N-confused oxahexaphyrin: oxidative furan ring-opening and fusion triggered chirality

Abstract

Oxidative ring-closure of N-confused hexapyrrane and thiahexapyrrane followed by oxidative opening of the pyrrole and thiophene rings has been shown to be effective in synthesizing novel porphyrinoids. In this work, a furan unit is incorporated with the purpose to generate a C=O double bond by oxidative furan ring-opening to disrupt the conjugation, resulting in structural distortion and chirality. Thus, a nonaromatic N-confused oxahexaphyrin (1) has been synthesized by the oxidative cyclization of N-confused oxahexapyrrane (N-O-P₅) with terminal confused pyrrole and furan units. Subsequent heating of 1 in MeOH triggered furan-opening and fusion reactions to give globally nonaromatic porphyrinoids 2 and 3 that both contain a carbonyl group and a multiply fused fragment. Thus, 2 possesses a [6.5.6.5]-tetracyclic fragment involving an sp³-C. In contrast, one tetrafluorophenyl unit is generated in 3 by fusion one of the meso-C₆F₅ groups with a furan-generated carbon through removing one HF unit, leading to the formation of a fused [5.6.5.7.6]-pentacyclic fragment. As expected, the C=O double bond present in the frameworks of 2 and 3 results in disrupted conjugation and structural distortion, and the fused rings are favorable for enhancing the stability of the distorted conformations, which enables chiral separation of enantiomers of 2 with central chirality and 3 with planar chirality via chiral high-performance liquid chromatography, with the maximum absorption dissymmetry factor (|g_abs|) of 0.0021 observed for 2. This work provides insight into developing chiral porphyrinoids by post-synthetic methods based on the combination of a highly reactive N-confused pyrrole unit with a terminal furan ring.