Reductive Insertion of Elemental Chalcogens into Boron–Boron Multiple Bonds

Abstract

The syntheses of sulfur- and selenium-bridged cyclic compounds containing boron stabilized by N-heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron-boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six-electron reduction reactions, resulting in the formation of [2.2.1]-bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron-boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC-stabilized trisulfidodiborolane. The demonstration of these six- and four-electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.

Dibora[2]ferrocenophane: A Carbene‐Stabilized Diborene in a Strained cis‐Configuration

“…elemental chalcogens). [27] We envisioned that dibora[2]ferrocenphane 3 might be especially prone to such reactivity due to the presumed strain energy that would be released by cleavage of the B=B bond. Given our previous success with chalcogens in the elucidation of the reactivity of boron-boron multiple bonds, [27,28] we chose a diorganyl diselenide to test this validity.…”

Highly Strained Heterocycles Constructed from Boron–Boron Multiple Bonds and Heavy Chalcogens

“…Both were structurally characterized by single-crystal X-ray crystallography ( Figure 1). [12] Though neither 77 Se nor 125 Te nuclei could be detected by NMR spectroscopy with 2 and 3, likely a result of extensive quadrupolar broadening induced by proximity to multiple boron nuclei, the 1 H NMR spectra of the compounds are consistent with the solid-state structures, as is the upfield shift of the 11 B NMR resonances from the resonance at 22 ppm found for 1, indicative of an increase in coordination number at boron. The B-B bonds in 2 (1.707(3) Å) and 3 (1.713(5) Å) are elongated with respect to the normal range of B=B lengths in diborene compounds (~1.58 -1.61 Å), [10,13] just slightly short of the normal range for B-B single bonds in base-stabilized neutral diboranes (1.72 -1.84).…”

“…Indeed, while direct oxidation of alkenes to thiiranes with elemental sulfur is rare, the reaction of diborenes with S8 results in the transfer of all four reducing equivalents of the double bond to three sulfur atoms in the formation of a five-membered trithiadiborolane. [11] Might these highly reductive double bonds be capable of reaction with elemental Se and Te to form stable diboraseleniranes and diboratelluriranes?When a thienyl-substituted, N-heterocyclic carbene (NHC) stabilized diborene (1, IMe2[5-(Me3Si)C4H2S]2B2, IMe = 1,3-dimethylimidazol-2-ylidene) was reacted with excess elemental selenium in benzene, a color change from the deep purple of 1 to yellow was observed, along with the emergence of a new signal at -14 ppm in its 11 B NMR spectrum. An equivalent reaction with elemental tellurium evidenced a similar change, with the emergence of a signal at -13 ppm in the 11 B NMR spectrum concomitant with the change of color from purple to yellow.…”

Anorganische Molekülchemie

“…Außerdem zeigte sich, dass mit einem Diboren eine Vier-Elektronen-Reduktion stattfindet, wobei sich ein NHC-stabilisiertes Trisulfidoborolan bildet. 16) Diborene waren bislang lediglich in der trans-Konfiguration bekannt. Jetzt gelang die Synthese der ersten cis-Diborenspezies (18) und (19) durch Reduktion von B 2 Br 2 Mes 2 mit Kaliumgraphit und den Chelatliganden dmpe oder 1,1-Bis(diphenylphosphino)methan.…”