The one-electron reduction of carbonium ions. Part 12. The conformational effect of the para-cyclopropyl group on the stability of phenyltropylium ions and on their reducibilities with chromium(II) ion

Abstract

A series of p-cyclopropylphenyltropylium ions with and without flanking methyl groups has been synthesized together with various methyl-substituted phenyltropylium ions as reference compounds. The conformational effect of the para-cyclopropyl group upon the cation stability has been studied by means of ¹H n.m.r. spectroscopy, pK_R+ measurements, and rate measurements of one-electron reduction with chromium(II) ion. Examination of the ¹H n.m.r. spectra and the pK_R+ values of these cations indicates that stabilization due to charge delocalization to the para-cyclopropyl group decreases with the introduction of the flanking methyl groups. A linear free-energy relationship is observed when log k₂ for the chromium(II) ion reduction is plotted against the summation of the substituent constants σ⁺ for unsubstituted, m- and p-methyl-, m,m-dimethyl-, and p-cyclopropyl-phenyltropylium ions, but log k₂ for the p-cyclopropyl-substituted cation with one or two flanking methyl groups deviates greatly from the linear correlation line, reflecting the loss of stabilization caused by conformational change of the cyclopropyl group.