Synthesis of Bis(indolyl)methanes Using Recyclable PEG-Supported Sulfonic Acid as Catalyst
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Synthesis of Bis(indolyl)methanes Using Recyclable PEG-Supported Sulfonic Acid as Catalyst Shou-Ri Sheng Æ Qiu-Ying Wang Æ Yan Ding Æ Xiao-Ling Liu Æ Ming-Zhong Cai
Received: 4 October 2008 / Accepted: 5 November 2008 / Published online: 5 December 2008 Ó Springer Science+Business Media, LLC 2008
Abstract An operationally simple, efficient and environmentally benign synthetic method to bis(indolyl) methanes has been developed in good yields (80–96%) by electrophilic substitution reactions of indole with aldehydes or ketones in methanol in the presence of 5 mol% of poly (ethylene-glycol) (PEG) supported sulfonic acid with respect to indole under mild conditions. Recovery and recycling of the poly (ethylene-glycol)-supported catalyst were also described. Keywords Bis(indolyl)methane PEG-supported sulfonic acid Indole Carbonyl compound Environmentally benign
1 Introduction The importance of indoles and their derivatives owing to their biological activities is well recognized by synthetic as well as biological chemists [1]. Bis(indolyl)methanes, which contain two indole or substituted indole units in a molecule, feature widely in bioactive metabolites of terrestrial and marine origin [2–6]. Therefore, there is a great deal of interest in the synthesis of this class of compounds. Generally, these molecules are obtained from the condensation of indoles with aldehydes and ketones in the
S.-R. Sheng (&) Q.-Y. Wang Y. Ding X.-L. Liu M.-Z. Cai College of Chemistry and Chemical Engineering, Jiangxi Normal University, 330022 Nanchang, People’s Republic of China e-mail: [email protected]
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presence of traditional protic [7–9] or Lewis acids [10–13]. Additionally, montmorillonite clay K-10 [14], and other catalysts such as InCl3, In (OTf)3, LnCl3, Dy(OTf)3, Ln(OTf)3, and Re (PFO)3 were also found to catalyze these reactions [15–18]. However, many of these Lewis acids are deactivated or sometimes decomposed by nitrogen containing reactants. Even when the desired reactions proceed, more than stoichiometric amounts of Lewis acids are required [19]. These problems can be solved to some extent by using expensive lithium perchlorate. But this catalyst requires longer reaction time for nitro-substituted aromatic aldehydes, giving the corresponding bis(indolyl)methanes in moderate yields [20]. In addition, PPh3HClO4, NaHSO4–SiO2, I2 and NBS [21–25], or ionic liquids [26, 27] have been used for such organic synthesis [26]. Recently, the use of protic solvent has been successfully developed to synthesis of bis(indolyl)methanes efficiently [28]. Although methods for preparation of bis(indolyl)methanes are well documented, some of the reported protocols suffered from the usage of expensive reagents, prolonged reaction times, low product yields, complicated manipulations along with the involving of environmentally toxic media, or use of an additional microwave or ultrasound irradiation. So, the development of simple, convenient and environmentally friendly approaches for the preparation of these pharmaceutically impo
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