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

Thermally induced dehydrogenation of the H-bridged cation L2B2H5+ (L=Lewis base) is proposed to be the key step in the intramolecular C-H 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 C-H insertion with B-B 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-. Catlike: In situ formation of a cationic sp2-sp3 diborane(4) is proposed to be the key step in the high-temperature intramolecular C-H borylation of amine boranes activated with catalytic amounts of strong electrophiles. Reaction mechanisms are discussed.