Synthesis of Novel Key Chromophoric Intermediates via C-C Coupling Reactions

The fundamentals of Pd-catalyzed Csp(2)-Csp(2) Miyaura borylation, Suzuki cross-coupling, and Stille cross-coupling reactions for a variety of borylated precursors based on phenothiazine (PTZ), phenoxazine (POZ), carbazole (Cz), and quinoxaline (QX) units have been explored. Three palladium-based catalysts were chosen for this study: Pd(PPh3)(4), Pd(PPh3)(2)Cl-2, and Pd(dppf)Cl-2, applying different reaction conditions. Around 16 desired chromophores were successfully designed and synthesized using C-C cross-coupling reactions in moderate to excellent yields, including PTZ, POZ, and Cz units coupled with QX, indolinium iodide, thienyl, phenyl, or triphenylamine moieties. Additionally, PTZ, POZ, and Cz have been employed in synthesizing various pinacol boronate ester derivatives in good to moderate yields. Interestingly, Pd(dppf)Cl-2 was found to be the best catalyst for borylation, and C-C cross-coupling reactions occurred in as little as 30 min, with an excellent yield exceeding 98%. Pd(PPh3)(4) and Pd(PPh3)(2)Cl-2 catalyzed the reaction to obtain the desired products in moderate to good yields after a long time (20-24 h). On the other hand, the Suzuki-Miyaura cross-coupling between N-(2-methyl)hexyl carbazole pinacol boronate ester derivative 10c and three halogenated quinoxaline derivatives-4-(3-(5-bromothiophen-2-yl)quinoxalin-2-yl)benzaldehyde (27), 4-(5-(3-(5-bromothiophen-2-yl)quinoxalin-2-yl)thiophen-2-yl)benzaldehyde (30), and 4-(3-chloroquinoxalin-2-yl)benzaldehyde (25) catalyzed by Pd(PPh3)(4)-afforded three carbazole-quinoxaline chromophores (28, 30, and 31, respectively) in 2-3 h, with good to excellent yields reaching 86%. The electron-deficient QX couplers proved to be coupled efficiently using the Stille coupling reaction, which involves the coupling between electron-rich orgaostannane and electron-deficient halide. The synthesized precursors and desired chromophores were characterized by FTIR, H-1-NMR, C-13-NMR, and HRMS.

Automated summary

The fundamentals of Pd-catalyzed Csp(2)-Csp(2) Miyaura borylation, Suzuki cross-coupling, and Stille cross-coupling reactions for a variety of borylated precursors based on phenothiazine (PTZ), phenoxazine (POZ), carbazole (Cz), and quinoxaline (QX) units have been explored. Different palladium-based catalysts were employed under various reaction conditions to design and synthesize desired chromophores and pinacol boronate ester derivatives. Pd(dppf)Cl-2 was found to be the most effective catalyst for borylation, allowing for rapid C-C cross-coupling reactions with excellent yields. The Suzuki-Miyaura and Stille cross-coupling reactions showed efficient coupling between electron-rich and electron-deficient compounds, resulting in the synthesis of various chromophores.