Asymmetric allylation of aldimines with indium and (+)-cinchonine

Article abstract of DOI:10.1081/SCC-200061721

By applying indium and allyl bromide, aldimines were converted into homoallyl amines with excellent yields. The addition of (+)-cinchonine to the reaction mixture yielded enantioselective allylation with 22-44% ee. Copyright Taylor & Francis, Inc.

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Asymmetric Allylation of
Aldimines with Indium and
(+)‐Cinchonine
Rongbi Han a b , Seung Hyuk Choi a , Kee In Son a ,
Young Moo Jun a , Byung Min Lee c & Byeong Hyo Kim
a
a Department of Chemistry , Kwangwoon University ,
Seoul, South Korea
b Department of Chemistry , Yanbian University ,
Jilin, China
c Korea Research Institute of Chemical Technology ,
Taejon, South Korea
Published online: 16 Aug 2006.
To cite this article: Rongbi Han , Seung Hyuk Choi , Kee In Son , Young Moo Jun ,
Byung Min Lee & Byeong Hyo Kim (2005) Asymmetric Allylation of Aldimines with
Indium and (+)‐Cinchonine, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 35:12, 1725-1733, DOI:
10.1081/SCC-200061721
To link to this article: http://dx.doi.org/10.1081/SCC-200061721
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Synthetic Communicationsw, 35: 1725–1733, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200061721
Asymmetric Allylation of Aldimines with
Indium and (1)-Cinchonine
Rongbi Han
Department of Chemistry, Kwangwoon University, Seoul, South Korea
and Department of Chemistry, Yanbian University, Jilin, China
Seung Hyuk Choi, Kee In Son, and Young Moo Jun
Department of Chemistry, Kwangwoon University, Seoul, South Korea
Byung Min Lee
Korea Research Institute of Chemical Technology, Taejon, South Korea
Byeong Hyo Kim
Department of Chemistry, Kwangwoon University, Seoul, South Korea
Abstract: By applying indium and allyl bromide, aldimines were converted into
homoallyl amines with excellent yields. The addition of (þ)-cinchonine to the
reaction mixture yielded enantioselective allylation with 22–44% ee.
Keywords: Enantioselective allylation, indium, aldimines, homoallylamines, chiral
ligand
Asymmetric allylations on prochiral faces of carbonyl and imine functional-
ities have been widely used for the preparation of optically active homoallyl
alcohols and amines.[1] Despite their popularity, the preparation of optically
active homoallyl amines remains a difficult procedure. To overcome this
challenge, a wide variety of metals and stereochemical controlling techniques
have been applied to the allylation of imine.[2] Among the many possible
candidate metals, indium appears to be the appropriate choice for this
Received in Japan December 16, 2004
Address correspondence to Byeong Hyo Kim, Department of Chemistry,
Kwangwoon University, Seoul 139-701, South Korea. E-mail: bhkim@daisy.kw.ac.kr
1725

1726
R. Han et al.
purpose. Compared with other transition metals, such as zinc and heavy metal
tin, indium shows incredible stability in a variety of atmospheres and has a
relatively low ionization potential.[3] The first ionization potential of indium
(5.78 V) is closer to alkali and alkali earth metals, which contrasts with that
of zinc (9.39 V) and tin (7.34 V). Particularly, indium is so stable in air and
humid conditions that even water can be used as a reaction medium in some
cases.[4] Many groups,[5] including the authors of this paper,[6] have recently
reported their utilization of indium for the synthetic transformation.
The studies on enantioselective additions to the imine C55N bond with
organometallic reagents can be grouped into two categories: 1) the introduc-
tion of intrinsic chiral substituents into the substrate imine molecule and 2) the
employment of chiral ligands as an external source of chiral auxiliary via com-
plexation. Most of the stereoselective manipulations of imine with allylindium
fall under the first category; however, it has a limitation resulting from the dif-
ficulty of the proper chiral ligand selection. Catalytic Lewis acid promoters
were able to enhance the reactivity in some cases,[7] but they also generated
the undesirable reverse reaction of converting imine back to carbonyl and
amine functionality.
In our previous study, SmI2-mediated allylation of imines was success-
ful.[8] Enatioselective allylation, however, did not work very well with
SmI2. As a simple approach to synthesizing the optically active homoallyl
amines, the results of the stereoselective allylation of aldimines with indium
and chinchona alkaloid chiral ligands are herein reported.
The Barbier-type allylation of imine to produce homoallyl amine was
carried out in an N-(benzylidene)aniline system to preclude potential compli-
cations during the reaction process, such as enamination or enolization within
the substrates. Among many reaction variables, the optimum molar ratio was
obtained when the excessive allyl bromide (6 equiv.), indium (6 equiv.) and
limiting amounts of the substrate were used in THF as shown in Table 1
(entry 3). The reaction in the cosolvent (THF–hexane ¼ 3:1) exhibited an
excellent yield similar to the reaction in the THF solution; however, the
reaction time was extended up to 12 h. The relatively mild reaction
condition produced a better yield than the cold temperature and was distinc-
tively noticeable even though the subsequent temperature elevation did not
help improve the reaction yield (Table 1, entries 5, 6).
With the optimized reaction condition, simple allylation of various N-
(benzylidene)aniline derivatives were examined prior to the asymmetric ally-
lation application. As shown in Table 2, excellent yields of homoallyl amine,
ranging from 81 to 97%, were obtained within 2–3 h. The yield was not sig-
nificantly affected by each substituent attachment to the rings, regardless of its
position.
For the asymmetric allylation of aldimines, the application of chiral
ligands to the reaction system was examined based on the results of the allyla-
tion of conventional allylation that was described previously. It was observed
that the application of ligands to the reaction system slowed down the reaction

Asymmetric Allylation of Aldimines
1727
Table 1. Controlled reactions for the formation of N-phenyl-a-2-propenyl-benzene-
methanamine under various reaction conditions
Molar ratio
1a:A:In
Time
(h)
Yield
(%)b
Entry
Solvent
Temp. (8C)
1
2
3
4
5
6
1:6:4
1:6:5
1:6:6
1:6:6
1:6:6
1:6:6
THF
THF
rt
11
10
2
51
53
98
96
37
33
rt
THF
rt
THF/hexanec
THF
THF
rt
12
4
2788C (2 h) ! rt
08C (2 h) ! rt
4
a0.5 mmol of substrate was used.
bGC yield with an internal standard (n-octane).
c3:1 v/v ratio.
process. As shown in Table 3, the additions of (þ)-cinchonine, (2)-cinchoni-
dine and N-(4-trifluoromethylbenzyl)cinchoninium bromide all required a
considerably prolonged reaction time, compared to the reaction without.
Screening of the performance of some of the suitable chiral ligands
indicated that the (þ)-cinchonine produced the best result in terms of the
Table 2. Allyation of N-(benzylidene)aniline derivatives by indium in optimal con-
dition, substrate (1 equiv.)/allyl bromide (6 equiv.)/indium (6 equiv.) in THF at
room temperature
Time
(h)
Yield
(%)a
Entry
Substrate
Product
1
2
3
4
5
6
7
8
1a; R1 ¼ H, R2 ¼ H
2
2
2
2
3
2
2
2
2a
2b
2c
2d
2e
2f
96
83
81
89
88
97
93
75
1b; R1 ¼ 4-chloro, R2 ¼ H
1c; R1 ¼ 4-ethoxy, R2 ¼ H
1d; R1 ¼ 3-bromo, R2 ¼ H
1e; R1 ¼ 4-methyl, R2 ¼ H
1f; R1 ¼ 4-isopropyl, R2 ¼ H
1g; R1 ¼ 3-chloro, R2 ¼ H
1h; R1 ¼ H, R2 ¼ 2-methoxy
2g
2h
aIsolated yield.

1728
R. Han et al.
Table 3. Formation of N-phenyl-a-2-propenyl-benzenemethanamine with various
chiral ligands
Molar ratio
1:A:In:B
Time
(h)
Yield
(%)a
ee
(%)
Entry
Chiral ligand
Temp.
1
2
3
1:6:4:2
1:6:6:2
1:6:6:2
(þ)-cinchonine
(þ)-cinchonine
(þ)-cinchonine
rt
22
20
18
73
76
60
24
28
22
rt
08C
(1 h) ! rt
4
5
1:6:6:2
1:6:6:2
(2)-cinchonidine
N-(4-trifluoro-
methylbenzyl)
cinchoninium
bromide
rt
rt
20
18
75
79
0
6
6
7
1:6:6:2
1:6:6:3
(þ)-cinchonineb
rt
rt
4
48
50
30
16
32
(þ)-cinchonine
aGC yield with an internal standard(n-octane).
bInCl3 was added.
enantiomeric excess (ee), whereas (2)-cinchonidine exhibited absolutely no
enantiomeric excess (Table 3).
This result was somewhat surprising because when the substrates had
carbonyl functionality, (2)-cinchonidine demonstrated the best catalytic
activity rather than the (þ)-cinchonine, which is quite contrary to the imine
case.[9] The anticipated chiral ligand, N-(4-trifluoromethylbenzyl)cinchoni-
nium bromide, gave only unsatisfactory results in view of its enantiomeric
excess (Table 3, entry 5).
Given (þ)-cinchonine as a catalytic ligand and other variables, the
optimal reaction condition was reached as illustrated in Table 3, when the
Figure 1. The chiral ligands tested in this study.

Asymmetric Allylation of Aldimines
1729
ratio of the substrate (1 equiv.), allylbromide (6 equiv.), indium (6 equiv.), and
the ligand (2 equiv.) produced the best result. It is noteworthy that the addition
of a Lewis acid promoter such as indium(III) chloride did not improve the
prospects; rather, it deteriorated both the chemical yield and the ee
(Table 3, entry 6). Lower temperature also lowered the yields, even though
the temperature was raised from the low initial temperature application
(Table 3, entry 3). The attempted experiment of increasing the chiral ligand
concentration (3 equiv.) gave rise to only a doubled reaction time and
lowered chemical yields and ee (Table 3, entry 7).
The optimized reaction condition was applied to various N-(benzylide-
ne)aniline derivatives for the asymmetric allylation application. As shown
in Table 4, moderate ee was observed in most cases. The yield and ee were
not significantly affected by each substituent attachment to the rings, regard-
less of its position.
There are some reports that the mixed solvent system drastically improved
both the chemical yields and the ee in the case of the enantioselective allylation
of aldehydes and ketones with base additives.[10] Different combinations of
hydrocarbon mixed solvents did not significantly improve the ee, and neither
did the application of ether lower the ee. The optimal condition for the use
of the mixed solvent for the allylation of N-(benzylidene)aniline was the
Table 4. Allylation of various N-(benzylidene)aniline derivatives by indium in opti-
mal condition, substrate (1 equiv.)/allyl bromide (6 equiv.)/indium (6 equiv.)/(þ)-
cinchonine (2 equiv.) at room temperature
Time
(h)
Yield
(%)a
ee
(%)
Entry
Substrate
Solvent
Product
1
2
1a; R1 ¼ H, R2 ¼ H
A
B
A
B
A
B
A
A
A
A
A
20
26
17
25
26
22
26
22
20
22
22
2a
2a
2b
2b
2c
2c
2d
2e
2f
76
89
86
91
84
68
88
84
83
93
70
28
33
28
30
24
44
20
26
24
24
34
3
1b; R1 ¼ 4-Cl, R2 ¼ H
1c; R1 ¼ 4-OEt, R2 ¼ H
4
5
6
7
1d; R1 ¼ 3-Br, R2 ¼ H
1e; R1 ¼ 4-Me, R2 ¼ H
1f; R1 ¼ 4-i-Pr, R2 ¼ H
1g; R1 ¼ 3-Cl, R2 ¼ H
1h; R1 ¼ H, R2 ¼ 2-OMe
8
9
10
11
2g
2h
aIsolated yield.
A ¼ THF, B ¼ THF/hexane (3/1, v/v).

1730
R. Han et al.
THF–hexane (3:1) system, which gave an 89% chemical yield and a 33% ee
(Table 4, entry 2), which was better than in the THF case. In the case of
4-isopropyl-N-(benzylidene)aniline, the allylation reaction in the THF/
hexane solution produced 44% ee with a low chemical yield (Table 4,
entry 6), whereas the allylation reaction in THF produced 24% ee with a
high yield (Table 4, entry 5). Except in these cases, the use of the THF/
hexane cosolvent system showed the chemical yield and ee results that were
similar to or worse than when the THF solvent was used.
In conclusion, the synthesis of homoallyl amine from aldimine with allyl
bromide in the presence of indium has presented excellent chemical yields
ranging from 75% to 97%. The stereoselectivity in terms of the ee in the
presence of a (þ)-cinchonine chiral ligand turned out to be 22–44% ee.
Further efforts to improve the ee, including pursuit of new chiral ligands
and employment of other possible methods, are underway.
EXPERIMENTAL
Typical Procedure for Asymmetric Allylation
To a mixture of indium powder (344mg, 3 mmol), (þ)-chinchonine (294mg,
1 mmol), and allyl bromide (260mg, 3 mmol) in 2 ml of THF was added N-(ben-
zylidene)aniline (0.5mmol) in 1 mL THF at room temperature. The mixture was
then stirred for 17–26h, quenched with water, and extracted with CH2Cl2
(3 ꢀ 20mL). The combined organic layer was washed with brine solution and
then dried over MgSO4, filtered, and concentrated. The residue was chromato-
graphed over silica gel (hexane/EtOAc, 99:1) to give the allylated product.
The product’s ee was determined with HPLC with Welk-O chiral column.
N-Phenyl-a-2-propenylbenzenemethanamine (2a)
Liquid, 1H NMR (300MHz, CDCl3) d 2.32–2.54 (m, 2H), 4.05 (bd s, 1H), 4.27
(dd, 1H, J ¼ 5.1, 8.0 Hz), 5.02–5.11 (m, 2H), 5.57–5.73 (m, 1H), 6.39 (d, 2H,
J ¼ 8.0 Hz), 6.51–6.57 (m, 1H), 6.94–7.00 (m, 2H), 7.08–7.15 (m, 1H) 7.16–
7.28 (m, 4H); IR (KBr) 3450, 3068, 3055, 2924, 1571, 1267cm21; GC-MS m/z
(rel. intensity) 223 (3, Mþ), 182 (100), 104 (16), 91 (4), 77 (19).
4-Chloro-N-phenyl-a-2-propenylbenzenemethanamine (2b)
1
Liquid, H NMR (300 MHz, CDCl3) d 2.42–2.57 (m, 2H), 4.18 (bd s, 1H),
4.34 (dd, 1H, J ¼ 5.1, 8.1 Hz), 5.13–5.21 (m, 2H), 5.66–5.77 (m, 1H), 6.45
(d, 2H, J ¼ 7.5 Hz), 6.63–6.68 (m, 1H), 7.04–7.10 (m, 2H), 7.29 (bd s,
4H); IR (KBr) 3419, 3081, 3059, 3020, 2981, 2913, 2846 cm21; GC-MS
m/z (rel. intensity) 257 (3, Mþ), 216 (100), 180 (7), 104 (16), 77 (25).

Asymmetric Allylation of Aldimines
1731
4-Ethoxy-N-phenyl-a-2-propenylbenzenemethanamine (2c)
White solid, mp 49–508C, 1H NMR (300 MHz, CDCl3) d 1.30 (t, 3H,
J ¼ 7.0 Hz), 2.38–2.52 (m, 2H), 3.91 (q, 2H, J ¼ 7.0 Hz), 4.15 (bd s, 1H),
4.24 (dd, 1H, J ¼ 5.5, 7.5 Hz), 5.02–5.11 (m, 2H), 5.58–5.74 (m, 1H), 6.42
(d, 2H, J ¼ 7.9 Hz), 6.53–6.59 (m, 1H), 6.76 (d, 2H, J ¼ 8.6 Hz), 6.96–
7.02 (m, 2H), 7.17 (d, 2H, J ¼ 8.6 Hz); IR (KBr) 3416, 3081, 3055, 3021,
2982, 2928, 2878 cm21; GC-MS m/z (rel. intensity) 267 (1, Mþ), 226
(100), 198 (18), 104 (7), 77 (6).
3-Bromo-N-phenyl-a-2-propenylbenzenemethanamine (2d)
1
Liquid, H NMR (300 MHz, CDCl3) d 2.39–2.63 (m, 2H), 4.13 (bd s, 1H),
4.32 (dd, 1H, J ¼ 5.0, 8.1 Hz), 5.15–5.22 (m, 2H), 5.66–5.80 (m, 1H), 6.47
(d, 2H, J ¼ 7.7 Hz), 6.64–6.70 (m, 1H), 7.06–7.38 (m, 5H), 7.52(s, 1H); IR
(KBr) 3413, 3081, 3055, 3014, 2984, 2907, 2835 cm21; GC-MS m/z (rel.
intensity) 301 (6, Mþ), 260 (100), 180 (38), 104 (27), 77 (39).
4-Methyl-N-phenyl-a-2-propenylbenzenemethanamine (2e)
Liquid, 1H NMR (300 MHz, CDCl3) d 2.31 (s, 1H), 2.41–2.62 (m, 2H), 4.12
(bd s, 1H), 4.34 (dd, 1H, J ¼ 5.3, 7.9 Hz), 5.11–5.22 (m, 2H), 5.68–5.82
(m, 1H), 6.48 (d, 2H, J ¼ 8.6 Hz), 6.59–6.65 (m, 1H), 7.03–7.12 (m, 4H),
7.23 (d, 2H, J ¼ 8.0 Hz); IR (KBr) 3420, 3054, 2985, 2919, 1263 cm21
GC-MS m/z (rel. intensity) 237 (3, Mþ),196 (100), 104 (16), 77 (19).
;
4-(1-Methylethyl)-N-phenyl-a-2-propenylbenzenemethanamine (2f)
1
Liquid, H NMR (300MHz, CDCl3) d 1.21 (d, 6H, J ¼ 7.0 Hz), 2.47–2.57
(m, 2H), 2.83–2.89 (m, 1H), 4.34 (dd, 1H, J ¼ 5.3, 7.9 Hz), 5.09–5.19
(m, 2H), 5.69–5.82 (m, 1H), 6.50 (d, 2H, J ¼ 8.6 Hz), 6.60–6.65 (m, 1H),
7.03–7.27 (m, 6H); IR (KBr) 3412, 3081, 3059, 3015, 2966, 2928,
2867cm21; GC-MS m/z (rel. intensity) 265 (1, Mþ), 224 (100), 104 (10), 77 (9).
3-Chloro-N-phenyl-a-2-propenylbenzenemethanamine (2g)
1
Liquid, H NMR (300 MHz, CDCl3) d 2.42–2.65 (m, 2H), 4.12 (bd s, 1H),
4.33 (dd, 1H, J ¼ 5.3, 7.9 Hz), 5.13–5.21 (m, 2H), 5.64–5.80 (m, 1H), 6.49
(d, 2H, J ¼ 7.7 Hz), 6.65–6.71 (m, 1H), 7.05–7.12 (m, 2H), 7.17–7.26
(m, 3H), 7.35 (s, 1H); IR (KBr) 3419, 3081, 3059, 3020, 2981, 2913,
2846 cm21; GC-MS m/z (rel. intensity) 257 (2, Mþ), 216 (100), 180 (5),
104 (12), 77 (21).

1732
R. Han et al.
(2-Methoyphenyl-1-phenylbut-3-enyl)amine (2h)
Liquid, 1H NMR (300 MHz, CDCl3) d 2.52–2.62 (m, 2H), 3.87 (s, 3H), 4.37
(s, 1H), 4.79 (s, 1H), 5.10–5.21 (m, 2H), 5.71–5.85 (m, 1H), 6.28–6.31 (dd,
1H, J ¼ 1.7, 6 Hz), 6.56–6.77 (m, 3H), 7.19–7.37 (m, 5H); IR (KBr) 3420,
3080, 2946, 1596, 1505, 1456, 1231 cm21; GC-MS m/z (rel. intensity) 253
(4, Mþ), 212 (100), 196 (14), 120 (16), 91 (9), 77 (4).
ACKNOWLEDGMENTS
This work was supported by the RRC program of MOST/Gyeonggi-do and
partly by BK21 program.
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