20π antiaromatic isophlorins without metallation or core modification

Isophlorins (two-electron-reduced porphyrins) with 20π antiaromaticity have been synthesized by metallation or core modification (replacing the inner NHs with less bulky atoms) thus far. The core-modification number (CMN) is an important factor for the stability of isophlorins against their oxidation to porphyrins. However, isophlorins with small CMN (≤ 2), i.e., without significant core modification, are difficult to synthesize and important for understanding their structure–antiaromaticity relationships. Herein, we report the synthesis of free-base (metal-free) antiaromatic isophlorins with no core modification (CMN=0) for the first time in the literature. β-Tetracyanoporphyrins were successfully reduced by hydrazine to their corresponding isophlorins under aerobic conditions. These porphyrin/isophlorin transformations are reversible. Their structure and antiaromaticity were studied via 1H NMR and UV-vis/near-infrared absorption spectroscopy, X-ray diffraction and theoretical calculations.