Metal-Catalyzed And Photocatalytic Systems For Oxidative Inter- And Intramolecular Phenol Coupling

Chapter 1: Considerable work in the area of catalytic, oxidative intermolecular phenol coupling has allowed for the efficient preparation of homo- and cross-coupled biphenol compounds. Despite this, little work has been done in the context of catalytic, oxidative intramolecular phenol coupling. Phenol-dienone compounds represent interesting candidates for the study of an intramolecular phenol coupling reaction, given their prevalence in natural product biosynthesis. Inspired by stoichiometric reports for the formation of phenol-dienone compounds from tethered phenol precursors and our own work in V(V)-oxo catalysis, a catalytic method was developed to this end. A broad scope of electronically diverse phenol-dienone products was prepared in good yield with low loadings of V(V)-oxo catalyst in the presence of dioxygen as a co-oxidant. This catalytic method was also used as the key step in the preparation of three naturally occurring compounds, as well as a salutaridine derivative relevant to the laboratory synthesis of morphinan alkaloid compounds. Chapter 2: Prior reports detailing the transition-metal catalyzed and electrochemical oxidation of phenol compounds enable the preparation of homo- and cross-coupled biphenol compounds. Despite this, the principles of photocatalysis have been applied only sparingly to the transformation of phenol compounds. Using a strongly oxidizing mesityl acridinium photocatalyst and air as a co-oxidant, the first photocatalytic phenol coupling was developed. Working with group alumnus Dr. Kyle Niederer, a broad collection of homo- and cross-coupled biphenol products were prepared in good yield with excellent regioselectivity. Detailed mechanistic and control experiments support a mechanism wherein the key C-C bond is formed between an oxidized phenoxy radical and a neutral, nucleophilic phenol, akin to electrochemical oxidation. Further study of this transformation also resulted in the first catalytic preparation of dityrosine using a ruthenium-centered photocatalyst. Chapter 3: The highly selective cross-coupling reaction of two distinct phenols using a chromium-salen catalyst in combination with dioxygen has been studied in detail. Interestingly, certain phenol substrates will undergo two C-C coupling events under these conditions to generate a triphenol product, due to the enhanced reactivity of the intermediate biphenol product. Taking advantage of this observation, we designed phenol substrates that allow for the preparation of symmetric, highly functionalized meta-terphenyl derivatives to be used as building blocks for the preparation of novel polycyclic aromatic hydrocarbon (PAH) frameworks. This approach has enabled the synthesis of a class of PAH compounds containing the building block linked to various hydrocarbon and heteroaromatic spacer compounds. Electrochemical and photophysical studies revealed their molecular properties and indicated potential applications of these molecules.