α-diazoacetates as carbene precursors: Metallosalen-catalyzed asymmetric cyclopropanation

Article abstract of DOI:10.1055/s-2006-926448

Readily available α-diazo compounds have been shown to be efficient carbene precursors for asymmetric cyclopropanation using ruthenium(NO)-salen and cobalt-salen complexes, as catalysts. The stereoselectivity of the cyclopropanation depends on the metal i

Full text of DOI:10.1055/s-2006-926448

SPECIAL TOPIC
1715
a-Diazoacetates as Carbene Precursors: Metallosalen-Catalyzed Asymmetric
Cyclopropanation
M
etallosalen-C
a
a
talyzedAs
t
ymmet
s
ric Cyclop
u
ropanation ya Uchida, Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University, 33, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
E-mail: katsuscc@mbox.nc.kyushu-u.ac.jp
Received 16 March 2006
a-Diazoacetate (ethyl a-diazoacetate) was first synthe-
Abstract: Readily available a-diazo compounds have been shown
sized by Curtius2 in 1883 and, since then, it and its deriv-
atives have been widely used as carbene precursors,
especially for cyclopropanations. Asymmetric carbene
to be efficient carbene precursors for asymmetric cyclopropanation
using ruthenium(NO)–salen and cobalt–salen complexes, as cata-
lysts. The stereoselectivity of the cyclopropanation depends on the
metal ion, its valency, the structural and electronic nature of the transfer to olefins poses two stereochemical issues: 1)
salen ligand, and the apical ligand of the salen complex used. Both
trans- and cis-selective asymmetric intermolecular cyclopropana-
tion can be realized under mild conditions by using a suitable met-
metal complex and the prochiral olefin. In 1965, Nozaki
allosalen complex. Furthermore, a wide range of alkenyl a-
diazoacetates and the related alkenyl diazomethyl ketones undergo
enantioface selection of a prochiral olefin and 2) diaste-
reoselection in the reaction between a prochiral carbene–
et al. for the first time reported transition-metal-mediated
stereoselective carbene transfer reactions, cyclo-
propanation3a and oxetane-ring opening,3b albeit with
modest enantio- and diastereoselectivity. In the wake of
this seminal work, much effort has been directed especial-
ly toward asymmetric cyclopropanation and various ex-
cellent catalysts such as Cu-Schiff base,3 Co-dioximato,4
Cu-semicorrin,5 Cu-bis(oxazoline),6 Ru-Pybox,7 Rh-ami-
no carboxylate,8 and Rh-MEPY,9 have been developed.
Although the enantioselectivities of these reactions are
high to excellent, the diastereoselectivities are moderate
and mostly trans-selective, except for some examples7,10
where both high trans-selectivity and high enantioselec-
tivity were observed. On the other hand, cis-selective
asymmetric cyclopropanation is limited in number. Cu-
mediated cyclopropanation of some specific substrates
shows good cis-selectivity11 and some catalysts show
moderate cis-selectivity.12,13 It can thus be seen that, al-
though asymmetric cyclopropanation chemistry has made
great advancement in the last three decades, some stere-
ochemical problems remain unresolved.
intramolecular cyclopropanation in a highly enantioselective man-
ner by using various metallosalen complexes, easily prepared in a
modular fashion, as catalyst.
Key words: diazoacetate, asymmetric cyclopropanation, metallo-
salen, cobalt, ruthenium
Diazo compounds, upon heating, UV-irradiation, or treat-
ment with some transition metal complexs, decompose to
highly reactive carbenes, which can undergo addition to a
p-bond (cyclopropanation and cyclopropenation), C–H
insertion (C–C bond formation), and ylide formation.
Most of these carbene transfer reactions are accompanied
by the generation of chiral center(s). Although these car-
bene transfer reactions provide unique tools for organic
synthesis, they will become truly useful, if the stereo-
chemistry of the reactions can be controlled at will. De-
composition of diazo compounds under heating or
irradiation give free carbenes and control of their reaction
stereochemistry is difficult. On the other hand, their de-
composition in the presence of a transition-metal complex
generally provides a carbene–transition-metal complex
(hereafter denoted as a carbenoid) and the stereochemistry
of its reaction can be controlled by the ligand coordinated
to the transition metal. Thus, much effort has been direct-
ed toward the development of transition-metal-mediated
carbene transfer reactions (Scheme 1).1 Of the various di-
azo compounds known, a-diazo esters are relatively stable
but can be decomposed, in the presence of a transition-
metal complex under mild conditions, to give the corre-
sponding carbenoid species, capable of carbene transfer to
a range of compounds. Due to the presence of an ester
group, the resulting product can be further transformed to
other useful compounds. Diazo esters have consequently
been extensively studied as carbene precursors.
cyclopropanation
or
cyclopropenation
R
R'
_
+
N
N
MLn
MLn
– N2
ylide formation
R"
Y = RO, RS, I
carbene–metal complex
R"
+
Y
Y
R
R'
_
R
R'
R
R'
X
H
X = C, O, N
H
X
σ-bond insertion
R
R'
Scheme 1
Recently, metallosalen complexes have been demonstrat-
ed to be efficient catalysts for asymmetric oxene and ni-
trene transfer reactions.14,15 Since carbene is an
isoelectronic species of oxene and nitrene, asymmetric
carbene transfer reactions, mainly cyclopropanation, us-
SYNTHESIS 2006, No. 10, pp 1715–1723
x
x.
x
x
.
2
0
0
6
Advanced online publication: 27.04.2006
DOI: 10.1055/s-2006-926448; Art ID: C00406SS
© Georg Thieme Verlag Stuttgart · New York

1716
T. Uchida, T. Katsuki
SPECIAL TOPIC
ing a-diazoacetates as precursors have been intensively Although the mechanism of Co(III)-salen catalyzed cy-
studied with metallosalen complexes as catalyst and it has clopropanation is unclear at present, we have proposed
been found that they show unique catalysis for asymmet- that olefins approach the putative carbenoid species per-
ric cyclopropanation. In this account, asymmetric cyclo- pendicularly along the downward Co–O bond and rotate
propanation using a-diazo esters or their related clockwise to give the major trans-product. In this propos-
compounds as carbene precursors in the presence of tran- al, the Co(V)-salen carbenoid species has been considered
sition-metal complexes, in particular, metallosalen com- to have adopted a stepped conformation (Figure 1).20
plexes, are briefly described.
Co(III)-salen complex (5 mol%)
t
As described above, various chiral metal complexes cata-
N2CHCO2 Bu
+
Ar
Ar
lyze asymmetric cyclopropanation using a-diazo esters as
carbene precursors. Several metallosalen complexes also
catalyze the asymmetric cyclopropanation, and the stere-
ochemistry of the cyclopropanation has been found to
strongly depend on the metal ion used, its valency and the
structure of the salen ligand. Thus, appropriate choice of
a combination of the central metal ion and salen ligand is
crucial for achieving high enantio- and diastereoselectivi-
ty.
Ar
t
t
CH2Cl2, N2, r.t.
CO2 Bu
CO2 Bu
cis
trans
Ar = Ph 1: 79%, trans:cis= 95:5, 64% ee
2: 0%
3: 76%, trans:cis = 98:2, 73% ee
4: 76%, trans:cis = 95:5, 75% ee
5: 55%, trans:cis = 94:6, 83% ee
6: 80%, trans:cis = 96:4, 93% ee
Ar = 4-ClC6H4
6: 86%, trans:cis = 97:3, 96% ee
Ph
N
Ph
N
1: R1, R2, R3 = H, X = I
In 1978, the first asymmetric cyclopropanation using a-
diazoacetates was examined in the presence of chiral
Co(II)-salen complex by the Otsuka and Nakamura
group.16 Although the enantioselectivity of the reaction
was modest (<10% ee), this study demonstrated that a-di-
azoacetates can be decomposed by a metallosalen com-
plex and an optically active salen ligand can serve as a
chiral inducer.
2: R1 = Me, R2, R3 =H, X =I
3: R1 = H, R2 = tBu, R3 = H, X = I
Co
X
5
R3
O
O
R3
4: R1, R2 = H, R3 = tBu, X = I
5: R1, R2 = H, R3 = tBu, X = Br
6: R1, R2 = H, R3 = OMe, X = Br
3
4
R2
R1
R1
R2
Scheme 2
Since our study on metallosalen-mediated oxene transfer
reactions revealed that the stereochemistry of the reaction
depends also on the nature of the apical ligand,17 we were
intrigued by the catalysis of Co(III)-salen complexes that
carry an apical ligand.18
tBuO2C
CH
OMe
Ph
N
O
Ph
N
O
t
CO2 Bu
Co
3'
MeO
Br
3
Ar
Ar
In the asymmetric oxene transfer reaction, the C3- and
C3¢-substituents of the salen ligand play an important
role.19 Thus, we first examined asymmetric cyclopropana-
tion of styrene with tert-butyl a-diazoacetate in the pres-
ence of a Co(III)-salen complex bearing 3,3¢-di-tert-butyl
groups. However, the Co(III)-salen complex showed no
catalytic activity. To our surprise, complex 2, possessing
small methyl groups at C3 and C3¢ was also found to be
catalytically inactive (Scheme 2). This result suggested
that the olefin approaches along the Co–O bond axis. In
accordance with this speculation, the reaction with com-
plexes (1, 3, and 4) bearing no substituent at C3(3¢) pro-
ceeded smoothly and showed high trans-selectivity and
moderate enantioselectivity. Introduction of a tert-butyl
group at C4(4¢) or C5(5¢) somewhat improved the enanti-
oselectivity. The reaction with complex 5, bearing an api-
cal bromo-ligand instead of an iodo-ligand, showed
further improved enantioselectivity and introduction of an
electron-donating methoxy group at C5(5¢) improved
enantioselectivity up to 93% ee without decreasing trans-
selectivity. The effects of the bromo-ligand and the meth-
oxy group have been attributed to a shift of the transition
state of the reaction to a more product-like one, due to the
weak trans effect of the apical ligand and the electron-do-
nating nature of the methoxy group.
trans-isomer
(1S,2S)
Figure 1
As mentioned above, most of the asymmetric cyclopropa-
nations are trans-selective. This is probably because the
incoming olefin rotates to avoid steric repulsion between
the olefinic substituent and the carbenoid ester group
(Figure 1). Therefore, we expected that cis-selective cy-
clopropanation should be realized if the olefin is forced to
rotate in the opposite direction by some means. Although
several moderately cis-selective catalysts and some cis-
selective cyclopropanation of limited substrates have
been reported,12,13 we wanted to develop an intrinsic and
highly cis-selective cyclopropanation. We speculated that
the olefin might rotate to give a cis-product, if some suit-
able chiral auxiliaries are positioned on either side of ole-
fins approach, with a space that allows access.
Fortunately, we recently found that a Ru(NO)-salen com-
plex served as an excellent catalyst for oxene transfer re-
action (epoxidation) under photoirradiation.21a Since
oxene and carbene are isoelectronic and the Ru–Oeq bond
is ca. 0.2 Å longer than the corresponding Co–Oeq
bond,21b,22 we expected that the Ru(NO)-salen complex 7
or 8 could provide a sufficiently open space for olefins ac-
cess, between the C3- and the C3¢-substituents. We thus
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

SPECIAL TOPIC
Metallosalen-Catalyzed Asymmetric Cyclopropanation
1717
Table 1 Asymmetric Cyclopropanation of Styrene with Ru(NO)–
Salen Complex as Catalyst
(S) or (R)
*
N
*
N
Ru(NO)–salen complex (5 mol%)
t
NO
N2CHCO2 Bu
+
Ph
Ru
Cl
Ph
Ph
t
t
solvent, hν, N2, r.t.
O
O
CO2 Bu
CO2 Bu
Ph Ph
cis
trans
Entry Cat.
Solvent
–
Yield (%) cis:trans % ee (cis)
(aR)
1
2
3
4
5
6
7
8
8
8
8
8
12
53
10
18
0
37:63
80:20
68:32
96:4
–
12 (1S, 2R)
81 (1S, 2R)
83 (1R, 2S)
99 (1S, 2R)
–
7: (S,aR)
8: (R,aR)
–
Figure 2
Hexane
THF
MeCN
examined the asymmetric cyclopropanation of styrene
with the Ru(NO)–salen complexes as catalyst (Table 1).23
We examined the cyclopropanation of styrene with com-
plexes 7 and 8 under photo-irradiation, and found that the
reaction in the presence of complex 8 showed good cis-
and enantioselectivity (Table 1, entry 2). The solubility of
these complexes in less polar solvents is poor, and the het-
erogeneous reactions in such solvents as hexane showed
moderate cis- and good enantioselectivity (entry 3). On
the other hand, homogeneous reactions in more polar sol-
vents such as THF, showed excellent cis- and enantiose-
lectivity (entry 4), though the reaction did not proceed in
coordinating solvents such as acetonitrile or pyridine (en-
try 5). It is, however, noteworthy that the sense of asym-
metric induction by complex 8 was found to depend on the
solvent used and change from a less-polar to a more-polar
solvent reversed the sense of asymmetric induction (en-
tries 3 and 4). Although the reason for this reversal is un-
clear, it is the most likely that complex 8 adopts different
conformations in heterogeneous and homogeneous states.
The reaction in THF showed excellent enantio- and cis-se-
lectivity but the yield of the product was unsatisfactory
because self-coupling of the diazo compound, giving a
mixture of fumaric and maleic acid esters, occurred com-
petitively under the reaction conditions (entry 4). The
yield was improved to some extent by increasing the ole-
fin concentration, and a moderate yield was achieved
without diminishing stereoselectivity, when the reaction
was carried out at 1:1 (v/v) substrate–THF ratio (entry
6).23b,c
THF–styrene
(1:1)
45
93:7
97 (1S, 2R)
with high enantio- and cis-selectivity (entry 1). In contrast
to the reaction with 8, self-coupling of the diazo ester was
slow in the reaction with 9 and only a trace amount of fu-
maric and maleic acid ester (<1%) was detected. Enantio-
and cis-selectivity was further improved to 98% ee and
98:2 respectively, by addition of NMI (entry 2). Another
advantage of the reaction was that excellent cis- and enan-
tio-selectivity could be attained even with commercial
ethyl a-diazoacetate (entry 3).26b It is noteworthy that
complexes 8 and 9, bearing the same ligand, showed the
opposite sense of enantioface selection to each other
(Tables 1 and 2). These results suggested that the access
route of the substrate to a carbenoid center is strongly af-
fected by the nature of the metal ion (vide infra). The re-
action with complex 10 also showed high cis- and
enantio-selectivity; however, the catalytic activity of 10
was low and its sense of asymmetric induction was oppo-
site to that of 9 (entries 2 and 5).
The cyclopropanation of other styrene derivatives also
showed excellent cis- and enantioselectivity, except for
the reaction of a-methylstyrene, in which cis-selectivity
was somewhat diminished (Table 3).
R1
R1
Cobalt(II)–salen complexes are flexible and they adopt
various ligand conformations, depending on the ligand
substituent and the apical ligand.24 Furthermore, Yamada
et al. reported that the cyclopropanation using a chiral al-
diminato cobalt(II) complex as the catalyst showed high
enantio- and trans-selectivity, and the stereoselectivity
and the reaction rate were enhanced by adding N-meth-
ylimidazole (NMI) to the reaction medium.25 Although
stereoselectivity of the previously reported cyclopropana-
tion using the Co(II)-salen complex was modest, we were
intrigued by the catalytic potential of the Co(II) complex
of our salen ligands. (Table 2).26
(S) or (R)
*
N
*
N
Co
O
O
R2 R2
3
2"
(aR)
9: R1, R1 = -(CH2)4- (R), R2 = Ph
10: R1, R1 = -(CH2)4- (S), R2 = Ph
11: R1 = H, R2 = Ph
12: R1, R1 = -(CH2)4- (R), R2 = Me
The Co(II) complex 9, carrying the same salen ligand as
complex 8, also catalyzed the cyclopropanation of styrene
Figure 3
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

1718
T. Uchida, T. Katsuki
SPECIAL TOPIC
Table 2 Asymmetric Cyclopropanation of Styrene with Co(II)–Salen Complex as Catalyst
Co(II)–salen complex (5 mol%)
N2CHCO2R, NMI (10 mol%)
+
Ph
Ph
Ph
THF, N2, r.t.
CO2R
CO2R
cis
trans
Entry
Cat.
9
THF/Styrene
100/1
100/1
10/6
R
Yield (%)
88
cis:trans
92:8
% ee (cis)
1a
2
3
4
5
6
7
8
t-Bu
t-Bu
Et
96 (1R, 2S)
98 (1R, 2S)
96 (1R, 2S)
98 (1R, 2S)
99 (1S, 2R)
2 (1S, 2R)
9
89
98:2
9
quant
90
99:1
9
10/1
t-Bu
t-Bu
t-Bu
t-Bu
Et
98:2
10
11
12
12
100/1
10/1
18
97:3
25
97:3
10/6
91
27:73
31:69
95 (1R, 2R)b
95 (1R, 2R)b
10/6
quant
a The reaction was carried out in the absence of NMI.
b The ee of the trans-isomer.
Table 3 Asymmetric Cyclopropanation of Styrene Derivatives with Co(II)–Salen Complex 9 as Catalyst
R1
R1
R1
Co(II)–salen complex 9 (5 mol%)
N2CHCO2R2, NMI (10 mol%)
+
Ar
Ar
Ar
THF, N2, r.t.
CO2R2
CO2R2
cis
trans
Entry
Ar
R1
H
R2
Yield (%)
85
cis:trans
97:3
% ee (cis)
1
2
3
4
5
6
p-ClC6H4
p-MeOC6H4
Ph
t-Bu
t-Bu
t-Bu
Et
96
97
99
96
93
94
H
84
97:3
Me
H
39
83:17
97:3
p-ClC6H4
p-MeOC6H4
Ph
quant
quant
quant
H
Et
95:5
Me
Et
85:15
Although sufficient structural information on complexes olefins approach along the Co(II)–N bond axis (Table 2,
9 and 10 is not yet available, we assumed that the Co(IV)- entry 6). When the phenyl group on the naphthalene ring
carbenoid species derived from them have a stepped con- of 9 is replaced by a small methyl group, the ester alkoxy
formation. Furthermore, the sense of salen ligand folding group of the carbenoid species derived from 12 shifts
seems to be dictated by the chirality of the ethylenedi- backward and, therefore, olefins approach the carbenoid
amine moiety, since 9 and 10 show the opposite sense of species with reversed orientation and show good trans-se-
asymmetric induction to each other. We also assumed that lectivity with excellent enantioselectivity (Figure 4; bot-
olefins would approach the carbenoid center derived from tom, Table 2; entry 7). The carbenoid species derived
9 along the Co–N(a) bond beyond the downward naphtha- from 10 is also considered to adopt a stepped conforma-
lene ring and the equatorial substituent at the diamine tion; however, the sense of the folding should be opposite
moiety and rotate counterclockwise to show the observed to that of the carbenoid species derived from 9 and olefins
cis- and enantioselectivity (Figure 4; top). It is noteworthy presumably approach along the Co–N(b) bond axis, re-
that complex 11, with no chirality at its ethylenediamine sulting in high cis- and enantioselectivity. In this case,
moiety, showed almost no enantioselectivity but high cis- however, the 2¢¢-phenyl group protrudes over this access
selectivity. This result also supports the suggestion that route and the reaction with 10 is slow.
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

SPECIAL TOPIC
Metallosalen-Catalyzed Asymmetric Cyclopropanation
1719
repulsion between the substituent and the ester unit of the
carbenoid. Thus, we examined the cyclopropanation with
complex 13 and 14 and found that the latter showed con-
siderably improved enantioselectivity. Encouraged by
these results, we next examined the cyclization with
Co(II)–salen complexes: though the reaction with com-
plex 9 was slow and only moderately enantioselective was
observed, those with complex 12 or 15 bearing no or small
substituent at C2¢¢ showed high enantioselectivity with ac-
ceptable yields.
R
O
CH
O
O
O
N
Ph
Co
N
(a)
(b)
CO2R
Ph
(1R,2S)-isomer
NMI
O
O
CH3
R
CH
Co
O
H
N2CH
O
O
O
N
Ru(NO)– or Co(II)–salen complex
Ph
N
Ph
O
(a)
Ph
O
(b)
CO2R
THF
CH3
Ph
(1R,2R)-isomer
H
Figure 4
(R)
Ru(NO)–salen complexes
N
O
N
Since Stork and Ficini reported the first intramolecular cy-
clopropanation in 1968,27 its asymmetric version has been
extensively studied. Doyle et al. have introduced a series
of chiral dirhodium(II) carboxamidate complexes and
achieved excellent enantioselectivity for intramolecular
cyclopropanations of various types of alkenyl diazoace-
tates.28 On the other hand, Pfaltz et al. have revealed that
copper-bis(oxazoline) complexes are excellent catalysts
for intramolecular cyclopropanation of alkenyl diazo ke-
tones.29 Later, Doyle et al. further demonstrated that cop-
per-bis(oxazoline) complexes serve as catalysts for
macrocyclization of alkenyl diazo esters.30 Pérez-Pietro et
al. introduced a dirhodium complex with a unique ortho-
metalated arylphosphine ligand and demonstrated that its
rhodium complex was an excellent catalyst for cyclization
of alkenyl diazo ketones.31 Davies et al. reported highly
enantioselective cyclization of alkenyl a-vinyl-a-diazoac-
tates.32 As these results indicate, although high enantiose-
lectivity has been achieved for a range of intramolecular
cyclopropanations, the scope of each reaction is rather
narrow, because the transition states of the intramolecular
cyclopropanations strongly depend on the substrate struc-
ture, olefinic substitution pattern, etc. We thought that the
use of catalysts readily assembled from modules would
provide a solution to this problem. Salen ligands can be
synthesized by condensation of salicyl aldehyde with eth-
ylenediamine and various derivatives are commercially
available or readily synthesized. Thus, we expected that
cyclization of a wide range of unsaturated diazo com-
pounds could be performed in a highly enantioselective
manner using a metallosalen complex as catalyst.
M
8: 66%, 70% ee
13: 58%, 67% ee
14: 54%, 82% ee
O
R
R
3
2"
Co(II)–salen complexes
9: 15%, 75% ee
12: 67%, 97% ee
15: 75%, 95% ee
13: R = Me, M = Ru(NO)
14: R = H, M = Ru(NO)
15: R = H, M = Co
Scheme 3
O
R1
R
O
H
C
R2
O
N
O
M
L
N
R3
R
Figure 5
Based on these results, we further examined the reaction
of various allyl a-diazoacetates with complexes 12 and 15
(Table 4). The reactions of most a-diazoacetates proceed-
ed with high enantioselectivity, except for 2-methylprope-
nyl a-diazoacetate (entry 10). This is in accord with the
model (Figure 5), where the presence of a geminal substit-
uent (R3) causes steric repulsion with the salen ligand and
reduces enantioselectivity.
Cyclization of 3-alkenyl diazomethyl ketones were also
examined (Scheme 4). In contrast to the reaction of alke-
nyl a-diazoacetates, Co(II)–salen complexes showed no
catalytic activity for this reaction, however, the Ru(NO)–
salen complex 8 was found to be effective.33c The reason
why complex 8 is the best catalyst is not very clear at
present, but it has been proposed that, in the cyclization of
alkenyl diazoketones, the alkenyl moiety approaches the
carbenoid center away from the chiral auxiliary.34 This
proposal could explain why the 2¢¢-phenyl group of com-
pound 8 seems to be essential for the cyclization.
We examined the cyclization with Ru(NO)–salen and
Co(II)–salen complexes as catalyst (Scheme 3).33 The cy-
clization of E-cinnamyl a-diazoacetate was first studied
with Ru(NO)–salen complex 8 as catalyst, but the enan-
tioselectivity of the reaction was only moderate (70% ee).
However, the configuration of the product suggested that
the cyclization proceeded through a transition state de-
scribed in Figure 5.33a This transition-state model suggest-
ed that a bulky 2¢¢-substituent would disturb the desired
transition state and reduce enantioselectivity due to steric
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

1720
T. Uchida, T. Katsuki
SPECIAL TOPIC
Table 4 Asymmetric Cyclopropanation of Allyl a-Diazoacetate Derivatives with Co(II)–Salen Complexes as Catalyst
O
O
CHN2
Co(II)–salen complex (5 mol%), NMI
H
R3
R2
R1
R1
O
O
THF, N2, r.t.
R2
R3
Entry
Cat.
12
12
12
12
12
15
15
12
12
15
R1
R2
R3
H
Yield (%)
ee (%)
98
1
2
p-ClC6H4
p-BrC6H4
p-MeOC6H4
PhC≡C
PhCH2CH2
i-Pr
H
72
70
70
32
10
34
47
35
16
12
H
H
97
3
H
H
98
4
H
H
93
5
H
H
91
6
H
H
75
7
Ph
Me
Ph
Ph
H
H
91
8
Ph
H
81
9
H
H
74
10
Ph
Me
38
O
We believe that the synthetic value of a-diazoacetates as
a carbene precursor has been remarkably enhanced by the
use of appropriate metallosalen complexes as catalyst.
H
H
R1
R2
R2
R1
complex (5 mol%), hν
CHN2
THF (5 mL), r.t., 16 h
O
R1 = Ph, R2 = H 8: 78%, 94% ee
13: 60%, 44% ee
1H NMR spectra were recorded at 400 MHz on a Jeol JNM-AL-400
instrument. All signals are expressed as ppm down field of TMS,
used as an internal standard, in CDCl3. IR spectra were obtained
with a Shimadzu FTIR-8400 instrument. Optical rotations were
measured with a Jasco P-1020 polarimeter. Column chromatogra-
phy was conducted on silica gel 60N (spherical, neutral), 63–210
mm, available from Kanto Chemical Co., Inc. Preparative thin layer
chromatography was performed on 0.5 mm × 20 cm × 20 cm E.
Merck silica gel plate (60 F-254). Enantiomeric excesses were de-
termined by HPLC analysis using Shimadzu LC-10AT-VP with an
appropriate optically active column. Solvents were dried and dis-
tilled shortly before use. Reactions were carried out under nitrogen
or argon, if necessary.
14: 62%, 58% ee
R1 = (CH3)2C=CHCH2CH2, R2= CH3
R1 = CH3, R2 = CH3
8: 72%, 93% ee
8: 65%, 87% ee
Scheme 4
a-Diazoacetate esters are useful carbene precursors and,
in this study, it has been demonstrated that they are also
useful precursors for metallosalen-catalyzed asymmetric
cyclopropanation. It was also found that the stereochem-
istry of this cyclopropanation depends on the metal ion, its
valency, the structure of the salen ligand, its electronic na-
ture, and the nature of the apical ligand. Both highly cis-
and trans-selective asymmetric cyclopropanations have
(R,R)-[N,N¢-Bis(5-methoxysalicylidene)-1,2-diphenylethylene-
diaminato]cobalt(II)
(1R,2R)-l,2-Diphenylethylenediamine (102 mg, 0.48 mmol) was
added to a solution of 2-hydroxy-5-methoxybenzaldehyde (145 mg,
been achieved by an appropriate combination of these fac- 0.96 mmol) in EtOH (2 mL) and stirred at r.t. overnight. The mix-
ture was concentrated in vacuo and to the residue were added de-
tors. Although cyclopropanation competes with the self-
gassed EtOH (8 mL) and a freshly prepared ethanolic solution of
coupling of diazoacetate, the self-coupling can be sup-
Co(OAc)2 (0.5 M, 0.96 mL, 0.48 mmol) under nitrogen. The mix-
pressed using an appropriate metallosalen complex. Fur-
ture was refluxed for 9 h, and then allowed to cool to r.t.. The result-
thermore, metallosalen complexes have been successfully
ing brown precipitate was separated from the solution by filtration,
washed with degassed ethanol under a nitrogen atmosphere, and
dried under vacuum to give the corresponding Co(II)–salen com-
applied to the intramolecular cyclopropanation of unsat-
urated diazo esters and ketones. The scope of known in-
tramolecular cyclopropanation reactions is generally plex (198 mg, 0.37 mmol, 77%).
narrow; however, the modular nature of these metallo-
salen complexes enables the cyclization of a wide range of
unsaturated diazo compounds.
IR (KBr): 3028, 3001, 2943, 2905, 2833, 1595, 1533, 1462, 1421,
1362, 1302, 1256, 1219, 1167, 1030, 824, 698 cm–1.
Anal. Calcd for C30H26N2O4Co: C, 67.04; H, 4.88; N, 5.21. Found:
C, 66.98; H, 4.90; N, 5.20.
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

SPECIAL TOPIC
Metallosalen-Catalyzed Asymmetric Cyclopropanation
1721
Asymmetric Cyclopropanation Using Complex 6; General Pro-
cedure
hexane, 0.5 mL/min). Further purification by preparative TLC (sil-
ica gel, hexane–i-Pr2O, 4:1) gave tert-butyl cis-2-phenylcyclopro-
pane-1-carboxylate as a single isomer (9.1 mg, 42 mmol, 42%).
To a CH2Cl2 solution of the Co(II)–salen complex (8.4 mM, 0.5
mL) was added a CH2Cl2 solution of Br2 (17 mL, 0.12 M, 2.1 mmol)
and the mixture was stirred for 1 h at r.t. to give the Co(III)–salen
complex 6. Styrene (49 mL, 0.42 mmol) was then added followed,
after 10 min stirring at r.t., by tert-butyl a-diazoacetate (11.9 mL, 85
mmol). The reaction was stirred at r.t. for 24 h then concentrated in
vacuo. The residue was passed through a short silica gel column
(hexane–AcOEt, gradient from l:0 to 9:1) to give a 96:4 mixture of
trans- and cis-tert-butyl 2-phenylcyclopropane-l-carboxylates, re-
spectively, in a combined yield of 80%.
(1S,2R)-tert-Butyl 2-Phenylcyclopropane-1-carboxylate
[a]D24 +18.0 (c 0.73, CHC13); ee = 97%.
The configuration was determined by chemical correlation.18b
Anal. Calcd for C14H18O2: C, 77.03; H, 8.31. Found: C, 77.00; H,
8.37.
(R,R)-{N,N¢-Bis[(R)-2-hydroxy-2¢-phenyl-1,1¢-binaphthyl-3-yl-
methylene]-1,2-cyclohexanediaminato}cobalt(II) (9)
Complex 9 was prepared in a manner similar to that described for
(R,R)-[N,N¢-bis(5-methoxysalicylidene)-1,2-diphenylethylenedi-
aminato]cobalt(II).
(1S,2S)-tert-Butyl 2-Phenylcyclopropane-1-carboxylate
[a]D20 +253.3 (c 0.73, CHC13); ee = 93% (HPLC; Daicel Chiralcel
OD-H; hexane).
The configuration was determined by chemical correlation.18b
HRMS–FAB: m/z [M + H]+ calcd for C14H19O2: 219.1385; found:
IR (KBr): 3443, 3051, 2934, 2860, 2363, 1603, 1547, 1491, 1450,
1331, 1298, 1225, 1186, 1148, 1028, 952, 870, 822, 758, 700, 581,
544 cm–1.
219.1385.
Anal. Calcd for C60H44O2N2Co·0.5 H2O: C, 80.7; H, 5.08; N, 3.14.
Found: C, 80.86; H, 5.08; N, 3.09.
(R,R)-{N,N¢-Bis[(R)-2-hydroxy-2¢-phenyl-1,1¢-binaphthyl-3-yl-
methylene]-1,2-cyclohexanediaminato}ruthenium(nitrosyl)
Chloride (8)
(R,R)-{N,N¢-Bis[(S)-2-hydroxy-2¢-methyl-1,1¢-binaphthyl-3-yl-
methylene]-1,2-cyclohexanediaminato}cobalt(II) (12)
IR (KBr): 3439, 3049, 3007, 2932, 2858, 1591, 1553, 1445, 1421,
1337, 1227, 1190, 1148, 1123, 955, 887, 860, 806, 781, 746, 567,
430 cm–1.
To a solution of (1R,2R)-1,2-diaminocyclohexane (0.17 g, 1.5
mmol) in EtOH (20 mL) was added (aR)-3-formyl-2-hydroxy-2¢-
phenyl-1,1¢-binaphthyl (1.1 g, 3.0 mmol) and the mixture was
stirred at r.t. for 6 h. The resulting light-yellow precipitate was sep-
arated from the solution by filtration, and dried under vacuum. This
precipitate was dissolved in DMF (30 mL) and added to a stirred
suspension of washed (hexane, 3 × 1.0 mL) NaH (60% dispersion
in mineral oil, 0.13 g, 3.3 mmol). Hydrogen evolved and the mix-
ture turned to a clear red solution. After 1 h, Ru(NO)Cl3·H2O (0.42
g, 1.6 mmol) was added and the mixture was stirred at 110 °C for
48 h. The reaction mixture was cooled, concentrated under vacuum
and the residue was purified directly by column chromatography
(CH2Cl2–acetone, 50:1) to give Ru(NO)–salen complex 8 as red-
brown crystals (892 mg, 0.9 mmol, 60%).
Anal. Calcd for C50H40O2N2Co: C, 79.04; H, 5.31; N, 3.69. Found:
C, 79.03; H, 5.35; N, 3.79.
(R,R)-{N,N¢-Bis[(S)-2-hydroxy-1,1¢-binaphthyl-3-ylmethylene]-
1,2-cyclohexanediaminato}cobalt(II) (15)
IR (KBr): 3443, 3049, 2932, 2856, 1595, 1551, 1423, 1352, 1329,
1227, 1188, 1150, 1123, 1043, 1018, 955, 891, 858, 797, 777, 748,
567, 503, 426 cm–1.
Anal. Calcd for C48H36O2N2Co·0.5 H2O: C, 77.83; H, 5.03; N, 3.78.
Found: C, 78.03; H, 5.08; N, 3.79.
IR (KBr): 3051, 2937, 2860, 1824, 1707, 1639, 1614, 1577, 1546,
1490, 1446, 1423, 1384, 1346, 1319, 1296, 1274, 1246, 1226, 1190,
1169, 1145, 1124, 1091, 1072, 1028, 951, 864, 818, 793, 775, 760,
696, 667, 640, 582, 548, 499, 470, 428 cm–1.
1H NMR (400 MHz, CDCl3): d = 8.25 (d, J = 2.0 Hz, 1 H), 8.22 (d,
J = 1.5 Hz, 1 H), 8.16 (d, J = 8.0 Hz, 1 H), 8.03 (d, J = 8.5 Hz, 1 H),
7.99 (d, J = 8.5, 1 H), 7.87–7.82 (m, 3 H), 7.67 (d, J = 8.0 Hz, 1 H),
7.63–7.58 (m, 3 H), 7.45–7.34 (m, 3 H), 7.29–6.95 (m, 9 H), 6.59–
6.55 (m 2 H), 6.30 (pseudo-d, J = 7.5 Hz, 2 H), 6.20–6.10 (m, 6 H),
3.99 (br t, J = 10.4 Hz, 1 H), 3.03 (br t, J = 10.4 Hz, 1 H), 2.68 (br
d, J = 12.0 Hz, 1 H), 2.60 (br d, J = 11.0 Hz, 1 H), 2.05–1.98 (br m,
2 H), 1.71–1.63 (br m, 1 H), 1.53–1.33 (br m, 3 H).
Asymmetric Cyclopropanation with Co(II)–Salen Complex 9
and Styrene; General Procedure
To a solution of complex 9 (44 mg, 50 mmol) in THF (5 mL) was
added a THF solution of N-methylimidazole (0.5 M, 0.2 mL, 0.1
mmol) and the mixture was stirred for 2 min. Styrene (550 mL, 4.8
mmol) was added to this solution and the mixture was stirred for an-
other 3 min before being treated with tert-butyl a-diazoacetate (140
mL, 1.0 mmol). The mixture was stirred for 24 h at r.t. and then con-
centrated in vacuo. The residue was chromatographed on silica gel
(hexane–EtOAc, 1:0 to 9:1) to give a 2:98 mixture of trans- and cis-
products (194 mg, 0.89 mmol, 89%). An aliquot of the mixture was
submitted to preparative TLC (silica gel, hexane–i-Pr2O, 4:1) to
yield the cis-product which was used to determination the enantio-
meric excess by HPLC analysis (Daicel Chiralcel OD-H, hexane,
0.5 mL/min). The configuration was determined by comparison of
the sign of optical rotation.35
HRMS–FAB: m/z [M]+ calcd for C60H44ClO3N3Ru: 991.2128;
found: 991.2114.
Anal. Calcd for C60H44ClO3N3Ru·H2O: C, 71.38; H, 4.59; N, 4.16.
Found: C, 71.24; H, 4.59; N, 4.13.
Intermolecular Asymmetric Cyclopropanation Using Ru(NO)–
Salen Complex 8 as Catalyst; Typical Procedure
(1R,2S)-tert-Butyl 2-Phenylcyclopropane-1-carboxylate
[a]D24 –18.4 (c 0.34, CHC13); ee = 98%.
To a solution of complex 8 (4.9 mg, 5 mmol) in THF (0.12 mL) was
added styrene (0.12 mL) under N2. tert-Butyl a-diazoacetate (14
mL, 0.1 mmol) was then added and the mixture was stirred for 48 h
under incandescent light (100 V, 57 W). The reaction mixture was
then concentrated in vacuo and the residue was passed through a
short silica gel column (hexane–i-Pr2O, 1:0 to 4:1) to give a 7:93
mixture of tert-butyl trans- and cis-2-phenylcyclopropane-1-car-
boxylates (9.8 mg, 45 mmol, 45%). Their enantiomeric excesses and
configuration were determined by HPLC (Daicel Chiralcel OD-H,
Anal. Calcd for C14H18O2: C, 77.03; H, 8.31. Found: C, 77.01; H,
8.32.
(1R,2S)-Ethyl 2-Phenylcyclopropane-1-carboxylate
[a]D24 –19.3 (c 0.48, CHC13); ee = 96% (Daicel Chiralcel OB-H).
HRMS–FAB: m/z [M + H]+ calcd for C12H15O2: 191.1057; found:
191.1058.
Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

1722
T. Uchida, T. Katsuki
SPECIAL TOPIC
Intramolecular Cyclopropanation of 2-Alkenyl a-Diazoacetates
with Co(II)–Salen Complex (12 or 15) as Catalyst; General Pro-
cedure
References
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2-Alkenyl a-diazoacetate (0.1 mmol) was placed in a Schlenk tube
and purged with nitrogen. A solution of N-methylimidazole (0.5 M,
0.2 mL) in THF followed by Co(II)–salen complex (5 mol%) was
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responding bicyclic lactone.
(1S,5R,6R)-6-Phenyl-3-oxabicyclo[3.1.0]hexan-2-one
Enantiomeric excess was determined by GLC analysis using a
chiral column (Supelco Chiral BETA-DEX column operated at
160 °C).
Mp 104 °C; [a]D22 –127 (c 0.25, CHCl3) {Lit.28b [a]D23 +130 (c 0.29,
CHCl3) for 1R,5S,6S-isomer}; ee = 97%.
6-Methyl-6-phenyl-3-oxabicyclo[3.1.0]hexan-2-one
Enantiomeric excess was determined by HPLC analysis using chiral
column (Daicel Chiralcel OB-H, hexane–i-PrOH, 15:1, 0.5 mL/
min).
[a]D22 –92 (c 0.25, CHCl3); ee = 90%.
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IR (KBr): 1762, 1440, 1357, 1188, 1070, 1045, 995, 968, 866, 833,
758, 698 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.35–7.23 (m, 5 H), 4.54 (dd,
J = 10.0, 5.6 Hz, 1 H), 4.36 (ddd, J = 10.0, 1.2, 0.8 Hz, 1 H), 2.54
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Intramolecular Cyclopropanation of 3-Alkenyl Diazomethyl
Ketones Using Ru(NO)-Salen Complex 8; General Procedure
Ru(NO)–salen complex 8 (4.9 mg, 5 mmol) was dissolved in THF
(4 mL) under N2 and the mixture was irradiated with incandescent
light whilst a solution of 3-alkenyl diazomethyl ketone (0.1 mmol)
in THF (1 mL), was added dropwise over a period of 12 h using a
syringe pump. The reaction mixture was stirred for a further 4 h then
concentrated in vacuo. The residue was chromatographed on silica
gel (hexane–AcOEt, 11:1) to yield the corresponding [3.1.0]bicy-
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6-Phenylbicyclo[3.1.0]hexan-2-one
Enantiomeric excess was determined by HPLC analysis using chiral
column (Daicel Chiralcel OD-H, hexane–i-PrOH, 9:1, 0.5 mL/min).
Mp 101 °C; [a]D22 –95 (c 0.275, CHCl3); ee = 94%.
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7.04.
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6-Methyl-6-(4-methyl-3-penten-1-yl)bicyclo[3.1.0]hexan-2-one
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[a]D24 +27 (c 0.46, CHCl3); ee = 93%.
IR (KBr): 2935, 2858, 1720, 1652, 1600, 1494, 1296, 1267, 1190,
1078, 1026, 968, 875, 802 cm–1.
1H NMR (400 MHz, CDCl3): d = 5.05 (tt, J = 7.1, 1.5 Hz, 1 H),
2.34–2.18 (m, 2 H), 2.09–2.00 (m, 3 H), 1.95–1.86 (m, 2 H), 1.68
(m, 4 H), 1.60 (s, 3 H), 1.33–1.20 (m, 2 H), 1.14 (s, 3 H).
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Synthesis 2006, No. 10, 1715–1723 © Thieme Stuttgart · New York

SPECIAL TOPIC
Metallosalen-Catalyzed Asymmetric Cyclopropanation
1723
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