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

Borohydride (BH₄^–) is a ubiquitous reducing agent in chemistry despite its apparent inability to transfer hydride fragments without assistance. In protic solvents such as methanol, B–H protonolysis and H₂ evolution furnish borane, which serves as the key hydride delivery agent in the reduction of cyclohexanone. Contrasting with widely accepted but inaccessible mechanisms involving direct (intermolecular) hydride transfer from BH₄^–, in this work, DFT calculations performed in close conjunction with H/D exchange studies unambiguously justify the use of alcohol as a necessary evil that renders substantial kinetic advantages in intramolecular hydride transfer, albeit requiring excess BH₄^– to compensate for the protonolytic loss of available B–H fragments as H₂.