Deamination of Indole Mannich Bases: An Efficient Route to 3Benzyl/ Alkylindoles via a Metal-Free Transfer Hydrogenation Under Microwave Irradiation

Background: 3-Benzyl/alkylindoles are one of the important classes of compounds having several applications. Microwave assisted organic synthesis (MAOS) has gained momentum recently due to the advantages like short reaction time and higher yields compared to conventional heating method. The aim of this paper is to introduce readers to a new methodology of synthesizing these compounds employing microwave assisted synthesis. Methods: We have done review of literature related to the various indole derivatives having medicinal applications and methods available to synthesize them. We have reviewed various methodologies for the C-3 alkylation of indoles including reductive alkylations developed to obtain 3-arylmethylindoles. We have used microwave irradiation to obtain 3benzyl/alkylindoles in very good yield. Results: A facile route to 3-benzyl/alkylindoles via deamination of tertiary amine followed by a metal-free catalytic transfer hydrogenation of Mannich bases of indoles under microwave irradiation has been developed. Deamination is catalyzed by p-TsOH and Hantzsch 1,4-dihydropyridine is used as the reductant. The reaction proceeded through an alkylideneindolenine ion. The reaction is simple, clean and offers very good yield of the products. Conclusion: An efficient method is developed for the synthesis of 3-benzylated/alkylated indoles. The reaction does not use any hazardous metal catalyst or reductant. The method uses p-TsOH as catalyst which is a convenient, easily available and cheap reagent. This method would offer easy and economic solution for synthesis of this important class of compounds augmenting the presently available procedures.