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

The tetraaryl μ‐hydridodiborane(4) anion [ 2 H] − possesses nucleophilic B−B and B−H bonds. Treatment of K[ 2 H] 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[ 2 H] (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, [ 2 H] − 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[ 2 H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[ 2 H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R 2 BFlu].