Selective ruthenium-catalyzed N-alkylation of indoles by using alcohols

Article abstract of DOI:10.1002/chem.200903144

(Chemical Equation Presentation) Alcohols for alkylation! The first homogeneous-catalyzed N-alkylations of indoles with aliphatic alcohols proceed under transfer hydrogenation conditions. By the use of the Shvo catalyst and p-toluenesulfonic acid (PTSA) t

Full text of DOI:10.1002/chem.200903144

DOI: 10.1002/chem.200903144
Selective Ruthenium-Catalyzed N-Alkylation of Indoles by Using Alcohols
Sebastian Bꢀhn,[a] Sebastian Imm,[a] Kathleen Mevius,[a] Lorenz Neubert,[a]
Annegret Tillack,[a] Jonathan M. J. Williams,*[b] and Matthias Beller*[a]
Indoles constitute important heterocyclic systems in
nature. Based on the broad structural diversity and due to
their biological activity, indoles have become an important
component in several current pharmaceutical drugs.[1]
Hence, the development of improved synthetic methodolo-
gies for the generation of the indole core, as well as func-
tionalization reactions on the aromatic ring system continue
to be of significant interest to organic chemists.[2] Among
the numerous known bioactive indoles, various N-alkylated
derivatives are known.[3] The classical approach for the in-
troduction of these alkyl chains involves treatment of the
indole with base and an alkyl halide.[4] A drawback of these
reactions is the quantitative formation of waste salts. Clear-
ly, this disadvantage can be overcome by using easily avail-
able alcohols as a more benign alkyl source. However, so
far, the N-alkylation of indoles with alcohols has only been
investigated in the presence of Raney nickel.[5] More recent-
ly, Grigg and co-workers[6] demonstrated that simple alco-
hols can be used for selective C-3-alkylation of indoles. This
reaction is based on the so-called “borrowing hydrogen”
methodology.[7] Thereby, an alcohol or amine is initially de-
hydrogenated, then undergoes a functionalization reaction,
and finally is rehydrogenated. Applications of this elegant
methodology have been developed by the groups of Yus,[8]
Fujita,[9] Grigg,[10] Kempe,[11] and us.[12,13]
ested in the selective N-alkylation of this important class of
natural products. At the start of our investigations we stud-
ied the reaction of indole with hexanol as a model system.
In general, this alkylation might result in N-, C-, and dial-
AHCTUNGTERGkNNUN ylated products (Scheme 1).
Scheme 1. Possible alkylations of indole by using hexanol.
An initial catalyst screening of the model reaction re-
vealed that the Shvo catalyst[16,17] 2 (Scheme 2) provided the
best activity and selectivity, whereas all other tested systems,
Based on our ongoing interest in the synthesis of novel in-
Scheme 2. Dissociation of the Shvo complex.
doles[14] and the activation of alcohols,[15] we became inter-
including [{Ru
Xantphos,[19]
N
ACHTUNGTRENNUNG
[a] S. Bꢀhn, S. Imm, K. Mevius, L. Neubert, Dr. A. Tillack,
Prof. Dr. M. Beller
Leibniz-Institut fꢁr Katalyse an der Universitꢀt Rostock e.V.
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-51113
E-mail: matthias.beller@catalysis.de
clohexylphosphino)-1-phenyl-1H-pyrrole,
Cp*=1,2,3,4,5-
[b] Prof. Dr. J. M. J. Williams
pentamethylcyclopentadienyl; yield: <15%, selectivity:
<40%). Therefore, we optimized the reaction conditions
employing 2 (Table 1). By using a small excess of hexanol
and toluene as the solvent at 1108C in the presence of 2
(0.5 mol%) we obtained good conversion (85%) and yield
(77%) of N-hexylindole (Table 1, entry 1). The conversion
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY (UK)
Fax : (+44)1225-383-942
E-mail: j.m.j.williams@bath.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903144.
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Chem. Eur. J. 2010, 16, 3590 – 3593

COMMUNICATION
Table 1. N-Alkylation of indole with hexanol.[a]
Entry Alcohol/ Shvo PTSA Solvent
Conv.[b] Yield[b]
indole
[%]
[%]
[%]
[%]
1
2
3
4
1.2
1.2
1.1
1.1
1.1
1.1
1.1
1.1
0.5
0.5
0.2
0.2
0.2
0.2
0.2
0.2
–
toluene
toluene
toluene
toluene
toluene
toluene
heptane
85
99
99
99
95
93
99
77
45
69
85
81
79
82
72
2
0.1
0.025
0.001
0.025
0.025
0.025
5
6[c]
7
8
t-amyl alcohol 90
[a] Reaction conditions: indole (1 mmol), solvent (0.5 mL), 24 h. [b] Con-
version and yield are determined by GC analysis with hexadecane as the
internal standard; conversion and yield are based on the indole. [c] 18 h.
Scheme 3. Possible mechanism for the N-alkylation of indoles with alco-
hols.
is increased further by adding p-toluenesulfonic acid
(PTSA; Table 1, entries 2–5). However, by using 2 mol% of
PTSA the selectivity of the reaction dropped considerably.
To our delight, lowering the amount of PTSA to
0.025 mol% (Table 1, entry 4) in the presence of only
0.2 mol% catalyst still resulted in quantitative conversion
and an excellent product yield of 85%. Further reduction of
the amount of PTSA (Table 1, entry 5) as well as a reduc-
tion of the reaction time resulted in a decrease of the con-
version. Variation of the solvent (Table 1, entries 7 and 8)
revealed that heptane can be used too, whereas tert-amyl al-
cohol reduced the reactivity of the system.
the optimized conditions (Table 1, entry 4). In fact, N-hex-
AHCTUNGTREGyNNUN lindole is formed in 72% yield. When the experiment was
repeated without catalyst, no product was formed, but the
enamine 3 was obtained as the major product. This shows
the necessity of the catalyst in the final isomerization step.
The groups of Bꢀckvall[22] and Casey[23] demonstrated that
transfer-hydrogenation processes in the presence of 2 pro-
ceed through a monohydride mechanism. This implies that
the deuterium removed from the electron-deficient
a
carbon of the butanol should be transferred to the electron
deficient C-2-position of indole. The isomerization step
should lead to an H:D ratio of 50/50 in the C-2-position. In
agreement with our mechanistic proposal, the resulting N-
butylindole contained 40% deuterium in the C-2-position
(Scheme 4).[24] The discrepancy of 10%, as well as 15% deu-
In addition to the desired transformation, small amounts
of C-3-alkylation and dialkylation of indole, as well as the
formation of hexyl hexanoate are observed as side reactions
(<5%).
Considering the previous work of Grigg and co-workers,[6]
who performed their reactions in the presence of KOH
(20 mol%), our selective N-alkylation is rather remarkable.
The difference in pH seems to be the main reason for the al-
tered regioselectivity. Apparently, the typical “borrowing hy-
drogen”[7] mechanism, in which hexanol is oxidized by the
catalyst to give hexanal, which is attacked by indole, cannot
be applied here. Indeed, direct reaction of hexanal with
indole gave a mixture of N-, C-, and disubstituted indoles,
but only a small amount of the N-substituted product.
Careful analysis of the side products revealed the possibil-
ity of another reaction mechanism. In addition to the N-al-
kylated product the formation of around 3% N-hexyl-2,3-di-
hydroindole is observed under the optimized conditions.
Evidently, this product requires hydrogenation of the indole
ring under the reaction conditions. As shown in Scheme 3,
we propose an initial transfer hydrogenation from the alco-
hol to the indole to give the corresponding aldehyde and in-
doline. Subsequent condensation and final isomerization
leads to the N-alkylated product. To the best of our knowl-
edge, transfer hydrogenations of indoles to indolines have
not been reported before. To support this novel mechanism
we performed the reaction of indoline and hexanal under
Scheme 4. Deuterium incorporation (%) during the alkylation reaction.
terium incorporation at the C-3-position can be explained
by H/D exchange reactions between water and the Shvo
monomer 2b (Scheme 2). In addition, hydrogen incorpora-
tion into the formerly completely deuterated butyl chain
was observed. The increase in reactivity by the addition of
PTSA can also be explained by the proposed mechanism.
Here, we presume that protonation at the C-3-position of
indole speeds up the indoline formation. Yet the question
remains open as to whether the 2,3-double bond is hydro-
genated directly or if subsequent formation of a 1,2-double
bond occurs.
Next, we investigated the scope and limitation of our cat-
alytic system. For this purpose, we studied the reaction of
indole and 5-methoxyindole with different alcohols
Chem. Eur. J. 2010, 16, 3590 – 3593
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M. Beller, J. M. J. Williams et al.
Table 3. N-Alkylation of different indoles with hexanol.[a]
(Table 2). We were pleased to find that indole was N-alkyl-
ACHTUNGTRENNUNGated with different aliphatic alcohols selectively and in very
good yields (Table 2, entries 1–5). Reaction of benzyl alco-
Table 2. N-Alkylation of indole and 5-methoxyindole with different alco-
hols.[a]
Entry
Indole
T
[8C]
Shvo
[%]
Conv.[b]
[%]
Yield[b]
[%]
1[c]
2[c]
120
130
0.5
0.5
89
96
82 (78)
89
3
4
120
130
0.5
0.5
91
96
80 (79)
88
Entry
R1
Alcohol
T
[8C]
Shvo
[%]
Conv.[b]
[%]
Yield[b]
[%]
1[c]
2[c]
H
H
110
110
0.2
0.5
99
97
85 (82)
84 (81)
5
6
7
8
110
110
120
120
0.2
0.2
0.5
0.5
99
97
97
95
93 (92)
83 (81)
86 (76)
68 (62)
3
H
H
110
110
0.2
0.2
89
98
77 (72)
84
4[d]
5
H
120
0.5
93
81 (74)
6
OMe
110
0.5
99
70 (19)
7
OMe
110
0.5
99
80 (53)
8
OMe
OMe
110
110
0.5
0.5
99
99
73 (62)
93 (83)
[a] Reaction conditions: indole (1 mmol), hexanol (1.1 mmol), PTSA
(0.025 mol%), toluene (0.5 mL), 24 h. [b] Conversion and yield are deter-
mined by GC analysis with hexadecane as the internal standard; conver-
sion and yield are based on the indole. [c] Alcohol (2 mmol).
9[c]
10
H
120
0.5
98
94 (81)
tuted double bond is more difficult. Indoles with electron-
rich substituents (Table 3, entries 5 and 6) are easily alkyl-
AHCTUNGTREGaNNNU ted, too. In contrast, indoles with electron-withdrawing
substituents (Table 3, entries 7 and 8) showed reduced reac-
tivity and required higher temperatures and catalyst loading.
Nevertheless, N-alkylations are achieved in good yields and
selectivities.
[a] Reaction conditions: indole or 5-methoxyindole (1 mmol), alcohol
(2 mmol), PTSA (0.025 mol%), toluene (0.5 mL), 24 h. [b] Conversion
and yield are determined by GC analysis with hexadecane as the internal
standard; conversion and yield are based on the indole. [c] Alcohol
(1.1 mmol). [d] Alcohol (3 mmol).
hol with indole resulted in a mixture of the N- and C-alkyl-
A
In conclusion, we have developed the first homogeneously
catalyzed N-alkylation of indoles with alcohols. In the pres-
ence of catalytic amounts of the Shvo catalyst and PTSA
this atom-efficient reaction proceeds highly selectively and
water is formed as the only byproduct. The protocol we
have developed is benign and operationally simple: no addi-
tional hydrogen is required and the reaction can be carried
out in commercially available pressure tubes. With respect
to the mechanism an unusual in situ transfer hydrogenation
of the indole takes place.
kylated with electron-rich, as well as electron-deficient ben-
zylic alcohols (Table 2, entries 6–8). Clearly, by using benzyl-
ic alcohols, the enamine 3 (Scheme 3) cannot be formed.
Thus, we assume that the alkylation proceeds via an imini-
um ion intermediate, which is more stabilized by the elec-
tron-rich 5-methoxyindole than by indole. To our delight,
the N-alkylation of indole with phenyl-1,2-ethandiol also
takes place with excellent selectivity (Table 2, entry 10).
Thereby, the primary hydroxyl group of the diol is converted
and the desired product is obtained in high yield. This reac-
tion constitutes the first example of a selective N-alkylation
of heterocycles with diols. In agreement with this finding,
low reactivity was observed when secondary alcohols were
employed.
Finally, different indoles were examined in reactions with
hexanol (Table 3). Substitution at the 2-position of the
indole ring is tolerated, but slightly increased temperature is
required to obtain high activity and yield (Table 3, entry 1).
Notably, substitution at the C-3-position is also tolerated but
the reactivity was reduced slightly. This is in line with our
proposed mechanism because the hydrogenation of a substi-
Experimental Section
General procedure for the selective N-alkylation of indoles: PTSA
(0.00025 mmol from a tert-butyl methyl ether stock solution) was placed
in an ACE pressure tube and the solvent was removed in an argon
stream. Then indole (1 mmol) and Shvo catalyst (2.2 mg, 0.002 mmol,
0.2 mol%) were added and the pressure tube was purged. Under an
argon atmosphere toluene (0.5 mL) and alcohol (1.1 mmol) were added.
The pressure tube was fitted with a Teflon cap and stirred at 1108C for
24 h. The solvent was removed in vacuo, and the crude product was puri-
fied by column chromatography.
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Chem. Eur. J. 2010, 16, 3590 – 3593

Ruthenium-Catalyzed N-Alkylation of Indoles
COMMUNICATION
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Acknowledgements
This work has been supported by the BMBF (Bundesministerium fꢁr Bil-
dung und Forschung) and the Deutsche Forschungsgemeinschaft (DFG
BE 1931/16-1, Leibniz prize).
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Keywords: alcohols · alkylation · homogeneous catalysis ·
hydrogen transfer · indoles · ruthenium
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1
[24] For H NMR spectra see the Supporting Information.
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Received: November 16, 2009
Published online: March 5, 2010
Chem. Eur. J. 2010, 16, 3590 – 3593
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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