Iodine-mediated synthesis of 3 H -indoles via intramolecular cyclization of enamines

Article abstract of DOI:10.1021/jo100796s

(Figure presented) The synthesis of 3H-indoles was achieved via the iodine-mediated intramolecular cyclization of enamines. A wide variety of 3H-indole derivatives bearing multifunctional groups were obtained in good to high yields under transition metal-free reaction conditions.

Full text of DOI:10.1021/jo100796s

pubs.acs.org/joc
3H-indole derivatives are very limited.4 Therefore, a general
Iodine-Mediated Synthesis of 3H-Indoles via
Intramolecular Cyclization of Enamines
and efficient method for the synthesis of 3H-indole deriva-
tives is an attractive and formidable challenge in synthetic
chemistry.5
Zhiheng He, Huanrong Li, and Zhiping Li*
In the course of our studies on C-H bond oxidation and
C-C bond formation,6 we unexpectedly found that elemen-
tal iodine (I2)7 could efficiently promote an intramolecular
cyclization of enamines.8,9 The current process presents a
novel and efficient method to construct the 3H-indole ske-
leton. Herein we report our efforts on the synthesis and
preliminary applications of 3H-indoles.
Department of Chemistry, Renmin University of China,
Beijing 100872, China
zhipingli@ruc.edu.cn
Received April 23, 2010
(Z)-Ethyl 2-methyl-3-phenyl-3-(phenylamino)acrylate (1a)
was used as a model substrate for optimization of the reaction
conditions (Table 1). An extensive investigation of a range of
oxidants, bases, and solvents was carried out. The desired
product 2a, ethyl 3-methyl-2-phenyl-3H-indole-3-carboxy-
late, was obtained in 82% yield when elemental iodine was
used as oxidant with K2CO3 as base and DMF as solvent
(entry 1). A lower yield of 2a (60%) was formed when IBr
was employed instead of I2 (entry 2). Hypervalent iodine
reagents, phenyliodine(III) diacetate (PIDA) and pheny-
liodine(III) bis(trifluoroacetate) (PIFA), were much less
effective for 3H-indole formation (entries 3 and 4). Other
inorganic bases, such as Na2CO3, NaHCO3, and Cs2CO3,
gave comparable yields of 2a, while an organic base, 2,6-
lutidine, afforded a lower yield of 2a (Table 1, entries 5-8).
Other solvents were inferior to DMF with regard to the
yield of the desired 3H-indole, for example, CH3NO2 (17%
yield) and toluene (33% yield) (entries 9 and 10). Product
2a was not observed in the absence of I2 or base (entries 11
and 12).
The synthesis of 3H-indoles was achieved via the iodine-
mediated intramolecular cyclization of enamines. A wide
variety of 3H-indole derivatives bearing multifunctional
groups were obtained in good to high yields under
transition metal-free reaction conditions.
Indole alkaloids are ubiquitous heterocycles in natural
products and play pharmacologically important roles
because of the broad spectrum of their biological activ-
ities. Many efforts have therefore been made to synthesize
the indole skeleton.1 3H-Indole is one of the most impor-
tant structural units found in nature2 and has been utilized
as a key scaffold to construct various indoline alkaloids.3
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indole derivatives, the methodologies for the synthesis of
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2008, 108, 3395. (c) Kruger, K.; Tillack, A.; Beller, M. Adv. Synth. Catal.
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4636 J. Org. Chem. 2010, 75, 4636–4639
Published on Web 06/04/2010
DOI: 10.1021/jo100796s
r
2010 American Chemical Society

He et al.
The substrate variation was then investigated. To our satis-
faction, the reaction shows a wide scope for the structural
JOCNote
tions (Table 2). Both electron-donating and electron-with-
drawing groups at the para- or ortho-position of N-phenyl
enamines 1 allowed smooth transformation of 1 into the
corresponding products with good yields (entries 1-6). It
should be noted that the reaction could be carried out on
2 mmol scale with similar efficacy (entry 2). R-Naphthalenyl
enamine was also transformed into the corresponding indole
(entry 7). The meta-substituted enamine 1g afforded two
regioisomers with a ratio of 1:2.2, where 4-methyl-substituted
product 2g0 was the major product (entry 8). Ethyl- and
benzyl-substituted enamines 1h and 1i afforded the desired
3H-indoles with moderate yields (entries 9 and 10), whereas
diester enamines1j,k gavethe corresponding productsingood
to excellent yields (entries 11-13). Aryl-substituted substrate
1m also led to 2m exclusively albeit in a lower conversion
(entry 14). Cyanoethyl product 2n formed in 74% yield;
however, a prolonged reaction time was necessary to consume
the starting enamine 1n (entry 15). Benzoyl enamine 1o was
also transformed into the corresponding product 2o in good
yield (entry 16).
variation of enamine 1 under the optimized reaction condi-
TABLE 1. Optimization of Reaction Conditionsa
entry
oxidant
base
solvent
yield (%)b
1
2
3
4
5
6
7
8
9
I2
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaHCO3
Cs2CO3
2,6-lutidine
K2CO3
K2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CH3NO2
toluene
DMF
DMF
82 (76)
60
IBr
PIDA
PIFA
I2
I2
I2
I2
I2
I2
5
N.D.c
78
77
d
79
39
17
33
N.D.
N.D.
10
11
12
K2CO3
I2
With the synthetic method for 3H-indole derivatives in
hand, preliminary applications of the obtained 3H-indoles
2 were investigated. The CdN double bond of 2a was
reduced by common reducing reagents effectively (eqs 1
and 2). A 1:1 ratio of the diastereomers, 310 and 30, was
obtained when NaBH4 was used (eq 1). In contrast, both
the CdN double bond and the ester group were reduced by
LiAlH4 (eq 2). Interestingly, only one isomer 4 was formed
together with decarboxylation product 5 when LiAlH4
was used. This result indicated that the ester group of 2a is
first reduced to a hydroxyl group and then acts as a
directing group for the reduction of the CdN bond via a
chelated transition state. This is supported by the outcome
of the reduction of a 1:1 ratio of 3 and 30 by LiAlH4, which
gave a 1:2.1 diastereisomeric mixture of 4. With regard to
the formation of 5, presumably the carboxylate formed
from 2a under basic condition is susceptible to decarbo-
xylation.
aConditions unless otherwise noted: 1a (0.25 mmol), oxidant (0.28
mmol), base (0.30 mmol), and solvent (1.0 mL). bNMR yields are
determined by 1H NMR spectroscopy with mesitylene as an internal
standard; isolated yield is given in parentheses. cNot detected by TLC.
d2.4 equiv.
TABLE 2. Synthesis of 3H-Indolesa
Polycyclic indoline alkaloids are frequently found in nat-
ural products and pharmaceuticals; direct cyclization from
the 3H-indole represents a convenient and powerful method
to construct such structures.11 The cycloadduct 6 was selec-
tively formed when a mixture of 3H-indole 2a and dim-
ethyl but-2-ynedioate was treated with trimethylsilylmethyl
aConditions unless otherwise noted: 1 (0.25 mmol), I2 (0.28 mmol),
K2CO3 (0.30 mmol), and DMF (1.0 mL), 100 °C, 1 h. bIsolated yields.
c1a (2.0 mmol), I2 (2.2 mmol), K2CO3 (2.4 mmol), and DMF (5.0 mL).
d2 h.
(10) Hodges, J. C.; Wang, W.; Riley, F. J. Org. Chem. 2004, 69, 2504.
(11) Fishwick, C. W. G.; Jones, A. D.; Mitchell, M. B.; Eggleston, D. S.;
Baures, P. W. Synlett 1990, 6, 359.
J. Org. Chem. Vol. 75, No. 13, 2010 4637

He et al.
JOCNote
SCHEME 1. An Initially Proposed Pathway for the Formation
of 2
SCHEME 2. A Proposed Mechanism for the Formation of 3H-
Indole 2
triflate in toluene (eq 3). The structure of 6 was confirmed by
NOE analysis.
role of I2 in this reaction, a catalytic amount of NaI instead of
stoichiometric I2 was used and 9 was obtained with a 92%
yield (eq 7). These results suggested that an iodide inter-
mediate E, presumably formed by nucleophilic displacement
of Cl by I from the tautomer of 8, is involved in the present
transformation. Moreover, the application of a catalytic
amount NaI in eq 7 also indicated that a three-membered
iodonium intermediate A or D is not involved in the forma-
tion of the indole ring.
Interestingly, the reaction of 2a with allylmagnesium chlo-
ride afforded ring-opening product 7 (eq 4).12 When the
reaction mixture was quenched by deuterated water, 7d
was obtained exclusively (eq 5). Presumably an addition-
elimination reaction occurs with addition of the first equiva-
lent of reagent to give an equilibrium between the expected
addition product and an ester enolate and imine formed from
cleavage of the C2-C3 bond. Attack of a second equivalent
of allylmagnesium chloride on the imine and quenching of
the enolate with D2O would generate 7d.
Furthermore, a series of competition experiments were
performed to address the influences of the electronic proper-
ties of enamines in the present transformation.14 The results
showed that the reaction rate of N-phenyl enamines 1 with
electron-donating groups at the para-position is faster than
those with electron-withdrawing groups, which agrees with
the Friedel-Crafts aromatic alkylation. On the basis of these
results and the literature,15 a tentative mechanism of the
formation of 3H-indole 2 is proposed (Scheme 2). The
oxidative iodination generates an iodide intermediate F.
Consequently, the intermediate G is formed via an intramo-
lecular Friedel-Crafts aromatic alkylation reaction, fol-
lowed by rearomatization to give 3H-indole 2.
The initial mechanistic proposal for the formation of 2 was
based on a general iodocyclization pathway (Scheme 1).13
Iodocyclization of the enamine CdC double bond forms a
three-membered iodonium intermediate A, followed by an
intramolecular electrophilic aromatic substitution to give C,
followed by removal of HI under basic condition to give 3H-
indole 2. However, when chloro-substituted enamine 8 was
employed in the reaction, des-chlorine indole product 9 was
formed under the standard reaction conditions (eq 6). The
expected product 10 was not observed. To investigate the
(12) (a) Rodriguez, J. G.; Urrutia, A.; de Diego, J. E.; Martinez-Alcazar,
M. P.; Fonseca, I. J. Org. Chem. 1998, 63, 4332. (b) Eberson, L.; Greci, L.
J. Org. Chem. 1984, 49, 2135.
(13) Selected reviews: (a) Ranganathan, S.; Muraleedharan, K. M.;
Vaish, N. K.; Jayaraman, N. Tetrahedron 2004, 60, 5273. (b) Kitagawa,
O.; Taguchi, T. Synlett 1999, 1191. (c) Robin, S.; Rousseau, G. Tetrahedron
1998, 54, 13681. (d) Dowle, M. D.; Davies, D. I. Chem. Soc. Rev. 1979, 8, 171.
(14) The details are provided in the Supporting Information.
(15) For a representative review, see: Stavber, S.; Jereb, M.; Zupan, M.
Synthesis 2008, 1487.
4638 J. Org. Chem. Vol. 75, No. 13, 2010

He et al.
In summary, we have demonstrated a general and efficient
method for the synthesis of 3H-indoles. A detailed mechan-
istic evaluation and the utilization of elemental iodine for the
synthesis of other heterocycles would further highlight the
advantages of the present methodology.
JOCNote
7.48-7.46 (m, 3H), 7.40 (t, J=7.2 Hz, 2H), 7.25 (t, J=7.6 Hz,
2H), 4.16-4.08 (m, 1H), 4.02-3.96 (m, 1H), 1.71 (s, 3H), 0.98
(t, J=7.2 Hz, 3H); 13C NMR (ppm) δ 178.0, 171.6, 154.9, 141.8,
132.0, 131.1, 129.0, 128.8, 128.4, 126.3, 121.3, 121.2, 62.1, 61.8,
21.0, 13.7; ATR-FTIR (cm-1) 3055, 2990, 1647, 1593, 1572,
1491, 1258, 1173, 1140, 1038, 777, 758, 696; MS (EI) m/z (%) 279
(Mþ), 234, 220, 206, 130, 128, 105, 86, 84 (100), 47, 35, 29;
HRMS (ESI) calcd for C18H18NO2 (Mþ þ H) 280.1332, found
280.1328.
Experimental Section
General Procedure for Products 2. To a mixture of N-aryl
enaminecarboxylate 1 (0.25 mmol), iodine (1.1 equiv), and
K2CO3 (1.2 equiv) was added 1.0 mL of N,N-dimethylforma-
mide (DMF) under nitrogen at room temperature. The reaction
temperature was raised to 100 °C for 1 h. The temperature of the
reaction was reduced to room temperature. The resulting reac-
tion solution was quenched with 20 mL of aqueous ammonia
(5%) and extracted with 3 ꢀ 20 mL of ethyl acetate. The extract
was washed with 2 ꢀ 20 mL of brine and dried over MgSO4. The
solvent was evaporated in vacuo and the residue was purified by
flash column chromatography on silica gel with ethyl acetate/
petroleum ether as an eluent to give the desired product 2a. 1H
NMR (ppm) δ 7.99-7.97 (m, 2H), 7.72 (d, J = 7.6 Hz, 1H),
Acknowledgment. We thank the NSFC (20832002) for
financial support. H.L. thanks RUC (2009030026) for finan-
cial support. We also thank Prof. Zhenfeng Xi at Peking
University and Prof. Matthew Todd at the University of
Sydney for helpful discussion.
Supporting Information Available: Representative experi-
mental procedure, characterization of all new compounds, and
1H NMR and 13C NMR data. This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 13, 2010 4639