Synthesis of novel phenothiazine, phenoxazine and carbazole derivatives via Suzuki-Miyaura reaction

The Suzuki-Miyaura cross-coupling reaction was utilized for the synthesis of 15 new compounds containing phenothiazine (PTZ), phenoxazine (POZ), carbazole (Cz), and triphenylamine (TPA) as good electron donors. The synthesized compounds were furnished with active carbaldehyde groups and decorated with phenyl, thienyl, furyl, imidazolyl, or oxadiazolyl units linked to either one or two sides of the central cores (PTZ, POZ, and Cz). A simple catalytic systems based on Pd(PPh3)4 were tried and the results showed better CC coupling efficiencies of the electron-rich phenothiazine halides over phenoxazine and carbazole analogues when coupled with either electron-rich or electron-deficient boronic acid precursors. Besides, the coupling process of substituted phenothiazine halides with different boronic acids precursors, based on phenyl (Ph), furyl (Fu) and thienyl (Th) units, was found to be dependent on the electronic nature of both PTZ-halide and boronic acid precursors. Coupling efficiency of Ph, Fu, and Th rings with the skeleton of imidazolyl-PTZ-Br coupler to bring up compounds 6a-c showed the order: Ph>Fu>Th (with the yield of 86, 85, and 80%, respectively), while coupling efficiency of the brominated PTZ-oxadiazolyl-phenyl coupler with different arylboronic acids showed the order of Th>Ph>Fu to obtain compounds 8a-c. Additionally, the TPA derivative 11b was obtained with an excellent yield of 96% using Pd(PPh3)4 . The successful activation of the synthesized derivatives with carbaldehyde groups and the integration of different spacers made them substantially suitable to be exploited as promising intermediates for the future synthesis of versatile π-conjugated systems.