Modular approach for the development of supported, monofunctionalized, salen catalysts

Article abstract of DOI:10.1021/jo051919+

We report a modular approach toward polymer-supported, metalated, salen catalysts. This strategy is based on the synthesis of monofunctionalized Mn- and Co-salen complexes attached to a norbornene monomer via a stable phenylene-acetylene linker. The resulting functionalized monomers can be polymerized in a controlled fashion using ring-opening metathesis polymerization. This polymerization method allows for the synthesis of copolymers, resulting in an unprecedented control over the catalyst density and catalytic-site isolation. The obtained polymeric manganese and cobalt complexes were successfully used as supported catalysts for the asymmetric epoxidation of olefins and the hydrolytic kinetic resolution of epoxides. All polymeric catalysts showed outstanding catalytic activities and selectivities comparable to the original catalysts reported by Jacobsen. Moreover, the copolymer-supported catalysts are more active and selective than their homopolymer analogues, providing further proof that catalyst density and site isolation are key toward highly active and selective supported salen catalysts.

Full text of DOI:10.1021/jo051919+

Modular Approach for the Development of Supported,
Monofunctionalized, Salen Catalysts
Michael Holbach and Marcus Weck*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
marcus.weck@chemistry.gatech.edu
ReceiVed September 13, 2005
We report a modular approach toward polymer-supported, metalated, salen catalysts. This strategy is
based on the synthesis of monofunctionalized Mn- and Co-salen complexes attached to a norbornene
monomer via a stable phenylene-acetylene linker. The resulting functionalized monomers can be
polymerized in a controlled fashion using ring-opening metathesis polymerization. This polymerization
method allows for the synthesis of copolymers, resulting in an unprecedented control over the catalyst
density and catalytic-site isolation. The obtained polymeric manganese and cobalt complexes were
successfully used as supported catalysts for the asymmetric epoxidation of olefins and the hydrolytic
kinetic resolution of epoxides. All polymeric catalysts showed outstanding catalytic activities and
selectivities comparable to the original catalysts reported by Jacobsen. Moreover, the copolymer-supported
catalysts are more active and selective than their homopolymer analogues, providing further proof that
catalyst density and site isolation are key toward highly active and selective supported salen catalysts.
Introduction
hetero Diels-Alder reactions,14,15 Pictet-Spengler reactions,16
and hydrocyanations.17-20 While a wide variety of different
chiral salen ligands have been synthesized, metalated, and their
catalytic properties studied in detail,21,22 the original metal-
salen complexes of the general structure 1 (Figure 1) developed
by Jacobsen are still the most selective catalysts for a wide
variety of catalytic reactions and a broad range of substrates.2
To utilize these catalysts in the pharmaceutical and fine
chemical industries, the removal of the metal species and the
reusability of the catalysts are key criteria. The most successful
strategy to achieve this goal is the use of the supported analogues
of these complexes.3,23-28 However, immobilized versions of
chiral salen complexes have been published only for a limited
Metalated chiral salen ligands such as 1 are among the most
successful and important asymmetric catalysts today.1-3 These
complexes were first introduced in the 1990s by Jacobsen and
Katsuki as highly enantioselective catalysts for the asymmetric
epoxidation (AE) of unfunctionalized olefins.4,5 Over the past
decade, metalated salen complexes have also been successfully
employed as catalysts in a large variety of other asymmetric
transformations1-3 including ring-opening of epoxides,6-10
hydrolytic kinetic resolution (HKR) of terminal epoxides,11-13
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10.1021/jo051919+ CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/01/2006
J. Org. Chem. 2006, 71, 1825-1836
1825

Holbach and Weck
fashion without the need for post-polymerization steps. Fur-
thermore, the catalytic sites can be randomly dispersed within
a copolymer, thereby allowing for controlled site isolation of
the catalytic moieties. Our poly(norbornene) complexes are
highly active and selective catalysts for the epoxidation of
olefins and the HKRs of terminal epoxides. By comparing the
catalytic performances of the polymeric complexes for these
two reactions, which have very diverse requirements (vida infra),
we set up general guidelines for the future development of
supported salen catalysts.
FIGURE 1. Structure of the most widely active and selective metal-
salen complex.
range of reactions such as AEs,10,23,29-77 HKRs,12,13,24,78-93 other
ring-opening reactions,9,10,94-98 hetero Diels-Alder reac-
tions,43,50 and others.99-108 Despite the success of some of these
strategies, no modular approach allowing a systematic and
straightforward development of various different supported salen
catalysts has been reported. In this article we describe our studies
toward such a system for the development of polymer-supported
salen catalysts by employing poly(norbornene)-immobilized
salen complexes of manganese and cobalt.
The use of poly(norbornene) complexes as support has a
number of advantages over other polymeric supports. As a result
of the polymerization characteristics of ring-opening metathesis
polymerization (ROMP),109-116 the fully characterized mono-
meric complexes can be polymerized in a highly controlled
Results and Discussion
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1826 J. Org. Chem., Vol. 71, No. 5, 2006

Supported, Monofunctionalized, Salen Catalysts
SCHEME 1. Proposed Mechanisms for (A) the AE of Olefins and (B) the HKR of Epoxides
supported complexes. Only these systems allow for a rational
design and optimization of the catalysts. For polymeric supports,
almost all studies report on the synthesis of a polymeric ligand
and the subsequent complexation of a catalytically active metal
in a post-polymerization process. When this strategy is used, a
uniform system can only be obtained when the complexation
step is quantitative and no side reactions occur, a prerequisite
that most post-polymerization strategies do not fulfill, especially
when using sterically congested polymeric ligands. The goal
of this study is, thus, to overcome these shortcomings by first
synthesizing well-characterized monomeric metal complexes that
can be polymerized subsequently using a controlled polymer-
ization method such as ROMP.109-116
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The two polymer-supported salen catalysts that have been
investigated in this study are active catalysts for the AE of
olefins (manganese-salen) and the HKR of terminal epoxides
(cobalt-salen). Besides their outstanding importance in industry,
these two catalysts have several contradicting properties. They
represent different methodologies for synthesizing chiral com-
pounds (AE, asymmetric catalysis; HKR, resolution of enanti-
omers) and are suitable for the synthesis of different kinds of
enantiomerically highly enriched epoxides (AE, highest enan-
tioselectivities for synthesizing cis-disubstituted epoxides; HKR,
highest selectivities for the resolution of terminal epoxides).
Most importantly, both reactions differ dramatically in their
reaction mechanisms (Scheme 1). In the case of AE, the Mn-
(III) complex is oxidized to Mn(V) by a single oxygen source,
such as a peroxide, and the Mn(V) species is responsible for
the epoxidation of the olefin, whereby the manganese is reduced
back to Mn(III).117,118 The most common deactivation pathway
for this system is the dimerization of a Mn(III) and a Mn(V)
complex, resulting in an oxo-bridged Mn(IV)-O-Mn(IV)
species (Scheme 1A).119 For this reason, it has been proposed
that a highly active Mn-salen epoxidation catalyst can only be
realized by immobilizing the complex in such a way that a
perfect site isolation of all catalytic moieties is guaran-
teed.27,51,52,74 The mechanism of the HKR, on the other hand,
is believed to involve a cooperative bimetallic pathway, with
one cobalt activating the epoxide while a second metal is
activating the nucleophile (Scheme 1B).12,13,88,120-122 In this case,
a system allowing an easy interaction of the two metal centers
is desired, whereas a system with site-isolated catalytic centers
should be less selective. Because of the very different mecha-
nistic properties of AE and HKR, we propose that, once a
successful modular approach for the synthesis of catalysts for
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J. Org. Chem, Vol. 71, No. 5, 2006 1827

Holbach and Weck
SCHEME 2. Synthesis of the Norbornene-Functionalized Manganese- and Cobalt-Salen Complexes 9 and 10
SCHEME 3. ROMP of the Norbornene-Functionalized Salen Complexes 9 and 10
these two reactions has been worked out, the development of
supported salen catalysts for other reactions should be straight-
forward by adapting to the mechanistic properties of the studied
reaction.
(norbornene) backbone. For the attachment, chemically inert
C-C bond linkages via a phenylene-acetylene linker have been
employed that have been proven to minimize catalyst degrada-
tion during the epoxidation reaction.50 Variations of the catalyst
density were realized by either homo- or copolymerizing the
metalated salen monomers with a terminal alkyl group contain-
ing monomer, using ROMP as the highly controlled polymer-
ization method.
Synthesis and Polymerization of the Mononorbornene
Functionalized Salens. The monofunctionalized salens were
obtained by the desymmetrization of 1,2-diaminocyclohexane
2 with HCl to yield the monoammonium salt 3.125 After the
reaction of 3 with 4, the resulting monoammonium imine 5 was
deprotected with NEt3 in a one-pot procedure and reacted with
the functionalized aldehyde 6. After an esterification of the
resulting unsymmetrically substituted salen 7 with 8 and the
subsequent metalation, the functionalized Mn- and Co-salen
norbornenes 9 and 10 were obtained in good yield (Scheme 2).
ROMP of the analytically pure and fully characterized mono-
mers using the 3°-generation Grubbs catalyst,109,110 yielded p-
(9) and p(10) (Scheme 3). Furthermore, we prepared copolymers
of 9 and 10 with the unfunctionalized norbornene 11126 (Scheme
We rationalized that a modular salen support system can be
developed for both systems by adapting criteria put forward by
Sherrington et al.52 These criteria include the following: (i) the
supported catalyst should resemble the optimized salen ligand
sphere 1 developed by Jacobsen et al.21,22 as closely as possible;
(ii) the salen ligand should be attached to the support via a single
linker to minimize steric restrictions; (iii) for Mn-salen species,
the catalyst loading should be sufficiently low to maximize site
isolation of the catalytic centers and thus minimize the formation
of catalytically inactive oxo-bridged dimers,119,123 while for the
Co species, catalyst density must be high enough to allow for
the proposed simultaneous activation of the epoxide and the
nucleophile via two different cobalt centers;12,88,120,124 and (iv)
the morphology of the supports should ensure free accessibility
to all active sites.
To fulfill these requirements, we employed monofunction-
alized salen cores attached via a single site to a soluble poly-
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1828 J. Org. Chem., Vol. 71, No. 5, 2006

Supported, Monofunctionalized, Salen Catalysts
TABLE 1. Elemental Analysis and ICP Data of the Polymers
% found (% calcd)
Oa
entry
1
polymer
p(9)
Ca
Ha
Na
Cl/Sb
n.d.e
Mec
Med
68.67
(70.71)
71.10
(72.13)
n.d.e
7.02
(6.92)
7.65
3.27
(3.44)
2.64
9.09
(7.85)
9.72
11.95f
(11.09)
5.68
5.00
(6.74)
4.68
(5.15)
3.86
(3.51)
1.39
2
3
4
p(9.11)11
p(9.11)13
p(9.11)19
2.26
(7.78)
(2.63)
(9.01)
(3.33)
(5.15)
n.d.e
n.d.e
n.d.e
n.d.e
n.d.e
75.05
9.63
0.84
11.69
0.96
1.83
(75.15)
(9.53)
(0.91)
(11.47)
(1.16)
(1.79)
(1.79)
5
6
7
8
9
10
73.33
(73.54)
71.06
(73.54)
72.88
(74.32)
74.25
(75.11)
75.50
(75.92)
7.21
(7.20)
7.06
(7.20)
7.89
(7.99)
8.73
(8.80)
9.50
(9.62)
3.49
(3.57)
3.57
(3.57)
2.76
(2.71)
1.95
(1.82)
0.90
(0.92)
8.33
(8.16)
8.95
(8.16)
9.89
(9.28)
11.15
(10.42)
11.96
(11.59)
7.64
(7.52)
9.36
(7.52)
6.58
(5.70)
3.92
(3.84)
2.14
(1.94)
8.00
(7.52)
6.96
(7.52)
5.96
(5.70)
3.95
(3.84)
2.02
(1.94)
p(10)
p(10.11)11
p(10.11)13
p(10.11)19
10
11
p(10c)
65.42
(69.16)
57.65
6.79
(6.65)
6.27
2.60
(2.93)
2.58
15.02
(11.73)
20.47
3.97
(3.36)
5.10
6.20
(6.17)
7.93
5.60
(6.17)
4.27
p(10c)_rec
(69.16)
(6.65)
(2.93)
(11.73)
(3.36)
(6.17)
(6.17)
a Determined by elemental analysis. b Determined by elemental analysis; S only for p(10c.11) and p(10c.11)_rec (entries 10 and 11). c Determined by
100 - (sum of percentages of other elements). d Determined by ICP. e Not determined as a result of lack of sample material. f Content of MnCl, determined
by 100 - (sum of percentages of other elements).
3) to (i) site-isolate individual catalyst sites and (ii) probe the
effect of catalyst loadings on the catalytic activity.
out ICP analyses of all compounds. Second, we analyzed the
C, H, N, and O contents by elemental analysis. Because the
monomers contain only the elements C, H, N, and O, but for
the metal and its ligands (such as chlorine, etc.), one can
calculate the metal content by subtracting the C, H, N, and O
(and in case of the Mn complexes, Cl) contents from 100
percent. The remaining contents must be the metal complex.
The results of these two methods are shown in Table 1.
Finally, if possible, we determined the chlorine content of
all polymers. The manganese-containing compounds contain one
Cl ligand on all metals. Therefore, by analyzing the chlorine
content, one can calculate the metal content.128 Using this
methodology, the manganese content of p(9.11)11 was calculated
to be 3.50% (calcd, 5.15%), and that of p(9.11)19 was calculated
to be 1.48% (calcd, 1.79%). While one should expect these three
methods to agree in the metal contents, we found that all three
methods slightly diverse from the theoretically expected metal-
content values (some methods suggest a higher metal content,
while others suggest a lower one), which we attribute to the
error of the three analytical methods as a result of the small
sample sizes and the low metal percentage of all polymers. With
these limitations in mind, the values for the metal loadings of
the polymers are within the error range (in most cases, (0.5%
for all three methods) and in good agreement with the theoretical
values. These results suggest that the metalated salen complexes
do not decompose during the ROMP (it is important to note
that always fully characterized and purified, i.e., by column
chromatography, metalated monomers were employed during
the ROMP) and that the co-monomer ratios are approximately
the same as the theoretical targeted ratios.
Characterization of the Polymers. Although the complexes
9 and 10 are paramagnetic, we were able to investigate the
1
polymerization rates using H NMR spectroscopy by solely
focusing on the signals of the olefin protons. In all cases,
monomer conversions (monomer-to-catalyst ratios up to 100:
1) were quantitative after 1-2 h. Moreover, following the
homopolymerizations of 10 equiv of 9, 10, or 11 by 1H NMR,
we found that the polymerization kinetics of all three monomers
using the 3°-generation Grubbs catalyst are similar with
complete monomer conversion after 2-5 min (see Supporting
Information). These results suggest that the copolymerization
of 9 and 11 and 10 and 11 yield statistical copolymers p(9.11)
and p(10.11), that is, the monomers containing the catalytic
moiety are randomly interdispersed in the alkyl-norbornene
matrix.
It is well-known that polyelectrolyte and metal-salt-containing
polymers cannot be characterized by gel-permeation chroma-
tography (GPC), most likely as a result of the interactions of
the polymers with the packing material and the formation of
aggregates during the process.127 The same limitation holds true
for our metal-containing polymers, and no GPC results of any
polymer containing more than 10 repeating units could be
obtained in either THF, chloroform, or methylene chloride.
Therefore, no polydispersities (PDIs) or molecular weights for
the high-molecular-weight polymers are reported. However, the
values for the molecular weights and the PDIs of 10-mers of
p(9) (Mw ) 5,900; Mn ) 4,400; PDI ) 1.34) and p(10) (Mw )
55,400; Mn ) 18,300; PDI ) 3.03) confirmed a successful and
quantitative ROMP of the monomeric complexes.
Asymmetric Epoxidation. For studying the properties of the
polymeric Mn-salen complexes as AE catalysts, we chose
styrene 12, 1,2-dihydronaphthalene 14, and cis-â-methylstyrene
16 as substrates, representing a terminal, a cyclic, and a cis-
We used three different analysis methods to determine the
metal content of all monomers and polymers. First, we carried
(127) Meier, M. A. R.; Lohmeijer, B. G. G.; Schubert, U. S. Macromol.
Rapid Commun. 2003, 24, 852.
(128) % Mn ) % Mn (calcd) × [% Cl/% Cl (calcd)].
J. Org. Chem, Vol. 71, No. 5, 2006 1829

Holbach and Weck
FIGURE 2. Kinetic study of the epoxidation of (A) styrene 12 (STY), (B) 1,2-dihydronaphthalene 14 (DHN), and (C) cis-â-methylstyrene 16
(M-STY) using the poly(norbornene)-based manganese catalysts via GC analysis of the reaction mixture (lines are just visual aids and not plots).
TABLE 2. Epoxidation of Unfunctionalized Olefinsa
SCHEME 4. Epoxidations of Unfunctionalized Olefins
entry olefinb epoxide
catalyst
temp (°C) conv.c (%) ee (%)
1
2
3
4
5
12
12
12
12
12
13
13
13
13
13
Jacobsen
p(9)
p(9.11)11
p(9.11)13
p(9.11)19
-20
-20
-20
-20
-20
100
100
100
100
100
34
32
33
32
33
6
7
8
14
14
14
14
14
14
14
15
15
15
15
15
15
15
Jacobsen
p(9)
-20
-20
-20
-20
-20
-20
-20
100
100
100
100
85
88
76
81
47
6
p(9.11)11
Cycle 2
Cycle 3
p(9.11)13
p(9.11)19
9
10
11
12
100
100
81
82
13
14
16
16
17
17
Jacobsen
p(9.11)11
-20
-20
100
100
93
92
a All epoxidations were carried out at -20 °C with m-CPBA (2 equiv),
NMO (5 equiv), and a 4 mol % Mn catalyst in CH2Cl2. b Styrene 12, cis-
â-methylstyrene 16, or 1,2-dihydronaphthalene 14. c After a period of 5
min.
disubstituted, noncyclic olefin. Following published proce-
dures,10,51,52,129 we dissolved the polymeric catalysts, N-meth-
ylmorpholine-N-oxide (NMO), the olefin, and chlorobenzene
or dodecane as an internal standard in methylene chloride,
cooled the solutions to -20 °C, and added meta-chloroperoxy-
benzoic acid (m-CPBA) in three equal portions over a period
of two minutes (Scheme 4).
Kinetic studies via the GC analysis of the reactions clearly
showed the outstanding high activities of the polymer-supported
salen systems with quantitative conversions using 4 mol % Mn
after 150-300 s (Figure 2, Table 2). The epoxidations of styrene
12 and cis-â-methylstyrene 16 are as fast as the control
experiments, using the original nonsupported Jacobsen complex
(Figure 2A,C). Only in the case of 1,2-dihydronaphthalene 14
is the epoxidation rate with p(9) (Figure 2B, green) somewhat
slower than the rate of the original Jacobsen catalyst (black line).
The copolymers, p(9.11), have catalytic activities (Figure 2B)
that are higher than their homopolymer analogue, p(9). All
reactions are quantitative after 5 min or less, suggesting a good
accessibility to all catalyst sites.
Our supported catalysts have also outstanding selectivities
(Figure 3, Table 2). For the epoxidation of styrene 12 and cis-
â-methylstyrene 16, the enantiomeric excesses (ee’s) are on par
with the unsupported Jacobsen catalyst (32-33 vs 34% ee for
13 and 92 vs 93% ee for 17). For the epoxidation of
1,2-dihydronaphthalene 14, the ee’s with the polymeric catalysts
are slightly lower than with the Jacobsen catalyst (76-82 vs
88% ee) but are among the highest ee's reported for any
immobilized salen complexes (10-84% ee).3,27,43,44,50 A possible
reason for the somewhat lower selectivities of our supported
systems is their slower reaction rate, which increases the amount
(129) Palucki, M.; McCormick, G. J.; Jacobsen, E. N. Tetrahedron Lett.
1995, 36, 5457.
1830 J. Org. Chem., Vol. 71, No. 5, 2006

Supported, Monofunctionalized, Salen Catalysts
SCHEME 5. Reaction Conditions of the HKR of
Epichlorohydrin 18
backbone olefins of the poly(norbornene) complexes could be
cross-linked or epoxidized) is yet unclear. However, Janda and
Reger reported similar problems for the solubility and recycla-
bility of monofunctionalized, polymeric, Mn-salen catalysts
with different linkers and different polymeric supports when
using 1,2-dihydronaphthalene as starting material,51 indicating
that these difficulties may not be a shortcoming of our support,
but a more general issue of monofunctionalized salen complexes
attached to soluble supports.
Our results with the Mn-poly(norbornene) complexes as
chiral epoxidation catalysts clearly reveal that, by following the
above-described criteria, immobilized catalysts with outstanding
activities and selectivities, that are on par with the catalytic
activities and selectivities of the original Jacobsen catalyst, can
be synthesized. Furthermore, the polymeric Mn-salen com-
plexes can be easily removed by precipitation and subsequent
centrifugation, allowing for the removal of the vast majority of
the metal species after complete reactions.
Hydrolytic Kinetic Resolution. The catalytic performance
of the polymeric cobalt complexes p(10) and p(10.11) were
studied in the HKR of racemic epichlorohydrin 18 (Scheme 5).
Because only the Co(III) complexes are catalytically active in
this reaction, we oxidized the obtained Co(II) polymers, p(10)
and p(10.11), by stirring methylene chloride solutions of the
polymers with acetic acid under an atmosphere of air (Scheme
6, X ) OAc). After removal of the solvent and the excess AcOH
in vacuo, the desired Co(III)-salen polymers p(10a) and p-
(10a.11), with acetates as counterions, were obtained. For the
HKR reactions, the Co(III) polymers were dissolved in a mixture
of methylene chloride, 18, and chlorobenzene as an internal
standard, followed by the addition of 0.7 equiv of water to start
the resolution. The addition of some methylene chloride as a
solvent was necessary, because the copolymers were not fully
soluble in 18.
The reaction kinetics of the HKR were studied via chiral GC
analysis. When either the homopolymer, p(10a), or the two
copolymers, p(10a.11)11 and p(10a.11)13, are used, epoxide (R)-
18 is fully converted after 5 h to its corresponding diol, leaving
pure (S)-18 in the reaction mixture in above a 99% ee (Figure
5). After this time period, 55% of the racemic 18 is converted,
that is, all of the unwanted (R)-enantiomer is converted to the
FIGURE 3. Enantiomeric excesses of the epoxidation of styrene 12
(blue bars), 1,2-dihydronaphthalene 14 (red bars), and cis-â-methyl-
styrene 16 (green bars) using the polymeric Mn-salen catalysts. Jac
1 (original Jacobsen catalyst) is included as standard.
of racemic epoxide produced by a slow background reaction
without the need of a catalyst (Figure 2B, dotted black line).
As already seen for the epoxidation rates, the enantioselectivities
of the copolymers p(9.11) are very similar (81-82% ee) and
higher than the homopolymer p(9) (76% ee), suggesting a good
site isolation of the manganese centers even for the 1:1
copolymer.
The polymeric Mn complexes could be separated easily from
the reaction mixtures by the precipitation into Et2O/MeOH and
subsequent centrifugation. In all cases, the polymer was
recovered quantitatively. For a first study of the reusability, we
used p(9.11)11, representing the copolymer system with the best
catalytic performance and the highest manganese loading.
Unfortunately, after separating the polymer by precipitation, the
residue was not completely soluble in methylene chloride
anymore. When the resulting suspension was used as a catalyst
for the epoxidation of 14, the epoxidation rate was slower than
the one with the nonrecycled polymer (Figure 4A, red lines).
This drop of activity after recycling is even more pronounced
in the third cycle, where we could get only 85% conversion of
14 after 300 s (Figure 4A). Even more dramatic was the decline
of the enantioselectivity from 81% ee to 47% ee (second cycle)
to only 6% ee after the third cycle (Table 2, Figure 4B), showing
that p(9.11)11 is not reusable. Whether the observed solubility
problems and the subsequent drops in activity and selectivity
are a result of the dimerization of Mn(III)s and Mn(V)dO
centers (leading to cross-links between different polymer chains)
or a degradation/chemical modification of the catalyst (i.e., the
FIGURE 4. (A) Conversion vs time and (B) enantioselectivities of the epoxidation of 1,2-dihydronaphthalene 14 (DHN) with p(9.11)11 (lines are
just visual aids and not plots).
J. Org. Chem, Vol. 71, No. 5, 2006 1831

Holbach and Weck
FIGURE 5. HKR of 18 with polymeric CoOAc-salen complexes (determined via chiral GC analysis of the reaction mixtures; lines are just visual
aids and not plots).
FIGURE 6. HKR of 18 with polymeric Co-salen complexes containing different counterions (determined via chiral GC analysis of the reaction
mixtures; lines are just visual aids and not plots).
SCHEME 6. Oxidation of the Polymer-Supported Cobalt-Salen Catalysts
diol, while only 5% of the desired epoxide has been converted,
indicating selectivities similar to the original Jacobsen CoOAc
catalyst (53% conversion, >99% ee under solvent-free condi-
tions). The epoxidation rates with p(10a.11)11 and p(10a.11)13
are slightly higher than the rates using p(10a) (Figure 5). This
finding, which on first sight contradicts the assumption of a
bimetallic mechanism for the HKR (see Scheme 1), may be
the result of a higher backbone flexibility of the copolymers in
comparison to the sterically more congested homopolymers.
However, further dilution of the salen moieties along the
polymer backbone (polymers p(10a.11)19) resulted in a dramatic
drop of the activity (Figure 5C, only 43% conversion and 80%
ee after 5 h), while the selectivities stay the same (e.g.,
p(10a.11)11, 44.5% conversion, 78.4% ee and p(10a.11)13,
45.5% conversion, 81.0% ee after 1 h). This result clearly
suggests that the extreme dilution of the catalytic moieties along
the polymer backbone results in the deactivation of the catalysts
as a result of the unlikelihood of two catalytic moieties being
in close proximity to each other, a prerequisite for the bimetallic
catalytic pathway.
polymeric catalyst is less selective under these reaction condi-
tions. When comparing the kinetics of the original Jacobsen
CoOAc-salen catalyst 1 (Me ) CoOAc) with the monomeric
catalysts 10a (Figure 6, black and blue lines), we found that
10a is slightly less selective than the original Jacobsen catalyst,
indicating that the lower selectivities of the polymers are
primarily a result of the different catalyst structure (a phe-
nylene-acetylene linker instead of a tert-butyl group in the 5
position of one of the aromatic rings) and not based on the
polymeric support. This result suggests that even small changes
in the structure of the salen core can have a significant effect
on the catalytic properties of the resulting complexes.
Another important variable that has been studied with Co-
salen catalysts is the choice of the counterion on the metal
center.83,88,122 It has been reported that very nucleophilic
counterions such as Cl can attack the epoxide very fast, resulting
in small amounts of byproduct, while catalyst activity can be
increased dramatically with more electronegative counterions
such as OTs.80,122 Finally, noncoordinating counterions such as
-
-
PF6 or BF4 can suppress the undesired reduction of Co(III)
to Co(II).83 To investigate if this counterion effect holds true in
our polymeric system and if we can tailor the activities and
selectivities of our polymeric catalysts through changes of the
counterion, we synthesized p(10b) and p(10c) with an iodide
and a tosylate as counterions, respectively. Polymer p(10b) was
obtained by oxidizing 10 with iodine and a subsequent ROMP
When p(10a) is used, the HKR of 18 can also be carried out
under solvent-free conditions (Figure 6, red line). In this case,
the reaction rates are faster than the rate with p(10a), using
CH2Cl2 as the solvent, as well as the rate of the original Jacobsen
complex, resulting in (S)-18 with >99% ee after less than 2 h.
However, an increase in conversion (62%) suggests that the
1832 J. Org. Chem., Vol. 71, No. 5, 2006

Supported, Monofunctionalized, Salen Catalysts
FIGURE 7. HKR of 18 with polymeric CoOTs-salen complexes (determined via chiral GC analysis of the reaction mixtures; lines are just visual
aids and not plots).
FIGURE 8. HKR of 18 with polymeric CoOAc-salen complexes (determined via chiral GC analysis of the reaction mixtures; lines are just visual
aids and not plots).
of 10‚I. In contrast, p(10c) was obtained by oxidizing the
polymer p(10) with O2/p-toluenesulfonic acid (Scheme 6).
Interestingly, when either p(10b) or p(10c) was employed as a
catalyst in the HKR of 18 under solvent-free conditions, both
catalysts showed higher activities than p(10a) (Figure 6). In
particular, p(10b) was highly active, with (S)-18 being obtained
in >99% ee after less than 1 h (Figure 6, purple line). However,
the selectivities of p(10b) and p(10c) are somewhat lower than
that of p(10a) (conversions for obtaining (S)-18 with >99%
ee, 66.7% using p(10b), 69.7% using p(10c), and 62.0% using
p(10a)). These results prove that a counterion effect exists also
in our polymer-supported Co-salen catalysts.
As a result of the outstanding activities of p(10c), we used
this polymeric catalyst to optimize our reaction conditions.12,122
We first tried to improve the selectivity by adding CH2Cl2 as a
solvent to the reaction (Figure 7, green line). Unfortunately,
the reaction was significantly slower, with only 49.0% of 18
converted after 11.5 h (87.9% ee of (S)-18). Nevertheless, higher
ee’s of (S)-18 at 50% conversion were observed under these
reaction conditions, which indicate an increase in selectivity of
p(10c) when adding a solvent. We also investigated the effect
of decreasing the catalyst loading on the activities and the
selectivities. When decreasing the catalyst loading of the
polymeric CoOTs catalyst to 0.2 mol % Co for the HKR of 18,
we observed a decrease in activity (the reaction takes 11 h for
a complete conversion of (R)-18) as well as a decrease in
selectivity (after 11 h, 67.2% of the racemic epoxide is
converted). Moreover, a comparison of the ee of the remaining
(S)-18 at similar conversions suggests an increased selectivity
with a decrease of the CoOTs amount (0.2 mol % p(10c), 47.4%
conversion, 78.2% ee; 0.5 mol %, 51.4% conversion, 64.4%
ee). These results demonstrate that, for the CoOTs-poly
(norbornene) catalyst, the use of less catalyst loading might be
advantageous.
All polymeric Co complexes could be separated easily from
the reaction mixtures by precipitation into Et2O and subsequent
centrifugation. By simply washing the obtained Et2O solution
with water, (R)-3-chloro-1,2-propandiol 19 can be removed
nearly quantitatively, resulting in pure (S)-18. This protocol is
considered to be an important advantage to the normally used
methodology of distilling off 18 from the crude reaction mixture,
because remaining Co(III)-salen complexes are reported to
catalyze the decomposition and the racemization of the epoxide
during the purification process.13,130,131 Because the polymeric
CoOAc complex p(10a) gave the best selectivities, we chose
this catalyst to study the reusability of our polymeric Co-salen
catalysts. After reoxidation with O2 in the presence of acetic
acid, recycled p(10c) showed the same resolution rate and
selectivity as the original polymeric catalyst p(10c) (Figure 8,
61.8% conversion, > 99% ee of (S)-18 after 150 min). However,
the need for ultrasonication to dissolve p(10a) fully in a solution
of 18 and chlorobenzene before the catalysis already pointed
toward a reduced solubility. The solubility of the polymeric
catalyst worsened for the third catalytic cycle. We were not able
to dissolve the twice-recycled p(10a) fully in the reaction
mixture, even after the addition of CH2Cl2 as a solvent and
ultrasonicating the mixture. As a result of the lower solubility,
the reaction rate dropped by a third, with only 78.8% ee after
3 h (49.6% conversion). However, after 11 h, 61.0% of the
racemic epoxide is converted and (S)-18 is obtained with 97.8%
ee, indicating that the polymeric catalyst is still active and very
(130) Aouni, L.; Hemberger, K. E.; Jasmin, S.; Kabir, H.; Larrow, J. F.;
Le-Fur, I.; Morel, P.; Schlama, T. Asymmetric Catal. Ind. Scale 2004, 165.
(131) Larrow, J. F.; Hemberger, K. E.; Jasmin, S.; Kabir, H.; Morel, P.
Tetrahedron: Asymmetry 2003, 14, 3589.
J. Org. Chem, Vol. 71, No. 5, 2006 1833

Holbach and Weck
FIGURE 9. HKR of 18 with polymeric CoOAc-salen complexes (determined via chiral GC analysis of the reaction mixtures; lines are just visual
aids and not plots).
selective and that after an appropriate reaction time enantio-
merically pure (S)-18 can be isolated in good yields.
the second catalysis cycle. In the third cycle, this residue showed
a catalytic performance that is comparable to the one described
for the second cycle (Figure 9). Interestingly, we did not observe
any solubility problems using this methodology, contrasting our
results with the precipitation method. While the reason for this
is still unclear and a matter of current investigations, it shows
that the method of recycling may be an important factor in
determining the properties and performance of a recycled
catalyst.
The polymeric cobalt-salen results clearly demonstrate that
we successfully synthesized a highly active and selective
supported version of the Jacobsen’s cobalt-salen catalyst.
Furthermore, we have shown that the copolymers are slightly
more active than their homopolymer analogues. Finally, we have
also observed the often-described counterion effect in our
supported catalyst system, allowing for the tailoring of activities
and selectivities. While the catalysts can be removed from the
reaction mixture allowing for the easy removal of metal species
from the product, using the precipitation method, our supported
catalysts can only be recycled once as a result of their decreased
solubility after the reoxidation step.
Similar results for the reusability (i.e., the solubility problems
and subsequent low resolution rates) were obtained with the
polymeric CoOTs-salen p(10c) (see Supporting Information
for kinetic data). Analysis of p(10c) by ICP before starting the
recycling experiments and after the third catalysis cycle showed
that the metal content of the polymer decreased from 5.60 to
4.27%. This decrease in metal content cannot be explained with
the error range of the elemental analysis. Because a leaching
of metallic cobalt is not reported in the literature, we suggest
that this slight decrease in metal content may be a result of a
cleavage of the ester bonds resulting in a loss of a complete
salen moiety from the polymer. Nevertheless, while we recently
reported on the stability of the norbornene ester linkages under
a variety of reaction conditions,132 we cannot exclude (also
unlikely) some metal leaching of the cobalt from the polymer
at this point. Additionally, the percentages of carbon, oxygen,
and sulfur are increasing (Table 1, entries 10 and 11), indicating
that either para-toluenesulfonic acid is not completely separated
during the precipitation or that the acid is reacting with the
double bond along the polymer backbone.
A second recycling method that has been reported in the
literature is the removal of the substrates by fractionated
distillation followed by the addition of more starting material
to the metal-containing residue.133 While this method suggests
repeated usability of the Co catalysts, it has several disadvan-
tages including the potential for the undetected decomposition
of the catalyst (i.e., leaching of cobalt) and that epoxides such
as (S)-18 may racemize during this process.13,130,131 However,
because of the solubility issues we encountered during the
separation of the catalyst by precipitation, we also studied this
recycling method, whereby we also used CH2Cl2 as a solvent.
Starting with 11 mg of p(10a) in the first cycle, we obtained
56 mg of a red-brown solid after distilling off CH2Cl2, (S)-18,
and 19. Subsequently, the residue was dried in vacuo, and the
Co(II) complex was reoxidized with O2/AcOH. The increased
mass indicates an incomplete removal of the substrates, in
particular, 3-chloropropane-1,2-diol (boiling point 213 °C). The
recycled polymeric catalyst showed a somewhat lower activity
in the second cycle. It took 11 h instead of 5 h to obtain (S)-18
with >99% ee (57% convervision; Figure 9). After recycling
and reoxidation, we isolated 99 mg of a brown residue after
Conclusion
In this contribution, we describe the synthesis of monofunc-
tionalized Mn- and Co-salen complexes 9 and 10 attached to
a norbornene monomer via a stable phenylene-acetylene linker.
The analytically pure and fully characterized monomeric
complexes were homo- and copolymerized using the controlled
polymerization method, ROMP, thus circumventing the usually
applied post-polymerization complexation step, allowing for the
synthesis of fully functionalized immobilized metal-salen
catalysts. The obtained polymeric manganese and cobalt com-
plexes were successfully used as supported catalysts for AEs
of different olefins and for the HKR of epoxides. All polymeric
catalysts showed outstanding catalytic activities and selectivities
that are comparable to the original catalysts reported by
Jacobsen, thus proving the effectiveness of our design criteria.
Interestingly, we found a dependence of the activity and
selectivity of the catalyst on the density of the catalytic moieties
along the polymer backbones. In general, we found that the
copolymers are slightly more active and selective catalysts than
their homopolymer analogues. This holds true for both the
manganese as well as the cobalt-based systems. Only when
diluting the cobalt-salen moieties to less than 15% along the
polymer backbone was a drop in catalytic activity observed.
Our polymeric catalysts can be easily removed from the reaction
mixtures, demonstrating the possibility of easy metal removal
(132) Carlise, J. R.; Kriegel, R. M.; Rees, W. S., Jr.; Weck, M. J. Org.
Chem. 2005, 70, 5550.
(133) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen,
K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. J. Am. Chem. Soc.
2002, 124, 1307.
1834 J. Org. Chem., Vol. 71, No. 5, 2006

Supported, Monofunctionalized, Salen Catalysts
Cl2 in a dry, 50-mL, Schlenk flask, equipped with a magnetic stir
bar and a septum. The red solution was cooled to 0 °C. A solution
of 204 mg of norbornene carbonyl chloride, 8 (1.31 mmol, 1.1
equiv), in 8 mL of anhydrous CH2Cl2 and 0.18 mL of anhydrous
NEt3 (1.31 mmol, 1.1 equiv) was added dropwise via a syringe,
respectively. The yellow solution was stirred at 0 °C for 30 min
and was then allowed to warm to room temperature over a period
of 30 min. The reaction mixture was filtered through a pad of dry
silica, and the silica was flushed with CH2Cl2. After the removal
of the solvent under reduced pressure, 913 mg of product (96%)
was obtained as a yellow solid. Usually, the crude product was
used in the following steps. For characterization purposes, a part
of the norbornene was purified by column chromatography (dry
from the products. The polymeric cobalt-salen catalysts can
be recycled at least once with retention of their outstanding
activities and selectivities. Currently, we are investigating the
use of other insoluble supports. Moreover, investigations into
the effect of the linker as well as polymer backbone flexibility
on the activity and selectivity of immobilized metal-salen
catalysts is being studied in our laboratory.
Experimental Section.
5-(4-Hydroxyphenylethinyl)-3-tert-butyl-2-hydroxybenzalde-
hyde, 6. An in vacuo flame-dried, 100-mL Schlenk flask under
argon, equipped with a magnetic stir bar and a septum was charged
with 1.714 g of 4-iodophenol (7.79 mmol, 1.0 equiv), 278 mg of
[PdCl2(PPh3)2] (0.397 mmol, 5 mol %), and 51 mg of triph-
enylphosphane (0.195 mmol, 2.5 mol %). The flask was evacuated
and subsequently flushed with Ar. After the evacuating-flushing
procedure had been repeated twice, 10 mL of anhydrous THF, a
solution of 1.576 g of 5-ethinyl-3-tert-butyl-2-hydroxybenzalde-
hyde50 (7.79 mmol, 1.0 equiv) in 15 mL of anhydrous THF, and
2.2 mL of NEt3 (1.58 g, 15.54 mmol, 2.0 equiv) were added via a
syringe, respectively, and the slurry was stirred at room temperature
for 20 min. CuI, in the amount of 28 mg (0.117 mmol, 1.5 mol
%), was added, and the dark red solution was stirred at room
temperature for 27 h. The mixture was diluted with 100 mL of
H2O and 50 mL of Et2O, and the phases were separated. The
aqueous layer was extracted with Et2O (2 × 50 mL), and the
combined organic layers were washed with brine (40 mL) and dried
over MgSO4. After removal of the solvent under reduced pressure,
4.16 g of the crude product was obtained as a red oil. Purification
by column chromatography (SiO2, 4 × 58 cm2, ethyl acetate/
hexanes ) 1:7) yielded 1.224 g of product (51%) as a yellow
powder. 1H NMR (500 MHz, CDCl3/CD2Cl2): δ 1.41 (s), 5.19 (br
s), 6.80 (d), 7.40 (d), 7.57 (d), 7.63 (dd), 9.84 (br s), 11.88 (dd).
13C NMR (125 MHz, CDCl3/CD2Cl2): δ 29.0, 35.0, 86.9, 88.1,
114.6, 115.4, 115.6, 120.5, 133.2, 134.9, 137.0, 138.8, 155.9, 161.0,
196.9. HRMS (ESI): calcd for C19H18O3, 294.1256; obsd, 295.1341
[M + 1]+. Anal. Calcd for C19H18O3: C, 77.53; H, 6.16; O, 16.31.
Found: C, 77.20; H, 6.37; O, 16.27.
Salen Linker, 7. Under an argon atmosphere in a flame-dried,
25-mL, three-necked flask, equipped with a magnetic stir bar, a
septum, and a gas inlet, 301 mg of (R,R)-1,2-diaminocyclohexane-
mono-aminochloride 3 (2 mmol, 1.0 equiv) and some 4 Å molecular
sieves were slurried in 7 mL of anhydrous ethanol and 7 mL of
anhydrous methanol. To this slurry was added 487 mg of 5-(4-
hydroxyphenylethinyl)-3-tert-butyl-2-hydroxybenzaldehyde, 6 (2
mmol, 1.0 equiv). The bright yellow solution was stirred at room
temperature. After 4 h, a solution of 589 mg of 3,5-di-tert-butyl-
2-hydroxybenzaldehyde, 4 (2 mmol, 1.0 equiv), in 15 mL of
anhydrous CH2Cl2, and 0.56 mL of anhydrous NEt3 (4 mmol, 2.0
equiv) was added. The red solution was stirred at room temperature
for an additional 4 h. The mixture was filtered, the solvent was
removed under reduced pressure, and the residue was purified by
column chromatography (dry SiO2, 2.5 × 40 cm2, ethyl acetate/
hexanes ) 1:5). Product, in the amount of 910 mg (64%), could
be isolated as a yellow-orange powder. 1H NMR (500 MHz, CDCl3/
CD2Cl2): δ 1.26 (s), 1.44 (s), 1.46 (s), 1.40-1.53 (m), 1.82-1.93
(m), 1.66-1.81 (m), 1.93-2.05 (m), 3.25-3.77 (m), 6.81 (d), 7.00
(d), 7.19 (d), 7.35 (d), 7.40 (d), 7.41 (d), 8.22 (s), 8.28 (s). 13C
NMR (125 MHz, CDCl3/CD2Cl2): δ 24.4, 29.4, 29.6, 31.5, 33.1,
33.2, 35.0, 34.2, 35.1, 72.0, 72.2, 87.2, 88.3, 112.5, 115.7, 115.7,
117.7, 118.4, 126.1, 127.3, 132.7, 133.2, 133.2, 136.7, 137.9, 140.1,
155.7, 158.4, 161.5, 165.2, 166.2. MS (ESI, %) m/z: 607.3888
((0.025; 56, [M + 1]+), 391.2 (C25H31N2O2, 100), 331.3 (C21H35N2O,
72). Anal. Calcd for C40H50N2O3: C, 79.17; H, 8.30; N, 4.62; O,
7.91. Found: C, 78.61; H, 8.26; N, 4.63; O, 8.06.
1
SiO2, Et2O/hexanes ) 1:10). H NMR (500 MHz, CDCl3/CD2-
Cl2): δ 1.24 (s), 1.42 (s), 1.43 (s), 1.35-1.39 (m), 1.45-1.50 (m),
1.83-1.94 (m), 1.50-1.64 (m), 1.68-1.82 (m), 1.94-2.01 (m),
2.01-2.10 (m), 2.97-3.02 (m), 3.19-3.25 (m), 3.22-3.37 (m),
3.37-3.41 (m), 6.09 (dd), 6.28 (dd), 6.18 (dd), 6.21 (dd), 6.96 (d),
7.01 (d), 7.08 (d), 7.19 (d), 7.32 (d), 7.39 (d), 7.47 (d), 7.80 (d),
8.25 (s), 8.28 (s), 13.62 (br s), 14.28 (br s). 13C NMR (125 MHz,
CDCl3/CD2Cl2): δ 24.4, 29.3, 29.6, 29.5, 30.7, 31.6, 33.1, 33.3,
34.2, 35.0, 35.1, 41.9, 42.8, 43.5, 43.8, 46.1, 46.5, 46.9, 49.9, 72.4,
72.5, 86.7, 89.7, 112.2, 117.8, 118.6, 121.2, 121.7, 126.0, 127.1,
132.3, 132.6, 132.7, 133.4, 135.8, 136.6, 137.9, 138.4, 140.2, 150.5,
158.0, 161.3, 165.1, 166.2, 173.2. MS (ESI, %) m/z: 727.5 (100,
[M + 1]+), 525.4 (19). Anal. Calcd for C40H58N2O4: C, 79.30; H,
8.04; N, 3.85; O, 8.80. Found: C, 79.07; H, 8.06; N, 3.79; O, 8.86.
Mn-Salen-Norbornene, 9. Under an argon atmosphere, an in
vacuo flame-dried, 100-mL, three-necked flask, equipped with a
magnetic stir bar, a septum, an addition funnel, and a reflux
condenser with gas inlet, was charged with 923 mg of Mn(OAc)2‚
4H2O (3.77 mmol, 3.0 equiv) and 15 mL of anhydrous ethanol.
The white slurry was heated to reflux (90 °C, oil-bath temperature),
and a solution of 913 mg of salen-norbornene (1.26 mmol, 1.0
equiv) in 15 mL of anhydrous toluene was filled into the addition
funnel. The ligand solution was added dropwise to the manganese
solution, and the addition funnel was washed with anhydrous
ethanol (2 × 5 mL). The reaction mixture was heated under reflux
for 2 h. Then a needle connected to an air cylinder was placed in
the solution through the septum, air was slowly bubbled through
the refluxing mixture, and the conversion of the ligand was
monitored by the TLC analysis of the red solution. After 1 h, TLC
analysis indicated a complete conversion of the free ligand. Then
160 mg of lithium chloride (3.77 mmol, 3.0 equiv) was added, and
the mixture was allowed to cool to room temperature over a period
of 1 h. The mixture was transferred into a separatory funnel, and
the toluene layer was washed with H2O (3 × 50 mL) and brine
(50 mL) and dried over MgSO4. After removal of the solvent under
reduced pressure, 1.93 g of crude product was obtained as a dark
red oil. Purification by column chromatography (dry SiO2, 4 × 30
cm2, CH2Cl2 f CH2Cl2/ethyl acetate ) 5:1) yielded 813 mg of
product (79%) as a dark red solid. MS (ESI, %) m/z: 779.35 (100,
[M - Cl]+). Anal. Calcd for C40H56ClMnN2O3: C, 70.71; H, 6.92;
N, 3.44. Found: C, 70.34; H, 7.11; N, 3.37.
Poly(Mn-salen-norbornene), p(9) (Synthesis of a 50-mer).
In an in vacuo flame-dried, 50-mL, three-necked flask equipped
with a magnetic stir bar, a septum, and a reflux condenser with a
gas inlet, 122 mg of Mn-salen-norbornene, 9 (150 µmol, 50
equiv), was dissolved in 3 mL of anhydrous CDCl3, and the solution
was heated to 40 °C. Grubbs catalyst (3°-generation) in the amount
of 2.7 mg (3 µmol, 1 equiv) was added as a solid, and the mixture
was stirred for 2.5 h at 40 °C. The polymerization was quenched
by adding 3 drops of ethyl vinyl ether, and the polymer was
precipitated by adding 30 mL of Et2O. The polymer was separated
by centrifugation and washed with Et2O three times (the polymer
was suspended in Et2O and subsequently separated by centrifuga-
tion). After drying the residue in vacuo, 96 mg (79%) of an ether-
insoluble polymer fraction was obtained as a dark red-brown
powder. Anal. Calcd for C40H56ClMnN2O3: C, 70.71; H, 6.92; N,
Salen-Norbornene. Under an argon atmosphere, 720 mg of 7
(1.186 mmol, 1.0 equiv) was dissolved in 20 mL of anhydrous CH2-
J. Org. Chem, Vol. 71, No. 5, 2006 1835

Holbach and Weck
3.44; O, 7.85. Found: C, 68.67; H, 7.02; N, 3.27; O, 9.09. ICP
calcd: Mn, 6.74. Found: Mn, 5.00.
Poly(Co-salen-norbornene-co-n-octyl-norbornenecarbon-
yl Ester) p(10.11) (x/y ) 1:1, x + y ) 50). In an in vacuo flame-
dried, 10-mL, Schlenk flask under Ar, equipped with a magnetic
stir bar and a septum, 78 mg of Co-salen-norbornene, 10 (100
µmol, 25 equiv), and 25 mg of n-octylnorbornenecarbonyl ester,
11 (100 µmol, 25 equiv), were dissolved in 5 mL of anhydrous
CDCl3. Then 3.5 mg of Grubbs catalyst (3° generation; 4 µmol, 1
equiv) was added as a solid, and the mixture was stirred at room
temperature. After 40 min, TLC analysis as well as an analysis of
an aliquot by 1H NMR spectroscopy indicated complete conversion
of both monomers. The polymerization was quenched by adding 2
drops of ethyl vinyl ether. The solvent was removed under reduced
pressure, the residue was redissolved in some CH2Cl2, and the
polymer was precipitated by adding 40 mL of cold Et2O. After the
separation of the solid by centrifugation and drying in vacuo, 94
mg (91 mol %) of an ether-insoluble polymer fraction was obtained
as a dark brown powder. Anal. Calcd for [C48H56CoN2O4]1
[C16H26O2]1: C, 75.11; H, 8.80; N, 7.82; O, 10.42. Found: C, 74.25;
H, 8.73; N, 1.95; O, 11.15. ICP calcd: Co, 3.84. Found: Co, 3.95.
Typical Procedure for the HKR of Racemic Epichlorohydrin
rac-18 with Poly(Co(III)(OTs)-salen-norbornene) p(10c). In
a vial with a magnetic stir bar, 48.7 mg of poly(Co(III)(OTs)-
salen-norbornene), p(10c) (50 µmol, 0.5 mol %) was dissolved in
100 µL of chlorobenzene (as an internal standard) and 784 µL of
epichlorhydrin (10 mmol, 1.0 equiv). After taking an aliquot (2
µL), 126 µL of water (7 mmol, 0.7 equiv) was added. Aliquots (2
µL) were taken after 5, 10, 15, 20, 30, 60, 120, and 180 min. All
aliquots were filtered through a pipet with cotton and some silica,
and the silica was flushed with 1.5-2 mL Et2O. The resulting
solutions were analyzed via chiral GC-FID (γ-TA). After 3 h, the
dark mixture was diluted with 2 mL of CH2Cl2 and added dropwise
to cold Et2O (ca. 40 mL). The resulting precipitate was separated
by centrifugation, “washed” with cold Et2O (the polymer was
suspended in Et2O and subsequently separated by centrifugation),
and subsequently dried in vacuo.
Recycling and Reoxidation of the Cobalt Polymers. All
polymeric Co complexes were separated from the reaction mixtures
by precipitation into Et2O and by subsequent centrifugation. The
residue obtained after centrifugation was dissolved in CH2Cl2; 9.5
mg of para-toluenesulfonic acid monohydrate (50 µmol, 1.0 equiv
according to Co) was added, and the mixture was stirred under an
atmosphere of air for 1 h. The CH2Cl2 solution was added dropwise
to cold Et2O (ca. 40 mL), and the precipitate was separated by
centrifugation and “washed” with cold Et2O. The obtained residue
was dried in vacuo to yield 45 mg (93%) of reoxidized p(10c)_rec
as a dark green powder.
The polymeric catalysts could be recovered quantitatively. For
example, for CoOAc-salen complex p(10a), we started with 23
mg for the first catalytic cycle and obtained 24 mg after recycling
and reoxidation in the second cycle and 24 mg for the third cycle.
These data clearly prove that the polymer was recycled quantita-
tively.
Poly(Mn-salen-norbornene-co-n-octylnorbornenecarbon-
yl Ester), p(9.11) (x/y ) 1:1, x + y ) 50). In an in vacuo flame-
dried, 25-mL, three-necked flask under Ar, equipped with a
magnetic stir bar, a septum, and a reflux condenser with a gas inlet,
were dissolved 79 mg of Mn-salen-norbornene, 9 (94 µmol, 25
equiv), and 24 mg of n-octylnorbornenecarbonyl ester, 11 (94 µmol,
25 equiv), in 4 mL of anhydrous CDCl3. Then 3.3 mg of Grubbs
catalyst (3°-generation; 3.75 µmol, 1 equiv) was added as a solid,
and the mixture was stirred at room temperature. After 20 min,
TLC analysis as well as analysis of an aliquot by 1H NMR
spectroscopy indicated complete conversion of both monomers. The
polymerization was quenched by adding 2 drops of ethyl vinyl ether,
and the polymer was precipitated by adding 20 mL of Et2O. After
the separation of the solid by centrifugation and drying in vacuo,
82 mg (82%) of an ether-insoluble polymer fraction was obtained
as a dark red-brown powder. Anal. Calcd for [C40H56ClMnN2O3]1
[C16H26O2]9: C, 72.13; H, 7.78; N, 2.63; O, 9.01; Cl, 3.33. Found:
C, 71.10; H, 7.65; N, 2.64; O, 9.72; Cl, 2.26. ICP calcd: Mn, 5.15.
Found: Mn, 4.68.
Typical Procedure for the Epoxidation of 1,2-Dihydronaph-
thalene, 14, with Poly(Mn-salen-norbornene), p(9). Under an
argon atmosphere in a dry, 10-mL, Schlenk flask, 13 mg of poly-
(Mn-salen-norbornene), p(9) (16 µmol, 4 mol %), and 234 mg
of NMO (2 mmol, 5.0 equiv) were dissolved in 2 mL of anhydrous
CH2Cl2. Then 52 µL of 1,2-dihydronaphthalene, 14 (52 mg, 0.4
mmol, 1.0 equiv), and about 0.04 mL of chlorobenzene as an
internal standard were added via a microsyringe, and the solution
was cooled to -20 °C. m-CPBA in the amount of 138 mg (0.8
mmol, 2.0 equiv) was added in 3 equal portions at t ) 0, 60, and
120 s. Aliquots (ca. 0.2 mL) were taken after 0, 30, 90, 150, 210,
and 300 s. All aliquots were filtered through a pipet with cotton
and some Al2O3, and the Al2O3 was flushed with about 1.5 mL of
CH2Cl2. The resulting solutions were analyzed via GC-FID (HP-
5). The aliquots at t ) 210 s and t ) 300 s were also analyzed by
chiral GC-MS (â-CD).
Co-Salen-Norbornene, 10. In an in vacuo flame-dried, 10-
mL, Schlenk flask under Ar, equipped with a magnetic stir bar and
a septum, 146 mg of salen-norbornene (0.20 mmol, 1.0 equiv)
was dissolved in 1.5 mL of anhydrous CH2Cl2. A solution of 60
mg of Co(OAc)2‚4H2O (0.24 mmol, 1.1 equiv) in 2 mL of
anhydrous MeOH was added dropwise via a syringe. The resulting
red suspension was stirred for 30 min at room temperature, cooled
in a ice bath, and stirred for another 30 min at 0 °C. The orange-
red precipitate was separated by vacuum filtration, washed with
cold MeOH, and dried in vacuo to yield 121 mg (77%) of 10 as an
orange-red powder. MS (ESI, %) m/z: obsd, 783.3517 ((0.01; 100,
[M]+); calcd, 783.3572. Anal. Calcd for C48H56CoN2O4: C, 73.54;
H, 7.20; N, 3.57; O, 7.52. Found: C, 73.33; H, 7.21; N, 3.49; O,
7.64. ICP calcd: Co, 7.52. Found: Co, 8.00.
Poly(Co(II)-salen-norbornene) p(10)′ (Synthesis of a 20-
mer). In an in vacuo flame-dried, 25-mL, three-necked flask under
Ar, equipped with a magnetic stir bar, a septum, and a gas inlet,
111 mg of Co(II)-salen-norbornene, 10 (151 µmol, 20 equiv),
was dissolved in 10 mL of anhydrous CDCl3. Then 6.7 mg of
Grubbs catalyst (3° generation; 7.5 µmol, 1 equiv) was added as a
solid, and the mixture was stirred at room temperature. After 2 h,
analysis of an aliquot by 1H NMR spectroscopy showed complete
conversion of the monomer (absence of monomeric olefin peaks).
The polymerization was quenched by adding 3 drops of ethyl vinyl
ether, and the mixture was poured into 50 mL of cold Et2O. The
precipitated polymer was separated by centrifugation and washed
with Et2O (the polymer was suspended in Et2O and subsequently
separated by centrifugation). After drying the residue in vacuo, 105
mg (94%) of an ether-insoluble polymer fraction was obtained as
a dark red powder. Anal. Calcd for C48H56CoN2O4: C, 73.54; H,
7.20; N, 3.57; O, 7.52. Found: C, 71.06; H, 7.06; N, 3.57; O, 8.16.
ICP calcd: Co, 7.52. Found: Co, 6.96.
Acknowledgment. Financial support of this work by the US
DOE Office of Basic Sciences (Catalysis Science Contract No.
DE-FG02-03ER15459) and the Deutsche Forschungsgemein-
schaft is acknowledged. The authors also thank Dr. Chris Jones
and his group for their helpful discussions. M.W. gratefully
acknowledges a 3M Untenured Faculty Award, a DuPont Young
Professor Award, an Alfred P. Sloan Fellowship, a Camille
Dreyfus Teacher-Scholar Award and a Blanchard Assistant
Professorship.
Supporting Information Available: Detailed experimental
procedures and characterization data are described. This information
is available free of charge via the Internet at http://pubs.acs.org.
JO051919+
1836 J. Org. Chem., Vol. 71, No. 5, 2006