Asymmetric Bronsted acid-catalyzed Friedel-Crafts reactions of indoles with cyclic imines - Efficient generation of nitrogen-substituted quaternary carbon centers

Article abstract of DOI:10.1002/adsc.201000952

A new enantioselective Bronsted acidcatalyzed Friedel-Crafts reaction of indole with cyclic imines has been develeoped. This organocatalytic reaction provides for the first time optically active indolindolinone derivatives in high yields and with excellen

Full text of DOI:10.1002/adsc.201000952

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DOI: 10.1002/adsc.201000952
Asymmetric Brønsted Acid-Catalyzed Friedel–Crafts Reactions of
Indoles with Cyclic Imines - Efficient Generation of Nitrogen-
Substituted Quaternary Carbon Centers
Magnus Rueping,a,* Sadiya Raja,a and Alberto NfflÇezb
a
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax : (+49)-241-80-92665; e-mail: magnus.rueping@rwth-aachen.de
Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid, Spain (part of a three-month predoctoral stay)
b
Received: December 16, 2010; Revised: January 25, 2011; Published online: March 2, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000952.
Abstract: A new enantioselective Brønsted acid-
catalyzed Friedel–Crafts reaction of indole with
cyclic imines has been develeoped. This organocata-
lytic reaction provides for the first time optically
active indolindolinone derivatives in high yields and
with excellent enantioselectivities (up to 91% ee)
under mild reaction conditions.
proved to be highly efficient for these transforma-
tions.[10–14]
However, despite an increasing number of success-
ful organocatalytic enantioselective approaches for
the reaction of indoles with acyclic imines,[10–13] an
analogous reaction involving cyclic imines has not yet
been described.[15] Based on our previous success on
Brønsted acid-catalyzed asymmetric reactions,[16] we
decided to examine the Friedel–Crafts reaction of in-
doles with aromatic cyclic imines to obtain optically
active indolindolinone derivatives with a quaternary
chiral center bearing a nitrogen atom (Scheme 1).[14b]
In this article we present our preliminary results.
Initially, we started our investigation by testing the
Friedel–Crafts reaction of indole (2a) with imine 1a in
Keywords: BINOL phosphate; enantioselective
Friedel–Crafts reaction; organocatalysis; quaternary
carbon centers
One of the most prominent structural motifs, which is the presence of 5 mol% of chiral phosphoric acids
found in various natural products and drugs with well 4a–j bearing bulky substituents in the 3,3’-positions
recognized pharmacological properties, is the indole (Table 1).
skeleton.[1] A frequently used method for the synthe-
Compound 4j (Ar=SiPh3) was found to be the best
sis of optically active indole derivatives is the enantio- catalyst with respect to enantioselectivity (Table 1,
selective Friedel–Crafts reaction with chiral metal entry 11). The desired product 3a was formed with
complexes or chiral organocatalysts.[2–5] Among the 77% ee when the reaction was performed at À608C in
vast array of explored processes, considerable efforts toluene. No reaction was observed using the same cat-
have been devoted to the development of enantiose- alyst, when the temperature was decreased to À788C
lective Friedel–Crafts reactions of indoles with imines (Table 1, entry 12).
as electrophilic partners, as these reactions offer easy
To enhance the enantioselectivity, our investiga-
access to chiral indolylalkylamine derivatives.[6] tions focused on varying the organic solvents for the
Brønsted acid catalysts, in particular the recently in- reaction. Carrying out the reaction in CHCl3 at
troduced chiral phosphoric acid organocatalysts,[7–9] À608C resulted in a higher selectivity (Table 2,
Scheme 1. Brønsted acid-catalyzed Friedel–Crafts reaction of indoles with cyclic imines.
Adv. Synth. Catal. 2011, 353, 563 – 568
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563

Magnus Rueping, et al.
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Table 1. Evaluation of different Brønsted acid catalysts.
revealed that the employed solvent has a substantial
influence on both selectivity and reactivity. Hence,
different mixtures of solvents and different tempera-
tures were investigated. When a 3:1 mixture of CHCl3
and toluene was used, the reactivity and selectivity
were improved considerably, with the best results
being obtained at À708C (Table 2, entry 6). Increasing
the CHCl3/toluene ratio to 8:1 had a deleterious
effect on both yield and enantioselectivity (Table 2,
entry 7). The same effect was observed when toluene
was replaced by CH2Cl2 in the solvent mixture
(Table 2, entry 8).
Entry[a]
4
Ar
T [8C]
ee [%][b]
With the optimal conditions in hand, the scope of
the Friedel–Crafts reaction was explored by first vary-
ing the aryl substituent of the imine 1 (Table 3). Vari-
ous cyclic imines 1a–j with electron-donating and
electron-withdrawing substituents were applied in this
1
2
3
4
5
6
7
8
a
b
c
c
d
e
f
g
h
i
2-naphthyl
À60
À60
À60
À78
À60
À78
À60
À78
À78
À78
À60
À78
36
38
45
53
50
56
71
31
20
44
77
–
2-naphthyl [H8]
1,3,5-iPr-C6H2
1,3,5-iPr-C6H2
9-phenanthryl
3,5-(CF3)2-C6H3
4-NO2-C6H4
Table 3. Scope of the Friedel–Crafts reaction with substitut-
ed imines.
anthracyl
9
4-C6H5-C6H4
3,5-tBu2-4-MeO-C6H2
SiPh3 [H8]
10
11
12
j
j
SiPh3 [H8]
[a]
Reactions were performed with imine 1a, indole 2a
(1.2 equiv.) and 5 mol% 4 in toluene.
Enantiomeric excess was determined by HPLC on a
[b]
chiral phase.
Table 2. Evaluation of the solvent and reaction temperature.
Entry[a] Product
Time
[h]
Yield
[%][b]
ee
[%][c]
1
3
6
91
92
91
86
Entry[a] Solvent
T [8C] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
toluene
toluene
CHCl3
CH2Cl2
CHCl3:toluene 3:1 À65
CHCl3:toluene 3:1 À70
CHCl3:toluene 8:1 À65
CHCl3:CH2Cl2 8:1 À65
À78
À65
À60
À65
–
–
2
86
80
75
90
91
74
74
77
86
81
89
91
83
78
3
6
4
96
87
86
79
[a]
[b]
[c]
Reactions were performed with imine 1a, indole 2a
(1.2 equiv.) and 5 mol% 4j.
Yield of the isolated product after column chromatogra-
phy.
Enantiomeric excess was determined by HPLC on a
chiral phase.
4[d]
entry 3), while the reaction in toluene led to a better
yield at À658C (Table 2, entry 2). These experiments
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Asymmetric Brønsted Acid-Catalyzed Friedel–Crafts Reactions of Indoles with Cyclic Imines
Table 4. Application of different substituted indole deriva-
tives in the Friedel–Crafts reaction.
Table 3. (Continued)
Entry[a] Product
Time
[h]
Yield
[%][b]
ee
[%][c]
5
6
5
4
92
94
88
89
Entry[a] Product
Time
[h]
Yield
[%][b]
ee
[%][c]
1
2
98
92
94
88
90
91
88
80
84
88
7
8
9
2
93
49
90
91
90
84
2
3
4
32
3
4
12
3
10
12
79
87
5
4
[a]
Reactions were performed with imine 1, indole 2a
(1.2 equiv.) and 5 mol% 4j.
Yield of the isolated product after column chromatogra-
[a]
Reactions were performed with imine 1a, indole 2
(1.2 equiv.) and 5 mol% 4j.
Yield of the isolated product after column chromatogra-
[b]
[c]
[d]
[b]
[c]
phy.
Enantiomeric excess was determined by HPLC on a
chiral phase.
The reaction was carried out at À658C.
phy.
Enantiomeric excess was determined by HPLC on a
chiral phase.
reaction. When imines with electron-withdrawing ble decrease in reactivity was observed in the case of
groups were applied, the indolindolinones were fur- meta substituted imine (Table 3, entries 7 and 8). Ac-
nished in high yields and good enantioselectivities. In cordingly, when the reaction was carried out with
contrast, imine 1d with a para-methoxy electron-do- meta bromo-substituted imine the desired product
nating group afforded the product with a slightly was generated in moderate yield of 49% (Table 3,
lower enantioselectivity (Table 3, entry 4).
entry 8) but with excellent enantioselectivity.
Despite comparable selectivities obtained with the
Subsequently, different substituted indoles 2 were
para and meta bromo-substituted imines, a considera- evaluated in the Brønsted acid-catalyzed Friedel–
Adv. Synth. Catal. 2011, 353, 563 – 568
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565

Magnus Rueping, et al.
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Crafts reaction (Table 4). In general, indoles with Experimental Section
electron-withdrawing and electron-donating groups
were both successfully applied providing the indolin- General Procedure for Friedel–Crafts Reaction of
dolinones 3k-o in high yields and with good enantio- Indole with Cyclic Imines
control. Apart from the indole bearing a substituent
at the C-7 position which required an extended reac-
tion time for completion (Table 4, entry 2), substitu-
In a typical experiment the imine 1 (0.040 g) and indole 2
(1.2 equiv.) were suspended at 0.05M in CHCl3/toluene
(3:1) in a screw-capped test tube and stirred at À708C for
tion did not influence the yield or selectivity. Whereas
the reaction with 5-methyl-indole was completed in
2 h, the reaction with 7-methylindole was 16 times
slower (Table 4, entries 1 and 2). Nevertheless, the de-
sired products were attained in both cases in very
high yields and with good enantioselectivities. When
N-methylindole was used as a nucleophile under the
optimized reaction conditions the product was isolat-
ed in good yield, but with lower enantioselectivity
(54% ee). Thus, the presence of a nitrogen-hydrogen
bond in the indole may be crucial in order to obtain
the products with high enantioselectivity.
10 min. The catalyst (5 mol%) was added to the solution
and the mixture was stirred for 2–32 h. The crude reaction
mixture was directly charged on silica gel and purified by
column chromatography (cyclohexane/ethyl acetate as
eluent) to afford the desired products.
Acknowledgements
Generous support by the Deutsche Forschungsgemeinschaft
(Priority Program 1179 Organocatalysis) is gratefully ac-
knowledged.
The absolute configuration of the indolindolinone
products was assigned by X-ray crystal structure anal-
ysis of compound 3m (Figure 1).
In summary, we have developed an asymmetric
Brønsted acid-catalyzed Friedel–Crafts reaction of
differently substituted indoles with cylic imines. This
method represents the first enantioselective aza-Frie-
del–Crafts reaction with cyclic imines as electrophiles
and provides the corresponding optically active indol-
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 563 – 568