Complexes between bicyclic boron derivatives and dihydrogen: the importance of strain

Protonolysis of BH4– Leads to Intermediacy of BH3-σ(H2) That Evolves H2 and Furnishes Borane as the Key Reducing Agent

“…These investigations unambiguously demonstrate that the commonly speculated mechanisms involving the intermolecular transfer of hydride ,− from BH 4 – to the keto fragment (Scheme , left), despite various modalities of assistance ( vide infra ), are not accessible. Instead, B–H protonolysis (Scheme , right), which is the microscopic reverse of H 2 -activation at B-centers, − furnishes σ­(H 2 )-borane adducts ,− and concomitantly borane, which allows facile intramolecular hydride transfer. , Isotope scrambling experiments, together with a comparison of previously reported rates and KIEs, were found to be consistent with this mechanistic hypothesis.…”

“…Immediately following autoionization, as shown in Scheme , the B–H fragment of 1 was found to exhibit a hydrogen-bonding interaction with MeOH 2 + in Int1 , which immediately leads to B–H protonolysis via TS2 associated with an accessible barrier of only 20.5 kcal/mol (relative to starting materials). Importantly, B–H protonolysis via TS2 leads to the formation of an elusive intermediate Int2 − featuring a unique B-σ­(η 2 -H 2 ) moiety that was dubbed pentahydroboron by Olah et al (see Section for discussion).…”