INDEPENDENT EXISTENCE IN THE GAS-PHASE OF THE 1-METHOXYCYCLOPROPYL AND 2-METHOXYALLYL CATIONS, AS OBSERVED IN AN ION-CYCLOTRON RESONANCE SPECTROMETER

Substitution reactions of cyclopropyl compounds which have a good leaving group × at C1 occur in the condensed phase, often with a disrotatory cleavage of the C2-C3 bond. However, this ring rupture can be suppressed or even prevented if an electron-donating group Y is present at C1 since it can supply electron density to the cyclopropyl cation center at C1 as the group × leaves. If this substituent effect predominates over the donation of electrons from the C2-C3 bond, then substitution of group × without ring cleavage can take place. This electronic stabilization of the intermediate cyclopropyl cation has been found for various electron-donating groups Y such as dialkylamino, phenylthio, o-hydroxyphenyl, cyclopropyl, vinyl, methylthio, p-methoxyphenyl, p-tolyl, methoxy, arylethynyl, and several aryl groups. For Y = N(CH3)23, 4 and NHCH3 the intermediate cyclopropyl cations have been observed in NMR. © 1979, American Chemical Society. All rights reserved.