A short synthesis of tetraalkoxydiborane(4) reagents

The synthesis of a series of bis(catecholato)diborane(4) compounds, B₂(1,2-O₂C₆H₄)₂, B₂(1,2-O₂C₆H₃Me-4)₂, B₂(1,2-O₂C₆H₂Me₂-3,5)₂, B₂[1,2-O₂C₆H₃Bu^t-4)]₂, and B₂[1,2-O₂C₆H₂Bu^t₂-3,5]₂, is reported. The compounds have been synthesized by reaction of 1% sodium/mercury amalgam with the corresponding halocatecholboranes, which are cleanly formed from the reaction of BCl₃ or BBr₃ and catechol. Combining these two steps in one pot, B₂[1,2-O₂C₆H₃Bu^t-4)]₂ was prepared from BCl₃ and 4-tert-butylcatechol, and B₂[1,2-O₂C₆H₂Bu^t₂-3,5]₂ was prepared from 3,5-di-tert-butylcatechol and BBr₃ on a multigram scale. Bis(pinacolato)diborane(4) was not formed from reaction of chloropinacolborane and Na/Hg, but it was formed by in situ addition of pinacol to either B₂[1,2-O₂C₆H₃Bu^t-4)]₂ or B₂[1,2-O₂C₆H₂Bu^t₂-3,5]₂. Cyclic voltammetry indicated that the reduction potentials of the chlorocatecholborane reactant and of the bis(catecholato)diborane(4) product are similar. These similar potentials make selective reduction of haloborane difficult and thus far unique to sodium−mercury amalgam. Read more on publisher's site.