Chiral bipyridine-copper(II) complex. Crystal structure and catalytic activity in asymmetric cyclopropanation

Article abstract of DOI:10.1039/a706754b

A chiral bipyridine-copper(II) complex has been prepared and its crystal structure determined. The structure involves a CuN₂Cl₂ compressed tetrahedral stereochemistry. Chiral bipyridine complexes of copper-(I) and -(II) triflate are

Full text of DOI:10.1039/a706754b

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Chiral bipyridine–copper(II) complex. Crystal structure and catalytic
activity in asymmetric cyclopropanation
Hoi-Lun Kwong,*,†,a Wing-Sze Lee,a Hung-Foon Ng,a Wing-Hong Chiua and Wing-Tak Wongb
a Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue,
Kowloon Tong, Hong Kong
b Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
A chiral bipyridine–copper(II) complex has been prepared and its crystal structure determined. The structure
involves a CuN2Cl2 compressed tetrahedral stereochemistry. Chiral bipyridine complexes of copper-(I) and -(II)
triflate are active catalysts for asymmetric cyclopropanation of alkenes with enantiomeric excesses up to 92%.
Despite the importance of bipyridine in catalysis, photo-
chemistry and supramolecular chemistry, little use has been
made of chiral bipyridine ligands in asymmetric catalysis, when
compared with other chiral N᎐N bidentate ligands.1 Cyclo-
propanes are useful building blocks for the construction of
organic molecules. The synthesis of chiral cyclopropanes from
achiral starting materials has been of considerable interest in
the last few decades.2 Chiral metal complexes, especially those
of copper, have been widely used as catalysts for asymmetric
cyclopropanation. Chiral ligands such as bis(oxazolines),3 semi-
corrins4 and Schiff bases5 are the most popular for copper-
catalysed cyclopropanation. Recently, Katsuki and co-workers6
reported the use of chiral bipyridine ligands synthesized from
2,3-cyclohexenepyridine in asymmetric cyclopropanation.6
These results clearly suggested the great potential of this type
of ligand. Bolm et al.7 reported the facile synthesis of the chiral
bipyridine L and its use in diethylzinc addition to aldehydes.
In order to increase the scope of this type of ligand, we report
the synthesis of its copper() chloride complex, the crystal
structure of the latter and the use of such chiral bipyridine–
copper-(), -() triflate (trifluoromethanesulfonate) compounds
in enantioselective cyclopropanation of alkenes. The few
examples of copper–chiral N᎐N bidentate ligand complexes
characterized are either mononuclear 2:18 or polymeric 1:1
ligand to copper complexes.9 The present compound has an
unusual co-ordination environment for CuII. Unlike the previ-
ously reported chiral N᎐N bidentate ligands, this chiral bipyrid-
ine ligand is unique in that both copper-() and -() triflate form
active catalysts for cyclopropanation.
ental analyses were performed by MEDAC Ltd. or on a Leco
CHN-900 micro carbon hydrogen nitrogen determinator.
Preparation of the copper(II) chiral bipyridine complex 1
Chiral compound L (0.14 g, 0.4 mmol) in CH2Cl2 (5 cm3) was
added dropwise to a solution of copper() chloride (0.068 g, 0.4
mmol) in absolute ethanol (5 cm3). An intense orange-red color
developed instantaneously. The solution was refluxed overnight
to ensure complete complexation. Addition of diethyl ether to
the cooled reaction mixture led to a deep orange microcrystal-
line solid. The mixture was placed in a refrigerator overnight
and the microcrystalline solid then filtered off and washed with
ether. It was recrystallized from CH2Cl2–ether to give deep
orange crystals, which were filtered off and dried in vacuo. The
complex was characterized by X-ray crystallography, elemental
analysis (C,H,N), IR and UV spectroscopy. Yield: 0.145 g
(74%) (Found: 53.80; H, 6.59; N, 5.66. Calc. for C22H32Cl2-
CuN2O2: C, 53.82; H, 6.52; N, 5.71%). IR(KBr): 3086.3w,
2967.6vs, 2946.0vs, 2830.5w, 1597.2m, 1570.2m, 1478.6s,
1458.0s, 1431.2m, 1396.0w, 1358.9m, 1225.0w, 1203.1w,
1165.6s, 1080.9vs, 1021.5m, 962.0m, 941.8w, 916.9w, 820.0m,
785.5w, 758.0w, 671.4w and 647.9w cmϪ1. Visible spectrum
(CH2Cl2): λmax(ε) 310 (22 125), 322 (19 075), 406 (881) and 919
nm (107 dm3 molϪ1 cmϪ1).
X-Ray crystallography
Suitable crystals were grown by dissolving complex 1 in CH2Cl2
and by diffusion of diethyl ether into it. These orange crystals
were air stable. Diffraction data were obtained on a Rigaku
AFC7R diffractometer at ambient temperature. Crystal data
and details of measurements are summarized in Table 1.
CCDC reference number 186/833.
N
N
N
N
Cu
Preparation of the copper(I)–chiral bipyridine complex
OCH3
CH3O
OCH3
CH3O
Cl
Compound L (0.071 g, 0.2 mmol) in CH2Cl2 (2.5 cm3) was
added dropwise to a solution of copper() chloride (0.018 g, 0.2
mmol) in CH3CN (2.5 cm3). An intense red colour developed
instantaneously. The solution was refluxed overnight under N2
to ensure complete complexation. Solvent was removed and the
red solid obtained was washed with ether. It was then recrystal-
lized from CH2Cl2–hexane to give red crystals which were
filtered off and dried in vacuo. The complex was characterized
by elemental analysis (C, H, N), IR, UV and mass spectrometry.
Yield: 0.067 g (67%) (Found: 58.25; H, 6.67; N, 6.29. Calc. for
C22H32ClCuN2O2: C, 58.02; H, 7.08; N, 6.15%). IR(KBr):
3077.5s, 2969.8vs, 2862.1s, 2814.3s, 1593.5s, 1569.5s, 1473.8s,
1455.8s, 1431.9s, 1396.0s, 1354.1m, 1336.2m, 1258.4s, 1222.5m,
1196.6m, 1162.7s, 1090.9vs, 1043.0s, 1013.1s, 965.3s, 815.7s,
779.6s, 749.9w and 666.1m cmϪ1. Visible spectrum (CH2Cl2):
Cl
L
1
Experimental
Chloroform was distilled under N2 from calcium chloride.
Chiral compound L was prepared according to the literature.7
tert-Butyl diazoacetate was prepared by modification of a lit-
erature procedure.10 All other chemicals were of reagent-grade
quality used as commercially obtained. Proton and 13C NMR
spectra were recorded on a JEOL 270 FT spectrometer, IR
spectra on a Perkin-Elmer Model FTIR-1600 spectrometer and
UV spectra on a Perkin-Elmer Lambda 19 spectrometer. Elem-
† E-Mail: bhhoik@cityu.edu.hk
J. Chem. Soc., Dalton Trans., 1998, Pages 1043–1046
1043

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Table 1 Crystallographic data for complex 1
Formula
C22H32Cl2CuN2O2
M
490.96
Crystal color, habit
Crystal dimensions/mm
Crystal system
Space group
Lattice type
a/Å
Orange, prism
0.22 × 0.22 × 0.25
Monoclinic
C 2 (no. 5)
C-centered
14.192(3)
b/Å
c/Å
8.684(3)
10.311(3)
β/Њ
105.92(2)
1221.9(7)
U/Å3
Z
2
Dc/g cmϪ3
1.334
Fig. 1 Molecular structure of complex 1
Radiation (λ/Å)
µ(Mo-Kα)/cmϪ1
2θmax/Њ
F(000)
No. reflections measured
Mo-Kα (0.710 69)
11.31
60.0
Table 2 Selected bond lengths (Å), angles (Њ) and torsion angles (Њ) for
complex 1
514.0
total: 1959
unique: 1889 (Rint = 0.046)
0.045, 0.028
Cu᎐Cl(1)
Cu᎐Cl(1*)
2.186(3)
2.186(3)
Cu᎐N(1)
Cu᎐N(1*)
2.017(6)
2.017(6)
Residuals R, RЈ
Cl(1)᎐Cu᎐Cl(1)
Cl(1)᎐Cu᎐N(1)
100.6(2)
132.8(2)
Cl(1*)᎐Cu᎐N(1) 106.1(2)
N(1)᎐Cu᎐N(1*) 83.0(3)
λmax(ε) 336 (2247) and 462 nm (95) dm3 molϪ1cmϪ1. Positive ion
FAB mass spectrum: m/z 454 (Mϩ) and 419 (Mϩ Ϫ Cl).
Torsion angles
Cyclopropanations
Cl(1)᎐Cu᎐N(1)᎐C(1) Ϫ130.9(5)
Cl(1*)᎐Cu᎐N(1)᎐C(1) 107.2(5)
Using complex 1. To a two-neck round-bottom flask were
added complex 1 (0.039 g, 0.08 mmol) and silver() triflate
(0.041 g, 0.16 mmol) under nitrogen. Chloroform (3 cm3) was
added and the solution stirred at room temperature for 2 h. The
mixture was filtered through a packed filter-paper to a solution
of an alkene (32 mmol) in CHCl3 (3 cm3). A solution of ethyl
diazoacetate (0.901 g, 7.9 mmol) in CHCl3 (3 cm3) was slowly
added over a period of 4 h using a syringe pump. The mixture
was then stirred for 16 h at room temperature, worked up by
removing the solvent and the crude product obtained purified
by column chromatography (hexane–ethyl acetate). All the
cyclopropanes obtained are known compounds and were char-
acterized by 1H, 13C NMR, IR spectroscopy and GC–MS. The
enantiomeric excesses (e.e.s) of the cyclopropanes in entries
1–6, 9 and 11–15 in Table 3 were determined by capillary GC
with a Chiraldex β-PH column (30 m × 0.25 mm) and by HPLC
with a Daicel Chiralcel OJ column. For substrates in entries 7,
8, 10 and 16 the e.e.s were determined as previously.4a Dia-
stereoselectivities (cis/trans ratio) were measured by GC with
an Ultra 2 cross-linked 5% phenyl methyl silicone column
(25 m × 0.2 mm × 0.33 µm).
bipyridine are slightly longer than such distances for other
achiral bipyridine complexes.13 The bite angle of 83.0Њ made by
the chiral bipyridine and copper is considered large when com-
pared with an average value of 77Њ cited in the literature.13 The
copper–chlorine bond distances are 2.19 Å. The dihedral angles
Cl(1)᎐Cu᎐N(1)᎐C(1) and Cl(1)᎐Cu᎐N(1*)᎐C(1*) of Ϫ130.9
and 107.2Њ clearly illustrate the distorted nature of the copper
centre which can be attributed to the bulkiness of L. The bulki-
ness of this ligand is also reflected by the fact that formation of
a 2:1 ligand to copper complex is not possible even with an
excess of L and prolonged heating whereas other bipyridines
form 2:1 ligand to copper complexes fairly easily.
The solution UV/VIS spectrum provided further evidence
that the distorted geometry comes from the bulkiness of the
ligands. Previous work has been done on relating the energies of
the copper() d–d transition to the geometries.14 For example
Cu(bipyam)Cl2 [bipyam = bis(2-pyridyl)amine], an analogous
copper() complex, has a typical green color and a peak in the
electronic spectrum at about 700 nm. The unusual color of
orange and the d–d transition of the present chiral copper()
complex at 919 nm in CH2Cl2 are consistent with a pseudo-
tetrahedral structure.
These results suggested that the distorted geometry observed
in the solid-state structure is retained in solution. Compound
L also forms a complex with CuICl, assigned the formula
Cu(bipy*)Cl, which is co-ordinatively unsaturated, based on
elemental and mass spectral analyses. No complex of the form
CuL2Cl was obtained even with the use of an excess of L and
prolonged heating.
The chloride complex 1 is not an active catalyst in cyclo-
propanation. However, the corresponding triflate complex,
formed by treating 1 with silver triflate, was active. The poten-
tial of this copper complex as a catalyst for asymmetric cyclo-
propanation was illustrated by the reaction of various alkenes
with diazoacetate (Table 3). The cyclopropanation of styrene by
ethyl diazoacetate in the presence of 1 mol % of copper catalyst
gave ethyl cyclopropanecarboxylates as the only products in
48% isolated yield. The GC analysis of the reaction mixture
indicated a 80:20 trans:cis ratio and no diethyl fumarate and
maleate were formed. The enantioselectivity was found to be
88% e.e. for the trans and 72% for the cis isomer. Alternatively,
Using the copper(I) complex. The complex was generated by
stirring copper() triflate and L for 2 h. All other steps were
similar to those with complex 1. For cyclopropane tert-butyl
diazoacetate was used instead of ethyl diazoacetate.
Results and Discussion
Although there are large numbers of copper–bipyridine com-
plexes in the literature,11 no crystallographically characterized
chiral bipyridine complex of copper of type Cu(bipy*)X2 has
been reported. The stereochemistries of CuII(bipy)nX2 (n = 1 or
2) type complexes are dominated by five- or six-co-ordinate
geometries.11 Four-co-ordinate copper() complexes only occur
with two chelating bidentate ligands and non-bridging anions.12
The chiral bipyridine L and copper() chloride form a complex
in dichloromethane, which was isolated, as an orange solid in
good yield.
The crystal structure, shown in Fig. 1, shows a highly dis-
torted copper() ion. Table 2 summarizes the structural param-
eters. The complex is C2- symmetric. The copper–nitrogen bond
distances of 2.02 Å for both nitrogen atoms of the chiral
1044
J. Chem. Soc., Dalton Trans., 1998, Pages 1043–1046

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Table 3 Catalytic asymmetric cyclopropanation*
% e.e.
Entry
Substrate
Product
trans:cis
trans
cis
Yield (%)
(a) Copper() complex
CO2Et
CO2Et
1
2
3
79:21
81:19
80:20
88
86
91
72
69
82
48
46
52
4
84:16
80:20
89
88
84
83
61
63
Cl
Cl
Me
5
CO2Et
CO2Et
Me
6
75:25
77
63
60
MeO
MeO
Me
Me
7
8
96:4
47
49
32
42
60
57
Ph
Ph
CO2Et
CO2Et
43:57
9
—
56
42
50
65
72
78
75
83
Ph
Ph
CO2Et
CO2Et
10
73:27
80:20
83:17
79:21
75:25
90:10
76
89
87
88
77
92
83
74
83
83
63
71
(b) Copper() complex
11
CO2Et
CO2Et
12
Cl
Cl
Me
13
CO2Et
CO2Et
Me
14
MeO
MeO
15
16
CO2But
—
87
56
Ph
Ph
CO2But
* Ethyl diazoacetate (0.2 equivalent) was used for reduction of the copper() catalyst before the start of cyclopropanation.
J. Chem. Soc., Dalton Trans., 1998, Pages 1043–1046
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the catalyst can be generated by stirring free L with copper()
triflate for 2 h (entry 2). However, a slight decrease in enantio-
selectivity was observed. Lowering the reaction temperature
from room temperature to 0 ЊC did not hamper the reaction but
increased the enantioselectivity and isolated yield (entry 3),
indicating that this catalyst is highly effective. For substituted
styrenes the trans:cis ratios were between 84:16 and 75:25
(entries 4–6) and the enantioselectivity between 89 and 77% e.e.
for the trans isomer and 84 and 63% e.e. for the cis isomer.
Styrenes with electron-withdrawing groups showed higher
enantioselectivity and trans–cis selectivity than those with
electron-donating groups. In the case of Katsuki’s chiral bipy-
ridine,6 styrene derivatives having electron-withdrawing groups
showed higher enantioselectivity but lower trans:cis ratio than
those with electron-donating groups. In fact, the trans:cis ratio
provided by L is the highest among copper catalysts for cyclo-
propanation. This may come from the unusual bulkiness of L.
Cyclopropanation of cis-β-methylstyrene showed high trans
selectivity but the enantioselectivities for the trans and cis
isomers were only moderate (entry 7). In contrast, trans-β-
methylstyrene showed reversed cis selectivity and the enan-
tiomeric excesses for both trans and cis isomers were again
moderate (entry 8). The copper complex 1 was also found to be
an efficient and highly enantioselective catalyst for 1,1-diphenyl-
ethene and hex-1-ene. 1,1-Diphenylethene reacted with 83% e.e.
(entry 9) and hex-1-ene, which has an isolated, non-activated
double bond, reacted to give a 73:27 trans:cis ratio and 76 and
83% e.e. for trans- and cis-cyclopropane respectively (entry 10).
Although the oxidation state of the copper catalyst in cyclo-
propanation has been the subject of some debate, it is generally
believed that the copper() complex is the catalytically active
form.4 In order to determine the actual catalyst involved in the
present case, the copper()–bipyridine complex was prepared
and used in cyclopropanation (Table 3, entries 11–16). The
results obtained were almost identical to those with copper(),
only with better yield. Thus, we believe the active catalyst is
copper(). However, unlike the previously reported chiral N᎐N
bidentate ligands where the copper() complexes were either
inactive or required activation by heating, stirring of ethyl
diazoacetate with this copper() complex at room temperature
reduced the copper() to active copper() catalyst. Thus, this
copper()–bipyridine complex, which is air stable, is a very
convenient precursor to the active copper() catalyst.
Employing L with either CuI or CuII, the absolute configur-
ations of cyclopropane ester products formed from styrene
were determined to be (1R,2R) and (1R,2S) for the trans and
cis isomers respectively. Since L can only form a 1:1 complex
with CuI, we believe that the catalytically active complex in the
cyclopropanation is the co-ordinatively unsaturated three-
coordinated CuIL(O3SCF3) complex. Although the structure of
the active intermediate in the carbene transfer is still unknown,
the sense of asymmetric induction observed here was the
same as those for other chiral (N᎐N-bidentate ligand) copper
catalysts reported previously. Hence, it is consistent with the
model cited by Pfaltz and co-workers.4a
Acknowledgements
Financial support for this research from the Hong Kong
Research Council and City University of Hong Kong is grate-
fully acknowledged.
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Received 17th September 1997; Paper 7/06754B
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J. Chem. Soc., Dalton Trans., 1998, Pages 1043–1046