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.

Electrophilic Reactivity of the α,α-Dimethylbenzyl (Cumyl) Cation

“…The rate constant calculated in this way is 400 times higher than the value reported previously by the Paris laboratory for the addition of the cumyl cation to α-methylstyrene at −65 °C, which was obtained from the initial slopes of appearance of a 348 nm absorbance band ascribed to the dimeric cation …”

“…The rate constant calculated in this way is 400 times higher than the value reported previously by the Paris laboratory for the addition of the cumyl cation to R-methylstyrene at -65 °C, which was obtained from the initial slopes of appearance of a 348 nm absorbance band ascribed to the dimeric cation. 37 Although previous investigations on styrene derivatives showed similar electrophilic reactivities of monomeric and dimeric cations, 17,18 we cannot generalize this observation. Because we were not able to synthesize suitable precursors for the laser flash photolytic generation of the dimeric cation, we leave the investigation of the oligomeric cations to the following paper.…”

“…A direct kinetic determination of the first step of the carbocationic polymerization of α-methylstyrene, that is, the reaction of the cumyl cation ( 1 + ) with α-methylstyrene ( 2e ) to form the dimeric cation, was not attempted because α-methylstyrene absorbs at the excitation wavelength of the laser and interferes with the photolytic generation of the cumyl cation. Furthermore, the UV-absorption spectra of the cumyl (λ max = 335 nm) and the dimeric cation (λ max = 348 nm) are very similar, which complicates measuring the rate of consumption or formation of the carbocations.…”

Isobutene Polymerization Using a Chelating Diborane Co-Initiator

“…By analogy, reaction with CumCl forms ion pair 2a , featuring the unknown [ 1 (μ-Cl)] anion partnered with the cumyl cation. On the basis of the extent of ionization, the Lewis acidity of 1 toward CumCl exceeds that of BCl 3 and is comparable to that of SbF 5 where ionization is essentially irreversible …”

“…On the basis of the extent of ionization, the Lewis acidity of 1 toward CumCl exceeds that of BCl 3 and is comparable to that of SbF 5 where ionization is essentially irreversible. 8 Ion-pair 2a is stable at -80 °C, but on warming, decomposition to the known indan 3 8 occurs while diborane 1 and HCl are the other major byproducts of this decomposition (Scheme 1). A similar process also ensues for 2b although competitive degradation of the counteranion is observed as will be described in detail elsewhere.…”