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Mechanistic Aspects of the Substrate Ionization Step in Aminium Salt Catalyzed Cyclopropanation

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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-dibromophenyl)aminium hexachloroantimonate (2*+) have been studied by competition kinetics. For the reactions catalyzed by the milder aminium salt (1*+), the Hammett-Brown p 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 p 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 p 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 p values which correspond nicely with the p values observed for equipotential aminium salt catalysts.

Affiliations: 1: Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712, USA

10.1163/156856797X00349
/content/journals/10.1163/156856797x00349
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/content/journals/10.1163/156856797x00349
1997-01-01
2017-03-24

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