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Lewis Acid Mediated Reactions: Electronic Modification of Platinum Complexes and Metal-Free Catalysis

  • Author(s): Liberman-Martin, Allegra Lauren
  • Advisor(s): Tilley, T. Don
  • Bergman, Robert G.
  • et al.
Abstract

Chapter 1. A strategy for the control of electron density at a metal center is reported, using a remote chemical switch involving second-sphere Lewis acid binding that modulates electron density in the first coordination sphere. Binding of the Lewis acid B(C6F5)3 at remote nitrogen positions of a bipyrazine-diarylplatinum(II) complex accelerates biaryl reductive elimination by a factor of 64,000.

Chapter 2. The silicon and zinc Lewis acids Si(cat)2 (cat = catecholato), Si(catF)2 (catF = tetrafluorocatecholato), and Zn(C6F5)2 bind to the remote ligand site of a 2,2´-bipyrimidyl–platinum diaryl complex. This platinum complex provides a platform to systematically evaluate electronic and reactivity differences triggered by Lewis acid binding. The electron density of the bipyrimidine ligand is substantially depleted upon Lewis acid binding, as evidenced by UV-vis spectroscopy and cyclic voltammetry. Biaryl reductive elimination studies allowed quantification of the effect of Lewis acid binding on reactivity, and Lewis acid binding accelerated reductive elimination rates by up to eight orders of magnitude. Kinetics experiments in combination with DFT studies support an unusual mechanism featuring complete dissociation of the bidentate bipyrimidine ligand prior to reductive elimination.

Chapter 3. Z-type interactions between bis(perfluorophenyl)zinc and platinum(II) diaryl complexes supported by phenanthroline (phen), bipyridine (bpy), and bis(dimethylphosphino)ethane (dmpe) ligands are reported. In the solid state, there is an unsupported Pt–Zn bond for the phen complex, while the dmpe derivative features additional bridging aryl interactions. The Pt–Zn complexes featuring phen and bpy ligands undergo facile reductive elimination, whereas aryl exchange between Pt and Zn is observed for the dmpe complex.

Chapter 4. Bis(perfluorocatecholato)silane Si(catF)2 was prepared, and stoichiometric binding to Lewis bases was demonstrated with fluoride, triethylphosphine oxide, and N,N'-diisopropylbenzamide. The potent Lewis acidity of Si(catF)2 was suggested from catalytic hydrosilation and silylcyanation reactions with aldehydes. Mechanistic studies of hydrosilation using an optically active silane substrate, R-(+)-methyl(1-napthyl)phenylsilane, proceeded with predominant stereochemical retention at silicon, consistent with a carbonyl activation pathway. The enantiospecificity was dependent on solvent and salt effects, with increasing solvent polarity or addition of NBu4BArF4 leading to a diminished enantiomeric ratio. The medium effects are consistent with an ionic mechanism, wherein hydride transfer occurs prior to silicon–oxygen bond formation.

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