In-Situ Electrolyte for Electrosynthesis: Scalable Anodically-Enabled One-Pot Sequence from Aldehyde to Isoxazol(in)es.

ABSTRACT

Electrochemical transformations are considered a green alternative to classical redox chemistry as it eliminates the necessity for toxic and waste producing redox reagents. Typical electrochemical reactions require the addition of a so-called supporting electrolyte - a salt bridge - and other additives, such as hexafluorisopropanol, to enhance conductivity and reaction outcomes, respectively. However, this is often accompanied by an increase in the amount of produced waste. Here, we report an "in-situ electrolyte" concept for facile, transition-metal-free, additive-free one-pot electrochemical preparation of isoxazol(in)es, important scaffolds for biologically active natural and synthetic molecules, from the respective aldehydes. The protocol utilizes no halogenated solvents and no external oxidants, while salt side-products provide the ionic conductivity necessary for the electrosynthesis. The electrolysis is performed in an undivided cell, using the state-of-the-art electrodes for the chlor-alkali industry dimensionally stable and scalable mixed metal oxide anode and platinized titanium anode of high durability. The cascade transformation comprises the condensation of aldehyde to oxime followed by its anodic oxidation and subsequent intra- and/or intermolecular [3+2] cycloadditions with an appropriate dipolarophile. Chemical yields up to 97%, and good Faradaic efficiency, scalability, and stability are observed for most substrates in a broad scope.