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

Abstract

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 NiIII/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–CF3 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 NiII/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 NiIV(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 NiIV 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.