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

meso-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 T0 and T0(4,D) Huckel structures. Mobius transition states are located in the interconversion pathways with activation barriers of 27 kcal mol(-1). Conversely, [24]pentaphyrins is able to switch between Huckel and Mobius conformers with very low activation barriers. However, nonfused [24]pentaphyrins are unstable and spontaneously undergo and N-fusion reaction driven by the strain release. On the contrary, nonfused [22]pentaphyrins could be isolated if a T0(4,D) conformation is adopted. Importantly, conformational control of pentaphyrins can be achieved by meso-substituents. Two stable conformations (T0(4,D) and T0(A,D)) are found for the nonfused [22]pentaphyrin, which are delicately balanced by the number of substituents. The T0(A,D) conformation is preferred by fully meso-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.