The following dissertation discusses the development and mechanistic understanding of reactions catalyzed by N-heterocyclic carbene-ligated Ni complexes. The focus of this work will be directed towards the activation and functionalization of C–O, C–H, and C–Br bonds in diaryl ethers, arenes and alkenes, and alkyl bromides, respectively.
Chapter 1 provides a review of the structural and electronic properties of N-heterocyclic carbenes (NHCs) as well as an introduction to the role of Ni catalysts in organometallic chemistry. A discussion of the nature of NHC–M bonds is discussed and representative classes of reactions catalyzed by NHC-ligated Ni complexes are presented.
Chapter 2 presents a detailed mechanistic analysis of the hydrogenolysis of diaryl ethers catalyzed by the combination of a Ni metal precursor and an N-heterocyclic carbene. Catalytic experiments revealed the key role of base in this catalytic reaction. The resting state of the catalyst was identified and kinetic analysis showed that the base is not involved in the rate-limiting C−O bond cleavage. Catalytically relevant Ni complexes have been independently prepared, fully characterized, and their role in the catalytic cycle has been studied.
Chapter 3 discusses the first undirected hydroarylation of unactivated alkenes with unactivated arenes that occurs with high regioselectivity for the anti-Markovnikov product. The reaction occurs with a Ni catalyst ligated by a highly sterically hindered NHC. Catalytically relevant arene- and alkene-bound Ni complexes have been characterized, and the rate-limiting step was shown to be reductive elimination to form the C-C bond. DFT calculations, combined with second-generation, absolutely-localized molecular orbital energy decomposition analysis (ALMO- EDA), suggest that the difference in activity between catalysts containing large and small carbenes results more from stabilizing intramolecular, non-covalent interactions in the secondary coordination sphere than from steric hindrance.
Chapter 4 discusses the development of a cross-coupling reaction between secondary alkyl bromides and an imine substrate catalyzed by an NHC-ligated Ni complex. Various reaction parameters have been investigated and an initial substrate scope of alkyl halides that participate in this reaction is presented.