Enantiotopic Discrimination by Coordination-Desymmetrized meso-Ligands

Article abstract of DOI:10.1002/cctc.201902243

The first examples of enantiopure catalysts that are chiral merely due to coordination of different metal ions at enantiotopic positions of an achiral meso-ligand are reported. These catalysts exhibit a pseudo-C_s symmetry and are able to catalyze reactions demanding simultaneous involvement of two catalytic sites. The latter was demonstrated by application in the asymmetric ring-opening of meso-epoxides.

Full text of DOI:10.1002/cctc.201902243

Accepted Article
Title: Enantiotopic Discrimination by Coordination-Desymmetrized
meso-Ligands
Authors: Yutang Li, Anna Lidskog, Helena Armengol-Relats, Thanh
Huong Pham, Antoine Favraud, Maxime Nicolas, Sami
Dawaigher, Zeyun Xiao, Dayou Ma, Emil Lindbäck, Daniel
Strand, and Kenneth Wärnmark
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To be cited as: ChemCatChem 10.1002/cctc.201902243
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ChemCatChem
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COMMUNICATION
Enantiotopic Discrimination by Coordination-Desymmetrized
meso-Ligands
Yutang Li,[a] Anna Lidskog,[a] Helena Armengol-Relats,[a] Thanh Huong Pham,[a] Antoine Favraud,[a]
Maxime Nicolas,[a] Sami Dawaigher,[a] Zeyun Xiao,[a] Dayou Ma,[a] Emil Lindbäck,[a,b] Daniel Strand,*[a]
Kenneth Wärnmark*[a]
Abstract: The first examples of enantiopure catalysts that are chiral
merely due to coordination of different metal ions at enantiotopic
positions of an achiral meso-ligand are reported. These catalysts
such as asymmetric cycloadditions, carbonyl-ene, aldol,
Mannich, Michael, Friedel-Crafts, hydogenations, hydrosilyations
of ketones, reductions and allylic oxidations.[12] Commonly, the
exhibit a pseudo-C
s
symmetry and are able to catalyze reactions
ligands are designed around privileged motives based on C
symmetry such as Jacobsen’s salens,[13a] boronic acids,[13b]
oxazolines,[13c] and 1,1’-binaphthyl-2,2’-diols.[13d] less-
developed approach is to use achiral ligands that are
desymmetrized by coordination of catalytically active metal
ions.[14] This concept would be particularly attractive in bimetallic
catalysis but no examples of such structures have been reported
to date.
2
demanding simultaneous involvement of two catalytic sites. The
latter was demonstrated by application in the asymmetric ring-
opening of meso-epoxides.
A
Molecular chirality and symmetry have inspired advances in
chemistry for over a century.[1] Chirality remains an important
tool in topics ranging from elucidation of reaction mechanisms,[2]
to synthesis of new enantioenriched compounds[3] and the
fundamental understanding of life.[4] It has even motivated
synthesis for aesthetic purposes.[5] Efficient breaking of
molecular mirror-symmetry by enantiomorphic or enantiotopic
discrimination has been accomplished by numerous methods.
In a C -symmetric bi-functional ligand with binding sites related
2
by the two-fold symmetry axis, the sites are homotopic (Figure 1,
left). In principle, enantiopure hetero-bimetallic complexes
(
pseudo-C -symmetric complexes) can thus be readily accessed
2
by sequential coordination of two different metal ions.[15] On the
other hand, formation of the corresponding enantioenriched
Commonly,
asymmetric
catalysis[3a-b,6]
and
chiral
chromatography are used,[7] but more esoteric methods have
also been reported such as the use of circularly polarized light,[8]
crystallization,[9] sublimation[10] and absolute asymmetric
catalysis.[11]
hetero-bimetallic complexes of homologous
Cs-symmetric
ligands (pseudo-meso complexes) where the binding sites are
instead related by a plane of symmetry[16] is more challenging as
such sites are enantiotopic (Figure 1, right). Sequential
coordination of different metal ions would thus, absent external
chiral influence, lead to a racemic product (Figure 2).
Figure 1. In a C2-symmetric ligand containing two binding sites (L), the binding
sites are homotopic (left) (previous work[15]). In a meso-ligand containing two
binding sites, the binding sites are enantiotopic (right) (this work).
Asymmetric catalysis using chiral ligands coordinated to Lewis
acidic metal ions has been an active research area for the past
decades with great importance for the development of reactions
Figure 2. Two enantiomeric bimetallic complexes based on one (prochiral)
meso-ligand (
) (top). A macroscopic analogy (bottom): An enantiomeric
pair is created by interchanging the position of the red and the green balls of
the meso-hands.
[
a]
Y. Li, A. Lidskog, H. Armengol-Relats, T. H. Pham, A. Favraud, M.
Nicolas, Dr. S. Dawaigher, Dr. Z. Xiao, Dr. D.-Y. Ma, Dr. E. Lindbäck,
Dr. D. Strand, Prof. K. Wärnmark
Centre for Analysis and Synthesis, Department of Chemistry Lund
University
P.O. Box 124, SE-22100, Lund, Sweden
E-mail: kenneth.warnmark@chem.lu.se,
daniel.strand@chem.lu.se
Present address: Department of Chemistry, Bioscience and
Environmental Engineering, Faculty of Science and Technology
University of Stavanger
Contingent that no metal-ion exchange occurs, one possible
tactic to solve this problem is to join two ligand-metal moieties
step-wise. That is, starting with an enantiopure ligand, inserting
the first metal ion, introducing another ligand, then inserting the
second metal ion. Using this approach, we here report the first
enantiopure hetero-bimetallic complexes with a meso-ligand
backbone,[17] and the proof-of-principle that such pseudo-meso
complexes can perform enantiotopic discrimination in meso-
substrates to induce substantial ee’s in the product of useful
catalytic processes.
[
b]
NO-4036 Stavanger, Norway
Supporting information for this article is given via a link at the end of the
document.
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COMMUNICATION
Scheme 1. The synthesis of the pseudo-meso bissalen complexes 3a and 3b.
Figure 3. Structures of pseudo-C
top), -symmetric monometallic monosalen complexes (middle), and
pseudo-meso bimetallic bissalen complexes (bottom).
2 symmetric bimetallic bissalen complexes
(
C2
Table 1. Ring-opening of cyclohexene oxide with TMSN
-bissalen complexes or 1:1 mixture of mono-metallic mono-salen
complexes.[a]
3 catalyzed by pseudo-
Cs
a
Drawing from Jacobsen’s
C2-symmetric homo-bimetallic
bissalen catalysts,[18] we recently reported a study of hetero-
bimetallic bissalen catalysts.[15] In this study, the bimetallic
pseudo-C
2
catalysts 1a and 1b (Figure 3, top) displayed
Entr
y
TOF[c]
Yield [d]
(%)
ee[e][j]
(%)
Catalyst (mol%)
Solvent[b]
(h-1
76
23
)
significantly increased reaction rates and enantioselectivies in
the ring opening of meso-epoxides compared the corresponding
1
[15]
1a(0.01)
None
None
THF
96[f]
> 99[g]
49[h]
66
94
62
50
32
36
57
35
7
1:1 mixtures of monometallic monosalen catalysts 2a and 2b or
2
[15]
1b(0.01)
2
a and 2c, respectively (Figure 3, middle),[15,18] and even higher
yields and ee’s were obtained compared to using Jacobsen’s
parent bimetallic C symmetric catalyst for the ring opening of
3
4
5
6
7
8
9
3a(1)
2.9
120
118
0.7
2
3a(0.1)
None
None
THF
> 99[i]
cyclohexene oxide.[15,18] The different metal ions in 1a and 1b
enable an efficient intramolecular activation of both the epoxide
and the nucleophile.[15,18,19] The success of this system suggests
that a bimetallic catalyst with a meso-backbone could also
provide an efficient entry to chiral discrimination. For such a
study, pseudo-meso complexes 3a and 3b (Figure 3, bottom)
were targeted. The principal design elements of 3a and 3b
3a(0.01)
> 99[i]
35[h]
3b(1)
3b(0.1)
None
None
THF
97
> 99[i]
> 99[i]
24[h]
3b(0.01)
144
0.61
7.2
(metal ions, tether length etc.) draws from prior work on the
2a(1)+4b(1)
2a(0.01)+4b(0.01)
2a(1)+4c(1)
2a(0.01)+4c(0.01)
optimization of pseudo-C symmetric catalysts 1a and 1b.[15]
2
1
0
1
2
None
THF
22[i]
4
1
1
0.52
10.6
24[h]
31[i]
4
None
4
[
[
a] Reaction conditions: r.t, cyclohexene oxide (0.5 mmol), TMSN
3 (0.6 mmol).
b] Reaction in THF (270 μL) or solvent-free. [c] Calculated as (initial
rate)/[catalyst]. [d] Determined by GC using 1,2,4,5-tetramethylbenzene as
internal standard. [e] The absolute configuration was determined by
comparison with the optical rotation reported in the literature. [f] Measured
after 72 h. [g] Measured after 108 h. [h] Measured after 24h. [i] Measured after
1
20 h. [j] The major configuration is (S,S).
Synthetically, catalysts 3a and 3b were readily prepared in
enantiopure form starting by the condensation of (1S,2S)-
cyclohexane-1,2-diamine mono-hydrochloride and (R,R)-
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COMMUNICATION
[
Cr(III)Cl]salen complex 5 (Scheme 1).[15] Triethylamine was
transition state (Figure 4).[15,18] In contrast to the bimetallic
pseudo-meso catalysts 3a and 3b, the corresponding mixtures
of monosalen catalysts follow a second order kinetic model with
respect to the catalyst.[15,18] From Fig. 4 it is clear that complex
3a catalyzes the reaction faster than 3b. The Cr-Mn is thus a
more efficient combination than Cr-Co in the pseudo-meso-case,
added to deprotonate the ammonium group which was then
reacted with 3,5-di-tert-butyl-2-hydroxy-benzaldehyde to give the
monometallic bissalen complex 6. Insertion of Mn(II) or Co(II)
followed by air oxidation quantitatively furnished the respective
analytically pure pseudo-meso bimetallic complexes [Cr(III)-
Mn(III)]bissalen 3a and [Cr(III)-Co(III)]bissalen 3b. The
complexes were characterized by HRMS and elemental analysis.
Both structures 3a and 3b, and their common precursor 6 were
found stable and do not undergo metal-ion scrambling,[18,19] as
confirmed by ESI-MS and CD spectroscopy (Figure S18-19).
which is in good agreement with both the C
2- and pseudo-C
2
systems.[18-19]
Table 2. Ring-opening of various meso-epoxides with TMSN
3 catalyzed by
pseudo-meso bissalen complexes under solvent-free conditions.[a]
The catalytic properties of the pseudo-meso complexes were
first investigated in the ring opening of cyclohexene oxide with
TMSN in THF (Table 1, entry 3 and 6). Both 3a and 3b gave
3
Catalyst
mol%)
Yield[b]
(%)
ee[c][d][e]
(%)
Entry
R
t(d)
moderate to good conversions into product with moderate to low
enantioselectivities (62% ee for 3a and 36% ee for 3b). All
reactions gave higher yields and higher turn-over frequencies
(
1
2
3
4
5
6
-CH
3
3
3a(0.1)
3b(0.1)
3a(0.1)
3b(0.1)
3a(0.1)
3b(0.1)
50
35
23
17
63
76
10
14
5
5
5
5
7
7
(TOFs) under solvent-free conditions compared to THF (Table 1,
-CH
entry 4, 5, 7 and 8). When compared to the pseudo-C catalysts,
2
-(CH
-(CH
-(CH
-(CH
2
2
2
2
)
)
)
)
3
-
-
-
-
> 99
> 99
50
the ee’s of the ring-opened products were however lower for the
pseudo-meso catalysts under identical conditions (Table 1, entry
3
5
5
1
vs. 5, entry 2 vs. 8). The performances of the bimetallic
complexes 3a and 3b were also compared to 1:1 mixtures of
R,R)-salen 2a and (S,S) salen 4b[20] or (S,S)-salen 4c.[21]
(
11
Notably, both 3a and 3b gave better yields, TOFs and ee’s than
the corresponding mixtures of monometallic catalysts in each
case (Table 1, entry 3 vs. 9, 5 vs. 10, 6 vs. 11, and 8 vs. 12).
This is in agreement with what was observed in a similar
[a] Reaction conditions: r.t, meso-epoxide (0.5 mmol), TMSN
3 (0.6 mmol). [b]
Determined by H NMR with 1,2,4,5-tetramethylbenzene as internal standard.
1
[c] Determined by GC. [d] The absolute configuration was determined by
comparison of the optical rotation with the literature value. [e] The major
configuration is (S,S).
comparison for pseudo-C catalysts 1a and 1b.[15]
2
The lower performance in terms of both reaction rate and
enantioselectivity for pseudo-meso 3a and 3b compared to that
The substrate scope was also explored. The ring-opening
of pseudo-C 1a and 1b[15] under otherwise identical conditions,
2
reaction of cyclopentene oxide with TMSN as the nucleophile
3
is indicative of a less favored bimetallic transition state in the
pseudo-meso case. We thus conclude that it is the relative
gave the highest observed selectivities for the pseudo-meso
system with 76% and 63% ee for 3b and 3a, respectively (Table
stereochemistry of the pseudo-C
2
system rather than the
2, entries 4 and 3). The acyclic meso-butene oxide gave
pseudo-meso system that constitutes the matched case in this
reaction.
moderate ee’s of 23% with 3a and 17% with 3b (Table 2, entries
2
1
and 1). With cycloheptene oxide, the ee’s dropped to around
0% for both catalysts (Table 2, entries 5 and 6).
5
4
3
2
1
00
00
00
00
00
0
3
a
Other nucleophiles than TMSN were also investigated. Within
all the entries, the asymmetric ring opening using 3a as catalyst
and water as the nucleophile gave the highest ee of 57% (Table
3
linear fit of 3a
3b
linear fit of 3b
3
, entry 1). The same reaction catalyzed by 3b (Table 3, entry 2),
gave both a low enantioselectivity and yield. For the other
nucleophiles studied (PhSH, TMSCN and TMSCl), both catalysts
3a and 3b gave low ee’s in the resulting products (Table 3,
entries 3-8). In terms of absolute configuration, for most
nucleophiles studied, the major enantiomers follows that of the
0
1
2
3
4
[
cat.](mM)
ring opening of epoxides by TMSN
O was employed as the nucleophile and 3a/3b as the catalyst,
the opposite major absolute configuration of the product, i.e.
R,R), was favored.
3
, i.e. (S,S). In contrast, when
H
2
Figure 4. Fitting of a first order kinetic model to the observed kinetic data for
the ring-opening of cyclohexene oxide with pseudo-Cs complexes 3a and 3b
(
under solvent-free conditions.
Table 3. Ring-opening of cyclohexene oxide with various nucleophiles
catalyzed by pseudo-C bissalen complexes under solvent-free conditions.[a]
The initial reaction rates for catalysis with 3a and 3b under
solvent-free conditions were then investigated with catalyst
concentrations varying from 0.01 to 0.1 mol%. The rates fit a
first-order kinetic model with respect to the concentration of the
catalyst, which is in agreement with a dominating bimolecular
s
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COMMUNICATION
[
1]
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Entry
NuR
Catalyst (mol%)
Yield[b]
ee[e] (%)
t(d)
Understanding Chemical Reactivity, Vol.
5 (Eds.: P. G. Mezey),
(%)
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1
2
3
4
5
6
7
8
H
2
O
O
3a(0.1)
3b(0.1)
3a(0.1)
3b(0.1)
3a(0.1)
3b(0.1)
3a(0.1)
3b(0.1)
5
57[c][g]
17[c][g]
15[d][h]
17[d][h]
5
5
7
7
6
6
1
1
2
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H2
31
> 99
> 99
10
5
PhSH
PhSH
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TMSCN
TMSCN
TMSCl
TMSCl
4
4
[c][f]
[c][f]
11
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[4]
Determined by H NMR spectroscopy with 1,2,4,5-tetramethylbenzene as
1
internal standard. [c] Determined by GC. [d] Determined by HPLC. e] The
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based on the desymmetrization of meso-ligands containing two
enantiotopic coordination sites by the coordination of two
different metal ions, have been designed, synthesized and
evaluated in catalysis. The system was shown capable of
enantiotopic discrimination in asymmetric nucleophilic ring
opening of meso-epoxides with up to quantitative yields, and
with up to 76% ee. As such, the study touches upon how
seemingly small modifications that break the symmetry of a
meso-ligand can be used to induce substantial ee’s in the
products of catalytic reactions. Beyond synthetic considerations,
we envision that catalysts of the described type could find
applications as mechanistic probes: The two metal ions are
surrounded by complimentary chiral environments, thus the
enantiomeric outcome of a reaction should reflect the role of
each metal ion in the stereo-discriminating step. Finally, a
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the presence of two different spheres at positions related by a
mirror plane is not without a certain aesthetic appeal (Figure 2,
bottom). Further investigation of this catalytic system in other
contexts as well as applications in synthesis are under way and
will be reported in due course.
9
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Acknowledgements
We thank the Swedish Research Council, the Royal
Physiographic Society of Lund and the Crafoord foundation for
grants. We thank Chinese Scholarship Council for PhD
scholarship to Yutang Li. We thank the Crafoord foundation for
postdoc fellowships to Zeyun Xiao, Dayou Ma, and Emil
Lindbäck.
2
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Conflict of interest
(
J. Am. Chem. Soc. 2015, 137, 14232.
15] D.-Y. Ma, Z.-Y. Xiao, J. Etxabe, K. Wärnmark, ChemCatChem 2012, 4,
The authors declare no conflict of interest.
Keywords: pseudo-meso • bissalen • asymmetric ring opening
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Entry for the Table of Contents
COMMUNICATION
Yutang Li, Anna Lidskog, Helena
Armengol-Relats, Thanh Huong Pham,
Antoine Favraud, Maxime Nicolas, Sami
Dawaigher, Zeyun Xiao, Dayou Ma, Emil
Lindbäck, Daniel Strand*, Kenneth
Wärnmark*
Page No. – Page No.
Title
Enantiotopic Discrimination by
Coordination-Desymmetrized meso-
Ligands
Just barely chiral: A meso-ligand, desymmetrized by coordination of two different
metal ions, gives a pseudo-meso catalyst. This catalyst system mediates opening of
meso-epoxides in moderate ee’s and excellent yields.
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