Isolation of a Diborane(6) Dication: Formation and Cleavage of an Electron-Precise B(sp~3)-B(sp~3) Bond

摘要

One-electron oxidation of organoboron L_2PhB: 1 (L = oxazol-2-ylidene) afforded a dicationic diborane(6) species [L_2PhB-BPhL_2]·2X (X = OTf, BF_4, AlCl_4) 3, representing a new strategy to construct a B(sp~3)-B(sp~3) covalent bond. Each boron atom in 3 is in the formal oxidation state +Ⅱ, and tetracoordinate with a Ph group and two oxazol-2-ylidenes. The cyclic voltammetry of 3 shows irreversible reduction and oxidation. Indeed, two-electron reduction of 3 with potassium graphite (KC_8) afforded 1, making a fully reversible 1 (⇄) 3 redox system, whereas two-electron oxidation with AuCl produced a boronium [L_2PhBCl]OTf 4. Moreover, the reactions of 3 with isonitrile derivatives RNC: under heating conditions gave a cyano-substituted boronium [L_2PhBCN]BF_4 5 and a 2- boranyl-indole derivative 6, depending on the substituent R. The proposed reaction mechanism involves a borinylium radical 1~(·+) which is generated via a homolytic cleavage of the B-B bond of 3.