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

Abstract

Borohydride (BH 4 − ) 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 H 2 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 BH 4 − , 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 BH 4 − to compensate for the protonolytic loss of available B−H fragments as H 2 .

Unravelling the underlying mechanism of the reduction of aldehydes/ketones with metal borohydride in an aprotic solvent

“…Conversely, an exchange of the proton of alcohol and hydride of borohydride is observed in THF, indicating that the B-coordinated η 2 -H 2 fragment may form as an intermediate in the process. 23 Thus, the six-membered ring mechanism proposed here may not be unique for the reduction of aldehydes/ketones by borohydrides in an aprotic solvent, and further studies are underway in our lab.…”