Theoretical Insight of Structural Effects on Bridge Mononitration of Tetrasubstituted p‐tert‐Butylcalix[4]Arenes

Abstract

In order to deeply understand calix[4]arene bridge mononitration, four representative tetrasubstituted p‐tert‐butylcalix[4]arenes were selected as substrates to theoretically explore calix[4]arene structural effects. Based on our proposed mechanism, the four corresponding calix[4]arene bridge radicals were artificially constructed. Then, three different levels of density functional theory (DFT) were used to calculate their pertinent species energies in radical generations and radical spin densities. The nitration differences of these four substrates were tentatively interpreted through comparison of their reaction activation energies in radical generations and radical stability. As a result, Gibbs activation free energies in radical generations cannot rationally interpret their nitration behaviors while radical stability can, which are characterized with radical stability energies (RSEs) and center carbon spin densities (SDcs). From radical structural analysis, it can be concluded that aromatic ring inversion is remarkably beneficial to enhance calix[4]arene bridge radical stability through strengthening spin delocalization onto their connecting aromatic rings, whereas a substituent with large steric hindrance is unbeneficial due to weakening spin delocalization. The theoretical analysis of radical intrinsic structural effects on their spin delocalizations provides guidance in designing new efficient calix[4]arene‐based substrates.