Conformational Control in [22]- and [24]Pentaphyrins(1.1.1.1.1) by Meso Substituents and their N‑Fusion Reaction

Abstract

<i>meso</i>-Substituted pentaphyrins(1.1.1.1.1) were unexpectedly isolated as N-fused species under Rothemund-type conditions. The reaction mechanism is unknown at present, but the first example of a nonfused [22]­pentaphyrin was reported in 2012. Here, the conformational preferences and N-fusion reaction of [22]- and [24]­pentaphyrins have been investigated using density functional calculations, together with their aromaticity-molecular topology relationships. Two global minima are found for the unsubstituted [22]­pentaphyrin corresponding to <i>T0</i> and <i>T0</i>^4,D Hückel structures. Möbius transition states are located in the interconversion pathways with activation barriers of 27 kcal mol^–1. Conversely, [24]­pentaphyrin is able to switch between Hückel and Möbius conformers with very low activation barriers. However, nonfused [24]­pentaphyrins are unstable and spontaneously undergo an N-fusion reaction driven by the strain release. On the contrary, nonfused [22]­pentaphyrins could be isolated if a <i>T0</i>^4,D conformation is adopted. Importantly, conformational control of pentaphyrins can be achieved by <i>meso</i>-substituents. Two stable conformations (<i>T0</i>^4,D and <i>T0</i>^A,D) are found for the nonfused [22]­pentaphyrin, which are delicately balanced by the number of substituents. The <i>T0</i>^A,D conformation is preferred by fully <i>meso</i>-aryl pentaphyrins, which is converted to the N-fused species. Interestingly, the removal of one aryl group prevents the N-fusion reaction, providing stable aromatic nonfused [22]­pentaphyrins in excellent agreement with the experimental results