2‐Chloro‐Azaborolyl Anion: A Source of 1,2‐Azaborole Isosteric to Cyclopentadienylidene

Azaborolyl anions, the five‐membered BN heterocycles, have attracted a considerable attention due to their aromaticity and isoelectronic relationship with ubiquitous cyclopentadienyl ligands. Besides their syntheses and applications in the preparation of metal complexes, the other aspects of their chemistry have been virtually unexplored. Reduction of the azabutadienyl chelate boron dichloride [ArNC(R)CHC(R)]BCl 2 ( 2 , Ar=2,6‐Me 2 C 6 H 3 , R= t Bu) with two equivalents of potassium yielded the novel 2‐chloro‐azaborolyl anion [ArNC(R)CHC(R)BCl]K(thf) ( 3 ) as a stable product in good yield. Reaction of 3 with 1,3,4,5‐tetramethylimidazol‐2‐ylidene (NHC) yielded the first NHC–azaborole adduct with the elimination of KCl. The salt elimination reaction was also observed in the reactions with H 2 O and the organic azide ArN 3 , leading to the formation of an oxo‐bridged 3 H ‐1,2‐diazaborole and an intramolecular donor‐stabilized iminoborane, demonstrating that 3 is a source of the unexplored 1,2‐azaborole isosteric to cyclopentadienylidene.