Enantioselective catalytic cyclopropanation of styrenes by copper complexes with chiral pinene-[5,6]-bipyridine ligands

Article abstract of DOI:10.1016/S0957-4166(00)00401-8

Substituted mono- and bis-pinene-[5,6]-bipyridines are useful ligands in asymmetric copper-catalyzed cyclopropanation by styrenes. Copper complexes were prepared either in situ or prior to the reaction. The catalytic reaction of styrene with ethyl diazoacetate and Cu-11b yields ethyl trans-phenylcyclopropane carboxylate in >99% yield and 87% e.e. at 0°C. The corresponding cis-configured cyclopropane was produced with an e.e. of 90%. The cis/trans ratio is 22:78. Other ligands of this series are less effective. Various olefins were tested as substrates but exo-methylene olefins show the best results. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00401-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4341–4357
Enantioselective catalytic cyclopropanation of styrenes by
copper complexes with chiral pinene-[5,6]-bipyridine ligands
a
a
b
a,
Didier L o¨ tscher, Stefan Rupprecht, Helen Stoeckli-Evans and Alex von Zelewsky *
a
Institute for Inorganic and Analytical Chemistry, University of Fribourg, P e´ rolles, CH-1700 Fribourg, Switzerland
b
Universit e´ de Neuch aˆ tel, Institute of Chemistry, Av. Bellevaux 51, CH-2000 Neuch aˆ tel, Switzerland
Received 14 September 2000; accepted 6 October 2000
Abstract
Substituted mono- and bis-pinene-[5,6]-bipyridines are useful ligands in asymmetric copper-catalyzed
cyclopropanation by styrenes. Copper complexes were prepared either in situ or prior to the reaction. The
catalytic reaction of styrene with ethyl diazoacetate and Cu–11b yields ethyl trans-phenylcyclopropane
carboxylate in >99% yield and 87% e.e. at 0°C. The corresponding cis-configured cyclopropane was
produced with an e.e. of 90%. The cis/trans ratio is 22:78. Other ligands of this series are less effective.
Various olefins were tested as substrates but exo-methylene olefins show the best results. © 2000 Elsevier
Science Ltd. All rights reserved.
1
. Introduction
In contrast to the earlier introduced catalytic systems containing phosphorus or oxygen donor
ligands, such as hydrogenation catalysts, nitrogen donor ligands have been studied in detail only
1
more recently. They have a wide potential in enantioselective catalysis, especially in zinc-medi-
2+
ated alkylations, where N,O-donor ligands are favored, due to the relative hardness of the Zn
ion.
2
–4
The first example of an enantioselective catalytic cyclopropanation dates back to Nozaki in
5
1
966, but shows only poor e.e. The potential of N,N-donor ligands was evaluated by the groups
of Evans and Pfaltz. Both published remarkable e.e. values in cyclopropanation reactions with
6
7
copper complexes of bis-oxazolines 1–3 (Scheme 1) or copper semicorrin complexes 4 and 5
8
(
Scheme 1). Similar work was done by the groups of Masamune with bis-oxazolines 6 (Scheme
9
,10
1
) and Katsuki,
who used chiral bipyridine derivatives (molecules 7 and 8) (Scheme 1).
Synthesis of 7 and 8 requires preparative HPLC separation of a racemic mixture of an
1
1
intermediate. Recently Kwong and Lee reported successful enantioselective cyclopropana-
*
Corresponding author. Tel: +41-26-300-87-32; fax: +41-26-300-97-38; e-mail: alexander.vonzelewsky@unifr.ch
0
957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00401-8

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D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Scheme 1. Ligands and complexes used for asymmetric cyclopropanations by other research groups
12
tions with chiral C -symmetric terpyridine ligands first synthesized in our laboratories. In
2
Kwong’s work, the pinene moiety of the terpyridine was substituted with several alkyl groups.
The best result for cyclopropanation of styrene with ethyl diazoacetate was shown by copper
complex 9 (94% e.e.) (Scheme 1).
We present herein our results from copper-mediated enantioselective cyclopropanations with
(
(
−)-pinene-[5,6]-bipyridine ligands of the type 10, 11, 12 and 13 and their copper complexes
Scheme 2).
2
. Results and discussion
2
.1. Ligand syntheses
13
The synthesis of the ligand 10 (R=H) has been published earlier; the synthesis of the
alkylated species 10a–c was achieved by lithiation of 10 and quenching with the proper alkyl
1
4
iodide (Scheme 3). Synthesis of the C -symmetric bipyridine 11 (R=R%=H) was first carried
2
1
5
out in our laboratories. Alkylation of this ligand was done in a similar way using LDA and
up to 3 equiv. of the appropriate alkyl iodide to yield either C - or C -symmetric bipyridines
1
2
1
1a–k depending on the number of equivalents of LDA (Scheme 3). The synthesis of the

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4343
Scheme 2.
Scheme 3. Alkylation of bipyridines 10 and 11
C -symmetric bipyridine 12 was carried out in the way shown in Scheme 4, starting from
1
2
-acetyl-6-phenylpyridine 14. Formation of the Kr o¨ hnke salt 15 using pyridine and iodine was
almost quantitative, however the formation of pyridinium iodide 16 could not be inhibited.
16
Annellation of the Kr o¨ hnke salt 15 with (+)-pinocarvone 17 yielded the desired bipyridine 12
in 40% yield. Ligand 12a was then synthesized in 84% yield by lithiation of 12 using LDA and
ethyliodide as the alkylating agent.
Ligand 13 was prepared according to a published procedure of our laboratories, starting from
1
7
(
−)-b-pinene.
There are two different possibilities for carrying out catalytic cyclopropanations. The first
method is a one-pot synthesis using either copper(I) trifluoromethanesulfonate CuOTf·0.5C H6
6

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D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Scheme 4. Synthesis of bipyridines 12 and 12a
or Cu(II)OTf as the copper source. The active species is formed in situ by reduction. This
method is reported in most of the publications wherein cyclopropanation reactions are studied.
18
The second method is described by Kwong et al. In this method, the copper(II)–dichloro
complex is synthesized prior to the catalytic cyclopropanation reaction. This complex is then
transferred into the ditriflate and reduced in situ by the cyclopropanation agent (Scheme 5).
2
.2. Complex syntheses
Copper(II) complexes were synthesized using CuCl ·2H O and the corresponding ligands.
2
2
CH Cl /EtOH
2
2
CuCl ·2H O+L ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢁ CuLCl +2H O
2
2
2
2
rt, reflux
The chiral bipyridine ligands 11, 11a–c, and 13 used for this purpose form complexes, soluble
in dichloromethane, which were isolated as brown to red solids in good yields. The results of
these complex syntheses are shown in Table 1.

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4345
Scheme 5. Two different ways to carry out catalytic cyclopropanations
Crystals suitable for X-ray diffraction could be obtained for complexes Cu–11, Cu–11a, and
Cu–13 from dichloromethane/diethyl ether solution. The molecular structure and crystallo-
20
graphic numbering scheme is illustrated in the PLATON drawings. (Figs. 1–3).
The copper center is coordinated in a distorted tetrahedral environment. Table 2 summarizes
the structural parameters and Table 3 shows selected bond angles and torsion angles for the
complexes. In the solid state, all of these complexes are C -symmetric. A remarkable property
1
of complex Cu–13 is seen in Fig. 2. The two chloride ligands attached to the copper(II) ion are
pointing towards the bulky methyl groups of the pinene moiety (carbons C12/C13 and
C32/C33), while in complex Cu–11, this is not the case. Here, the chlorides of the copper ion are
going in the opposite direction to the bulky methyl groups.
2
.3. Enantioselective cyclopropanations
The chloride complexes mentioned above are not active catalysts in cyclopropanation.
However, the corresponding triflate complexes, formed by treating the chloride complexes with
silver triflate and reduction in situ with ethyl diazoacetate, were active. The potential of these
ligands and copper complexes was tested with styrene as substrate using both methods for the
catalytic reaction (Scheme 6). The reaction of ethyl diazoacetate leads to the formation of all
four possible cyclopropanes 18. The results are shown in Tables 4 and 5.
The results of the catalytic cyclopropanations with copper complexes of ligands 10a–c show
increasing e.e. values with increasing steric demand of the alkyl group. With R=iso-propyl an
e.e. of 58% for the trans-cyclopropane was achieved. The best results with up to 84% e.e. for the
C -symmetrical ligands 11a–f were obtained with small substituents such as methyl 11a and ethyl
2

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D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Table 1
Synthesis of Cu(II) complexes
1
1b. However, more bulky substituents reduce the e.e. significantly or show no e.e. at all (11d),
but still give up to 80% yield. Either there is no complexation of copper(II) triflate by the ligands
1c and 11d or the two pyridine rings complex the copper cation differently, e.g. in an
1
9
intermolecular way. This differs from the results published by Katsuki, who shows that ligand
systems 7 and 8 (Scheme 1) produce the best results with rather bulky triethylsilyl groups. In our
case, the trimethylsilyl substituted ligand 11f shows already reduced e.e. values compared to
ligand 11b. The diastereoselectivity ranges from 45:55 to 27:73. The best result was also achieved
with ligand 11b.
The mono-alkylated bis-pinene-[5,6]-bipyridines 11g–k show similar results. The most efficient
ligand is the mono-ethylated ligand 11h with an e.e. of 71% for trans-cyclopropane. The other

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4347
Figure 1. ORTEP plot of Cu–11
Figure 2. ORTEP plot of Cu–11a
Figure 3. ORTEP plot of Cu–13
derivatives show lower e.e. values between 40 and 60%. It seems that the lower spatial
requirements of the mono-alkylated ligands compared to the bis-alkylated ligands allow the
formation of the copper complex. However, the substrates can bind more flexibly to the catalytic
center, thus lowering the enantioselectivity and diastereoselectivity (Table 6). The unsubstituted
ligands 10 and 13 induce almost no e.e. in the catalytic reaction. This shows clearly that the
formation of a stereogenic center on C(7) and/or C(7%) is crucial for the induction of chirality to
the cyclopropanes. The reactions of ligand 11b with different olefins are shown in Table 7. The
ligand system works best on cyclopropanation of exo-methylene double bonds. Cis-substituted

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D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Table 2
Crystallographic data for complexes Cu–11, Cu–11a, and Cu–13
Cu–11a
Cu–11b
Cu–13
Empirical formula
Formula weight (g mol
Crystal color, habit
Crystal size (mm)
Crystal system
C H Cl CuN ·CH Cl C H Cl CuN ·0.4(CH Cl ) C H Cl CuN ·CH Cl
24 28 2 2 2 2 26 32 2 2 2 2 24 28 2 2 2 2
−
1
)
563.85
Red, block
540.95
Purple, plate
563.85
Red, block
0.34×0.34×0.19
Monoclinic
P21
0.50×0.30×0.15
Orthorhombic
P2 2 2
0.40×0.25×0.20
Orthorhombic
P2 2 2
Space group
1
1
1
1 1 1
a (A
,
)
)
)
8.6296(5)
15.4946(12)
10.4406(7)
90
109.470(10)
90
1316.20(15)
2
1.423
12.2293(7)
14.048(7)
16.5056(13)
90
90
90
2835.5(14)
4
1.267
1.050
1127
10.3549(7)
13.6271(9)
17.9559(14)
90
90
90
2533.7(3)
4
1.478
1.300
1164
b (A
,
c (A
,
h (°)
i (°)
g (°)
3
V (A
,
)
Z
−
3
Dcalc (g cm
Absorption coefficient (mm
F (000)
)
−
1
)
1.252
582
q min. and max. (°)
Reflections collected/unique
Refinement method
2.07–25.49
5102/2551
Full-matrix
least-squares on F
2.07–25.91
13867/5284
Full-matrix
least-squares on F
2.27–26.03
19903/4924
Full-matrix
least-squares on F
2
2
2
Final R indices [I\2|(I)]
R indices all data
R =0.0468,
R =0.0576,
R =0.0357,
1
1
1
wR =0.0931
wR =0.1679
wR =0.0856
2
2
2
R =0.0696,
1
R =0.0662,
1
R =0.0450,
1
wR =0.1037
wR =0.1767
wR =0.0889
2
2
2
Absolute structure parameter
0.00(3)
0.01(2)
0.000(14)
Largest difference peak and hole 0.345 and −0.424
1.823 and −0.471
1.176 and −0.709
−
3
(
e A
,
)
alkenes show moderate e.e. values, while in trans-configured alkenes the enantioselectivity is
further reduced.
If we compare the catalytic reactions of the one-pot synthesis (method 1) with method 2 where
the pre-catalysts are formed prior to the reaction, one can see, that method 2 gave slightly better
results in enantioselectivity and in diastereoselectivity. This could be explained by the fact, that
the copperꢀnitrogen bonds—necessary for the formation of the active species—are already
formed. Decrease of the reaction temperature to 0°C has only a slight influence on enantio- and
diastereoselectivity for cyclopropanation.
Based on the absolute configuration of the products obtained, the sense of asymmetric
induction observed here can be explained by the model shown in Scheme 7 and first proposed
7
by Pfaltz et al. In the proposed mechanism, the metal carbenoid attacks the olefinic double
bond according to the pathways a and b. In the case of pathway a, repulsive steric interaction
is built up between the ester group and the adjacent bulky alkyl group at C(7) of the ligand. In
the case of pathway b, no such steric interaction, leading to the cis-(1R)- or to the trans-(1R)-
cyclopropyl esters, is consistent with the experimental results. Therefore, pathway a is more

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4349
Table 3
Selected bond lengths (A), angles (°), and torsion angles (°) for Cu–11, Cu–11a, and Cu–13
,
Cu–11a
Cu–11b
Cu–13
Cu(1)ꢀN(2)
Cu(1)ꢀN(1)
Cu(1)ꢀCl(1)
Cu(1)ꢀCl(2)
2.002(5)Cu(1)ꢀN(1)
2.036(5)Cu(1)ꢀN(21)
2.197(2)Cu(1)ꢀCl(1)
2.245(2)Cu(1)ꢀCl(2)
2.002(3) Cu(1)ꢀN(2)
2.023(3) Cu(1)ꢀN(1
2.2220(10)Cu(1)ꢀCl(1)
2.2271(9) Cu(1)ꢀCl(2)
2.006(4)
2.036(4)
2.2110(15)
2.2492(15)
N(2)ꢀCu(1)ꢀN(1)
N(2)ꢀCu(1)ꢀCl(1)
N(1)ꢀCu(1)ꢀCl(1)
N(2)ꢀCu(1)ꢀCl(2)
N(1)ꢀCu(1)ꢀCl(2)
Cl(1)ꢀCu(1)ꢀCl(2)
82.7(2) N(1)ꢀCu(1)ꢀN(21)
139.03(18)N(1)ꢀCu(1)ꢀCl(1)
107.70(15)N(21)ꢀCu(1)ꢀCl(1)
100.81(17)N(1)ꢀCu(1)ꢀCl(2)
130.44(17)N(21)ꢀCu(1)ꢀCl(2)
101.22(8) Cl(1)ꢀCu(1)ꢀCl(2)
82.35(12) N(2)ꢀCu(1)ꢀN(1)
84.09(17)
137.74(12)
104.98(13)
100.00(14)
128.90(13)
104.94(6)
101.76(9)
136.40(9)
133.03(8)
104.12(8)
103.82(4)
N(2)ꢀCu(1)ꢀCl(1)
N(1)ꢀCu(1)ꢀCl(1)
N(2)ꢀCu(1)ꢀCl(2)
N(1)ꢀCu(1)ꢀCl(2)
Cl(1)ꢀCu(1)ꢀCl(2)
N(2)ꢀCu(1)ꢀN(1)ꢀC(9) −179.7(6) N(21)ꢀCu(1)ꢀN(1)ꢀC(6)
Cl(1)ꢀCu(1)ꢀN(1)ꢀC(9) −40.4(6) Cl(1)ꢀCu(1)ꢀN(1)ꢀC(6)
174.0(3)
−50.1(3)
71.8(3)
−5.6(2)
130.3(2)
N(2)ꢀCu(1)ꢀN(1)ꢀC(10) 178.0(5)
Cl(1)ꢀCu(1)ꢀN(1)ꢀC(10) 40.0(5)
Cl(2)ꢀCu(1)ꢀN(1)ꢀC(10) −83.9(5)
N(2)ꢀCu(1)ꢀN(1)ꢀC(2) 0.7(3)
Cl(1)ꢀCu(1)ꢀN(1)ꢀC(2) −137.3(3)
Cl(2)ꢀCu(1)ꢀN(1)ꢀC(2) 98.7(3)
Cl(2)ꢀCu(1)ꢀN(1)ꢀC(9)
N(2)ꢀCu(1)ꢀN(1)ꢀC(1)
Cl(1)ꢀCu(1)ꢀN(1)ꢀC(1)
82.3(6) Cl(2)ꢀCu(1)ꢀN(1)ꢀC(6)
4.5(5) N(21)ꢀCu(1)ꢀN(1)ꢀC(2)
143.8(4) Cl(1)ꢀCu(1)ꢀN(1)ꢀC(2)
Cl(2)ꢀCu(1)ꢀN(1)ꢀC(1) −93.5(5) Cl(2)ꢀCu(1)ꢀN(1)ꢀC(2) −107.8(2)
N(1)ꢀCu(1)ꢀN(2)ꢀC(21) 174.2(6) Cu(1)ꢀN(1)ꢀC(2)ꢀC(3) −176.3(3)
N(1)ꢀCu(1)ꢀN(2)ꢀC(30) −177.6(4)
Cl(1)ꢀCu(1)ꢀN(2)ꢀC(30) −71.8(5)
Cl(1)ꢀCu(1)ꢀN(2)ꢀC(21)
65.6(7) Cu(1)ꢀN(1)ꢀC(6)ꢀC(5)
176.4(3)
−5.7(5)
Cl(2)ꢀCu(1)ꢀN(2)ꢀC(21) −55.9(6) Cu(1)ꢀN(1)ꢀC(6)ꢀC(10)
Cl(2)ꢀCu(1)ꢀN(2)ꢀC(30)
N(1)ꢀCu(1)ꢀN(2)ꢀC(22)
53.9(4)
4.7(3)
N(1)ꢀCu(1)ꢀN(2)ꢀC(13) −5.2(5) N(1)ꢀCu(1)ꢀN(21)ꢀC(26) −174.7(3)
Cl(1)ꢀCu(1)ꢀN(2)ꢀC(13)−113.9(5) Cl(1)ꢀCu(1)ꢀN(21)ꢀC(26) 86.2(3)
Cl(2)ꢀCu(1)ꢀN(2)ꢀC(13) 124.7(5) Cl(2)ꢀCu(1)ꢀN(21)ꢀC(26) −42.2(3)
Cl(1)ꢀCu(1)ꢀN(2)ꢀC(22) 110.5(3)
Cl(2)ꢀCu(1)ꢀN(2)ꢀC(22)−123.8(3)
Scheme 6. Catalytic cyclopropanation of styrene with ethyl diazoacetate
favored than pathway b. This explains the enantioselectivity. The diastereoselectivity could also
be explained in a similar way (Scheme 8). If the mechanism follows pathway c, that means a
cisoid position of the R group on the olefin according to the ester group, the steric hindrance
1
between these two groups is very high. Therefore the approach shown in pathway d will be
preferred. This is again consistent with our experimental results.

4350
D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Table 4
Cyclopropanation of styrene with ethyl diazoacetate and Cu(I) catalysts of (−)-mono-pinene-[5,6]-bipyridine
a
(
one-pot method)
Catalyst
Yield (%)
D.e. (cis/trans)
E.e (cis) (%)
E.e. (trans) (%)
1
1
1
0a R=Methyl
0b R=Ethyl
0c R=i-Propyl
73
83
39
36:64
37:63
37:63
20
45
51
31
55
58
a
Reaction conditions: 1 mol% catalyst; 3 equiv. styrene; methylene chloride at rt for 7 h. All d.e. and e.e. values
were determined via chiral GC. Ligands 10a–c all 92% (e.e.).
Table 5
Cyclopropanation of styrene with ethyl diazoacetate and Cu(I) catalyst of (−)-bis-pinene-[5,6]-bipyridine (one-
a
pot method)
Catalyst
Yield (%)
D.e. (cis/trans)
E.e. (cis) (%)
E.e. (trans) (%)
1
1
1
1
1
1
1: R=R%=H
80
78
78
91
77
85
33:67
40:60
33:67
40:60
42:58
36:64
3
48
64
43
50
42
4
54
71
42
60
55
1g: R=H; R%=Me
1h: R=H; R%=Et
1i: R=H; R%=i-Pr
1j: R=H; R%=Bn
1k: R=H; R%=TMS
11a: R=R%=Me
11b: R=R%=Et
11c: R=R%=Pr
11d: R=R%=IꢀPr
11e: R=R%=Bn
11g: R=R%=TMS
13
93
94
80
78
79
71
38:62
27:73
43:57
37:63
45:55
42:58
36:64
78
83
1
B1
40
31
79
84
16
B2
18
41
8
c
c
b
74
5.5
a
Reaction conditions: 1 mol% catalyst; 3 equiv. styrene; methylene chloride at rt for 7 h; all d.e. and e.e. values
were determined via chiral GC; d.e. values of ligands 11a–k: 98% (11b, 11c, 11d, 11e, 11h, 11i, 11j); 92% (11a, 11g),
8
4% (11f, 11k).
b
Not isolated.
c
Other enantiomer was formed; absolute configuration: trans-(1S,2S)-1-phenylcyclopropane carboxylate, cis-
(
1S,2R)-phenylcyclopropane carboxylate.
3
. Conclusions
The ligand systems and the complexes derived from enantiopure natural compounds, which
are described in this paper are able to produce cyclopropanes with moderate to good enantio-
selectivities and diastereoselectivities. The ligand 11b and especially complex Cu–11b shows by
far the best results with respect to enantioselectivity and diastereoselectivity. Further investiga-
tions in this subject are in progress.

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4351
Table 6
Cyclopropanantion of styrene with ethyl diazoacetate and copper complexes Cu–11, Cu–11a, Cu–11b, Cu–12a,
a
and Cu–13
Catalyst
Yield (%)
D.e. (cis/trans)
E.e. (cis) (%)
E.e. (trans) (%)
b
Cu–11
52
34:66
37:63
39:61
23:77
22:78
37:63
39:61
3
85
88
85
90
9
3
81
87
84
87
17
8
b
Cu–11a at rt
Cu–11a at 0°C
Cu–11b at rt
Cu–11b at 0°C
Cu–12a
72
b
77
77
b
b
\99
b
86
b
Cu–13
72
5.5
a
Reaction conditions are according to Kwong et al.: 2 equiv. of AgOTf according to the complex (1 mol%), styrene
equiv. according to ethyl diazoacetate.
Not isolated.
4
b
Table 7
Cyclopropanation of different olefins with ethyl diazoacetate and Cu(I) catalyst 11b (one-pot method)
Substrate
Yield (%)
D.e. (cis/trans)
E.e. (cis) (%)
E.e. (trans) (%)
Styrene
94
64
75
91
37:63
34:66
45:55
83
75
46
52
84
79
11
a
a
a-Methyl styrene
b-Methyl styrene
Dihydronaphthalene
a
a
a
a
Absolute configuration not determined.
Scheme 7. Explanation of the enantioselectivity

4352
D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
Scheme 8. Explanation of the diastereoselectivity
4
. Experimental
4
.1. Products
Solvents and reagents were purchased from Fluka or Aldrich. Iodine was sublimed before
utilization. Pyridine was dried over KOH and freshly distilled prior to use. Ammonium acetate
was dried for 5–6 h before use in a vacuum oven at 40°C. Diethyl ether was distilled from
sodium/benzophenone prior to use. Dichloromethane was dried over CaH and distilled prior to
2
use.
4
.2. Measurements
NMR spectra were recorded on a ‘Varian Gemini 300’ (300.075 MHz) or on a ‘Bruker
Avance DRX500’ (500.13 MHz) spectrometer, chemical shifts are given in ppm using TMS or
the solvent itself as internal standards, coupling constants J are given in hertz. Attribution of the
1
13
H and C signals was performed by COSY, DEPT, and HECTOR techniques. Diastereotopic
protons are labeled as H for the endo oriented protons and H for the exo oriented protons.
a
b
The exo methyl groups of the pinene moieties are assigned as 12 and 12%, the endo oriented ones
as 13 and 13%, respectively. The numbering scheme for the ligands is shown in Scheme 9.
UV–vis spectra were measured on a Perkin–Elmer Lambda 40 spectrometer. Mass spectral
data were acquired on ‘VG Instrument 7070E’ equipped with a FAB inlet system. Elemental
analysis were obtained from EIF (Ecole d%ing e´ nieurs de Fribourg, Switzerland). CD spectra were
measured on a ‘Jasco J-715’ spectropolarimeter.
X-Ray diffraction: Intensity data were collected at room temperature on a Stoe AED2 4-circle
diffractometer using Mo Ka graphite-monochromated radiation (u=0.71073 A
,
) with ꢀ–2[
scans in the 2[ range 4–51°. The structure was solved by direct methods using the program

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4353
Scheme 9. Numbering scheme for the ligands
1
9
SHELXS-97. The refinement and all further calculations were carried out using SHELXL-97.
H atoms were included in calculated positions and treated as riding atoms using SHELXL-97
default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix
2
least-squares on F . The coordinates correspond to the absolute structure of the molecule in the
crystals.
Chiral GC: Hewlett–Packard 5890 Series II gas chromatograph and Supelco Beta-Dex 110
column. Toven=120°C; (1S,2R)-18: t 46.4 min, (1R,2S)-18: t 47.7 min, (1R,2R)-18: t 55.3
R
R
R
min, (1S,2S)-18: t 56.2 min.
R
4
.3. Synthesis of the ligands
13
The synthesis of the ligand 10 (R=H) has been published earlier. The synthesis of the
alkylated species 10a–c was achieved by lithiation of 10 followed by quenching with the
1
4
respective alkyl iodide. Synthesis of the C -symmetric bipyridine 11 (R=R%=H) was first
2
1
5
carried out in our laboratories. Alkylation of this ligand was done in a similar way using LDA
and up to 3 equiv. of the appropriate alkyl iodide to yield either C - or C -symmetric bipyridines
1
2
1
1a–m depending on the number of equivalents of LDA and alkylating agent. Synthesis of
ligand 12 started from 2,6-dibromopyridine, which is commercially available.
4
.3.1. 2-Acetyl-6-bromopyridine
3.69 g (0.1 mol, 1.0 equiv.) of 2,6-dibromopyridine were solubilized in 300 ml of dry diethyl
2
ether. At −60°C, 77 ml (0.1 mol, 1.0 equiv.) of n-BuLi (1.3 M in hexane) were added within 30
min. The resulting yellow solution was stirred for 30 min at −40°C and 13.9 ml of dimethylac-
etamide (freshly distilled from BaO) were added at −60°C. The solution was stirred and heated
to room temperature. The resulting yellow mixture was hydrolyzed by 100 ml of saturated
NH Cl. Extraction of the aqueous solution by diethyl ether, drying over Na SO , and evapora-
4
2
4
tion of the solvent gave 37.67 g of a yellow to brown solid. Recrystallization in diethyl
1
ether/pentane afforded 11.60 g of a yellow solid (yield 58%). H NMR (300 MHz, CDCl ): l
3

4354
D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
3
3
13
7
.95 (dd, J=6.9 and 6.8 Hz, 1H); 7.65 (2d, J=6.9 and 7.0 Hz, 2H); 2.69 (s, 3H). C NMR
(
75 MHz, CDCl ): l 198.6 (C), 154.3 (C, 1C), 141.4 (C, 1C), 139.2 (CH, 1C), 131.8 (CH, 1C),
3
1
4
20.5 (CH, 1C), 25.8 (CH , 1C). MS-EI: 201 (31), 199 (30), 173 (36), 78 (88), 76 (100).
3
.3.2. 2-Acetyl-6-phenylpyridine
7
.75 g (38.74 mmol, 1.0 equiv.) of 2-acetyl-6-bromopyridine were dissolved in dry xylene (75
ml). Phenylboronic acid (7.09 g, 58.11 mmol, 1.5 equiv.) was added and the resulting suspension
was degassed three times. Tetrakis(trisphenylphosphine)palladium (5% mol, freshly purified) and
potassium carbonate (10.71 g, 77.48 mmol, 2.0 equiv.) were added. The solution was degassed
again three times and stirred for 2 h at room temperature. Water (500 ml) was added and the
mixture was extracted by dichloromethane (500 ml). The resulting organic phases were dried
over sodium sulfate and solvents were evaporated to afford after recrystallization in hexane/
1
ethyl acetate 6.95 of the desired compound (91% yield). H NMR (300 MHz, CDCl ): l 8.03
3
13
(
dd, 2H), 7.91–7.8 (m, 3H), 7.44–7.38 (m, 3H), 2.75 (s, 3H). C NMR (75 MHz, CDCl ): l
3
2
00.9 (C, 1C), 156.9 (C, 1C), 153.8 (C, 1C), 138.8 (C, 1C), 138.0 (C, 1C), 129.8 (C, 1C), 129.2
(
CH, 2C), 127.3 (CH, 2C), 123.8 (CH, C), 120.2 (CH, 1C), 26.1 (CH ), 1C). MS-EI: 197 (100),
3
1
54 (80), 127 (25), 77 (15).
4
.3.3. 1-(2-Acetyl-6-phenylpyridine)pyridinium iodide 15
Under an argon atmosphere, in 25 ml one-necked round-bottomed flask, equipped with a
magnetic stirring bar and reflux condenser, 3.0 ml of pyridine (distilled over KOH, 14.6 equiv.)
was added to 634 mg of iodine (2.53 mmol, 1.0 equiv.) and 500 mg of 2-acetyl-6-phenylpyridine
(
2
2.53 mmol, 1.0 equiv.). The resulting dark solution was kept at 130°C for 3 h and at 0°C for
0 h. The pyridine was then removed by distillation in vacuo to afford a black solid. After
addition of dry ether, stirring, filtration under an argon atmosphere, and drying, 1.45 g of a
1
black solid was obtained. H NMR determination of the two most shifted doublet signals gave
a ratio of 2:1 between the desired product 15 and pyridinium iodide 16 (100% yield). This crude
material was stored at 20°C under an argon atmosphere and used without further purification
in the next step.
4
.3.4. 6%-Phenyl-5,6-pinene-2,2%-bipyridine 12
Accessible from (+)-pinocarvone and 1S-(−)-a-pinene. In a 25 ml two-necked-bottomed flask,
a solution of 1.45 g (2.53 mmol, 1.0 equiv.) of crude product 15, 380 mg of (+)-pinocarvone
(
obtained in one step from (−)-a-pinene, 2.53 mmol, 1.0 equiv.) and 3.2 ml of glacial acetic acid
was heated to 55°C for 15 min. Ammonium acetate (1.70 g) as a solid (22.0 mmol, 1.0 equiv.)
was added in portions and the reaction mixture was heated to 110°C for 16 h. After cooling to
room temperature, 15 ml of water were added to the reaction mixture and the aqueous phase
was extracted with 5×50 ml of CH Cl . The organic layers were combined and washed with 2×50
2
2
ml of water (pH 5). After drying over MgSO and filtration the organic phase was treated with
4
active charcoal (1 h, 25°C). Filtration over Celite gave a pale yellow solution which was
evaporated in vacuo to yield 836 mg of a yellow solid. Recrystallization from acetone (7.0 ml)
1
gave 407 mg of the pure desired bipyridine 12 (55% yield). H NMR (300 MHz, CDCl ): l 8.37
3
3
3
3
(
d, J =8.0 Hz, 1H, HꢀC(3%)); 8.31 (d, J =8.0 Hz, 1H, HꢀC(3)); 8.13 (dd, J =8.0 Hz,
2
%,3%
3,4
8%,9%
4
3
3
J8%,10%=2.1 Hz, 2H, HꢀC(8%,12%)); 7.85 (dd, J=7.4 Hz, 2H, HꢀC(4%)); 7.72 (dd, J=7.8 Hz,
4
3
3
J=0.8 Hz, 1H, HꢀC(5%)); 7.48 (dd, J=7.4 Hz, 2H, HꢀC(9%,11%)); 7.42 (dd, J=7.9 Hz, 1H,
3
3
HꢀC(10%)); 7.37 (d, J=8.0 Hz, 1H, HꢀC(4)); 3.22 (m, 2H, HꢀC(7)); 2.83 (dd, J=6.0 and 5.6

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4355
3
Hz, 1H, HꢀC(10)); 2.71 (ddd, 1H, HꢀC(9b)); 2.40 (sept, J=3.2 Hz, 1H, HꢀC(8)); 1.42 (s, 3H,
3
13
HꢀC(12)); 1.32 (d, J
=9.5 Hz, 1H, HꢀC(9a)); 0.68 (s, 3H, HꢀC(13)). C NMR (75 MHz,
9
a,9b
CDCl ): l 156.6 (C, 1C); 156.5 (C, 1C), 154.0 (C, 1C); 142.5 (C, 1C); 139.8; 137.8 (CH, 1C);
3
1
1
3
3
34.0 (CH, 1C); 129.1 (CH, 1C); 128.9 (CH, 1C), 127.2 (CH, 1C), 119.9 (CH, 1C); 119.3 (CH,
C), 118.4 (CH, 1C), 46.8 (CH, 1C), 40.6 (CH, 1C); 39.8 (C or CH ; 1C); 37.0 (C or CH , 1C);
2
2
−
6
2.2 (C or CH , 1C), 26.4 (CH , 1C); 21.6 (CH , 1C). UV–vis (CHCl , 8.94×10 M): u
04.0; 253.5 (29000). EI-MS: 326 (100, M ), 311 (50, M−CH ); 297 (36, M−C H ); 283 (93,
(m)
2
3
3
3
max
+
+
3
+
2
5
+
+
+
M−C H ); 246 (10); 231 (2), 205 (7); 154 (15, C H N ); 128 (35); 77 (20, C H ); 51 (9).
3
7
11
8
6
5
Elemental analysis: calculated for C H N : C 84.63, H 6.79, N 8.58%; found C 83.16, H 6.89,
2
3
22
2
N 8.19.
4
.3.5. Bipyridine 12a
To a 25 ml Schlenk flask 15 ml of dry THF (distilled over Na/benzophenone) and 0.29 ml
2.083 mmol, 1.7 equiv.) of dry diisopropylamine (distilled over KOH) were added. This mixture
(
was then cooled down to −40°C and 1.3 ml of n-BuLi (1.6 M in hexane, 1.96 mmol, 1.6 equiv.)
was added via a syringe. The weakly yellow colored solution was allowed to warm up to 0°C
using an ice bath. After having stirred this solution for 30 min, it was cooled down again to
−
40°C and ligand 12 (400 mg, 1.225 mmol, 1 equiv.) dissolved in 5 ml of dry THF was added
using a syringe pump within 1 h. After addition of one drop of this ligand solution, the reaction
mixture became immediately blue. After this addition, the resulting dark blue solution was
stirred at −40°C for 2 h. Then a solution of ethyliodide in 5 ml of dry THF was added using
a syringe pump during 1 h. This resulting reaction mixture was allowed to warm up to room
temperature overnight, and was quenched with 1 ml of water. THF was removed under reduced
pressure and the residue was taken up with 50 ml of CH Cl . More water was added and the
2
2
water phase was extracted with four 20 ml portions of CH Cl . The combined organic phases
2
2
were dried over MgSO , filtered, and evaporated to yield a brown solid, which was then purified
4
by column chromatography (SiO , hexane/ether/triethylamine=10:1:0.1). A pale yellow solid
2
1
3
could be isolated (yield 84%). H NMR (300 MHz, CDCl ): l 8.37 (d, J =8.0 Hz, 1H,
3
3%4%
3
3
4
HꢀC(3%)); 8.31 (d, J =8.0 Hz, 1H, HꢀC(3)); 8.13 (dd, J =8.0 Hz, J
=2.1 Hz, 2H,
3
,4
8%,9%
8%,10%
3
3
4
HꢀC(8%,12%)); 7.85 (dd, J=7.4 Hz, 2H, HꢀC(4%)); 7.72 (dd, J=7.8 Hz, J=0.8 Hz, 1H,
3
3
HꢀC(5%)); 7.48 (dd, J=7.4 Hz, 2H, HꢀC(9%,11%)); 7.42 (dd, J=7.9 Hz, 1H, HꢀC(10%)); 7.37 (d,
3
3
J=8.0 Hz, 1H, HꢀC(4)); 3.22 (m, 1H, HꢀC(9a)); 2.83 (dd, J=6.0 and 5.6 Hz, 1H, HꢀC(10));
3
2
.71 (ddd, 1H, HꢀC(9b)); 2.51 (m, 1H, 14b); 2.40 (sept, J=3.2 Hz, 1H, HꢀC(8)); 1.55 (m, 1H,
3
3
HꢀC(14a); 1.42 (s, 3H, HꢀC(12)); 1.32 (d, J
=9.5 Hz, 1H, HꢀC(9a)); 1.23 (t, J=7.45 Hz,
9a,9b
1
3
3
1
1
1
1
H, HꢀC(15); 0.68 (s, 3H, HꢀC(13)). C NMR (75 MHz, CDCl ): l 159.5 (C, 1C); 156.27 (C,
3
C), 154.0 (C, 1C); 139.5 (C, 1C); 137.6 (CH, 1C); 134.0 (CH, 1C); 128.9 (CH, 1C); 128.7 (CH,
C), 126.9 (CH, 1C), 119.8 (CH, 1C); 119.4 (CH, 1C), 118.4 (CH, 1C), 46.9 (CH, 1C), 45.7 (CH2
C); 42.8 (CH, 1C); 41.1 (C or CH ; 1C); 28.41 (C or CH , 1C), 26.4 (CH , 1C); 25.65 (CH ,
2
2
3
2
C); 20.98 (CH , 1C). Elemental analysis: calculated for C H N : C 84.70, H, 7.39, N 7.90;
found C 84.24, H 7.66, N 7.93. MS-FAB: 355.3 m/z (100, M +H), 325.2 (18, M−C H ), 297.2
3
25 26
2
+
2
5
(
12), 283.2 (18), 147.1 (15).
4
1
.3.6. Typical procedure for the synthesis of mono- and bis-substituted bipyridines 10a–c and
1a–m ( for details see Refs. 14 and 15)
A Schlenk flask was charged with 15 ml of dry THF and diisopropylamine and cooled down
to −40°C. Then n-BuLi (1.6 M in hexane) was added. This mixture was stirred for 30 min at
°C, and cooled again to −40°C. The corresponding bipyridine was dissolved in 5 ml of dry
0

4356
D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
THF and added to the LDA solution by a syringe pump within 1 h. This deep blue solution was
then stirred for 2 h, followed by slow addition (during 1 h) of the respective alkyl halide
(
chloride, bromide or iodide) by a syringe pump. This reaction mixture was then allowed to
warm up slowly to room temperature. After quenching with water and evaporating, more water
was added and the water phase was extracted with dichloromethane. The combined organic
layers were dried over MgSO or Na SO , filtered, and evaporated to dryness. The crude
4
2
4
product was then purified by column chromatography to yield the desired alkylated bipyridine.
4
.3.7. Complex syntheses
In a typical procedure CuCl ·2H O was dissolved in 5 ml EtOH, and the corresponding
2
2
bipyridine was added as a solution in CH Cl . The green solution turned red-brown immedi-
2
2
ately. The reaction mixture was stirred for 4 h. The reaction was controlled by TLC. The
solution was then evaporated to dryness and to the residue was added diethyl ether in order to
form a suspension. The solid was then filtered off and dried in high vacuo to yield the desired
complex.
Cu–11: yield 93%. Elemental analysis: calculated for C H Cl CuN ·CH Cl : C 61.2, H 6.16,
24
28
2
−
2
2
2
4
5
(
.71; found C 60.2, H 6.15, N 5.03. UV–vis (CH Cl , 5.0234×10 M): umax (m) 379 (1110); 341
2
2
+
1861); 328 (20666); 274 (14750). FAB-MS: 407.1 (100, M −2Cl), 351 (20), 309.0 (12), 257.0 (10),
2
04.1 (10), 142.0 (7).
Cu–11a: yield 92%. Elemental analysis: calculated for C H Cl CuN ·0.4(CH Cl ): C 58.6, H
26
32
2
2
2
2
−
5
6
3
3
.1, N 5.18; found C 59.9, H 6.43, N 5.23. UV–vis (CH Cl , 6.1410510 M): u
(m) 882 (127),
2
2
max
+
92 (1345), 327 (18777), 274 (15519), 230 (11888). FAB-MS: 435.2 (100, M −2Cl), 387.2 (68),
73.2 (96, L+1), 311.1 (14), 297.1 (12), 271.1 (11), 154.0 (12), 128.0 (11).
Cu–11b: yield 64%. Elemental analysis: calculated for C H Cl CuN ·0.4CH Cl : C 59.95, H
28
36
2
2
2
2
−
5
6
(
.52, N 4.92; found C 60.38, H 6.59, N 4.69. UV–vis (CH Cl , 4.34514×10 M): u
(m) 886
2
2
max
+
158), 393 (1388), 329 (19957), 273 (16144), 227 (14049). FAB-MS: 498.0 (14, M −Cl), 463.1 (67,
+
+
M −2Cl), 401.2 (100, L +1), 154.1 (28).
Cu–11c: yield 65%. Elemental analysis: calculated for C H Cl CuN ·CH Cl : C 57.46, H
30
40
2
2
2
2
−
5
6
4
.53, N 4.32; found C 59.51, H 6.59, 4.42. UV–vis (CH Cl , 5.17905×10 M): u
(m) 847 (250),
2
2
max
+
58 (903), 327 (17576), 274 (16069), 227 (15251). FAB-MS: 526.1 (8, M +2−Cl), 491.1 (12,
+
+
M +2−2Cl), 463.1 (8), 429.2 (100, L +1).
Cu–12a: yield 72%. Elemental analysis: calculated for C H Cl CuN : C 61.41, H 5.28, N
25
26
2
5
2
−
5
(
(
.30; found C 59.86, H 5.28, N 5.30. UV–vis (CH Cl , 3.7763·10 M): umax (m) 922 (174), 331
2
2
+
18152), 260 (17739). FAB-MS: 417.0 (100, M −2Cl), 355.1 (38, L), 332.9 (8), 271.0 (6), 218.0
4), 154.1 (7).
Cu–13: yield 68%. Elemental analysis: calculated for C H Cl CuN ·CH Cl : C 53.3, H 5.36,
2
4
28
2
2
2
2
−
4
N 4.97; found C 55.83, H 5.83, N 5.30. UV–vis (CH Cl , 4.84804·10 M): u
(m) 832
2
2
max
(
228.3397), 377 (1858), 339 (17328), 328 (18587), 270 (11170), 267 (11283). FAB-MS: 751.3 (98,
+
+
CuL −H), 442.1 (10, M −Cl), 407.2 (100, M −2Cl), 363.2 (19), 309.2 (9), 217.2 (6), 154.1 (12),
2
1
4
36.1 (14).
.3.8. Cyclopropanation of styrene—method 1 (one-pot)
In a typical catalytic reaction, 10.9 mg (0.030 mmol) of Cu(II) triflate and 0.033 mmol of the
(
−)-ligand were dried in vacuo for 20 minutes, set under an inert gas atmosphere and dissolved
in 4 ml of dry CH Cl . Upon addition of 937 mg (9.0 mmol) of styrene the slightly green
2
2
solution turned yellow-brown. Ethyl diazoacetate (368 mg of a commercially available solution

D. L o¨ tscher et al. / Tetrahedron: Asymmetry 11 (2000) 4341–4357
4357
in CH Cl ; 3.0 mmol) dissolved in about 3 ml of CH Cl was added using a syringe pump over
2
2
2
2
a period of 7 h. First the diazoacetate reduces the Cu(II) species to a Cu(I) species, forming the
2
1
active catalyst. After addition the reaction mixture was stirred for an additional 2 hours. The
solvent was removed and the liquid residue, mainly excess styrene, 2-phenylcyclopropane-1-car-
boxylates 18 and catalyst, was purified on silica gel (20 g SiO 100 ml of hexane/ethyl ether 20:1,
2
then 100 ml of ether).
4
.3.9. Cyclopropanation of styrene—method 2 (using the corresponding Cu(II) complex)
To a two-necked round-bottomed flask were added the corresponding Cu(II) complexes
(
0.048 mmol) and silver(I) triflate (0.096 mmol) under argon. Dry CH Cl was added (3 ml) and
2
2
the solution was stirred at room temperature for 2 h. The precipitated AgCl was filtered through
a packed filter paper to a solution of the alkene (4.8 mmol) in CH Cl (5 ml) in a 15 ml Schlenk
2
2
flask. This solution was then degassed by the pump-and-freeze technique. A degassed solution
of ethyl diazoacetate (1.185 mmol) in 5 ml of dry CH Cl was then added via a syringe pump
2
2
within 4 h. The reaction mixture was then stirred at room temperature or at 0°C for 16 h.
Enantiomeric excess determination was carried out by chiral column chromatography without
working up the reaction mixture. Yields were determined in the same way.
Acknowledgements
We thank Dr. Catherine Goujon-Ginglinger for the synthesis of ligand 12. This work was
supported by the Swiss National Science Foundation.
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