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

5 years ago

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

Rajendra Rathore, Denan Wang, Shriya H. Wadumethridge, Maxim V. Ivanov

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, ^MeOIpt and ^BOHE/^BHHE, molecules have drastically different ordering of bisallylic and quinoidal FMOs. They are almost degenerate in ^BOHE/^BHHE, while in ^MeOIpt, the bisallylic orbital lies far below the quinoidal HOMO. Oxidation of ^BOHE/^BHHE induces coplanarization of the methoxy group and destabilizes the bisallylic HOMO, leading to a relatively low oxidation potential. In ^MeOIpt, 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 ^BOHE/^BHHE. 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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpclett.7b01971

DOI: 10.1021/acs.jpclett.7b01971