Conformational Control in - and Pentaphyrins(184.108.40.206.1) by Meso Substituents and their N‑Fusion Reaction
journal contributionposted on 19.02.2016, 10:11 by Mercedes Alonso, Paul Geerlings, Frank De Proft
meso-Substituted pentaphyrins(220.127.116.11.1) 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 pentaphyrin was reported in 2012. Here, the conformational preferences and N-fusion reaction of - and pentaphyrins have been investigated using density functional calculations, together with their aromaticity-molecular topology relationships. Two global minima are found for the unsubstituted pentaphyrin corresponding to T0 and T04,D Hückel structures. Möbius transition states are located in the interconversion pathways with activation barriers of 27 kcal mol–1. Conversely, pentaphyrin is able to switch between Hückel and Möbius conformers with very low activation barriers. However, nonfused pentaphyrins are unstable and spontaneously undergo an N-fusion reaction driven by the strain release. On the contrary, nonfused pentaphyrins could be isolated if a T04,D conformation is adopted. Importantly, conformational control of pentaphyrins can be achieved by meso-substituents. Two stable conformations (T04,D and T0A,D) are found for the nonfused pentaphyrin, which are delicately balanced by the number of substituents. The T0A,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 pentaphyrins in excellent agreement with the experimental results.