Desymmetrizing Electron‐Deficient Diboranes(4): Diverse Products and Their Reactivity

Abstract

A comprehensive study of the reactivity of Lewis bases with dihalodiboranes(4) is presented. Diaryldihalodiboranes provide rearranged monoadducts when treated with cyclic (alkyl)(amino)carbenes, but halide-bridged adducts when treated with a range of pyridyl bases. Alternatively, the combination of diaminodihalodiboranes with strong carbene donors leads to boraborenium salts. The reduction and halide-abstraction reactivity of these adducts was also explored, leading to intramolecular C-H activation and the first 1,2-bis(borenium) dication.

Synthesis and Reduction of Sterically Encumbered Mesoionic Carbene‐Stabilized Aryldihaloboranes

“…The B1−C1 bond to Ph L is nearly aligned with the plane of the sp 2 borane residue (torsion angle (C1,B1,B2,C22) 15.0(8) o ). The B−B bond length (1.772(9) Å) is typical for diborane(5) compounds and falls within the range observed for the analogous SIMes and Me cAAC adducts (1.762(4) and 1.772(5) Å, respectively) …”

“…C−H activation at the isopropyl methine position of a Dipp substituent had also been observed by Robinson and co‐workers upon the reduction of [(IDipp)BCl(N( i Pr) 2 )]Cl with KC 8 in THF . Similarly, our group reported that the reduction of the diboranes(5) [Mes 2 BB(SIMes)Cl 2 ] and [Mes 2 BB( Me cAAC)Br 2 ] led to C−H activation at the ortho ‐methyl position of one mesityl ligand, in both cases leading to a bicyclic hydride‐bridged diborane …”

“…The product distribution was found to be dependent on the steric demands of the phosphine ligand, with larger PMeCy 2 favoring C and smaller PEt 3 favoring B . Complete selectivity for B was obtained with PMe 3 , as well as a range of pyridine donors, whereas the use of sterically demanding NHCs or cAACs selectively afforded C . Furthermore, the tendency towards rearrangement increased for the heavier halide analogues, in line with a weakening of the B‐X bond .…”

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

“…An example for a dication with three‐coordinate boron(II) atoms was presented by Siebert, Brothers and co‐workers, however, its structural characterization was merely based on theoretical methods . Very recently, Braunschweig and co‐workers structurally characterized a related three‐coordinate diboron(II) dication which was isolated in low yield after chloride abstraction from an unsymmetrical diborane(4) adduct precursor …”

Konstruktion linearer und verzweigter Tetraborane durch 1,1‐ und 1,2‐Diborierung von Diborenen

“…Viele Arbeiten haben gezeigt, dass die Desymmetrisierung von Diboranen(4) [17a, 18] zu erhçhter Reaktivität, sowohl in der Aktivierung von kleinen Molekülen [19] als auch in Borylierungs-und Diborierungsreaktionen [20] sowie intramolekularen Umlagerungsprozessen [21] führt. Kürzlich berichteten wir über das Benzylphosphan-substituierte,u nsymmetrische Diboren 3 und sein gem-Isomer 4 (Schema 2), [22] das Borylborylen-Eigenschaftenaufweist.…”