Dinuclear zinc catalyzed asymmetric Friedel-Crafts amidoalkylation of indoles with aryl aldimines

Article abstract of DOI:10.1039/c0ob01200a

The asymmetric Friedel-Crafts amidoalkylation of indoles with aryl aldimines could be efficiently catalyzed by Trost's bis-ProPhenol dinuclear zinc complexes to attain 3-indolyl methanamine derivatives in good to excellent yields (85-98%) with moderate to high enantiomeric ratios (from 70:30 up to 95:5 er). Remarkably, this approach provides efficient access to enantiomerically enriched 3-indolyl methanamines, which avoids the formation of the undesirable bis- and tris(indolyl)methanes (BIMs and TIMs) byproduct.

Full text of DOI:10.1039/c0ob01200a

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Dinuclear zinc catalyzed asymmetric Friedel–Crafts amidoalkylation of
indoles with aryl aldimines†
Bei-Lei Wang, Nai-Kai Li, Jin-Xin Zhang, Guo-Gui Liu, Teng Liu, Qi Shen* and Xing-Wang Wang*
Received 18th December 2010, Accepted 9th February 2011
DOI: 10.1039/c0ob01200a
The asymmetric Friedel–Crafts amidoalkylation of indoles
with aryl aldimines could be efficiently catalyzed by Trost’s
bis-ProPhenol dinuclear zinc complexes to attain 3-indolyl
methanamine derivatives in good to excellent yields (85–98%)
with moderate to high enantiomeric ratios (from 70 : 30 up
to 95 : 5 er). Remarkably, this approach provides efficient
access to enantiomerically enriched 3-indolyl methanamines,
which avoids the formation of the undesirable bis- and
tris(indolyl)methanes (BIMs and TIMs) byproduct.
Due to their ability to tightly bind metal ions and to achieve
high levels of molecular recognition, the use of enantiopure
multidentate crown compounds as ligands has turned out to be
effective for designing chiral catalysts.7 In 2000, Trost and Ito de-
signed and reported the well-organized semi-crown bis-ProPhenol
dinuclear zinc catalysts (Scheme 1). So far, Trost’s dinuclear zinc
catalysts as mimic metalloenzymes have been utilized to catalyze
various asymmetric reactions, such as aldol reaction, Mannich
reaction, Michael addition, and alkynylation, with excellent
enantioselectivities.8 Recently, Wang’s group applied the Trost
dinuclear zinc catalyst to conjugate additions of phosphites to a, b-
unsaturated N-acylpyrroles and imines, affording aminophospho-
nates in high yields and excellent enantioselectivities.9 Moreover,
Ding’s group found that the bis-ProPhenol dinuclear Zn/Mg
complexes could catalyze copolymerization of cyclohexene oxide
with CO2, which displayed completely alternating polycarbonate
selectivity and high efficiency.10 Furthermore, Da’s group designed
other semicrown chiral BINOL-derived zincate catalysts, which
were employed to catalyze direct aldol addition with up to 80%
ee.11 Herein, we report that Trost’s dinuclear zinc catalysts can
efficiently catalyze the Friedel–Crafts amidoalkylation reaction
of indoles with aryl aldimines, affording 3-indolyl methanamines
in good to excellent yields (85–98%) with moderate to high
enantiomeric ratios (from 70 : 30 to 95 : 5).
The Friedel–Crafts alkylation reaction is a powerful carbon–
carbon bond forming process in modern organic synthetic
chemistry.1 In view of the significance of the chirality, enantios-
elective Friedel–Crafts alkylation reactions, promoted by chiral
metal complexes or organocatalysts, have been rapidly developed.2
Among them, enantioselective Friedel–Crafts alkylation of indoles
with aldimines have proven to be an atom-economic method
for the preparation of optically active 3-indolyl methanamine
derivatives, which show biologically significant activities in phar-
maceuticals, agrochemicals, and indole alkaloids.3 Accordingly,
great progress has been made in the last ten years. Johannsen
and Zhou’s group,4 respectively, disclosed that copper-BINAP or
copper-bisoxazoline complexes could catalyze amidoalkylation of
indoles with an imino ester or N-sulfonyl aldimines in high yields
and enantioselectivities. Deng et al.5 reported that 9-thiourea
cinchona alkaloids, as bifunctional organocatalysts, catalyzed the
Friedel–Crafts amidoalkylation with broad substrate generality
in high yields with excellent enantioselectivities. Recently, You,
Terada and Antilla, respectively, reported that the chiral binaph-
thyl derived phosphoric acids were applied into the Friedel–Crafts
reaction of indoles with imines, affording the optically enriched
3-indolyl methanamine derivatives in good yields and excellent
enantioselectivies.6 Given that the asymmetric Friedel–Crafts
amidoalkylation of indoles with imines provides an easy access
to enantiomerically enriched 3-indolyl methanamine derivatives,
the cultivation of catalytic enantioselective variants of this reaction
is still in high demand.
Scheme 1 Trost’s dinuclear zinc catalysts.
Initially, several semi-crown dinuclear zinc catalysts were ex-
amined for the Friedel–Crafts alkylation of indole 2a with N-
Ts aldimine 3a in toluene at rt under a catalyst loading of 10
mol%. As shown in Table 1, the ligands of (R,R)-L1,12a (R,R)-
L212b and (R,S,S)-L311 were found to be ineffective for the reaction,
affording essentially only trace amounts of the desired products
(entries 1–3). Subsequently, we tested Trost’s dinuclear zinc
catalysts for this transformation. The bis-ProPhenol ligands (S,S)-
L4–8, varied with different substituents on (S)-prolinol, were
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chem-
istry, Chemical Engineering and Materials Science, Soochow University,
Suzhou, 215123, P. R. China. E-mail: wangxw@suda.edu.cn; Fax: +86-
512-65880378
† Electronic supplementary information (ESI) available: Experimental
procedures and analysis data. See DOI: 10.1039/c0ob01200a
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Table 1 Examination of the catalysts
Table 2 Optimization of reaction conditions
Entry
Solvent
Time
Yield (%)a
erb
1
2
3
4
5
6
Toluene
Xylene
Benzene
CHCl3
DCM
ClCH2CH2Cl
THF
2 h
2 h
2 h
2 h
2 h
2 h
2 h
6 h
95
90
82
91
92
90
65
91
95
92 : 8
82 : 18
71 : 29
77 : 23
90 : 10
87 : 13
83 : 17
64 : 36
83 : 17
7
8c
9d
Toluene
Toluene
40 min
a Isolated yields. b Determined on a chiralcel OD-H column. c The reaction
was performed at 0 ◦C. d The reaction was performed at 50 ◦C.
Cu(OTf)2 was used as the metal source, the reaction gave the
bis(indolyl)methanes (BIMs)13c as the sole product in 90% yield
(Table 1, entry 11). In addition, when the catalyst loading was
decreased to 5 mol%, the reaction gave a reduced yield and an
enantiomeric ratio (entry 12). When increasing the catalyst loading
to 15 mol%, the yield was not influenced too much but the er also
dropped (entry 13). Therefore, 10 mol% of ZnEt2/(S,S)-L4 as
the catalyst and five equivalents of indole 2a were chosen for the
Friedel–Crafts amidoalkylation of 3a with 2a.
Entrya
Ligand
Cat. (mol%)
Mb
Yield (%)c
erd
1
2
3
L1
L2
L3
L4
L4
L4
L5
L6
L7
L8
L4
L4
L4
10
10
10
10
10
10
10
10
10
10
5
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
ZnEt2
Cu(OTf)2
ZnEt2
ZnEt2
Trace
Trace
Trace
95
90
96
90
95
92
—
—
—
Further results on the optimization of the other parameters for
the reaction of 3a and 2a catalyzed by dinuclear zinc ZnEt2/(S,S)-
L4, including reaction solvent and temperature, are summarized in
Table 2. The reaction in toluene and dichloromethane, compared
with the other solvents examined, gave the highest yields and
enantiomeric ratios (entries 1–6). The reaction in THF proceeded
in an obviously sluggish manner affording the desired product
in 65% yield and 83 : 17 er (entry 7). Furthermore, when the
reaction temperature was decreased to 0 ◦C, the reaction became
obviously slow, and the enantiomeric ratio was dramatically
reduced to 64 : 36 (entry 8). However, when increasing the reaction
4
92 : 8
75 : 25
90 : 10
76 : 24
48 : 52
77 : 23
88 : 12
—
5e
6f
7
8
9
10
11
12
13
90
0 (90g)
82
5
15
69 : 31
80 : 20
92
a The reactions were performed under N2 at rt for two hours. b ZnEt2, 1.0 M
in toluene. c Isolated yields. d Determined on a chiralcel OD-H column.
e Indole : 3a = 2 : 1. f Indole : 3a = 6 : 1. g Yield of bisindolylmethanes
(BIMs).
◦
temperature to 50 C, the reaction could be complete in 40 min.
The desired product 4a was obtained in 95% yield, albeit with
moderate enantiomeric ratio (83 : 17) (entry 9). Thus, toluene as
the reaction media and room temperature were optimal conditions
for the Friedel–Crafts amidoalkylation.
smoothly synthesized according to the literature reports.8,10 Then
the dinuclear zinc catalyst derived from ZnEt2 and (S,S)-L4 was
prepared in situ by the addition of a solution of diethylzinc into a
solution of (S,S)-L4 in toluene according to the molar ratio of 2 : 1
at 0 ◦C.8b After adding aldimine 3a and five equivalents of indole
2a into the above catalyst solution, the mixture was stirred at room
temperature for two hours. To our delight, the desired product 4a
could be isolated in 95% yield and 92 : 8 er (Table 1, entry 4).
Then decreasing the amount of indole 2a to two equivalents was
found to considerably affect the yield and the enantiomeric ratio
(90% and 75 : 25 er) (entry 5). When increasing the amount of
indole 2a to six equivalents, the desired product 4a was obtained
in 96% yield and 90 : 10 er (entry 6). After screening the ligands
(S,S)-L4–8, the results indicated that ligand (S,S)-L4 was the best
choice in terms of enantioselectivity and yield (entry 4 vs. entries
7–10). It should be noted that, for the dinuclear zinc complex
catalyzed Friedel–Crafts amidoalkylation, the undesirable bis-
and/or tris(indolyl)methanes (BIMs and TIMs)13 byproducts were
not observed during the reaction course. However, when the
Having established the optimized reaction conditions, we then
explored the substrate scope and limitation of the present protocol.
The results are summarized in Table 3. It should be noted that
most of the reactions proceeded very quickly. The reaction mixture
generally becomes cloudy and thick within 10 min (photographs
i and ii for entry 1). Substrates 3b–g bearing either electron-
withdrawing substituents (-F, -Cl, -Br, -CF3) or electron-donating
substituents (-Me, -OMe) at the para-position on the phenyl
group were well tolerated and led to their desired products 4b–
g in good to excellent yields (90–98%), albeit accompanied with
varying levels of enantiomeric ratios (entries 2–7). Among them,
the substrate 3g bearing a para-CF3 group gave the desired product
4g in 98% yield with 94 : 6 er (entry 7). For substrates 3h–m
bearing ortho- and meta-substituents on the phenyl group, the
desired products 4h–m could be obtained in 92–98% yields but
also with the varying levels of enantiomeric ratios from 76 : 24
to 92 : 8 (entries 8–13). To our delight, substrate 3n bearing 2,4-
dichloro groups furnished the desired product 4n in 98% yield
with 95 : 5 er (entry 14). Substrates 3o–q bearing naphthyl and
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Table 3 Substrate scope in the reaction
Entry
Indoles (R1, R2)
Aldimines (R3, Ar)
Product
Yield (%)b
ercd
1
2
3
4
5
6
7
8
R1, R2 = H (2a)
Tse, C6H5 (3a)
4a
4b
4c
4d
4e
4f
4g
4h
4i
95
97
98
98
90
98
98
98
96
97
98
92
98
98
96
90
85
94
98
90
91
87
Trace
Trace
92 : 8
R1, R2 = H (2a)
Ts, p-FC6H4 (3b)
Ts, p-ClC6H4 (3c)
Ts, p-BrC6H4 (3d)
Ts, p-MeOC6H4 (3e)
Ts, p-MeC6H4 (3f)
Ts, p-CF3C6H4 (3g)
Ts, o-ClC6H4 (3h)
Ts, o-FC6H4 (3i)
Ts, o-BrC6H4 (3j)
Ts, o-MeOC6H4 (3k)
Ts, m-MeC6H4 (3l)
Ts, m-ClC6H4 (3m)
Ts, 2,4-Cl2C6H3 (3n)
Ts, 1-Naphthyl (3o)
Ts, 2-Naphthyl (3p)
Ts, thiophen-2-yl (3q)
Ts, C6H5 (3a)
90 : 10
74 : 26
86 : 14 (R)
92 : 8
78 : 22
94 : 6 (R)
92 : 8
82 : 18
84 : 16
90 : 10
88 : 12
76 : 24
95 : 5
85 : 15
78 : 22
81 : 19
72 : 28
70 : 30
85 : 15
87 : 13
85 : 15
—
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
9
R1, R2 = H (2a)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
R1, R2 = H (2a)
4j
4k
4l
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
R1, R2 = H (2a)
R1 = 5-CH3, R2 = H (2b)
R1 = 5-Br, R2 = H (2c)
R1 = 7-CH3, R2 = H (2d)
R1 = 7-CH3, R2 = H (2d)
R1, R2 = H(2a)
Ts, C6H5 (3a)
Ts, C6H5 (3a)
Ts, o-ClC6H4 (3h)
Bsf, C6H5 (3r)
R1, R2 = H(2a)
p-ClC6H4, C6H5 (3s)
Ts, C6H5 (3a)
R1 = H, R2 = CH3 (2e)
—
a Photographs: i before reaction and ii after reaction for entry 1. b Isolated yields. c Determined on a chiralcel OD-H column. d Absolute configuration
was determined by comparison of the optical rotation with known compounds in the literature.4b,5,6 e Ts = p-MeC6H4SO2, f Bs = C6H5SO2.
thiophen-2-yl groups were also suitable for this transformation
to give the desired products 4o–q in high yields and moderate
enantioselectivities (entries 15–17). The indole derivatives 2b–c
bearing 5-CH3 and 5-Br groups were also investigated for the
reaction to provide the corresponding products 4r–s in high yields
(94% and 98%) and modest er (72 : 28 and 70 : 30) (entries 18–19).
By comparison, the reactions of the indole derivative 2d bearing
a 7-CH3 group with 3a and 3h, respectively, led to the desired
products in slightly lower yields but with better enantiomeric ratios
(85 : 15 and 87 : 13) (entries 20–21). Furthermore, when N-Bs aryl
aldimine 3r was used in the reaction with indole 2a, the reaction
ran smoothly with high yield and good enantiomeric ratio (entry
22). In addition, the reactions of indole 2a with aldimine 3s and
N-methyl indole 2e with 3a were subsequently tested, affording
essentially only trace amounts of the products after prolonged
reaction time of 36 h (entries 23–24).
Based on Ding’s single-crystal X-ray analysis for the bis-
ProPhenol zinc complex10a and the mechanism of Friedel–Crafts
alkylation of pyrroles with nitroalkenes proposed by Trost,8e the
mechanism of this reaction is speculated to be that one zinc
atom coordinates to nitrogen atom of an indole through the
deprotonation by the formation of one equivalent of ethane, and
another zinc atom coordinates to the oxygen atom of sulfonyl
group. Then the Friedel–Crafts amidoalkylation of the indoles
with aldimines occurs. Finally, the exchange of proton with
another indole promotes the catalytic cycle and regenerates the
active catalysts (Scheme 2).
In conclusion, we have developed one new approach to
synthesis of 3-indolyl methanamine derivatives through the
asymmetric Friedel–Crafts amidoalkylation of indoles with N-
sulfonyl aldimines, catalyzed by the intramolecular dinuclear
zinc catalysts. The corresponding adducts have been obtained
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Scheme 2 Proposed catalytic cycle.8e
in good to excellent yields (up to 98%) with moderate to high
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Under an argon atmosphere, a solution of diethylzinc (50 mL,
1.0 M in toluene, 0.05 mmol) was added to a stirred and cooled
solution of L4 (0.025 mmol) in toluene (0.5 mL) at 0 ◦C. After the
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allowed to stir at rt for 30 min. Then a solution of N-sulfonyl imine
3 (0.25 mmol) and 2 (1.25 mmol) in 1.0 mL toluene were added.
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the reaction was complete (monitored by TLC), 10% NaHCO3 (3
mL) was added. The mixture was extracted with ethyl acetate (3 ¥
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Acknowledgements
We are grateful for the financial support from National Natural
Science Foundation of China (21072145), Foundation for the
Author of National Excellent Doctoral Dissertation of PR China
(200931), Natural Science Foundation of Jiangsu Province of
China (BK2009115).
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