Radical Ion Probes. 6. Origin of the High Intrinsic Barrier to Nucleophile-Induced Ring Opening of Arylcyclopropane Radical Cations

Radical cations generated from 1-cyclopropylnaphthalene ( 1 ), 1-bromo-4-cyclopropylnaphthalene ( 2 ), and 2-cyclopropylnaphthalene ( 3 ) were studied electrochemically. Oxidation of all these substrates in CH 3 CN in the presence of CH 3 OH leads to cyclopropane ring-opened products, i.e., the corresponding (1,3-dimethoxypropyl)naphthalenes. However, the rate constant for methanol-induced ring opening (Ar-c-C 3 H 5 • + + CH 3 OH → ArCH( • )CH 2 CH 2 O(H + )CH 3 ) is extremely small (<20 M - 1 s - 1 for the α-cyclopropylnaphthalenes) despite the fact that ring opening is exothermic by nearly 30 kcal/mol. These results are explained on the basis of a product-like transition state for ring opening wherein the positive charge is localized on the cyclopropyl group, and thus unable to benefit from potential stabilization offered by the aromatic ring.