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

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 B₃ 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 [B₃ H₈ ]^- 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 Me₃ SiCl leads to equimolar amounts of 9-R-9-borafluorene and HBFlu (R=Me or Me₃ Si). 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[R₂ BFlu].