The one-electron reduction of carbonium ions. Part 13. The conformational effects of cyclopropyl, isopropyl, and phenyl substituents flanked with methyl groups on the reduction rate of the tropylium ion with the chromous ion

Abstract

Methyl-, phenyl-, cyclopropyl-, and isopropyl-tropylium ions and the respective tropylium ions with flanking methyl groups have been synthesized, and the second-order rate constants, k2, for the reduction of the tropylium ions with chromous ion have been determined in 10% hydrochloric acid at 25.0 °C. A linear free-energy relationship is observed when log k2 values for phenyl-, H-, and methyl-tropylium ions flanked with one [(2e), (1b), and (2b)] and also two methyl groups [(3e), (3a), and (3b)] are plotted against those for the corresponding ions without flanking methyls [(1e), (1a), and (1b)]. However, the log k2 values for cyclopropyl- or isopropyl-tropylium ions with flanking methyls [(2c), (3c), and (3d)] significantly deviate upward from the linear correlation lines, reflecting the loss of stabilization caused by restriction on the conformation of the isopropyl and cyclopropyl groups. A linear correlation has also been found between the log k2 values and the charge-transfer energies with pyrene for all the tropylium ions [(1a)–(3e)], indicating that the reducibility of the tropylium ions reflects the electron affinity inherent to the respective ions, irrespective of the presence or absence of the flanking methyl groups.