Electrochemical oxidation of [1.1.1.1]pagodanes, bisseco-, seco- and dodecahedra(di)enes: stability of caged 4c/3e radical cations and 4c/2e dications
Abstract
The anodic oxidation of variously functionalized[1.1.1.1]pagodanes A and bissecododecahedradienes B(π,π-distance, ca. 2.8 Å) has been studied by cyclic voltammetry (CV). For nine out ot ten pagodanes A, independent of their functionalization and of the scan rate, an irreversible two-electron oxidation wave (Ep= 1.20–2.23 V) was observed in line with the sequence A→A˙+→B˙+→B2+(ECE) in which the chemical step C is very fast on the CV timescale. For seven of the corresponding valence isomeric dienes B, the first one-electron oxidation wave (B→B˙+), was found to be reversible(Eo′= 0.66–1.39 V) whilst the second one-electron oxidation wave (B˙+→B2+) was found to the irreversible (Ep= 1.20–1.80 V). The standard half-wave potentials allowed a thermodynamically meaningful estimate of the conjugative stabilization of the unusually persistent caged, non-classical 4c/3e radical cations B˙+(ΔEo′= 0.91 V for the parent diene B and the monoene reference) and of the destabilization by various functionalization patterns (0.40–0.73 V). For the secododecahedradienes D(π,π-distance ca. 2.85/3.2 Å) and the dodecahedradienes E(π,π-distance ca. 3.5 Å), only irreversible one-electron oxidation waves were recorded [Ep=(0.8)–1.46 V and (1.0)–1.39 V, respectively]. The degree of conjugative stabilization in the increasingly spherical, yet still non-classical 4c/3e radical cations D˙+ and E˙+ is approximated by ΔEp= 0.62 and 0.39 V, respectively.