Xanthene-to-fluorene skeletal editing via oxygen deletion mediated by boron and aluminium radicals

Single-atom skeletal editing via selective oxygen deletion from diarylethers remains an underdeveloped transformation, despite its potential to directly access new carbon frameworks. Here, we report a boron- and aluminium-mediated O-deletion reaction that converts xanthene and diphenylether motifs into fluorene and biphenyl architectures through concomitant C-C bond formation. Lithium metal reduction of diamido arylether boron halides affords lithium boryloxy complexes in high yield and on a multigram scale, with both the new C-C bond and terminal B-O^- unit formed in a single step via a transient open-shell B(ii) intermediate. Hydrolysis furnishes fluorene- and biphenyl-based [1,3,2]diazaborepin-2-ols, representing previously inaccessible boron-containing fluorophores that exhibit high photoluminescence quantum yields. Extension of this strategy to aluminium allows clean hydrolytic release of the organic scaffold and provides a concise, scalable synthesis of functionalised 4,5-diaminofluorenes. These results establish O-deletion as a viable skeletal editing strategy for arylethers and highlight the role of main-group radical intermediates in selective framework reorganisation.