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(catCl) synthon (catCl = tetrachlorocatecholate) in the form of the corresponding adduct Ph3PO→SbCl3(catCl). The same reaction in the presence of a chloride salt affords the corresponding antimonate anion [Cl4Sb(catCl)]-. Computational studies indicate that the putative SbCl3(catCl) 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.