Synthesis of Stable Nonaromatic Fluorenonephyrin Macrocycle

Abstract

AbstractA new derivative of our recently reported nonaromatic fluorenophyrin 5 named fluorenonephyrin 6 was synthesized over a sequence of six steps starting with commercially available fluorene as precursor. Our attempts to direct oxidation of 5 to 6 were not successful. Hence, we synthesized 2,7‐bis(phenyl(1H‐pyrrol‐2‐yl)methyl)‐9‐fluorenone (fluorenone tripyrrane) in five simple steps from fluorene, and condensed it with pentafluorobenzaldehyde under mild acidic conditions in CH2Cl2 followed by oxidation with DDQ and chromatographic purification to afford fluorenonephyrin (9%). 6 was characterized and studied by HR‐MS, 1D and 2D NMR spectroscopy, absorption, electrochemical and DFT/TD‐DFT techniques. The DFT studies revealed that 6 was more distorted than 5 and that introduction of C=O in the macrocyclic core remarkably distorts the pyrrole rings of the dipyrrin unit from the mean plane of macrocyclic framework. The macrocycle showed a band at around 390 nm and a broad band at around 650 nm. The redox properties indicated that the macrocycle was highly electron‐deficient in nature. TD‐DFT studies were in accordance with the experimental observations.