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20π Antiaromatic Isophlorins without Metallation or Core Modification

Von Wiley-VCH zur Verfügung gestellt

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.


Abstract

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.

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