Advances in the Fluorine Chemistry of Transition Metals and N-Heterocyclic Carbenes: Understanding Perfluoroalkyl and Fluoroalkene Reactivity
Creators: name: Leclerc, Matthew
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2017
Subjects: Fluorine
Perfluoroalkyl
Fluoroalkene
Carbene

description: The importance of fluorine in a wide array of different areas within chemistry and biochemistry has been demonstrated time and time again. Fluorine-containing products range from essential substituents in pharmaceuticals and relatively long-lived tracers for PET imagining, to fluoropolymers with outstanding properties, to essential components in most of the strongest acids available to chemists today. Fluorine’s extreme electronegativity makes it a truly unique element, but its acute toxicity in its elemental F2 form makes it difficult to handle, prompting researchers to explore different options for incorporating this important element into a variety of different molecular scaffolds. Due to the remarkable thermodynamic and kinetic stabilities of C-F bonds, methods for forming and breaking these cleanly, and under relatively mild conditions, are in high demand. Fortunately, transition metals have greatly aided in this process. However, fluoroorganometallic chemistry is much less developed than transition metal chemistry involving hydrocarbons, and certainly less understood. One of the primary reasons for this relative dearth of fluoroorganometallic complexes is the difficulty associated with their synthesis. In this work, important steps towards perfluoroalkyl chain-growth within the coordination sphere of a transition metal will be presented, stemming in part from the synthesis and characterization of novel cobalt fluoride and bis(perfluoroalkyl) complexes. As important electrophiles, fluoroalkenes have primarily been used as monomers for the formation of important fluoropolymers. However, their direct reactivity with organics remains rare and is usually difficult to control, with limited substrate scopes. Herein, the formation of stable N-heterocyclic fluoroalkene adducts as versatile synthons for the incorporation of fluoroalkene fragments into various chemical environments will be introduced. By forming these adducts, the inconvenience of manipulating fluorinated gases in further reactions can be avoided, and the N-heterocyclic fragment is shown to aid in directing substitutions involving polyfluoroalkenyl imidazolium salts and organic nucleophiles to form a variety of C-E (E = C, N, O, S) and C-M bonds (M = Mn, Mo). The ease with which C-F bonds are manipulated in these systems is quite remarkable, as the substitution reactions occur cleanly and efficiently at room temperature, to form a variety of new bonds without the need for a transition metal. By expanding on the fundamental reactivity between N-heterocyclic carbenes and fluoroalkenes, attempts were made to correlate the observed reactivity with certain electronic and steric parameters unique to the utilized carbenes. Although a correlation has not yet been established, the effects of atypical steric constraints in a cyclic (alkyl)(amino)carbene were demonstrated, wherein the initial point of attack by the carbene on the fluoroalkene was modified. It is hoped that this work will eventually lead to new roles for organocatalysts in fluoroalkene transformations.