Palladium-Catalyzed Arene Functionalization via Single-Electron Reduction of Selectfluor
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CitationMazzotti, Anthony Renato. 2016. Palladium-Catalyzed Arene Functionalization via Single-Electron Reduction of Selectfluor. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractPalladium-catalysis is commonly used in the functionalization of aromatic rings, ranging from prefunctionalized substrates such as aryl metals or aryl (pseudo)halides to the direct functionalization of aromatic C–H bonds. Palladium-catalyzed functionalizations typically proceed through a (trans)metallation step to generate an organometallic intermediate. The work in this thesis describes two palladium-catalyzed arene functionalization methodologies that forego the formation of an organometallic species. Instead, the catalysts reduce Selectfluor by a single electron to generate reactive intermediates: Selectfluor radical cation, which retains the fluoride and is an active fluorinating reagent, and TEDA2+*, which loses the fluoride and is an active reagent for the functionalization of arene C–H bonds.
Chapter 1 describes the development of a method for the palladium(III) catalyzed fluorination of arylboronic acid derivatives. The reaction is tolerant of air and moisture, able to be carried out in an open flask and was successfully performed on up to decagram scale. The reaction tolerates protic functional groups and bulky ortho,ortho’-disubstitution, and can be applied to the synthesis of natural product derivatives. We observe single electron oxidation of a bis terpyridyl palladium(II) complex to generate a mononuclear palladium(III), the first mononuclear palladium(III) catalyst in the literature. Fluorination via a radical pathway renders the reaction tolerant of protic functionality and circumvents formation of protodeborylated side product commonly observed in other transition metal-mediated fluorination reactions.
Chapter 2 describes the development of an arene C–H functionalization method for the incorporation of the doubly cationic residue of Selectfluor, TEDA, with high para-selectivity. The substitution of arene C–H bonds by TEDA selectively creates a protected aniline moiety and is tolerant of air and moisture, able to be performed with substrate as the limiting reagent and up to decagram scale without loss of yield. Subsequent reduction of the aryl-TEDA species by sodium thiosulfate provides pharmaceutically-relevant aryl piperazines directly in a one-pot, two-step sequence from the starting arene C–H bond. In the vast majority of cases, aryl piperazines are furnished as a single constitutional isomer across a wide variety of carboarene and heteroarene substrates.
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