Investigating Reaction Pathways and Non-Covalent Interactions from Diarylhalonium Salts Using Density Functional Theory

Diaryliodonium salts are very useful for organic synthesis. From diaryliodoniums, one can generate arynes and aryl radicals, as well as transfer aryl groups and form useful intermolecular associations as a Lewis acid. The variety of reactivity and uses for diaryliodonium salts makes improved understanding of their properties and behavior advantageous. Using density functional theory (DFT), this work will present a series of models including diaryliodoniums, bromoniums, and chloraniums, to help expand our understanding of halonium interactions and reactions. The first study undertaken here uses chlorine, bromine, iodine, and astatine analogues to investigate halogen bond strength to each halogen center. Using a combination of molecules including hypervalent and monovalent species, as well as charged and neutral compounds, this work probes the variables influencing the association between halogen centers and chloride anions. The final model for this system found the sigma hole potential and orbital contributions to bonds by halogen centers to have the greatest impact on the DFT-calculated halogen-chloride bond strength. Next we studied phenyl(mesityl)iodonium, bromonium, and chloranium salts and the mesityl vs. phenyl transfer ratios from each to m-methoxyphenoxide. The synthesis and coupling reaction trial results are reported, and DFT-based models of both possible reaction pathways are presented. Transition state energy comparison to product ratios found reasonable agreement with the iodonium, but not for the bromonium or chloranium. We then present models for chlorophenyl(trimethoxyphenyl)iodonium triflate reactions with methoxide, exploring both ligand coupling and aryne extrusion reaction pathways. The iodonium cation, triflate anion, methoxide anion, and sodium cation combined to produce the lowest energy intermediate identified for each system. Ligand coupling transition states were accessible from this lowest-energy intermediate, but rearrangement of the intermediate was necessary to access aryne forming transition states. Aryne formation is the focus of the final portion of this work. Phenyl(mesityl)iodonium and phenylthianthrenium salt aryne extrusion reactions were studied using a variety of substitution patterns on the aryne-forming aryl group. We saw concerted deprotonation and carbon-iodine bond cleavage in the iodonium salt reactions. The thianthrenium salts showed deprotonation as a separate reaction step, followed by a zwitterionic intermediate, before sulfur-carbon bond cleavage and aryne formation. Finally, we found correspondence between the DFT-calculated kinetic isotope effect (KIE) for 3-bromo-6-methoxyphenylthianthrenium and experimental deuterium kinetic isotope effect (DKIE), supporting the two-step aryne extrusion mechanism for thianthrenium salts.