Lewis Acidity of the SbCl₃/o-chloranil System

While SbCl₃ is typically inert toward oxidation by ortho-quinones, we use o-chloranil to show that the outcome of such reactions may be altered by the presence of a donor such as triphenylphosphine oxide, which readily traps the SbCl₃(cat^Cl) synthon (cat^Cl = tetrachlorocatecholate) in the form of the corresponding adduct Ph₃PO→SbCl₃(cat^Cl). The same reaction in the presence of a chloride salt affords the corresponding antimonate anion [Cl₄Sb(cat^Cl)]^-. Computational studies indicate that the putative SbCl₃(cat^Cl) synthon has a higher chloride ion affinity than SbCl₅, suggesting significant Lewis acidity. This property is further demonstrated by the use of the SbCl₃/o-chloranil system for both THF polymerization and a Friedel-Crafts-type alkylation of benzene using 1-fluorooctane. Finally, the reaction of E-stilbene with o-chloranil in the presence of SbCl₃ affords the corresponding benzodioxene, suggesting that SbCl₃ may also operate as a redox-active catalyst.