Discovery of Ruthenium Tris-phosphine Complexes for Poly-alcohol Functionalization and Alkene Hydroamination
This dissertation describes the discovery and development of new ruthenium cis-triphosphine coordination complexes and their application towards catalytic functionalization of alcohols, ketones, and alkenes. Ruthenium-mediated transfer dehydrogenation of alcohols and subsequent complex ketone functionalization reactions enable the modification of the structures of complex molecules bearing multiple alcohol functionalities. Alteration of the ruthenium catalyst X-type scaffold generates a coordination complex that activates select amines for the direct Markovnikov hydroamination of terminal alkenes. Unactivated terminal alkenes bearing a diversity of functional groups and limiting amounts of alkene can be employed with acceptable yield. Internal alkenes undergo hydroamination with a selectivity that targets tandem isomerization-hydroamination.
Chapter 1 surveys ruthenium-catalyzed hydrogenation and borrowing hydrogen chemistry with an emphasis on hydrogen transfer-mediated alcohol functionalization. Possible mechanisms of hydrogen transfer are considered and kinetic and thermodynamic considerations regarding hydrogen transfer and hydrogenation reactions are analyzed. This chapter then presents a survey of direct hydroamination reactions of alkenes with late metals and known hydroamination systems with ruthenium catalysts. Precedent in the fields of hydrogen transfer chemistry and hydroamination is described with an emphasis on the synthetic systems of highest utility or closest analogy to the transformations presented in this thesis. The utility, scope, and mechanism of known hydrogen transfer mediated functionalization reactions and seminal contributions to this field are described
Chapter 2 presents the discovery and development of a new class of ruthenium cis-trisphosphine complexes bearing triflate ligands which catalyze a diverse array of organic transformations. The synthesis of this class of complexes is described in addition to select stoichiometric reactions of these complexes. This chapter presents the application of newly developed ruthenium catalysts towards the selective dehydrogenation of secondary alcohols in diverse complex polyhydroxylated natural products. The epimerization of select secondary alcohols in complex natural products is described. Several mechanistic experiments are given which support insight into the hydrogen transfer catalysts we have developed. Furthermore, this chapter covers the development of complex ketone functionalization chemistries to convert complex ketones to diverse functionalized complex molecules. In particular the description of chemistries for the incorporation of nitrogen based functional groups into complex ketones are presented.
Chapter 3 presents the development of a new catalytic hydroamination system of terminal alkenes which tolerates limiting amounts of alkene and which enables the modification of a diverse scope of alkene substrates. The synthesis of a new class of catalysts based on a ruthenium trisphosphine scaffold with triflimide X-type ligands is presented along with their catalytic application. A synthetic route a novel osmium analog of the primary ruthenium scaffold is described. Several mechanistic experiments are presented to interpret the high reactivity of the system relative to previous systems in order to explain how a ruthenium catalyst has an activity that supersedes previous rhodium, iridium, and gold-based systems for hydroamination. Empirical observations including the generation of catalytic quantities of unsaturated intermediates in the hydroamination reaction, the tolerance of diverse solvents of widely varied dielectric properties, and the incorporation of deuterium at numerous locations in the starting materials and products the reaction are analyzed. Nuclear magnetic resonance spectroscopic and X-ray crystallographic studies are shown as preliminary support of a borrowing hydrogen type hydroamination mechanism and to support a preliminary proposal regarding the identity of ruthenium complexes along the catalytic cycle of hydroamination.