Poly(styrene)-supported co-salen complexes as efficient recyclable catalysts for the hydrolytic kinetic resolution of epichlorohydrin

Article abstract of DOI:10.1002/chem.200500786

Here we describe an unprecedented synthetic approach to poly-(styrene)-supported chiral salen ligands by the free radical polymerization of an unsymmetrical styryl-substituted salen monomer (H₂salen = bis(salicylidene)ethylenediamine). The new method allows for the attachment of salen moieties to the polymer main chain in a flexible, pendant fashion, avoiding grafting reactions that often introduce ill-defined species on the polymers. Moreover, the loading of the salen is controlled by the copolymerization of the styryl-substituted salen monomer with styrene in different ratios. The polymeric salen ligands are metallated with cobalt(II) acetate to afford the corresponding supported Co-salen complexes, which are used in the hydrolytic kinetic resolution of racemic epichlorohydrin, exhibiting high reactivity and enantioselectivity. Remarkably, the copolymer-supported Co-salen complexes showed a better catalytic performance (>99% ee, 54 % conversion, one hour) in comparison to the homopolymeric analogues and the small molecule Co-salen complex. The soluble poly(styrene)-supported catalysts were recovered by precipitation after the catalytic reactions and were recycled three times to afford almost identical enantiomeric excesses as the first run, with slightly reduced reaction rates.

Full text of DOI:10.1002/chem.200500786

DOI: 10.1002/chem.200500786
Poly(styrene)-SupportedCo–Salen Complexes as Efficient Recyclable
Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin
[a]
Xiaolai Zheng,[a, b] Christopher W. Jones,*[a, b] andMarcus Weck*
Abstract: Here we describe an unpre-
cedented synthetic approach to poly-
(styrene)-supported chiral salen ligands
by the free radical polymerization of
an unsymmetrical styryl-substituted
salen monomer (H2salen=bis(salicyli-
zation of the styryl-substituted salen
monomer with styrene in different
ratios. The polymeric salen ligands are
metallated with cobalt(ii) acetate to
afford the corresponding supported
Co–salen complexes, which are used in
the hydrolytic kinetic resolution of rac-
emic epichlorohydrin, exhibiting high
reactivity and enantioselectivity. Re-
markably, the copolymer-supported
Co–salen complexes showed a better
catalytic performance (>99% ee, 5 4%
conversion, one hour) in comparison to
the homopolymeric analogues and the
small molecule Co–salen complex. The
soluble poly(styrene)-supported cata-
lysts were recovered by precipitation
after the catalytic reactions and were
recycled three times to afford almost
identical enantiomeric excesses as the
first run, with slightly reduced reaction
rates.
dene)ethylenediamine).
The
new
method allows for the attachment of
salen moieties to the polymer main
chain in a flexible, pendant fashion,
avoiding grafting reactions that often
introduce ill-defined species on the
polymers. Moreover, the loading of the
salen is controlled by the copolymeri-
Keywords: asymmetric catalysis
·
cobalt · kinetic resolution · poly-
merization · supported catalysts
Introduction
development of immobilized salens with a scaffold mimic to
1 has drawn considerable attention in the past decade.[18–23]
In addition to the potential of reusing catalysts many times
and the ease of separating metals from products, immobili-
zation of salens onto supports of desirable morphologies
may potentially lead to the discovery of more efficient sup-
ported catalysts for asymmetric catalysis. While the litera-
Chiral salen complexes (H2salen=bis(salicylidene)ethylene-
diamine) represent a powerful family of catalysts for a spec-
trum of important asymmetric organic transformations,[1–5]
including the epoxidation of olefins,[6,7] the hydrolytic kinetic
resolution of epoxides,[8–10] other epoxide ring-opening reac-
tions,[11–13] hetero Diels–Alder reactions,[14] and conjugate ad-
dition reactions.[15–17] Among the many catalytic reactions
that salen complexes can promote, the asymmetric epoxida-
tion and the hydrolytic kinetic resolution (HKR) of epox-
ides are of particular value, given the fact that enantiopure
epoxides are versatile intermediates for asymmetric organic
syntheses. Since Jacobsenꢀs salen 1 has been recognized as a
universal ligand for many of those transformations,[1–5] the
[a] Dr. X. Zheng, Prof. Dr. C. W. Jones, Prof. Dr. M. Weck
School of Chemistry and Biochemistry
Georgia Institute of Technology, Atlanta, Georgia 30332 (USA)
Fax : (+1)404-894-7452
E-mail: christopher.jones@chbe.gatech.edu
marcus.weck@chemistry.gatech.edu
[b] Dr. X. Zheng, Prof. Dr. C. W. Jones
School of Chemical & Biomolecular Engineering
Georgia Institute of Technology, Atlanta, Georgia 30332 (USA)
Fax : (+1)404-894-2866
576
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Chem. Eur. J. 2006, 12, 576 – 583

FULL PAPER
ture in this field has mainly focused on the immobilization
of salen complexes for epoxidation reactions,[18–32] the devel-
opment of dendrimer,[33] oligomer,[34–36] and polymer-sup-
ported[37–42] Co–salen catalysts for the HKR has been report-
ed in recent years. Inorganic supports including silica[42–44]
and zeolite[45] and liquid-immobilization methods, such as
the fluorous biphasic system (FBS)[46] and ionic liquids,[47]
were also applied to investigate recyclable HKR catalysts.
Interestingly, kinetic studies of the HKR reactions with the
homogeneous Co–salen catalyst 2 indicated a second-order
dependence of the reaction rates on the Co–salen species;
this dependence strongly supports a cooperative bimetallic
mechanism for the ring-opening step.[8,48] In this context,
supported Co–salen catalysts may possess a higher local con-
centration of metal catalysts and, if the neighboring catalytic
sites can cooperate with each other, could exhibit improved
catalytic reactivity in comparison with their homogeneous
analogues.
Results andDiscussion
In general, the syntheses of the polymer-supported
salens[18] involve the following two strategies: A) grafting re-
actions of salen ligands onto insoluble supports such as
resins,[30–32,42] or B) polymerizations of salen mono-
mers.[24–29,37–41] Method A, often realized by using a multistep
route, suffers from the coexistence of ill-defined species in
the polymers and relatively low catalyst loading. Therefore,
method B might be considered advantageous with respect to
method A, yet it has been practiced only with symmetrical
salen ligands as monomers. While salen ligands with C2 sym-
metry are readily available from a synthetic point of view,
polymerization or copolymerization of such monomers in-
troduces the salen cores along the main chain or as a cross-
link of the polymer matrix, respectively, which undesirably
hinders the accessibility and flexibility of the catalytic sites.
Therefore, in comparison with their homogeneous counter-
parts, the polymer-bound salen complexes often exhibit
poor enantiocontrol and reduced reactivity.
In contrast, the polymerization of unsymmetrical salen
monomers can be used to immobilize the salen moieties on
the polymer backbone in a flexible, pendant fashion that
can overcome the aforementioned drawbacks. There are no
reports in the literature of the synthesis of monotethered or-
ganic polymer-supported Co–salen derivatives, presumably
due to the lack of efficient synthetic pathways to unsymmet-
rical salens. In this contribution, we report that the free radi-
cal homo- and copolymerization of an unsymmetrical mono-
styryl-substituted salen monomer (3) afforded polymers 4
with the salen moieties being bound as side arms with re-
spect to the poly(styrene) backbone. The corresponding sup-
ported Co–salen complexes (5) exhibited high reactivity and
enantioselectivity for the HKR of epichlorohydrin, with the
best catalytic performance obtained by the copolymer-im-
mobilized catalysts.
One-pot synthesis of unsymmetrical salen 3: The condensa-
tion of one equivalent of a diamine and two equivalents of a
salicylaldehyde has been established as a standard synthetic
methodology for symmetrical tetradentate Schiff base li-
gands.[4,49] However, it turned out to be very difficult to pre-
pare salen ligands that are unsymmetrical in terms of the
substituents on the two aromatic rings.[50–52] Since the con-
densation reactions of the first and the second amino groups
of a diamine often proceed at comparable rates, it becomes
almost impossible to control the reaction to stop after a
single condensation step. As a result, condensations with
two different salicylaldehydes afford inevitably a statistical
mixture of three salens, of which one is the targeted unsym-
metrical product while the two other byproducts are sym-
metrical.[42] Recently, we have developed a novel one-pot
practical protocol for the synthesis of a variety of unsym-
metrical salens.[53] This method was successfully applied to
the preparation of the monostyryl-substituted unsymmetri-
cal salen 3 in high yield. The condensation of the ammoni-
um salt 6[50] with 3,5-di-tert-butyl-2-hydroxybenzaldehyde
and 3-tert-butyl-2-hydroxy-5-(4’-vinylphenyl)benzaldehyde[30]
afforded 3 in 85% yield as a bright yellow solid (Scheme 1).
Free-radical polymerization: Having monomer 3 in hand,
we set forth to prepare polymer-supported salen ligands 4
by means of free-radical polymerization (Scheme 2). It was
anticipated that the presence of phenolic hydroxyl groups in
the monomer could present a problem, because it is widely
known that phenols are free-radical inhibitors.[54] For exam-
ple, 2,6-di-tert-butylphenol has been used as an antioxidant
or a radical trap in polymeric materials.[55] Nevertheless, the
free-radical polymerization of 3 with 2,2’-azobis(isobutyroni-
trile) (AIBN) as an initiator readily afforded the target
polymers 4. We propose that the success of this polymeri-
zation can be attributed to the stabilization of the salen
À
phenol moieties by the intramolecuar O H···N hydrogen
bonds, as evident by the downfield shifts of the phenolic
1
protons in both 3 and 4 to d=13.6–14.0 ppm in the H NMR
spectrum, and the relatively low tendency of the AIBN-
based tertiary radicals for the extraction of hydrogen radi-
Chem. Eur. J. 2006, 12, 576 – 583
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577

M. Weck, C. W. Jones, and X. Zheng
Figure 1. Kinetics of the copolymerization of 3 and styrene in 1:1 ratio.
1
statistical in nature (Figure 1). By means of H NMR spec-
troscopy, we found that both monomers were consumed at
almost identical rates from the initial stage to the end of the
polymerization. Therefore, the ratios of monomer units in
the copolymers remained almost constant and comparable
to the loading ratios regardless of conversion levels (3/sty-
rene loading: 50:50, found: 53:47–51:49). These results indi-
cated that the copolymerization proceeded in a random
fashion that excluded unambiguously the possibility of gen-
erating block or blocky copolymers.
Polymers 4a–e were isolated in yields of 75–87% by re-
peated precipitation from methanol. All the polymers were
characterized by 1H and 13C NMR, UV-visible, and FT-IR
spectroscopy and GPC analysis. As shown in Figure 2, the
signals in the 1H NMR spectra of 4 showed characteristic
broadening features associated with the polymers. No resid-
ual signals corresponding to the vinyl protons from mono-
mer 3 were detectable, indicating all the remaining mono-
mer was removed during the workup after polymerization.
According to GPC determinations performed with poly-
(styrene)s as standards, polymers 4a–e have number-average
Scheme 1. Synthesis of styryl-substituted salen monomer 3.
cals. By varying the ratio of the initiator AIBN to the mono-
mer, the number-average molecular weights (Mn) of the re-
sulting polymers could readily be controlled. Gel-permea-
tion chromatography (GPC) analyses showed that a polymer
with on average 12 repeating units (4a) was obtained with
an initiator loading of 10 mol%, whereas a polymer with ap-
proximately 24 repeating units (4b) was generated with a
loading of 2.5mol%.
To elucidate whether the dilution of the salen along the
polymer backbone would have an effect on the catalytic
properties of the supported complexes, we synthesized co-
polymers 4c–e by the free-radical copolymerization of sty-
rene and 3 in different molar ratios (3/styrene loading: 4c=
50:50, 4d=20:80, 4e=10:90). A kinetic study on the 1:1 co-
polymerization of 3 and styrene was carried out to deter-
mine if the resulting copolymers are blocky, alternating, or
Scheme 2. Synthesis of poly(styrene)-supported Co–salen complexes.
578
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Chem. Eur. J. 2006, 12, 576 – 583

Asymmetric Catalysis
FULL PAPER
Figure 3. UV-visible spectra of salen monomer 3, polymer 4b, and metal-
lated polymer 5b.
spectra indicated that, in addition to the appearance of two
new metal d–d migration bands at 370 and 422 nm, a blue
shift of the band at 254 nm (4b) to 265nm ( 5b) was ob-
served upon complexation (Figure 3). The loading of cobalt
in the polymers was characterized by elemental analyses.
The final metal contents ranged from 0.59 to 1.37 mmolgÀ1,
indicating that 84–89% of the salen centers were loaded
with cobalt. The obtained Co–salen polymers are soluble in
THF and halogenated solvents, such as dichloromethane,
but insoluble in methanol and hexane.
Figure 2. 1H NMR spectra of salen monomer 3, homopolymer 4b, and co-
polymer 4d.
molecular weights ranging from 7200 to 14600 with polydis-
persity indices (PDI) of 1.6 to 2.6 (Table 1). It is known that
salen ligands can exchange the two salicylideneimine moiet-
ies especially in solution as a result of the disproportiona-
tion of the C=N bonds.[56,57] This imine metathesis reaction
hardly plays any role with our polymers. A solution of 4a in
CDCl3 can be stored at room temperature for a week with-
Hydrolytic kinetic resolution of epichlorohydrin: The poly-
(styrene)-supported Co–salen complexes 5a–e were exam-
ined for their catalytic efficiency in the HKR of racemic epi-
chlorohydrin (Scheme 3). With multiple functional groups in
the structure, enantiopure epichlorohydrin represents an ex-
tremely useful intermediate for asymmetric syntheses.[58]
This substrate was suspected to undergo chlorine-catalyzed
racemization that can be promoted in the presence of CoIII–
salen species.[59–61] Therefore, supported Co–salen catalysts
are advantageous, because the catalysts can readily be re-
moved from the reaction mixture after the HKR.[42]
1
out any detectable changes according to the H NMR spec-
troscopy and GPC analysis.
Table 1. Free radical polymerization characterization of salen monomer
3.
[c]
Product AIBN
Yield m/n
loading exptl exptl
m/n[a] m, n[b] Mn
PDI[c]
1.6
[mol%] [%]
1
2
3
4
5
4a
4b
4c
4d
4e
10 87
2.585100:0
2.5 78
2.57520:80
2.580
100:0 n.a. 12, n.a. 7200
n.a. 24, n.a. 14600 1.9
50:50 48:52 15, 14
22:78 11, 40 11000 2.6
10:90 11:89 6, 48 8600
Prior to the catalytic reaction, CoII precatalysts 5a–e were
oxidized to the corresponding CoIII active species in the
open air with the help of excessive acetic acid. The oxida-
tion process was evidenced by a dramatic color change from
deep red to dark brown, which is well documented in the lit-
erature.[9] The HKRꢀs were carried out at ambient tempera-
tures in the presence of 0.5mol% catalyst calculated on the
basis of cobalt. The conversions and enantiomeric excesses
(ee) of the substrate were monitored by GC analysis. The
catalytic data are compiled in Table 2 and a kinetic plot of
ee versus the reaction time is presented in Figure 4. All the
10200 2.1
2.0
[a] Determined by 1H NMR spectroscopy. [b] Calculated with data of
m/n exptl and Mn. [c] Determined by GPC in THF using poly(styrene)s
as standards.
Metallation of salen polymers with cobalt(ii) acetate: The
salen polymers 4a–e were converted to the corresponding
CoII complexes 5a–e by refluxing them in the presence of
cobalt(ii) acetate tetrahydrate
under the protection of an at-
mosphere of argon. The color
of the polymers changed from
yellow to deep red, a character-
istic of CoII–salen species, after
the transformation. UV-visible Scheme 3. Hydrolytic kinetic resolution of epichlorohydrin.
Chem. Eur. J. 2006, 12, 576 – 583
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579

M. Weck, C. W. Jones, and X. Zheng
Table 2. Hydrolytic kinetic resolution of epichlorohydrin.
A key motivation to develop immobilized metal com-
plexes lies in their potential for facile recovery and reuse in
subsequent reactions. The recycling of the copolymer-bound
Co–salen complex 5d (Table 3) was studied by precipitation
of the catalyst after the HKR of epichlorohydrin by the ad-
dition of diethyl ether. The precipitated catalyst was reacti-
vated with acetic acid and then reused under strictly identi-
cal conditions to the first run. Whereas the enantioselectivi-
ty of the reused catalyst remained almost unchanged after
four cycles, the catalytic reactivity fell gradually. The reac-
tion time had to be extended to two hours in the fourth
cycle to obtain an ee of 98%. The same phenomenon of
longer reaction times has been observed before when isolat-
ing supported Co–salen catalysts in main-chain polymers
through precipitation methods.[38] To evaluate whether the
deactivation was due to leaching of catalyst, more racemic
epichlorohydrin and water were charged into the pale yel-
lowish organic phase from the workup of the first catalytic
run of precatalyst 5d. About 4% additional epichlorohydrin
was consumed in an hour. Control experiments showed that
no background reaction was detected in the absence of the
catalyst or in the presence of the unmetallated salen poly-
mer. These results indicated that, at least in part, the loss of
catalysts during workup was responsible for the observed
deactivation on recycle, which is quite a common phenom-
enon for soluble polymer-supported catalysts.[62] It is not
clear what the role of other factors such as potential mor-
phological changes of the polymers have on the long-term
performance characteristics of the catalysts.
Catalyst
t [h]
Conv.[a] [%]
ee[a] [%]
1
2
3
4
2a
2a
1.0
1.55
49
93
>99
2
5a
5a
1.0
2.0
47
55
81
>99
5b
5b
1.0
2.0
48
55
83
>99
5c
5c
1.0
1.55
50
90
>99
4
5
6
5d
1.0
54
54
>99
5e
1.0
>99
[a] Determined by GC analyses using a Chiraldex G-TA column. The ee
refers to the enatiomeric excess of the remaining epichlorohydrin.
Table 3. Recycling of catalyst 5d in the HKR of epichlorohydrin.
Conv.[a] [%]
ee[a] [%]
Figure 4. Plot of ee vs. reaction time in the HKR of epichlorohydrin.
Cycle
t [h]
1
2
3
4
1.0
1.55
2.0
54
>99
>99
>99
98
5
poly(styrene)-supported catalysts are highly reactive and
enantioselective for the HKR of epichlorohydrin. As shown
in Table 2, the copolymer-supported catalysts 5d (entry 5)
and 5e (entry 6) showed the most desired catalytic perform-
ances. The remaining epichlorohydrin was determined to
have enantiomeric excesses higher than 99% within one
hour with a conversion of 54%. In comparison, the Jacob-
senꢀs catalyst 2a (entry 1) gave 93% ee in 49% conversion
under the same reaction conditions and it took 1.5h for it to
reach >99% ee.
It is worth noting that the copolymer-supported catalysts
5c and d in general exhibited improved reactivity and enan-
tioselectivity compared with their homopolymer analogues
5a and 5b. For example, for the homopolymer catalyst 5a,
the reaction time had to be prolonged to two hours
(entry 2) to obtain >99% ee with a conversion of 55%. We
attributed this observation to the greater complex mobility
in the copolymer-bound salen catalysts. Dilution of the salen
moieties in the poly(styrene) main chain might make the
catalytic sites more accessible to the substrate. In addition,
the copolymers might have more flexible polymer back-
bones that would increase the possibility of intramolecular
cooperation between cobalt catalytic sites.
55
53
2.0
[a] Determined by GC analyses using a Chiraldex G-TA column; The ee
refers to the enatiomeric excess of the remaining epichlorohydrin.
By means of enforcing the intramolecular bimetallic coop-
eration for the ring-opening step, the Jacobsenꢀs cyclic oligo-
meric salen complexes displayed superior reactivity in the
epoxide ring-opening reactions.[34–36] However, the possibility
of easy recycling of these oligomeric systems by using pre-
cipitation methods is limited due to the low molecular
weight of these catalysts. Normally, these low-molecular-
weight catalysts have been recycled by removing the volatile
reactants followed by the addition of more starting material
to the reaction vessel without the isolation of the catalytic
species.[8] In contrast, our polymeric Co–salen complexes,
besides having the desirable catalytic performance in the
HKR, hold advantages of facile product separation and cat-
alyst recycling. Hence, these supported catalysts are particu-
larly suitable for the kinetic resolution of epoxides (e.g., epi-
chlorohydrin) that are prone to racemization in the presence
of the catalysts.
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Asymmetric Catalysis
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ether/hexanes (1/50) afforded the target compound 3 as a yellow solid
(5.05 g, 85.2%). M.p.: 177–1788C; 1H NMR (400 MHz, CDCl3): d=1.22
(s, 9H; CMe3), 1.42 (s, 9H; CMe3), 1.44–1.51 (m, 2H; CH2), 1.46 (s, 9H;
CMe3), 1.70–1.84 (m, 2H; CH2), 1.88–1.91 (m, 2H; CH2), 1.97–2.02 (m,
2H; CH2), 3.30–3.78 (m, 2H; 2NCHCH2), 5.25 (d, J=11.0 Hz, 1H; CH=
CH2), 5.77 (d, J=17.6 Hz, 1H; CH=CH2), 6.74 (dd, J=11.0, 17.6 Hz, 1H;
CH=CH2), 6.97 (d, J=2.5Hz, 1H; ArH), 7.21 (d, J=2.5Hz, 1H; ArH),
7.31 (d, J=2.5Hz, 1H; ArH), 7.40–7.45 (m, 4H; ArH), 7.49 (d, J=
2.5Hz, 1H; ArH), 8.30 (s, 1H; N =CH), 8.35(s, 1H; N =CH), 13.69 (brs,
1H; OH), 14.01 ppm (brs, 1H; OH); 13C,1H NMR (100.6 MHz, CDCl3):
d=24.53, 24.54, 29.59, 29.62, 31.60, 33.32, 33.37, 34.23, 35.15, 35.17, 72.58
(2overlapping lines, 2CHN), 113.64, 117.98, 118.94, 126.22, 126.74,
126.88, 127.11, 128.23, 128.33, 130.51, 135.99, 136.59, 136.68, 137.76,
140.21, 140.70, 158.15, 160.26, 165.81, 166.24 ppm; IR (KBr): n˜ =3082,
2999, 2952, 2933, 2860, 1628, 1467, 1440, 1390, 1360, 1271, 1252, 1171,
840 cmÀ1; UV/Vis (THF): lmax =262, 300, 340 nm; MS (70 eV, FAB+):
m/z (%): 592 (100) [M+]; elemental analysis calcd (%) for C40H52N2O2
(592.85): C 81.04, H 8.84, N 4.73; Found: C 81.06, H 8.95, N 4.72.
Concluding Remarks
In this contribution, we have demonstrated that a novel
family of polymer-supported salen catalysts can readily be
formed by means of the free radical homo- and copolymeri-
zations of an unsymmetrical monostyryl-substituted salen
monomer. The advantage of this methodology lies in the
fact that the salen moieties are immobilized onto the poly-
mers in a pendant fashion and hence possess a higher
degree of flexibility and accessibility. The corresponding
cobalt-loaded salen catalysts are highly active and selective
in the HKR of racemic epichlorohydrin. We were able to
prove that diluting the Co–salen catalysts along the polymer
backbone through copolymerizations with unfunctionalized
comonomers resulted in increased activity and selectivity in
comparison to the homopolymer analogues. This difference
might be due to dilution effects or better catalyst accessibili-
ty as a result of more flexible polymer backbones. Ongoing
research in this laboratory has been directed to the design
and immobilization of chiral salen complexes on polymers
with relatively flexible linkers and/or main chains.
Synthesis of homopolymers 4a,b: A Schlenk tube was charged with mon-
omer
3
(237 mg, 0.40 mmol) and 10 mol% of AIBN (6.6 mg,
0.040 mmol). The system was purged several times with argon and de-
gassed chlorobenzene (2 mL) was added. The reaction mixture was stir-
red at 808C for 48 h and then cooled to RT. The mixture was slowly
poured into methanol (20 mL) to precipitate the crude product as a
yellow power and the suspension was stirred at RT for 30 min. The
powder was collected by filtration and washed with 1:20 dichlorome-
thane/methanol (3ꢂ10 mL). The crude product was dissolved in dichloro-
methane (2 mL) and reprecipitated with methanol (20 mL). The solid
was collected on a frit, washed with methanol (10 mL), and dried under
high vacuum to afford polymer 4a as a yellow powder (206 mg, 87%).
Following the aforementioned procedure, polymerization of 3 (237 mg,
0.40 mmol) in chlorobenzene (2 mL) using 2.5mol% of AIBN (1.7 mg,
0.010 mmol) as an initiator afforded polymer 4b as a yellow powder
(201 mg, 85%). 1H NMR (400 MHz, CDCl3): d=1.14 (s, 9H; CMe3), 1.33
(br, 18H; 2 CMe3), 1.40–1.80 (brm, 11H), 3.25(brm, 2H; 2 NC HCH2),
6.20–7.45(brm, 8H; ArH), 8.01 (brs, 1H; N =CH), 8.27 (brs, 1H; N=
CH), 13.62 (brs, 1H; OH), 13.99 ppm (brs, 1H; OH); 13C,1H NMR
(100.6 MHz, CDCl3): d=24.46 (br), 29.66, 31.60, 33.10 (br), 34.14, 35.00,
35.10, 40.87 (br), 71.47, 72.62, 117.91, 118.82, 126.16 (br), 127.01, 128.41
(br), 131.06, 136.45, 137.36, 138.50 (br), 140.00, 143.23 (br), 158.08,
159.71, 165.76, 166.19 ppm; IR (KBr): n˜ =3045, 3020, 2997, 2952, 2862,
1628, 1470, 1441, 1393, 1362, 1271, 1252, 1171, 827 cmÀ1; UV/Vis (THF):
lmax =254, 294, 335 nm.
Experimental Section
General: Reagents were purchased from Aldrich, Acros, or Alfa, and
used as received unless noted below. Dichloromethane and THF were
dried by passing through columns of activated copper and alumina suc-
cessively. Chlorobenzene was distilled under an atmosphere of argon
prior to use. (1R,2R)-1,2-Diaminocyclohexane monohydrochloride salt,[50]
3-bromo-5-tert-butyl-2-hydroxybenzaldehyde,[63] and 3-tert-butyl-2-hy-
droxy-5-(4’-vinylphenyl)benzaldehyde[30] were prepared according to pub-
lished procedures. NMR spectra were acquired with a Varian Mercury
400 (1H, 400.0 MHz; 13C, 100.6 MHz) spectrometer. Chemical shifts are
reported in ppm and referenced to the corresponding residual nuclei in
deuterated solvents. IR and UV-visible spectra were recorded with a Shi-
madzu FTIR-8400S and a Shimadzu UV-2401PC spectrometer, respec-
tively. Mass spectra were recorded with a VG 7070 EQ-HF hydrid
tandem mass spectrometer. Gel-permeation chromatography (GPC)
analyses were performed with American Polymer Standards columns
equipped with a Waters 510 pump and a UV detector, using poly-
Synthesis of copolymers 4a–c: A Schlenk tube was charged with mono-
mer 3 (119 mg, 0.20 mmol) and AIBN (1.7 mg, 0.010 mmol). The system
was purged several times with argon. Freshly distilled styrene (23 ml,
10.8 mg, 0.20 mmol) and degassed chlorobenzene (1 mL) were added.
The reaction mixture was stirred at 808C for 48 h, cooled to RT, and
slowly poured into methanol (20 mL) to precipitate the crude product as
a yellow powder. After the suspension was stirred at RT for 30 min, the
power was collected by filtration and washed with 1:20 dichloromethane/
methanol (3ꢂ10 mL). The crude product was dissolved in dichlorome-
thane (2 mL) and reprecipitated with methanol (20 mL). The solid was
collected on a frit, washed with methanol (10 mL), and dried under high
vacuum to afford the target copolymer 4c as a yellow powder (109 mg,
78%). Following the aforementioned procedure, copolymerization of 3
(119 mg, 0.20 mmol) and styrene (92 mL, 83 mg, 0.80 mmol) in chloroben-
zene (1 mL) using AIBN (4.1 mg, 0.025mmol) as an initiator afforded
the copolymer 4d as a yellow powder (152 mg, 75%). Copolymerization
of 3 (119 mg, 0.20 mmol) and styrene (206 mL, 188 mg, 1.8 mmol) in
chlorobenzene (2 mL) using AIBN (3.3 mg, 0.02 mmol) as an initiator af-
forded the copolymer 4e as a yellow powder (246 mg, 80%). Copolymers
4c–e have very similar spectroscopic properties and, hence, only the data
for 4d is listed in the following. 1H NMR (400 MHz, CDCl3): d=0.82–
2.18 (m), 1.21 (s, CMe3), 1.40 (s, CMe3), 1.45(s, CMe 3), 3.34 (br,
2NCHCH2), 6.22–7.45(br, ArH), 8.32 (br, 2 N =CH), 13.68 (brs, OH),
13.97 ppm (brs, OH); 13C,1H NMR (100.6 MHz, CDCl3): d=24.54, 29.68,
31.63, 33.54, 34.23, 35.1, 40.36 (br), 72.72 (br), 118.01, 118.91, 125.87,
126.23, 127.05, 128.17 (br), 131.17 (br), 136.55, 137.51, 138.44 (br), 140.15,
(styrene)s as standards for calibration and THF at
a flow rate of
1.0 mLminÀ1 as a mobile phase. Enantiomeric excesses were determined
by capillary gas-phase chromatography (GC) analysis on a Shimadzu GC
14 A instrument equipped with a FID detector and a Chiraldex G-TA
column (30 mꢂ0.25mm) with helium as a carrier gas. Melting points
were determined with a Laboratory Devices MEL-TEMP II apparatus
and are uncorrected.
(R,R)-N-(3,5-Di-tert-butylsalicylidene)-N’-(3-(4’-vinylbenzene)-5-tert-bu-
tylsalicylidene)-1,2-cyclohexanediamine (3): A 250 mL flask was charged
with (1R,2R)-1,2-diaminocyclohexane monohydrochloride salt (1.51 g,
10 mmol), activated 4 ꢃ molecular sieves (4.0 g), anhydrous methanol
(40 mL), and anhydrous ethanol (40 mL). 3,5-Di-tert-butyl-2-hydroxyben-
zaldehyde (2.34 g, 10 mmol) was added in one portion and the reaction
mixture was stirred at RT for four hours. After complete consumption of
the aldehyde as monitored by TLC, a solution of 3-tert-butyl-2-hydroxy-
5-(4’-vinylphenyl)benzaldehyde (2.74 g, 10 mmol) in dichloromethane
(80 mL) was added to the reaction system, followed by the slow addition
of triethylamine (2.8 mL, 20 mmol). The reaction mixture was stirred at
RT for additional four hours followed by the removal of the solvents.
The residue was dissolved in dichloromethane (100 mL), washed with
aqueous hydrochloric acid (1m, 50 mL) and water (2ꢂ50 mL), and dried
with magnesium sulfate. Flash chromatography of the crude product with
Chem. Eur. J. 2006, 12, 576 – 583
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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581

M. Weck, C. W. Jones, and X. Zheng
145.40 (br), 158.18, 159.85, 165.78, 166.20 ppm; IR (KBr): n˜ =3082, 2059,
3024, 2951, 1930, 2860, 1628, 1601, 1470, 1441, 1391, 1362, 1273, 1264,
1173, 829, 750, 698 cmÀ1; UV/Vis (THF): lmax =254, 293, 335 nm.
03ER15459. 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. C.W.S. gratefully acknowledges a DuPont Young Professor
Award, a Shell Faculty Initiation Award and a J. Carl and Sheila Pirkle
Faculty Fellowship.
Kinetic study of the copolymerization of 3 and styrene: Salen monomer 3
(237 mg, 0.40 mmol) and AIBN (3.3 mg, 0.020 mmol) were charged to a
Schlenk tube and the system was purged with argon several times. Fresh-
ly distilled styrene (46 mL, 41.7 mg, 0.40 mmol) and degassed chloroben-
zene (2 mL) were added under the protection of argon. The system was
immersed into an oil bath preheated at 808C. At each designed time,
0.1 mL of the reaction mixture was withdrawn from the system under the
protection of argon. The sample was diluted with CDCl3 and a 1H NMR
spectrum was acquired. The total conversion of 3 and styrene was calcu-
lated by comparing the integrals of the signals at d=3.34 ppm (2H,
NCHCH2 from 3) and 5.77 ppm (1 + 1H, CH=CH2 from overlapping
signals of 3 and styrene). The volatile including remaining styrene and
solvents was completely removed from the sample under high vacuum.
The 1H NMR spectrum of the residue was acquired to give the conver-
sion of 3. The conversion of styrene was calculated based on the total
conversion and the conversion of 3.
Synthesis of CoII–salen-immobilizedpolymers 5a–e : Polymer 4a (95mg,
0.16 mmol) was charged into a 50 mL flask equipped with a condenser.
After the system was thoroughly purged with argon, degassed CH2Cl2
(2 mL) was added to dissolve the polymer. A solution of cobalt(ii) ace-
tate tetrahydrate (50 mg, 0.20 mmol) in degassed methanol (2 mL) was
transferred into the flask by means of a cannula with careful exclusion of
air. A red powder formed immediately in the reaction mixture. After the
suspension was heated at reflux under an atmosphere of argon for 24 h,
additional degassed methanol (2 mL) was added and the reaction mixture
was stirred at RT for twelve hours. The solid was collected by filtration
under the protection of argon, washed with 1:10 degassed dichlorome-
thane/methanol (2ꢂ10 mL) and methanol (10 mL), and dried in vacuo to
give 5a as a dark red solid (99 mg, 95%). Elemental analysis (ICP) indi-
cated that 5a contains 7.85% of elemental cobalt, corresponding to a
loading of 1.33 mmolgÀ1. Using a similar procedure, metallation of 4b,c
with cobalt(ii) acetate tetrahydrate afforded 5b,c.
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3018, 2951, 2866, 1597, 1526, 1461, 1421, 1387, 1360, 1338, 1321, 1254,
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Complex 5d: 95% yield; Co loading: 0.82 mmolgÀ1; IR (KBr): n˜ =3082,
2059, 3024, 2949, 2864, 1599, 1526, 1492, 1452, 1421, 1392, 1360, 1338,
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Complex 5e: 91% yield; Co loading: 0.59 mmolgÀ1
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General procedure for the hydrolytic kinetic resolution of (rac)-epichloro-
hydrin: The precatalyst 5 (0.025mmol on the basis of cobalt) was dis-
solved in dichloromethane (1 mL) in a 10 mL flask. Glacial acetic acid
(0.10 mL) was added and the reaction mixture was stirred in the open air
for 30 min. The solvent and the excess acetic acid were roughly removed
in vacuo. The brown-black residue was pumped under vacuum (10 mbar)
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activated catalyst and the flask was immersed into a water bath at RT.
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system to start the reaction. Samples (2 mL) were taken from the reaction
mixture with a micro-syringe at each designed time, diluted with anhy-
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Received: July 7, 2005
Published online: September 29, 2005
Chem. Eur. J. 2006, 12, 576 – 583
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583