Alkynylated phenothiazines 1, 2, 5, 7, and 12−15 can be synthesized in moderate to excellent yields by sequences of aldehyde−alkyne transformations and/or Sonogashira crosscoupling reactions from suitable phenothiazine aldehydes or bromides. The electronic properties of (hetero)aryl ethynylsubstituted N-methyl-phenothiazines (UV/Vis absorption, fluorescence, redox potentials) strongly correlate with Hammett σ P parameters and indicate that remote substituents
Alkynylated and butadiynyl-bridged phenothiazines with variable functionalization can be synthesized in good yields by cross-coupling and condensation approaches. In addition, the structure of the diethynylated phenothiazine (7a) has been corroborated by an X-ray structure analysis. These oligofunctional heterocycles are fluorescent with modest quantum yields (Phi(f) = 20-35%) and represent suitable building blocks for novel photoexcitable molecular wires.
Phenothiazinylboronic acid derivatives 3,7 and 9, useful building blocks for the construction of oligophenothiazines, are readily synthesized in good yield from brominated phenothiazines 5 and 6 by bromine-lithium exchange followed by trapping with trialkylborate (route A) or by palladium-catalyzed borylation with tetramethyl dioxoborolane (8) (route B). The novel class of tetrakis(phenothiazinylphenyl)methanes 11, showing remarkably large Stokes shifts and a reversible low oxidation potential, can be prepared in good yield by Suzuki coupling of tetrakis(p-bromophenyl)methane (10) with 3a.Scheme 1 Retrosynthetic analysis of (hetero)aryl bridged oligo(phenothiazines) Scheme 2 Synthesis of phenothiazinylboronic acid derivatives 3 and 7 via halogen-metal exchange and electrophilic borylation (route A) Downloaded by: University of Arizona Library. Copyrighted material.
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