Asymmetric inter- and intramolecular cyclopropanations of alkenes catalyzed by rhodium D4-porphyrin: A comparison of rhodium- and ruthenium-centred catalysts

Article abstract of DOI:10.1016/S0957-4166(03)00047-8

Iodo-(5,10,15,20-tetrakis(1,2,3,4,5,6,7,8-octahydro-1,4:5,8- dimethanoanthracen-9-yl)porphyrinatorhodium(III), designated as [Rh(P*)(I)], was prepared and its catalytic activity in the asymmetric cyclopropanation of alkenes with ethyl diazoacetate (EDA) w

Full text of DOI:10.1016/S0957-4166(03)00047-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 837–844
Asymmetric inter- and intramolecular cyclopropanations of
alkenes catalyzed by rhodium D4-porphyrin: a comparison of
rhodium- and ruthenium-centred catalysts
Pang-Fei Teng,a Tat-Shing Lai,a,b Hoi-Lun Kwonga,* and Chi-Ming Cheb,*
aDepartment of Biology and Chemistry and Open Laboratory of Chirotechnology of the Institute of Molecular Technology for Dr
ug Discovery and Synthesis, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
bDepartment of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discover
y and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Received 24 October 2002; accepted 17 December 2002
Abstract—Iodo-(5,10,15,20-tetrakis(1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracen-9-yl)porphyrinatorhodium(III), desig-
nated as [Rh(P*)(I)], was prepared and its catalytic activity in the asymmetric cyclopropanation of alkenes with ethyl diazoacetate
(EDA) was examined. High catalyst turnovers (TON >103) and moderate enantioselectivities (up to 68% ee) were observed.
However, the obtained trans/cis ratios are low. Competition experiments revealed that electron-donating substituents on styrene
accelerate the cyclopropanations. The log(kX/kH) versus |+ plot for substituted styrenes exhibits a good linearity with a small
negative z+ value (−0.14). [Rh(P*)(I)] is also active in the intramolecular cyclopropanation of allyl diazoacetates. A comparison
between rhodium and ruthenium porphyrin complexes was made. © 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
catalysts for group- and atom-transfer reactions4 and the
use of metalloporphyrins to catalyze alkene cyclopropa-
nations has made important advances in recent years.5–8
Because of the utility of cyclopropanes as building blocks
for the construction of organic molecules, there has been
much interest in the synthesis of chiral cyclopropanes
from achiral starting materials.1 Chiral metal complexes,
particularly those of copper(I) and rhodium(II), have
been widely used as catalysts for asymmetric cyclopropa-
nations.2,3 In general, metalloporphyrins provide robust
The catalytic activity of rhodium porphyrins toward
carbene transfer reactions was first reported by Cal-
lots.5a,b Subsequently, Kodadek and co-workers reported
the asymmetric version of alkene cyclopropanations with
chiral rhodium porphyrin catalysts.6a,c While thousands
* Corresponding authors. E-mail: bhhoik@cityu.edu.hk; cmche@hkucc.hku.hk
0957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00047-8

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P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
of turnovers of the cyclopropane products can be
obtained, the enantioselectivity is generally poor. For
example, the cyclopropanation of styrene with the
iodorhodium(III) catalysts containing the ‘chiral wall’6a
and ‘chiral fortress’6c porphyrins produce cis-cyclo-
propyl ester in 10 and 15% ee, respectively. Recently,
Che and Berkessel independently reported that high ee
values (up to 90%) of trans-cyclopropyl ester can be
obtained with the [Ru(P*)(CO)] catalyst.7a,c It is there-
fore of interest to prepare the rhodium(III) complex
with the D4 porphyrin ligand and compare its perfor-
mance with that of [Ru(P*)(CO)]. Herein, we report
our findings on the use of [Rh(P*)(I)] in intermolecular
and intramolecular cyclopropanation.
turnovers with [Rh(P*)(I)] are >1000. For styrene the ee
of the trans- and cis-cyclopropyl esters are 61 and 36%,
respectively, which are much higher than those
obtained with Kodadek’s catalyst, though a lower
trans/cis ratio of 1:1.5 was obtained.6a,c The results
using [Ru(P*)(CO)] as catalyst for comparison are sum-
marized in Table 1. In contrast to the high trans/cis
ratios and high ee values for trans-isomers produced by
[Ru(P*)(CO)], reactions with [Rh(P*)(I)] have lower
trans/cis ratios and low ee values for trans-cyclo-
propane products. However, [Rh(P*)(I)] gave a signifi-
cant amount of cis-isomer with higher ee than that
obtained with [Ru(P*)(CO)] catalyst. Another impor-
tant observation is that the absolute configuration of
the major enantiomers in the Rh porphyrin-catalyzed
reactions (trans-(1S,2S) and cis-(1S,2R)), are the same
as those from Ru porphyrin-catalyzed reactions. All
other styrene derivatives gave similar observations.
2. Results and discussion
The D4-porphyrin, H2P*, was synthesized according to
the literature procedure.9 The rhodium catalyst
[RhIII(P*)I] used in this work was prepared from tetra-
carbonylbis(m-chloro)dirhodium in 37% yield. Asym-
metric cyclopropanation of various styrenes with EDA
were performed in CH2Cl2 at 40°C in the presence of
[Rh(P*)(I)]. The results are summarized in Table 1.
Similar to other rhodium porphyrin systems, all the
To find out more about the active intermediate in the
reaction, the relative rates of cyclopropanation of sub-
stituted styrenes using [Rh(P*)(I)] with EDA were
determined through competition experiments. The reac-
tion was enhanced by electron-donating groups but
retarded by electron-withdrawing groups. A Hammett
plot of log(kX/kH) versus |(+) is shown in Fig. 1. Good
|(+) correlation is obtained with a very small z+ of
Table 1. Comparative yields and enantioselectivities of [Rh(P*)(I)]- and [Ru(P*)(CO)]-catalyzed intermolecular cyclopropana-
tion of alkenes

P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
839
NMR and GC–MS analyses of the crude reaction
mixture after removal of the solvent. At least several
hundred turnovers of the cyclopropane product were
obtained in each case. However, only moderate enan-
tioselectivities were observed. The best enantioselectiv-
ity comes from allylic acetates 1c and 1j, which gave 49
and 48% ee, respectively. Nonetheless, these results
indicate that the catalytic system reported here could be
extended to the cyclization of unsaturated diazoacetates
in which the double bond is mono-, di-, and tri-substi-
tuted with alkyl and aryl groups. The results using
[Ru(P*)(CO)] as catalyst are included in Table 2. A
very interesting aspect when comparing [Rh(P*)(I)] and
[Ru(P*)(CO)] is that the former smoothly converts a-
substituted diazoacetates to the corresponding lactones
in good yields and with moderate enantioselectivities,
while there is no reaction with [Ru(P*)(CO)] even when
the reaction was carried at 40°C. Also, lower stereo-
chemical induction was observed for allylic acetate 1d
with both catalysts. The product cis:trans ratios are 2.8
and 0.75 with [Rh(P*)(I)] and [Ru(P*)(CO)], respec-
tively, which might indicate the formation of an inter-
mediate which can lead to loss of stereochemistry.
Figure 1. Log(kX/kH) versus |+ plot for [Rh(P*)(I)]-catalyzed
cyclopropanation of substituted styrenes with EDA.
−0.14. This value is lower than that of [Ru(P*)(CO)]
(−0.44)7b and much lower than that of copper(I) ter-
pyridine (−0.79)10 and copper(I) tris(pyrazolyl) borate
(−0.85)11 reported previously. The negative value sup-
ports formation of an electrophilic metal–carbene com-
plex intermediate with a small positive charge build-up
at the benzylic carbon in the transition state. This may
also indicate an early transition state compared with
other catalytic systems.
Catalyst [Ru(P*)(CO)] induced good yield and high
enantioselectivity in the catalytic intramolecular cyclo-
propanation of 1g (77% yield and 85% ee). However, its
activity and enantioselectivity dropped dramatically in
reactions with diazoacetates bearing a methyl or ethyl
group at the R3 position (1e,f). Another important
finding is that with 1g, [Ru(P*)(CO)] gave the opposite
enantiomer as the major product when compared with
[Rh(P*)(I)]. In fact, if we compare the absolute configu-
ration of the products formed from different allylic
acetates, some dependence of the steric influences of the
olefinic substituents on the absolute configuration of
the bicyclic lactones has been found. The absolute
configuration of lactones 2a formed from allylic acetate
1a with [Rh(P*)(I)] and [Ru(P*)(CO)] catalysts are both
Recently, we reported the first metalloporphyrin-cata-
lyzed intramolecular cyclopropanation of alkenes using
[Ru(P*)(CO)].7b Using [Rh(P*)(I)], we studied similar
catalytic reactions and the results are shown in Table 2
(see Scheme 1). Most of the substrates were smoothly
converted to the corresponding lactones in moderate to
good yields, with no detectable side products by 1H
Table 2. Comparative yields and enantioselectivities of [Rh(P*)(I)]- and [Ru(P*)(CO)]-catalyzed intramolecular cyclopropana-
tion of 1a–j
Allylic diazoacetate 1
Cyclopropane 2
[Rh(P*)(I)]
Ee (%)b,c
[Ru(P*)(CO)]a
Ee (%)b,c
Yield (%)
Yield (%)
a
b
c
a
b
c
d
g
e
f
g
h
i
23
79
59
31
11
33
78
84
81
89
65
20 (1R,5S)
24 (1R,5S)
49 (1R,5S)e
31 (1S,5R)
11 (1R,5S)
24 (1R,5S)
25 (1R,5S)e
20 (1R,5S)e
37 (1R,5S)
12 (1R,5S)
48 (1R,5S)
48
ndd
ndd
18
24
67
63
77
65
82
85
24 (1R,5S)
–
–
d
22 (1S,5R)
57 (1S,5R)
28 (1S,5R)
22 (1S,5R)e
85 (1S,5R)e
36 (1R,5S)
18 (1R,5S)
30 (1R,5S)
e
f
g
h
i
j
j
a Some of the results for [Ru(P*)(CO)] have been reported previously.7b
b Enantiomeric excesses were determined by a chiral GC column: Chiraldex G-TA, 30 m.
c Absolute configurations were assigned by comparing the elution orders of enantiomers reported by Doyle.13
d Not determined.
e Absolute configurations were assigned by comparing the elution orders with those of a similar allylic diazoacetate.

840
P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
Scheme 1.
(1R,5S) whereas for allylic acetates having substituents
at R2 (1d), the absolute configuration of lactones are both
(1S,5R). For allylic acetates with substituents at both R2
and R3 (1h–j), both [Rh(P*)(I)] and [Ru(P*)(CO)] gave
lactone products with the (1R,5S) configuration only.
However, the absolute configuration of lactones formed
from allylic acetates having substituent at R3 (1e–g) have
were recently reported. If we assume that the Rh(III)–
and Ru(II)–porphyrin catalysts react with EDA in the
intermolecular cyclopropanation reactions to form active
metallocarbene intermediates with similar structures, the
alkene should approach the active metallocarbene com-
plexes from either the si or re face as represented by 3
and 4, respectively (which are viewed along the MꢀC
bond axis of the presumed metallocarbene). In these two
cases, the CꢀH bond of the metallocarbene bisects the
NꢀRhꢀN angle. The enantioselectivity of the intermolec-
ular alkene cyclopropanation is determined by the inter-
action between the chiral porphyrin ligand and the
substituents on the alkene as it approaches the metallo-
carbene center in the transition state. In 3, the reacting
double bond approaches the carbene from the less
hindered re face of the rhodium carbene moiety. On the
other hand, the reacting double bond approaches the
carbene from the more hindered si face of the rhodium
carbene in 4. If the orientations of the double bond with
respect to the face of the catalyst are the same in 3 and
4, these two spatial arrangements necessarily lead to the
formation of enantiomeric cyclopropyl lactones. Fur-
thermore, based on the intermolecular cyclopropanation
opposite
configuration
when
[Rh(P*)(I)]
and
[Ru(P*)(CO)] were used. For example, with a Ph group
at the R3 position (1g), the configuration of the lactone
formed from [Ru(P*)(CO)] and [Rh(P*)(I)] are (1S,5R)
and (1R,5S), respectively.
The mechanism of the rhodium porphyrin-catalyzed
cyclopropanation has been studied by Kodadek and
co-workers.6b,e An intermediate complex observed at
−40°C, identified as the diazonium ion adduct was found
to be inactive in the cyclopropanation. Although the
mechanism is still elusive, a Rh porphyrin carbene
complex had been proposed as the active intermediate6b,e
though such a species has never been isolated or properly
characterized. However, X-ray crystal structures of chiral
Ru(II)–7b and Os(II)–porphyrin carbene complexes5e

P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
841
results, the observed enantioselectivity and stereoselec-
tivity of the Rh–porphyrin-catalyzed reactions is consis-
tent with the alkene approach trajectories depicted in
representations 5 and 6. In both cases, the alkene
approaches the carbene in a perpendicular fashion.
These proposals are consistent with the mechanism
previously proposed.6b,e
propanations observed with catalyst [Rh(P*)(I)] are
consistent with the alkene approach trajectories
depicted in representations 9 and 10. In these two
representations, the carbonꢀcarbon double bond of the
substrate approaches to the carbene centre perpendicu-
lar to the metal carbene bond. For cis-disubstituted and
trisubstituted allylic diazoacetates, configuration
9
In intramolecular cyclopropanation, if allylic acetate 1
reacts with metalloporphyrin to form active metallocar-
bene intermediates, there should be two possible
configurations 7 and 8. In 7, the reacting double bond
approaches the carbene from the less hindered re face
of the rhodium carbene bond. On the other hand, the
reacting double bond approaches the carbene from the
more hindered si face of the rhodium carbene bond in
8. If the orientations of the double bond with respect to
the face of the catalyst are the same in 7 and 8, these
two spatial arrangements necessarily lead to the forma-
tion of enantiomeric cyclopropyl lactones.
should be favored, as the interaction of the R2 group
with the catalyst face is very pronounced in 10. trans-
Disubstituted allylic diazoacetates should favor 10
because interactions of the R1 and R3 groups with the
catalyst face are most pronounced in 9. For ruthenium–
porphyrin-catalyzed reactions, the absolute configura-
tions of the products obtained are the same with
rhodium–porphyrin catalyst except for trans-disubsti-
tuted allylic diazoacetates. Since the enantioselectivities
observed with this class of substrates are low, we do not
want to speculate about the mechanism of the reaction
at the moment. Work studying the mechanism of the
ruthenium porphyrin-catalyzed cyclopropanation reac-
tion is now underway.
According to the intramolecular cyclopropanation
results, we propose that the selectivities for allylic cyclo-

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P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
3H), 3.49–3.62 (m, 8H), 7.35 (s, 4H), 8.63–8.78 (m, 8H).
Anal. calcd for C84H76N4RhI·2H2O·CH3CN: C, 71.31;
H, 5.78; N, 4.83. Found: C, 71.45; H, 6.10; N, 4.72%.
4.3. General procedure for catalytic cyclopropanation
A typical procedure is given for the reaction of styrene
with EDA in the presence of [Rh(P*)(I)] (cata-
lyst:EDA:substrate=1:2000:10000). The reaction was
carried out under nitrogen. A solution of [Rh(P*)(I)] (2
mg, 2.45 mmol) and styrene (2.65 g, 25.4 mmol) in
dichloromethane (10 ml) was stirred at ambient temper-
ature for 30 min. To the mixture was added EDA (0.58
g, 5.08 mmol) dropwise with the aid of a syringe pump
over a period of 8 h and the mixture was stirred for a
further period of ca.12 h. The trans/cis ratio was deter-
mined by GC–MS after purification by flash column
chromatography (15:1 hexane:Et2O as eluent).
3. Conclusion
The intermolecular cyclopropanations catalyzed by
[Rh(P*)(I)] proceed in good yields with moderate to
good enantioselectivities. Moderate enantioselectivities
were observed for intramolecular cyclopropanation of
allylic diazoacetate using [Rh(P*)(I)] as catalysts. The
reactivities of the Ru and Rh catalysts using the same
chiral D4 porphyrin are quite different. The metal-
dependant ‘switch’ in enantioselectivity for the trans-
allyl diazoacetate indicates that the catalysts might
react through a different mechanism.
4.4. General procedure for cyclopropanation of allylic
diazoacetates in the presence of [Ru(P*)(CO)]
A solution of the allylic diazoacetate (0.25 mmol) in
anhydrous CH2Cl2 (6 mL) was added dropwise to a
stirred solution of catalyst [Ru(P*)(CO)] (0.2 mol%) in
CH2Cl2 (1.5 mL) at room temperature. During this time
the initial red-orange color of the reaction solution
imparted by [Ru(P*)(CO)] catalysts become brown.
After addition was complete, the reaction mixture was
then stirred for about 15 h under a nitrogen atmo-
sphere. The crude product was purified by flash chro-
matography eluting with hexane/ Et2O mixtures.
GC–MS and NMR analyses were performed to charac-
terize the product.
4. Experimental
4.1. Materials
Tetracarbonylbis(m-chloro) dirhodium and ethyl dia-
zoacetate were purchased from Strem and Aldrich,
respectively. H2P* was prepared according to the litera-
ture method.9 Alkenes are purified by either vacuum
distillation or chromatography just prior to use. Allylic
and homoallylic diazoacetates were prepared according
to the literature procedure.12
4.5. General procedure for cyclopropanation of allylic
diazoacetates in the presence of [Rh(P*)(I)]
A solution of the diazoacetate (0.25 mmol) in anhy-
drous CH2Cl2 (6 mL) was added dropwise to a well-
stirred solution of Rh(P*)(I) catalyst (0.2 mol%) in
CH2Cl2 (1.5 mL) at 40°C. Over this time the initial light
brown color of the Rh(P*)(I) catalysts often turned
dark brown. After addition was complete, the reaction
mixture was then stirred for about 15 h under a nitro-
gen atmosphere. The crude product was purified by
flash chromatography eluting with hexane/Et2O mix-
4.2. Preparation of [Rh(P*)I]
A mixture of H2P* (50 mg), tetracarbonylbis(m-chloro)
dirhodium (50 mg) and anhydrous sodium carbonate
(100 mg) in decalin (20 ml) was heated under reflux
under an inert atmosphere for 2 days. An orange
fraction was obtained by column chromatography with
CH2Cl2 as eluent. To the concentrated solution, iodine
(50 mg) was added. The mixture was stirred at room
temperature for 6 h. Removal of the solvent gave a
brown solid, which was washed and purified by column
chromatography with toluene as the eluent. A red
orange solid characterized as Rh(P*)I was obtained in
37% yield. +ve FAB MS: m/z cluster at 1371(M+),
1244(M+−I); UV–vis/CH2Cl2 umax (log m): 526(18),
412(94), 374(35), 296(39); IR (KBr): 2959.9, 2920.1,
1
tures. GC–MS and H NMR analyses were performed
to characterize the product.
4.6. Ee analysis and comparison of absolute configura-
tions of intramolecular cyclopropanation products
4.6.1. (1a,5a)-3-Oxabicyclo[3.1.0]hexan-2-one, 1a. Enan-
tiomer separation was carried out on a 30 m Chiraldex
G-TA column operated at 80°C for 2 min and then
programmed to 150°C at 1°C/min: 50.8 min for the
(1R,5S) enantiomer, 54.8 min for the (1S,5R)
enantiomer.
1
2867.7, 1701.03, 1295.6, 1015.71; H NMR (300 MHz,
CDCl3): l 1.05–1.50 (m, 32H), 1.75–1.96 (m, 16H),
2.62–2.65 (m, 3H), 2.72–2.75 (m, 2H), 2.88–2.91 (m,

P.-F. Teng et al. / Tetrahedron: Asymmetry 14 (2003) 837–844
843
4.6.2.
(1a)-5a-Methyl-3-oxabicyclo[3.1.0]hexan-2-one,
under nitrogen. The solution was stirred at ambient
temperature for 30 min. Ethyl diazoacetate (1 mmol) in
dichloromethane (1 ml) was added dropwise to the
reaction mixture. The mixture was allowed to stir for 12
h at room temperature. The ratios of the resulting
cyclopropanes were determined by GC analysis.
1b. Enantiomer separation was carried out on a 30 m
Chiraldex G-TA column operated at 110°C: 44.6 min
for the (1S,5R) enantiomer, 45.8 min for the (1R,5S)
enantiomer.
4.6.3. (1a)-5a-Phenyl-3-oxabicyclo[3.1.0]hexan-2-one, 1c.
Enantiomer separation on a 30 m Chiraldex G-TA
column operated at 125°C: 243.6 min and 251.7 min for
two enantiomers.
Acknowledgements
4.6.4. (1a,5a)-6a-Phenyl-3-oxabicyclo[3.1.0]hexan-2-one,
1d. The two diastereomers were separated by column
chromatography on silica gel using hexane/ Et2O (1:1).
Enantiomeric excess for each diastereomer was deter-
mined using a 30 m Chiraldex G-TA column operated
at 150°C: (i) for trans isomer: 88.3 min for the (1S,5R)
enantiomer, 96.6 min for the (1R,5S) enantiomer; for
cis isomer: 86.4 min and 92.1 min for the two
enantiomers.
This research was supported by the Research Grants
Council of Hong Kong (CERG Grant No. CityU 1095/
01P) and partially supported by the Area of Excellence
Scheme established under the University Grants Com-
mittee of the Hong Kong Special Administrative
Region, China (Project No. AoE/P-10/01).
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and 36.3 min for the two enantiomers.
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1
respectively; H NMR (300 MHz) l 4.41 (dd, 1H), 4.15
(d, 1H), 2.30–2.18 (comp, 2H), 1.48–1.38 (m, 2H),
1.12–1.05 (m, 3H).
4.6.7. (1a,5a)-6b-Phenyl-3-oxabicyclo[3.1.0]hexan-2-one,
1g. Enantiomer separation was carried out on a 30 m
Chiraldex G-TA column operated at 150°C: 91.15 min
for the (1S,5R) enantiomer and 99.7 min for the
(1R,5S) enantiomer.
4.6.8. (1a,5a)-6,6-Dimethyl-3-oxabicyclo[3.1.0]hexan-2-
one, 1h. Enantiomer separation was carried out on a 30
m Chiraldex G-TA column operated at 120°C: 16.6 min
for the (1S,5R) enantiomer, 21.4 min for the (1R,5S)
enantiomer.
4.6.9. (1a,5a)-6a-Methyl-6b-(4-methyl-3-penten-1-yl)-3-
oxabicyclo[3.1.0]hexan-2-one, 1i. Enantiomer separation
was carried out on a 30 m Chiraldex G-TA column
operated at 150°C: 62.4 min for the (1S,5R) enantiomer
and 67.9 min for the (1R,5S) enantiomer.
4.6.10. (1a,5a)-6b-Methyl-6a-(4-methyl-3-penten-1-yl)-3-
oxabicyclo[3.1.0]hexan-2-one, 1j. Enantiomer separation
was carried out on a 30 m Chiraldex G-TA column
operated at 150°C: 59.3 min for the (1S,5R) enantiomer
and 63.8 min for the (1R,5S) enantiomer.
4.7. General procedure for competition reactions
To a two-necked round-bottomed flask were added
[Rh(P*)I] (0.05 mol%), styrene (2.5 mmol) and substi-
tuted styrene (2.5 mmol) in dichloromethane (2 ml)

844
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