Study On The Aromaticity And Cyclization Mechanism Of ?-Conjugated Helical Oligomers

The aromatic hydrocarbon compounds are essentially artefacts of conjugated cyclic molecules(closed-shell system),e.g.annulenes,that are stabilized by the delocalization of either[4n+2]? electrons according to the Hückel rule,or[4n]?electrons with M(?)bius topology.Heilbronner's theoretical prediction for Mobius topology in twisted molecules has stimulated extensive studies of various isomers and conformations of higher annulenes.Since the twist in the ? system is usually introduced by a suitable arrangement of E and Z double bonds,the direct synthesis or isolation of these compound is expected to be a great challenge due to a large number of energetically closed isomers.Moreover,twisted cyclic molecules are destabilized by large ring strains and suppressing overlap of the p orbitals involved in electron delocalization and stabilization.In larger cyclic molecules,ring strain is less pronounced but the structures are very flexible and flip back to the less-strained Hückel topology.However,rational control the flexible twisted conformation in macrocyclic molecules is still a great challenge in the construction of Mobius strip.Focusing on how to induce Mobius aromaticity,numerous types of sign inversion have been proposed and introduced into cyclic conjugated structures.However,all these reported Mobius strips are based on a two-dimensional cyclic molecular structure,which is also the basic cognition for Hückel systems.Recently,through-space electronic delocalization in acyclic systems via noncovalent interactions has been flourished.As one of the typical noncovalent interactions widely existed in biological systems,?-? stacking is expected to be a valid strategy to achieve the through-space ?-electron delocalization,and can be further utilized for the development of advanced materials and supramolecular systems.Herein we report the first example of a highly stable antiaromatic Mobius strip based on an acyclic helical pyrrole oligomer,in which through-space ?-? stacking participate in the conjugation pathway.The calculated very high through-space electron transfer rate(1012·S-1)as well as the anisotropic ring current deduced from NMR and magnetic circular dichroism(MCD)spectroscopies,provide evidences for the direct transmission of ? conjugation across a ?-? stacking unit.This work paves an avenue for stabilizing helical topology via noncovalent through-space electron delocalization.Besides,we also designed and synthesized a carbonyl rhenium complex of the ?conjugated helical oligomer.Its molecular structure is similar to that of the ligand,possessing a ?-conjugated helical structure with an intramolecular ?-? stacking unit.The stability of the conjugated helical oligomer structure is further illustrated and the anti-aromatic properties of the system are demonstrated.In the acid titration experiment of the conjugated helical oligomer,we found that the long-term solution will undergo a ring-forming reaction to form a porphyrin derivative.Subsequently,it was characterized by absorption spectroscopy and electron paramagnetic resonance spectroscopy,and it was found that the conjugated helical oligomer has double-radical properties,and the corresponding double-radical cyclization mechanism was proposed.