Structure–Activity Relationships for Hypervalent Iodine Electrocatalysis

Abstract

The design and optimization of novel electrocatalysts requires robust structure-activity data to correlate catalyst structure with electrochemical behavior. Aryl iodides have been gaining attention as metal-free electrocatalysts but experimental data is available for only a limited set of structures. Here we report electrochemical data for a family of 70 aryl iodides. Half-peak potentials are utilized as proxies for reduction potentials and reveal that despite differences in electrochemical reversibility, the potential for one-electron oxidation of 4-substituted aryl iodides to the corresponding iodanyl radicals is well-correlated with standard Hammett parameters. Additional data is presented for 3- and 2-substituted aryl iodides, including structures with potentially chelating 2-substituents that are commonly encountered in hypervalent iodine reagents. Finally, potential decomposition processes relevant to the (in)stability of iodanyl radicals are presented. We anticipate the collected data will advance the design and application of aryl iodide electrocatalysis.

Iodanyl Radical Catalysis

“…The results of this initial foray into hypervalent iodine electrocatalysis resulted in two tantalizing conclusions. First, because the primary anodic oxidation event removes an electron from the aryl iodide and not acetate, the structure of the aryl iodide can be used as a tool to control the potential at which catalysis proceeds . Second, in the absence of biaryl amide 12 , electrolysis of 2b did not result in the formation of I­(III) species; rather, bulk electrolysis of 2b resulted in deiodinative coupling to afford 4,4′-dimethoxy-1,1′-biphenyl .…”

“…First, because the primary anodic oxidation event removes an electron from the aryl iodide and not acetate, the structure of the aryl iodide can be used as a tool to control the potential at which catalysis proceeds . Second, in the absence of biaryl amide 12 , electrolysis of 2b did not result in the formation of I­(III) species; rather, bulk electrolysis of 2b resulted in deiodinative coupling to afford 4,4′-dimethoxy-1,1′-biphenyl . Together, these observations raised the unexpected prospect that iodanyl radicals, and not I­(III) intermediates, may be on-path for substrate functionalization.…”

Hypervalent Halogen Compounds in Electrochemical Reactions: Advantages and Prospects

“…This method represents a powerful method for generating (diacetoxyiodo)arenes in situ . More recently, the group disclosed an extensive cyclic voltammetry analysis for a family of 70 aryl iodide mediators [12] . The results revealed the potential for the one electron oxidation of 4‐substituted aryl iodides is well‐correlated with standard Hammet parameters.…”

“…[7] Ex‐cell applications, where the electrochemical generation of iodine(III) reagents is performed in the absence of a substrate, are the most prevalent approach. This circumvents the relatively high oxidation potentials of aryl iodides [12] . The groups of Hara, Waldvogel, Francke, Lennox and Wirth have disclosed efficient electrochemical methods for the generation of iodine(III) reagents, including (difluoroiodo)arenes, (diacetoxyiodo)arenes and (dialkoxyiodo)arenes.…”

“…This circumvents the relatively high oxidation potentials of aryl iodides. [12] The groups of Hara, Waldvogel, Francke, Lennox and Wirth have Thomas Wirth is professor of organic chemistry at Cardiff University. He received his PhD after studying chemistry at Bonn and the Technical University of Berlin.…”