Fuctionalization of Linear and Angular Phenothiazine and Phenoxazine Ring Systems via Pd(0)/XPhos Mediated Suzuki-Miyaura Cross-coupling Reactions

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

The fundamentals of Pd-catalyzed Csp2−Csp2 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)2Cl2, and Pd(dppf)Cl2, 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)Cl2 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)2Cl2 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, 1H-NMR, 13C-NMR, and HRMS.

trans-Dichlorobis(XPhos)palladium(II) Precatalyst for Suzuki-Miyaura Cross-Coupling Reactions of Aryl/Vinyl Sulfonates/Halides: Scope, Mechanistic Study, and Synthetic Applications

Suzuki-Miyaura cross-coupling reactions of aryl/vinyl sulfonates/halides with various boron species were performed using an easily available trans-dichlorobis(XPhos)palladium(II) precatalyst. Under microwave assistance, more than 30 coupling products were obtained with yields ranging from 23 to 99%, including the synthesis of two bioactive compounds, dubamine and tamoxifen. A mechanistic investigation of the Suzuki-Miyaura reaction was conducted notably by nuclear magnetic resonance (NMR) and high-resolution mass spectroscopy, revealing the nature of the active Pd⁰ species and of the reducing entity.

Novel Phenothiazine-Bridged Porphyrin-(Hetero)aryl dyads: Synthesis, Optical Properties, In Vitro Cytotoxicity and Staining of Human Ovarian Tumor Cell Lines

We report here the synthetic procedure applied for the preparation of new AB₃-type and trans-A₂B₂ type meso-halogenophenothiazinyl-phenyl-porphyrin derivatives, their metal core complexation and their peripheral modification using Suzuki-Miyaura cross coupling reactions with various (hetero)aryl (phenothiazinyl, 7-formyl-phenothiazinyl, (9-carbazolyl)-phenyl and 4-formyl-phenyl, phenyl) boronic acid derivatives. The meso-phenothiazinyl-phenyl-porphyrin (MPP) dyes family was thus extended by a series of novel phenothiazine-bridged porphyrin-(hetero)aryl dyads characterized by UV-Vis absorption/emission properties typical to the porphyrin chromophore, slightly modulated by increasing the size of peripheral substituents. Three phenothiazine-bridged porphyrin-heteroaryl dyads with fluorescence emission above 655 nm were selected as fluorophores in red spectral region for applications in cellular staining of human ovarian tumors. In vitro experiments of cell metabolic activity displayed a moderate toxicity on human ovarian tumor cell lines (OVCAR-3, cisplatin-sensitive A2780 and cisplatin-resistant A2780cis respectively). Visualization of the stained living cells was performed both by fluorescence microscopy imaging and by fluorescence lifetime imaging under two photon excitation (TPE-FLIM), confirming their cellular uptake and the capability of staining the cell nucleus.

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)₂ catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd⁰ complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)₂ initially associates with the ligand to form a complex in solution, which has been characterized as Pd^II (OAc)₂ (XPhos). This Pd^II center is then reduced to the Pd⁰ (XPhos)₂ species by an intramolecular process. This study also sheds light on the formation of Pd^I -Pd^I dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd⁰ (XPhos) as the reactive species in equilibrium with the unreactive Pd⁰ (XPhos)₂ . Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.