Alkene cyclopropanation catalyzed by Halterman iron porphyrin: Participation of organic bases as axial ligands

Article abstract of DOI:10.1039/b606757c

With the iron(iii) complex of the Halterman iron porphyrin [P*Fe(Cl)] and ethyl diazoacetate (EDA) as catalyst and carbene source, respectively, styrene-type substrates were converted to cyclopropyl esters with high trans/cis ratio (not less than 12) and high enantioselectivity for the trans-isomers (74-86% ee). The isomeric distribution of the cyclopropyl esters so obtained is akin to that obtained from the previously reported Ru(ii) counterpart [P*Ru(CO)]. A linear Hammett correlation log(k_X/k_H) = σ⁺ρ was observed with ρ = -0.57 suggesting the involvement of an electrophilic cyclopropanating species derived from the iron(ii) center as the reactive intermediate in the catalytic cycle. This is further supported by a dramatic decrease in the enantioselectivity and trans/cis ratio observed in an experiment of styrene cyclopropanation when the reaction mixture was deliberately exposed to air. Axial ligand effects on the selectivities was also investigated. Substantial improvement in trans/cis ratios could be achieved by addition of organic bases such as pyridine (py) and 1-methylimidazole (MeIm) to the catalytic reaction. The existence of axially ligated iron carbene moieties, [P*Fe(CHCO₂Et)(py)] and [P*Fe(CHCO₂Et)(MeIm)], was established by electrospray mass spectrometry. Study of secondary kinetic isotope effect indicated that a more product-like transition state was generated by addition of MeIm. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606757c

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www.rsc.org/dalton | Dalton Transactions
Alkene cyclopropanation catalyzed by Halterman iron porphyrin:
participation of organic bases as axial ligands
a
a
a
a
a
Tat-Shing Lai,* Fung-Yi Chan, Pui-Kin So, Dik-Lung Ma, Kwok-Yin Wong* and
Chi-Ming Che
b
Received 15th May 2006, Accepted 7th August 2006
First published as an Advance Article on the web 22nd August 2006
DOI: 10.1039/b606757c
With the iron(III) complex of the Halterman iron porphyrin [P*Fe(Cl)] and ethyl diazoacetate (EDA) as
catalyst and carbene source, respectively, styrene-type substrates were converted to cyclopropyl esters
with high trans/cis ratio (not less than 12) and high enantioselectivity for the trans-isomers
(
74–86% ee). The isomeric distribution of the cyclopropyl esters so obtained is akin to that obtained
from the previously reported Ru(II) counterpart [P*Ru(CO)]. A linear Hammett correlation
+
log(k
X
/k
H
) = r q was observed with q = −0.57 suggesting the involvement of an electrophilic
cyclopropanating species derived from the iron(II) center as the reactive intermediate in the catalytic
cycle. This is further supported by a dramatic decrease in the enantioselectivity and trans/cis ratio
observed in an experiment of styrene cyclopropanation when the reaction mixture was deliberately
exposed to air. Axial ligand effects on the selectivities was also investigated. Substantial improvement in
trans/cis ratios could be achieved by addition of organic bases such as pyridine (py) and
1
-methylimidazole (MeIm) to the catalytic reaction. The existence of axially ligated iron carbene
moieties, [P*Fe(CHCO Et)(py)] and [P*Fe(CHCO Et)(MeIm)], was established by electrospray mass
2
2
spectrometry. Study of secondary kinetic isotope effect indicated that a more product-like transition
state was generated by addition of MeIm.
Introduction
excess for the trans-isomer obtained with the chiral iron porphyrin
was significantly lower than that in the case of Ru(II) counterpart.
The highest enantioselectivity of alkene cyclopropanation using
chiral iron porphyrin as catalyst was reported by Woo and co-
workers, and the enantiomeric excess of the trans-cyclopropyl ester
from styrene was up to 42% with ethyl diazoacetate (EDA) as
Iron and ruthenium porphyrins have been an important class of
catalyst for group and atom transfer reactions such as alkene
cyclopropanation, aziridination and aldehyde olefination. Com-
1
2
3
4
pared to other excellent transition metal catalysts, ruthe-
nium porphyrins feature high catalytic turnovers and enticingly
high diastereoselectivity for the preferential formation of trans-
cyclopropyl ester in alkene cyclopropanation using diazo esters as
1r
carbene source.
This report describes the competent catalytic performance of
8
Halterman iron porphyrin [P*Fe(Cl)] (Fig. 1) in enantioselective
1
p
carbene sources. In fact, unrelenting efforts have been directed to
alkene cyclopropanation without adding one-electron reductant
although the addition of reductant for converting iron(III) to
iron(II) or directly employing the iron(II) complex is generally
necessary to turn on the efficient catalysis. Axial ligand effect was
1,5
carbene transfer reactions mediated by metalloporhyrins. This
may be one of the reasons that trans-cyclopropyl esters are of
high synthetic utility as they are important organic intermediates
for drug production, as in the case of tranylcypromine (an
antidepressant) and a melantolin agonist for the treatment of
sleep disorders. Despite the periodic relationship between iron
6
7
and ruthenium, asymmetric versions of iron-catalyzed alkene
cyclopropanation are unexpectedly sporadic. In fact, chiral iron
carbene complexes were shown to be efficient stoichiometric
1r,s
cyclopropanating reagents.
In a report of comparing iron and ruthenium metal centers
with the same chiral D -symmetrical porphyrin ligand in styrene
2
1n
cyclopropanation by Gross et al., the same level of trans/cis
ratio for the cyclopropyl esters were obtained but the enantiomeric
a
Department of Applied Biology and Chemical Technology and Central
Laboratory of the Institute of Molecular Technology for Drug Discovery
and Synthesis, The Hong Kong Polytechnic University, Hunghom, Kowloon,
Hong Kong, China. E-mail: bcringo@polyu.edu.hk; Fax: + (852)2364-9932
b
Department of Chemistry and Open Laboratory of Chemical Biology of the
Institute of Molecular Technology for Drug Discovery and Synthesis, The
University of Hong Kong, Pokfulam Road, Hong Kong
Fig. 1 Structure of 1.
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also examined because of the trans-relationship to the transferable
carbene moiety and the common phenomenon that participation
of axial ligand can fundamentally determine or alter the reactivity
and selectivity of various similar catalytic systems including alkene
of involving Fe(II) intermediacy was further supported by the
dramatic decreases in both the % ee of the trans-isomer and
trans/cis ratio (entry 2) when the reaction mixture was exposed to
air. The negative effects were attributed to that Fe(III) porphyrin
became the major catalytic species in aerobic conditions.
9
2
1
epoxidation and aziridination and cyclopropanation.
Lowering the catalyst loading to 0.02% lead to a product
turnover over 1000 and improvements in both trans/cis ratio and
percentage enantiomeric excess of the trans-isomer were observed.
It is speculated that the improvements in the selectivities may be
due to a structural modification of the porphyrin ligand in the high
Results
Catalytic cyclopropanation catalyzed by P*Fe(Cl)
According to the shape preference for terminal alkene substrates
in cyclopropanation with achiral iron porphyrin as catalyst
reported by Woo and co-workers, styrene, p-substituted styrenes,
a-methylstyrene and 1,1-diphenylethylene were chosen as the
substrate spectrum in this study. Ethyl diazoacetate (EDA) was the
limiting reactant for the purpose of suppressing the formation of
diethyl furmurate and diethyl maleate and 0.2% catalyst was used.
Under an inert atmosphere, monosubstituted aromatic alkenes
were smoothly converted to cyclopropyl esters and the product
yields range from 56 to 72% (Table 1). GC analyses indicated very
clean product profiles and only trace diethyl fumarate and maleate
were detected in all cases. For styrene, the addition of cobaltocene
gave almost the same enantiomeric excess (ca. 80% ee) (entry 3) but
a moderate increase in trans/cis ratio when compared to the case
without the one-electron reductant (entry 1). So, iron(II) porphyrin
could be generated in situ with EDA, which could be a reductant,
to account for the high % ee of the trans-isomer and the high
trans/cis ratio. In fact, trans/cis higher than 8 could be considered
as an indication of the involvement of an Fe(II) intermediate for
sterically unhindered tetraarylporphyrin systems. This suggestion
10
turnover condition. In an experiment of 0.002% catalyst loading,
4
10
turnovers were attained and also no detrimental effects
on the selectivities were observed! Similarly, p-chlorostyrene, p-
methylstyrene and p-methoxystyrene were cyclopropanated af-
fording similar product isomeric distribution (high trans/cis ratio,
high % ee for the trans-isomer) which is akin to those obtained
with the ruthenium(II) analogue [P*Ru(CO)] previously reported
11
by our laboratory. Satisfactory enantioselectivity (not less than
8
0% ee) was also obtained with 1,1-disubstituted alkenes including
a-methylstyrene and 1,1-diphenylethylene (entries 8 and 9).
Hammett kinetic study
The cyclopropanation rates of para-substituted styrenes p-
XC
6
H
4
CH=CH
2
(X = OCH
3
, CH
3
, Cl, NO ) relative to that
2
of styrene were determined through competition experiments
for EDA cyclopropanation in dichloromethane by employing
equimolar amounts of styrene and the para-substituted derivatives.
The relative rates k
cyclopropyl esters derived from p-XC
X
/k , defined as the molar ratio of resultant
H
H
4
CH=CH
2
to that derived
6
a
Table 1 Enantioselective cyclopropanation of alkenes with EDA using Fe(P*)(Cl) as catalyst
e
g
Entry
Substrate
Styrene
Product
Isolated yield (%)
trans/cis
trans ee (%)
cis ee (%)
TON
f
f
f
1
2
3
4
5
6
7
8
9
Ethyl 2-phenylcyclopropane
carboxylic acid ester
Ethyl 2-phenylcyclopropane
carboxylic acid ester
Ethyl 2-phenylcyclopropane
carboxylic acid ester
Ethyl 2-phenylcyclopropane
carboxylic acid ester
Ethyl 2-(4-chlorophenyl)cyclopropane
carboxylic acid ester
Ethyl 2-(4-methylphenyl)cyclopropane
carboxylate
2-(4-Methoxyphenyl)cyclopropane
carboxylic acid ester
Ethyl 2-methyl-2-phenylcyclopropane
carboxylic acid ester
71
64
60
70
57
56
65
68
72
12 : 1
80
1.1
368
200
c
f
f
f
Styrene
4 : 1
43
1.6
d
f
f
f
Styrene
18 : 1
81
3.4
405
b
f
f
f
Styrene
23 : 1
86
6
1218
284
g
g
g
4-Chlorostyrene
4-Methylstyrene
4-Methoxystrene
a-Methylstyrene
1,1-Diphenylethene
18 : 1
75
32
g
g
12 : 1
79
Not determined
424
416
390
410
g
g
g
13 : 1
74
43
f
f
f
3 : 1
81
16
h
Ethyl 2,2-diphenylcyclopropane
carboxylic acid ester
—
83
a
Reaction conditions: catalyst : EDA : alkene = 1 : 500 : 1500, CH
2
Cl
2
, room temperature, stirring during 4 h addition time followed by 1 h stirring
Cl , room temperature, stirring during 4 h addition time followed by 1 h
b
under N
2
.
Reaction conditions: catalyst : EDA : alkene = 1 : 2000 : 6000, CH
2
2
c
d
e
f
2
stirring under N . Under air. Adding cobaltocene. Isolated product yields based on EDA. Determined by chiral GC-FID (column: Varian cyclodex
g h i
B, length 25 m). Determined by GC-MS after conversion to the L-menthyl ester. Determined by chiral HPLC (Daicel OJ). Calculated as the amount
of cyclopropyl esters divided by the amount of catalyst.
4
846 | Dalton Trans., 2006, 4845–4851
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Fig. 3 The side view of diastereoselectivity of intermolecular cyclo-
propanation.
Fig. 2 Hammett plot for the cyclopropanation of para-substituted
styrenes with EDA using Fe(P*)(Cl) as the catalyst.
propanation. Apart from solvation effect for stabilizing polar
diastereomeric intermediates to different degrees, axial ligation
of the donor solvent onto the metallo-carbene moiety could be
another plausible explanation for the enhancement. In this work,
various organic bases were screened for seeking an alternative
way to improve the trans-selectivity and to verify the involvement
of axially ligated monocarbene as the active cyclopropanating
agent in the catalytic cycle. Results of styrene cyclopropanation
with P*Fe(Cl) in the presence of various nitrogen-containing
heterocycles, 4-phenylpyridine N-oxide and dimethyl sulfoxide
(DMSO) are collected in Table 2. Obviously, substantial increases
in trans/cis ratio were observed in all cases of adding the axial
ligand and the best result (trans/cis = 33) was obtained with
pyridine (entry 2). The use of 4-dimethylaminopyridine (DMAP)
and MeIm, both of which possess high p-electron density, gave
trans/cis ratios (ca. 25) lower than that in the case of using pyridine.
The trans-selectivity obtained with N-methylpyrrolidine, being
from p-HC
plot [log(k
6
H
/k
4
CH=CH
2
, styrene. The corresponding Hammett
) vs. r ] is shown in Fig. 2 with q = −0.57. This
+
H
X
1i
value is comparable to that obtained with Fe(TTP) (−0.68)
10
and P*Ru(CO) (−0.44). The negative slope is consistent with
the proposal of an electrophilic iron carbene intermediate and
suggests an accumulation of positive charge of some degree on
the benzylic carbon of the styrenes. It is the trend that the rate of
cyclopropanation is accelerated with electron-donating groups on
the para position. p-Nitrostyrene being the most electrodeficient
alkene gave the slowest reaction rate among the four substrates. p-
Chlorostyrene, however, reacted somewhat faster than styrene and
this may indicate that the rate-limiting transition state, of which the
benzylic carbon carrying some cationic character, was stabilized
by the chloro substituent via mesomeric interaction. Together with
the high trans-selectivity, it may also indicate a more product-
like transition state when compared with rhodium(III) porphyrin
1e
3
catalysts. The transition state previously proposed by Woo and
a sp nitrogen donor, produced no further improvement when
co-workers to account for the high trans-selectivity and positive
charge accumulation on the substrate part is depicted in Fig. 3.
compared with DMAP and MeIm. Using heteroatom oxides, 4-
phenylpyridine N-oxide and DMSO could also afford significant
enhancement in trans-selectivity.
1i
Effect of axial ligand on styrene cyclopropanation
Secondary kinetic isotope effect
The use of donor solvents such as tetrahydrofuran (THF) and
diethyl ether was found to enhance the trans-selectivity for
both iron and ruthenium porphyrin-catalyzed styrene cyclo-
Woo and co-workers probed the nature of transition state of iron-
mediated alkene cyclopropanation by investigating the secondary
1i
1p
1i
a
Table 2 Cyclopropanation of styrene with various organic bases as axial ligands
b
b
b
c
Entry
Organic base
trans/cis
trans ee (%)
cis ee (%)
TON
1
2
3
4
5
6
7
Cl
12
33
24
26
27
23
17
80
82
81
83
86
83
82
1.1
6.8
13.0
2.3
13.0
1.2
3.6
368
307
321
275
293
209
385
Pyridine (py)
DMAP
1-Methylimidazole (MeIm)
1-Methylpyrrolidine
4-Phenylpyridine N-oxide
DMSO
a
Reaction conditions: catalyst : organic base : EDA : alkene = 1 : 20 : 500 : 1500, CH
2 2
Cl , room temperature, stirring during 4 h addition time followed
Determined by chiral GC-FID (column: Varian cyclodex B, length 25 m). Calculated as the amount of cyclopropyl esters
b
c
by 1 h stirring under N
2
.
divided by the amount of catalyst.
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kinetic isotope effect (SKIE) in a competition experiment between
styrene and d -styrene and made an important comparison to
8
1e
the case of a rhodium porphyrin. This provided information
of rehybridization in the rate-limiting transition state. With this
methodology, the effect of axial ligand on the nature of transition
state could therefore be probed by examining the k
obtained with and without axial ligand in styrene cyclopropa-
nation catalyzed by P*Fe(II). An inverse SKIE (k /k ) of 0.87
H
/k
D
values
H
D
(
± 0.01) was observed in the absence of axial ligand. Interestingly,
the SKIE value was lowered to 0.81 (± 0.01) in presence of MeIm.
2
This indicated that a higher degree of rehybridization from sp to
3
sp of styrene in the transition state occurred in the case of adding
axial ligand. Since SKIE is a kinetic phenomenon not involving
the cleavage of C–H bond, the change in the magnitude in the case
of no fundamental mechanistic change for a particular reaction
is therefore bound to be small. In our case the difference between
−
0.87 and −0.81 is significant enough to indicate the direction of
the shift of transition state towards product side. In other words,
a more-product like transition is developed in the presence of
axial ligand. This mechanistically explains the improvements of
trans/cis ratio in cyclopropanation of styrenes upon adding axial
ligands.
Six-coordinate iron monocarbene as the active intermediate for
alkene cyclopropanation
Electrospray ionization as a very soft ionization method in
mass spectrometry was employed to detect the target mole-
III
cular ions generated from [P*Fe(CHCO
2
Et)(py)] and [P*Fe-
Fig. 4 (a) The simulated spectra of Fe (P*)(C H N); (b) the most
5
5
III
(
CHCO Et)(MeIm)]. EDA, P*Fe(Cl) and pyridine or imidazole
2
abundant ions in the ES mass spectra of Fe (P*)(C
5
H
5
N).
were mixed in dichloromethane in a small reaction vial equipped
with a rubber septum. The mixtures were then subjected to ESMS
+
analyses. The mass spectra of the molecular ion targets (M + H)
are shown in Fig. 4(a) and 5(a). The declustering voltage was
lowered to 10.0 V in order to observe the axially ligated iron
monocarbene moiety. Simulations of isotopic distributions for
the target ions C93
and C92
H
87
N
5
O
2
Fe [P*Fe(CHCO
2
Et)(py)] (Fig. 4(b))
H
88FeN
6
O
2
[P*Fe(CHCO Et)(MeIm)] (Fig. 5(b)) were per-
2
formed for confirming the identity, and good agreements between
the simulation and experimental results were obtained. ESMS
detection of [P*Fe(CHCO
2
Et)(py)] and [P*Fe(CHCO Et)(MeIm)]
2
and the improvements in trans-selectivity upon addition of the
organic bases could therefore be ascribed to the formation
of axially ligated iron monocarbene in the catalytic cycle for
cyclopropanating alkenes.
Discussion
1p,11
With reference to the previous report of using (+)-P*Ru(CO),
the same antipode of the D -symmetrical porphyrin [(+)-P*H ]
4
2
was used in this work. Because of the preference of produc-
ing trans-isomer and that the absolute configuration of major
stereoisomer of cyclopropyl ester derived from styrene is (1S,2S),
the asymmetric induction model for styrene cyclopropanation
III
Fig. 5 (a) The simulated spectra of Fe (P*)(C
4
H
6
N
2
); (b) the most
1p,11
III
mediated by (+)-P*Ru(CHCO
the stereochemical outcome obtained with (+)-P*Fe(CHCO
as the reactive species. It is also a logical extension of the
2
Et)
could be used to explain
abundant ions in the ES mass spectra of Fe (P*)(C H N ).
4 6 2
2
Et)
with the same chiral environment to produce the characteristic
distribution set of stereoisomers. Again, the subtle steric difference
between the methano and ethano bridges on the chiral wings
2
+
2+
mechanistic aspect that Ru and Fe should be capable of
generating transition states of similar geometry for interacting
4
848 | Dalton Trans., 2006, 4845–4851
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operates with high enantiocontrol in cyclopropanation of aromatic
terminal alkenes (Fig. 6).
Fig. 6 The enantioselectivity of the intermolecular alkene cyclo-
propanation.
Scheme 1 The proposed catalytic cycle of alkene cyclopropanation and
participation of organic base.
There are some fully characterized stable monocarbene com-
12
1p,13
1f ,h,q,r,14
plexes of iron, ruthenium
by porphyrin ligands in the literature. They are, however,
disubstituted carbenes such as M=CPh , M=CPh(CO
Et),
M=C(CO
Et) , whose stability may be attributed to the steric
and osmium
supported
1
6
In the examples of iron disubstituted carbene (Fe=CPh
)
2
13a
and (Fe=CCl
supported by porphyrin ligands, the iron(IV)
state was assigned for the iron monocarbene moieties based
)
2
2
2
2
2
12
nature around the electrophilic carbene and the C=C double bond
of the alkene substrates are so hindered that a significantly higher
activation energy barrier has to be overcome for the formation of
a product-like transition state in the course of carbene transfer. In
fact, the ligated disubstituted carbene moiety of the stable metal
complexes can, in general, be rendered transferable to organic
substrates by employing relatively high reaction temperature in
the presence of axial ligands (e.g. in refluxing toluene and in
on M o¨ ssbauer spectroscopic studies. A high ‘aminophilicity’
can therefore be anticipated for the high valent metal center. The
improvements in trans/cis ratio upon addition of axial ligands
suggest a more product-like transition state is generated for the
catalytic reaction. This is supported by the effect of axial ligand on
the inverse secondary kinetic isotope effect. Attempts were made
to establish the Hammett plots in the presence of axial ligands and
this would directly give clues of the effect of axial ligation on the
electrophilicity of the iron monocarbene moiety. Unfortunately,
the data points obtained in duplicate runs were obviously scattered
and rather irreproducible rendering the study inconclusive. With
the presence of triphenylphosphine for (TPP)Ru=C(CO
Et) ).
2
2
On the other hand, metalloporphyrins ligated with mono-
14
substituted carbene (:CHR), such as (TPP)Os(CHCO Et),
2
1
r
1r
(
TPP)Fe=CH(mesityl) and (TPP)Fe=CH(TMS) could be iso-
N-methylimidazole as axial ligand, the log(k
X
H
/k ) values for
1
lated as impure forms and characterized by H NMR and undergo
various para-substituted styrenes were 0.25 ± 0.02 (p-MeO),
stoichiometric alkene cyclopropanation. It should be pointed out
0.16 ± 0.03 (p-Me), 0.31 ± 0.08 and 0.0063 ± 0.005 (p-NO
2
).
that lengthening of Ru=CPh
environments with the sixth vacancy occupied by axial ligands was
and Fe=CPh
bonds in porphyrin
2
2
Conclusion
15
reported previously by Che and co-workers.
Based on the aforementioned past findings, P*Fe(CH-
CO Et)(py) (3a) is thought to be the active cyclopropanating
Despite the inferior results of asymmetric cyclopropanation
with chiral iron porphyrins so far documented, P*Fe(Cl) was
demonstrated as a competent catalyst for alkene cyclopropanation
in terms of not only catalytic turnovers but also enantio- and
diastereo-controls in this work. Moreover, a prominent axial
ligand effect on trans/cis ratio for styrene cyclopropanation
was described and axially ligated iron monocarbene complexes
supported by the chiral porphyrin were observed with ESMS. This
is the first experimentally based mechanistic proposal of hexa-
coordinate iron monocarbene as a catalytically active species.
This provides a useful insight for the development of iron-based
catalysts for alkene cyclopropanation and other synthetically
useful carbene transfer reactions. In addition, the future of using
iron complexes as catalysts is particularly important because
2
agent when using pyridine as additive. As the participation of
pyridine in the sixth coordination of the monocarbene entity
[
P*Fe(CHCO
ment in trans/cis ratio and by electrospray mass spectrometry,
P*Fe(CHCO Et)(py) (3a) is unambiguously assigned to be the
2
Et)(py)] was revealed by the substantial enhance-
2
cyclopropanating agent, while P*Fe(py) (4) is regarded as the
resting state of the catalyst ready for another catalytic turnover
(
Scheme 1). Moreover, the proposal that donor solvents such as
THF and diethyl ether play a role of axial ligand in promoting
the trans-selectivity claimed in the previous reports of iron
and ruthenium porphyrin-catalyzed styrene cyclopropanation, is
further consolidated.
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the use of cheaper, non-toxic and environmentally benign metal
centers has been a trend in catalyst development for industrial
synthesis of organic compounds.
ligand for the same reaction was similarly determined by adding
MeIm to the mixture of complex 1 and cobaltocene.
Acknowledgements
Experimental
This work was supported by the Area of Excellence Scheme
established under the University Grants Committee of the Hong
Kong Special Administrative Region, China (AoE/P-10/01).
Materials
Styrene, 4-methylstyrene, 4-chlorostyrene, 4-methoxystyrene,
4
-nitrostyrene, 1,1-diphenylethene, a-methylstyrene, ethyl dia-
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1p
literature reported methods. Dichloromethane (A.R., ACROS)
was first washed with conc. H SO (aq.), neutralized and then
2
4
distilled from CaH. Styrene and 4-methoxystyrene were distilled
under vacuum immediately before use. Other alkenes were passed
through a short column of silica gel immediately before use.
All reactions with Fe(P*)(Cl) were carried out under a nitrogen
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001, 123, 4119; (q) Y. Li, J.-S. Huang, Z.-Y. Zhou and C.-M. Che,
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189; (t) J.-L. Zhang, H.-B. Zhou, J.-S. Huang and C.-M. Che, Chem.
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3
1
.6 × 10 mol) in dichloromethane (5 mL) over a period of 4 h
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mixture was stirred for 1 h and then evaporated to dryness in vacuo.
The crude cyclopropyl esters were purified by chromatography on
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2
3
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(
1.5 mmol each) and complex 1 (0.5 mg) were dissolved in
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1
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8
(4.86 ×
−
3
−4
10
mol each) and EDA (1.62 × 10 mol) were added. The
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analyzed by GC-MS in selected ion monitoring mode (detecting
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8
H
D
/k of ionization. The experiment was repeated
1
995, 117, 5763.
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