Investigations into the Nucleophilic meso-Substitution of F-BODIPYs and Improvements to the Synthesis of 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene

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A series of three F-BODIPYs, with varying levels of steric crowding about the meso-position were selected to investigate nucleophilic meso-substitution of F-BODIPYs. The synthesis of one of these F-BODIPYs, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (totally unsubstituted dipyrrin skeleton), was optimized to give higher yields over routine literature procedures. This modified procedure involves oxidn. of a dipyrromethane using p-chloranil, instead of DDQ, to give a dipyrrin which is then trapped in situ as its BF2 complex. Nucleophilic meso-alkylation of the series of F-BOIDPYs with n-butyllithium gave meso-Bu F-BODIPYs in moderate to good yields. This work represents a new, synthetically viable method for the synthesis of meso-alkylated F-BODIPYs. Extension of the nucleophilic substitution methodol. to meso-arylation was possible. However, the reaction was unselective: substitution at boron, to give the boron-diaryl C-BOIDPYs, occurred preferentially to nucleophilic meso-substitution and thus a mixt. of products was obtained.