Reversal of enantioselectivity by tuning the conformational flexibility of phase-transfer catalysts

Article abstract of DOI:10.1002/anie.200906814

(Figure Presented) Towards perfect asymmetric catalysis: When binol-derived N-spiro quaternary ammonium salts were used as phase-transfer catalysts in the conjugate addi-tion of nitroalkanes to chalcones and its analogues, an intriguing reversal of enan-t

Full text of DOI:10.1002/anie.200906814

Communications
DOI: 10.1002/anie.200906814
Phase-Transfer Catalysis
Reversal of Enantioselectivity by Tuning the Conformational
Flexibility of Phase-Transfer Catalysts**
Ming-Qing Hua, Han-Feng Cui, Lian Wang, Jing Nie, and Jun-An Ma*
Catalytic asymmetric synthesis provides one of the most
powerful and economical approaches for the preparation of a
variety of enantiomerically enriched compounds that are
critical to developments in medicine, biology, and materials
science.[1] In this scenario, the development of environmen-
tally friendly, highly efficient, and selective chiral catalysts is
important. Therefore, one crucial objective is the design and
synthesis of new chiral catalysts, which enable challenging
and/or previously unknown asymmetric transformations to
occur in a highly efficient way. The requirement of maximum
conformational rigidity is central to the design of a chiral
catalyst. The rigid structure of the catalyst would enhance the
enantiofacial differentiation by minimizing the possibilities of
different conformers available to the coupling partners, and
thus, deliver the maximum asymmetric induction from the
chiral catalyst.[2] These semiempirical criteria have been
applied to the creation of thousands of chiral catalysts in
accord with the increasing need for enantiopure medicinal
agents and the rapid advancement of the field of asymmetric
synthesis.[3] Furthermore, most efficient catalysts with rigid
structures, such as cinchona alkaloids, salen complexes, and
binol (1,1’-bi-2-naphthol) derivatives, have demonstrated
useful levels of enantioselectivity for a wide range of different
asymmetric reactions.[4] On the other hand, the conforma-
tional flexibility is another fundamental characteristic of a
chiral catalyst. Keeping the conformational flexibility at an
optimal level is also essential to the catalyst reactivity and
stereoselectivity. However, the effect of an appropriate
balance between the conformational rigidity and flexibility
in asymmetric catalysis has largely been ignored.[5] Herein we
report the development of new chiral quaternary ammonium
salts as phase-transfer catalysts (PTC) based on the concept of
a linker-dictated structure that tunes rigidity and flexibility.
This strategy led to the discovery of two catalysts that give
access to both enantiomeric products of a catalyzed addition
reaction from a common chirality source.
In recent years, chiral phase-transfer catalysis has
emerged as an area of intense interest in asymmetric synthesis
owing to its operational simplicity and mild reaction con-
ditions.[6] Although impressive new advances have been
made, there appears a growing number of challenging
substrates and difficult transformations that cannot be
promoted by existing phase-transfer catalysts. One such
instance involves the conjugate addition of nitroalkanes to
enones to give products that are useful and versatile
precursors for a variety of structures such as aminocarbonyl
compounds, aminoalkanes, and pyrrolidines.[7] Although the
reactions can be successfully carried out with simple, unhin-
dered linear nitroalkanes in high enantioselectivity,[8] utiliza-
tion of sterically more demanding nitroalkanes for the
reaction of bulky b-aryl-substituted enones such as the
chalcones can be less rewarding.[9] Variable levels of asym-
metric induction with unpredictable stereoselectivity were
obtained when cinchona-alkaloid-derived quaternary ammo-
nium salt 3[9b] and the sugar-based azacrown ether 4[9c,d] were
used as chiral catalysts (Scheme 1). The structurally rigid and
highly reactive chiral phase-transfer catalysts based on the
binaphthyl scaffold, such as the N-spiroammonium salts 5 and
6, which were recently developed by Maruoka et al.,[6e] and
the analogous phosphonium salt 7,[6s] gave poor performance
for the addition of 2-nitropropane to chalcone. The reactions
were plagued with extremely low yield and disappointing
ee values (< 10%) as indicated by the results shown in
Scheme 1.
We surmised that dual activation of both the nucleophile
and the electrophile is necessary for this particularly chal-
lenging reaction. As depicted in Scheme 2, the modular
dimeric structure 8 was anticipated to provide a potential
entry into a chiral catalyst to fulfill such a goal. It is further
expected that the key to creating a positive synergy between
the two chiral fragments is the choice of a flexible linker. The
linker we have chosen to explore can provide us with the
following opportunities for the fine-tuning of the reactivity
and stereoselectivity of the catalysts: 1) to regulate the
distance between the quaternary ammonium centers by
varying the chain length, so that the reactants can be
synergistically stabilized and/or activated; 2) to maintain an
optimal relative orientation of the two chiral binaphthyl
moieties, by virtue of the flexibility of the -(CH2)n- chain
[*] M.-Q. Hua, H.-F. Cui, L. Wang, J. Nie, Prof. J.-A. Ma
Department of Chemistry, Tianjin University
Tianjin 300072 (China)
Fax: (+86)22-2740-3475
E-mail: majun_an68@tju.edu.cn
Prof. J.-A. Ma
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Science
Shanghai 200032 (China)
À
through C C bond rotation, to achieve a high level of
enantioselectivity. The latter requires the nitronate nucleo-
phile to be delivered to either the Re or the Si face of the
enone. The use of the piperidine rings for the construction of
the dimeric structure was based on the anticipation that the
resulting N-spiroammonium centers, which bear the
binaphthyl chiral elements, help preserve the rigidity of the
[**] This work was financially supported by the NSFC (no. 20772091 and
20972110). We thank Prof. Qingwei Yao, Northern Illinois Univer-
sity, for his helpful discussions and suggestions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906814.
2772
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Angew. Chem. Int. Ed. 2010, 49, 2772 –2776

Angewandte
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chiral motifs. The dicationic nature of 8 could also be used to
its advantage in catalyst recycling and reuse.
Starting from the commercially available (S)-binol, a
series of (S)-3,3’-disubstituted 2,2’-bi(bromomethyl)-1,1’-
binaphthyl derivatives were readily synthesized through an
established protocol.[6j] The final quaternary ammonium
bromides 8 were prepared by treatment with the correspond-
ing appropriate bipiperidines in the presence of K2CO3 in
CH3CN and were purified by simple chromatography on silica
gel or recrystallization, and gave the desired products
(Scheme 2). Crystals suitable for X-ray crystallographic
analysis were obtained for two of the bisquaternary ammo-
nium salts, compounds 8j and 8l (see the Supporting
Information).[10] As expected, the structural rigidity of the
two chiral centers in both catalysts are well preserved and the
two structures differ mainly in the size of the open cavity and
the relative orientation of the chiral binaphthyl units, the
latter could be a crucial element responsible for the sense of
the stereoselectivity of each catalyst.
Preliminary screening on their reactivity as chiral phase-
transfer catalysts was performed using the conjugate addition
of 2-nitropropane with chalcone 1a in the presence of K2CO3
in toluene (see the Supporting Information). Pleasingly,
catalysts 8d–l (Table 1, entries 4–17) with bulky substituents
at the 3,3’-position of the binaphthyl scaffold significantly
outperformed all of the monomeric catalysts listed in
Scheme 1. Catalysts 8a–c, which lack substituents at the
3,3’-position, were much less effective (Table 1, entries 1–3),
thus clearly demonstrated the beneficial effect of the steric
bulkiness at these positions as a necessary feature of the
binaphthyl unit. Notably, a reversal of enantioselectivity that
is directly related to the nature of the linker was achieved: the
reaction gave adduct (R)-2a as the major enantiomer with
catalysts 8a, 8d, 8g, or 8j (n = 0; Table 1, entries 1, 4, 7, and
10), whereas (S)-2a was the predominant enantiomer with
catalysts 8h, 8i, 8k, or 8l (n = 2 or 3; Table 1, entries 8, 9, 11,
and 12).[11] The linker-dependent enantioselectivity switching
became most obvious with catalysts 8j and 8l, for which
product 2a was obtained in essentially quantitative yields and
with completely reversed enantioselec-
Scheme 1. Catalytic asymmetric conjugate addition of 2-nitropropane
to chalcones 1 with the reported phase-transfer catalysts 3–7.
tivity (e.r. = 4:96 for 8j and e.r. = 97:3 for
8l). Therefore, catalysts 8j and 8l, which
differ only in the length of the methylene
chain of the linker, can be used as a
surrogate of each otherꢀs enantiomer.
This outcome represents a rare example
of accessing both enantiomers of an
asymmetric transformation by using cat-
alysts possessing a common chiral ele-
ment. Further improvement in the enan-
tioselectivity was achieved using 8l when
the reaction was run at À208C (e.r. =
99.9:0.1; Table 1, entry 14). It was also
found that the loading of 8l could be as
low as 0.25 mol% without sacrificing the
enantioselectivity (Table 1, entries 15 and
16). Similarly, adduct (R)-2a was
obtained in 98.5:1.5 e.r. using catalyst 8j
Scheme 2. Synthesis of a series of new phase-transfer catalysts 8.
Angew. Chem. Int. Ed. 2010, 49, 2772 –2776
at À208C (Table 1, entry 17).
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2773

Communications
After 8l and 8j were identified as highly reactive and
sal of the absolute configuration along with high e.r. was also
observed (Table 2, entries 25–28).[12]
enantioselective catalysts for the conjugate addition of 2-
nitropropane to 1a, the scope of this reaction was further
examined under the optimized conditions with a series of
other chalcone derivatives or analogues as the electrophile
(Table 2). The reaction proved to be exceptionally general
These adducts 2 are versatile synthetic intermediates and
can be readily transformed into functionalized amine deriv-
atives that are otherwise difficult to access. For example,
direct hydrogenation of both enantiomers of 2a in the
presence of Raney Ni provided
(S)- and (R)-aminoalkanes 9 in
high yields with excellent enantio-
meric purities.[13] Reduction with
Zn in AcOH delivered the highly
functionalized substituted pyrroli-
dine 10 in both enantiomeric from
(S)- and (R)-2a in a one-pot trans-
formation with high diastereo- and
enantioselectivity (Scheme 3).
These results show that com-
pounds 8j and 8l are very efficient
phase-transfer catalysts for enantio-
selective synthesis. However, owing
to the relatively high molecular
weight of these quaternary ammo-
nium salts, the mass of the cata-
Table 1: Screening and optimization of new phase-transfer catalysts 8 for enantioselective conjugate
addition of 2-nitropropane to chalcone 1a to give 2a.[a]
Entry
PTC (mol%)
T [8C]
t [h]
Yield [%][b]
e.r.[c]
30:70
50:50
52.5:47.5
44.5:55.5
50:50
52.5:47.5
24.5:75.5
66.5:33.5
92.5:7.5
4:96
64:36
97:3
99.5:0.5
99.9:0.1
99:1
Config.[d]
1
2
3
4
5
6
7
8
8a (2.5)
8b (2.5)
8c (2.5)
8d (2.5)
8e (2.5)
8 f (2.5)
8g (2.5)
8h (2.5)
8i (2.5)
8j (2.5)
8k (2.5)
8l (2.5)
8l (2.5)
8l (1.0)
8l (0.5)
8l (0.25)
8j (1.0)
0
0
0
0
0
0
0
0
0
72
72
72
24
24
24
24
24
24
24
24
24
48
60
72
96
60
40
80
61
98
99
99
97
98
99
99
99
99
99
99
85
84
95
R
–
S
R
–
S
R
S
S
R
S
S
S
S
S
S
R
lyst—even at
a
loading of
9
0.25 mol%—is still high for their
large-scale application. Fortunately,
this drawback can be overlooked
owing to the practical advantage
that the chiral catalyst is easily
recovered in excellent yield after
one simple regenerating process.
The recovered catalyst can be
reused without any loss of reactivity
and enantioselectivity.[14]
10
11
12
13
14
15
16
17
0
0
0
À20
À20
À20
À20
À20
99:1
1.5:98.5
[a] See the Supporting Information for details. [b] Yield of isolated product. [c] Determined by HPLC
analysis on a chiral stationary phase. [d] The absolute configuration was assigned according to
references [9b–d].
As a result of high activity and
stereoselectivity, catalysts 8j and 8l
and their variants represent a prom-
and efficient. It tolerated the inclusion of a variety of
sterically and electronically different substituent groups on
both aryl rings (Table 2, entries 1–16). Catalysts 8l and 8j
gave very comparable yields and e.r. while they delivered the
product with a universal reversal of absolute configuration for
all substrates tested. The scope of this reaction was not limited
to chalcones, as substitution of the benzene rings with
heterocycles, such as furan and thiophene, proved uneventful
(Table 2, entries 17–22). When an
ising new generation of chiral phase-transfer catalysts for
asymmetric synthesis. The conformational rigidity and flex-
ibility are two indispensable features of a chiral catalyst.
These studies indicate the broad potential for regulating the
conformational rigidity and flexibility of chiral molecules in
future catalyst design. Further investigation of the mecha-
nism[15] and the application of these catalysts to other
reactions are ongoing.
enone derived from aliphatic ketone
was employed, the reversal of enan-
tioselectivity was also realized, albeit
with modest yields and e.r. (Table 2,
entries 23 and 24). To verify further
the reversal of the enantioselectivity
by using these catalysts, other nucle-
ophilic donors with bulkier groups,
such as nitrocyclopentane and nitro-
cyclohexane, which have rarely been
used in conjugate addition owing to
their intrinsic low reactivity, were also
tested. Gratifyingly, successful rever- Scheme 3. Synthetic transformations of the chiral nitroketones 2a.
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Table 2: The scope of conjugate addition catalyzed with 8j and 8l.[a]
Entry
Product
PTC Yield [%][b] e.r.[c]
Entry Product PTC
Yield [%][b] e.r.[c]
1
2a
2a
2b
8l
8j
8l
99
95
94
99.9:0.1
1.5:98.5
97.9:2.1
15[d]
16[d]
17[d]
2h
2h
2i
8l 90
8j 87
8l 98
98.9:1.1
2.4:97.6
96.3:3.7
2
3
4
2b
8j
92
2.9:97.1
18[d]
2i
8j 95
7.6:92.4
5
6
2c
2c
8l
8j
92
89
96.7:3.3
3.2:96.8
19
20
2j
2j
8l 99
8j 90
98.5:1.5
4.1:95.9
7
2d
2d
2e
2e
2 f
2 f
8l
8j
8l
8j
8l
8j
99
92
97
83
91
88
99.1:0.9
2.3:97.7
99.1:0.9
1.8:98.2
99.6:0.4
1.9:98.1
21[d]
22[d]
23
2k
2k
2l
8l 97
8j 86
8l 43
8j 54
8l 98
8j 80
96.7:3.3
0.8:99.2
83.3:16.7
11.5:88.5
98.3:1.7
3.8:96.2
8
9
10
11
12
24
2l
25[d]
2m
2m
26[d]
13[d]
2g
2g
8l
8j
99
84
99.1:0.9
6.6:93.4
27[d]
2n
2n
8l 98
98.0:2.0
14[d]
28[d]
8j 88
14.7:85.3
[a] Unless otherwise noted, all reactions were carried out with 1 mol% of catalyst 8l or 8j at À208C for 48–96 h. [b] Yield of isolated product.
[c] Determined by HPLC analysis on a chiral stationary phase. The absolute configuration of adducts 2b–n were assigned on the basis of analogy with
studies of 2a. [d] Reaction carried out at 08C.
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Received: December 3, 2009
Revised: February 9, 2010
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Keywords: asymmetric synthesis · conformational flexibility ·
.
conjugate addition · enantioselectivity · phase-transfer catalysis
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[12] For the addition of nitroethane to chalcone: adduct isolated in
94% yield, d.r. = 33:67 (syn/anti), e.r. = 93:7 (syn), e.r. = 95.5:4.5
(anti) (by using catalyst 8l).
[13] Interestingly, under these conditions the aryl ketone function-
ality underwent a complete deoxygenative reduction to form a
methylene group.
[14] For example, catalyst 8l (1 mol%) was used in the reaction of 2-
nitropropane with 1a on a 30 mmol scale, and delivered adduct
(S)-2a in 99% yield with e.r. = 99.0:1.0; it was recovered in pure
form in 97% yield. See the Supporting Information for
experimental details for the recycling and reuse of the catalyst.
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methylpiperidine were prepared and used for the addition of 2-
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