Electrochemistry of Hypervalent Halogen Compounds

Conspectus: Hypervalent halogens (I(III) and Br(III) derivatives) represent a versatile and effective class of reagents that are widely used in synthetic organic chemistry. Many of these compounds, however, are unstable and pose risks during handling, which limits their practical application, especially on a larger scale. In this context, the electrochemical in situ generation of hypervalent halogen species represents an interesting alternative to conventional methods. Inspired by these ideas, we launched a collaborative “trans-Baltic” research program 10 years ago that is still ongoing. Our efforts focus on developing electrochemical methods for synthesis of these reactive species, their synthetic application and mechanistic elucidation. After initial studies on the electrochemical generation of dialkoxy-λ³-iodanes, our activities have expanded to include analogous hypervalent bromine compounds as well as diaryliodonium and bromonium salts. Our work has resulted in a large number of new species, most of which exhibit interesting, useful, and versatile reactivity. Their synthesis, electrochemical properties, and reactivity are the focus of this article.Our review starts with the discussion of fluorinated dialkoxy-λ³-iodanes, which are readily formed via anodic oxidation of iodoarenes in fluorinated alcohols such as HFIP or TFE, the latter acting both as solvent and stabilizing ligands. To date, no nonelectrochemical access to such species has been reported, likely due to their sensitivity toward nucleophiles. Although the λ³-iodanes derived from iodoarene conversion in HFIP cannot be isolated, they are effective in various oxidative coupling reactions when generated in situ. Incorporating ionic tags enables dual functionality as mediator and supporting electrolyte and allows facile recovery after electrolysis. In contrast, analogous dialkoxy-λ³-bromanes could not be synthesized electrochemically. However, singly and doubly chelation-stabilized bromanes can be prepared by anodic oxidation in HFIP. The doubly chelated species is significantly more stable and isolable. The less stable singly chelated form can be converted into a stable [Br–O–Br] dimer, which is suitable both as a precursor for other hypervalent bromine compounds and as a reagent for synthetic applications. The intrinsic Br(III) reactivity of the doubly chelated bromane is moderate but can be activated either thermally or with TfOH, enabling both ionic and single-electron transfer (SET) reactions. In contrast, the [Br–O–Br] dimer undergoes homolytic cleavage upon heating or near-UV irradiation, enabling radical coupling.When using noncoordinating solvents such as acetonitrile, anodic oxidation of iodoarenes yields iodonium salts. An acid-free, anion-flexible method was developed, allowing counterion variation via the supporting electrolyte and enabling aryl transfer reactions. Compared to the iodonium species, the bromonium analogues are much more difficult to access and are limited to cyclic species formed from 2,2′-dibromobiphenyls. Together with other newly identified hypervalent species, these compounds offer promising starting points for future research.