MECHANISTIC ASPECTS OF THE SUBSTRATE IONIZATION STEP IN AMINIUM SALT CATALYZED CYCLOPROPANATION

The cyclopropanations of a series of m- and p-substituted trans-beta-m ethylstyrenes (3) by ethyl diazoacetate (4), catalyzed by tris(4-bromo phenyl)aminium hexachloroantimonate (1(.+)) and also by tris(2,4-dibro mophenyl)aminium hexachloroantimonate (2(.+)) have been studied by com petition kinetics. For the reactions catalyzed by the milder aminium s alt (1(.+)), the Hammett-Brown rho values and the fact that the absolu te rates are independent of the concentration of 4 establish that ioni zation to 3(.+) is not reversible, but rate-determining. The dependenc e of the magnitude of rho upon the absolute concentration of 3 indicat es the operation of competing chain and catalytic mechanisms, i.e. the ionization of 3 by both product cation radicals and by the catalyst. The extremely low rho value observed in the reactions catalyzed by 2(. +) indicates the exclusive operation of a relatively unselective chain mechanism. These mechanistic assignments are further supported by the observation of the formation of the same products under electrochemic al conditions, in the absence of a chemical catalyst, in closely compa rable diastereoisomer ratios and with rho values which correspond nice ly with the rho values observed for equipotential aminium salt catalys ts.