Merging Shuttle Reactions and Paired Electrolysis: E-Shuttle Unlocks Reversible Halogenations

Abstract

<p>Polyhalogenated molecules have found widespread applications as flame retardants, pest-control agents, polymers and pharmaceuticals. They also serve as versatile synthetic intermediates in organic chemistry due to the inherent reactivity of carbon-halogen bonds. Despite these attractive features, the preparation of polyhalogenated molecules still mainly relies on the use of highly toxic and corrosive halogenating reagents, such as Cl<sub>2</sub> and Br<sub>2</sub>, which are hazardous compounds to transport, store, and handle. Moreover, the use of such highly reactive reagents inherently makes the development of the reverse reactions, <i>retro</i>-dihalogenations, highly challenging, despite their potential for the recycling of persistent halogenated pollutants. Here, we introduce an electrochemically-assisted shuttle<i> (e-shuttle)</i> paradigm for the facile and scalable interconversion of alkenes and vicinal dihalides, a class of reactions which can be used both to synthesize useful polyhalogenated molecules from simple alkenes and to recycle waste material through <i>retro</i>-dihalogenation. The power of this reaction is best highlighted by an example, in which different soils contaminated with a persistent environmental pollutant (Lindane), could be directly used as Cl<sub>2</sub>-donors for the transfer dichlorination of simple feedstock alkenes, merging a recycling process with a synthetically relevant dichlorination reaction. We further demonstrate that this paired electrolysis-enabled shuttle protocol, which uses a simple setup and inexpensive electrodes, is applicable to four different, synthetically useful transfer halogenation reactions, and can be readily scaled-up to a decagram scale. In a broader context, the symbiotic merging of shuttle reactions and electrochemistry introduced in this work opens new horizons for safer transfer functionalization reactions that will address important challenges across the molecular sciences.</p> <div><br><div> </div> </div>

Taming Electron Transfers: From Breaking Bonds to Creating Molecules

“…Importantly, together with the Waldvogel group, Morandi and co-workers showed that dissociative electron transfer pathways can be used to overcome bottlenecks in conventional shuttle catalysis schemes. [25] In constant current electrolysis conditions, 1,2-dichloro species were reductively cleaved to mono-chlorinated radicals, that readily expel chloride after further reduction. Mn(II)-electrocatalysts could then oxidize (at the anode) the alkene co-substrate to the corresponding dichloride in a domino paired electrolysis setup.…”

“…Development of an e-shuttle catalytic scheme (two functional groups shuttling) to overcome limitations of current (hydro)functionalization shuttle catalysis. [25] These examples highlight the important role that dissociative electron transfer schemes exert in modern organic synthesis. They also show how a detailed mechanistic picture of the elementary steps and rate constants may allow to fine-tune the reactivity and to develop highly selective processes under mild conditions.…”