Asymmetric heterogeneous carbene transfer catalyzed by optically active ruthenium spirobifluorenylporphyrin polymers

Article abstract of DOI:10.1016/j.tetasy.2005.01.051

Anodic oxidation of chiral ruthenium complexes of spirobifluorenylporphyrins leads to the coating of the working electrode by insoluble optically active films whose electrochemical behaviour and physicochemical properties are described. After removal from

Full text of DOI:10.1016/j.tetasy.2005.01.051

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 1463–1472
Asymmetric heterogeneous carbene transfer catalyzed by
optically active ruthenium spirobifluorenylporphyrin polymers
Yann Ferrand,a Cyril Poriel,a Paul Le Maux,a
a,
Joe¨lle Rault-Berthelotb, and Gerard Simonneaux
*
*
´
a
´ ´
Laboratoire de Chimie Organometallique et Biologique, UMR CNRS 6509, Institut de Chimie, Universite de Rennes1,
35042 Rennescedex, France
b
´ ´
Laboratoire d’Electrochimie Moleculaire et Macromoleculaire, UMR CNRS 6510, Institut de Chimie, Universite de Rennes1,
´
35042 Rennescedex, France
Received 7 January 2005; accepted 31 January 2005
Available online 11 March 2005
Abstract—Anodic oxidation of chiral ruthenium complexes of spirobifluorenylporphyrins leads to the coating of the working elec-
trode by insoluble optically active films whose electrochemical behaviour and physicochemical properties are described. After
removal from the electrode, the ruthenium-complexed polymers were evaluated as enantioselective catalysts for the cyclopropana-
tion of olefins by ethyl diazoacetate. The results show the reactions proceeded very efficiently at room temperature with excellent
yields (80–90%) and moderate enantioselectivities (up to 53% at ꢀ40 ꢀC). The chiral electrosynthesized polymer catalysts can be
recycled by simple filtration and reused even up to the seventh cycle with only a slight decrease of activity and enantioselectivity.
ꢁ 2005 Published by Elsevier Ltd.
1. Introduction
Numerous methods for immobilizing metalloporphyrins
under insoluble materials have been reported11 but very
few under optically active forms12–15 and even less with
success for asymmetric catalytic reactions.14,15 We have
recently shown that poly(tetraspirobifluorenylporphy-
rin) complexed by ruthenium showed potential applica-
tions as heterogeneous cyclopropanation catalysts of
olefins with ethyl diazoacetate.16 These polymers can
be prepared by oxidative electropolymerization of
metalloporphyrin complexes bearing spirobifluorene
groups. Thus polymerization of Ru spirobifluorenepor-
phyrins leads to very efficient catalysts that can be easily
recovered and reused. We anticipate that these heteroge-
neous catalytic systems would potentially be applicable
to practical organic synthesis with a special focus on
asymmetric catalysis using chiral polymers. As an exten-
sion of our previous work on polymers,16 the spirobiflu-
orenyl group was designed to allow polymerization and
the chiral group for asymmetric induction. We chose a
C2-symmetric group, which contains two norbornane
groups fused to the central benzene ring, previously re-
ported by Halterman and Jan (Scheme 1).17 This chiral
group is particularly interesting since the enantiomeric
excesses are quite high (90%) when the catalytic reac-
tions are undertaken under homogeneous condi-
tions.18,19 Herein, we report the preparation of
The synthesis of enantiomerically pure compounds is
one of the major challenges in heterogeneous cataly-
sis.1–4 The use of heterogeneous catalysts under ambient
conditions offers several advantages compared with their
homogeneous counterparts, such as ease of recovery and
recycling and enhanced stability. Distinct methodologies
have been developed for the immobilization of homo-
geneous catalysts or the creation of heterogeneous
catalysts.1 Thus, some of the commonly used metal-
catalysts for asymmetric carbene transfer reactions have
been immobilized on organicor inorganicsupports,
with a particular focus on bis(oxazoline)5 and pyri-
dine-bis(oxazoline) (pybox)6,7 ligands. There are some
recent reviews on this topic.1,2,8,9 However most of the
strategies have relied on covalent attachments between
chiral ligands and the solid supports. A new strategy
for the immobilization of rhodium catalysts has also
been recently reported for cyclopropanation reaction.10
*
Corresponding authors. Tel.: +33 02 99286285; fax: +33 02
99281646; e-mail: gerard.simonneaux@univ-rennes1.fr
0957-4166/$ - see front matter ꢁ 2005 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2005.01.051

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Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
H
H
O
3 x
4 x
1 x
+
+
N
H
O
2
1
1. BF3OEt2
2. DDQ
NH
N
N
NH
N
N
NH
N
N
HN
HN
HN
3
4
5
Scheme 1. Mixed aldehyde condensation of chiral dinorbornabenzaldehyde and 9,90-spirobifluorene-2-carbaldehyde.
optically active electropolymers bearing chiral metallo-
porphyrins and asymmetricheterogeneous acrbene
transfer catalyzed by these polymers. To the best of
our knowledge, this system is the first example, which
shows asymmetric cyclopropanation with chiral poly-
mers obtained by anodicoxidation as catalysts.
products was undertaken using a mixture of the two
aldehydes, 9,90-spirobifluorene-2-carbaldehyde 1 and
dinorbornabenzaldehyde 2 in the ratio of 1/3 and pyr-
role. This method gave a mixture of three porphyrins
3, 4 and 5 in moderate yields (see Experimental). These
porphyrins were readily separated to produce the two
expected optically active porphyrins, 4 and 5 bearing
three and two chiral groups, respectively (Scheme 1).
The ruthenium complexes were then prepared by treat-
ment of the porphyrins with Ru3CO12 in degassed
o-dichlorobenzene at 180 ꢀC for 2 h to give 3-RuCO,
4-RuCO and 5-RuCO. 3-RuCO has previously been
reported in 1997 by Berkessel and Frauenkron25 and
Che and co-workers.26
2. Results
2.1. Porphyrin and metalloporphyrin syntheses
We have previously reported the synthesis of the ruthe-
nium complex of tetraspirobifluorenylporphyrin TSP-
RuCO.16 Due to possible difficulties (vide infra) with
the polymerization of porphyrins bearing only one
9,90-spirobifluorene group, we decided first to target
porphyrins bearing two spirobifluorene groups. trans-
A2B2 porphyrins are typically synthesized from 5-sub-
stituted dipyrromethane derivatives and aldehydes
following the MacDonald [2+2] condensation.20,21 Con-
densation of the 5-spirobifluorenyl-dipyrromethane with
the chiral dinorbornabenzaldehyde in dichloromethane,
2.2. Electropolymerization and film formation
As recently reported with the tetraspirobiflu-
orenylporphyrin ruthenium complex TSP-RuCO,16
anodicoxidation of 4-RuCO and 5-RuCO yields the
two reversible one-electron Ru-porphyrin oxidation
(E1 ꢁ 0.4 V and E2 ꢁ 0.8 V vs Fc/Fc+), corresponding
to porphyrin and metal oxidation, respectively (see
Table 1 and Fig. 1).27,28 The DE between E1 and E2 is
22
in presence of a catalytic amount of BF3OEt2 gave a
mixture consisting of at least four different tetraarylpor-
phyrins, as shown by TLC. The major compounds were
isolated in low yield (overall yields <10%) and identified
Table 1. Three oxidation potentials of monomers 5-RuCO, 4-RuCO
and TSP-RuCO
1
by H NMR spectroscopy and HRMS to be chiral por-
phyrins bearing 1–4 chiral groups.19 It is almost certain
that the mixture of porphyrins was produced as a result
of acid redistribution reactions common of many dipyr-
romethanes, as previously reported by Smith and
co-workers.23
E1a
E2
E3
5-RuCO
4-RuCO
TSPRuCO
0.44
0.37
0.46
0.86
0.84
0.89
1.34
Shoulderb
1.37
All potentials were obtained during cyclic voltammetric investigations
of 5-RuCO (1.92 · 10ꢀ3 M), 4-RuCO (4.58 · 10ꢀ3 M) and TSP-RuCO
(2.2 · 10ꢀ3 M) in CH2Cl2 containing 0.2 M Bu4NPF6. Platinum elec-
trode diameter 1 mm, sweep-rate: 100 mV/s.
a Potentials are referred to the ferrocene/ferrocenium.
b E3 is a shoulder of a wave whose maximum is at 1.51 V.
An alternate route to these chiral porphyrins is also pos-
sible through a mixed-aldehyde condensation of spirobi-
fluorene aldehyde, chiral aldehyde and pyrrole.24 This
method, which should afford a mixture of all the desired

Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
1465
wave. In contrast, recurrent sweeps between ꢀ0.3 and
1.5 V versus Fc/Fc+ led to a gradual modification of
the cyclic voltammograms (CVs). After five recurrent
sweeps, the CVs show the appearance of a new revers-
ible wave around 1.0 V and the regular increase of the
three waves E1, E2 and E3.
The polymerization of 4-RuCO (Fig. 6), bearing only
one spirobifluorene unit, also leads to the appearance
of the new reversible wave around 1.0 V versus Fc/Fc+
but with a decrease in the intensity of the E1 wave, which
shifts towards a more positive potential. This is due to
an increase of the threshold oxidation potential of this
polymer, which is less easily p-doped because of the
small amount of spirobifluorene units. After cycling,
the electrode is covered by an insoluble Ôgold likeÕ film
(Scheme 2), which can be studied in an electrolytic cell
free of any electroactive species or can be scratched from
the electrode to be used in heterogeneous catalytic reac-
tions. Anodicocpolymerizations of
4-RuCO and 5-
RuCO were also performed in presence of 9,90-spirobi-
fluorene SBF using SBF/4-RuCO ratio of 1/4.2, 1/2.1
or 1/1.4, and SBF/5-RuCO of 1/4.8 or 1/2.3, respectively.
The electrochemical behaviour, physicochemical proper-
ties and catalytic efficiencies of these copolymers were
studied by comparison to poly(4-RuCO) and poly(5-
RuCO).
2.3. Characterization of the polymer film
As shown in Figure 2, the CVs recorded in CH2Cl2 and
Bu4NPF6 0.2 M with a platinum working electrode cov-
ered by poly(5-RuCO) gave two first oxidation waves in
a potential range fitting the two one-electron oxidation
waves of the Ru-porphyrin followed by two additional
waves, which correspond to the p-doping process of
the polyspirobifluorene units. The threshold oxidation
potential of poly(5-RuCO) is 0.1 V versus Fc/Fc+. In
contrast, CVs recorded with a platinum electrode cov-
ered by poly(4-RuCO) (Fig. 3) show a threshold oxida-
tion potential of poly(4-RuCO) at 0.58 V versus Fc/Fc+.
Therefore, the oxidation of the Ru-porphyrin unit
occurs at a higher potential than in monomer 4-RuCO,
which is related to the beginning of the p-doping process
of the polyspirobifluorene units. Thus, only one wave,
including both oxidation processes, is observed in the
CVs.
Figure 1. Cyclic voltammograms (CVs) recorded during the anodic
oxidation of a 1.92 · 10ꢀ3 M solution of 5-RuCO in CH2Cl2 and 0.2 M
Bu4NPF6 using a Pt working electrode. Five sweeps between ꢀ0.32
and 1.5 V versus Fc/Fc+. Sweep-rate: 100 mV/s.
ꢁ0.45 V for the three compounds. Then, irreversible spi-
robifluorene oxidation occurs at E3 (ꢁ1.4 V). The inten-
sity of this E3 wave decreases from TSP-RuCO > 5-
RuCO > 4-RuCO according to a decrease of the number
of spirobifluorene units in the three compounds. Poten-
tials of these redox couples are summarized in Table 1.
No polymer film was observed on successive cycling if
the potential sweep did not reach the third oxidation
(
(
CO
Ru
CO
Ru
N
N
N
N
N
N
N
N
(
n
Anodic Oxidation
(
n
(
(
n
n
5-RuCO
poly(5-RuCO)
Scheme 2. Anodicpolymerization of 5-RuCO.

1466
Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
Poly(5-RuCO)
Copoly(5-RuCO) (25%)
5-RuCO
414 nm : Soret Band
360 nm poly(spirobifluorene)
526 nm : Q Band
564 nm : Q band
300
400
500
600
λ (nm)
700
800
900
Figure 4. UV–vis spectra of 5-RuCO, poly(5-RuCO) and copoly(5-
RuCO) (25%) recorded in CH2Cl2. The insoluble deposits were
previously coated on a platinum disc, which acts also as reflector for
the optical beam.
Copoly(4-RuCO) (25%), obtained by copolymerization
of 4-RuCO with SBF (see Experimental), shows a
threshold oxidation potential, which is shifted to a less
positive potential: 0.48 V versus Fc/Fc+ (Fig. 3). This
is due to the higher amount of spirobifluorene units in
the material and probably to the higher polyspirobifluo-
rene chain length. The CVs present three oxidation
waves. The first one, relatively sharp, was assigned to
the first oxidation wave of the Ru-porphyrin. The sec-
ond one, which appears larger, may include the second
oxidation of the porphyrin and the first oxidation of
the polyspirobifluorene p-doping process. Finally, the
third sharper wave corresponds to the second wave of
the p-doping process of the polyspirobifluorene units.
Figure 2. Cyclic voltammograms (CVs) recorded between ꢀ0.30 and
1.5 V in CH2Cl2 and 0.2 M Bu4NPF6 using a Pt working electrode
modified by poly(5-RuCO), previously deposited by anodicoxidation
at 1.44 V of a 1.92 · 10ꢀ3 M solution of 5-RuCO in CH2Cl2 and 0.2 M
Bu4NPF6, amount of charge used for the oxidation: 2 · 10ꢀ4 C.
Sweep-rate: 100 mV/s.
UV–vis absorption spectra of poly(5-RuCO) and co-
poly(5-RuCO) (25%) were performed for the compari-
son with the monomer 5-RuCO (Fig. 4). The
electronic spectrum of the polymers showed that the
bands of the Ru-porphyrin at 414, 526 and 564 nm are
maintained in the polymers together with an additional
absorption band at 360 nm corresponding to the
poly(spirobifluorene) units. The intensity of this band
is higher in copoly(5-RuCO) (25%) than that in
poly(5-RuCO) due to a higher content of spirobifluorene
groups in the former.
2.4. Catalytic cyclopropanation reactions
We first choose to investigate the catalytic properties of
the chiral ruthenium porphyrin monomers using the
cyclopropanation of styrene (Table 2). Reactions were
monitored by gas chromatography with asymmetric
inductions determined by chiral gas chromatography.
To assign the signals to the corresponding cyclopro-
pane, we carried out the previously described homoge-
neous reaction,29 allowing us to determine the
enantiomericexcess in both trans and cis cyclopropanes,
and also the absolute configuration of the products.
With these reactive substrates, cyclopropanation yields
Figure 3. Cyclic voltammograms (CVs) recorded between ꢀ0.30 and
1.5 V in CH2Cl2 and 0.2 M Bu4NPF6 using a Pt working electrode
modified by poly(4-RuCO) or copoly(4-RuCO) (25%), previously
deposited by anodicoxidation at 1.44 V of a 1.92 · 10ꢀ3 M solution of
4-RuCO in CH2Cl2 and 0.2 M Bu4NPF6, amount of charge used for
the oxidation: 2 · 10ꢀ4 C. Sweep-rate: 100 mV/s.

Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
1467
Table 2. Cyclopropanation of styrene with ethyl diazoacetate catalyzed by ruthenium catalystsa
CO2Et
H
aryl
CO2Et
H
Ru-catalyst
+
N2
+
aryl
H
aryl
CO2Et
Catalyst
Temperature (ꢀC)
Yield (%)b
trans/cis
c
Eetrans (%) (1S,2S)
c
Eecis (%) (1S,2R)
1
2
3
4
5
6
3-RuCO
25
ꢀ20
25
96
95
95
90
94
81
95/5
96/4
93/7
97/3
90/10
95/5
87
91
65
76
41
47
4
3
4-RuCO
5-RuCO
3
ꢀ20
25
1
8
16
ꢀ20
a Reactions were performed in CH2Cl2 for 24 h with a catalyst:diazo:substrate molar ratio of 1:200:1000.
b Yields were based on the styrene consumed.
c Determined by gas chromatography.
Table 3. Asymmetric cyclopropanation of styrene and ethyl diazoacetate catalyzed by porphyrins polymersa
c
Eetrans (%) (1S,2S)
c
Eecis (%) (1S,2R)
Catalyst
Solvent
Toluene
Temperature (ꢀC)
Yieldb (%)
trans/cis
1
Copoly(4-RuCO) (25%)
25
0
82
50
35
89
80
86
79
31
89
85
85
80
82/18
85/15
77/23
88/12
89/11
87/13
81/19
89/11
89/11
90/10
86/14
82/18
25
40
53
29
30
22
28
46
29
29
23
28
7
9
7
5
8
5
5
4
9
6
4
5
2
3
ꢀ40
25
0
4
Copoly(5-RuCO) (25%)
Copoly(4-RuCO) (25%)
5
6
CH2Cl2
25
0
7
8
ꢀ40
25
0
9
Copoly(5-RuCO) (25%)
Copoly(4-RuCO) (50%)
10
11
12
25
0
a Reactions were performed in CH2Cl2 for 24 h with a catalyst:diazo:substrate molar ratio of 1:200:1000.
b Yields were based on the styrene consumed.
c Determined by gas chromatography.
were higher than 90% with the two catalysts using ethyl
diazoacetate. Obviously, monomer 3-RuCO gave the
highest enantioselectivity, due to the presence of four
chiral groups (ee = 87%).26 This enantioselectivity was
increased at lower temperature (ee = 91%, ꢀ20 ꢀC).
Interestingly, complex 4-RuCO, bearing only three chi-
ral groups catalyzed the cyclopropanation with a mod-
erate decrease of the ee (76%, ꢀ20 ꢀC) by comparison
with complex 3-RuCO. A larger decrease was observed
with complex 5-RuCO, which bears only two chiral
groups.
with spirobifluorene gave the best results in toluene, but
the most significant difference with the monomer is the
reduction of the enantioselectivity, decreasing from
76% (ꢀ20 ꢀC) to 53% (ꢀ40 ꢀC). If we expect this result
at a low temperature, surprisingly, the two polymers
gave similar enantioselectivity and diastereoselectivity.
To illustrate the role that the polymericmatrix can play
in the stereochemical outcome of reactions, copolymer-
ization of 4-RuCO with two different amounts of spiro-
bifluorene was undertaken at two different temperatures
(Table 3, entries 6, 7; 11, 12). Similar catalytic results
were observed, showing that the dilution of the active
site in the polymer seems not to modify the reactivity
and the enantioselectivity.
The solid polymers resulting from the electrocopoly-
merization of 4-RuCO and 5-RuCO with SBF, copoly-
(4-RuCO) and copoly(5-RuCO), respectively, were then
tested as catalysts in the cyclopropanation reactions of
styrene with ethyl diazoacetate at three different temper-
atures (Table 3). Tables 2 and 3 allow the comparison of
the results obtained with the homogeneous and the two
different polymericcatalysts. The two polymers led to
diastereoselectivities and yields (Table 3) close to those
obtained with the homogeneous catalysts (Table 2).
The solid obtained by the copolymerization of 4-RuCO
The recovery and recyclability of copoly(5-RuCO)
(25%) polymer were also examined. The polymer tested
for enantioselectivity and reactivity in the cyclopropana-
tion of styrene with ethyl diazoacetate could be used for
seven recycling steps without a significant decrease in
enantioselectivity or reactivity (Fig. 5). The yield decrea-
sed thought from 89% to 78% after seven runs while the
enantioselectivity was maintained at ꢁ30%.

1468
Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
enantiomeric excess
yield
3. Discussion
100
90
80
70
60
50
40
30
20
10
0
Asymmetric electrocatalytic reactions have recently been
the subject of intense studies.30,31 Thus, electrochemical
asymmetric induction can be achieved by the use of chi-
ral electrodes.32–34 Various chiral polymers, mainly
polythiophene35 and polypyrrole derivatives,30,36 have
been described for the preparation of chiral electrodes
capable of enantioselective recognition or electrosynthe-
ses.37 Although catalytic asymmetric reactions with chi-
ral electropolymers have given some satisfactory
results,38 in no case were the polymers removed from
the electrode. We report herein the development of
novel chiral electropolymers for asymmetric hetero-
geneous carbene transfer to olefins.
1
2
3
4
5
6
7
Run #
Figure 5. Recovery–reuse outcome from the cyclopropanation of
styrene with ethyl diazoacetate using copoly(5-RuCO) (25%) as
catalyst (CH2Cl2; 25 ꢀC; catalyst loading: 5 mg).
Optically active ruthenium porphyrin complexes have
been the focus of intense studies by us19,29,39 and oth-
ers26,40–42 as catalysts for asymmetric cyclopropanation.
In contrast, immobilizing chiral metalloporphyrins
under insoluble materials is still rare.14,15 Herein, we report
a new method, using electropolymerization, to produce
insoluble optically active polymers bearing ruthenium
porphyrins, which can be used as heterogeneous asymmet-
riccatalysts after being removed from the electrode.
3.1. Electrosynthesis and characterization
The mechanism of ruthenium porphyrin film formation
is described below:
E1
1e-
ð1Þ
SBFx
SBFx
PRu+
SBFx
SBFx
PRu
+1e-
E2
1e-
ð2Þ
ð3Þ
PRu+
PRu2+
+1e-
E3
1e-
SBFx
PRu2+
SBFx+
(SBFx
PRu2+
Figure 6. Cyclic voltammograms (CVs) recorded during the anodic
oxidation of a 4.58 · 10ꢀ3 M solution of 4-RuCO in CH2Cl2 + 0.2 M
Bu4NPF6 using a Pt working electrode. Five sweeps between ꢀ0.2 and
1.7 V. Sweep-rate: 100 mV/s.
-2H+
-1e-
PRu2+
)
2
+
2 SBFx+
PRu2+
dimer
ð4Þ
reduction
+
m+
(SBFx
PRu2+
)
(SBFx
PRu2+
)
(SBFx
PRu)
2
n
ð5Þ
p-doped polymer
Metalloporphyrin film formation is suggested to be the
two electron oxidation of the ruthenium porphyrin
(Eqs. 1 and 2) followed by the oxidation of the spirobi-
fluorene units to their radical cations (Eq. 3), which cou-
ple with other radicals via C2–C7 bonds. Next, two
protons are eliminated at the C2 and C7 carbon atoms
forming a dimer (Eq. 4). This dimer is immediately oxi-
dized at this potential, since it is more easily oxidized

Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
1469
than the monomer, leading to a new dimer radical cat-
ion. Thus, the oligomer chain grows on the electrode
surface, yielding chiral ruthenium polyspirobi-
fluorenylporphyrin with the ruthenium porphyrin in a
high oxidation state. Before scratching the film out of
the electrode, the oxidized polymer was electrochemi-
cally reduced at 0 V to give a neutral polymer.
3.2. Heterogeneous catalysis
a
We reported in a preliminary communication16 that
ruthenium spirobifluorenylporphyrins can be electro-
polymerized and then used as carbene transfer catalysts.
Herein, we have prepared new optically active polymers
by electropolymerization of chiral ruthenium porphy-
rins. The good stability of these catalysts is obvious if
we consider the recovery and the enantioselectivity after
seven cycles, as reported in Figure 5. No significant
metal porphyrin leaching was observed during these het-
erogeneous reactions. To prepare the catalytic system, it
was necessary to introduce a nonchiral group, the spiro-
bifluorene, to polymerize the monomer. Obviously, a
decrease in the enantiomeric excess is to be expected
if we compared the catalytic efficiency, 3-RuCO > 4-
RuCO > 5-RuCO under homogeneous conditions,
according to the number of chiral groups. It was
thought that complex 4-RuCO, bearing only three chiral
groups catalyzed the cyclopropanation with only a mod-
erate decrease in the ee (76%, ꢀ20 ꢀC) by comparison
with complex 3-RuCO (91%, ꢀ20 ꢀC). In contrast, the
enantioselectivity is reduced to 53% as the best result
(Table 3, entry 3) after copolymerization of 4-RuCO.
It is difficult to give an explanation for this effect,
although it is clear that the polymer has a different influ-
ence on the energy of the transition states of the reac-
tion. This effect may also be related to a diminished
access of the substrates to the catalytic sites because of
the cross-linked structure of the polymers. It should be
noted that atropisomers were detected with ruthenium
spirobifluorenylporphyrins.44 Another explanation
would be the presence of high oxidation state ruthe-
nium species in the polymer, such as paramagnetic
Ru(III) and Ru(IV) as reactive species, in the case of
incomplete reduction after the electropolymerization.
This reactive sites may also decrease the enantio-
selectivity.
It should be noted that the question of metal versus ring
oxidation of ruthenium porphyrins for the second oxi-
dation step (Eq. 2) is not obvious.28,43 The doubly oxi-
dized species is only stable on a cyclic voltammetry
time scale, and it has been suggested that a rapid chem-
ical reaction occurs after electrogeneration of this spe-
cies.28,43 Thus modified Pt disc electrodes, which were
prepared by controlled-potential oxidation at 1.6 V,
can be coated with a film containing ruthenium porphy-
rin with a partial lost of the carbon monoxide ligand.
Accordingly, the IR spectrum of poly(TSP-RuCO) poly-
mer showed a CO vibration at 1949 cmꢀ1 in KBr very
close to the value observed for the monomer, but with
a relative reduced intensity (Fig. 7). The UV–vis spec-
trum suggests however that most of the porphyrin units
in the polymers have a ruthenium atom in the +2 oxid-
ation state since the Soret band is detected at 414 nm, as
observed for the starting ruthenium monomer.
Copolymerization with free spirobifluorene was neces-
sary to obtain larger amount of polymer with 4-RuCO,
the monomer containing only one spirobifluorene
group. As expected, the polymerization ability of the dif-
ferent monomers increases with the number of spiro-
bifluorene groups linked to the porphyrin ring. More-
over, the comparison of the values of the threshold oxi-
dation of poly(4-RuCO) (0.58 V) and poly(5-RuCO)
(0.1 V) suggests a possible conjugation through the
metalloporphyrin units, which exists only in poly(5-
RuCO). The less important negative shift of potential
(0.1 V), observed from poly(4-RuCO) to copoly(4-
RuCO) is due only to an increase of the polyspirobiflu-
orene chain length.
4. Conclusion
Despite the good stability and recyclability, the poly-
meric catalysts suffer from the drawback of moderate
enantioselectivity compared to the homogeneous coun-
terpart. Compared to another chiral ruthenium porphy-
rins immobilized in a sol–gel matrix used for asymmetric
epoxidation in which catalyst leaching and deactivation
were observed,14 the system herein seems however to
offer better recyclability. An alternative method to in-
crease the enantioselectivity will be to attach the spirobi-
fluorene groups at the periphery of the chiral porphyrin
to maintain the local D4 symmetry. Accordingly, recent
promising results obtained with a D4-chiral porphyrin
encapsulated in ordered mesoporous molecular sieves,15
have been reported for intramolecular cyclopropana-
tion. We have developed a simple and efficient method-
ology for the immobilization of chiral porphyrin
systems, which may also be used for the oxidation reac-
tion. This new route opens the way for the preparation
of a range of different supported chiral porphyrin-
based catalysts for a wide variety of enantioselective
reactions.
Figure 7. Infra-red spectra of the monomer TSP-RuCO and its
polymer: poly(TSP-RuCO).

1470
Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
5. Experimental
allowed to react at room temperature in dry and de-
gassed chloroform (31 mL) under an argon atmosphere
and protected from light, with acid catalysis (BF3OEt2:
70 lL, 0.49 mmol). Under these conditions, the reaction
reached an equilibrium in about 2 h. A stoichiometric
amount of DDQ was then added to irreversibly oxidize
the porphyrinogene. After the addition of several drops
of triethylamine and ordinary work-up, concentration,
washing, drying, the crude reaction mixture was purified
by chromatography on basic alumina (CH2Cl2/pentane:
1/1). The purple fraction, which eluted first, was a
mixture of three different porphyrins based on TLC
(pentane/CH2Cl2: 8/2): (Rf 3: 0.82; Rf 4: 0.56; Rf 5:
0.30), which consisted, after chromatography on silica
of porphyrins 3 (10%), 4 (18%) and 5 (8%).
5.1. General
All reactions were performed under argon and were
magnetically stirred. Solvents were distilled from appro-
priate drying agent prior to use: Et2O and THF from so-
dium and benzophenone, toluene from sodium, CH2Cl2
from CaH2, CHCl3 from P2O5 and all other solvents
were HPLC grade. Commercially available reagents
were used without further purification unless otherwise
stated. All reactions were monitored by TLC with
Merck pre-coated aluminium foil sheets (Silica gel 60
with fluorescent indicator UV254). Compounds were
visualized with UV light at 254 and 365 nm. Column
chromatographies were carried out using silica gel from
Merck (0.063–0.200 mm). H NMR and 13C NMR in
5.3. 5,10,15,20-Tetrakis-[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-
octahydro-1,4:5,8-dimethanoanthracene-9-yl]-porphyrin 3
1
CDCl3 were recorded using Bruker (Advance 500dpx
and 300dpx spectrometers) at 500 and 75 MHz, respec-
tively. High-resolution mass spectra were recorded on
a ZabSpec TOF Micromass spectrometer in ESI positif
mode at the CRMPO.
The synthesis of this porphyrin was previously
reported.17
5.4. 5,10,15-Tri-[(1S,4R,5R, 8S)-dimethanoanthracen-9-
yl-]20-monospirobifluorenylporphyrin 4
All electrochemical experiments were performed using a
platinum disc electrode [a Pt working electrode (dia-
meter 1 mm) for CVs or a platinum sheet area (2 cm2)
for preparative electrosynthesis]. The counter electrode
was a vitreous carbon rod and the reference electrode
was a silver wire in a 0.1 M AgNO3 solution in CH3CN.
Ferrocene was added to the electrolyte solution at the
end of a series of experiments. The ferrocene/ferroce-
nium (Fc/Fc+) couple served as internal standard and
all reported potentials are referenced to its reversible
formal potential. Activated Al2O3 was added in the elec-
trolyticsolution to remove excess moisture. The three
electrodes cell was connected to a PAR Model 173
potentiostat monitored with a PAR Model 175 signal
generator and a PAR Model 179 signal coulometer.
The cyclic voltammetry traces (CVs) were recorded on
an XY SEFRAM-type TGM 164.
1H NMR (CDCl3, ppm): ꢀ2.60 (s, 2H); 1.02 (m, 6H);
1.41 (m, 18H); 1.79 (m, 6H); 2.06 (m, 6H); 2.84 (m,
6H); 3.62 (s, 6H); 6.92 (d, 2H); 7.18 (td, 2H); 7.27 (m,
2H); 7.33 (t, 1H); 7.39 (t, 1H); 7.43 (m, 3H); 7.54 (t,
1H); 7.83 (m, 3H); 8.13 (d, 1H); 8.19 (dd, 2H); 8.76
(m, 8H). UV–vis (CH2Cl2): kmax/nm (loge): 224 (4.78);
274 (4.54); 423 (5.38); 517 (4.17); 553 (3.87); 592
(3.79); 647 (3.61); Mass (ESI, CH2Cl2/MeOH 9/1)
(m/z): calculated for C93H77N4 [M+H]+: 1249.6148;
found: 1249.6129.
5.5. 5,15-Bis-[(1S,4R,5R,8S)-dimethanoanthracen-9-yl-
]10,20-bis-spirobifluorenylporphyrin 5
1H NMR (CDCl3, ppm): ꢀ2.78 (s, 2H); 1.18 (m, 4H);
1.35 (m, 12H); 1.86 (m, 4H); 2.00 (m, 4H); 2.98–
2.42 (m, 4H); 3.57 (s, 4H); 6.89 (t, 2H); 7.06 (m, 5H);
7.26 (m, 5H); 7.38 (m, 5H); 7.52 (t, 2H); 7.69 (d, 1H);
7.78 (m, 5H); 7.98 (d, 1H); 8.09 (m, 3H); 8.16 (m, 3H);
8.66 (m, 8H). UV–vis (CH2Cl2): kmax/nm (loge) 227
(4.83); 270 (4.58); 308 (4.42); 423 (5.41); 517 (4.11);
553 (3.90); 591 (3.65); 647 (3.60). Mass (ESI, CH2Cl2/
MeOH 9/1) (m/z): calculated for C102H75N4 [M+H]+:
1355.7142 found: 1355.7135. Since compound 5 and
the major compound (containing two chiral groups)
obtained from the MacDonald condensation showed
the same NMR spectrum, it is assumed that the two
chiral groups are in the trans-position.
Dichloromethane with less than 100 ppm of water (ref.
SDS 02910E21) and tetrabutylammonium hexafluoro-
phosphate from FLUKA were used without any purifi-
cation. Aluminium oxide was obtained from Woe¨lm,
activated by heating at 300 ꢀC under vacuum for 12 h
and used at once under argon pressure.
Liquid UV–vis spectra were recorded on a UVIKON
XL from Biotech. Solid UV–vis spectra were recorded
using a Guided Wave model 150 spectrophotometer
with optical fibres; a concave platinum surface acted
as a reflector for the optical beam. Infrared spectra were
performed in KBr discin a IFS 28 Bruker. All catalytic
reactions were controlled on a Varian CP-3380 Gas
5.6. 5,10,15-Tri-[(1S,4R,5R,8S)-dimethanoanthracen-9-
yl-]20-monospirobifluorenyl porphyrinatorutheniumcar-
bonyl 4-RuCO
Chromatograph equipped with
Column.
a CP-Chirasil-Dex
5.2. Synthesis of porphyrins
To a solution of 4 (100 mg, 0.08 mmol) dissolved in de-
gassed o-dichlorobenzene heated at 180 ꢀC, was added
by portion, Ru3CO12 (153 mg, 0.24 mmol). The mixture
was stirred under an argon atmosphere for 2 h until the
reaction was completed. The solvent was removed under
9,90-Spirobifluorene-2-carbaldehyde (0.191 g, 0.556
mmol), dimethanoanthracene-9-carbaldehyde (0.4 g,
1.667 mmol) and pyrrole (155 lL, 2.2 mmol) were

Y. Ferrand et al. / Tetrahedron: Asymmetry 16 (2005) 1463–1472
1471
high vacuum and the crude product was purified by
5.10. Catalytic conditions for polymers
chromatography on silica gel using first pentane, then
a mixture of pentane/dichloromethane (1/1). The major
red-orange band was collected and dried under vacuum
Conditions are similar to those with monomers. Catalyst
amounts used were calculated from the Ru contents in
the heterogeneous catalysts and determined by elec-
tronicmicroanalysis. After 2 h, the mixture was filtered
and the residue washed several times by acetone and
dichloromethane. The polymer was dried under vacuum
and used for another run under the same experimental
conditions.
1
to afford a red powder. Yield 90%. H NMR (CDCl3,
ppm): 1.30 (m, 24H); 1.87 (m, 6H); 1.99 (m, 6H); 2.71
(m, 6H); 3.58 (s, 6H); 6.87 (t ; 1H); 7.12 (m; 2H); 7.24
(m, 2H); 7.36 (m, 6H); 7.53 (m, 1H); 7.65 (d, 1H); 7.77
(dd, 2H); 8.20 (m, 3H); 8.54 (m, 8H). UV–vis (CH2Cl2):
k
(4.35). Mass (ESI, CH2Cl2/MeOH 9/1) (m/z): calculated
max/nm (loge): 224 (4.88); 308 (4.47); 413 (5.38); 529
for C94H75N4O 102Ru [M+H]+: 1377.4984 found:
5.11. Enantiomeric excess determination
1377.5053; IR (KBr): mCO = 1943.9 cmꢀ1
.
For the cyclopropyl esters obtained from cyclopropana-
tion of styrene with ethyl diazoacetate: GC-column
CP-Chirasil-Dex column, injector 200 ꢀC (pulsed
split mode), detector (FID 220 ꢀC, oven: 120 ꢀC;
2.5 ꢀC minꢀ1), pressure = 15 psi. cis (1R,2S) sr =
13.30 min; cis (1S,2t) sr = 13.78 min; trans (1R,2R)
sr = 14.21 min; trans (11S,2S) sr = 14.36 min.
5.7. 5,15-Bis-[(1S,4R,5R,8S)-dimethanoanthracen-9-yl-
]10,20-bis-spirobifluorenyl porphyrinatorutheniumcar-
bonyl 5-RuCO
The complex was prepared using the same method as for
compound 4-RuCO. Yield 90%. 1H NMR (CDCl3,
ppm): 8.49 (m, 8H); 8.18 (m, 4H); 8.09 (m, 2H); 7.75
(m, 4H); 7.52 (m, 4H); 7.35 (m, 6H); 7.24 (m, 5H);
7.11 (m, 5H); 6.87 (m, 2H); 3.55 (br s, 4H); 2.74–2.52
(m, 4H); 1.91 (m, 8H); 1.34 (m, 12H); 1.11 (m, 4H).
UV–vis (CH2Cl2): kmax/nm (loge) 228 (5.15); 308
(4.81); 415 (5.35); 530 (4.34). Mass (ESI, CH2Cl2/MeOH
9/1) (m/z): calculated for C103H72N4ONa 102Ru
[M+Na]+: 1505.4647 found: 1505.4610; IR (KBr):
Acknowledgments
We thank Girex and the MENRT for financial support
(Y.F. and C.P.).
m
CO = 1943.9 cmꢀ1
.
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