The Stabilisation of Non-Classical Boron Architectures by N-Heterocyclic Carbenes: Synthesis and Potential Applications

• Applicant: Dr. René Frank
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410908941

• Final Report Year: 2024

Final Report Abstract

Based on preliminary results from our laboratories the chemistry of N-heterocyclic carbenes (NHCs) in the stabilization of boron architectures is investigated. Stimulated by our earlier reported boryl anions with essential cyanide substituents, the chemistry is expanded to alkylnyl moieties. Alkylnyl boranes of various substitution patterns are difficult to produce and were obtained in high yield by us with stabilization of NHC ligands, albeit the subsequent reduction towards boryl anions was not successful. In further approaches our research was expanded to NHC-stabilized benzyl boranes. The selective functionalisation of the aromatic core in benzyl moieties was found to strongly depend on the nature of the NHC, for which alkylated backbones afforded excellent results. The simplest benzyl boranes of type NHC-BH2-CH2-Ph were demonstrated to undergo facile complete halogenation of the BH2 entity with boron trichloride and boron tribromide, while boron triiodide afforded borylation of the aromatic core directly by the BH2 entity as an outstanding result. The reactions of a four-membered bora heterocycle was performed with carbon donors (isonitrile, NHCs) as well as carbenoid analogues of heavier elements (Ga, Si). In all cases ring expansion reactions were observed, which afforded unprecedented annulated and spiro-type bora heterocycles.

Publications

• Completing the series of boron-nucleophilic cyanoborates: boryl anions of type NHC–B(CN)2−. Chemical Science, 8(9), 6274-6280.
  Böser, Richard; Haufe, Lisa C.; Freytag, Matthias; Jones, Peter G.; Hörner, Gerald & Frank, René
  
• Benzyl Borane NHC Adducts: Beyond B−C Bond Scission. Chemistry – A European Journal, 25(45), 10575-10579.
  Böser, Richard; Denker, Lars & Frank, René
  
• N-Heterocyclic Carbene Adducts of Alkynyl Functionalized 1,3,2-Dithioborolanes. Molecules, 24(9), 1690.
  Böser, Richard; Denker, Lars & Frank, René
  
• Metal Halide Exchange in Benzylborane NHC‐Adducts: The Effect of Backbone Alkylation in the N‐Heterocyclic Carbene. Zeitschrift für anorganische und allgemeine Chemie, 648(7).
  Wullschläger, Daniela; Denker, Lars & Frank, René
  
• Reactions of a Four‐Membered Borete with Carbon, Silicon, and Gallium Donor Ligands: Fused and Spiro‐Type Boracycles. Chemistry – A European Journal, 28(32).
  Güven, Zeynep; Denker, Lars; Dolati, Hadi; Wullschläger, Daniela; Trzaskowski, Bartosz & Frank, René