Catalyst-free assembly of giant tris(heteroaryl)methanes: synthesis of novel pharmacophoric triads and model sterically crowded tris(heteroaryl/aryl)methyl cation salts

A series of giant tris(heteroaryl)methanes are easily assembled by one-pot three-component synthesis by simple reflux in ethanol without catalyst or additives. Diversely substituted indoles (Ar(1)) react with quinoline aldehydes, quinolone aldehydes, chromone aldehydes, and fluorene aldehydes (Ar(2)CHO) and coumarins (Ar(3)) in 1:1:1 ratio to form the corresponding tris(heteroaryl)methanes (Ar(1)Ar(2)Ar(3))CH along with (Ar(1)Ar(1)Ar(2))CH triads. A series of new 2:1 triads were also synthesized by coupling substituted indoles with Ar(2)CHO. The coupling reactions could also be carried out in water (at circa 80 °C) but with chemoselectivity favoring (Ar(1)Ar(1)Ar(2))CH over (Ar(1)Ar(2)Ar(3))CH. The molecular structure of a representative (Ar(1)Ar(2)Ar(3))CH triad was confirmed by X-ray analysis. Model tris(heteroaryl/aryl)methylium salts were generated by reaction with DDQ/HPF(6) and studied by NMR and by DFT and GIAO-DFT.