From alcohols to indoles: A tandem Ru catalyzed hydrogen-transfer Fischer indole synthesis

Article abstract of DOI:10.1021/ol3030956

In a new version of the Fischer indole synthesis, primary and secondary alcohols have been catalytically oxidized in the presence of phenylhydrazines and protic or Lewis acids to give the corresponding indoles. The overall reaction can be accomplished in one step, and the use of alcohols instead of aldehyes or ketones as starting materials has several advantages in terms of a large selection of reagents, easy handling, and safety of the process.

Full text of DOI:10.1021/ol3030956

ORGANIC
LETTERS
2012
Vol. 14, No. 23
6112–6115
From Alcohols to Indoles: A Tandem
Ru Catalyzed Hydrogen-Transfer Fischer
Indole Synthesis
Andrea Porcheddu,*,† Manuel G. Mura,† Lidia De Luca,† Marianna Pizzetti,‡ and
Maurizio Taddei‡
ꢀ
Dipartimento di Chimica e Farmacia, Universita degli Studi di Sassari, via Vienna 2,
07100 Sassari, Italy, and Dipartimento di Biotecnologie, Chimica e Farmacia,
ꢀ
Universita di Siena, Via A. Moro 2, 53100 Siena, Italy
anpo@uniss.it
Received November 13, 2012
ABSTRACT
In a new version of the Fischer indole synthesis, primary and secondary alcohols have been catalytically oxidized in the presence of phenyl-
hydrazines and protic or Lewis acids to give the corresponding indoles. The overall reaction can be accomplished in one step, and the use of
alcohols instead of aldehyes or ketones as starting materials has several advantages in terms of a large selection of reagents, easy handling, and
safety of the process.
The indole skeleton is a privileged molecular scaffold in
nature and plays a prominent role in drug discovery due
to its high receptor binding affinity.1 Although there are
many powerful methodologies for the construction and
functionalization of indoles, there is still today a strong
demand for a versatile, efficient, and regioselective synth-
esis of these heterocycles.2 The Fischer cyclization continues
to be one of the most popular approaches to indoles 130
years after its discovery due to its operational simplicity.3
A potential bothersome restriction of the reaction isthe use
of enolizable aldehydes that may be unstable toward the
acids required in the process, causing side reactions.4 Con-
sequently, synthetic equivalents such as acetals or imines
are often used in placeof aldehydes toget in situgeneration
of the free carbonyl group.5
†
ꢀ
Universita degli Studi di Sassari.
‡
ꢀ
Universita di Siena.
(1) For some selected recent examples of biological activities of
indoles, see: (a) Sundberg, R. J. Indoles; Academic Press: London,
1996. (b) Petit, S.; Duroc, Y.; Larue, V.; Giglione, C.; Leon, C.; Soulama,
C.; Denis, A.; Dardel, F.; Meinnel, T.; Artaud, I. ChemMedChem 2009, 4,
261–275. (c) Sharma, V.; Kumar, P.; Pathak, D. J. Heterocycl. Chem.
2010, 47, 491–502. (d) Kochanowska-Karamyan, A. J.; Hamann,
M. T. Chem. Rev. 2010, 110, 4489–4497. (e) Patil, S. A.; Patil, R.;
Miller, D. D. Curr. Med. Chem. 2011, 18, 615–37. (f) Biswal, S.; Sethy,
S.; Kumar, H. K. S.; Banerjee, M. Asian J. Pharm. Clin. Res. 2012, 5,
1–6. (g) Ali, N. A.; Dar, B. A.; Pradhan, V.; Farooqui, M. Mini Rev.
Med. Chem. 2012, 12, 1–16. (h) Johansson, H.; Jørgensen, T. B.;
In an effort to improve this well-established reaction, we
decided to investigate the possibility of completing the
overall Fischer indole synthesis starting from readily
€
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r
10.1021/ol3030956
Published on Web 11/28/2012
2012 American Chemical Society

available and inexpensive alcohols, applying a “hydrogen
auto-transfer”6 based protocol (also denoted as “borrowing
hydrogen”,7 Scheme 1).
Table 1. Synthesis of Indoles: Optimization of the Reaction
Conditionsa
entry
catalyst
solvent t (°C)
Hþ
yield (%)b
1
Pd(PPh3)4
toluene
toluene
toluene
toluene
toluene
toluene
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
150 H2SO4
ꢀ
Scheme 1. Protocol for Indole Synthesis Starting from Alcohols
and Arylhydrazines
2
PdCl2(PPh3)2
RhCl(PPh3)3
RhH(CO)(PPh3)3
Rh(acac)3
ꢀ
3
7
4
12
15
35
18
15
11
53
77
75c
61d
69
65
84
81
89
86f
92g
93h
90i
22j
5
6
[Cp*Cl2Ir]2
7
[RuCl2(p-cymene)]2 toluene
8
RuHCl(CO)(PPh3)3 toluene
With this approach, sensitive carbonyl compounds could
be generated in situ with high efficiency and in low concen-
tration starting from alcohols, minimizing both unwanted
side reactions and tedious purifications.8 We report here
for the first time that it is possible to directly transform a
primary and a secondary alcohol into an indole, in the
presence of phenylhydrazine, through a one-pot tandem
Ru catalyzed H-transfer Fischer indole synthesis.9 To
study the influence of different parameters such as the
nature of the metal, ligands, solvents, and temperature, the
reaction between N-methyl-N-phenylhydrazine 1a with
1-propanol 2a was investigated as a model system (Table 1).
9
RuCl2(PPh3)3
Ru3(CO)12
toluene
toluene
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Ru3(CO)12/BIPHEP toluene
Ru3(CO)12/BIPHEP toluene
Ru3(CO)12/BIPHEP toluene
Ru3(CO)12/BIPHEP CPME
Ru3(CO)12/BIPHEP dioxane 150 H2SO4
Ru3(CO)12/BIPHEP TAAe
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
Ru3(CO)12/BIPHEP TAA
150 H2SO4
150 AcOH
150 ZnCl2
150 ZnCl2
150 ZnCl2
130 ZnCl2
170 ZnCl2
100 ZnCl2
(6) (a) Guillena, G.; Ramon, D. J.; Yus, M. Angew. Chem., Int. Ed.
ꢁ
a Unless otherwise specified, the reactions were carried out in a closed
vessel inserted in a preheated oil bath (one-pot reaction): N-Methyl-N-
phenylhydrazine (1.0 mmol), crotononitrile (1.0 mmol), catalyst (5 mol %),
ligand (15 mol %), acid additive (1.0 mmol) solvent (2.5 mL), under Ar,
150 °C, overnight. b Yields of isolated pure product. c The catalyst
loading has been reduced to 2 mol % (BIPHEP: 3 mol %). d Reaction
performed using 1 mol % of catalyst. e TAA = 2-methyl-2-butanol (tert-
amyl alcohol). f One-pot, two-step procedure. g Reaction performed under
microwave dielectric heating (MW) at 150 °C (3 h). h MW at 130 °C (3 h).
i MW at 170 °C (3 h). j MW at 100 °C (3 h).
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J. F.; Kim, I. S.; Patman, R. L.; Krische, M. J. Angew. Chem., Int. Ed.
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Berlin, 2011; pp 77ꢀ106.
The screening started by examining different Pd, Rh, Ir,
and Ru based complexes, which are the most successfully
and widely used catalytic systems in the N-alkylation of
amines by alcohols via borrowing hydrogen methodology
(Table 1, entries 1ꢀ10).10 Preliminary results showed that
the ruthenium carbonyl cluster [Ru3(CO)12] was the most
active catalyst giving 1,3-dimethyl indole in moderate yield
after heating reagents 1a and 2a in the presence of croto-
nonitrile (as the hydrogen acceptor) at 150 °C in toluene
overnight (Table 1, entry 10). The addition of phoshine
based ligands improved the yield, and BIPHEP11 gave the
best results in terms of yield and purity of the reaction
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2039–2042. (j) De Luca, L.; Porcheddu, A. Eur. J. Org. Chem. 2011,
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Taddei, M. Adv. Synth. Catal. 2012, 354, 2453–2464. (l) Watson, A. J. A.;
Maxwell, A. C.; Williams, J. M. J. Org. Biomol. Chem. 2012, 10, 240–243.
(9) Although in 1987 an indole synthesis through a ruthenium catalyzed
reaction of anilines with 1,2-diols was described, this procedure showed
several drawbacks. Moreover, to promote the indole ring synthesis a
considerable excess of arylamines or alternatively toxic SnCl2 is often
required. In addition, all these noteworthy strategies are suffering from
the scarce commercial availability of diols, limiting the construction of
highly substituted indoles. See: (a) Tsuji, Y.; Huh, K.-T.; Watanabe, Y.
J. Org. Chem. 1987, 59, 1673–1680. (b) Cho, C. S.; Lee, M. J.; Shim, S. C.;
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Org. Lett., Vol. 14, No. 23, 2012
6113

mixture (Scheme 2). It is worthy of note that some
phosphines promoted the NꢀN bond breakage with for-
mation of the aniline and contemporary decrease of the
indole yield.12 The catalyst loading was further reduced to
2 mol % without significantly affecting the yield of 3a
(Table1, entry 12). Conversely, with 1 mol % Ru3(CO)12, a
sharp drop in yield occurred (Table 1, entry 13). Also, no
indole ring was obtained in the absence of the metal
catalyst.
Scheme 3. Effect of Substituent on the Phenylhydrazine Ring
Scheme 2. Influence of Various Phosphine Ligands on the
Indolization Reactiona
These conditions allowed aryl functionalized indole deri-
vatives to be prepared in high yields. In general, both
electron-donating and -withdrawing groups, irrespective
of the position on the aryl ring, were tolerated and had no
significant impact on the yield, allowing the preparation of
different functionalized indoles (3bꢀ3f in Scheme 3). As
expected, a meta substituted phenylhydrazine led to an
approximately equimolar mixture of both regioisomers 3f1
and 3f2 in 80% overall yield (Scheme 3).14
a Ratio determined by GC/MS.
Encouraged by these results, the reaction of several
arylhydrazines with an array of commercially available
primary alcohols (C2ꢀC3 indole fragment) was investi-
gated (Scheme 4). No remarkable variations in terms of
yields and purities of the corresponding indoles were
observed using a primary alcohol higher than 1-propanol
(Scheme 4, indoles 3gꢀ3i). Good yields were also achieved
using benzyl alcohols and other aliphatic alcohols contain-
ing an aryl ring (Scheme 4, indoles 3jꢀ3l). The use of the
unsubstituted phenylhydrazines was also possible provid-
ing NH-free indoles, albeit in moderate yields (Scheme 4,
indoles 3mꢀ3o). Secondary alcohols have also been used
to introduce additional substituents at both C-2 and C-3 of
the indole ring. Reaction of N-methyl-phenylhydrazine
with various commercially available symmetrical aliphatic
and cyclic secondary alcohols was carried out in the
presence of the Ru catalyst and ZnCl2. All of the substrates
were transformed into the corresponding 2,3-dialkylin-
doles in good yields (Scheme 4, indoles 3p and 3v).
2-Hydroxyalkanes exclusively gave the indole15 derived
by cyclization on the more highly substituted side of the
corresponding hydrazones (Scheme 4, indoles 3r and 3s).
Conversely, other unsymmetrical secondary alcohols led
to the corresponding indoles as a 65/35 mixture of two
regioisomers16 (Scheme 4, indoles 3t1 and 3t2). This pro-
tocol was alsoapplicable toa variety of alcohols bearing an
additional functional group on the aliphatic chain, and
indoles 3uꢀ3z were obtained in moderate to good yields
(Scheme 4).
Although several olefins can be used as a hydrogen
acceptor, crotononitrile was preferred as a hydrogen sca-
venger on the basis of volatility of the alkene and its
hydrogenated product. At least 1.0 equiv of crotononitrile
was required for good results.13 Next, the influence of
different solvents on the reaction efficiency was checked,
observing that 2-methyl-2-butanol and toluene gave the
best results in terms of yield and purity (Table 1 entries 11
and 16). After these optimizations, a variety of acidic
catalysts were investigated (Table 1, entries 17ꢀ18), and
ZnCl2 was found to be superior and of general applicability.
To these optimized reaction conditions, microwave di-
electric heating was applied finding that the reaction
worked well heating the reagents at 130 °C for 3 h in
2-methyl-2-butanol (93% see Table 1, entry 21). With
microwaves, a temperature increase had no appreciable
effects on the reaction yields, while reducing the tempera-
ture or shortening the reaction time reduced the amounts
of 3a obtained (Table 1, entries 22 and 23). Based on these
results, the reaction conditions described in entry 21 were
chosen as optimal.
Having developed a reliable method for the catalytic
synthesis of indoles from alcohols and arylhydrazines, the
reaction scope and limitations of the protocol were inves-
tigated. In order to evaluate the influence of substituents
on the aromatic ring, the reaction of 1-propanol with
various commercially available aryl hydrazines was ex-
plored (Scheme 3).
The present method was mild enough to tolerate a wide
variety of common functional groups on starting materials,
(11) (a) BIPHEP = 2,20-bis(diphenylphosphanyl)-1,10-biphenyl. (b)
dppp = 1,3-Bis(diphenylphosphino)propane. (c) Xantphos = 4,5-Bis-
(diphenylphosphino)-9,9-dimethylxanthene. (d) Davephos = 2-Dicy-
clohexylphosphino-20-(N,N-dimethylamino)biphenyl.
(14) The regioisomer ratio (6/4) of indoles 3f1 and 3f2 was determined
by 1H NMR.
(12) The NꢀN bond breaking of arylhydrazine may occur, as a side
reaction, when the alcohol oxidation is slowed down by different factors
producing the anilines as byproducts. However, this behavior is mini-
mized using the [Ru3(CO)12]/BIPHEP catalyst.
(13) Control experiments indicated that no reaction occurred with-
out the hydrogen acceptor.
(15) Indoles 3r and 3s were prepared starting from 2-pentanol and
4-phenyl-2-butanol respectively.
(16) Indoles 3t1 and 3t2 were prepared starting from 3-heptanol. The
regioisomer ratio was determined by 1H NMR.
6114
Org. Lett., Vol. 14, No. 23, 2012

catalyzed aromatic [3 þ 3] sigmatropic rearrangement
gives access to the desired indole ring 3 (Scheme 5).
Scheme 4. Different Indoles Prepared from Arylhydrazines and
Alcoholsa
Scheme 5. Mechanistic Proposals for the Catalyzed Synthesis of
Indoles
In summary, we developed a simple, convenient, and
efficient method for the preparation of indole derivatives
combining the classical Fischer indole synthesis with the
innovative hydrogen autotransfer technology. The most
remarkable advantages in using alcohols in the place of
aldehydes or ketones as starting material are the easy
handling, the lower toxicity, the higher stability, and the
large selection of commercially available alcohols to be
employed in the reaction.18 The ability to easily incorpo-
rate on the indole ring a further element of molecular
diversity, which may be subsequently modified, makes this
synthetic protocol particularly attractive for increasing the
molecular complexity. In addition, microwave irradiation
significantly increases the reaction rate enhancing the final
indole yields.
a Reaction conditions: N-substituted-N-arylhydrazine (1.0 mmol),
crotononitrile (1.0 mmol), [Ru3(CO)12] (2 mol %), BIPHEP ligand
(3 mol %), ZnCl2 (1 equiv), 2-methyl-2-butanol (2.5 mL), under Ar, MW
130 °C, 3 h. Yields of isolated products after column chromatography.
which may be subsequently subjected to further processing
in order to increase the molecular diversity of final pro-
ducts. Inspired by previously published mechanisms,7 the
most probable reaction pathway for the conversion of aryl
hydrazines and alcohols into indoles is outlined in Scheme 5.
We believe that the reactions may proceed via an initial
oxidation of alcohol 2 into the corresponding carbonyl
compound 5 by formal transfer of a hydrogen molecule to
crotononitrile with concomitant regeneration of the cat-
alyst. The in situ generated carbonyl compound 5, in the
presence of arylhydrazine, is immediately converted into
the corresponding hydrazone 6.17 The subsequent acid
Acknowledgment. Thisworkwas supported byRegione
Autonoma della Sardegna (Grant No. J71J1 0000250002
through the Basic Research Program).
(17) The hydrazone intermediate was isolated in the reaction per-
formed without the presence of the acid catalyst.
(18) Although suffering from the use of phenylhydrazines, this reac-
tion has the advantage of the use of a Ru catalyst (2 mol %), and it could
be considered a valid alternative to the recently developed Pd catalyzed
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
€
(5ꢀ10 mol %) Fischer type indole synthesisbased on anilines. (a) Wurtz,
€
S.; Rakshit, S.; Neumann, J. J.; Droge, T.; Glorius, F. Angew. Chem.,
€
Int. Ed. 2008, 47, 7230–7233. (b) Neumann, J. J.; Rakshit, S.; Droge, T.;
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Wurtz, S.; Glorius, F. Chem.;Eur. J. 2011, 17, 7298–7303. (c) Shi, Z.;
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The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 23, 2012
6115