Stopped-Flow and 1H NMR Study of the Ionization of Cumyl Chloride by Boron Trichloride.

Abstract

Study of the ionization of cumyl chloride by boron trichloride was carried out using stopped-flow UV−visible and 1H NMR spectroscopy in methylene chloride. Cumyl chloride is very weakly ionized to form the cumyl cation (λmax = 330 nm) which is stable at low temperature (<−60 °C) with tetrachloroborate anion since no elimination could be observed. At that temperature, owing to the presence of traces of α-methylstyrene in the cumyl chloride solutions, the ionization step was immediately followed by the addition of this monomer, resulting in the formation of the dimeric (or oligomeric) cation (λmax = 348 nm), which slowly cyclized into the corresponding indanic derivatives. At −65 °C, the equilibrium constant of ionization of cumyl chloride is much smaller than that of the chlorinated dimeric (or oligomeric) species. At room temperature, dehydrochlorination of the cumyl cation leading to α-methylstyrene was evidenced and the indanic dimer was the only final product.