Mechanistic insights into the role of silver in asymmetric gold(I) catalyzed cycloisomerization OF 1,6-ene allenes Public Deposited

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  • March 22, 2019
Creator
  • Hulchanski, Allison J.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • This thesis encompasses a series of studies carried out to ascertain the mechanism that axially chiral bis(gold) phosphine catalysts undergo to stereoselectivity cycloisomerize 1,6-ene allenes. The thesis also discusses the role in which silver salts play to not only activate the gold catalysts but their role in catalysis that affects catalyst speciation, stereoselectivity, and formation of dinuclear intermediates. While previous studies into the mechanism of gold(I) catalyzed reactions have focues primarily on monodentate ligated gold catalysts, little is known about the mechanism involved with bis(gold) phosphine based catalysts. In addition, only recently has it been acknowledged that silver may play an active role in catalysis rather than the notion that silver activates gold to become an inactive silver halide byproduct. The thesis first uses Non-linear effects experiments to determine if oligomers often seen in gold coordination chemistry occur during catalysis. Attempts were made at synthesizing 3-center-2-electron digold species often seen in mono-dentate gold catalyzed reaction failed. However, the inability to form stable or isolable digold model compounds suggests that axially chiral bis(gold) phosphine catalysts are too strained to form this type of intermediate. Also described in this thesis is the study of the role of silver in catalysis. The effect of the amount of silver used to activate the gold catalyst on the enantiomeric excess of the product as well as the catalysts speciation directly shows silver is more than an inactive bystander in catalysis. Titration of silver with bis(gold) phosphine aryl model compounds with silver at low temperatures demonstrate the potential for dinuclear Au-Ag intermediates. Finally, studies removing AgCl byproduct and addition of silver salts with less coordinating counter ions that chloride directly impacted enantiomeric excess of the products suggesting that the proposed dinuclear Au-Ag species is involved in the stereochemistry determining step of the mechanism.
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  • In Copyright
Advisor
  • Gagne, Michel
Degree
  • Master of Arts
Graduation year
  • 2013
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