Asymmetric neutral amination of nitroolefins catalyzed by chiral bifunctional ammonium salts in water-rich biphasic solvent

Article abstract of DOI:10.1002/anie.201101307

It′s just a phase: Environmentally benign title reaction was achieved under neutral phase-transfer conditions in the presence of 0.05 mol% of a chiral bifunctional ammonium bromide. The importance of bifunctional design of the chiral phase-transfer catalysts (PTC) was clearly shown in the transition-state model of the reaction based on the single-crystal X-ray structure. Bn=benzyl, Boc=tert-butoxycarbonyl.

Full text of DOI:10.1002/anie.201101307

DOI: 10.1002/anie.201101307
Phase-Transfer Catalysis
Asymmetric Neutral Amination of Nitroolefins Catalyzed by Chiral
Bifunctional Ammonium Salts in Water-Rich Biphasic Solvent**
Lijia Wang, Seiji Shirakawa, and Keiji Maruoka*
The development of new, catalytic asymmetric transforma-
tions using a chiral metal-free catalyst in water solvent under
neutral conditions with excellent atom economy is one of the
most ideal approaches in current asymmetric synthesis in view
of environmental awareness.[1] Accordingly, we are interested
in the possibility of realizing such an ideal transformation, and
hence want to develop an environmentally benign asymmetric
conjugate amination to nitroolefins[2,3] under essentially
neutral conditions to fulfill the ideals of the four important
keywords—metal-free, water, neutral, and atom economy—
as described above. This transformation allows the efficient
asymmetric synthesis of chiral 1,2-diamino compounds, which
are useful chiral building blocks in pharmaceutical areas.[4]
Herein, we report our initial study on this subject by
developing an asymmetric neutral amination to nitroolefins
catalyzed by chiral bifunctional tetraalkylammonium salts of
Scheme 1. Asymmetric neutral amination catalyzed by chiral ammoni-
um salts.
type 2 with very low catalyst loading (0.05 mol%) in water-
rich biphasic solvent (Scheme 1).
At first, we examined the screening of binaphthyl-
modified chiral ammonium salts as phase-transfer catalysts[5]
for conjugate amination[6–8] of nitroolefins under neutral
conditions in water-rich biphasic solvent (Scheme 1).[9] An
attempted reaction of tert-butyl benzyloxycarbamate and b-
nitrostyrene in H2O/toluene (10:1) with 1 mol% of (S)-1a[10]
as chiral phase-transfer catalyst at room temperature (258C)
for 24 hours afforded an amination product in low yield with
low enantioselectivity (Table 1, entry 1). Replacing 3,4,5-
trifluorophenyl groups of catalyst (S)-1a by 3,5-bis(trifluor-
omethyl)phenyl groups (1b) did not improve the result
(Table 1, entry 2). The use of a morpholine-derived catalyst
(S)-1c slightly improved the enantioselectivity but with low
yield (39% ee; Table 1, entry 3). Next, we examined bifunc-
tional ammonium bromides of type 2[9,11] possessing diary-
lhydroxymethyl groups. Pleasingly, a dramatic improvement
in both the yield and enantioselectivity was attained in the
reaction with a bifunctional-type catalyst (S)-2a (69% yield,
75% ee; Table 1, entry 4). Exchange of the morpholine
skeleton of catalyst (S)-2a to the piperidine skeleton
improved the yield and enantioselectivity (91% yield,
82% ee; Table 1, entry 5). Switching the catalyst to (S)-2c,
which possesses radially extended aromatic substituents (Ar),
further improved the selectivity (88% ee; Table 1, entry 6).[12]
Table 1: Screening of the optimum condition.[a]
Entry Catalyst
(mol%)
Amination agent t [h] Yield [%][b] ee [%][c]
(R1NHR2)
1
2
3
4
5
6
(S)-1a (1)
(S)-1b (1)
(S)-1c (1)
(S)-2a (1)
(S)-2b (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
(S)-2c (1)
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNHOBn
BocNH2
24
24
24
24
13
4
4
4
36
4
4
5
17
12
69
91
90
90
82
90
ꢀ0
99
56
73
91
76
16
14
39
75
82
88
88
84
93
–
7[d]
8[e]
9[f]
10
11
12
13
14
15
BnONH2
TsNHOBn
MeO2CNHOBn
ꢀ0
[*] Dr. L. Wang, Dr. S. Shirakawa, Prof. Dr. K. Maruoka
Laboratory of Synthetic Organic Chemistry and Special
Laboratory of Organocatalytic Chemistry
4
4
8
72
3[g]
25[g]
90
(S)-2c (0.05) BocNHOBn
(S)-2c (0.01) BocNHOBn
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo, Kyoto 606-8502 (Japan)
Fax: (+81)75-753-4041
91
[a] Reaction conditions: b-nitrostyrene (0.050 mmol) and amination
agent (0.25 mmol) in the presence of phase-transfer catalyst in H2O
(2.0 mL)/toluene (0.20 mL) at room temperature (258C). [b] Yield of
isolated product. [c] Determined by HPLC on a chiral stationary phase.
[d] Reaction was performed in H2O/toluene=50:1. [e] Reaction was
performed in H2O/toluene=100:1. [f] Reaction was performed at 08C.
[g] Absolute configuration of the major isomer was not determined. Bn=
benzyl, Boc=tert-butoxycarbonyl, Ts=4-toluenesulfonyl.
E-mail: maruoka@kuchem.kyoto-u.ac.jp
[**] This work was supported by a Grant-in-Aid for Scientific Research
from the MEXT (Japan) and The Association for the Progress of New
Chemistry. We are grateful to Dr. Hiroyasu Sato (Rigaku Corpo-
ration) for X-ray crystallographic analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101307.
Angew. Chem. Int. Ed. 2011, 50, 5327 –5330
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5327

Communications
Table 2: Asymmetric amination of nitroolefins catalyzed by (S)-2c under
Additionally, even in the further water-rich biphasic solvent
neutral conditions.[a]
system (H2O/toluene = 50:1–100:1), catalyst (S)-2c efficiently
promoted the reaction and gave an amination product with
high enantioselectivity (Table 1, entries 7 and 8).[13] The
highest enantioselectivity was attained when lower temper-
ature (08C) was employed with catalyst (S)-2c (93% ee;
Table 1, entry 9).
Other amination agents were also examined in the
asymmetric amination of b-nitrostyrene in the presence of
chiral ammonium bromide (S)-2c. The use of tert-butyl
carbamate did not give any amination product (Table 1,
entry 10). On the other hand, the reaction using N-benzyloxy-
amine proceeded smoothly and amination product was
obtained in high yield, however, with almost no selectivity
(Table 1, entry 11). The reactions with N-benzyloxytosyl-
amine or methyl benzyloxycarbamate also gave the corre-
sponding amination products with low enantioselectivities
(Table 1, entries 12 and 13). These results indicate that both a
tert-butoxycarbonyl and benzyloxy group on the nitrogen
atom of the amination agent are necessary to obtain high yield
with high selectivity.
The practicability of the present reaction was shown in
decreasing the catalyst loading of chiral ammonium bromide
(S)-2c. The reaction using only 0.05 mol% of catalyst (S)-2c
promoted the amination efficiently at room temperature, and
gave the product in high yield with high enantioselectivity
(91% yield, 90% ee; Table 1, entry 14). Further decrease of
the catalyst loading to 0.01 mol% still gave the product in
good yield (76% yield, TON 7600) and high selectivity
(91% ee) with prolonged reaction time (Table 1, entry 15).
It should be noted that the present reaction does not work
well under the ordinary phase-transfer reaction conditions
using aqueous base solutions, such as aqueous solutions of
KOH, K2CO3, and PhCOOK, and under the homogeneous
reaction conditions without aqueous solution (Scheme 2).
Entry
R
Conditions[b]
t [h]
Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
8
Ph
Ph
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
8
36
16
36
16
36
36
72
10
68
15
72
18
72
16
48
8
91 (3a)
90 (3a)
93 (3b)
90 (3b)
91 (3c)
89 (3c)
70 (3d)
62 (3d)
94 (3e)
92 (3e)
85 (3 f)
70 (3 f)
93 (3g)
81 (3g)
90 (3h)
82 (3h)
95 (3i)
99 (3i)
97 (3j)
99 (3j)
90
93
90
91
91
94
90
93
92
95
90
91
90
94
90
93
77
82
79
83
4-MeC6H4
4-MeC6H4
4-BrC6H4
4-BrC6H4
3-FC6H4
3-FC6H4
4-TBSOC6H4
4-TBSOC6H4
2-naphthyl
2-naphthyl
2-thienyl
2-thienyl
2-furyl
9
10
11
12
13
14
15
16
17
18
19
20
2-furyl
(CH3)2CHCH2
(CH3)2CHCH2
tBu
32
6
17
tBu
[a] Reaction conditions: nitroolefin (0.050 mmol) and tert-butyl benzy-
loxycarbamate (0.25 mmol) in the presence of catalyst (S)-2c (0.05 or 1
mol%) in H2O (2.0 mL)/toluene (0.20 mL) at room temperature or 08C.
[b] Conditions A: (S)-2c (0.05 mol%) at room temperature (258C);
conditions B: (S)-2c (1 mol%) at 08C. [c] Yield of isolated product.
[d] Determined by HPLC on a chiral stationary phase.
loading (0.05 mol%; conditions A). The reactions of nitro-
olefins having various aromatic and heteroaromatic groups
under the conditions A gave the corresponding amination
products 3 in high enantioselectivities (90–92% ee) with good
to high yields (Table 2, entries 1–15 (odd-number entries)).
The reactions with nitroolefins possessing alkyl group gave
the products in good enantioselectivities (77–79% ee) with
high yields (Table 2, entries 17 and 19). The reactions at lower
temperature (08C) with 1 mol% of catalyst (S)-2c (condi-
tions B) slightly improved the enantioselectivities and gave
the amination products 3 in 91–95% ee for nitroolefins with
aromatic and heteroaromatic groups, and 82–83% ee for
nitroolefins with alkyl group (Table 2, entries 1–15 (even-
numbered entries)). Notably, substrates containing unstable
functional groups under basic and acidic conditions, such as
siloxy groups,[14] are tolerated in the present neutral phase-
transfer conditions (Table 2, entries 9 and 10). Furthermore,
the amination was examined in the presence of other
compounds containing base- and/or acid-sensitive functional
groups, such as 4–6, and we found that the asymmetric
amination was performed without any decomposition (e.g.
retro-aldol reaction, dehydration) and loss of stereochemical
information in the presence of compounds 4–6 under the
Scheme 2. Effect of a base and aqueous solution.
Hence, the highly enantioselective conjugate amination of
nitroolefins was only achieved when the reaction was
performed under the base-free, neutral phase-transfer con-
ditions in water-rich biphasic solvent.
With optimal reaction conditions in hand, we further
studied the generality of the asymmetric conjugate amination
to nitroolefins under the neutral conditions in the presence of
chiral bifunctional ammonium bromide (S)-2c. Two standard
reaction conditions were examined for each substrate (con-
ditions A and B in Table 2). Various types of nitroolefins were
found to be employable for the reaction with low catalyst
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Angew. Chem. Int. Ed. 2011, 50, 5327 –5330

neutral phase-transfer conditions. These results support the
fact that complex substrates containing base- and/or acid-
sensitive functional groups can be applied to this neutral
phase-transfer reaction system.
The resulting amination products 3 can be readily trans-
formed into the corresponding 1,2-diamines, which are
versatile chiral building blocks from synthetic as well as
pharmaceutical viewpoints.[4] For example, exposure of a
mixture of 3a in MeOH to a hydrogen atmosphere in the
presence of Raney Nickel at room temperature for 24 hours
resulted in the formation of N-Boc-protected 1,2-diamine 7[15]
in 72% yield with complete preservation of the enantiopurity
(Scheme 3).
hydroxy group in the binaphthyl unit and the oxygen atom of
amide anion (O1) is clearly observed in the crystal structure
of (S)-2d.
Based on the X-ray structure of (S)-2d, plausible tran-
sition-state models have been proposed to account for the
observed absolute configuration of amination products 3
(Figure 1). In the generation of a chiral ammonium amide
derived from tert-butyl benzyloxycarbamate and the catalyst
(S)-2c under neutral conditions, the amide anion would be
stabilized by both the ionic interaction of the ammonium
amide and the hydrogen bond between the hydroxy group in
catalyst (S)-2c and the oxygen atom of the amide moiety (A
in Figure 1). Then, nitrostyrene might approach from the
upper side, thereby avoiding the steric repulsion between tert-
butoxycarbonyl group of amide and phenyl group of nitro-
styrene (B @ C in Figure 1), to account for the observed
absolute configuration in 3a.[17] This explanation on the steric
effect of the tert-butoxycarbonyl group for obtaining the high
enantioselectivity was supported by the result of the reaction
using less-hindered methyl benzyloxycarbamate, which gave
low enantioselectivity (Table 1, entry 13).
Scheme 3. Transformation of the amination product into 1,2-diamine.
In summary, we have successfully developed a highly
efficient catalytic asymmetric amination of nitroolefins under
neutral phase-transfer conditions under the influence of a
chiral bifunctional tetraalkylammonium bromide in water-
rich biphasic solvent. The advantage of the base-free, neutral
phase-transfer reaction system for asymmetric amination was
clearly demonstrated in comparison with the reaction under
ordinary phase-transfer conditions using aqueous base solu-
tion. Furthermore, the role of hydroxy groups in the bifunc-
tional catalyst was clearly shown in the transition-state model
of the reaction based on the single-crystal X-ray structure of
an ammonium amide. Further application to other asymmet-
ric reactions and the mechanistic study of the neutral phase-
transfer reaction system using a chiral bifunctional ammoni-
um salt are currently underway.
To obtain insights into a transition-state structure for the
present asymmetric amination, we carried out X-ray crystal
structure analysis of ammonium amides prepared from chiral
bifunctional ammonium salts of type 2. Several kinds of
ammonium amides were prepared to obtain crystals suitable
for X-ray diffraction analysis, and finally we succeeded to
obtain a single-crystal X-ray structure of (S)-2d (Figure 1).[16]
The crystal structure of (S)-2d provides important structural
information. The bond lengths of amide moiety (O1-C1:
1.304 ꢀ, N1-C1: 1.309 ꢀ) indicate that the negative charge of
the amide anion is delocalized as shown in Figure 1.
Importantly, the hydrogen-bonding interaction between the
Received: February 22, 2011
Published online: April 28, 2011
Keywords: amination · asymmetric synthesis ·
.
conjugate addition · organocatalysis · phase-transfer catalysis
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Angew. Chem. Int. Ed. 2011, 50, 5327 –5330
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