Synthesis, Characterization, and Trapping of a Cyclic Diborylcarbene, an Electrophilic Carbene

Abstract

A carbene bearing two geminal boryl substituents, called diborylcarbene (DBC), has been predicted to be highly Lewis acidic in sharp contrast to the well-studied persistent carbenes stabilized by π-donating substituents. Studies on DBC have been limited to either the base-trapping or theoretical calculations. Herein, we developed chemical equivalents for DBC, namely, K/X-diborylcarbenoids 2 X (X = F or Cl). Treatment of 2 F with Al(C 6 F 5 ) 3 yielded [AlF(C 6 F 5 ) 3 ] − -stabilized DBC 1-FAl, which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with 19 F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of 2 X only in the presence of Al(C 6 F 5 ) 3 . Generated DBC 1 from 1-FAl or 2 Cl was successfully trapped with excess amounts of trialkylphosphines (PR 3 , R = Me or Et), which afforded the corresponding DBC-PR 3 adducts. In addition, the Lewis acidity of DBC 1 was evaluated both experimentally and theoretically to reveal that 1 is one of the most Lewis acidic species among neutral molecules.

Trendbericht Anorganik 2024: Hauptgruppen‐ und Molekülchemie

“…12) Nozaki und Kusumoto konnten experimentell die Vorhersage bestätigen, dass flankierende Boryl-Substituenten an einem Carbenzentrum zur Entstehung eines Lewis-sauren Diborycarbens führen. 13) Sie stellten Diborylcarbenoide her, die sich als Vorläufer für basenstabilisierte Diborylcarbene einsetzen lassen (Abbildung 11). Diese Veröffentlichung wirft Licht auf diese bisher wenig betrachtete Klasse von Carbenen und bewertet die Lewis-Azidität der Diborylcarbenspezies: Diese Substanzklasse gehört nun zu den stärksten neutralen Lewis-Säuren.…”

Designing a σ0π2 singlet ground state carbene from dicationic carbones

“…8 In principle, π-acceptors can be gainfully employed for this task, but this still often leaves the triplet as the ground state like in a recently reported diboryl carbene with a gap of more than 10 kcal mol −1 to the σ 0 π 2 singlet. 9 Hoffmann and Borden suggested that the experimentally known 2 H -imidazol-2-ylidene can be characterized as a σ 0 π 2 singlet ground state carbene while it was previously rather thought of as a carbodiimide (Fig. 1B).…”

A stable rhodium-coordinated carbene with a σ 0 π 2 electronic configuration

“…1C). Diborylcarbenes exemplify this conceptual approach; however, attempts to isolate these molecules have hitherto remained elusive ( 26 – 30 ). Other attempts to isolate σ 0 π 2 carbenes involve exploring postulated metallodiphosphinocarbenes with high electrophilicity ( 31 – 34 ).…”