Synthesis of highly fluorescent helical quinolizinium salts by a Rh-catalyzed cyclotrimerization/C–H activation sequence

Abstract

A series of helical quinolizinium salts were prepared utilizing Rh-catalyzed [2+2+2]cyclotrimerization and C–H activation processes as the crucial synthetic steps. The cyclotrimerization of appropriately substituted diynes with trimethylsilylethyne under Rh-catalyzed conditions provided the 1-arylisoquinolines in up to 61% isolated yields. Their Rh-catalyzed C–H activation/annulation with various aryl and alkyl disubstituted alkynes gave rise to [7]-helical quinolizinium salts in high isolated yields (up to 93%). Enantioselective C–H activation was also tried with asymmetric induction up to 62% ee. The respective boron and platinum complexes of 1-arylisoquinolines were prepared as well. All prepared compounds exhibit fluorescence in the orange-red light region (606–682 nm) with Φ_Fs 28–99%.