The one-electron reduction of carbonium ions. Part 14. Effect of successive introduction of methyl substituents on the reducibility of tropylium ion in chromium(II) ion and cathodic reductions

Abstract

The second-order rate constants (k₂) of one-electron reduction with Cr^II ion of methylated tropylium ions [C₇H_{7 –n}(CH₃)_n⁺ClO₄^–, n= 0–7] were determined in 10% hydrochloric acid at 25.0°C. Reduction peak potentials determined by means of triangular wave cyclic voltammetry and charge transfer energies with pyrene were compared with the rates of Cr^II ion reduction in terms of the effect of methyl substituents. The log k₂ values decrease linearly with an increase in the number of methyl groups up to n= 4. However, the decelerating effect per extra methyl substituent becomes smaller when n is 5 or 6, and the seventh methyl substituent accelerates the reduction. The methyl substituent effect on the reduction peak potentials of the carbocations is similar to that found for the Cr^II ion reduction, giving rise to a linear correlation. A Marcus treatment suggests that Cr^II ion reduction proceeds through an outer-sphere mechanism. The charge transfer energies with pyrene are linearly correlated with reduction potentials for ions with n= 0–6, but considerable deviation is noted when n is 7. The charge transfer energies are linearly correlated with the LUMO energy levels of the methylated tropylium ions, suggesting that the unexpectedly pronounced reducibilities of hexa- and hepta-methyltropylium ions are attributable to steric factors. The deviation from linearity in the Cr^II ion reduction and the reduction potential are reasonably explained by assuming that non-bonded repulsive interactions among congested methyl substituents in the carbocations are relieved upon reduction to their corresponding radicals. This explanation is consistent with our previous conclusion from a ¹³C n.m.r. study that the methyl substituents of 1,2,3,4,5-penta-, hexa-, and hepta-methyltropylium ions are highly congested and distorted out of plane.