Influence of Carbon Group Substituents on Bond Shift and Electrochemical Reduction of Cyclooctatetraene

Abstract

The free energies of activation for bond shift in the carbon group substituted cyclooctatetraenes (COT−M(CH3)3) in THF-d8 at 298 K have been determined to be 16.4, 16.2, 16.2, and 18.1 kcal/mol for M = Si, Ge, Sn, and C, respectively, and 15.6 kcal/mol for CH3−COT. These data permit an interpretation of the previously reported opposite orders for the ease of the first and second electrochemical reductions in the Si, Ge, and Sn compounds. It is postulated that the order of the first reduction potential is controlled by a decrease in overlap between the substituent and the ring π orbitals in the order Si > Ge > Sn, whereas the second reduction potential is controlled by the energy gap between the symmetric π HOMO of the COT radical anion and an interacting substituent σ* orbital of π symmetry (εσ*π − επ), which increases in the order Sn < Ge < Si. HF/3-21G molecular orbital calculations indicate that the high barrier for t-Bu−COT primarily reflects steric effects in the transition state.