Solvent-Free Non-Covalent Organocatalysis: Enantioselective Addition of Nitroalkanes to Alkylideneindolenines as a Flexible Gateway to Optically Active Tryptamine Derivatives

catalytic asymmetric addition of nitroalkanes to alkylideneindolenines, generated in situ from arylsulfonylindoles, is presented. Despite the weakness of the non-covalent H-bond interactions between catalyst and substrates, the performance of the bifunctional organocatalyst used was found to be essentially unaffected by the polarity of the reaction medium. Nitroalkanes, mostly used in nearly stoichiometric amounts, could thus function both as solvents and reagents, resulting in a truly solvent-free reaction. The broad substrate scope shown by the present transformation allowed the preparation of some optically active tryptamine precursors that are not accessible through the previous catalytic asymmetric methods.