Rhodium-catalyzed asymmetric addition of arylboronic acids to indolylnitroalkenes

Article abstract of DOI:10.1002/ejoc.201101648

Indolylnitroethanes and their derivatives are key intermediates to many bioactive structures. Most approaches to access chiral indolylnitroethanes involve organocatalyzed or metal-catalyzed asymmetric Friedel-Crafts reaction of indoles with nitroalkenes.

Full text of DOI:10.1002/ejoc.201101648

FULL PAPER
DOI: 10.1002/ejoc.201101648
Rhodium-Catalyzed Asymmetric Addition of Arylboronic Acids to
Indolylnitroalkenes
Junwei Xing,[a,b,c] Guihua Chen,[a,b,c] Peng Cao,[b] and Jian Liao*[a,b,c]
Keywords: Synthetic methods / Asymmetric synthesis / Homogeneous catalysis / Rhodium / Heterocycles
Indolylnitroethanes and their derivatives are key intermedi-
ates to many bioactive structures. Most approaches to access
chiral indolylnitroethanes involve organocatalyzed or metal-
catalyzed asymmetric Friedel–Crafts reaction of indoles with
nitroalkenes. We have developed an efficient approach to
optically pure α-aryl-3-indolylnitroethanes through rhodium-
catalyzed asymmetric 1,4-addition of arylboronic acids to
indolylnitroalkenes. Excellent yields (up to 99%) and enan-
tiomeric excesses (up to 99% ee) of chiral indolylnitro-
ethanes were achieved under mild conditions.
Introduction
Indolylnitroethanes and their derivatives, which are key
intermediates to many bioactive structures, are an impor-
tant class of heterocyclic compounds.[1] To date, most ap-
proaches to access chiral indolylnitroethanes involve the use
of organo- or metal-catalyzed asymmetric Friedel–Crafts
reactions of indoles with nitroalkenes;[2–3] other strategies
are less developed. Originally developed by Hayashi and
Miyaura, rhodium-catalyzed asymmetric 1,4-addition of
organoboron reagents to electron-deficient olefins is an ef-
ficient transformation for the construction of new carbon–
carbon bonds.[4–6] Many successful results have been
achieved on the asymmetric addition of organoboron rea-
gents to α,β-unsaturated ketones, esters, amides, and so on.
However, less studies have focused on nitroalkenes during
the past decade.[7–11] Recently, Lin’s group[10] and our
group’s[11] developed Rh/chiral diene and Rh/chiral sulfox-
ide–phosphane complexes, respectively, to realize the ad-
dition of arylboronic reagents to styrene-type nitroalkenes.
To the best of our knowledge, asymmetric catalytic addition
of arylboronic acids to heteroarylnitroalkenes has not been
reported. The impetus of this work was to find another ef-
fective strategy to access chiral indolylnitroethanes
(Scheme 1).
Scheme 1. Synthetic strategies for 3-indolylnitroethane.
Results and Discussion
We initially carried out the reaction of N-benzyl-pro-
tected 3-indolylnitrioalkene (1a) with phenylboronic acid
(2a) in the presence of rhodium/sulfoxide–phosphane com-
plex L1 (Scheme 2) under our previously reported reaction
conditions:[11] ethanol, triethylamine (TEA) as the base,
and stirring at 40 °C for 10 h. The expected indolylnitro-
ethane adduct was afforded in excellent yield and enantio-
selectivity (98% yield and 97% ee; Table 1, entry 1). The
effect of N-protecting groups was further evaluated: N-
methyl showed similar results to the N-benzyl protecting
group and the N-benzenesulfonyl group had a negative ef-
fect on the ee value (Table 1, entries 2 and 3). Surprisingly,
we found that 3-indolylnitroalkene 1d, which was without
an N-protecting group, worked well in our system. Com-
pared with styrene-type nitroalkenes, indolylnitroalkenes
(without N-protecting groups), as a class of N-heteroarylni-
[a] Key Laboratory of Asymmetric Synthesis and Chirotechnology
of Sichuan Province, Chengdu Institute of Organic Chemistry, troalkenes, are more challenging substrates than styrene-
Chinese Academy of Sciences,
type nitroalkenes because the nitrogen atom might ligate to
Chengdu 610041, P. R. China
the metal, and therefore, lead to a loss in reactivity.[12] In
[b] Natural Products Research Center, Chengdu Institute of
Biology, Chinese Academy of Sciences,
Chengdu 610041, P. R. China
this reaction, we were pleased to find that the desired prod-
uct was achieved with excellent results (96% yield and 98%
ee; Table 1, entry 4). Compound 1d was then chosen as the
model substrate for screening of ligands and other condi-
tions. Both sulfoxide–phosphane ligands L2 and L3 can
generate similar results to L1, however, the classical rho-
Fax: +86-28-8522-9250
E-mail: jliao@cib.ac.cn
[c] Graduate School of Chinese Academy of Sciences,
Beijing 100049, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101648.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1230–1236

Asymmetric Addition of Arylboronic Acids to Indolylnitroalkenes
dium–BINAP complex L4 (BINAP = 2,2Ј-bis(diphenyl- Table 1. Rhodium-catalyzed asymmetric 1,4-addition of phenyl-
boronic acid to 3-indolylnitroalkene.[a]
phosphanyl)-1,1Ј-binaphthyl) does not work in this trans-
formation (Table 1, entries 5–7). After systematic solvent
screening, we found that alcohols were the optimal solvents
in terms of yield and ee values. The reaction did not work
well in CH3CN, toluene, Et2O, dioxane, and THF (Table 1,
entries 8–18), only trace of product was obtained. Next,
various inorganic/organic bases were examined, such as
KOH, KHCO3, KF, CsF, N,N,NЈ,NЈ-tetramethylethylene-
diamine (TMEDA), 1,5-diazabicyclo[5.4.0]undec-5-ene
(DBU),
pyridine
and
1,4-diazabicyclo[2.2.2]octane
(DABCO) (Table 1, entries 19–26); TEA gave the best yield
and ee value (Table 1, entry 4).
Entry
1
Ligand Solvent Base
Yield[b]
[%]
ee[c] [%]
1
2
3
4
1a
1b
1c
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
L1
L1
L1
L1
L2
L3
L4
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
dioxane KOH
CH2Cl2 TEA
acetone TEA
EtOAc TEA
MeOH TEA
nPrOH TEA
nBuOH TEA
CH3CN TEA
toluene TEA
TEA
TEA
TEA
TEA
TEA
TEA
98
96
99
96
96
95
trace
25
57
68
95
93
95
trace
trace
trace
trace
trace
trace
trace
77
97
98
86
98(R)
97
96
n.d.
88
95
96
5
6
7[d]
8
9
Scheme 2. Ligands for this work.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
97
98
98
With the appropriate rhodium catalyst, which was
formed in situ from [{Rh(C2H4)2Cl}2] and L1, and optimal
reaction conditions in hand, various 3-indolylnitroalkenes
and arylboronic acids were investigated to examine the gen-
erality of the reaction. The results are showed in Table 2.
Most of the arylboronic acids and 3-indolylnitroalkene re-
acted smoothly under mild conditions to give the corre-
sponding products in high yields (up to 99%) and excellent
enantioselectivities (up to 99% ee). Arylboronic acids with
different substituents in the meta and para positions can be
all employed successfully to afford the desired products in
high yields (95–99%) and excellent enantioselectivities (94–
99% ee; Table 2, entries 1–5, 7, 9, 10, 12, and 13). In con-
trast, steric hindrance of the substituents on the arylboronic
acids, such as (o-methyl-, (o-methoxyphenyl)boronic acid,
and 1-naphthaleneboronic acid, had a significant effect on
the activity and only traces of products were obtained. In
addition, electron-withdrawing arylboronic acids had a
slightly negative impact on the activity and enantio-
selectivity (Table 2, entries 6, 8, and 11). Moderate yield
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
76
n.d.
97
n.d.
n.d.
97
Et2O
dioxane TEA
THF
TEA
TEA
KOH
KHCO3
KF
CsF
TMEDA 68
DBU
Pyridine
DABCO
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
trace
trace
trace
47
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), [{Rh(C2H4)-
Cl}2] (1.5 mg, 0.004 mmol), L (0.0096 mmol), base (0.1 mmol), and
EtOH (1.5 mL) at 40 °C for 10 h. [b] Isolated yield. [c] Determined
by HPLC analysis with chiral columns. The absolute configuration
was determined by comparison of the specific rotation to literature
data.[2a] n.d. = not determined. [d] Reaction temperature 100 °C
for 10 h.
(72%) and a slightly decreased enantioselectivity (83% ee) heterocyclic substrates, such as 2-nitrovinylfuran and -thio-
was provided with (4-trifluoromethylphenyl)boronic acid phene, were also chosen in this catalyst system and excellent
(Table 2, entry 8). As expected, electron-donating aryl- yields (98–99%) and modest enantioselectivities (50–76%
boronic acids are efficient for this reaction. Excellent yields ee) were achieved (Table 3).
and enantioselectivities were also obtained when phenyl-
Finally, we investigated the catalyst loading of this trans-
boronic acid was treated with substituted 3-indolylnitroalk- formation. Under the optimized conditions, in the presence
enes (Table 2, entries 14 and 15). One exception was 1g, for of 1 mol-% Rh catalyst, the reaction did not proceed at
which a modest yield (62%) was observed (Table 2, entry 40 °C, fortunately, the desired product could be obtained
16).
with 96% yield and without loss of enantioselectivity when
To further extend the scope of the reaction, this catalyst the temperature was increased to 80 °C (oil bath tempera-
system was used in the asymmetric addition of 2-indolylni- ture, Scheme 3). However, continually reducing the catalyst
troalkenes with arylboronic acids to afford the correspond- loading to 0.5 mol-% Rh or scaling up the reaction to five-
ing 2-indolylnitroethanes, which cannot be realized through fold in the presence of 1 mol-% Rh led to failure of the
direct Friedel–Crafts reactions.[13] Two other five-membered transformation. These phenomena indicated that the ac-
Eur. J. Org. Chem. 2012, 1230–1236
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1231

J. Xing, G. Chen, P. Cao, J. Liao
FULL PAPER
Table 2. Scope of the rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to 3-indolylnitroalkenes.[a]
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), [{Rh(C2H4)Cl}2] (1.5 mg, 0.004 mmol), L1 (0.0096 mmol), 50 mol-% TEA, and
EtOH (1.5 mL) at 40 °C for 10 h. [b] Isolated yield. [c] Determined by HPLC analysis with chiral columns. [d] Reaction was carried out
for 20 h.
tivity of indolylnitroalkene was lower than that of styrene-
type nitroalkenes, which could be smoothly converted into
Conclusions
adducts with a substrate/catalyst ratio 500 under the same
We have developed an efficient rhodium-catalyzed asym-
metric addition of arylboronic acids to indolylnitroalkenes
by using (tert-butylsulfinyl)phosphane as a ligand. The re-
action works well with various indolynitroalkenes and aryl-
boronic acids under mild conditions to give excellent yields
(up to 99% yield) and enantioselectivities (up to 99% ee)
of the products. This route provided an efficient option to
access optically pure α-aryl-3-indolylnitroethanes. Further
studies focused on rhodium-catalyzed asymmetric addition
of arylboronic acids to other nitroalkenes are underway in
our laboratories.
conditions.[11]
Scheme 3. Catalyst-loading test.
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1230–1236

Asymmetric Addition of Arylboronic Acids to Indolylnitroalkenes
Table 3. Rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to 2-indolylnitroalkenes and heterocyclnitroalkenes.[a]
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), [{Rh(C2H4)Cl}2] (1.5 mg, 0.004 mmol), L1 (0.0096 mmol), 50 mol-% Et3N, and
EtOH (1.5 mL) at 40 °C for 10 h. [b] Isolated yield. [c] Determined by HPLC analysis with chiral columns.
(t, 1 H, J = 7.2 Hz), 5.10–5.01 (m, 2 H), 4.95–4.88 (m, 1 H) ppm.
Experimental Section
13C NMR (75 MHz, CDCl3): 137.7, 137.3, 135.4, 133.9, 129.4,
General Procedure for the Enantioselective Rhodium-Catalyzed Ad-
dition of Arylboronic Acids to 3-Indolylnitroalkenes: Under an ar-
gon atmosphere, at room temperature, [{Rh(C2H4)2Cl}2] (1.5 mg,
0.004 mmol) and (tert-butylsulfinyl)phosphane ligand L1 (4.1 mg,
0.0096 mmol), followed by 1.0 mL CH2Cl2, were added to a 10 mL
Schlenk tube and the mixture was stirred at room temperature for
30 min, then CH2Cl2 was removed under reduced pressure and 3-
indolylnitroalkenes (0.2 mmol) and arylboronic acids (0.4 mmol)
were added, after purging with argon, then ethanol (1.5 mL) and
TEA (14.0 μL, 0.1 mmol) were added successively. The mixture was
stirred at 40 °C for 10 h, then the solvent was removed in vacuo
and the residue was purified by flash chromatography on silica gel
with petroleum ether/ethyl acetate, 10:1, as the eluent. The ee value
was determined by HPLC on a chiral phase.
129.3, 129.1, 128.0, 127.6, 126.6, 125.3, 123.6, 123.1, 121.1, 119.7,
113.8, 78.9, 41.1 ppm. HPLC: Daicel Chiralcel, IB, n-hexane/2-
propanol = 70:30, 1.0 mLminϪ1, 254 nm, retention time: 18.19
(minor), 28.93 min (major). HRMS (EI) for C22H18N2NaO4S [M
+ Na]: calcd. 429.0885; found 429.0861.
3-(2-Nitro-1-phenylethyl)-1H-indole (3da): Yield 96%. [α]2D0 = –12 (c
= 0.114, CHCl3) for 98% ee. 1H NMR (300 MHz, CDCl3): δ =
8.07 (s, 1 H), 7.47 (d, J = 7.9 Hz, 1 H), 7.36–7.29 (m, 6 H), 7.26–
7.21 (m, 1 H), 7.11 (t, J = 7.3 Hz, 1 H), 7.01 (d, J = 2.3 Hz, 1 H),
5.20–5.18 (m, 1 H), 5.11–5.04 (m, 1 H), 4.98–4.91 (m, 1 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 139.1, 136.4, 128.9, 127.7, 127.5,
126.0, 122.6, 121.6, 119.9, 118.9, 114.3, 111.4, 79.5, 41.5 ppm.
HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol
1.0 mLminϪ1
254 nm, retention time: 15.22 min (major),
16.86 min (minor).
= 90:10,
,
1-Benzyl-3-(2-nitro-1-phenylethyl)-1H-indole (3aa): Yield 98%.
[α]2D0 = –38 (c = 0.168, CHCl3) for 97% ee. 1H NMR (300 MHz,
CDCl3): δ = 7.52 (d, J = 7.9 Hz, 1 H), 7.39–7.19 (m, 10 H), 7.14–
7.12 (m, 3 H), 7.02 (s, 1 H), 5.29–5.23 (m, 3 H), 5.11–5.05 (m, 1
H), 5.00–4.93 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
139.3, 137.2, 136.9, 128.84, 128.75, 127.79, 127.6, 127.5, 126.8,
126.6, 125.7, 122.4, 119.7, 119.1, 113.4, 110.0, 79.5, 50.0, 41.5 ppm.
3-[2-Nitro-1-(p-tolyl)ethyl]-1H-indole (3 db): Yield 98%. [α]2D0 = –2
(c = 0.150, CHCl3) for 98% ee. 1H NMR (300 MHz, CDCl3): 8.05
(s, 1 H), 7.87 (d, 1 H, J = 7.9 Hz), 7.34 (d, 1 H, J = 8.1 Hz), 7.25–
7.19 (m, 3 H), 7.15–7.07 (m, 3 H), 6.99 (d, 1 H, J = 2.1 Hz), 5.19–
5.14 (m, 1 H), 5.09–5.02 (m, 1 H),4.96–4.89 (m, 1 H), 2.33 (s, 3 H)
ppm. 13C NMR (75 MHz, CDCl3): 137.2, 136.4, 136.1, 129.5,
127.6, 126.0, 122.6, 121.5, 119.8, 114.5, 111.3, 79.6, 41.1, 21.0 ppm.
HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol
= 90:10,
1.0 mLminϪ1, 254 nm, retention time: 7.98 (major), 9.13 min
(minor).
HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol
= 90:10,
1.0 mLminϪ1, 254 nm, retention time: 14.15 (major),16.23 min
(minor).
1-Methyl-3-(2-nitro-1-phenylethyl)-1H-indole (3ba): Yield 96%.
[α]2D0 = –10 (c = 0.172, CHCl3) for 98% ee. 1H NMR (300 MHz,
CDCl3): δ = 7.50 (d, J = 7.9 Hz, 1 H), 7.39–7.26 (m, 7 H), 7.12 (t,
J = 6.8 Hz, 1 H), 6.89 (s, 1 H), 5.24–5.19 (m, 1 H), 5.10–4.99 (m,
1 H), 4.96–4.92 (m, 1 H),3.74 (s, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 139.4, 137.2, 128.8, 127.7, 127.4, 126.5, 126.3, 122.2,
119.4, 118.9, 112.7, 109.5, 79.5, 41.5, 32.7 ppm. HPLC: Daicel Chi-
ralcel, IA, n-hexane/2-propanol = 90:10, 1.0 mLminϪ1, 254 nm, re-
tention time: 7.31 (major), 7.74 min (minor).
3-[1-(4-Methoxyphenyl)-2-nitroethyl]-1H-indole (3dc): Yield 98%.
[α]2D0 = –9 (c = 0.118, CHCl3) for 99% ee. 1H NMR (300 MHz,
CDCl3): δ = 8.09 (s, 1 H), 7.39 (d, J = 7.9 Hz, 1 H), 7.35 (d, J =
8.1 Hz, 1 H), 7.26–7.23 (m, 3 H), 7.10–7.06 (m, 1 H), 7.00 (d, J =
2.0 Hz, 1 H), 6.86–6.83 (m, 2 H), 5.14–5.10 (m, 1 H), 5.05–5.01 (m,
1 H),4.93–4.86 (m, 1 H), 3.77 (s, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 158.8, 136.4, 131.1, 128.8, 126.0, 122.6, 121.4, 119.9,
118.9, 114.6, 114.2, 111.3, 79.7, 55.2, 40.8 ppm. HPLC: Daicel Chi-
ralcel, IB, n-hexane/2-propanol = 75:25, 0.8 mLminϪ1, 254 nm, re-
tention time: 20.26 (minor), 21.53 min (major).
3-(2-Nitro-1-phenylethyl)-1-(phenylsulfonyl)-1H-indole (3ca): Yield
99%. [α]2D0 = –17 (c = 0.128, CHCl3) for 86% ee. 1H NMR
(300 MHz, CDCl3): 8.00 (d, 1 H, J = 8.2 Hz), 7.87–7.84 (m, 2 H), 3-{1-[4-(Methoxymethoxy)phenyl]-2-nitroethyl}-1H-indole
7.56–7.52 (m, 2 H), 7.45–7.42 (m, 2 H), 7.33–7.25 (m, 7 H), 7.20
(3dd):
1
Yield 99%. [α]2D0 = –17 (c = 0.114, CHCl3) for 99% ee. H NMR
Eur. J. Org. Chem. 2012, 1230–1236
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1233

J. Xing, G. Chen, P. Cao, J. Liao
FULL PAPER
(300 MHz, CDCl3): 8.11 (s, 1 H), 7.45 (d, 1 H, J = 7.9 Hz), 7.33 3-[1-(3-Methoxyphenyl)-2-nitroethyl]-1H-indole (3dj): Yield 98%.
(d, 1 H, J = 8.1 Hz), 7.26–7.18 (m, 3 H), 7.10 (t, 1 H, J = 7.1 Hz), [α]2D0 = –10 (c = 0.110, CHCl3) for 98% ee. 1H NMR (300 MHz,
6.99–6.97 (m, 3 H), 5.15–5.12 (m, 3 H), 5.07–5.00 (m, 1 H), 4.92–
4.85 (m, 1 H), 3.47 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3):
156.5, 136.4, 132.4, 128.8, 126.0, 122.6, 121.4, 119.8, 118.9, 116.5,
114.5, 111.3, 94.3, 79.6, 56.0, 40.8 ppm. HPLC: Daicel Chiralcel,
IB, n-hexane/2-propanol = 70:30, 1.0 mLminϪ1, 254 nm, retention
time: 13.07 (minor), 14.04 min (major). HRMS (EI) for
C18H18N2NaO4 [M + Na]: calcd. 349.1164; found 349.1156.
CDCl3): δ = 8.09 (s, 1 H), 7.49 (d, J = 7.7 Hz, 1 H), 7.33 (d, J =
7.9 Hz, 1 H), 7.28–7.18 (m, 2 H), 7.10 (t, J = 7.0 Hz, 1 H), 6.98–
6.89 (m, 3 H), 6.81 (d, J = 7.9 Hz, 1 H), 5.20–5.15 (m, 1 H), 5.08–
5.01 (m, 1 H), 4.96–4.89 (m, 1 H), 3.76 (s, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 159.8, 140.8, 136.4, 129.9, 126.0, 122.6,
121.6, 120.0, 119.9, 118.8, 114.1, 114.0, 112.4, 111.4, 79.4, 55.1,
41.4 ppm. HPLC: Daicel Chiralcel, IB, n-hexane/2-propanol =
70:30, 1.0 mLminϪ1
16.77 min (major).
, 254 nm, retention time: 12.65 (minor),
3-{1-[4-(tert-Butyl)phenyl]-2-nitroethyl}-1H-indole (3de): Yield 96%.
1
[α]2D0 = +15 (c = 0.126, CHCl3) for 98% ee. H NMR (300 MHz,
3-[1-(3-Chlorophenyl)-2-nitroethyl]-1H-indole (3dk): Yield 93%.
[α]2D0 = –18 (c = 0.116, CHCl3) for 90% ee. 1H NMR (300 MHz,
CDCl3): 8.12 (s, 1 H), 7.45 (d, 1 H, J = 7.9 Hz), 7.37–7.32 (m, 2
H), 7,25–7.20 (m, 4 H), 7.11 (t, 1 H, J = 7.5 Hz), 7.00 (d, 1 H, J
= 2.1 Hz), 5.20–5.15 (m, 1 H), 5.07–5.00 (m, 1 H), 4.94–4.87 (m, 1
H) ppm. 13C NMR (75 MHz, CDCl3): 141.3, 136.4,134.7, 130.1,
127.9, 127.8, 125.9, 125.8, 122.8, 121.5, 120.0, 118.7, 113.5, 111.5,
79.1, 41.1 ppm. HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol
= 90:10, 1.0 mLminϪ1, 254 nm, retention time: 14.24 (major),
15.99 min (minor).
CDCl3): δ = 8.07 (s, 1 H), 7.50 (d, J = 7.9 Hz, 1 H), 7.36–7.18 (m,
6 H), 7.10 (t, J = 7.3 Hz, 1 H), 7.03 (s, 1 H), 5.21–5.51 (m, 1 H),
5.16–5.02 (m, 1 H), 4.97–4.91 (m, 1 H), 1.29 (s, 9 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 150.3, 136.4, 136.0, 127.3, 126.1,
125.8, 122.6, 121.6, 119.8, 118.9, 114.5, 111.3, 79.5, 41.0, 34.4, 31.3
ppm. HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol = 90:10,
1.0 mLminϪ1, 254 nm, retention time: 9.74 (major), 11.15 min
(minor). HRMS (EI) for C20H22N2NaO2 [M + Na]: calcd.
345.1579; found 345.1570.
3-[1-(4-Chlorophenyl)-2-nitroethyl]-1H-indole (3df): Yield 96%.
[α]2D0 = –2 (c = 0.128, CHCl3) for 94% ee. 1H NMR (300 MHz,
CDCl3): δ = 8.11 (s, 1 H),7.41 (d, J = 7.9 Hz, 1 H), 7.35 (d, J =
8.1 Hz, 1 H), 7.31–7.20 (m, 5 H), 7.10 (t, J = 7.3 Hz, 1 H), 6.99 (d,
1 H), 5.19–5.14 (m, 1 H), 5.08–5.01 (m, 1 H), 4.93–4.86 (m, 1 H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 137.7, 136.4, 133.3, 129.1,
129.0, 125.8, 122.8, 121.5, 120.0, 118.7, 113.8, 111.5, 79.2, 40.9
ppm. HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol = 90:10,
1.0 mLminϪ1, 254 nm, retention time: 17.07 (major), 21.41 min
(minor).
3-[1-(3,5-Dimethylphenyl)-2-nitroethyl]-1H-indole (3dl): Yield 97%.
[α]2D0 = –2 (c = 0.128, CHCl3) for 99% ee. 1H NMR (300 MHz,
CDCl3): δ = 8.06 (s, 1 H), 7.52 (d, J = 7.9 Hz, 1 H), 7.34 (d, J =
8.0 Hz, 1 H), 7.26–7.19 (m, 1 H), 7.12 (t, J = 7.1 Hz, 1 H), 7.01–
6.91 (m, 4 H), 5.17–5.11 (m, 1 H), 5.08–5.01 (m, 1 H), 4.96–4.89
(m, 1 H), 2.29 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ =
139.0, 138.3, 136.4, 129.2, 126.1, 125.5, 122.5, 121.5, 119.8, 118.9,
114.4, 111.3, 79.5, 41.3, 21.3 ppm. HPLC: Daicel Chiralcel, IA, n-
hexane/2-propanol = 95:5, 0.8 mLminϪ1, 254 nm, retention time:
21.50 (major), 23.19 min (minor). HRMS (EI) for C18H18N2NaO2
[M + Na]: calcd. 317.1266; found 317.1254.
3-[1-(4-Fluorophenyl)-2-nitroethyl]-1H-indole (3dg): Yield 95%.
[α]2D0 = –29 (c = 0.118, CHCl3) for 97% ee. 1H NMR (300 MHz,
CDCl3): 8.11 (s, 1 H), 7.43 (d, 1 H, J = 7.9 Hz), 7.37–7.32 (m, 3
H), 7.32–7.28 (m, 1 H), 7.23–7.10 (m, 1 H), 7.04–7.00 (m, 3 H),
5.18–5.16 (m, 1 H), 5.09–5.02 (m, 1 H), 4.93–4.86 (m, 1 H) ppm.
13C NMR (75 MHz, CDCl3): 162.0 (d, JC-F = 244.6 Hz), 136.4,
134.9 (d, JC-F = 3.2 Hz), 129.3 (d, JC-F = 8.1 Hz), 125.9, 122.7,
121.4, 120.0, 118.8, 115.7 (d, JC-F = 21.3 Hz), 114.1, 111.4, 79.5,
40.8 ppm. HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol =
3-[1-(Naphthalen-2-yl)-2-nitroethyl]-1H-indole (3dm): Yield 98%.
1
[α]2D0 = +14 (c = 0.096, CHCl3) for 98% ee. H NMR (300 MHz,
CDCl3): δ = 8.09 (s, 1 H), 7.81–7.78 (m, 4 H), 7.49–7.42 (m, 4 H),
7.35 (d, J = 8.2 Hz, 1 H), 7.20 (t, J = 7.7 Hz, 1 H), 7.09–7.04 (m,
2 H), 5.40–5.34 (m, 1 H), 5.18–5.02 (m, 2 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 136.6, 136.5, 133.4, 132.7, 128.8, 127.9,
127.6, 126.4, 126.3, 126.1, 125.8, 122.7, 121.7, 120.0, 118.9, 114.3,
111.4, 79.4, 41.6 ppm. HPLC: Daicel Chiralcel, IB, n-hexane/2-
propanol = 70:30, 1.0 mLminϪ1, 254 nm, retention time: 15.42
(minor), 20.41 min (major).
90:10, 1.0 mLminϪ1
19.52 min (minor).
, 254 nm, retention time: 15.95 (major),
2-Methyl-3-(2-nitro-1-phenylethyl)-1H-indole (3ea): Yield 97%.
3-{2-Nitro-1-[4-(trifluoromethyl)phenyl]ethyl}-1H-indole
(3dh):
1
[α]2D0 = +30 (c = 0.112, CHCl3) for 99% ee. H NMR (300 MHz,
Yield 72%. [α]2D0 = –3 (c = 0.110, CHCl3) for 83% ee. 1H NMR
(300 MHz, CDCl3): δ = 8.15 (s, 1 H), 7.58 (d, J = 8.2 Hz, 2 H),
7.48–7.44 (m, 2 H), 7.41–7.36 (m, 2 H), 7.24 (t, J = 7.4 Hz, 1 H),
7.14–7.09 (m, 1 H), 7.02 (d, J = 2.1 Hz, 1 H), 5.26 (t, J = 7.8 Hz,
1 H), 5.12–5.05 (m, 1 H), 4.99–4.92 (m, 1 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 143.3, 136.5, 129.9 (q, JF,C = 32.3 Hz),
128.2, 125. 9 (q, JF,C = 3.8 Hz), 124.0 (q, JF,C = 270.6 Hz), 123.0,
121.6, 120.2, 118.8, 113.5, 111.5, 41.3, 29.7 ppm. HPLC: Daicel
Chiralcel, IA, n-hexane/2-propanol = 90:10, 1.0 mLminϪ1, 254 nm,
retention time:14.32 (major), 18.06 min (minor).
CDCl3): 7.84 (s, 1 H), 7.41 (d, 1 H, J = 7.7 Hz), 7.36–7.23 (m, 6
H), 7.16–7.06 (m, 2 H), 5.26–5.12 (m, 3 H), 2.36 (s, 3 H) ppm. 13C
NMR (75 MHz, CDCl3): 139.5, 135.3, 132.8, 128.7, 127.2, 127.0,
126. 8, 121.2, 119.7, 118.5, 110.7, 108.7, 78.6, 40.4, 11.9 ppm.
HPLC: Daicel Chiralcel, IB, n-hexane/2-propanol
= 70:30,
1.0 mLminϪ1, 254 nm, retention time: 9.43 (major), 28.15 min
(minor).
4-Bromo-3-(2-nitro-1-phenylethyl)-1H-indole (3fa): Yield 97%. [α]2D0
= +27 (c = 0.100, CHCl3) for 98% ee. 1H NMR (300 MHz,
CDCl3): δ = 8.24 (s, 1 H), 7.35–7.25 (m, 7 H), 7.02 (t, J = 7.8 Hz,
1 H), 6.91 (s, 1 H), 6.01–5.96 (m, 1 H), 5.18–5.11 (m, 1 H), 4.94–
4.86 (m, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 139.3, 137.7,
128.8, 128.0, 127.4, 124.7, 124.2, 123.5, 114.9, 113.7, 110.8, 79.6,
40.4, 29.6 ppm. HPLC: Daicel Chiralcel, AD-H, n-hexane/2-
propanol = 90:10, 1.0 mLminϪ1, 254 nm, retention time: 13.46
(major), 15.74 min (minor).
3-[2-Nitro-1-(m-tolyl)ethyl]-1H-indole (3di): Yield 98%. [α]2D0 = –5
(c = 0.110, CHCl3) for 98% ee. 1H NMR (300 MHz, CDCl3): 8.06
(s, 1 H), 7.50 (d, 1 H, J = 7.8 Hz), 7.35 (d, 1 H, J = 8.0), 7.25–7.08
(m, 6 H), 7.00 (s, 1 H), 5.20–5.15 (m, 1 H), 5.09–5.05 (m, 1 H),
4.97–4.90 (m, 1 H), 2.33 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3):
139.0, 138.5, 136.4, 128.7, 128.5, 128.3, 126.0, 124.6, 122.6, 121.5,
119.8, 118.8, 114.3, 111.3, 79.5, 41.4, 21.4 ppm. HPLC: Daicel Chi-
ralcel, IA, n-hexane/2-propanol = 95:5, 0.8 mLminϪ1, 254 nm, re-
tention time: 29.13 (major), 31.83 min (minor).
5-Bromo-3-(2-nitro-1-phenylethyl)-1H-indole (3ga): Yield 62%.
1
[α]2D0 = +37 (c = 0.152, CHCl3) for 97% ee. H NMR (300 MHz,
1234
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1230–1236

Asymmetric Addition of Arylboronic Acids to Indolylnitroalkenes
CDCl3): 8.17 (s, 1 H), 7.56 (s, 1 H), 7.36–7.24 (m, 6 H), 7.17 (d, 1
H, J = 8.6 Hz), 7.02 (d, 1 H, J = 2.2 Hz), 5.15–5.09 (m, 1 H), 5.04–
4.97 (m, 1 H), 4.94–4.87 (m, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): 138.6, 135.0, 129.0, 127.8, 127.7, 127.6, 125.5, 122.7,
121.3, 113.8, 113.1, 112.9, 79.3, 41.2 ppm. HPLC: Daicel Chiralcel,
IA, n-hexane/2-propanol = 90:10, 1.0 mLminϪ1, 254 nm, retention
time: 13.66 (major), 15.33 min (minor).
Acknowledgments
We are grateful for support from the National Natural Science
Foundation of China (NSFC) (grant numbers 21072186 and
20872139), the West Light Foundation of CAS, the National Basic
Research Program of China (973 Program, 2010CB833300), and
the Chengdu Institute of Biology, CAS (Y0B1051100).
1-Methyl-2-(2-nitro-1-phenylethyl)-1H-indole (3ha): Yield 99%
[α]2D0 = –121 (c = 0.12, CHCl3) for 50% ee. 1H NMR (300 MHz,
CDCl3): δ = 7.68 (d, J = 7.8 Hz, 1 H), 7.38–7.32 (m, 3 H), 7.28–
7.24 (m, 4 H), 7.21–7.15 (m, 1 H), 6.56 (s, 1 H), 5.16–5.08 (m, 2
H), 4.91–4.82 (m, 1 H), 3.51 (s, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 137.7, 137.1, 136.9, 129.2, 128.1, 128.0, 127.2, 121.8,
120.5, 119.8, 109.0, 79.0, 99.1, 79.0, 42.0, 29.7 ppm. HPLC: Daicel
Chiralcel, IB, n-hexane/2-propanol = 70:30, 1.0 mLminϪ1, 254 nm,
retention time:31.67 (minor), 69.17 min (major). HRMS (EI) for
C17H16N2NaO2 [M + Na]: calcd. 303.1109; found 303.1103.
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1-Methyl-2-[2-nitro-1-(p-tolyl)ethyl]-1H-indole (3hb): Yield 98%.
[α]2D0 = –145 (c = 0.13, CHCl3) for 62% ee. 1H NMR (300 MHz,
CDCl3): δ = 7.66 (d, J = 7.8 Hz, 1 H), 7.27–7.22 (m, 2 H), 7.18–
7.10 (m, 5 H), 6.53 (s, 1 H), 5.10–5.04 (m, 2 H), 4.87–4.78 (m, 1
H), 3.50 (s, 3 H), 2.35 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 137.9, 137.7, 137.2, 134.1, 129.9, 127.2, 121.7, 120.4, 119.7,
109.0, 99.0, 79.1, 41.69, 29.7, 21.0 ppm. HPLC: Daicel Chiralcel,
IB, n-hexane/2-propanol = 70:30, 1.0 mLminϪ1, 254 nm, retention
time: 26.61 (minor),52.09 min (major). HRMS (EI) for
C18H18N2NaO2 [M + Na]: calcd. 317.1266; found 317.1262.
2-[1-(4-Methoxyphenyl)-2-nitroethyl]-1-methyl-1H-indole
(3hc):
Yield 98%. [α]2D0 = –157 (c = 0.116, CHCl3) for 75% ee. H NMR
(300 MHz, CDCl3): δ = 7.65 (d, J = 7.8 Hz, 1 H), 7.29–7.22 (m, 2
H), 7.18–7.13 (m, 3 H), 6.86 (d, J = 8.6 Hz, 2 H), 6.51 (s, 1 H),
5.11–5.03 (m, 2 H), 4.86–4.77 (m, 1 H), 3.79 (s, 3 H), 3.49 (s, 3 H)
ppm. 13C NMR (75 MHz, CDCl3): δ = 159.3, 137.7, 137.2, 129.1,
129.0, 127.2, 121.7, 120.4, 119.7, 114.5, 109.0, 99.0, 79.1, 55.2, 41.3,
29.6 ppm. HPLC: Daicel Chiralcel, IA, n-hexane/2-propanol =
1
90:10, 1.0 mLminϪ1
, 254 nm, retention time: 10.14 (minor),
11.09 min (major). HRMS (EI) for C18H18N2NaO3 [M + Na]:
calcd. 333.1215; found 333.1207.
3-(2-Nitro-1-phenylethyl)furan (3ia): Yield 99%. [α]2D0 = –57 (c =
0.098, CHCl3) for 60% ee. 1H NMR (300 MHz, CDCl3): δ = 7.38–
7.28 (m, 6 H), 6.33–6.32 (m, 1 H), 6.14–6.13 (m, 1 H), 5.03–4.99
(m, 1 H), 4.96–4.91 (m, 1 H), 4.84–4.81 (m, 1 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 152.0, 142.5, 136.9, 129.0, 128.1, 127.8,
110.4, 107.4, 78.0, 43.5 ppm. HPLC: Daicel Chiralcel, AS-H, n-
hexane/2-propanol = 90:10, 0.5 mLminϪ1, 254 nm, retention time:
16.34 (major), 18.04 min (minor).
3-(2-Nitro-1-phenylethyl)thiophene (3ja): Yield 99%. [α]2D0 = –7 (c =
0.080, CHCl3) for 76% ee. 1H NMR (300 MHz, CD3COCD3):
7.48–7.44 (m, 2 H), 7.40–7.30 (m, 4 H), 7.12–7.11 (m, 1 H), 7.01–
6.98 (m, 1 H), 5.28–5.20 (m, 3 H) ppm. 13C NMR (75 MHz,
CD3COCD3): 144.1, 140.7, 129.7, 128.6, 128.6, 127.9, 126.0, 125.9,
80.2, 45.4 ppm. HPLC: Daicel Chiralcel, OD-H, n-hexane/2-
propanol = 70:30, 1.0 mLminϪ1, 254 nm, retention time: 13.61
(minor), 26.03 min (major).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization of the prepared
compounds, copies of the NMR spectra, and chiral HPLC spectra
of the Michael addition products.
Eur. J. Org. Chem. 2012, 1230–1236
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1235

J. Xing, G. Chen, P. Cao, J. Liao
FULL PAPER
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Received: November 15, 2011
Published Online: January 4, 2012
1236
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 1230–1236