The cyclopropanations of a series of m- and p-substituted trans-β-methylstyrenes (3) by ethyl diazoacetate (4), catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (1 ·+) and also by tris(2,4-dibromphenyl) aminium hexachloroantimonate (2 ·+) have been studied by competition kinetics. For the reactions catalyzed by the milder aminium salt (1 ·+), the Hammett-Brown ρ values and the fact that the absolute rates are independent of the concentration of 4 establish that ionization to 3 ·+ is not reversible, but rate-determining. The dependence of the magnitude of ρ upon the absolute concentration of 3 indicates 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 ρ 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 electrochemical conditions, in the absence of a chemical catalyst, in closely comparable diastereoisomer ratios and with ρ values which correspond nicely with the ρ values observed for equipotential aminium salt catalysts.
Research on Chemical Intermediates – Springer Journals
Published: Apr 14, 2009
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