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.
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Müller et al. (2002) studied this question.