Osmylated macroporous resins: safe, highly efficient and recyclable
catalysts for asymmetric aminohydroxylation of olefins†
Cheon Hee Jo,a Sien-Ho Han,a Jung Woon Yang,b Eun Joo Roh,b Ueon-Sang Shina and Choong Eui
Song*b
a Department of Chemical Engineering and Biotechnology, Korea Polytechnic University, Shihung-Si,
Kyongki-Do, 429-793, Korea
b Life Sciences Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul
130-650, Korea. E-mail: s1673@kist.re.kr
Received (in Cambridge, UK) 19th March 2003, Accepted 8th April 2003
First published as an Advance Article on the web 6th May 2003
Osmylated macroporous resins displayed excellent catalytic
performances in the asymmetric aminohydroxylation of
olefins and, moreover, these resins were easily recovered and
reused without any significant decrease in catalytic effi-
ciencies.
In searching for the more efficient and practical immobilisa-
tion method for OsO4, we have very recently found that OsO4
can be simply immobilised by osmylation onto the resins
bearing residual vinyl groups such as Amberlite XAD-4, XAD-
7 and XAD-16, etc. (Fig. 1).8 According to XPS (X-ray
Photoelectron Spectroscopy) analysis of the osmylated resins,
Today, because of increasingly stringent environmental and
social regulations, the international chemistry community is
under increasing pressure to change current working practices
and to find greener and safer alternatives, i.e., chemical
manufacturers should develop such processes that produce less
waste and avoid, as much as possible, the use of toxic and/or
hazardous reagents.
osmium exists in forms of Os(VI) monoglycolate and Os(IV)
bisglycolate.8 These osmylated resins are air-stable, nonvolatile
and thus much easier to handle than their homogeneous
counterpart (OsO4). Moreover, the resin-bound OsO4 exhibited
excellent catalytic performances in asymmetric dihydroxylation
of olefins, and could be easily recovered by simple filtration and
reused for consecutive reactions without any significant loss of
catalytic efficiency. These results encouraged us to examine the
efficiency of our osmylated resins in the AA reactions. We
report here our preliminary findings.
The osmium-catalysed aminohydroxylation (AA) of olefins
provide one of the most elegant methods for the synthesis of
chiral aminoalcohols in enantiomerically enriched form.1
Although this reaction offers a number of processes that could
be applied to the synthesis of chiral drugs, natural products, fine
chemicals, etc., the high cost of osmium and the chiral alkaloid
ligands as well as the high toxicity and volatility of osmium
component has made their large-scale industrial applications
with these reagents difficult. Another serious problem of
homogeneous Os-catalysed reactions is that toxic osmium metal
can exist as an impurity in the prepared products which will be
a deathblow especially to drug intermediates.2 Thus, it would be
highly desirable to develop the immobilised catalyst system that
could be readily separated from the reaction mixture and
provide the possibility of reuse of catalyst. Surprisingly, in spite
of the importance of this issue, only a few attempts to solve the
above-mentioned problems have been made.3–5 Several years
ago, we first reported heterogeneous asymmetric aminohydrox-
ylation of olefins using silica-supported bis-cinchona alkaloid.3
After our report, another example of a cinchona alkaloid
immobilised onto polymer supports came from Nandana4 and
Salvadori.5 Although using our silica-supported bis-cinchona
alkaloid, up to > 99% ee was achieved in the AAs of trans-
cinnamate derivatives, this type of attempt to immobilise
osmium on solid-supported alkaloid ligands failed due to severe
osmium leaching. Very recently, Choudary and his coworkers
employed their K2OsO4·2H2O anchored by the ion exchange
method onto layered double hydroxides (LDH) for the AAs of
olefins.6 Although it was reported that LDH–OsO4 was used for
three cycles in AA of trans-stilbene (ca. 60% ee per each cycle),
the activity of catalyst decreased drastically upon reuse. On the
other hand, the microencapsulated osmium tetroxide in a
polymer matrix developed by Kobayashi7 might be used as a
recyclable catalyst for asymmetric aminohydroxylation of
olefins. However, it has been known that the activity of this
polymeric osmium tetroxide is too low and thus the amount (5
mol%) of this catalyst needed to achieve reasonable reaction
rate is too large compared to that (0.2–1 mol%) needed in
homogeneous AD reactions.
The AA reactions of olefins using the osmylated macro-
porous resins (XAD-4-OsO4 and XAD-16-OsO4, ~ 0.36 mmol
Os per gram resin) were carried out with AcNHBr–LiOH as the
oxidant/nitrogen source under the same reaction conditions
adopted for the analogous homogeneous process.† The results
are summarised in Table 1. The data show that all reactions
employing the osmylated resins gave excellent ees ( > 99% ee
for all trans-cinnamate derivatives). Moreover, in the case of
cinnamate derivatives, the chemical yields obtained in this
study were significantly higher than those9 achieved in
homogeneous conditions (entries 1 and 2 in Table 1).
The efficiency with which the osmylated resin can be
recycled has also been examined. For these experiments,
osmylated resins are loaded onto a commercially available
small cylindrical container with mesh wall (MicroKan™), in
order to minimise grinding of osmylated resin through stirring
during the reaction. The dark-coloured Amberlite XAD-4-Os
complex was recovered by simple filtration after each reaction,
which is not possible in a homogeneous process. The alkaloid
ligand was also almost quantitatively recovered by simple acid–
base extraction. The AA reactions were repeated with these
recovered resins without any further addition of osmium. As
shown in Table 2, the recovered catalyst could be used for three
times without any significant loss of yields and enantioselectiv-
ity. However, leaching of OsO4 into solution has been found,
causing increased turnover time upon reuse.10
On the basis of all these results it can be assumed that osmium
anchored to resins is highly exposed to the oxidant, and once
† Electronic supplementary information (ESI) available: experimental
Fig. 1 Osmylated macroporous resins.
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CHEM. COMMUN., 2003, 1312–1313
This journal is © The Royal Society of Chemistry 2003
Jo, Cheon Hee
