Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges
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A large porphyrin analogue, cyclo[6]pyridine[6]pyrrole, containing no meso bridging atoms, has been synthesized through Suzuki coupling. In its neutral form, this macrocycle exists as a mixture of two figure-eight conformers that undergo fast exchange in less polar solvents. Upon protonation, the dynamic twist can be transformed into species that adopt a ruffled planar structure or a figure-eight shape depending on the extent of protonation and counteranions. Conversion to a bisboron difluoride complex via deprotonation with NaH and treatment with BF3 acts to lock the macrocycle into a figure-eight conformation. The various forms of cyclo[6]pyridine[6]pyrrole are characterized by distinct NMR, X-ray crystallographic, and spectroscopic features.
