Syntheses of phenothiazinylboronic acid derivatives - Suitable starting points for the construction of redox active materials

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.