Rapid Sequentially Palladium Catalyzed Four-Component Synthesis of Novel Fluorescent Biaryl-Substituted Isoxazoles

A series of novel 3- and 5-biaryl-substituted isoxazoles was prepared by a rapid microwave-assisted four-component three-step synthesis: concatenating Sonogashira coupling, cyclocondensation, and Suzuki coupling in a one-pot fashion. The Pd-catalyst was successfully employed in the sense of a sequentially catalyzed process, i.e., without the addition of further catalyst loading. Biaryl-substituted isoxazoles with donor–acceptor decoration possess remarkable photophysical properties, such as high fluorescence quantum yields in solution up to ΦF = 0.86 and large Stokes shifts up to 10,000 cm−1. The experimental absorption and emission characteristics can be reproduced and rationalized by computations on the DFT (density functional theory) and TDDFT (time-dependent density functional theory) level of theory.