A short synthesis of tetraalkoxydiborane(4) reagents

The synthesis of a series of bis(catecholato)diborane(4) compounds, B-2(1,2-O2C6H4)(2), B-2-(1,2-O2C6H3Me-4)(2), B-2(1,2-O2C6H2Me2-3,5)(2), B-2[1,2-O2C6H3But-4)](2), and B-2[1,2-O(2)C(6)H(2)Bu(t)2-3,5](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, B-2[1,2-O2C6H3But-4)](2) was prepared from BCl3 and 4-tert-butylcatechol, and B-2[1,2-O2C6H2Bu2t-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 B-2[1,2-O2C6H3But-4)](2) or B-2[1,2-O(2)C(6)H(2)Bu(t)2-3,5](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.