Isolation of a Diborane(6) Dication: Formation and Cleavage of an Electron-Precise B(sp³)–B(sp³) Bond

Abstract

One-electron oxidation of organoboron L2PhB: 1 (L = oxazol-2-ylidene) afforded a dicationic diborane(6) species [L2PhB-BPhL2]·2X (X = OTf, BF4, AlCl4) 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 (KC8) afforded 1, making a fully reversible 1 ↔ 3 redox system, whereas two-electron oxidation with AuCl produced a boronium [L2PhBCl]OTf 4. Moreover, the reactions of 3 with isonitrile derivatives RNC: under heating conditions gave a cyano-substituted boronium [L2PhBCN]BF4 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.

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

“…Although cationic monoboron(III) species, which are significantly more Lewis acidic than neutral boranes, are well‐known and have found applications in catalysis, in particular, cationic diboron species like these remain relatively rare. Most known examples are dicationic bis(base)‐stabilized sp 2 –sp 2 or sp 3 –sp 3 diboron(II) compounds, in which the electron‐deficient boron centers are stabilized through π‐donation by neutral and/or anionic nitrogen ligands, whereas Kinjo reported a unique tetrakis(carbene)‐stabilized sp 3 –sp 3 diboron dication . Prior to the [B 2 X 3 L 2 ]A compounds presented herein, only a couple of examples of monocationic sp 2 –sp 2 ‐diboron(II) compounds had been reported, which were obtained by adding a single equivalent of IMes or IDip to B 2 Br 2 (NMe 2 ) 2 , resulting in the displacement of one bromide ligand …”

Three‐Coordinate Boron(III) and Diboron(II) Dications

“…In seminal studies Himmel and co‐workers isolated the bicyclic guanidinate‐stabilized diboron(II) dication III (Figure ) . In 2016, Kinjo and co‐workers described the diboron(II) dication IV stabilized by oxazolin‐2‐ylidene ligands (Figure ) . As in III the metalloid centers in IV are four‐coordinate.…”

On the Dual Reactivity of a Nucleophilic Dihydrido‐Diborane: Reaction at the B−B Bond and/or the B−H Bond

“…They can be regarded as boryl‐substituted boranes that react as electrophiles. Lately several spectacular results were reported with sp 3 –sp 3 ‐hybridized diborane(4) and also sp 3 –sp 3 ‐diborane(6) derivatives, but nevertheless their chemistry is still underdeveloped. Braunschweig et al.…”