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. 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, UV‐vis/near‐infrared absorption spectroscopy, X‐ray diffraction and theoretical calculations. For the first time, the synthesis of free‐base (metal‐free) isophlorins with 20π antiaromatic character was achieved without core modification. The reduction of β‐tetracyanoporphyrins by hydrazine under aerobic conditions yielded isophlorins. These 18π aromatic porphyrin/20π antiaromatic isophlorin redox interconversions are reversible.