First enantioselective catalyst based on a chiral terpyridine: Synthesis of new C2-symmetric 2,2':6',2''-terpyridine ligands and copper-catalyzed enantioselective cyclopropanation of alkenes

Article abstract of DOI:10.1016/S0957-4166(00)00182-8

New C₂-symmetric chiral 2,2':6',2''-terpyridines were prepared from readily available homochiral materials. Copper complexes of these ligands were prepared in situ and their catalytic activities in cyclopropanations of alkenes with alkyl diazoacetate to give cyclopropyl esters were studied. In all cases, the cyclopropyl ester yields were excellent and enantioselectivities up to 94% ee were observed. Competition experiments revealed that electron-donating substituents on styrene accelerate the reaction. Hammett plot exhibited a good linearity with negative ρ⁺ value (-0.79). Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00182-8

Tetrahedron: Asymmetry 11 (2000) 2299±2308
First enantioselective catalyst based on a chiral terpyridine:
synthesis of new C2-symmetric 2,20:60,200-terpyridine ligands
and copper-catalyzed enantioselective cyclopropanation
of alkenes
Hoi-Lun Kwong* and Wing-Sze Lee
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Received 28 February 2000; accepted 8 May 2000
Abstract
New C2-symmetric chiral 2,20:60,200-terpyridines were prepared from readily available homochiral materials.
Copper complexes of these ligands were prepared in situ and their catalytic activities in cyclopropanations
of alkenes with alkyl diazoacetate to give cyclopropyl esters were studied. In all cases, the cyclopropyl ester
yields were excellent and enantioselectivities up to 94% ee were observed. Competition experiments
revealed that electron-donating substituents on styrene accelerate the reaction. A Hammett plot exhibited a
good linearity with a negative r+ value (^0.79). # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The presence of a C2-symmetry axis within a chiral auxiliary can serve the very important
function of dramatically reducing the number of possible competing diastereomeric transition
states.1 Although chiral C2-symmetric bidentate N,N-coordinating nitrogen heterocycles such as
bisoxazoline,2 semicorrine3 and 2,20-bipyridine4 are among the most useful ligands in asymmetric
catalysis, the development of chiral C2-symmetric tridentate N,N,N-coordinating ligands represents
a new challenge for chemists. One type of these ligands, 2,6-pyridinediyl-2,20-bisoxazoline 1, has
been shown to be highly useful in a number of catalytic reactions. For example, the Rh(III)
complexes are ecient enantioselective catalysts for the hydrosilylation of ketones;5 the Ru(II)
complexes are active catalysts for enantioselective cyclopropanation and epoxidation of alkenes;6,7
and the Cu(II) complexes are ecient catalysts for asymmetric carbon±carbon bond-forming
reactions8 and allylic oxidation of cycloalkenes.9 Terpyridines, having the general structure 2, are
* Corresponding author. Fax: (852)-2788-7406; e-mail: bhhoik@cityu.edu.hk
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00182-8

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structurally similar to these ligands and are well known to form complexes with various metal
ions.10 They play important roles in supramolecular chemistry,11 photochemistry12 and electro-
chemistry.13 The most common coordination mode is meridional,14 although they could also
form helicating or bridging structures with certain metals.15 However, very few chiral terpyridine
ligands are known and most of them have not been used in asymmetric catalysis.16 To the best of
our knowledge, only one example of utilizing chiral terpyridine in asymmetric catalysis has been
reported where the cyclopropanation of styrene with ethyl diazoacetate gave an almost racemic
product.17 Recently, von Zelewsky et al. reported the synthesis of C2-symmetric chiral terpyridine
ligands 3 and 4 and their ruthenium and rhodium complexes.18 Herein, we report the synthesis
of a number of new chiral terpyridine ligands (ligands 5±8) and their application in the copper-
catalyzed asymmetric cyclopropanation of alkenes with alkyl diazoacetates.
2. Results and discussion
Terpyridine ligands 5±8 (Scheme 1) were prepared from the terpyridine 4, which is readily
accessible by the Krohnke condensation of (1R)-(+)-pinocarvone with 2,6-bis(pyridinioacetyl)-
pyridine diiodide.18 The dark-blue solution of dilithiated 4, obtained by treatment with lithium
diisopropylamide (LDA) at ^40ꢀC for 2 h, was quenched with the proper alkyl iodide to give
ligands 5±8.
Scheme 1.

H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
2301
The ligands were all obtained as sole diastereomers as shown by NMR analysis of the product
mixtures, and the con®gurations are shown in Scheme 1. The yields were between 39 and 57%
which depended only slightly on the nature of the alkyl group. Similar stereoselective lithiation
and alkylation have been reported previously with pinocarvone-derived bipyridine ligands.19
Having prepared these terpyridine ligands, we tested their eciency in the copper-catalyzed
asymmetric cyclopropanation of alkenes (Scheme 2). Although it is generally believed that the
copper(I) species is the active form in the cyclopropanation, copper(II) catalysts were used in this
study because of their stability in air. Unlike the previously reported catalytic systems where
heating was required for the activation process,3a these catalysts can be easily activated by stirring
with a few equivalents of alkyl diazoacetates for 30 min at room temperature.
Scheme 2.
The results of asymmetric cyclopropanation of alkenes with the copper(II)±terpyridine complexes,
which were generated from ligands 3±8 and copper(II) tri¯ate, are shown in Table 1. All the
copper±terpyridine complexes were found to be active catalysts and the yields of the isolated
cyclopropyl esters were excellent (87±98%). The enantioselectivities obtained were high for
ligands 5±8. Analysis of the reaction mixtures by GC indicated that the trans/cis ratios were
between 59:41 and 78:22 (entries 1±5 a0nd 7). The results obtained here were much better than the
previously studied 2,6-pyridinediyl-2,2 -bisoxazoline ligand.20
The introduction of alkyl groups at the 8-position of the tetrahydroquinoline ring of ligand 4
was crucial for the enantioselectivity of the reaction. Thus, ligand 4 gave a much lower enantio-
meric excess for both the cis- (29%) and the trans-isomer (30%) than the alkyl-substituted ligands
5±8. The best enantioselection was achieved with the butyl-substituted ligand 7, where enantio-
meric excesses of 86% for the trans- and 90% for the cis-isomer were obtained (entry 5).
With terpyridine 7 being the best ligand, it is used to optimize reaction conditions. Lowering
the reaction temperature from room temperature to 0ꢀC did not hamper the reaction but
increased the enantioselection to 90% for the trans- and 94% for the cis-isomer (entry 6). Changing
the solvent has a great e€ect on both the enantioselectivity and isolated yield, but only a small
e€ect on the diastereoselectivity. Dichloromethane (entry 5) was found to be the best solvent,
slightly lower enantioselectivities were obtained in toluene (entry 9) and ether (entry 10), and the
enantioselectivity and isolated yield decreased signi®cantly in THF (entry 8).
Variation on the structure of the diazo ester has a great e€ect on the trans/cis diastereo-
selectivity. As can be seen from entries 5 and 11±13 in Table 1, when the R group changes from
the ethyl to bulkier groups (tert-butyl, l-menthyl and d-menthyl), all reactions give higher trans:cis

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H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
Table 1
Catalytic asymmetric cyclopropanation with chiral copper(II) terpyridine complexes
ratios for the cyclopropanation of styrene. The best result was obtained with the d-menthyl
group, where a trans:cis ratio of 91:9 (entry 13) was obtained. This is consistent with the trend
previously observed with other copper catalysts.4a d However, the enantioselectivities for both
the trans- and cis-isomers decreased when the bulkiness of the ester group increased.
Apart from styrene, the complex derived from ligand 7 was capable of catalyzing the cyclo-
propanation of various alkenes with di€erent electronic (entries 14 and 15) and steric properties
(entry 16). Higher trans/cis ratios were observed with 4-chlorostyrene (entry 14) and 4-methoxy-
styrene (entry 15). However, there was no improvement on the enantioselectivity for all the

H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
2303
substituted styrenes. By employing ligands 3±8, the absolute con®guration of cyclopropyl ester
products formed from styrene and substituted styrenes were determined to be (1R,2R) and
(1R,2S) for the trans- and cis-isomers, respectively. Based on the absolute con®guration of the
products obtained, the sense of asymmetric inductions observed here can be explained by the
model shown in Scheme 3. This model is in agreement with the one previously cited by P¯atz for
other N,N-bidentate ligands.3a In the model, the metal carbenoid attacks the ole®nic double bond
according to the pathways a and b. In the case of pathway a, a repulsive steric interaction builds
up between the ester group and the adjacent bulky alkyl group of the ligand. In the case of
pathway b, no such steric interaction exists and it leads to the cis-(1R)- or to the trans-(1R)-
cyclopropyl esters that are consistent with the experimental results. Therefore, pathway b is more
favored than pathway a.
Scheme 3.
In order to know more about the active intermediate in the reaction, the relative reaction rates
were measured in the competitive cyclopropanation of EDA with substituted styrenes in the
presence of the catalyst derived from ligand 7. The reaction was enhanced by electron-donating
groups but retarded by electron withdrawing groups. A Hammett plot of log(kX/kH) versus s(+)
is shown in Fig. 1; a good s(+) correlation is obtained with r=^0.79. This value is similar to that
reported by Kodadek et al.21 (^0.68) and Perez et al.22 (^0.85) for the cyclopropanation of substituted
Figure 1. Hammett plot for the cyclopropanation of styrene with EDA using Cu(OTf)2 and terpyridine 7 as the catalyst

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H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
styrenes using EDA/iron(II) porphyrin and EDA/copper(I) tris(pyrazolyl) borate systems,
respectively. However, in both of the previous systems, correlation to s instead of s(+) was
found. This negative value of r obtained supports the formation of an electrophilic metal±carbene
complex intermediate with only a moderate positive charge build-up at the benzylic carbon in the
transition state.
In summary, we have successfully synthesized four new C2-symmetric 2,20:60,200-terpyridine
ligands in good yields. The copper complexes of these ligands are excellent catalysts for enantio-
selective cyclopropanation. Ee's up to 94% were observed. We are continuing our e€orts to study
the use of these ligands in other catalytic asymmetric reactions.
3. Experimental
3.1. General methods
Benzyl iodide was prepared by a literature method.23 Toluene was distilled under N2 from
sodium. Dichloromethane and acetonitrile were distilled over calcium hydride. Diethyl ether and
THF were distilled under N2 over sodium/benzophenone. All other chemicals were of reagent
grade. Infrared spectra in the range 500±4000 cm^1 were recorded as KBr plates on a Perkin±
Elmer Model FTIR-1600 spectrometer. Proton and 13C NMR spectra were recorded on a Varian
300 MHz Mercury instrument. Positive ion FAB mass spectra as 3-nitrobenzylalcohol matrix
were recorded on a Finnigan MAT 95 spectrometer. Electron ionization mass spectra were
recorded on a Hewlett±Packard 5890II GC instrument coupled with a 5970 mass selective detec-
tor. Elemental analyses were performed on a Vario EL elemental analyzer. Optical rotation was
measured by a JASCO DIP-370 digital polarimeter. Melting point was measured by an Electro-
thermal digital melting point apparatus.
3.2. 2,6-Bis(pyridinioacetyl)pyridine diiodide
To a pyridine solution (10 ml) of 2,6-diacetylpyridine (8 mmol, 1.31 g) was added a solution
of iodine (16 mmol, 4.06 g) in pyridine (10 ml). The mixture was heated at 110ꢀC for 3 h and,
after cooling, the dull yellow solid was ®ltered and washed with cold ethanol. This product was
1
1
characterized by H NMR and IR. Yield=85% (3.82 g). H NMR (CDCl3): ꢀ 6.98 (s, 4H), 8.26
(dd, 4H), 8.43 (s, 3H), 8.76 (dd, 2H, J=7.8 Hz), 9.14 (d, 4H, J=5.7 Hz); IR (KBr): 3022.9 vs,
2969.5 vs, 2946.6 vs, 1719.6 vs, 1635.7 s, 1491.1 s, 1344.5 s.
3.3. (1R)-(+)-Pinocarvone
Following the literature procedure,24 (1R)-(+)-pinocarvone was obtained in 50% yield from
SeO2 oxidation of b-pinene. This product was characterized by 1H NMR and IR.
3.4. Terpyridine ligand 3
This ligand was synthesized by Krohnke condensation.25 2,6-Bis(pyridinoacetyl)pyridine diiodide
(1.5 mmol, 0.84 g), (1R)-(^)-myrtenal (3 mmol, 0.45 g) and ammonium acetate were dissolved in
glacial acetic acid (2 ml). The mixture was re¯uxed overnight under a N2 atmosphere. The solvent

H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
2305
was removed under reduced pressure and the brown residue obtained was washed with ethanol,
and recrystallized from dichloromethane to give 0.40 g (63%) of 3. The product was characterized
by IR, 1H NMR, 13C NMR and MS analyses. ‰ꢁŠD25=^106.7 (c 0.50, CHCl3); IR (KBr): 2992.4 vs,
2969.5 vs, 2931.3 vs, 2893.1 vs, 1550.5 s, 1455.2 s; 1H NMR (CDCl3): ꢀ 0.69 (s, 6H), 1.27 (d, 2H,
J=9.9 Hz), 1.44 (s, 6H), 2.37 (m, 2H), 2.74 (m, 2H), 2.90 (m, 2H), 3.14 (d, 4H), 7.93 (t, 1H,
J=7.8 Hz), 8.24 (s, 2H), 8.37 (d, 2H, J=8.1 Hz), 8.40 (s, 2H); 13C NMR (CDCl3): ꢀ 21.46, 26.00,
31.81, 33.02, 39.23, 40.07, 44.45, 120.20, 120.31, 137.54, 142.74, 145.12, 145.26, 154.36, 155.56;
positive ion FAB mass spectra m/z: 422 (M++H). Anal calcd for C29H31N3: C, 82.62; H, 7.41; N,
9.97. Found: C, 81.89; H, 7.20; N, 9.86.
3.5. Terpyridine ligand 4
This ligand was synthesized by Krohnke condensation.25 2,6-Bis(pyridinoacetyl)pyridine diiodide
(1.5 mmol, 0.84 g), (1R)-(+)-pinocarvone (3 mmol, 0.45 g) and ammonium acetate were dissolved
in glacial acetic acid (2 ml). This mixture was re¯uxed overnight under a N2 atmosphere. The solvent
was removed under reduced pressure and the brown residue obtained was washed with ethanol,
and recrystallized from dichloromethane to give 0.37 g (59%) of 4. The product was characterized
by IR, 1H NMR, 13C NMR and MS analyses. ‰ꢁŠD25=^149.3 (c 0.40, CHCl3); IR (KBr): 2969.5 s,
1
2908.4 vs, 2862.6 s, 1561.9 vs, 1432.4 vs; H NMR (CDCl3): ꢀ 0.70 (s, 6H), 1.33 (d, 2H, J=9.3
Hz), 1.44 (s, 6H), 2.42 (m, 2H), 2.72 (m, 2H), 2.83 (dd, 2H, J=5.4 Hz), 3.21 (d, 4H, J=2.1 Hz),
7.37 (d, 2H, J=7.8 Hz), 7.90 (t, 1H, J=7.8 Hz), 8.29 (d, 2H, J=7.8 Hz), 8.37 (d, 2H, J=7.8 Hz);
13C NMR (CDCl3): ꢀ 21.31, 26.03, 31.91, 36.64, 39.49, 40.17, 464.4, 117.75, 119.86, 133.50,
137.42, 141.94, 153.51, 155.53, 156.04; positive ion FAB mass spectra m/z: 422 (M++H). Anal
calcd for C29H31N3: C, 82.62; H, 7.41; N, 9.97. Found: C, 81.58; H, 7.38; N, 9.65.
3.6. General procedures for the synthesis of terpyridine ligands 5±8
A 100 ml ¯ask was ®lled with dry THF (10 ml) and cooled to ^40ꢀC. Diisopropylamine (3.1
mmol, 0.44 ml) and n-butyllithium (1.6 M solution in hexane, 3.72 mmol, 2.3 ml) were added
slowly with stirring. The temperature was then raised to 0ꢀC and stirred for 30 min. The mixture
was again cooled to ^40ꢀC and terpyridine 4 (0.77 mmol, 0.27 g) in dry THF (5 ml) was added
slowly. The mixture was kept at ^40ꢀC for 2 h and a dark blue solution was obtained. The desired
iodoalkane (6.14 mmol) was added slowly to this mixture. The temperature was raised to room
temperature over 2 h and the mixture was stirred overnight. The reaction was quenched by the
addition of water (20 ml) and the aqueous phase was extracted with dichloromethane (three times
by 50 ml). The combined organic layer was dried with Na2SO4. The solvent was removed under
reduced pressure and the crude product obtained was puri®ed by silica gel column chromatography.
1
All ligands were characterized by spectral (IR, H NMR, 13C NMR and MS) analyses.
3.6.1. Terpyridine 5
The above procedure was followed using methyl iodide. After workup and puri®cation by
column chromatography with petroleum ether:EtOAc (60:1, Rf=0.3), 0.16 g (46%) of terpyridine
5 was obtained: ‰ꢁŠ2D5=^8.0 (c 0.50, CHCl3); IR (KBr): 2969.5 s, 2903.4 s, 2862.6 s, 1558.1 vs,
1
1424.8 vs; H NMR (CDCl3): ꢀ 0.69 (s, 6H), 1.35 (d, 2H, J=9.6 Hz), 1.44 (s, 6H), 1.49 (d, 6H,
J=7.2 Hz), 2.20 (m, 2H), 2.59 (m, 2H), 2.83 (t, 2H, J=5.4 Hz), 3.27 (m, 2H), 7.34 (d, 2H, J=7.8
Hz), 7.91 (t, 1H, J=7.8 Hz), 8.31 (d, 2H, J=7.5 Hz), 8.45 (d, 2H, J=7.5 Hz); 13C NMR

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H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
(CDCl3): ꢀ 18.28, 20.93, 26.31, 28.59, 38.85, 41.39, 46.71, 47.09, 117.62, 119.79, 133.22, 137.30,
141.78, 153.30, 155.57, 159.79; positive ion FAB mass spectra m/z: 450 (M++H). Anal calcd for
C31H35N3: C, 82.81; H, 7.85; N, 9.35. Found: C, 82.65; H, 7.58; N, 9.03.
3.6.2. Terpyridine 6
The above procedure was followed using isopropyl iodide. After workup and puri®cation by
column chromatography with petroleum ether:EtOAc (80:1, Rf=0.3), 0.22 g (57%) of terpyridine
6 was obtained: ‰ꢁŠ2D5=+28.9 (c 0.50, CHCl3); IR (KBr): 2954.2 vs, 2931.3 vs, 2870.2 s, 1558.1 vs,
1428.6 vs; 1H NMR (CDCl3): ꢀ 0.66 (s, 6H), 0.89 (d, 6H, J=6.9 Hz), 1.20±1.70 (m, 2H), 1.27 (d,
6H, J=6.9 Hz), 1.41 (d, 2H), 1.44 (s, 6H), 2.40 (m, 2H), 2.60 (m, 2H), 2.79 (t, 2H, J=5.4 Hz),
2.93 (m, 2H), 7.35 (d, 2H, J=7.8 Hz), 7.90 (t, 1H, J=7.8 Hz), 8.34 (d, 2H, J=7.8 Hz), 8.44 (d,
2H, J=7.8 Hz); 13C NMR (CDCl3): ꢀ 20.21, 21.06, 22.37, 26.37, 29.33, 30.37, 41.39, 41.86, 46.74,
49.08, 117.47, 119.80, 133.23, 137.28, 142.26, 153.03, 155.68, 158.42; positive ion FAB mass
spectra m/z: 506 (M++H). Anal calcd for C35H43N3: C, 83.12; H, 8.57; N, 8.31. Found: C, 83.28;
H, 8.43; N, 8.08.
3.6.3. Terpyridine 7
The above procedure was followed using n-butyl iodide. After workup and puri®cation by
column chromatography with petroleum ether:EtOAc (80:1, Rf=0.3), 0.22 g (54%) of terpyridine
7 was obtained: ‰ꢁŠ2D5=+25.9 (c 0.51, CHCl3); IR (KBr): 2954.2 vs, 2931.3 vs, 2862.6 s, 1558.1 vs,
1
1432.4 vs; H NMR (CDCl3): ꢀ 0.60±1.60 (18H), 0.67 (s, 6H), 1.34 (d, 2H, J=9.6 Hz), 1.45 (s,
6H), 2.39 (m, 2H), 2.56 (m, 2H), 2.81 (t, 2H, J=5.4 Hz), 3.06 (m, 2H), 7.34 (d, 2H, J=8.1 Hz),
7.92 (t, 1H, J=7.8 Hz), 8.31 (d, 2H, J=7.5 Hz), 8.45 (d, 2H, J=7.8 Hz); 13C NMR (CDCl3): ꢀ
14.23, 20.96, 23.02, 28.43, 29.69, 30.14, 32.24, 41.02, 43.33, 44.21, 46.86, 117.50, 119.78, 133.16,
137.31, 141.4, 153.19, 155.58, 159.47; positive ion FAB mass spectra m/z: 534 (M++H). Anal calcd
for C37H47N3: C, 83.25; H, 8.87; N, 7.87. Found: C, 83.09; H, 8.80; N, 7.65.
3.6.4. Terpyridine 8
The above procedure was followed using benzyl iodide. After workup and puri®cation by column
chromatography with petroleum ether:EtOAc (80:1, Rf=0.3), 0.18 g (39%) of terpyridine 8 was
obtained: ‰ꢁŠ2D5=^94.8 (c 0.25, CHCl3); IR (KBr): 2977.1 s, 2916.0 vs, 2862.6 s, 1558.1 vs, 1428.6
vs; 1H NMR (CDCl3): ꢀ 0.64 (s, 6H), 1.36 (s, 6H), 1.46 (d, 2H, J=9.9 Hz), 2.14 (m, 2H), 2.59 (m,
2H), 2.78 (m, 4H), 3.41 (m, 2H), 3.89 (m, 2H), 7.34 (m, 5H), 7.40 (d, 2H, J=7.5 Hz), 7.94 (t, 1H,
J=8.1 Hz, J=7.8 Hz), 8.37 (d, 2H, J=7.5 Hz), 8.50 (d, 2H, J=8.1 Hz); 13C NMR (CDCl3): ꢀ
20.88, 26.26, 28.26, 38.71, 41.09, 42.45, 46.19, 46.85, 117.86, 119.91, 125.58, 128.07, 129.14,
133.41, 137.36, 140.98, 141.97, 153.24, 155.49, 158.23; positive ion FAB mass spectra m/z: 602
(M++H). Anal calcd for C43H43N3: C, 85.82; H, 7.20; N, 6.98. Found: C, 85.28; H, 7.10; N, 6.65.
3.7. Procedure for copper-catalyzed cyclopropanation
To a two-neck round-bottomed ¯ask were added Cu(OTf)2 (0.072 g, 0.02 mmol), CH2Cl2 (2.0
ml) and ligand (0.022 mmol) under nitrogen. The solution was stirred at room temperature for 2
h. Alkene (4 mmol) and diazoacetate (0.2 mmol) were then added and the mixture was stirred at
room temperature for a further 0.5 h. A solution of diazoacetate (1 mmol) in CH2Cl2 (0.5 ml) was
then added to the reaction mixture over a period of 4 h using a syringe pump. After the addition
of diazoacetate, the mixture was stirred for 16 h at room temperature. The mixture was then

H.-L. Kwong, W.-S. Lee / Tetrahedron: Asymmetry 11 (2000) 2299±2308
2307
worked up by removing the solvent and the crude product obtained was puri®ed by column
chromatography (hexane/EtOAc). All the cyclopropanes obtained are known compounds and
1
were characterized by H NMR, 13C NMR, IR and GC±MS. The enantiomeric excesses of the
cyclopropanes in entries 1±10 and 16 were determined by HPLC with a Daicel Chiralcel OJ
column. For entry 11 enantiomeric excesses were determined by GC analysis using a chiral column
Chiraldex b-PH column (30 mÂ0.25 mm). For entries 12 and 13, enantiomeric excesses were
determined by GC analysis using an Ultra 2-crosslinked 5% PhMesilcone (25 mÂ0.2 mmÂ0.33 mm)
column. For entries 14 and 15, enantiomeric excesses were determined by a literature procedure.6c
Absolute con®gurations were determined by comparing the order of elution with samples of
known con®gurations.6c Diastereoselectivities (cis:trans ratio) were measured by GC with an
Ultra 2-crosslinked 5% PhMesilcone (25 mÂ0.2 mmÂ0.33 mm) column.
3.8. General procedure for competition reactions
To a two-neck round-bottomed ¯ask were added Cu(OTf)2 (0.036 g, 0.01 mmol), CH2Cl2 (1.0
ml) and terpyridine 5 (0.011 mmol) under nitrogen. The solution was stirred at room temperature
for 2 h. Styrene (1.0 mmol) and substituted styrene (1.0 mmol) were added to the stirred solution.
Ethyl diazoacetate (0.5 mmol) in CH2Cl2 (0.5 ml) was then added to the reaction mixture in one
portion. The mixture was allowed to stir for 16 h at room temperature. The relative amounts of
the resulting cyclopropanes were determined by GC.
Acknowledgements
Financial support for this research from the Hong Kong Research Grants Council and City
University of Hong Kong (grant no. 7100039) is gratefully acknowledged.
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