Thermal Dehydrogenation of Base-Stabilized B2H5+ Complexes and Its Role in CH Borylation

Thermally induced dehydrogenation of the H-bridged cation L2B2H5+ (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 H2 from L2B2H5+ generates the highly reactive cation L2B2H3+, which in its sp2-sp3 diborane(4) form then undergoes either an intramolecular CH insertion with BB bond cleavage, or captures BH3 to produce L2B3H6+. The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH2C6F5 complexes through disproportionation of L2B2H5+ HB(C6F5)3-.