The cationic dimerization of 1,1‐diphenylethylene. I. Nature of active centers in the case of protonic and lewis acids initiation, at low temperature, in dichloromethane solution

The reaction of 1,1‐diphenylethylene with titanium tetrachloride or with trifluoromethane‐sulfonic acid has been studied at −30 and −70°C in dichloromethane solution, and has given, in both cases, relatively stable diphenyl alkyl carbocations. PMR spectra show that these cationic species are mainly monomeric, i.e., (I). In the electronic spectra, a main absorption band at 435 nm is assigned to both monomeric (I) and dimeric (II) cations, whereas, a 520 nm absorption is assigned to monomer solvated forms of I and II. A third absorption at 456 nm was detected by resolution of the overlapping absorptions at 435 and 520 nm and might be assigned to the dimeric cation, autostabilized through a ß‐phenyl ring interaction. Constants of the equilibria between the different species were determined at − 30°C.