N-Heterocyclic Carbene Borenium Ions: Decomposition Pathways and Applications in Organic Synthesis

Abstract

Borocations have been synthetically accessible entities for over 35 years, thought of as chemical curiosities too reactive to be synthetically useful until recently. Our group has contributed to the development of a class of three-coordinate dialkyl borocations, known as borenium ions, stabilized by N-heterocyclic carbenes (NHCs). Our NHC is based on the 1,2,3-triazolylidene ring system, a modular motif that we previously exploited in borenium-catalyzed hydrogenation reactions. This thesis describes our work on borenium ion catalysis as well as insight into unexpected decomposition pathways. First, we established a facile hydrosilylation protocol of imines and quinolines with triazolylidene borenium ion 1+ under mild conditions. With respect to quinoline reductions, 1+ does not completely reduce the N-heterocyclic ring, giving rise to a cyclic N-silyl enamine, which we applied in a Mukaiyama aldol-like reaction with aldehydes to produce unique γ-aminoalcohols. Interestingly, control experiments showed that 1+ catalyzed both the reduction and nucleophilic addition. Next, we attempted to reduce pyridines with dihydrogen, a classically difficult substrate class. Control reactions indicated that catalyst decomposition was favoured at high temperatures in the presence of a bulky secondary amine. To mitigate this problem, a hydrosilane was injected into the reaction, allowing for more mild hydrogen pressures and temperatures. Thus far, 1+ reductions are racemic and introduction of chiral NHCs or boranes has resulted in low stereoinduction. The avenue presented in this thesis to overcome this challenge is the preparation of NHC-boranes containing chiral phosphate substituents. Strong thermodynamic and kinetic hydricity of the NHC-borane was critical for facile synthesis of these molecules. Finally, we discovered that imidazolylidene-stabilized carbene boranes ring expand at elevated temperature to yield cyclic azaborines. Subsequent hydride abstraction of these cyclic systems resulted in 6-membered cyclic aromatic boron carbocations. The data presented in this thesis challenge the preconceived notions of high NHC stability with regard to exotic main group molecules and approaches to overcome these obstacles has resulted in new synthetic methodologies.