B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

Abstract

The tetraaryl μ‐hydridodiborane(4) anion [ 2H] ⁻ possesses nucleophilic B−B and B−H bonds. Treatment of K[ 2H] with the electrophilic 9‐H‐9‐borafluorene (HBFlu) furnishes the B ₃ cluster K[ 3], with a triangular boron core linked through two BHB two‐electron, three‐center bonds and one electron‐precise B−B bond, reminiscent of the prominent [B ₃H ₈] ⁻ anion. Upon heating or prolonged stirring at room temperature, K[ 3] rearranges to a slightly more stable isomer K[ 3 a]. The reaction of M[ 2H] (M ⁺=Li ⁺, K ⁺) with MeI or Me ₃SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me ₃Si). Thus, [ 2H] ⁻ behaves as a masked [:BFlu] ⁻ nucleophile. The HBFlu by‐product was used in situ to establish a tandem substitution‐hydroboration reaction: a 1:1 mixture of M[ 2H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[ 2H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R ₂BFlu].

Abstract

Happy divorce: The hydride adduct of a tetraaryl diborane(4) has been shown to possess a nucleophilic B−B bond. Upon heterolysis, a diaryl boryl anion is released, which can act as a boron‐centered nucleophile or undergo [4+1] cycloaddition reactions to furnish spiroborates.