3
conducted using inexpensive graphite and iron plate electrodes
in a beaker open to air (Sheme 1) and LiClO4 was used as the
supporting electrolyte. The reaction procedure was simple,
efficient and the reaction conditions were mild.
a For experimental procedure, see supporting information.
b All compounds are known and were characterized by comparison of their
spectral data with those reported in the literature.8
c Yields of isolated pure compounds 4a.
With the optimized electrolysis parameters in hand, we
applied the method to a range of substrates in order to elucidate
the scope of the reaction (Table 2). We were pleased to find that
reaction works with wide range of aromatic aldehydes (Table 2,
entries 1a-k), aliphatic aldehyde (Table 2, entries 1l), various
types of indoles, viz., indole (Table 2, entry 3a), N-methyl indole
(Table 2, entry 3b), and 5-methoxy indole (Table 2, entry 3c).
Aromatic aldehydes bearing electron withdrawing substituents
seemed to be beneficial for the electrolysis, and the steric
hindrance seemed to have few effects on the results (Table 2,
entries 4h-i). Aliphatic aldehydes, such as pivalaldehyde (Table
2, entry 4n) also formed the desired products in 76% yield,
though it takes longer time 36h to complete the reaction . To
extend the scope of the methodology, two other kinds of indoles
were tested. 5-Methoxy indole reacted well with various
Acknowledgments
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra. Vinay K. Singh is grateful
to UGC, New Delhi, for the award of a DS Kothari postdoctoral
fellowship award No. F.4-2/2006 (BSR)/CH/14-15/0020 and
Rahul Dubey, A. Upadhyay thanks UGC for financial support.
Supplementary data
Supplementary data (detailed experimental procedures,product
characterization, and NMR spectra of the products) associated with this
article can be found,in the online version,at http://
References and Notes
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gave slightly lower yields, around 83% (Table 2, entry 4m).
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Scheme 2. A plausible mechanism for the formation 3-indole
We anticipated that anion of malononitrile II act as
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reaction of malononitrile and in situ generation ethoxide ion in
ethanol solvent. Knoevenagel condensation of aldehyde with
malononitrile anion takes place in the solution with the
elimination of water and formation of the corresponding
intermediate 2-arylidene malononitrile III with in 3 min with
95% yield. Michael addition of indole to the adduct III furnishes
the desired product 4 in the 90 % yield, though the Michael
addition reaction is takes place in 12 h.
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In conclusion, we are demonstrating here an
environmentally friendly, electrochemical reaction protocol for
the synthesis of 3-substituted indoles. These reactions were