Xanthene-to-Fluorene Skeletal Editing via Oxygen Deletion Mediated by Boron and Aluminium Radicals

Abstract

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-6-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.