Accessibility, Reactivity, and Fluoroalkylation Reactions of High-Oxidation-State Organonickel Complexe

A recent resurgence in nickel catalysis research has demonstrated that nickel-based catalyst systems are promising candidates to solve many outstanding problems in cross-coupling catalysis. Mechanistic studies of these transformations often reveal complicated interconversions of short-lived and consequently poorly characterized organometallic nickel intermediates. This observation is particularly true for highly oxidized nickel centers, which rapidly eliminate C--C and C--X bonds. Thus the rational development of methodologies based on high-valent nickel intermediates remains difficult. This dissertation seeks to address these uncertainties through detailed studies on the accessibility, reactivity and interconversions of model NiIII/IV complexes with a specific focus on fluoroalkylation elimination reactions from Ni III/IV centers.;Chapter 2 details the synthesis and 1e-- oxidation chemistry of [NiII(CF3)(Ph)] complexes bearing diphosphine or tridentate nitrogen donor ligands. Our studies demonstrate that with a judicious choice of ligand, nickel is able to efficiently mediate the formation of Ar--CF 3 bonds under oxidatively and thermally mild conditions. Stabilization of the proposed intermediates with a tridentate ligand is found to yield the first example of an isolable diorganonickel(III) complex that undergoes C--C coupling. Detailed mechanistic studies of this transformation rule out the potential intermediacy of NiIV in this reaction.;Chapter 3 describes the design and reactivity of a model system for a two-part study on elementary organometallic reactions pertinent to Ni II/IV catalysis. Various aryl and alkyl electrophiles are examined for their ability to effect the 2e-- oxidation of NiII to NiIV. The C--C and C(sp3) --X coupling of the reactions of resultant Ni IV(alkyl/aryl) compounds is investigated. Mechanistic studies differentiating 1e-- vs 2e-- pathways of these transformations are described.;In Chapter 4 the interconversion of organonickel(III/IV) complexes through their reactions with carbon-centered radicals (CCRs) is reported. First we demonstrate that CCRs effect the oxidation of NiIII to Ni IV through inner-sphere radical addition to the nickel centers. Secondly, we show that select NiIV alkyl complexes are susceptible to homolytic abstraction of a carbon donor ligand by a free carbon-centered radical. This non-traditional C--C coupling pathway opens up previously unprecedented types of reactivity, including mild C--C coupling to form H3C--CF3.;Chapter 5 describes the synthesis and reactivity the first isolated examples of a copper(I) difluoromethyl complexes. Key to the realization of this strategy was the implementation of a bulky N-heterocyclic carbene ligand to slow bimolecular decomposition. The stoichiometric reactions of these complexes with a variety of organic electrophiles are described culminating with the catalytic difluoromethylation of aryl iodides.