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 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(3)H(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(3)SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me(3)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[R(2)BFlu].