Cp*-Substituted Boron Cations: The Effect of NHC, NHO, and CAAC Ligands

Abstract

The effect of a ligand on the electron deficiency and Lewis acidity of the Cp*-substituted boron dication has been investigated experimentally and theoretically. In addition to the reported IMes- and N-heterocyclic olefin (NHO)-stabilized boron dications, the related cyclic alkylamino carbene (CAAC)-coordinated boron mono- and dications have also been synthesized and structurally characterized. An electrochemical study of dications [3a-3c] confirms the higher electron deficiency of the dicationic system than the related boron monocations. Moreover, the presence of a π-acidic CAAC ligand is critical for realizing stable radical species generated from the chemical reduction of boron cations. The nature of the axial ligand also significantly affects the selectivity of the hydride addition reaction of boron dications. While bulky superhydride reacts with [3a-3c] in the same manner to give the cyclic boreniums, [BH] attacks three different electrophilic sites of boron dications: the sp carbon of Cp* of the IMes-coordinated system ([3a]), the central boron atom of the NHO-stabilized analogue ([3b]), and the ylidene carbon of the CAAC-containing boron dication ([3c]).

Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor

“…7 Similarly, Chiu and co-workers showed that a cAAC adduct of the [Cp*B] 2+ dication underwent hydride addition at the ylidene carbon upon reaction with [ n Bu 4 N] + [BH 4 ] – . 15 To our knowledge, however, a 1,2-hydrogen shift from boron to a carbene unit has never been reported.…”

Cyclic (Alkyl)(amino)carbenes (CAACs): Recent Developments

“…Therole of CAACsinGroup 13 element chemistry is not limited to the preparation of nucleophilic organoborons.They have been used for the stabilization of neutral boron and aluminum radicals 13 and 14, [80] and radical cation 15 [66] (Scheme 8). Interestingly,C hiu et al [81] were not only able to isolate the CAAC-stabilized boron cation 16 and dication 18,b ut also the NHC analogues.H owever,w hile the reduction of the CAAC complexes 16 and 18 gave rise to the corresponding neutral radical 17 and radical cation 19, respectively,n op aramagnetic species could be characterized with the NHC complexes.T his is one of the numerous illustrations of the superiority of CAACs over NHCs for the stabilization of radicals.…”

“…They have been used for the stabilization of neutral boron and aluminum radicals 13 and 14 , and radical cation 15 (Scheme ). Interestingly, Chiu et al . were not only able to isolate the CAAC‐stabilized boron cation 16 and dication 18 , but also the NHC analogues.…”

Cyclische Alkylaminocarbene (CAACs): Neues von guten Bekannten

“…Auch die neutralen Bor‐ und Aluminiumradikale 13 und 14 sowie das Radikalkation 15 konnten mithilfe von CAACs stabilisiert werden (Schema ). Chiu und Mitarbeiter konnten, außer dem CAAC‐stabilisierten Borkation 16 und Bordikation 18 , auch die entsprechenden NHC‐Analoga isolieren. Dass die CAACs bezüglich der Stabilisierung von Radikalen den NHCs überlegen sind, zeigt sich daran, dass die Reduktion der CAAC‐Verbindungen 16 und 18 zum neutralen Radikal 17 bzw.…”