ELECTRON-TRANSFER REACTIONS WITH SIGNIFICANT INNER REORGANIZATION ENERGIES - REDUCTION OF SOME FULLY ALPHA-METHYLATED CYCLOALKANE-1,2-DIONES

Electron-transfer reactions of 3,3,omega,omega-tetramethyl-1,2-cycloalkanediones with 5-, 6-, 7-, and 8-membered rings (1-4, respectively) have been investigated. The standard heterogeneous electron-transfer rate constants, measured with mercury electrodes in acetonitrile solvent, decrease by 3 orders of magnitude along the series 1-3 and then increase a factor of 2 on going from 3 to 4. The self-exchange rate constant for the 3,3- was found to be 70 L mol-1 s-1. The cause of these changes is thought to be larger inner reorganization energies for the 7- and 8-membered ring diketones compared with those of 1 and 2. Qualitatively, the largest change in structure on going from neutral 3 and 4 to the radical anion is movement of the O=C-C=O dihedral angle from values near 80-100-degrees toward coplanarity of the diketone function. 1 and 2 undergo smaller changes upon reduction. The radical anion of 4 exists in a transoid conformation. Semiempirical molecular orbital calculations (AM1) give inner shell contributions to the enthalpic barrier for electron transfer in agreement with the order found by experiment, 3 > 4 > 2 > 1. Linear correlation of calculated electron affinities and reversible reduction potentials was observed, and deviation of the result for 4 was explained by weaker solvation of the less polar transoid anion as compared to 1-3, whose anions have a polar cis structure. A linear log-log correlation of heterogeneous and self-exchange electron-transfer rate constants was found for 3 and three acyclic 1,2-diketones. The electron-transfer reactions of 1-4 appear to be one-step reactions without formation of a discrete high energy intermediate anion. The factors that favor two-step electron transfer reactions are discussed in terms of the energetics of reduction of bianthrone.