Thermal Dehydrogenation of Base‐Stabilized B 2 H 5 + Complexes and Its Role in CH Borylation

Thermally induced dehydrogenation of the H‐bridged cation L 2 B 2 H 5 + (L=Lewis base) is proposed to be the key step in the intramolecular CH borylation of tertiary amine boranes activated with catalytic amounts of strong “hydridophiles”. Loss of H 2 from L 2 B 2 H 5 + generates the highly reactive cation L 2 B 2 H 3 + , which in its sp 2 ‐sp 3 diborane(4) form then undergoes either an intramolecular CH insertion with BB bond cleavage, or captures BH 3 to produce L 2 B 3 H 6 + . The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH 2 C 6 F 5 complexes through disproportionation of L 2 B 2 H 5 + HB(C 6 F 5 ) 3 − .