heated
under gentle reflux for 7 days. After cooling to rt, the
mixture was diluted with CHCl3, washed with water
and brine. The organic layer was dried over anhydrous
Na2SO4, and filtered through a short silica gel column
to give 5. The crude products were pure enough for
direct use in the next step.
4. Experimental
4.1. General
4.3.2.
6,6%-Bis-{1-[N-((1S)-2-hydroxy-1-benzyl-ethyl)]-
carbamoyl-1-methyl-ethyl}-[2,2%]bipyridinyl 5a. 78%
1
yield. H NMR: 8.14 (d, J=7.8 Hz, 2H), 7.84 (t, J=7.8
THF and xylene were distilled from sodium–benzophe-
none, CHCl3 and CH2Cl2 were distilled from CaH2. All
enantiomerically pure amino alcohols were prepared by
reduction of the corresponding commercially available
amino acids with NaBH4/H2SO4 in THF.12 6,6%-
Dibromo-2,2%-bipyridine was synthesized according to
the literature method.6 Melting points were measured
with a Yanaco MP-500 apparatus and are uncorrected.
1H NMR spectra were recorded on a Bruker AMX-
300AC instrument using tetramethylsilane as an inter-
nal standard in deuterochloroform. IR spectra were
obtained as KBr plates on a Shimadzu 435 spectropho-
tometer. Mass spectra were measured on a VG-7070E
spectrometer using a solid probe at 70 eV. Elemental
analyses were carried out on a Yanaco MT-3 analyzer.
Optical rotations were measured on a Perkin–Elmer
241 rotation apparatus.
Hz, 4H), 7.46 (d, J=7.8 Hz, 2H), 7.05–7.15 (m, 10H),
4.03–4.10 (m, 2H), 3.47–3.53 (m, 4H), 2.73–2.77 (m,
2H), 1.67 (s, 12H).
4.3.3. 6,6%-Bis-{1-[N-((1S)-2-hydroxy-1-isopropyl-ethyl)]-
carbamoyl-1-methyl-ethyl}-[2,2%]bipyridinyl 5b. 75%
1
yield. H NMR: 8.31 (d, J=7.8 Hz, 2H), 7.86 (t, J=7.8
Hz, 2H), 7.49 (d, J=7.8 Hz, 2H), 6.94 (d, 6.9 Hz, 2H),
3.48–3.66 (m, 6H), 1.70–1.79 (m, 14H), 0.82 (d, J=6.9
Hz, 6H), 0.74 (d, J=6.6 Hz, 6H).
4.3.4.
6,6%-Bis-{1-[N-((1S)-2-hydroxy-1-phenyl-ethyl)]-
5c. 81%
carbamoyl-1-methyl-ethyl}-[2,2%]bipyridinyl
1
yield. H NMR: 8.15 (d, J=7.8 Hz, 2H), 7.71–7.74 (m,
4H), 7.36 (d, J=7.8 Hz, 2H), 7.09–7.26 (m, 10H),
4.94–4.98 (m, 2H), 3.68–3.74 (m, 4H), 1.77 (d, J=6.6
Hz, 12H).
4.2. Synthesis of 6,6%-bis-(1-ethoxycarbonyl-1-methyl-
ethyl)-[2,2%]bipyridinyl 4
4.3.5.
6,6%-Bis-{1-[N-((1R)-2-hydroxy-1-phenyl-ethyl)-
carbamoyl]-1-methyl-ethyl}-[2,2%]bipyridinyl 5d. 79%
1
yield. H NMR: 8.15 (d, J=7.8 Hz, 2H), 7.68–7.76 (m,
A solution of n-BuLi/hexane (1.5 M, 16 mL, 24 mmol)
was added dropwise into a solution of diisopropyl-
amine (2.42 g, 24 mmol) in anhydrous THF (24 mL) at
4H), 7.46 (d, J=7.8 Hz, 2H), 7.08–7.20 (m, 10H),
4.93–4.98 (m, 2H), 3.70–3.72 (m, 4H), 1.76 (d, J=6.6
Hz, 12H).

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Y.-Z. Zhu et al. / Tetrahedron: Asymmetry 13 (2002) 161–165
4.4. General procedure for synthesis of ligands 6
2H), 3.98–4.01 (m, 2H), 3.19 (dd, J=13.6 Hz and 4.0
Hz, 2H), 2.73 (dd, J=13.6 Hz and 8.8 Hz, 2H), 1.69 (d,
J=18.0 Hz, 12H). MS (m/z, %): 558 (4, M+), 467 (100),
399 (22), 333 (20), 306 (12), 264 (10), 237 (26), 91 (54).
Anal. calcd for C36H38N4O2: C, 77.42; H, 6.81; N,
10.04. Found: C, 77.34; H, 6.84; N, 9.85%.
To a solution of 5 (3 mmol) in CHCl3 (15 mL) was
added a solution of SOCl2 (60 mmol) in CHCl3 (15 mL)
at −5°C over 0.5 h, and the mixture was allowed to stir
overnight at rt. Then water (20 mL) was added slowly,
the organic phase was separated, and the aqueous
phase was extracted with chloroform. The combined
organic phase was washed with aqueous NaHCO3 and
saturated NaCl solution, and dried over anhydrous
Na2SO4. The organic phase was filtered through a short
silica gel column and concentrated under reduced pres-
sure to give 6. The crude material was used directly to
the next step.
4.5.3. 6,6%-Bis-{1-[(4S,4%S)-4-isopropyl-4,5-dihydro-oxa-
zol-2-yl]-1-methyl-ethyl}-[2,2%]bipyridinyl
1b.
White
solid, 79% yield, mp 47–49°C. [h]2D0 −62 (c 1.0, CHCl3).
IR: 2958s, 1655s, 1572s, 1432s, 1382m, 1366w, 1138m,
1
1113s, 977s, 809s, 757m. H NMR: 8.35 (d, J=7.8 Hz,
2H), 7.72 (t, J=7.8 Hz, 2H), 7.29 (d, J=7.8 Hz, 2H),
4.14–4.19 (m, 2H), 3.93–4.04 (m, 4H), 1.83–1.95 (m,
2H), 1.70 (s, 12H), 0.97 (d, J=6.6 Hz, 6H), 0.89 (d,
J=6.6 Hz, 6H). MS (m/z, %): 462 (8, M+), 419 (100),
351 (68), 334 (25), 264 (19), 237 (31). Anal. calcd for
C28H38N4O2: C, 72.73; H, 8.22; N, 12.12. Found: C,
72.72; H, 8.20; N, 11.96%.
4.4.1. 6,6%-Bis-{1-[N-((1S)-2-chloro-1-benzyl-ethyl)car-
bamoyl]-1-methyl-ethyl}-[2,2%]bipyridinyl 6a. 90% yield.
1H NMR: 8.28 (d, J=7.8 Hz, 2H), 7.81 (t, J=7.8 Hz,
4H), 7.6 (d, J=7.38 Hz, 2H), 7.04–7.24 (m, 10H),
4.36–4.44 (m, 2H), 3.34–3.58 (m, 4H), 2.75–2.79 (m,
2H), 1.69 (d, J=9.0 Hz, 12H).
4.5.4. 6,6%-Bis-{1-[(4S,4%S)-4-phenyl-4,5-dihydro-oxazol-
2-yl]-1-methyl-ethyl}-[2,2%]bipyridinyl 1c. White solid,
75% yield, mp 150–152°C. [h]2D0 −151 (c 1.0, CHCl3).
IR: 2972m, 1662vs, 1563m, 1431s, 1136s, 1121s, 979m,
4.4.2.
6,6%-Bis-{1-[N-((1S)-2-chloro-1-isopropyl-ethyl)-
carbamoyl]-1-methyl-ethyl}-[2,2%]bipyridinyl 6b. 86%
1
1
yield. H NMR: 8.39 (dd, J=7.8 Hz and 0.9 Hz, 2H),
810m, 759s, 702s. H NMR: 8.42 (d, J=7.8 Hz, 2H),
7.84 (t, J=7.8 Hz, 2H), 7.46 (dd, J=7.8 Hz and 0.9 Hz,
2H), 7.06 (d, 9.0 Hz, 2H), 3.85–3.91 (m, 2H), 3.53–3.64
(m, 4H), 1.73–1.80 (m, 12H), 0.85 (d, J=6.6 Hz, 6H),
0.69 (d, J=6.9 Hz, 6H).
7.77 (t, J=7.8 Hz, 2H), 7.40 (d, J=7.8 Hz, 2H),
7.26–7.30 (m, 10H), 5.23–5.32 (m, 2H), 4.56–4.63 (m,
2H), 4.04–4.11 (m, 2H), 1.80 (d, J=4.8 Hz, 12H). MS
(m/z, %): 530 (12, M+), 385 (100), 264 (34), 237 (38),
103 (28), 91 (25). Anal. calcd for C34H34N4O2: C, 76.98;
H, 6.42; N, 10.57. Found: C, 76.81; H, 6.36; N, 10.34%.
4.4.3. 6,6%-Bis-{1-[N-((1S)-2-chloro-1-phenyl-ethyl)car-
bamoyl]-1-methyl-ethyl}-[2,2%]bipyridinyl 6c. 82% yield.
1H NMR: 8.44 (d, J=7.8 Hz, 2H), 7.91–7.96 (m, 4H),
7.62 (d, J=7.8 Hz, 2H), 7.24–7.33 (m, 10H), 5.38–5.46
(m, 2H), 3.87–3.90 (m, 4H), 1.91 (d, J=8.7 Hz, 12H).
4.5.5. 6,6%-Bis-{1-[(4R,4%R)-4-phenyl-4,5-dihydro-oxazol-
2-yl]-1-methyl-ethyl}-[2,2%]bipyridinyl 1d. White solid,
75% yield, mp 150–152°C. [h]2D0 +149 (c 1.0, CHCl3).
IR: 2972m, 1662vs, 1563m, 1455m, 1431m, 1383w,
1
4.4.4. 6,6%-Bis-{1-[N-((1R)-2-chloro-1-phenyl-ethyl)car-
bamoyl]-1-methyl-ethyl}-[2,2%]bipyridinyl 6d. 83% yield.
1H NMR: 8.29 (d, J=7.8 Hz, 2H), 7.76–7.83 (m, 4H),
7.49 (d, J=7.8 Hz, 2H), 7.14–7.19 (m, 10H), 5.23–5.31
(m, 2H), 3.74–3.76 (m, 4H), 1.78 (d, J=8.4 Hz, 12H).
1136s, 1121s, 979m, 810m, 759s, 702s. H NMR: 8.38
(d, J=7.8 Hz, 2H), 7.74 (t, J=7.8 Hz, 2H), 7.35 (d,
J=7.8 Hz, 2H), 7.23–7.29 (m, 10H), 5.20–5.28 (m, 2H),
4.50–4.60 (m, 2H), 3.98–4.07 (m, 2H), 1.78 (d, J=4.8
Hz, 12H). MS (m/z, %): 530 (12, M+), 385 (100), 264
(32), 237 (37), 103 (26), 91 (24). Anal. calcd for
C34H34N4O2: C, 76.98; H, 6.42; N, 10.57. Found: C,
76.86; H, 6.38; N, 10.49%.
4.5. Synthesis of ligands 1
4.5.1. General procedure. A mixture of 6 (2.7 mmol) and
NaOH (0.32 g, 8 mmol) in methanol (27 mL) was
heated under reflux for 6 h. After removal of methanol,
chloroform (60 mL) and water (15 mL) were added to
the residue, the organic phase was separated, and the
aqueous phase was extracted with chloroform. The
combined organic phase was washed with saturated
NaCl solution, dried over anhydrous K2CO3 and con-
centrated under reduced pressure. The crude material
was purified by column chromatography on silica gel
with PE/EtOAc (4:1) to provide ligands 1.
4.6. Enantioselective cyclopropanation catalyzed by Ru
complex of ligand 1a
To a Schlenk flask were added [Ru(p-cymene)Cl2]2 (3.1
mg, 0.005 mmol), ligand 1a (5.6 mg, 0.01 mmol) and
CHCl3 (2 mL) under nitrogen. The mixture was stirred
for 3 h at rt, then filtered to a three-neck flask equipped
with a dropping funnel. To the filtrate AgBF4 (8.0 mg,
0.02 mmol) was added, the mixture was stirred for 20 h
at rt, then filtered to a three-neck flask equipped with a
dropping funnel. After addition of styrene, ethyl dia-
zoacetate (114 mg, 1 mmol) in CHCl3 (10 mL) was
added over 3 h. The resulting mixture was stirred for 24
h at rt, and the solvent was evaporated under reduced
pressure. The residue was purified by a column chro-
matography on silica gel to give cyclopropanation
products. Diastereoselectivities (cis:trans ratio) of prod-
ucts were analyzed by GC using a capillary column
4.5.2. 6,6%-Bis-{1-[(4S,4%S)-4-benzyl-4,5-dihydro-oxazol-
2-yl]-1-methyl-ethyl}-[2,2%]bipyridinyl 1a. White solid,
84% yield, mp 110–112°C. [h]2D0 −13 (c 1.0, CHCl3). IR:
2974m, 1664vs, 1649s, 1575s, 1435s, 1382m, 1138m,
1
1109s, 1083s, 976s, 810s. H NMR: 8.36 (d, J=8.0 Hz,
2H), 7.70 (t, J=8.0 Hz, 2H), 7.22–7.33 (m, 10H), 7.17
(d, J=8.0 Hz, 2H), 4.44–4.49 (m, 2H), 4.12–4.17 (m,

Y.-Z. Zhu et al. / Tetrahedron: Asymmetry 13 (2002) 161–165
165
(HP-1, 30 m, 0.32 mm ID). The enantiomeric excesses
were measured by GC after re-esterification with (−)-
menthol.13
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Acknowledgements
Financial supports from the National Natural Science
Foundation of China, the Major Basic Research Devel-
opment Program (grant No. G2000077506), the Min-
istry of Education of China and the Hong Kong
Polytechnic University ASD Fund are gratefully
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,
13.053(2), b=13.053(2), c=21.702(5) A; V=3697.4(13)
3
,
A , Z=4, crystal dimensions=0.30×0.28×0.25 mm, T=
,
293(2) K, radiation=MoKa, u=0.71073 A, unique
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