From Bis(borylene)-Substituted Xanthenes as Reactive Intermediates to Diboraoxirane Complexes

Area of Science:

• Organoboron Chemistry
• Main Group Chemistry
• Catalysis

Background:

• N-heterocyclic carbenes (NHCs) are versatile ligands in stabilizing reactive main group species.
• Diboraoxiranes are a class of strained boron-containing heterocycles with limited synthetic accessibility.

Purpose of the Study:

• To synthesize and characterize the first NHC-stabilized diboraoxirane complex.
• To investigate the reactivity and ring-expansion potential of this novel boron heterocycle.

Main Methods:

• Reduction of a bis(dichloroboryl-IPr)xanthene precursor using potassium graphite.
• Isolation and characterization of a bis(borylene)xanthene intermediate.
• Ring-expansion reactions of the diboraoxirane with various small molecules (e.g., O-oxides, O2, S, isocyanides, carbonyls).

Main Results:

• Successful synthesis of the first NHC-stabilized diboraoxirane (4) via C-O-C deoxygenation of a xanthene spacer.
• Isolation of a bis(borylene)xanthene (6) when using a sterically less demanding NHC ligand (IMe).
• Demonstration of versatile ring-expansion reactions of complex 4 with diverse small molecules, yielding unprecedented boron heterocycles (7-12).

Conclusions:

• The study reports the first synthesis of an NHC-stabilized diboraoxirane, highlighting a novel deoxygenation pathway.
• The strained B-B bond in the diboraoxirane enables diverse ring-expansion reactions, leading to new classes of boron heterocycles.
• This work expands the scope of organoboron chemistry and provides access to novel heterocyclic scaffolds.