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 stu died electrochemically. Oxidation of all these substrates in CH3CN in the presence of CH3OH leads to cyclopropane ring-opened products, i.e. , the corresponding (1,3-dimethoxypropyl)naphthalenes. However, the ra te constant for methanol-induced ring opening (Ar-c-C3H5.(+) + CH3OH - -> ArCH-((.))CH2CH2O(H+)CH3) is extremely small (<20 M-1 s(-1) for the alpha-cyclopropylnaphthalenes) despite the fact that ring opening is exothermic by nearly 30 kcal/mol. These results are explained on the b asis of a product-like transition state for ring opening wherein the p ositive charge is localized on the cyclopropyl group, and thus unable to benefit from potential stabilization offered by the aromatic ring.