Catalytic Chemistry Relating to the Au(I)/Au(III) Redox Couple
- Author(s): Levin, Mark Daniel
- Advisor(s): Toste, F. Dean
- et al.
This thesis represents a progression of ideas surrounding chemistry of the two dominant oxidation states for homogeneous complexes of gold. As with all science, a steady combination of successful ideas, happy accidents, and illuminating failures led to the evolution of a perspective on the nature of the two electron transformation between Au(III) and Au(I) that is the focus of this dissertation (though the exact typology of this focus varies, depending on the chapter). Along the way, a foray into platinum chemistry served to highlight both the similarities between these sister elements and their often stark differences. The thread between gold-catalyzed reactivity in Chapter 2 and transformations in which this paradigm is flipped such that a catalyst acts upon a gold complex in Chapters 3 and 4 is a direct result of the intervening failures which pointed the way towards new chemistry. Accordingly, in each chapter, an honest account of the travails leading to the ultimate discovery is attempted.
Chapter 1 discusses the historical perspective on the Au(I)/Au(III) redox couple both in the context of catalytic chemistry and in stoichiometric organometallic transformations. An attempt is made to showcase, by means of this survey, the unique features of gold that give rise to its unusual behavior among its transition metal cohort.
Chapter 2 summarizes the development of an Au-catalyzed C(sp3)-C(sp2) bond-forming crosscoupling reaction between allylic bromides and arylboronic acids. The scope of the process was examined revealing an unusual tolerance for steric hindrance and novel chemoselectivity profile compared to competitive technologies. Mechanistic studies are presented which support oxidative addition of the C-Br bond as the central elementary transformation enabling catalysis, including a model system in which cyclometallation enables observation of each intervening elementary step. This discovery was among the first reports of a gold-catalyzed reaction proceeding through Au(I)/Au(III) redox without the use of a sacrificial oxidant; several reports by other groups taking advantage of this reactivity paradigm since this initial disclosure are discussed, alongside unsuccessful attempts at extension of this reactivity to other substrate classes which ultimately informed the remaining chapters.
Chapter 3 focuses on the discovery that C(sp3)-C(sp3) reductive elimination from Au(III) can be effected by the use of a [Ga4L6]12- supramolecular catalyst which recognizes and selectively stabilizes the transition state for C-C reductive elimination. This behavior was observed to extend to Pt(IV) complexes, and a series of mechanistic and structural studies support the formation of a coordinatively unsaturated cationic species within the supramolecular cavity prior to rate-limiting reductive elimination for both metals. A dual catalytic cross-coupling was developed in which iodomethane and tetramethyltin are coupled to afford ethane, notably requiring the collaborative action of both platinum and supramolecular catalysts to achieve efficient turnover, and a complete mechanistic picture for the overall process is delineated on the basis of stoichiometric experiments.
Chapter 4 describes a serendipitous discovery of C(sp3)-CF3 reductive elimination from Au(III) complexes in the presence of catalytic B(C6F5)3 and the subsequent application of this discovery to the synthesis of PET tracers. Mechanistic experiments revealed a novel “fluoride rebound” mechanism for reductive elimination involving C-F abstraction, migratory insertion, and C-F reductive elimination. The parent complexes were found to tolerate a surprising breadth of synthetic protocols, enabling the synthesis of complex organometallic derivatives via standard organic synthesis methodologies without cleavage of the Au-C bond. Interception of the rebounding fluoride in synthetically advanced complexes enabled the development of a protocol for the synthesis of [18F] radiolabeled CF3-containing compounds with substantial structural complexity.