Investigation of Gold-Vinyl Intermediates Opens New Avenues in Gold Catalysis
Public Deposited- Last Modified
- March 22, 2019
- Creator
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Weber, Dieter
- Affiliation: College of Arts and Sciences, Department of Chemistry
- Abstract
- The mechanism of gold(I)-catalyzed intramolecular hydroarylation of allenes was studied in detail. While monitoring the reaction by NMR, an unexpected dinuclear gold-vinyl intermediate was observed, that was proposed to contain a geminally diaurated carbon center stabilized by an aurophilic interaction. Isolation of this intermediate was achieved by inhibiting catalytic turnover with a base. Addition of suitable ligands abstracted one gold unit and yielded a monogold-vinyl complex, which was structurally confirmed by X-ray diffraction analysis. The reactivity of both intermediates was studied. To better understand the formation of digold-vinyl intermediates, a variety of arylgold(I) model complexes were synthesized. A mixture of mono- and digold-aryl complexes revealed averaged proton signals by NMR. The chemical shift of these resonances could be used to determine the percentage of monogold bound as digold, which allowed for quantification of anion and ligand effects on digold formation. In addition, it was observed that Brønsted acids affected the coordinating ability of anions through homoconjugate acid/base pairs. Trends observed in model systems were confirmed by mechanistic studies on the intramolecular hydroalkoxylation of allenes. Depending on the reaction conditions a digold or a monogold resting state was observed. Kinetic isotope effect studies and isotope labeling experiments were conducted to identify the catalytic role of digold. Data suggested that the digold intermediate operated off cycle and acted as an inactive catalyst reservoir, which seemed inhibitory for efficient catalysis. Further mechanistic studies revealed that digold formation altered kinetic data as well, which complicated the interpretation of Hammett studies and the development of rate laws. It was also found that silver and palladium intercepted gold-vinyl intermediates as well. Silver affected the mechanism of the intramolecular hydroarylation of allenes, which could explain previously observed silver effects in gold catalysis. Organogold complexes could engage in cross-coupling chemistry by transmetallating their organic group to Pd. Further studies led to a palladium-catalyzed homo-coupling of gold-aryl model complexes was observed, which was driven by a bimetallic Au(I)/Pd(0) RedOx process. Other new avenues in gold catalysis include the Lewis acid mediated activation of silver-free catalyst precursors and a new mechanistic proposal for the origin of chirality in gold-catalyzed reactions.
- Date of publication
- May 2012
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- In Copyright
- Advisor
- Gagne, Michel
- Degree
- Doctor of Philosophy
- Degree granting institution
- University of North Carolina at Chapel Hill
- Graduation year
- 2012
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