Catalytic and asymmetric cyclopropanation of styrenes catalysed by ruthenium porphyrin and porphycene complexes

Article abstract of DOI:10.1039/a608080d

Ruthenium porphyrin and porphycene complexes catalyse stereospecific cyclopropanation of styrenes; high product turnovers with up to 90.8% ee were achieved with the [Ru(P*)(CO)(EtOH)] catalyst (H₂P* = chiral D₄ porphyrin).

Full text of DOI:10.1039/a608080d

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Catalytic and asymmetric cyclopropanation of styrenes catalysed by ruthenium
porphyrin and porphycene complexes
Wai-Cheung Lo,a Chi-Ming Che,*a Kin-Fai Cheng*a and Thomas C. W. Makb
a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
b Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
Ruthenium porphyrin and porphycene complexes catalyse
stereospecific cyclopropanation of styrenes; high product
turnovers with up to 90.8% ee were achieved with the
[Ru(P*)(CO)(EtOH)] catalyst (H2P*
phyrin).
direction and is in the centre of the mean plane defined by the
four nitrogen atoms of the porphyrin ligand. There are two
CH2Cl2 and one EtOH solvent molecules in the unit cell with
0.5 site occupancy. Hydrogen bonding interactions exists
between the coordinated EtOH and the EtOH solvent molecules
with O–O distance of 2.8 Å in the unit cell. The measured
C(85)–O(1) and Ru–O(Et) distances of 1.188(2) and 2.241(2)
Å, respectively are comparable to the corresponding values of
1.16(3) and 2.21(2) Å found in [Ru(TPP)(CO)(EtOH)].18
The [Ru(P)(CO)(EtOH)] complexes are excellent catalysts
for the cyclopropanation of some styrenes by ethyl diazoester
(EDA) with very good diastereoselectivities. The results are
listed in Table 1. At a catalyst:EDA:alkene ratio of
1:300:600, the product yields based on EDA are moderate to
good for 1 and 2 but the reactions become less effective for the
less bulky catalysts 3 and 5. A notable point is the high trans
product selectivity. For example, the trans:cis ratio of 9.2
obtained from the reaction between styrene and EDA catalysed
by 1 is comparable to the value of 9.5 obtained from the same
reaction catalysed by [Os(TPP)(CO)(py)]3 but is considerably
higher than that of 1.1 by Rh(TPP)I.4
In order to optimize the product yield, the reaction between
1,1-diphenylethylene and EDA catalysed by 1 was investigated.
The product yield was found to increase with increasing ratio of
alkene:EDA. The effectiveness of the catalyst in cyclopropana-
tion is clearly demonstrated by the very high turnover number of
1855 and yield of 92.3% achieved at a catalyst:EDA:diphenyl-
ethylene ratio of 1:2000:20 000.
The ruthenium porphyrins catalysed the cyclopropanation of
alk-1-enes and 1,1-disubstituted olefins very efficiently, but
alkenes with other substitution patterns were poor substrates.
cis- or trans-b-Methylstyrene could not be cyclopropanated.
This shape selectivity is reminiscent of that observed for
Os(TTP)-catalysed cyclopropanation3 but constrasts with the
much broader substrate compatibility of rhodium porphyrin-
catalysed reactions.5 In the latter case, only tetrasubstituted
alkenes are sluggish substrates.
= chiral D4 por-
Metalloporphyrins are well known to be effective catalysts for
alkane hydroxylation,1 olefin epoxidation2 and cyclopropana-
tion.3–5 When compared to metallosalens, porphyrin catalysts
usually have a greater catalytic stability resulting in higher
product turnover numbers.6 However, their practical applica-
tions would require high regio-, stereo- and enantio-selectivities
to be achieved. To our knowledge, the best reported chiral
metalloporphyrins are the manganese threitol-strapped porphyr-
ins7 and the iron ‘twin-coronet’ porphyrins8 which gave 88 and
89% ee in the epoxidation of 1,2-dihyronaphthalene and
2-nitrostyrene by iodosobenzene, respectively. Herein is de-
scribed that ruthenium porphyrin and porphycene complexes
are effective catalysts for stereospecific cyclopropanation of
styrenes. With the D4 porphyrin (H2P*) reported by Halterman
and Jan,9 it is possible to achieve high product turnovers and
with up to 90.8% ee which is comparable to the best ee reported
with the non-porphyrin chiral metal catalysts.10–14
The [Ru(P)(CO)(EtOH)] catalysts (Fig. 1) used in this work
were prepared by literature methods.15,16 Refluxing
[Ru3(CO)12] with H2P* in decalin for 8 h gave [Ru(P*)(CO)] in
a 90% yield, which was recrystallised from a CH2Cl2–EtOH
mixture to give [Ru(P*)(CO)(EtOH)]. Its structure determined
by a X-ray crystal analysis features one of the few structures of
chiral ruthenium porphyrins.17 As shown in Fig. 2, the
porphyrin ligand has a pseudo D4 symmetry. The ruthenium
atom coordinates with one CO and one EtOH in the axial
R2
R2
R1
R2
R1
R2
R1
R2
CO
N
N
N
N
CO
N
N
N
N
Ru
Ru
EtOH
EtOH
R2
R1
R2
R2
1
2
3
[Ru(TPP)(CO)(EtOH)]
R1 = Ph, R2 = H
5 [Ru(TPrPc)(CO)(EtOH)]
N(2)
O(1)
[Ru(TMP)(CO)(EtOH)]
R1 = Mes, R2 = H
N(1)
C(85)
Ru(1)
N(3)
[Ru(OEP)(CO)(EtOH)]
R1 = H, R2 = Et
C(86)
O(2)
N(4)
CO
N
N
N
N
Ru
EtOH
Fig. 2 Perspective view of [Ru(P*)(CO)(EtOH)]. Selected bond lengths (Å)
and angles (°): Ru(1)–N(1) 2.082(2), Ru(1)–N(2) 2.068(2), Ru(1)–N(3)
2.043(2), Ru(1)–N(4) 2.057(2), Ru(1)–C(85) 1.776(2), Ru(1)–O(2),
2.241(1), O(2)–C(86) 1.437(2), C(85)–O(1) 1.188 Å; N(1)–Ru(1)–N(2)
89.6(1), N(1)–Ru(1)–N(3) 173.7(1), N(2)–Ru(1)–N(3) 90.6(1),
N(1)–Ru(1)–N(4) 88.9(1), N(2)–Ru(1)–N(4) 174.4(1), N(3)–Ru(1)–N(4)
90.3(1), Ru(1)–C(85)–O(1) 178.4(2)°.
4 [Ru(P*)(CO)(EtOH)]
Fig. 1 Ruthenium porphyrinoid complexes used in the catalytic cyclopropa-
nation of alkenes
Chem. Commun., 1997
1205

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The catalytic enantioselective cyclopropanation of alkenes
was investigated using the ruthenium complex of
D4-symmetric porphyrin (Fig. 1). The results are shown in
Table 2. The absolute configuration of the product from the
reaction between styrene and EDA was determined by compar-
ison with literature data.10
With styrene as substrate and at room temperature, a product
trans:cis ratio of 18:1 was obtained and the ee of the trans
product was 87% with absolute configuration of (1S,2S). A high
catalyst turnover number of 1665 was achieved. At 0 °C, the
trans–cis selectivity improved to 24:1 and ee of the trans
product increased to 91%. Similar results with very high trans–
cis selectivities and high enantioselectivities for the trans
product have also been found with other substituted styrenes.
Thus, the bulky aryl groups appended on the porphyrin ring of
the catalyst 4 can produce a good steric and chiral environment
around the reaction site. The reactive intermediate of the
cyclopropanation has also been studied. The reaction of 4 with
EDA in benzene at room temperature immediately gave a
species showing lmax at 407 and 534 nm observed by UV–VIS
spectroscopy. This species was unstable and was observed to
decay gradually. When monitored with 1H NMR, singlets at d
13.79 and 8.99 were observed. This intermediate species can be
tentatively assigned as the carbene complex [Ru(P*)(CH-
CO2Et)], with the NMR peaks at d 13.79 and 8.99 being
assigned to the carbene and the pyrrolic protons, respectively.
These values are similar to the respective NMR peaks at d13.79
and 8.55 of [Ru(TMP)(CHCO2Et)].19 The present work high-
lights that chiral ruthenium carbene complexes of porphyrins
could be generated and spectroscopically characterised.
We acknowledge support from the University of Hong Kong
and the Hong Kong Research Grants Council.
a
Footnotes
* E-mail: cmche@hkucc.hku.hk
†
C
a
Crystal data for [Ru(C84H76N4)CO·EtOH]·CH2Cl2·0.5(EtOH):
89H87Cl2N4O2.5Ru, 1428.6, monoclinic, space group P21,
9.849(1), b 28.164(2), c 14.399(1) Å, b 102.83(1)°,
M
=
=
=
=
=
U = 3894(2) Å3, Z = 2, Dc = 1.214 g cm23, l (Mo-Ka) = 0.71073 Å,
F(000) = 1492, m = 3.20 cm21, crystal dimensions 0.40 3 0.40 3 0.40
mm. Intensity data (3.0 < 2q < 55.0°) were collected on a Rigaku RAXIS
IIc imaging-plate system using Mo-Ka radiation (l = 0.71073 Å) from a
RU-200 rotating-anode X-ray generator at room temperature. The data were
corrected for absorption and Lorentz polarisation effects. The structure was
solved by direct methods and refined by full-matrix least-squares using
Siemens SHELXTL PLUS (PC Version). The H-atoms were generated
geometrically. 13493 Independent reflections were obtained. 12118 Reflec-
tions with ıF0ı > 6.0s(ıF0ı) were considered observed and used in the
structural analysis. The last least-squares cycle was calculated with 933
parameters giving R = 0.071, Rw = 0.089 and goodness of fit = 2.24. The
Table 1 Ruthenium catalysed cyclopropanation of alkenes with ethyl
diazoacetate (EDA)a
H
CO2Et
R
CO2Et
catalyst
+ N2CHCO2Et
+
R
R
H
H
H
trans
cis
Product
Yield
(%)b
Catalyst
Alkene
Styrene
Catalyst
trans:cisc
turnoversd
2
weighing scheme used was w21 = s (F) + 0.0006F2. The final Fourier-
1
2
3
5
1
2
3
5
1
2
3
5
1
2
3
5
1
2
3
5
45
68
25
39
58
66
36
52
71
81
41
44
44
53
23
21
50
68
36
23
9.2:1
7.7:1
12:1
135
198
62
difference map showed residual extrema in the range of +1.37 to 20.86
e Å23. Atomic coordinates, bond lengths and angles, and thermal
parameters have been deposited at the Cambridge Crystallographic Centre
(CCDC). See Information for Authors, Issue No. 1. Any request to the
CCDC for this material should quote the full literature citation and the
reference number 182/396.
11:1
117
176
208
114
162
214
252
121
133
138
164
67
4-Methylstyrene
4-Methoxystyrene
4-Chlorostyrene
a-Methylstyrene
11.2:1
7.5:1
9.6:1
9.9:1
8.1:1
7.3:1
5.8:1
7.2:1
13.7:1
10.5:1
11.0:1
12.3:1
2.8:1
1.7:1
2.0:1
2.7:1
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a Reaction conditions: catalyst:EDA:alkene = 1:300:600, CH2Cl2, room
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Turn-
over
% eec number
Yield
(%)
trans
cis
Alkene
trans:cis % eec
Styrene
Styreneb
a-Methylstyrene
1,1-Diphenyl-
ethylene
83
63
69
76
17.8:1
23.6:1
3:1
86.5 (1S,2S)
90.8 (1S,2S)
87
3.8
4.0
1665
1267
1384
1532
35
—
81
4-Chlorostyrene
4-Methylstyrene
4-Methoxystyrene 61
66
78
23.1:1
18.0:1
15.3:1
90.4
80.8
85.4
4.0
8.6
8.0
1328
1570
1235
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a
Reaction conditions same as Table 1, except catalyst:EDA:alkene
b
c
Received in Cambridge, UK, 29th November 1996; Com.
6/08080D
= 1:2000:10 000. Reaction temp. = 0 °C. Determined by chiral
HPLC.
1206
Chem. Commun., 1997