Nanosheet-enhanced enantioselectivity in the vanadium-catalyzed asymmetric epoxidation of allylic alcohols

Article abstract of DOI:10.1002/chem.201201659

The use of suitable chiral ligands is an efficient means of producing highly enantioselective transition-metal catalysts. Herein, we report a facile, economic, and effective strategy for the design of chiral ligands that demonstrate enhanced enantioselectivity and catalytic efficacy. Our simple strategy employs naturally occurring or synthetic inorganic nanosheets as huge and rigid planar substituents for, but not limited to, naturally available α-amino-acid ligands; these ligands were successfully used in the vanadium-catalyzed asymmetric epoxidation of allylic alcohols. The crucial role of the inorganic nanosheets as planar substituents in improving the enantioselectivity of the reaction was clearly revealed by relating the observed enantiomeric excess with the distribution of the catalytic centers and the accessibility of the substrate molecules to the catalytic sites. DFT calculations indicated that the LDH layer improved the enantioselectivity by influencing the formation and stability of the catalytic transition states, both in terms of steric resistance and H-bonding interactions. Handy tricks for asymmetric catalysis: The inorganic layer serves as a huge and rigid planar substituent to effectively enhance the enantioselectivity in the vanadium-catalyzed asymmetric epoxidation of allylic alcohols. Copyright

Full text of DOI:10.1002/chem.201201659

FULL PAPER
DOI: 10.1002/chem.201201659
Nanosheet-Enhanced Enantioselectivity in the Vanadium-Catalyzed
Asymmetric Epoxidation of Allylic Alcohols
Li-Wei Zhao, Hui-Min Shi, Jiu-Zhao Wang, and Jing He*[a]
Abstract: The use of suitable chiral li-
gands is an efficient means of produc-
ing highly enantioselective transition-
metal catalysts. Herein, we report
a facile, economic, and effective strat-
egy for the design of chiral ligands that
demonstrate enhanced enantioselectivi-
ty and catalytic efficacy. Our simple
strategy employs naturally occurring or
synthetic inorganic nanosheets as huge
and rigid planar substituents for, but
not limited to, naturally available a-
amino-acid ligands; these ligands were
successfully used in the vanadium-cata-
lyzed asymmetric epoxidation of allylic
alcohols. The crucial role of the inor-
ganic nanosheets as planar substituents
in improving the enantioselectivity of
the reaction was clearly revealed by re-
lating the observed enantiomeric
excess with the distribution of the cata-
lytic centers and the accessibility of the
substrate molecules to the catalytic
sites. DFT calculations indicated that
the LDH layer improved the enantio-
selectivity by influencing the formation
and stability of the catalytic transition
states, both in terms of steric resistance
and H-bonding interactions.
Keywords: asymmetric catalysis
·
heterogeneous catalysis ligand
·
design · nanosheets · substituent
effects
Introduction
active metal centers. This strategy was successfully applied
to the vanadium-catalyzed asymmetric epoxidation of allylic
alcohols,[27] which is a reaction of great value in organic syn-
thesis.[28] The inorganic nanosheets were layered double hy-
droxides (LDHs) that contained positively charged brucite-
like layers and interlayer anions. The brucite-like layer con-
sisted of metal cations that were surrounded in an approxi-
mately octahedral manner by hydroxide ions. The octahe-
dral units were linked by edge-sharing to form infinite
layers, which are typical of the CdI2 structure,[29] in which
the metal cations were orderly distributed throughout the
layer and the hydroxide ions sat perpendicular to the layer
plane.[30] The positive charges, which were acquired from the
substitution of a fraction of the divalent cations in a brucite
lattice by trivalent cations, allowed for the charge-balancing
anions to be intercalated between the layers. The divalent
and trivalent cations were Mg2+ and Al3+ ions in the natural-
ly occurring sample with carbonate intercalated as anions,
whilst both the metal cations and the intercalated anions
could be varied over a wide range in the synthetic cases.[31]
We used anions of pristine a-amino acids as the intercalated
anions and the brucite-like layer that was used for intercala-
tion consisted of ZnII and AlIII. a-Amino acids are highly at-
tractive as chiral ligands[32,33] because they are the building
blocks of enzymes that promote asymmetric reactions in
nature. To obtain satisfactory chiral induction, covalent
modification[34,35] or derivation[36–38] of a-amino acids is com-
monly performed. However, in our case, pristine a-amino
acids are simply attached as anions onto the inorganic nano-
sheets by using electrostatic forces, which could improve the
chiral induction whilst requiring minimal synthetic modifica-
tion of the chiral ligands.
The growing demand for enantiopure functional molecules
has emphasized the need to carry out chemical transforma-
tions in an enantioselective manner.[1] Because asymmetric
catalysis allows the generation of enantiopure chiral prod-
ucts from non-chiral starting materials, catalytic asymmetric
synthesis has become an important topic of research for
chemists in both academia and in industry.[2–4] One of the
most common methods for inducing asymmetry into a transi-
tion-metal-catalyzed reaction is through the use of chiral li-
gands.[5–9] Thus, widespread efforts have been devoted to the
design of active chiral ligands.[10–13] The validity of chirality
inducing ligands is largely dependent on the geometry and
rigidity of the ligand, the steric and electronic effects of sub-
stituent, and the metal–ligand chelate.[14–18] Typically, poten-
tial chiral ligands are bulky and complicated entities[19–25]
that require laborious synthesis and are often very expen-
sive. Although a variety of chiral ligands have been devel-
oped, only a select few have been effective in a variety of
transformations.[26] This shortage provides opportunities for
the discovery of facile, economic, and effective methods for
the development of chiral ligands for various transition-
metal-catalyzed reactions. We have previously reported[27]
a simple ligand-design strategy that employs inorganic nano-
sheets for modifying a-amino acids as chiral ligands for
[a] L.-W. Zhao, H.-M. Shi, J.-Z. Wang, Prof. J. He
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Box 98, 15 Beisanhuan Dong Lu
Beijing 100029 (P. R. China)
Fax : (+86)10-64425385
E-mail: jinghe@263.net.cn
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One prominent characteristic of LDH compounds is their
flexible interlayer spacing. The gallery height of the interlay-
er could match the size and arrangement of intercalated
anions[29,31] or be expanded to afford delamination.[39] There-
fore, when used as the hosts for asymmetric catalysts, LDH
compounds not only demonstrate confinement to boost
enantioselectivity[40] but also allow the catalytic reactions to
be carried out under pseudo-homogeneous conditions.[27]
The catalytic efficacy of pseudo-homogeneous systems has
recently approached the level in homogeneous systems.[27]
Attached to the brucite-like layers of LDHs through elec-
trostatic attractions, the a-amino acids are deprotonated and
the protons are replaced by positively charged LDH layers
(Figure 1). Thus, the brucite-like layers can be supposed to
serve as a “huge” and “rigid” planar substituent for the at-
tached a-amino acid anions, just like in a carboxylic ester
(Figure 1). Here, the “huge” and “rigid” planar substituent
imposes both steric and electronic effects to promote the
enantioselectivity in the vanadium-catalyzed asymmetric ep-
oxidation of allylic alcohols, which are important substrates
because the additional functional group of the resulting
epoxy alcohol makes these compounds more useful as start-
ing materials in the synthesis of many natural products and
pharmaceuticals.[41]
a cationic brucite-like layer, gave a well-defined stacking
structure with a basal spacing of 1.23 nm, as well as a hexag-
onally shaped crystal morphology (Figure 2a). The unit-lat-
tice parameter of the brucite-like layer was 3.02 ꢁ (Fig-
ure 2b). The gallery height between two layers was deter-
mined to be 0.75 nm (Figure 2c) by subtracting the layer
thickness (0.48 nm) from the basal spacing. According to el-
emental analysis, one l-glutamate anion was estimated to
balance ten octahedrons (equal to five lattice units with
a Zn/Al molar ratio of 1.5); X-ray diffraction (XRD) and
FTIR spectroscopy revealed that the anion was attached to
the layer through monodentate[42] interactions of its carbox-
ylate group (Figure 2d). The l-glutamate anion was ar-
ranged with its backbone tilting towards the planar layer at
an angle of 488 (Figure 2c) based on elemental analysis,
XRD, and FTIR spectroscopy. Next, the intercalation was
performed on l-alanine and l-serine. The intercalation of l-
alanine or l-serine gave a basal spacing of 0.90 nm, with
a gallery height of 0.42 nm in each case. Both l-alanine and
l-serine anions showed monodentate[42] interactions with the
planar substituent through its carboxylate group, with one
anion balancing four octahedron units. The l-serine anion
was arranged with its backbone tilting towards the planar
layer at an angle of 368 and the l-alanine anion at an angle
of 458.
Enantioselectivity boost by the LDH layer: First, an l-gluta-
mate anion that was attached to brucite-like layers of LDHs
was used as the ligand in the vanadium-catalyzed asymmet-
ric epoxidation of allylic alcohols (Table 1). In the asymmet-
ric epoxidation of 2-methyl-cinnamyl alcohol (Table 1, en-
tries 1 and 2), a preliminary investigation revealed that, by
using VOACTHNURTGENNG(U OiPr)3 as the precursor of the catalytic centers
and with l-glutamate intercalated in the interlayer region of
the LDHs as a chiral ligand, enhanced enantioselectivity
was afforded for both the trans- and cis-epoxidation prod-
ucts (38–89% ee for the cis isomers, 60–93% ee for the tran-
s isomers in similar yields). The enantioselectivity was simi-
lar to the level (97% ee) that was obtained previously by
using hydroxamic acid as the ligand.[43] Hydroxamic-acid de-
rivatives have been shown to be effective ligands in the va-
nadium-catalyzed asymmetric epoxidation of allylic alcohols.
The yield of the epoxidation product increased whilst the
enantiomeric excess remained unchanged (Table 1, cf. en-
tries 3 and 4 and entries 5 and 6). Because the l-glutamate
anions intercalate in the interlayer region of the LDHs to
form a stacked structure, the enhancement in enantioselec-
tivity could originate from either the spatial confinement of
the interlayer region, as observed previously in other sys-
tems,[40] or simply from the steric/electronic effects of the
single layer.
Figure 1. Brucite-like layer as a huge and rigid planar substituent for a-
amino acids.
Results and Discussion
Attachment of a-amino acids onto the LDH layers: First,
the attachment of LDH layers onto l-glutamic acid was per-
formed by intercalation through the ion-exchange. The Zn/
Al molar ratio for the intercalated precursor was 2.0. In the
final intercalation, the Zn/Al molar ratio was 1.5. As shown
in Figure 2, the intercalation of l-glutamate, in which the
carboxylic acid proton of l-glutamic acid was substituted by
To elucidate this point, the l-glutamate-intercalated
LDHs were delaminated to form exfoliated brucite-like
layers. This result clearly demonstrated that a boost in enan-
tioselectivity could be accomplished by attaching l-gluta-
mate anions onto single LDH layers (Table 1, cf. entries 7
and 8). Control experiments (Table 1, entries 9, 10, and 11)
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Nanosheet-Enhanced Enantioselectivity Epoxidation Reactions
FULL PAPER
tion of 2-methyl-cinnamyl alco-
hol (Table 1, entries 15–18).
Using VOACTHUNGRTENUNG(OiPr)3 as the precur-
sor of the catalytic centers and
the LDH layer as a planar sub-
stituent for either l-alanine
(Table 1, entries 15 and 16) or
l-serine (Table 1, entries 17 and
18) afforded the trans epoxide
(major epoxidation product)
with excellent enantioselectivity
(97% and 96% ee) in extreme-
ly high yields (95% and 94%).
In addition to the enhance-
ment in enantioselectivity, two
other interesting results were
also observed: First, as the sub-
stituent on l-glutamate, the
planar brucite-like layer im-
proved the diastereoselectivity
in the epoxidation of 2-methyl-
cinnamyl alcohol in most cases,
thereby giving a higher propor-
tion of the trans epoxide (major
product) than that observed
when using H as the substitu-
ent. Second, when the ligand
content
increased
from
a ligand/vanadium molar ratio
of 0 to 1:7 and then to 1:1
(Table 1, entries 10 to 8), the
yield of the epoxide increased
from 61% to 74% in 1440 min
and 93% in 640 min.
Figure 2. a) XRD pattern for l-glutamate-intercalated LDHs; inset: representative SEM image of the interca-
lated crystal and an illustration of the stacking structure. b) HRTEM image of a brucite-like layer lattice.
c) Arrangement of the interlayer l-glutamate. d) One l-glutamate anion compensates for five lattice units.
e) Hydrogen bonding between l-glutamate and the planar poly(octahedron) substituent. f) Calculated hydro-
gen bonds between the brucite-like layer and vanadium-coordinated l-glutamate and between the brucite-like
layer and the O=V moiety (V: light gray, Zn: light blue, Al: pink, N: dark blue, C: dark gray, O: red, H:
white).
Distribution of the active cen-
ters: The LDH layers demon-
strated diversity in the enhance-
ment of the enantioselectivity
depending on the precursor of
revealed that the enantioselective induction originated from
the nanosheet-attached l-glutamate and the epoxidation ac-
tivity was due to the vanadium centers. By using VOSO4 as
the precursor of the catalytic centers (Table 1, entries 13 and
14), a visible enhancement in the enantioselectivity of the
trans-epoxidation product was also observed (from 22% to
84% ee). These results clearly revealed that the huge and
rigid planar brucite-like layers of the LDHs played a crucial
role in achieving a high level of chiral induction. Herein, we
propose that the LDH layer serves as a giant planar sub-
stituent on the chiral a-amino-acid ligand, just like in a car-
boxylic ester. This giant planar substituent enhances the
chiral induction of chiral ligands through either steric or
electronic effects, or both.
the metal centers (Table 1). For example, at the same
ligand/vanadium molar ratios, the LDH layer improved the
enantioselectivity less-remarkably by using VOSO4 (84% ee
for the trans isomer and 8% for the cis isomer) than with
VOACTHUNTRGENNG(U OiPr)3 (at least 91% ee for the trans isomer and 68%
for the cis isomer) as the precursor of the catalytic center.
To reveal the reasons for the difference in the enhancement
of enantioselectivity, the distribution of the vanadium cen-
ters in the catalytic system was explored (Figure 3a–d). As
shown in Figure 3b,c, the vanadium centers could be re-
solved in a homogeneous distribution along the 2D hexago-
nal plane by using either VOSO4 or VOACHTNUGTRNEUNG(OiPr)3 as the pre-
cursor of the catalytic centers, which was in accord with the
observed improvement in enantioselectivity in either case.
However, the population of the vanadium centers in the in-
terlayer region was lower when using VOSO4 (V/Zn=0.1,
To demonstrate the validity of our proposal over a broad
range, the proposed planar substituents were then applied to
l-alanine and l-serine in the vanadium-catalyzed epoxida-
Figure 3b) than VOACHTNUTRGNE(NUG OiPr)3 as the precursor (V/Zn=0.58,
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J. He et al.
moieties.[a]
Table 1. Vanadium-catalyzed
asymmetric
epoxidation
of
allylic
alcohols
by
using
a-amino
acids
as
chelate
Entry a-Amino acid
Substituent on
the a-amino
acid
Vanadium
precursor
Major
t
Yield d.r.
ee[d] [%]
product
[min] [%][c] trans/cis
ACHTUNGTRENNUNG
A
N
ACHTUNGTRNE(NUNG 2R,3S)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
H
Rꢂ
H
Rꢂ
H
Rꢂ
H
VO
VO
VO
VO
VO
VO
VO
VO
VO
VO
N
70
520
170
640
280
1050
400
640
1440
1440
1440
1440
400
1440
1440
1440
1440
1440
2160
2160
2160
66
64
72
73
84
83
94
93
74
61
–
28
86
83
98
95
98
94
95
68
76
64:36
83:17
51:49
85:15
54:46
66:34
52:48
83:17
96:4
60
93
55
92
56
91
54
94
16
–
38
89
37
84
26
68
18
71
13
–
–
9
7
8
24
51
20
51
l-glutamic acid
Rꢂ[e]
Rꢂ[f,g]
Rꢂ[e,g]
Rꢂ
91:9
–
--
–
Rꢂ[e,h]
H
VO
N
57:43
77:23
91:9
70:30
61:39
69:31
64:36
80
22
84
68
97
56
96
33
39
53
VOSO4 (3.0)
VOSO4 (3.0)
Rꢂ
15
16
H
VO
VO
VO
VO
VO
VO
VO
l-alanine
Rꢂ[e]
H
17
18
19
20
21
l-serine
Rꢂ[e]
H
--
--
--
Rꢂ
Rꢂ[e]
l-glutamic acid
[a] All data were reproduced at least twice. [b] The number of moles of catalyzed substrate are given in parentheses; the amino-acid/vanadium molar
ratio was 1:1 if not otherwise indicated. [c] Yield of the isolated epoxy alcohol after chromatographic purification. [d] Determined by HPLC (Daciel
Chiral AD-H column) or by GC (Astec G-TA chiral capillary column). [e] The layers were exfoliated. [f] The vanadium/amino-acid molar ratio was 7:1.
[g] Only the poly(octahedron) was employed; no amino acids were present. [h] The vanadium/amino-acid molar ratio was 1:7.
Figure 3c). That is, a fraction of the vanadium centers from
the VOSO4 precursor possibly made no use of the planar
substituent, which was consistent with the lower ee value
was not enough to coordinate with the a-amino acid in the
central interlayer region. In accordance, ee values of 80%
for the trans isomers and 9% for the cis isomers were af-
forded (Table 1, entry 12), which were lower than those ob-
served at higher vanadium dosages. The vanadium distribu-
tion agreed well with the observed enantioselectivity, thus
clearly demonstrating the validity of LDH brucite-like
layers as an efficient planar substituent.
that was observed for VOSO4 than for VOACTHNUTRGNEG(NU OiPr)3. Thus,
this diversity in the enhancement of the ee value can be at-
tributed to the difference in the location of the catalytic
sites. The vanadium centers that are coordinated to the a-
amino-acid anions that contain planar substituents are sup-
posed to accomplish the catalysis with excellent enantiose-
lectivity and the vanadium centers that are coordinated to
the a-amino-acid anions around the central region of the
planar substituent benefit more from the steric effects than
those that are situated at the edges and corners of the crys-
tallites or at the gallery entrances. This observation is good
evidence for the crucial role of the planar substituent in the
enhancement of chiral induction.
To confirm this conclusion, the vanadium dosage was de-
creased to as low as 0.14 mol%, which gave a ligand/vanadi-
um molar ratio of 7:1. The vanadium centers were found to
concentrate at the edge regions of the hexagonal crystallites
(Figure 3d) because the vanadium dosage was so low that it
Catalyzed substrates: Next, the scope of the substrates for
the vanadium-catalyzed epoxidation reaction was extended
to the bulky 2-(2,2-dimethypropyl)allylic alcohol, which was
selected for its limitations in the diffusion into the interlayer
region but less hindrance of its hydroxy moiety in accessing
the vanadium center (Table 1, entry 19-21). An improve-
ment in the enantioselectivity was only observed for the ex-
foliated system (from 33% to 53% ee). The enantiomeric
excess that was afforded in the layer-stacked (intercalated)
system was similar to the value that was produced by its ho-
mogeneous counterpart (33–39% ee). The dependence of
the increase in enantioselectivity on the catalyzed substrates
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Nanosheet-Enhanced Enantioselectivity Epoxidation Reactions
FULL PAPER
loidal and epoxidation medium
and water-soluble VOSO4 as
the source of the catalytic
center, the catalyst could be di-
rectly recycled by simple liquid/
liquid separation without the
loss of catalytic activity and
enantioselectivity.
Theoretical investigation on the
role of the LDH layers: Ac-
cording to the previously re-
ported mechanism,[44] the
V
center was first activated by
using tert-butylhydroperoxide
(TBHP) to form VOACHTUNGTRENNUNG(OiPr)2-
AHCTUNGTRENNUNG
atom on the allylic alcohol co-
ordinated to the V center, with
the C=C bond of the allylic al-
cohol positioned perpendicular
to the peroxide bond to facili-
tate the formation of the epox-
ide. When using the nanosheet-
attached a-amino acid anions
as the chiral ligands on the V
center, the attacking directions
of the allylic alcohol were re-
stricted, owing to the steric re-
pulsion of the LDH nanosheet;
that is, the allylic alcohol could
only attack the active V center
in the direction away from the
LDH nanosheet. For example,
to form the (2R,3R) transition
state, the catalyzed allylic alco-
hol had to access the activated
vanadium site in its concave
Figure 3. Images of vanadium distribution visualized by EDS energy profiles and the ee values that were ob-
served with their corresponding catalysts. a) The scanning line through the 2D plane for (b–d). b) A VOSO4
precursor was coordinated with the l-glutamate intercalated between the LDH layers with a vanadium loading
of 3.0 mol%. c) A VO
ACHTUNGTRENNUNG
layers with a vanadium loading of 3.0 mol%. d) A VOAHCTUNGTRENNUNG
intercalated between the LDH layers with a vanadium loading of 0.14 mol%. In each case, the uncoordinated
vanadium moieties were excluded by washing prior to the characterization.
clearly confirmed the role of the LDH layers as the substitu-
ent on the chiral ligands. In contrast to the case with smaller
substrates, it was difficult for the bulky substrate to diffuse
into the interlayer region to access the interlayer vanadium
centers. In this case, the LDH layer hardly played a role as
the substituent in the epoxidation reaction, thereby leading
to an enantiomeric excess that was similar to its homogene-
ous counterpart. When the stacked layers were exfoliated to
expose the catalytic sites to the bulky substrates, an im-
provement in enantioselectivity was achieved because, in
this case, the LDH layer could demonstrate at least steric ef-
fects in the epoxidation reaction.
conformation to keep the C=C bond perpendicular to the
peroxide bond, whilst, to form the (2S,3S) transition state,
the alcohol would have to access the V site in its convex
conformation (Figure 4). The diverse attacking directions of
the allylic alcohol are thought to affect the H-bonds be-
tween the amino-acid anions and the LDH layer in different
ways, with a general interaction in the hydrotalcite-like
structures[29] because of the abundant hydroxy groups on the
layer surface. The H-bonds could be enhanced by forming
additional H-bonds or weakened by cleaving some of the H-
bonds, which might be regarded as electronic effects that
are concomitant with the steric effects. In addition, the allyl-
ic alcohols that attack in concave or convex directions are
arranged at various distances from the LDH layer and, ac-
cordingly, form various H-bonding interactions with the
LDH layer, which is thought to influence the formation and
stability of the transition states. To make this issue clearer,
density functional theory (DFT)[45] calculations were per-
formed. All of the geometries were fully optimized with the
Previously, we have demonstrated[27] that the heterogene-
ous catalyst with the intercalated ligand can be easily sepa-
rated by simple filtration with comparable yield (83%
versus 80%) and ee value for the major isomer (92% versus
96%). The colloidal catalyst with the delaminated ligand
gave rise to a slight decrease in the yield (from 98% to
88%) by using simple filtration. By using water as the col-
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J. He et al.
layer (Figure 2 f). The H-bonding interactions were thought
to facilitate an appropriate rigid linkage between the bru-
cite-like layer and the attached anions, which is necessary
for the excellent enantioselective induction. In the catalytic
epoxidation of, for example, 2-methyl-cinnamyl alcohol, the
access of 2-methyl-cinnamyl alcohol in either its concave or
convex conformation turned the H-bond between the LDH
layer and the O=V moiety from V=O3···H4 in 1.80 ꢁ into
V=O3···H3 in 1.66 or 1.61 ꢁ (Figure 5a) because of the
steric inhibition of the rigid LDH layer. The concave access
to form the (2R,3R) transition state strengthened the H-
bonds between the LDH layer and the attached l-glutamate
À
À
from 1.90 ꢁ (C2 O1···H1) and 1.70 ꢁ (C2 O2···H2) to
1.85 ꢁ and 1.61 ꢁ, respectively. But, the steric inhibition of
Figure 4. Proposed attacking directions of 2-methyl-cinnamyl alcohol by
using LDH-layer-attached a-amino acids as chiral ligands for the V
center (V: light gray, Zn: light blue, Al: pink, N: dark blue, C: dark gray,
O: red, H: white).
À
the rigid LDH layer weakened the C2 O1···H1 bond (to
1.95 ꢁ) when the catalyzed substrate accessed the activated
vanadium site in its convex conformation to form the
(2S,3S) transition state. Moreover, the H-bond between the
alcoholic moiety (O4) in 2-methyl-cinnamyl alcohol and the
brucite-like layer in the (2R,3R) transition state was also
stronger than that in the (2S,3S) transition state (1.72 ꢁ
versus 1.81 ꢁ). As a result, the energy difference between
the (2S,3S) and (2R,3R) transition states increased to
14.08 kJmolÀ1 for the LDH-layer-attached system (Fig-
ure 5a) from 9.49 kJmolÀ1 for its homogeneous analogue
(Figure 5b); this result accounted for the observed enhance-
ment in enantioselectivity.
Gaussian 09 package of programs (revision B.01)[46] and the
LANL2DZ[47] basis set was used for the V, Zn, and Al
atoms, whereas the 6-31G[48] basis set was used for the re-
maining atoms.
Theoretical optimization of the heterogeneous vanadium/
l-glutamate catalyst revealed that, in addition to the H-
bonds between the LDH layer and the attached l-glutamate
À
À
(C2 O2···layer-OH 1.70 ꢁ, C2 O1···layer-OH 1.90 ꢁ), the
LDH layer also afforded H-bonding interactions (1.80 ꢁ)
with the O=V moiety through the hydroxy groups on the
Conclusions
In summary, we have demon-
strated a highly efficient strat-
egy for enhancing the enantio-
selectivities in vanadium-cata-
lyzed asymmetric reactions.
This strategy, which simply in-
volves pristine a-amino acid as
a chelating moiety and synthet-
ic- or naturally occurring inor-
ganic brucite-like layers as
a planar substituent, was suc-
cessfully used in the vanadium-
catalyzed epoxidation of allylic
alcohols and can be further ex-
tended to other transition-
metal-catalyzed asymmetric re-
actions.
Experimental Section
Materials: The hydrotalcite-like struc-
ture with carbonate anions was first
Figure 5. The calculated energies and primary bonding distances for the (2R,3R) and (2S,3S) transition states
in the vanadium-catalyzed epoxidation of 2-methyl-cinnamyl alcohol by using a) LDH-layer-attached l-gluta-
mate and b) l-glutamic acid as chiral ligands (V: light gray, Zn: light blue, Al: pink, N: dark blue, C: dark
gray, O: red, H: white). The distances are reported in ꢁ.
synthesized according to a modified
homogeneous procedure.[39] The ex-
change of carbonate anions with ni-
trate anions and, subsequently, the ex-
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Nanosheet-Enhanced Enantioselectivity Epoxidation Reactions
FULL PAPER
change of nitrate anions with l-glutamate anions were performed to pro-
duce a hydrotalcite-like structure in which l-glutamate is sandwiched be-
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l-serine to the planar poly(octahedron) layers was achieved by co-precip-
itation.[49] The input Zn/Al molar ratio was about 2:1 in each case. The
final chemical composition was determined to be [Zn0.60Al0.41(OH)2] (l-
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0.17A
[Zn0.61Al0.39(OH)2]
(l-alanine)0.13
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A
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1
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Acknowledgements
The authors thank the NSFC and the 973 Project (2011CBA00504) for fi-
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Received: May 11, 2012
Published online: && &&, 0000
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Nanosheet-Enhanced Enantioselectivity Epoxidation Reactions
FULL PAPER
Asymmetric Catalysis
L.-W. Zhao, H.-M. Shi, J.-Z. Wang,
J. He* .......................... &&&& — &&&&
Nanosheet-Enhanced Enantioselec-
tivity in the Vanadium-Catalyzed
Asymmetric Epoxidation of Allylic
Alcohols
Handy tricks for asymmetric catalysis:
The inorganic layer serves as a huge
and rigid planar substituent to effec-
tively enhance the enantioselectivity in
the vanadium-catalyzed asymmetric
epoxidation of allylic alcohols.
Chem. Eur. J. 2012, 00, 0 – 0
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