Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges

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.

一种不含中位桥原子的大型卟啉类似物——环[6]吡啶[6]吡咯（cyclo[6]pyridine[6]pyrrole），已通过铃木偶联（Suzuki coupling）反应合成得到。该大环化合物在中性状态下以两种八字形构象的混合物形式存在，在低极性溶剂中二者可发生快速构象交换。经质子化处理后，其动态扭转结构可转化为不同物种：具体结构取决于质子化程度与抗衡阴离子（counteranions），可呈现褶皱平面结构或八字形构象。通过氢化钠（NaH）去质子化并与三氟化硼（BF₃）反应，可将其转化为二氟化二硼配合物，该过程可将大环锁定为八字形构象。环[6]吡啶[6]吡咯的多种存在形式均具有独特的核磁共振波谱（NMR）、X射线晶体学及光谱学特征。