S. S. Thakur et al. / Tetrahedron Letters 46 (2005) 2263–2266
2265
OH
Table 1. Kinetic data for the HKR of racemic ECH catalyzed by the
monomeric and dimeric [Co(salen):AlCl3] catalysts
O
O
HCl
(1.0 M soln.
in ether)
Cl
chiral catalyst
(1b-5b) 5 mol %
o
+
+
Catalyst
No. of
(salen)Co unit
kintra
(minꢀ1 · 10ꢀ2 a
kinter
(Mꢀ1 minꢀ1 a
R
,
TBME
0-5 C
R
R
)
)
+_
(
)
H3 ,CH2 CH3,CH2 Cl
R=C
85% ee
80% ee
1a
1a0
1b
1b0
5b
1
1
2
2
2
1.0
1.1
100
OH
Cl
44.4
45.5
48.9
10.2
15.7
17.7
H3
C
80
60
40
20
0
O
a Calculated from Figure 5 using Eq. 1 (see Supplementary data).
H3
C
C
OH
unit through the Al induce the cooperative mechanism,
albeit through a far less enantiodiscriminating transition
state than that attained with the monomeric (salen)CoAl
catalysts. Kinetic studies (Table 1) of the HKR of ECH
shows the two-term rate equation involving both intra-
and intermolecular components (Eq. 1),13,14
Cl
H3
O
H3
C
2
rate / kintra½catalyst þ kinter½catalyst
ð1Þ
plots of rate/[catalyst] versus [catalyst] should be linear
with slopes equal to kinter and y-intercepts correspond-
ing to kintra. Analysis of such plots with rate data ob-
tained with dimeric catalysts 1b, 1b0 and 5b revealed
linear correlations with positive slopes and nonzero
y-intercepts, consistent with participation of both inter-
and intramolecular pathways in the HKR (Figure 5; see
Supplementary data). Similar analysis of rate data
obtained with monomeric catalysts 1a and 1a0 revealed
y-intercepts of zero, reflecting the absence of any first-
order pathway for these catalysts.
0
5
10
15
20
25
Time (h)
Figure 3. Asymmetric ring opening of 1,2-epoxypropane EP with HCl
catalyzed by monomer (i.e., 1a) [CoIII(salen)(Cl)AlCl3] and dimer (i.e.,
1b) [CoIII(salen)(Cl)AlCl3CoIII(salen)(Cl)] catalysts.
The structural stability of catalysts in acidic medium
makes possible asymmetric ring opening of terminal
epoxides with HCl. It is hoped that these dimeric cata-
lysts will shed light to ARO with HCl and allow their
extension for other terminal epoxides. Further design,
crystal structure and recyclability of the catalyst is cur-
rently under investigation for the generality of asym-
metric ARO.
Thus, the dimer catalysts provide appropriate relative
proximity and orientation, which eventually reinforces
the reactivity and selectivity relative to monomeric com-
plex. The relative reactivity of dimeric catalysts (1b–5b)
could be explained by the ease of cooperativity of the
two (salen)Co units. Nonlinearity of the HKR reactions
is also consistent with cooperative reactivity between
two (salen)Co centres.
Supplementary data
Supplementary data associated with this article can be
The asymmetric ring opening of EP, ECH and EB with
HCl are taken for the present study (Fig. 3 for 1,2-
epoxypropane, EP). To date no examples have been re-
ported for the ARO of terminal epoxide with HCl. The
nonlinear effect is more pronounced in HCl than H2O.
In contrast to HKR of the ECH and EB, it need high
loading of the catalysts (5 mol %) and tert-butyl methyl
ether (TBME) as solvent. The ee% of the ECH decreases
with respect to time after attaining the maximal (79%).
The ring-opened product of ECH shows the reversibility
and it forms again ECH in the presence of catalyst after
prolonged time. This problem can be overcome by stop-
ping the reaction after getting high ee%. The asymmetric
ring opening reaction of EP, ECH and EB may occur by
the analogous cooperative bimetallic mechanisms
(Scheme 1).
References and notes
1. (a) Nielson, L. P. C.; Stevenson, C. P.; Backmond, D. G.;
Jacobson, E. N. J. Am. Chem. Soc. 2004, 126, 1360; (b)
Kim, S. K.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2004,
43, 3952; (c) Larrow, F.; Jacobsen, E. N. Top. Organomet.
Chem. 2004, 6, 123, and references cited therein; (d)
Jacobsen, E. N. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-
Verlag, Berlin, Germany, 2004; p 135, Supplement 2.
2. Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E.
N. Science 1997, 277, 936.
3. Furrow, M. E.; Schaus, S. E.; Jacobsen, E. N. J. Org.
Chem. 1998, 63, 6776.
4. (a) White, D. E.; Jacobsen, E. N. Tetrahedron: Asymmetry
2003, 14, 3633; (b) Breinbauer, R.; Jacobson, E. N. Angew.
Chem., Int. Ed. 2000, 39, 3604.
5. (a) Shin, C. K.; Kim, S. J.; Kim, G.-J. Tetrahedron Lett.
2004, 45, 7429; (b) Kim, G.-J.; Lee, H.; Kim, S.-J.
Tetrahedron Lett. 2003, 44, 5005.
In conclusion, the easily synthesized dimeric chiral (salen)Co
catalysts show several orders of magnitude times more
reactive than monomeric analogues without loss in
enantioselectivity. It provides high enantiomerically
pure valuable terminal epoxides in one-step process.