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Frontier molecular orbitals (FMOs) have played a critical role in predicting reactivity/selectivity of pericyclic reactions. Here we show that the structurally similar iptycene-based hydroquinone ether (HE), that is, MeO Ipt and BO HE/ BH HE, molecules have drastically different ordering of bisallylic and quinoidal FMOs. They are almost degenerate in BO HE/ BH HE, while in MeO Ipt, the bisallylic orbital lies far below the quinoidal HOMO. Oxidation of BO HE/ BH HE induces coplanarization of the methoxy group and destabilizes the bisallylic HOMO, leading to a relatively low oxidation potential. In MeO Ipt, considerable energy must be invested in coplanarization of the methoxy group to bring about orbital swapping, resulting in an oxidation potential higher than that in structurally similar BO HE/ BH HE. As the quinoidal HOMO density does not extend to the substituent-bearing carbon in H-, alkyl-, and alkoxy-substituted iptycenes, their redox potentials remain invariant. This case study involving a simple visual inspection of the nodal arrangement as well as energetics of the FMOs and Walsh analysis could serve as a tool for the design of organic molecules with a desired redox potential.

the journal of physical chemistry letters

When Substituents Do Not Matter: Frontier Orbitals Explain the Unusually High and Invariant Oxidation Potential in Alkoxy-, Alkyl-, and H-Substituted Iptycenes